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Author Topic:   Height of Engine
onlyawhaler posted 06-03-2003 09:51 PM ET (US)   Profile for onlyawhaler   Send Email to onlyawhaler  
I have an 80s era 18 Outrage with a 150 Johnson. The cavitation plate on the engine when trimmed all the way down is exactly level with the lowest point of the V of the transom. Iis this the right height? I am looking for a little more speed and would raising the engine a bit help?


Sal DiMercurio posted 06-03-2003 11:00 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
If your all the way down on the 1st set of holes, your to low.
The only way to really tell if your antivent plate is to low is to run the boat on plane & go to the stern & look down at it, if it's getting water on it or if it's not above the water, your to low.
I would raise the engine to the 3rd set of holes, take it out & run her, if she blows out or vents [ cavitates ] in a turn or in choppy water, bring her back down 1 set of holes.
I'v never seen a dealer mount an engine right yet, always right down on the lowest set of holes.
After you raise the engine, your going to gain a couple hundred rpms so don't let her over rev.
I think that engine is rated for 5,500 rpms, & thats where you want her to reach at wot & trimmed out.
If she runs 56 - 5,700 thats ok, but if she runs at 5,100 your over propped & need to drop down a couple inches of pitch.
What size prop are you running on her now ?
Please don't tell me a 15 x 16 or 17, cause a 15 diameter is far to much wheel to swing on that boat, i'd say the right prop would be a 14.25 x 17 or possibly a 19 if the boat bottom is clean & the boat isn't water logged.
JohnJ80 posted 06-03-2003 11:34 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
Whaler recommends 0-3/4" high of the bottom of the boat for the AV plate. They won't say higher. However, reading everything I can find on this on this forum, the consensus seems to be the best position is 3/4" to 1.5" high of the bottom of the boat.

Mine was about 1/4" below, I raised it one hole so it is 1/2" higher and noticed a stark change in ride. I am going to go up another hole and see what happens. I think it will be better.


NHKatama posted 06-03-2003 11:51 PM ET (US)     Profile for NHKatama  Send Email to NHKatama     
My dealer mounted my 90HP Merc, on my T&H marine Jackplate with the the engine most of the way down the CP is about 1/2" above bottom of boat. (I told them to mount it 2" above) since this is a 5" setback bracket.

However they did not listen. I have about 3" of upward travel left in my jackplate, do you guys think I have enough travel left to raise my engine as high as I would ever want to go. Also they redrilled the bottom hole on the jackplate so that the botom hole is now about and 1" higher, so I no longer have a standard pattern. Some people just do not listen. Anyway I hope they have not screwed my boat up.

You guys that have experience with jackplates let me know what you think.

Thanks, ~~~~Pete

Sal DiMercurio posted 06-04-2003 01:07 AM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
You know Pete, I really feel most if not all the dealers dio not know much about performance from these boats, they barely know the very basic things on mounting & props.
The dealer who hooked up [ not mounting ] my engine [ had to for warranty ] insisted I needed a 4 blade s/s 15 x 17 prop, I told him he dosen't even know the first thing about performance & I'd take him for a ride when I got her fine tuned.
About 2 months after he timed my computerized engine for warranty, I saw him testing an engine on the river, I had him tie up & take a ride with me.
Took him up to 45 mph & he said, thats about right for this setup, I looked at him , trimmed out & put the hammer down, the boat litterally leaped almost completely out of the water & the speedo went to 60 & the tach also went to 6,100 rpms, all he could say was,....holy sh!! what the hell did you do to that engine?
Told him, I didn't listen to you, thats what I did.
Now he calls me for my opinion on which prop he should try on his customers boats.
I mean I went there to buy some oil & he was mounting a long shaft engine on a short shaft boat & says it runs & handles just fine, I took him aside from the customer & explained he put the wrong engine on the wrong boat because thats a 20" shafy engine & the boat is a short shaft at 15", he said it dosen't make any difference, the customer is happy,.....DUH....
Yes, they know how to work on engines, but not how to get the combination of boat & engine to get the optimum performance from the setup.
Sorry to get so windy, but it really ticks me off when they do that, & your a perfect example.
NHKatama posted 06-04-2003 10:24 AM ET (US)     Profile for NHKatama  Send Email to NHKatama     

Do you think I will have enough height left with 3" of upward travel still left to raise my engine enough for best performance. I can lift the engine up one or two more holes as well. Again they have the jackplate adjusted up about 1/2" and the CP is about 1/2 above the keel. What do you think?

Thanks, ~~~~Pete

Sal DiMercurio posted 06-04-2003 10:41 AM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Pete, it sounds like it should be ok, but without seeing it, can't be certain.
All you can do is try it.
lhg posted 06-04-2003 12:14 PM ET (US)     Profile for lhg    
Mercury has very specific engine mounting height information furnished with the installation manual that comes with each engine. This would be applicable to any brand engine. I have a copy of this, and maybe I could get JimH to publish it on CW. Some boat manufacturers also specify normal engine mounting height, as did BW during the 80's. For most applications, BW always seemed to recommend one bolt hole up over dead low, which is usually 3/4".

Engine mounting height, which usually translates to the height the anti-VENTILATION plate runs above the surface of flow coming off the hull bottom, can only be so high, based on keeping the engine's water intakes submerged, and to not take in air. This dimension is easy to measure on your engine. For most, it is no more than 3", nor would you want more than that except for an all out racing application.

Since Whalers are not high performance nor racing boats, for most applications the optimum range would be 3/4" - 2" max. For twin engine installations, a little less, since each prop is cutting the water off a sloping Vee, rather than the bottom of a Vee.
The inward side of the prop is, in effect, running much higher than the outward side.

If you are elevated 3/4", as was recommended by Whaler, a standard cupped aluminum or SS "elephant ear" prop will suffice, as long as your top speed doesn't exceed 40 mph or so. My own expeience on Whalers is that aluminum props are strictly bottom of the line, and don't perform very well over 30 MPH when compared to an SS. For the 30-40 MPH top speed range, a standard SS is usually fine, engine in the second bolt hole pattern .

It's when your boat is capable of 40mph or better that you should be looking into a "performance grade", higher rake, vented, surfacing capable, SS prop, and get your engine up another bolt hole to 1 1/2". These will perform at elevated transom heights, such as 1 1/2" or 2" max, and the tips will "surface pierce" the water at high running speeds, with no slippage. For almost any Whaler, up to 65MPH speeds, 1 1/2" is about all the height you need for good performance in all conditions. For really high running heights of 2" - 3", really only recommend when WOT top speed is the only objective, a prop like the Mercury 4-bladed Trophy Plus or Tempest is called for.

Unless the engine is mounted directly on the transom, visually sighting the anti-ventilation plate against the hull bottom doesn't work. With setback brackets of any kind, you actually have to look at the flow coming past the engine to see where you need to be. Setback engines can always run higher RELATIVE TO THE TRANSOM, because the water flow immediately begins to swell back up once the boat passes through it. The further back the engine is, the higher the swell rises back up. Because they are higher, they can run faster with less gear case drag resistance. On my 25 Outrage, with 26" setback Armstrong bracket, sighting against the transom, the engines appear to be up about 4", but actually are only about 1" high at the water flow past the engines themselves. So the water is swelling up the other 3" as the boat passes through it.

I don't think one should be too hard on the Dealers. If the boat won't do over 42, or if they are installing a price point aluminum prop, or standard SS, on a new rig, you don't want the engine up any more than 3/4". Otherwise the "average Joe boater" will be back complaining the prop slips.

Sal DiMercurio posted 06-04-2003 12:26 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Ihg, excellent post.
Bigshot posted 06-04-2003 02:29 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Pete....jack that puppy all the way up and tun it. If she handles great then jack it up a hole. If it starts to blow out on slight turns or holeshots like a slipping clutch you are at top height and can leave be or go down to make it more "normal". If your prop sounds like it is slightly out of the water at cruise then you may want o go down a tad. My 70 is on a 5" setback and jacked about 4+" up. I could probably go another inch but I doubt it would gain me any speed being I only run 39 and I like the normal steering it has which I would lose being up another inch. At 60mph that extra inch can mean big gains. At 40 it might mean 1/2mph, not worth blowing out in heavy seas for that.

i jacked my 70 up on 15 to the top hole. you could see the whole trim motor. It was actually 1mph sower than down a hole being I was not getting the bite needed. A $400 prop could have fixed that but again....what is the point. 48.6gps in a 15' is fast enough for me.

knothead posted 06-04-2003 04:49 PM ET (US)     Profile for knothead  Send Email to knothead     
I've got a question.

I have an '87 Montauk with a 2001 90 hp Johnson. From everything I've read here I've deduced that I need to raise the motor. Currentley it's mounted all the way down, touching the top of the transom. Upper bolts are in the top holes which is no big deal to change, but the lower bolts screw into a blind mounting on the motor mount. The motor has mounting slots below the blind holes, but they seem to be a fair distance down.

My question is; Since this seems to be a common set-up for '80's vintage Montauks with OMC 90's, has anyone raised thir motor high enough to use the lower slots instead of the blind holes? And if so, what were the results?

At some point I will install a set back bracket, but for now I just want to raise the motor.


PSW posted 06-05-2003 10:30 AM ET (US)     Profile for PSW  Send Email to PSW     
After recently raising my Merc 115 on my Montauk to about 1 1/3" above keel I have been very pleased. So pleased that I have thought about doing the same to some of the other boats I use. After getting the Zodiac inflatable ready for the summer duties, as dinghy on the pursuit, I wondered since it is powered by the long shaft trolling motor off the Pursuit what should I do? I always change over the prop, but how would a 5 inch set back raised, oh say, 7 1/2 inches work? How do you think a inflatable would look with a jackplate. I am actually joking but after seeing how much better I like the all around performance of the Montauk with proper adjustment I wonder how the zodiac would feel if the prop wasn't buried under water. At a 22mph top end it does not matter much, but it was just a thought. And yes the Pursuit needs a long shaft trolling motor for when fishing in the ocean.


Bigshot posted 06-05-2003 11:29 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
If asthetics are not important then bolt a 2x8 or 2x10 to the zodiacs transom raised up 5". I did this on an aluminum boat that I had a 20" shaft on, worked great, probably stronger than the transom itself.
bsmotril posted 06-05-2003 11:58 AM ET (US)     Profile for bsmotril  Send Email to bsmotril     
What about a twin engine installation. Do you follow the same guidlines and measure from the front center of the cav plate to the hull directly in front of the engine? Do you have to adjust downward to make certain the outside engine won't suck air in a turn? Other than toe in (read that thread many times), any other special considerations for tuning a twin rig?
where2 posted 06-05-2003 05:33 PM ET (US)     Profile for where2  Send Email to where2     
Despite your comment BS, a Transom is NOT the weakpoint of a Zodiac... They're usually about 7 layer Marine Mahogany Plywood. I've got a transom off one I'll sell you...

As for the setback/jackplate concept for the Zodiac, I'd go with a straight jackplate, or BS's 2x10 bolted through the transom. I only ever got 22mph out of my own Zodiac, but I only had 9.5hp (at powerhead). I could never find a prop with more than 8.5" of pitch (I had that one cupped). I always wanted to find an Evinrude Racing 15Hp to mount on it with the racing lower unit and a short shaft.

David Jenkins posted 06-05-2003 08:39 PM ET (US)     Profile for David Jenkins  Send Email to David Jenkins     
When you don't use a jack plate and you are raising an engine so that it no longer rests on the top of a Whaler's transom, do you reduce stress on the transom by bolting/epoxying wood (a 2 x something) to the top of the transom so that some of the weight of the engine is supported by something other than the mounting bolts?
Sal DiMercurio posted 06-05-2003 09:23 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Dave, the bolts take all the brunt of the force while in reverse, the bracket on the engine spreds the force out equally while in forward.
As long as you have good thick washers on the inside of the boat theres not a problem.
No the hooks on the top of the engine don't take any of the stress away if mounted all the way down, which is usually wrong anyhow.
Don't worry about the bolts not being enough to hold the engine secure, the transom is designed for just that.
pstollie posted 06-11-2003 11:34 AM ET (US)     Profile for pstollie    
All this motor height discussion has really got my curiosity going. My 16 Ventura/90 Merc 4S set up has been back to the dealer 2X for a reprop. Can't get over 5300RPMS, now with a 14 pitch, 2 persons, 20 gallons fuel. I've asked the dealer if the motor can be causing drag at it looks too low to me. Measured last night and I'm at the 3/4" 'standard'. The dealer says he sets them all that way. I'm really tempted to raise the motor up a notch, but how do I do that? Seems I'd need an engine hoist for the 400LBS or so. Any suggestions? I do not want to go back to the dealer.
Bigshot posted 06-11-2003 12:04 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Rent an engine hoist(cherry picker) for $20/day and it is easy. If you have a friend with davits, use that instead. Strong rafter in Garage works too.
Sal DiMercurio posted 06-11-2003 03:42 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Yes, most dealers put the engine all the way down on the transom, & 99% of the people don't know any better thinking the dealer knows what he's doing.
Yes they usually do, but they set the boat up for the very basics, not bothering to fine tune the rig by raising the engine & putting a performance prop on.
Most people are satisfied if their boat runs 38 - 44 mph not knowing it's very capable of running 50+ mph if needed or wated.
Raising the engine is very simple.
2 ways.
You can put the boat on a trailer & dissconnect it from the vehicle with the wheel or stand in the down position, put a 4 x 4 with a floor jack under the bracket thats connected to the boat, un-bolt the 2 top bolts, [ should be in holes ] & "loosen" [ don't remove ] the 2 bottom bolts, [ should be in a channel or slot ] with the skeg on a 4 x 4 & the jack on the other 4 x 4 have a couple buddies steady the engine so it dosen't tilt back as the deadrise angle will push it away from the transom.
Now just pump the jack thats under the 4 x 4 thats under the bracket just a little, only 5.8" between holes, until the second set of holes lines up with the holes in the transom.
TYhen insert the bolts & tighten them, then tighten the bottom ones & your done.
The other way to raise the engine is, put a 4 x 4 under the skeg & jack the front of the trailer "up" so the skeg goes down & touches the 4 x 4, unbolt the 2 top bolts & have a couple buddies steady the engine while you "loosen" the lower bolts, then jack the front of the trailer up a hair, this forces the skeg to stop moving in the down direction & as you rraise the front of the trailer the skeg will hit the 4 x 4 & stop moving, but the transom will still be free to move in the down direction & the engine holes will now start to go higher, when the second or third set of holes on the engine lines up with the top holes in the transom, walla, put the bolts back in, tighten the crap out of em & go do it.
One hole can make a huge difference in your performance, like adding 200 rpms to the top end of your rpms & stop the boat from porpoising if it was before.
Many people don't know but, if your boat porpoises, raise the engine a hole & it will usually stop or reduce it from leaping around like a frog on a lily pad, if the one hole dosen't do it, raise it again, that should cure it.
TampaTom posted 06-11-2003 04:13 PM ET (US)     Profile for TampaTom  Send Email to TampaTom     
I lift my engine with a "come along" rachet (Home Depot $35) thrown over an oak tree branch.
lhg posted 06-11-2003 04:32 PM ET (US)     Profile for lhg    
I statically "hang" mine from an overhead beam, using the Mercury flywheel attached eye hook and steel cable/chain. Once the engine weight is on the cable, using the trailer tonque jack, the bolts can be removed. Then the trailer jack can very easily be used to either lower or raise the transom relative to the fixed height of the hanging engine. Mercury engines do not have the slots for the lower mounts, but rather 5 hole positions, so all four bolts have to come out at one time.

I also used this method to add transom setback jacks.

pstollie posted 06-12-2003 10:48 AM ET (US)     Profile for pstollie    
Thanks for all of the replys and information. Looks like I have a project this weekend. I'll report back on the results.
Eric posted 06-18-2003 09:13 PM ET (US)     Profile for Eric  Send Email to Eric     
How does raising the engine a moderate amount (like up one hole from the bottom) affect rough water ride?
JohnJ80 posted 06-18-2003 10:49 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
It was a big difference on my D15 - like huge.

Whaler recommends that the AV plate be 0-3/4" above the bottom of the boat. Seems that the consensus on CW has been that 3/4" to 1.5" is the place to be.

I'm moving mine tomorrow nite - currently within Whaler's recommendation (one hole up from bottom) putting it at 1/2" above bottom. I'll post results assuming I get this done tomorrow.


Jarhead posted 06-19-2003 08:22 AM ET (US)     Profile for Jarhead  Send Email to Jarhead     
I was looking thru the installation manual for my 115 Merc the other day and found the following that seems to support the advice offered here.

And I quote: "Increasing the height of outboard generally will provide the following: 1) Less steering torque, 2) more top speed, 3) greater boat stability, but, 4) will cause more prop "break loose" which may be particularly noticeable when planing off or with heavy load". End quote.

Source; installation manual, 115hp (4-Stroke) Mercury Marine, copy right 2001.

I would say the answer to your question, "will raising the engine a bit help?", is yes.

JohnJ80 posted 06-19-2003 10:59 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
I just finished raising my motor all the way up to the top hole. AV plate is now about 1.25-1.5" over bottom of the boat.

Side rooster tails are gone, boat has better trim range and she goes better through boat wakes (calm nite tonite). I'll do more testing later but it looks like I picked up about 200 rpm and about 2mph, motor doesn't sound like it dogs at full down trim. The whole thing just feels better, she goes better through the water in general.

The ride seems much better. When motor (75HP FICHT Evinrude) was put on the boat, it was all the way down. Terrible pounding ride. Picked it up one hole, improved it alot. Went the rest of the way tonite, worked fine. No prop blowout, motor pees water fine.

I still don't have the trim range that I want. Talking to Stilleto prop guys based on Sal's comments about them, I'm going to order a new prop and put that on too.

I used the jacking the trailer tongue method of moving the motor up. I put the trailer tongue on the pavement and strapped the boat to the trailer, removed the top bolts then loosened the bottom sliders. jacking the tongue up moves the motor up on the transom. Insert bolts when they are where you want them and your done. Whole thing from start to finish took about 45 minutes in the parking lot at the ramp.

Easy experiment, go for it if you have any doubts at all. I think its likely that you will like the result.


Drydock posted 06-19-2003 11:32 PM ET (US)     Profile for Drydock  Send Email to Drydock     
Very interesting subject. What about the hulls with the notch in the transom?
Should you reference the aftmost section of transom or the bottom of the notch which in my case is about 4 inches lower and 10 inches forward of the transom. The previous owner complained about porposing and put on doal fins. It seems to perform pretty good but only when trimmed in. (I also don't have a lot to compare to. All I know is that tt beats the pants off my sailboat1) This thread made me think perhaps raising the engine would have had better effect than the doal fins.
JohnJ80 posted 06-20-2003 08:55 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
I would do both - raise the motor and add the fin. From the talking that I have done with prop guys, alot of them say that any boat will benefit from a fin anyhow. I know there are others that don't agree, but my experience with two motors on my D15 really convinced me of the benefit.

Raising the is relatively easy to do and has a big effect. Between the two of them, you are talking a one hour project.

My personal preference for fins is the Grand Island Turbo Lift. You don't need to drill the AV plate, it is stainless steel and I found better results with it than with a Stingray (on a different motor). YMMV.


NHKatama posted 06-23-2003 12:04 PM ET (US)     Profile for NHKatama  Send Email to NHKatama     
Where do you find the Grand Island Turbo Lift? Does anyone have their telephone number and the website address.

Thanks, ~~~~~Pete

JohnJ80 posted 06-23-2003 05:11 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
Grand Island Marine can be reached at (321) 452-8126. They are located in FL.

Each unit is specific to each motor so you need to get the specific one that meets your motor's dimensions. It is made out of stainless and it is bullet proof (maybe literally).

They don't have a website since most of what they sell is OEM and goverment (US Customs and USCG boats is my understanding). However they will sell to you directly.

This was recommended to me by Chuck B. at Whaler. He said that they have used them in a number of applications with great results. This is how I heard about them.

I suppose it depends on your application, but I had great results with one of these. You wouldn't even know it is the same boat. Big difference.

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