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Author Topic:   engine raising questions
montaukman posted 06-08-2005 09:41 PM ET (US)   Profile for montaukman   Send Email to montaukman  
Hi Everyone,

I am posting this again on a brand new topic so that it gets visibility. Please forgive my redudancy as i am desperate to change my engine height before the weekend.

Jimh, I apologize for this and if you feel it necessary to remove it to save bandwidth, i will understand. Thanks and sorry about being a pest.
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I measured my stock engine height positioning compared to the keel of the transom and it appears my cav plate is approx in line with the keel. I have the Tigershark foil so i will def need to raise the engine.

I am not sure if I will do one or two holes. It is currently mounted second from the top.

That being said and reading all the other posts and directions, i am curious about a few things.

1.)The bracket seems to be integrated to the engine(one piece) and is mounted to the transom via 4 bolts. if you raise the bracket and engine up, the bracket will over extend the top of the transom. Also, it will leave a sealant and trim edge. Am I missing something or is the engine separate from the bracket. Does the bracket stay put while the engine mounts higer or lower? I can't imagine that that bracket rises with the engine. It will look odd extending an inch above the top of the transom.

2.) Is it necessary to reseal the holes and bracket? What gets sealed? The Merc manual shows just the screw shanks getting sealled.

3. There are four bolts on that hold the engine to the transom. As mentioned in an earlier reply, the merc 90 uses predrilled holes for adjusting (not slots) so the concept of sliding the engine higher or lower eludes me. Don't all the bolts have to be removed completely and then you raise the engine/bracket. Clamping is mentioned for keeping the engine attached to the transon (when not using a hoist). How is this done. I imagine you need a hefty c-clamp.

The whole process in theory seems simple. I just don't see how, if at all, the engine separates from the bracket. that might answer some of my questions.

Thanks for any help. I put on my fins and i still have 25 hours before i have my maintenence done so I want to do this myself rather than wait till i eat up 25 hours.

Thanks again,


2manyboats posted 06-08-2005 10:14 PM ET (US)     Profile for 2manyboats  Send Email to 2manyboats     
boy oh boy
montaukman posted 06-08-2005 10:35 PM ET (US)     Profile for montaukman  Send Email to montaukman     

I know, I know...Indulge me a little. Need all my facts before i do this. We all started somewhere.


2manyboats posted 06-08-2005 10:46 PM ET (US)     Profile for 2manyboats  Send Email to 2manyboats     
Engine stays with the bracket

top of bracket is above the top of the transom

reseal any bolt or screw you remove and reinstall

If you have no bolts in slots , you will need something to hold up the motor

And remember the skinny end of the engine stays pointing down

montaukman posted 06-08-2005 11:20 PM ET (US)     Profile for montaukman  Send Email to montaukman     

Glad you pointed that out to me. I couldn't for the life of me figure out why the prop wasn't working just spinning in the air like that. Worked great at trimming my goatee though.

Tomorrow I'll post my next question on which edge of the key needs to be facing up when you start the engine.

Thanks for your help on the other info. It was helpful.

All the best,


2manyboats posted 06-08-2005 11:25 PM ET (US)     Profile for 2manyboats  Send Email to 2manyboats     
If you plan to run prop up you will need a larger prop, you will also need a very long hose for the muffs.
bigjohn1 posted 06-09-2005 01:58 AM ET (US)     Profile for bigjohn1  Send Email to bigjohn1     
you guys are killing me! I very nearly fell out of my chair in laughter!
Marlin posted 06-09-2005 07:23 AM ET (US)     Profile for Marlin  Send Email to Marlin     
2many certainly gave you the succinct version of your answers. But seriously, move the engine one hole at a time and see how it performs. You'd like to have your foil above the water surface at fast planing speeds (say, 4000+ RPM), but too high and the prop will ventilate when trimmed out, in tight turns, or in chop.

Some people use an engine hoist to hold up the outboard while it's unbolted. I use a rafter in my garage. The boldest folks, which I'm not and you shouldn't try to become, just let it lean against the transom. Let the motor stay still, and ajust the transome height with your trailer's tongue jack or wheel until the next set of holes lines up.

No need to seal the bracket to the transom, just the bolts so that there's no water intrusion into the transom, which can result in rotted plywood.

lin posted 06-09-2005 07:56 AM ET (US)     Profile for lin  Send Email to lin     
What is your strategy for holding up the motor? Do you have access to a hoist? It can be done with out a hoist, but I've seen it done and it was really scary to watch-- and that was a lighter 2 stroke engine and it wasn't mine!
jimh posted 06-09-2005 08:20 AM ET (US)     Profile for jimh  Send Email to jimh     
Alan writes:

"I can't imagine that [the] bracket rises with the engine."

You are going to have to start imagining this, because the engine mounting bracket and the engine are rather firmly attached to each other. This is what holds the engine on the boat.

I recommend that during the entire process of unbolting the engine, moving it to a new location, and re-bolting the engine on the transom, that you have the engine firmly supported from an overhead lift so that it cannot possible fall.

The many questions you have asked about how to perform this procedure have imparted in us all a presage of an unfortunate outcome.

montaukman posted 06-09-2005 09:00 AM ET (US)     Profile for montaukman  Send Email to montaukman     
Ok.....well i have all my answers and i want to thank all of you for this. Even you 2manyboats. You were very helpful even though I had to wade through the sarcasm. luckily I just happened to have some chest waders in the garage to go with the long hose for the muffs.

Jimh, I am a novice and I take all the advice and warnings offered by this forum very seriously. I do plan on raising the engine. I think what I will do though is buy or rent a hoist and buy an pulling eyelet from mercury so that I can do this safely. My friends are a selfish bunch and are only available when it comes time for fishing.

I see now that the bracket raises with the motor and that it will extend over top of the transom. I think thats gonna look lousy but if thats how it was designed, it is what it is.

So that being the case, i will be purchaseing a ring, renting a hoist and making the change myself.

I have one last question and it is probably not an issue. the steering rodcoming out of the back of the inside wall of the transom is just touching the top of inside circular cutout in the transom. It is a non flexible metal rod used to push and pull the engine in either direction (turning).

My question is, will the rod retard the raising of the engine or will it still rise regardless of the rod jutting up against the inside of the cutout.

Thank you all again for your help, patience and advice. I know that I tend to need very simplistic instructions on some topics but it all stems from making sure that i am completely on top of what must be done. thank you again for understanding and putting up with it.

All the best,


P.S. Caught my first Striped bass last Saturday night. Actually two. for a while i thought i ws curse..hadn't brought a fish into the boat since purchasing it. Good to know the fishing gods shined on me. It was awesome....cigar in mouth, rock station on the radio, (playing low) and a gorgeous night. a beer or two and a few bass. Mybe i'll name my boat "BASSHOLE" -

Sheila posted 06-09-2005 01:54 PM ET (US)     Profile for Sheila  Send Email to Sheila     
Alan, I'm glad you caught some fish.

Do I not recall that recently (perhaps right after you took delivery) you agreed that running the boat for a season before modifying the factory set-up was a good idea? I seem to remember that Jimh (I think it was Jim) pointed out that Boston Whaler had put some time and effort into determining the proper engine height.

I'm finding, after 7 months of boat ownership, that improvements in performance come from my increasing skill at handling the boat, adjusting trim, and so forth.

I might humbly suggest that as your skills continue to develop, you'll reach the same conclusion.

fishgutz posted 06-09-2005 02:25 PM ET (US)     Profile for fishgutz  Send Email to fishgutz     
I have a Tigershark VG foil on my Dauntless 14. My cav plate is exactly in line with the keel. Personally I would not move the engine up.

Why would you think you have to get the fins out of the water?

After many sea(lake) trials in Lake Michigan around Milwaukee with the unbelievable chop we have, I've found the boat to be much more stable at higher speeds with NO loss of top end. The boat doesn't get bounces around nearly as much as it used to and "powers" through the chop much better. The foil actually stablizes the boat at all speeds and in all conditions.

My top end IN CHOP has actually increased and top end in calm conditions has not suffered at all.

My wife used to complain, "You're going too fast" when we'd be bouncing around. Not any more.

At the MOST, I'd maybe go up one hole, but keep some if not most of the fin in the water

I hope this helps.

montaukman posted 06-09-2005 02:48 PM ET (US)     Profile for montaukman  Send Email to montaukman     
Hi Sheila,

I am impressed. Gotta you have such an incredible memory that you remembered my posting from the many that are submitted? Either that or I must have made an impression......bad or good, a memorable

Yes I do recall saying that but I have to tell you, when I wrote that stuff, I had yet to take my wife and daughter out and yet to hear them bitch and whine about my going too fast (19 MPH) just to get on plane.

This was a progression. I read about foils helping lower plane speeds. Decided to buy one. Then I read about how it has to be level with the water when on plane and mine certainly was not so I looked and begged for information on rasing the engine. So, when I wrote that, I fully expected my family to take to the boat like I had. Things change and as such, so do plans. To be honest, i don't think they wil be on the boat much due to a scare they received when traveling through 2 major wakes at 5 mph and the bow rise that occured from these monster wakes hitting simultaneously. They do not want to go on the boat anymore and are asking me to trade it up to a cruiser (if you can believe that). Sea Ray forum, here I come....just kidding.

Fishgutz, I must tell you that since installing the Tigershark, i haven't noticed any improvement at all. I have been out three times in various conditions and loads and if anything, it seems to make the boat a tad more sluggish. I def don't see getting on plane faster with it. Still doing the 19 MPH required.

Is it possible that the hull shapes are different between the dauntless and montauk and maybe the fins work more agressively on your shape hull rather than mine. I will be raising it one notch to see. I can alwys go back and remove the fins as well. Just epoxy and paint.

Thanks for all you concerns and help.


rtk posted 06-09-2005 06:06 PM ET (US)     Profile for rtk  Send Email to rtk     
Alan, I can certainly understand your efforts trying to improve the ride of the boat to deal with sea conditions, especially to make the boat more comfortable for family.

The reality of the situation is it is a small and relatively light boat. Large boat wakes are going to pound you regardless of what you do with the boat. A boat wake is a very steep wave. If you slow your planing speed down to even 13 or 15 miles per hour a large boat wake is going to pound and launch the boat.

With a small boat you really need to approach the boat wake at an angle or speed that will cause the least amount of pounding. Running into it at a 90 degree angle will lift the bow with or without the fin, trim tabs on a light boat, no matter what speed you are going. In a small boat you need to learn how to take the wakes.

The bottom line is running a small and light boat in a short chop or alot of boat wakes, there will be a fair amount of pounding.

I live in an area that on the weekends the bay is significantly chopped up due to the amount of boats out. In a small boat the ride is brutal. It is my reason for going to the 21 Outrage I have.

Sounds like a 21 or 23 Outrage, Conquest/Walkaround or Ventura is more suited to your boating needs to make the family more comfortable.


jimh posted 06-09-2005 08:44 PM ET (US)     Profile for jimh  Send Email to jimh     
The OEM Lifting Ring and an engine hoist are one way. A good stout tree limb and some decent rope, properly tied, are another. You don't have to change the engine position at all; you just move the boat by using the tongue jack to raise or lower as needed.
chopbuster posted 06-09-2005 08:54 PM ET (US)     Profile for chopbuster  Send Email to chopbuster     
I could never define Great Humor, but I know it when I see it. :)


bigjohn1 posted 06-09-2005 09:26 PM ET (US)     Profile for bigjohn1  Send Email to bigjohn1     
Since you plan to purchase the special ring and rent a hoist, you may now be talking about expending more cash than letting the dealer charge you one hour labor to do it for you. Just an idea....but I generally like to do as much as possible myself to make sure its done correctly.
montaukman posted 06-09-2005 10:49 PM ET (US)     Profile for montaukman  Send Email to montaukman     
Hi Everyone,

Yeah we got pounded that day. I was ok with it but the family realy got affected. I know about handling wakes at 3/4 but when you have two major wakes coming at you in right degrees to each other, it is kinda hard to take both at a comfortable angle. I never felt like i was in danger but it was hairy for them. Hopefully they will venture out with me again.

Jimh, The tree limb idea is a good one. I spoke with my local mercury dealer and he said that the special ring used is only for the 2 stroke. He said that I need three pieces of chain and need to attach each one to the three loops situated around the engine. They should then meet in the middle and a hoisting hook can attached.

I am going to check my engine manual to see if he was right about the special eye that supposedly gets screwed into the fly wheel.

I found a place to rent a hoist for $50 a day. I can pick it up on a Saturday and return it early Monday and still only get charged for a day.

Inexpesive cheap hoists are about $120, which isn't alot but the shipping is close to that as well. I can't find a local place selling one fo rless than $160.

They also sell the type of hoist that you can attach to a joist or tree limb and raise and lower. via a mechanical ratchet. Sears sells one for $25. Thats a possibility too.

You all have been great on here and i want to thank you for putting up with my questions. If any of you are on Long island and in need of a hoist, a beer or a combination of the two (not at the same time), feel free to look me up.

All the best,


Knot at Work posted 06-10-2005 08:31 AM ET (US)     Profile for Knot at Work  Send Email to Knot at Work     

Forgo the grief, it is simply a little bantering form the community. I appreciate your intrepid spirit and moxie as you do it yourself.

Like Gen Patton said, "do not take counsel of your fears."

Sheila posted 06-10-2005 11:42 AM ET (US)     Profile for Sheila  Send Email to Sheila     
Alan, I'm either blessed, or cursed, depending on how you look at it, with that kind of memory.

I'll leave the suggestions on adjusting your motor's height to those with experience in that area.

Regarding your family's comfort (or lack thereof) on the boat:
Have you taught your wife how to operate the boat? Some familiarity with her (the boat's, not the wife's!) operation might make her more comfortable. It could also save you all should you ever become incapacitated while underway....the same goes for your daughter, if she's of an appropriate age.

The ideal would be for the two (or three) of you to take a course together from the USCG Auxiliary or the US Power Squadron. You might also consider buying her a copy of Powerboating: A Woman's Guide by Sandy Lindsey.

Best of luck!

montaukman posted 06-10-2005 01:06 PM ET (US)     Profile for montaukman  Send Email to montaukman     
Hi Sheila,

My wife is of the type that just wants to get in the boat and not be bothered. I don't think i could ever sell her on taking a course, let alone getting behind the wheel. I will suggest it as it is a very good (and smart) idea.

i should have married my boat.....less arguing and easier to get rid my boat will hold her value better over the years.....ouch. Hope no one knows my wife.

Thanks for the help Sheila.

Knot at work, thanks for your suppost. I am looking into a lifting ring now. I looked up the part in my mercury manual and it is a $250 item. It is a flywheel puller/lifting ring combo. I don't think i wil go that route if it is that expensive. Anybody wanna share the ewxpense? It could visit each house for a period of a month just like the stanley cup. I am lookinginto other lifting rings. We'll see how it goes

All the best,


Sheila posted 06-11-2005 03:49 AM ET (US)     Profile for Sheila  Send Email to Sheila     
Hi Alan,

You, of course, know your wife, and your relationship. But perhaps you can find a way to persuade her. Appeal to "togetherness." Tell her how proud you'll be of her. Appeal to her sense of safety onboard. Offer to find someone else to teach her if you're not the right guy to teach your wife to drive the boat (a wife can be so eager to not mess up that she gets nervous with her husband teaching her). Tell her she'll be setting a good example for your daughter....

If she's a reader, start with the book I suggested.

If all else fails, have her call me.

If you can find a way to encourage her, I'd almost guarantee you that her confidence and sense of comfort onboard will improve if she can handle some basic maneuvers on the boat.

montaukman posted 06-13-2005 09:30 PM ET (US)     Profile for montaukman  Send Email to montaukman     
Hi Sheila,

I admire your optimism on this subject but unfortunately, there is no way my wife will ever want to learn to drive the boat or help in any way. I love her but she is a pip to say the least. She refuses to drive highways and still to this day can not ride a bicycle.

Due to finances, another boat (bigger)at this point is out of the question (a consequence i am happy about). She and my daughter will learn to deal. They have to. In the mean time, i will continue to take them in the 5 mph areas and gradually gravitate towards faster more populated waterways.

So for now, I am still a devoted 170 owner. Thanks so much for your advice and your offer to speak with her. That is over and above forum help and you have once again proven yourself to be an amazing woman..strike that...amazing person.

All the best,


P.S. Mercury 4 stroke eyelift/flywheel puller is about $275. The standard screw-in lift ringfor older 2 strokemercs will not work. Supposedly there are two liftng rings permanently attached to the engine already. I have to locate them. A simple chain and shackle setup should work. I haven't loked for them yet. Will try tomorrow most likely. Wanna get the engine the right height for my big Dad's day excursion. 4 dads fishing. Gonna be crowded, messy and a blast.

bigjohn1 posted 06-13-2005 10:03 PM ET (US)     Profile for bigjohn1  Send Email to bigjohn1     
What about installing a quality overhead hoist and lifting beam in the'll always have it and can use it anytime you want to raise and lower the engine.
Sheila posted 06-14-2005 02:41 AM ET (US)     Profile for Sheila  Send Email to Sheila     
Thank you for that gracious response to what, in hindsight, struck me as an egocentric, childish post from me. If she prefers to remain a passenger, well, that's her preference, and not for me to judge, not to attempt to "change."

Best of luck dialing your boat in just the way you want her.

montaukman posted 06-15-2005 10:33 AM ET (US)     Profile for montaukman  Send Email to montaukman     
Hi Sheila,

I never saw your reply in that way. No worries. Thanks again,


I like your idea. I have to see if the montauk will fit in my garage. It might be tight with the trailer. I'll measure it today.

I removed the engine cowl, took off the flywheel cover and saw the two mounted lifting rings on the 4 stoke. It seems pretty easy to just buy some chain and couplings, attach them to the rings and a hoist and just hoist the engine up.

Is anyone familiar with a method of rigging a hoist to the beams on the ceiling of the garage. Will two beefy eyebolts with a chain attached to them hold into the lumber if the hoist is then attached to the that chain?

Maybe i can just loop the chain over the beam. Is a 2 X 6 beam stong enough to hold 400 lbs if looped in the most stressful, bendable area on the beam?

Jimh mentioned a stron tree limb but unfortunately i would have to drive the trailer across may lawn to get to one.

Thanks for the help,


Marlin posted 06-15-2005 11:34 AM ET (US)     Profile for Marlin  Send Email to Marlin     
I wouldn't use screw eyes, at least not screwed into the underside of the joist, since it would take far less force to tear them out than it would to break the 2x6. I'd just loop the chain over the joist.

I'm sure somebody here can look up the yield strength of a 2x6 that spans over 10 feet or so, but keep in mind that it doesn't need to support 400 pounds. You don't need to suspend the engine, you just need to support it to keep it from falling over. Most of the weight can be on the skeg. The joist will be plenty strong enough for that, no problem.

With the motor detached and remaining still, you'll use your trailer tongue jack to move the height of the transom until the desired set of holes lines up.


montaukman posted 06-15-2005 11:39 AM ET (US)     Profile for montaukman  Send Email to montaukman     

Good point...I forgot that the engine wasn't going to be totally supported by the beam. still....would be a good back up to know just in case the wood under the skeg mozes or travels past the skeg.

Thanks for your help


montaukman posted 06-15-2005 11:39 AM ET (US)     Profile for montaukman  Send Email to montaukman     

Good point...I forgot that the engine wasn't going to be totally supported by the beam. still....would be a good back up to know just in case the wood under the skeg mozes or travels past the skeg.

Thanks for your help


Plotman posted 06-15-2005 02:14 PM ET (US)     Profile for Plotman  Send Email to Plotman     
You don't need an engine lift, and I wouldn't hang that much weight from your garage joists. Depending on how your garage is built, the horizontal members of a truss aren't designed to do anything other than hold the walls from spreading.

In any event, forget the engine lift, chains etc. IMHO using the trailer tounge jack is the safest way to do this because you are never lifting anything. Just chock the wheels to keep the trailer from moving, and use ratchet tie-down straps betwen the transom eyes and the motor mounting bracket to keep the motor standing upright after you remove the bolts.

I have used this method a number of times to change the height on a 225, all by me lonesome. It takes 15 minutes or so from start to finish. It is almost silly how simple it is.

If your skeg still stands well clear of the ground with the tounge all the way up and you are uncomfortable devising a solid support under the skeg, you can back your trailer perpendicular to a curb and use that added height to your advantage.

I still say that raising the engine is not likely to materially lower the speed at which you can hold the boat on plane, and will likely have other uninted consequences, but it's your boat.


JohnJ80 posted 06-16-2005 10:09 AM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
Ditto - I agree.

Don't waste your time measuring the AV plate to the hull. There are too many variables to getting it right and you have no decent datum to measure against. A 1 degree variance in motor angle to the transom turns into something like a 1.04" error at a 12" separation (do the math and trig). Since the bolt holes are 3/4" a piece, then - well you see - it is a waste.

Do this experimentally. Higher is better than lower. I would probably go to the hole below the max raised hole and see how that is. If that worked fine I would probably go up all the way.

If I operated in big chop, I would stay one down from the top.

Anyhow the point is that getting the hole position right for the WAY YOU LOAD AND USE THE BOAT is an experimental process.

I basically spent a whole summer fooling with this. I bet I moved my motor (trailer method) 7 or 8 times. I researched engineering papers on boat porpoising and water dynamics, I added fins, I installed trim tabs.

What I can tell you is that hole position has a dramatic impact on your performance and boat ride. If it is wrong, you will hammer through chop - to the point where you think your fillings are coming out. If it is right, the boat will have a sweet ride. The range from bad to good can be one hole position.

Do some searches on my screen name and this subject, repowering and porpoising. There has been a ton of good discussion on this.


davej14 posted 06-17-2005 01:05 AM ET (US)     Profile for davej14  Send Email to davej14     

After reading your posts, I am convinced that you would be well advised to take your 170 to the local dealer the first time you change the motor height. This will give you some first hand experience with the process of removing and resealing it to the transom. You can then decide if you want to take this on for yourself in the future.

You also may be expecting too much from the changes. I have tried several motor positions, changed props and added a fin. My experience is that you will achieve incremental improvement but nothing "dramatic". You will not be able to get the 170 to plane at 10 MPH no matter what you do.

My wife was also unconfortable at planing speeds when we first got the boat. It is getting better each trip, be careful not to "push" too hard or it will get worse.

Let us know how you make out.

garijt posted 06-17-2005 08:42 PM ET (US)     Profile for garijt  Send Email to garijt     
Hi Alan

I have read your request with interest and have concern for safety and other issues. The last thread was right on. Know this though. That the boat will not take rough water as well and could become a safety issue if you are not able to maintain headway or proper steerage in a storm. This problem is amplified by boat load. The act of raising the lower unit out of the water was being done by racers (1960’s maybe earlier) to reduce the drag the engines lower unit imparts from the water. Alas at a sacrifice - hole shot and time to plane. Here in Florida, the fellow’s who fish the skinny water use a jack plat on their skiffs. The jack plate electrically lowers the motor for initial plan, than you electrically raise (and trim) the motor (while on a plan) to achieve maximum speed. This is usually good for an increase in speed for any giving (on plan) power setting. A guy might call it overdrive for their boat. How can the motor be raised out of the water while on a plan without cavitating? While a boat is on a plane (moving forward) water displaced by the weight of the hull rushes in to fill the space the hull just left behind. In other words the water comes of the back of the transom in an upward direction. The faster the boat goes the further back the motor must be off set to achieve the drag reduction correctly. A careful look a properly set up transom bracket will revel the use of motors with shorter shaft lengths. For recreational boats, this really equates to fuel savings for a given speed. Now the rub -with the hydrofoil installed you are effectively lifting the rear of the boat (reducing both hull drag and motor drag). You will not see a substantially safe increase in hull speed by raising the engine and adding the foil. The reason for this will be the loss of lift from the foil as it rises into a turbulent flow path caused by the water leaving the hull. A bit of additional information: Arneson makes a surface drive that actually pivots at the transom mount point. These drives actually run the upper prop blade out of the water to reduce rotational drag on the engines and still allow the pilot to lower the drive to get the boat on a plane. I have attached a web site to Arneson for you interest.

Finally, I recommend not raising the motor with the use of the foil. Improperly set up the motor will cavitate in turns and fall of plane easily. Please feel free to contact me at



kingfish posted 06-18-2005 10:47 AM ET (US)     Profile for kingfish  Send Email to kingfish     

1. Raising (or lowering) the motor in and of itself will not lower the speed at which your boat will hold plane, and is not the reason that exercise is considered.

2. Raising the motor can improve performance and efficiency, if your motor is currently set such that the AV plate is below the surface of the water when your boat is on plane. In fact if your AV plate is below the surface of the water when your boat is on plane, it is probably a good idea to raise it until that is no longer the case. You will find that your boat will handle better, too.

3. Your motor can be raised too high, where it ventilates too easily on turning and in chop. Higher still, and you can start to lose cooling under some circumstances.

4. Hydrofoils (like DoelFins) can help a boat to get up on plane more quickly and can help a boat hold plane at lower speeds, if the AV plate is not below the surface of the water when your boat is on plane.

5. Hydrofoils (like DoelFins) do not appreciably affect performance at high speeds, and are not intended to, if the AV plate is not below the surface of the water when your boat is on plane.

6. Hydrofoils (like DoelFins) will adversely affect the performance and handling of your boat when on plane and at high speeds, if the AV plate *is* below the surface of the water when your boat is on plane.

7. An estimate of whether your AV plate is below the surface of the water when your boat is on plane can easily be accomplished on dry land with your boat on the trailer as follows: A)Set the trim of your motor to approximate neutral trim when in the water. One way that can be done is to set the angle of the motor perpendicularly to the line you will establish in the next step. The set-back angle of the transom is frequently between 14 and 18 degrees; B)Find a way to develop a line, possibly with a straight edge, that extends the line of the keel of the boat back at least to the lower unit, or beyond; C)set the final "measuring" trim angle of the motor by gauging that the AV plate is parallel with the line established in step B; D)With the AV plate set parallel with the extended keel line, note the vertical relationshp between the keel line and the AV plate. If the AV plate is at or below that line, move the motor up one bolt hole (3/4"), and try it for a while. If there is no tendency for ventilation, it could probably be raised another bolt hole, but it would be hard to imagine raising the motor such that the AV plate is any more than 2 bolt holes above the surface of the water when your boat is on plane. Leaving it in the first bolt hole up would probably be fine; E)if the AV plate is already above the keel line, you may already have the motor height set right where it should be, unless the dimension by which the AV plate is above the keel line is well in excess of an inch, in which case you might want to lower the motor one bolt hole and try it there for a while.

8. The previous method of estimating the vertical relationship between your AV plate and the surface of the water when your boat is on plane, done on dry land, is both easy and well within a range of accuracy to make the exercise worth while. Earlier claims that a one-degree differeence in the angle of your lower unit against the transom would render vertical inaccuracies in excess of 3/4" are simply fantastic and inaccurate. In fact, a ten degree difference *either way* from perpendicular with the keel line renders a vertical difference of less than 1/2", and there is no way that your actual neutral trim angle when under way is not within ten degrees one way or the other of perpendicular with the keel line.


JohnJ80 posted 06-18-2005 09:22 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
Actually, you just made my point for me. 10 degree difference is at the beginning of where one can ascertain an eyball difference in angle with any accuracy. 1/2" variation when you are looking for a 3/4" resolution is exactly the problem. That would be an error of 66%. Going through all these gyrations to get to a +/- accuracy of 66% is a huge waste of time. Anyhow, I did the math carefully after looking at the geometry so I am pretty confident in my numbers.

But, anyhow, this measurement just completely doesn't matter anyhow. You have 5 choices on a merc for hole position, 4 on most Johnson and Evinrudes. Put in in the middle and start experimenting, I mean, how wrong could you be with that few choices? You would most likely be within a hole or two at the most.

The proper position is something that is completely different for each boater and boat because it depends on (a) how you load the boat, (b) where the weight it (c) the sea conditions you operate in (d) how you drive it (i.e. lots of turns, long and straight etc...), (e)lots of other environmental conditions such as wind.

All the engineering papers that I read through last year when I was researching this problem, after all the math was said and done, pretty much said that practically speaking it was virtually impossible to predict porpoising (or other ride characteristics) because of the influence of so many factors. That also explains why there is so much conflicting information about whether fins work or not etc... The ride characteristics are not something that is as simple as measuring (by whatever method) your AV plate height relative to your hull.

The only practical and useful way to do this is plan on moving your motor a couple of times and experimenting. Just as an aside, when I did this, I moved it through EVERY hole to make sure I understood what was happening. The changes to my configuration were very noticeable hole to hole. For what it is worth, I also went through adding and removing a fin, different props, trim tabs - all one at a time and recording the effect so I KNEW what the differences where. Needless to say, I learned a ton about my boat but it all boiled down to the fact that the solution is largely found on a boat by boat basis and through experimentation ON THAT BOAT in the manner in which it is used and loaded.


Perry posted 06-18-2005 10:13 PM ET (US)     Profile for Perry  Send Email to Perry     
I agree with Kingfish. An ESTIMATE of the height of the AV plate in relation to the hull is a good place to start. I set the angle of my motor so the AV plate appeared perpendicular the bottom of the hull and measured the distance. I used this measurement as a baseline and remeasured using the same method after raising the motor one hole. The net gain was very close to 3/4" which put the motor where I wanted it. The AV plate ran on the surface at planning speed and performance improved. If I would have put the motor in the middle holes to start, I would have had to lower it back down one hole and that would have been a waste of time.

I think performance (improved steering, better top end, reduced porpoising etc) is the main reason people raise their motors and getting your prop to bite better is the main reason why people lower their motor. Variables like how you load your boat and environmental conditions such as wind make little difference.

JohnJ80 posted 06-18-2005 10:52 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
The way you load your boat makes a big difference, including the weight of your engine. Do a search on this forum on porpoising and see the various discussions - some have had problems by moving weight forward, some by moving it aft. Some have reported that weight forward made porposing worse, others by moving it back. Anyhow, loading was a factor without a doubt. It certainly was in mine. In your particular case, it may not be a factor but to say it isn't factor (period) is incorrect.

Also, when I started with my motor mounted all the way down, the boat was very sensitive to porpoising. When I went out in flat calm, I could get it to perform adequately. In 5kn of breeze, the boat would porpoise much easier going into the wind than downwind - I could add much more trim angle downwind than up. So, yes, wind does make a difference.

In a flat calm seaway, I could run wide open. When even the slightest wave was present, the boat would not ride over it well but you could feel each wave hit the boat hard and I had to slow down. So, yes seaway does matter (another environmental condition).

I did this for a variety of trim angles and speeds as measured on my GPS, on my pitot tube speedo and my fishfinder speedo (I recorded all three since I was interested in the accuracy between them). I recorded all this data so that I knew what effect each change brought. I did this also after adding a fin to see what would happen and then with two different props.

I also then researched a number of engineering articles by marine architects and engineers on this subject and they confirmed what I had already learned. I posted links to these articles on this forum - you can find this by searching. See:

Which also contains alot of other good discussion about the technology surrounding porpoising, caueses and effects etc... I highly recommend reading it.

Admittedly, I started off in an extreme case - motor all the way down. I did this because I got tired of all the armchair advice on this subject and because I wanted to know what the effects would be. Being an engineer, I got interested in the problem and wanted to be able to quantify what I had learned so that I could repeat the success I had achieved with other boats.

Bottom Line - in order to achieve optimum results, you must arrive at the solution experimentally, You can guess (you have really only 2-3 choices if you disregard the all the way down setting) and you stand a pretty good chance of getting into an acceptable range with your first try. To optimize, you have to experiment.

If you are going to measure, you have chosen a good way of doing it by measuring a distance from the hull and setting the tilt this way each time. My point is that it really doesn't matter, a guess in the middle of the range is just as accurate, takes less time and effort, and you still have to find the optimum point through experimentation (unless you don't care about optimum but close enough is ok).


Perry posted 06-18-2005 11:47 PM ET (US)     Profile for Perry  Send Email to Perry     
Wind conditions effect performance
Loading your boat effects performance
Sea conditions effect performance

Performance (or lack of) is the main reason why people change motor height.

kingfish posted 06-18-2005 11:49 PM ET (US)     Profile for kingfish  Send Email to kingfish     
I didn't make any point for you, with the possible exception that you don't understand the math you are trying to describe.

Maybe your difficulties stemmed from trying to make a complex problem out of something that is not complicated. I described in terms that I thought might be basic enough for you to comprehend, a simple and common procedure. A lot of people have done it that way effectively and a lot more will continue to do so. I'm sorry it doesn't work for you.

JohnJ80 posted 06-19-2005 02:00 AM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
The math isn't important - that was the point. Measure all you want and it doesn't really matter - you can see where you are by counting the holes.

Experimentation is the proper way to find a solution because it is way beyoned the means of a whaler owner to model the statics and dynamics of their boat.

Getting it right is important, since per all the research, you do not approach the hull instability (porpoising) linearly - you reach a point and it gets unstable. Unstable means you can create a situation where operating the boat is dangerous.

Incidentally, this isn't new news, since the paper below was written in 1942. This agreed pretty closely with the results I saw on the lowest hole with the slightest trim added. A trim could be added but the oscillation amplitude got bigger faster than trim was added.


This paper also makes references to where the thrust is and its position to the hull (above the hull, since a seaplane, but the effect has some similarity to a prop below the surface.)

There are tons of papers written on developing simulation models and getting them to agree mathematically with actual results. Note, that getting this agreement is difficult (hence all the research into simulation models) again pointing to the experimentation solution unless you can accurately characterize your boat. To characterize what is happening, try this paper:

Boat performance IS a complex problem, which the references I cite, prove. Heck, just the VOLUMES of discussion on this topic alone in this forum ought to be proof enough it isn't as simple as it appears. Did you read the information I posted in my last post? I thought that discussion laid it out pretty clearly.

This is all important since you can configure a whaler, easily, that can be dangerous to operate.

If you want to debate the facts, I'm all for that. One of the main purposes, as I understand it, of this forum is to increase the body of knowledge about whalers. If your intention, in the complete absence of any other argument or fact, is to trade insults or to be patronizing, then I'm not for that and done with this discussion.

Up to you.


kingfish posted 06-19-2005 11:36 AM ET (US)     Profile for kingfish  Send Email to kingfish     
I'm for simple, relevant and accurate answers to questions that beginners ask on this forum.

I'm against suggesting that a simple, common and widely used procedure that answers one of the questions a beginner has asked, is a waste of time.

I'm even more against quoting faulty and unnecessarily complex mathematics to support a contention that an accepted procedure is a waste of time.

And I couldn't be more against shifting the focus from faulty and unnecessarily complex mathematics when called on them to a suggestion that you are being insulted or patronized. (You might consider whether others might have found your remarks to be insulting and patronizing. Apparently you don't like it - maybe others don't like it either.)

Three suggestions; you can take them or leave them:
1. Keep it simple, and keep it relevant; don't try to swim against the tide unless you really have found a good reason to do so.
2. Check your math - out of curiosity, I did. I found that your claim that 1 degree of difference in the angle of the lower unit to the transom renders a vertical difference in excess of 3/4" was incorrect and was off by a magnitude of more than 10. Not fractionally off but off by more than 10 times (close to 20 as a matter of fact). The fact is that swinging the lower unit 10 degrees either way from perpendicular to the keel line renders a vertical difference of less than 1/2". Go out to your trailer, in the real world, and try it, like I did. That may not point out the exact nature of your error, but it should suggest to you in the way of proof that something went wrong.
3. Admit your error and go on rather than trying to obfuscate it - we all make mistakes. I have made one here in using all this bandwidth trying to clarify such an obscure inaccuracy.


JohnJ80 posted 06-19-2005 12:30 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
reat this carefully:


Regardless of whether we agree on how on the math or not.

The "difficult" process I am advocating is to set the motor in the middle of the range. Up one from middle if in calm waters, down one from middle if lots of chop.

Then take your boat out and drive it around loaded with people and gear like you would normally. Try raising up first one notch and seeing how you like it. Look for optimum, plan on two tries. Use the trailer method and the whole thing takes less than two hours and you get the optimum setting to boot.

That sound difficult to you?

What I did was supply a lot of information on why that is important.


Perry posted 06-19-2005 01:30 PM ET (US)     Profile for Perry  Send Email to Perry     
Did you read my post where I used the measuring method to determine the motor height. I got it right first try and did not have to waste time with sea trials and taking motor off and putting it back on.
JohnJ80 posted 06-19-2005 03:02 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
That is what is called a lucky guess, which has a reasonable chance of success (1 in 3 or so if you disgregard all the way down and all the way up). If you get it to work that way, good for you!

I measured the first time I tried it with mine - in fact I put it exactly back where my old motor was when I re-powered, and wound up moving it. Point of fact, I actually used a method pretty similar to what kingfish describes to confirm it was in the same spot.

The new motor was heavier so the boat was now loaded differently and the motor required repositioning. So, the measuring thing is not definitive and not repeatable over every boat configuration.

Because it didn't work and, in spite of the common advice here, I wanted to know why.

The generalizations you CAN make are that if you have the exact same boat and motor as someone else and you load it the same and operate in the same waters/conditions and in approximately the same manner, you can utilize the same adjustments. But, that clearly doesn't work for everyone's setup.


Perry posted 06-19-2005 03:23 PM ET (US)     Profile for Perry  Send Email to Perry     
You still don't get it. You had to reposition the motor because your boat porpoised not because of the way it was loaded.
kingfish posted 06-19-2005 03:24 PM ET (US)     Profile for kingfish  Send Email to kingfish     
That's right - "it" doesn't work the same on every one's boat - choosing, say, the second hole up is *not* going to give the same relationship beween the AV plate and the keel line (approximated water flow at planing speeds) on one boat that it will on another. That is why, rather than just popping the motor into any set of holes and flailing around until you figure out where you are, most people estimate how the AV plate relates to the keel line *on their boat* so when they make a move, they have some kind of an idea where they're starting from and some kind of an idea what the move is going to do.

You don't want to do it that way, fine; it didn't work for you, fine. I'm glad you found something you could make work. You're not going to have much luck convincing the correspondents here that you have discovered a better way to slice bread though.

JohnJ80 posted 06-19-2005 11:12 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
Lets just drop it. I'm getting tired of this "see who gets the last word" thing.

We don't agree - fine. Let the folks decide and call it a day. I'm happy I understand the problem and its root causes and you are happy with your measurement. Whatever.


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