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Author Topic:   20' Ventura porpoising
northwhaler posted 02-23-2004 01:25 PM ET (US)   Profile for northwhaler   Send Email to northwhaler  
I have a 99 Whaler powered by a Merc Optimax 200. The boat has a porpoising problem. Once on plane, I can't trim it up at all without it starting to porpoise. I changed props to a Quicksilver prop (smaller and less pitch) because the prop that came with the boat wouldn't let the engine rev to proper WOT. This smaller prop helped a little. I have also added a turbolift, which helped a little, but doesn't eliminate the problem. The engine is mounted on the second hole, which puts the anti-vent plate about 1" above the bottom of the boat.

Anyone who has a similar set-up (I believe that the Dauntless is the same hull) who had this problem, and cured it, I would appreciate your help. For that matter, I would appreciate anyones help!

At 526 lbs dry weight, the Merc is lot of weight on the back on the boat. Love the boat, but tired of spending money on fixes that don't seem to work.

Thanks in advance for any and all help!

Richard Quinlivan posted 02-23-2004 01:39 PM ET (US)     Profile for Richard Quinlivan  Send Email to Richard Quinlivan     

Does the porpoising stop as you get up to speed? Porpoising occurs because the boat is trimmed to to high an angle with the water (angle of attack) As the boat speeds up it comes out of the water and the boat flattens out and the porpoising generally quits even with the motor trimmed out. My Dauntless 17 had that sort of problem. My solution was to raise the engine as high as I could get it. This reduces the leverage the engine has to pitch the boat and therefore allows you to get the engine trimmed out for without raising the bow as much.
Last year I added a Turbolift and that cured my problem altogether. I havened had a chance to find out what it did to my max speed.


northwhaler posted 02-23-2004 02:26 PM ET (US)     Profile for northwhaler  Send Email to northwhaler     

Thanks for your reply. Porpoising occurs more at higher speeds. With the Turbo lift, I now have some degree of trim available at lower planing speeds. The closer I get to WOT, the less trim I have available, as the boat starts porpoises at any trim angle except all the way down. With your motor mounted all the way up, how high is it above the bottom of the boat?

Also, at what trim angle do you have the motor set at to measure the distance. Merc Marine told me to trim my engine until the anti-vent plate was parallel with the bottom of the keel at the back end of the boat. That is, I taped a thin flat board on the keel so that it stuck out the back of the boat, and trim the engine until it was parallel with the board. I measured the distance from the bottom of the anti-vent plate to the surface of the board. It is about 1 1/8" above the board.


Moe posted 02-23-2004 03:35 PM ET (US)     Profile for Moe  Send Email to Moe     
Perhaps a better understanding of the problem will help.

Every hull has an optimal angle to the water at any given speed. If the angle (and the bow) is too low, too much of the hull will be in the water creating additional drag. If the angle (and the bow) is too high, the angle of attack is too steep and the stern is deeper in the water, creating additional drag.

The motor thrust is also optimal when the prop shaft is parallel with the water surface and all of its thrust is in a forward direction, rather than trimmed in (down) from that, using some of the thrust to lift the stern and lower the bow, or trimmed out (up) from that, doing the opposite.

However, the motor line of thrust is below the hull's support at the surface, naturally tending to lever the bow upward. The lower the motor's line of thrust, the longer the lever arm, and the stronger the tendency to lift the bow.

An optimally balanced boat will have sufficient weight forward, relative to the weight in the stern, to hold the bow down to the optimal hull angle, against the motor thrust's tendency to lift the bow, when the motor is trimmed so that the prop shaft is parallel to the water. SWEET!

However, the motor thrust varies, especially with differences in speed and water conditions (which also influence the hull angle), so the optimal balance of the boat will vary! As a compromise, it can only be setup (i.e. weight distributed) to be optimal for the most commonly used speed and conditions.

On either side of this optimum, if the thrust is too high for the amount of relative bow weight, the motor is trimmed in (down) from vertical and forward thrust sacrificed to lift the stern and lower the bow. This is typically the case when accelerating hard with very high thrust, especially from a stop. And if the thrust is too low for the amount of relative bow weight, the motor will have to be trimmed out (up) from vertical and forward thrust sacrificed to lower the stern and lift the bow. This might be required to keep the bow from digging in the back side of a wave in following seas when thrust is relatively low.

Porpoising occurs when the bow is significantly too light relative to the stern. This requires the motor to be trimmed in to hold the bow down with much less thrust than if it were relatively heavier. If the motor is trimmed out (up) to vertical, the thrust may then lift the bow. When the bow is lifted, the motor is no longer vertical and the prop shaft no longer parallel to the water surface, but is in a similar tilt as it was when trimmed in, angled to the forward line of direction, so forward thrust and bow lift decreases. As a result, the bow drops. But then the prop shaft is parallel to the water surface again, and it lifts the bow again, and the process repeats itself... over and over.

If redistributing the weight is not possible, the solution is to use something other than angling the motor thrust to lift the stern and lower the bow.

If the porpoising isn't severe, a motor hydrofoil plate sometimes helps. But there are two problems with these. One problem is that if the motor is lifted to reduce the lever arm, the plate then may run too far out of the water, where it is providing insufficient opposition to bow lift. The other is that even when down on or in the water, the plate's angle of attack, and hence its lift, is increasingly reduced as the motor is trimmed out (up), heading in the porpoising direction. That's just when you need more lift. In fact, if the motor is trimmed out (up) past vertical, a submerged motor plate can contribute to porpoising. If the porpoising isn't too bad, and the plate can be set to just run at the surface when the motor is trimmed vertically, the aft end of the plate should contact the water when the bow starts to lift. This might be enough to resolve the porpoising if it isn't too strong.

In the end, real trim tabs, hydraulic or electric, may be required to keep the bow down against the thrust when the motor is trimmed vertically. These can be used to achieve the optimal hull angle, while keeping the motor optimally vertical, over a wide range of speeds and conditions. They can also be adjusted asymmetrically to offset listing due to uneven side to side balance, or motor torque.

So check your turbo lift setup, and if it appears as good as it can be, do a search here for Lenco and Bennett trim tabs.

Hope this helps,

northwhaler posted 02-23-2004 04:36 PM ET (US)     Profile for northwhaler  Send Email to northwhaler     

Thanks for taking the time to post your reply. It was very informative.

I think that the ideal situation would be to fix the problem with weight distribution. But with the engine in the rear, the oil tank in the rear, the livewell in the rear, fuel going to the back of the tank under accelaration, etc., it would be hard to counterbalance all of that weight. The only equipment mounted 'up front' is the battery and anchor. I typically run the boat with 2 adults on board.

Interesting side-bar...I contacted Whaler about mounting a kicker motor on the boat, and the first question they asked was does your boat porpoise? I said yes it does, and they said with the weight of the 200 Opti, don't mount a kicker, or it will porpoise worse. (Weight issue) They recommended the turbo-lift, which helps some, but is not a cure. Seemed like a telling question, like it is a well known fact that the boat porpoises with the heavy outboard on the back. When the Ventura 20 first came out, the max HP was 175. Maybe it should have stayed at that rating.

Wonder if anyone out there is running a 20 Ventura that doesn't porpoise, and if so, what engine are you running??

Thanks again Moe...I suppose that trim tabs are going to have to be my next attempt at fixing the problem.

CHRISWEIGHT posted 02-23-2004 05:04 PM ET (US)     Profile for CHRISWEIGHT  Send Email to CHRISWEIGHT     

we also had a porposing whaler SL-16 when purchased last september and by way of encouragement have learned to use the trim and carefull balancing of the boat,to actually drive around the problem.

speeds around 25-30mph would produce scary oscillations and at the time I had grave reservations that this whaler was going to a new home and soon.

currently we suffer no porposing and can drive right through to WOT @ 42mph.

We have fitted a Teleflex pro-trim ($50) to the steering hub which enables two hands on the wheel whilst adjusting the trim, this does mean that should the boat go into a bouncing moment at least you don't have to let go of the steering wheel and grab the undulating gear change to change trim (a little trim IN is usually all that is required)

stick with it, hope this helps

regards chris

Richard Quinlivan posted 02-23-2004 10:09 PM ET (US)     Profile for Richard Quinlivan  Send Email to Richard Quinlivan     

As Moe points out real trim tabs are the optimum solution since they let you independently trim the boat at a stable angle and trim the motor for optimum thrust. Moving weight forward should also help since it will trim the bow down.

My engine is mounted up about 1 1/2 inches. The measurement is to be taken with the anticavitation plate parallel to the keel.

By the way I have two SS props and they had no effect on the porpoising. One is a High 5 and the other a Laser 2.

Porpoising is a classic problem with planing hulls and there is technical literature from as far back as the 1920s. The early literature was from NACA and was connected with seaplanes. Interestingly enough there is no clear explanation of the physics that causes the porpoising. The literature is clear on the fix - trim down! That is to bad wince the angle for max lift to drag for most hulls is highter then the angle that brings on porpoising.

Inception of porpoising in my experience is connected with trimming the bow to high at mid speed. With my boat it is extremely repeatable with trimming the motor out. At high speed the center of pressure of the planing lift is far enough forward of the cg so the bow cannot be trimmed into a pitch angle that causes porpoising.

northwhaler posted 02-24-2004 08:07 AM ET (US)     Profile for northwhaler  Send Email to northwhaler     
Chris and Richard

Thanks for your posts.

Looks like I'll be doing some research on trim tabs in the near future.

Richard, I wonder what model boat you have, and what it is powered with. I can keep my boat from porpoising, but only when the trim is all the way down. My Ventura runs a lot better with the Quicksilver prop that I switched to than it ran with the Mirage that it came with. Pitch went from 21 on the Mirage to 19 on the Quicksilver. The smaller prop gives me better hole shots, and it did slow the onset of porpoising down some. It also added about 600 RPM's to WOT, which was the principal reason for switching. (Went from 4800 to 5400.)

I've written to Whaler and requested the size trim tabs that they would recommend for the boat. I've owned other boats with tabs, (SeaRay 270 and a 310) but only needed them to level the ride, not counteract porpoising, so I don't have any experience as to how well they work on that problem. I've always heard that they add drag when you use them to keep the bow down, and therefore don't lighten the load on the engine that trimming the motor up is supposed to do, which is why I'm going to them as a last resort.

Again, I wonder if there are any 20' Ventura or Dauntless owners out there that do not have a porpoising problem, and what their boats are powered with.

Thanks again for your input.


Richard Quinlivan posted 02-24-2004 10:04 AM ET (US)     Profile for Richard Quinlivan  Send Email to Richard Quinlivan     

My boat is a 1995 Dauntless 17 Dual Console. It was the for runner of the Ventura series of boats. It is powered with a Merc 125 engine.

It has always had a mid speed porpoising problem. In the 25 to 35 mph range it is quite limited in how much bow up trim I could use without porpoising. I have tried different props without success. Raising the engine as high as I could without drilling new holes has helped in increasing the useable trim range. The biggest improvement I have made has been the Turbolift planing aid.

I have never had a problem at 40 plus mph. Unfortunately I boat on the Finger Lakes in Upstate NY and the water is often rough enough that 40 plus mph is not an option. I have considered tabs but have never done it and have no experience with them.

Your problem at top speed puzzles me. I think that your cg must be to far aft or you wouldn't be able to trim the bow high enough to porpoise.


jimh posted 03-05-2004 11:22 PM ET (US)     Profile for jimh  Send Email to jimh     
If your boat has a tendency to porpoise, you should carefully check the hull bottom near the transom for any sign of "hook."

"Hook" is any concavity to the hull bottom just in front of the transom. A good way to check is to take a long, flat, and truly straight reference surface and press it upward against the bottom of the boat, from the transom forward for several feet. A good carpenter's level that is 3 or 4 feet long makes a handy gauge for checking. You don't use it as a level, just as a straight and true surface that you might have around your shop already.

The hull should not show any sign of a hook downward to the transom. If you have concavity in your hull in this area, it could be contributing to a tendency to porpoise. If you do observe a hook, you may need to fair the hull surface back to a flat plane. You can do this with a fairing compound of low density micro-balloons. Sailboat racing hulls are faired like this all the time, so you should not have a problem finding the materials.

Boats which have been sitting on bunk style trailers where there is no keel roller supporting the keel of the hull and rigged on trailers where the bunks end short of the transom and leave the transom in a cantilever extending out from the bunk support may be more likely to have been distorted slightly and have introduced a hook.

It has also been mentioned that because of a problem in the original molding of the hull that certain boats were produced with slight amounts of hook in the hull right out of the mold.

The opposite of hook is called "rocker" and describes the hull being slightly convex in the region near the transom. This may also contribute to some porpoise tendency in the hullform. I think that rocker may be harder to fair out to a flat surface.

I don't mean these comments to be specific to the 20 Ventura model; they apply to any Boston Whaler as far as I understand.

Sal DiMercurio posted 03-06-2004 12:52 AM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Lift that engine another hole.
It will make a difference.
If it continues to leap like a frog, then get tabs.
I think you will se a big difference after lifting the engine .
Tom W Clark posted 03-06-2004 01:05 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

I must disagree with your observation. A "hook" in the hull will tend to reduce porpoising. On Whaler models that have a reputation for porpoising, like the 1986-1989 20' models equipped with Whaler Drive, adding a "hook" is sometimes the solution.

The original 13' Whaler was actually developed and molded with a "hook" in the bottom. Flip one over and see for yourself.

jimh posted 03-06-2004 01:29 AM ET (US)     Profile for jimh  Send Email to jimh     
Hmm... looking for another article I ran across this one:
jimh posted 03-06-2004 01:36 AM ET (US)     Profile for jimh  Send Email to jimh     
Then compare to this article (see comment from doobee re 20/22 foot hulls)

Tom W Clark posted 03-06-2004 08:16 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

Doobee's comment on the 20/22 hull indicates "a high spot just forward of the transom". This suggests to me a convex surface which is the opposite of the concave surface that is created by a "hook".

Think of a hook as a tim tab in the down position. It will tend to bring the bow of a boat down which is one way you can dampen porpoising. You can trim the motor in to bring the bow down. If you trim out, the bow will rise and porpoising will increase.

jimh posted 03-06-2004 08:32 AM ET (US)     Profile for jimh  Send Email to jimh     
I think he is referring to the mold, so the effect is reversed. A high spot in the mold makes a low spot in the boat.
Tom W Clark posted 03-06-2004 01:02 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

You are right. I was reading his comments the wrong way. Still very odd. I suspect he may have misstated that.

northwhaler posted 03-06-2004 05:23 PM ET (US)     Profile for northwhaler  Send Email to northwhaler     

Thanks for your input into my post. Running a straight edge on the bottom shows that I have both conditions that you wrote about in your post. I have some very slight hooking forward of the transom. More pronounced on the port side within a foot of the transom, and two areas of hooking forward of the transom on the starboard side, one area right in front of the other. The boat is on a bunk style trailer, but the bunks protrude beyond the stern of the boat by about 4 inches.

There is also a "rocker" area about 3 1/2 feet ahead of the transom. I had noticed this area before when putting a straight edge on the bottom of the boat so that I could measure the height of the AV plate on my outboard.

The hooked areas are very slight. Probably in the area of 1/32" off of true flatness. After reading the discussion after your original post, and the other articles that you gave the addresses to, I'm not sure what "just forward of the transom" is. Does that mean within a foot of the transom???, or is the rocker area I have, 3 1/2 feet ahead of the transom considered just ahead of the transom, and therefore a problem. To smooth out the rocker, I would definetely have to have a fiberglass shop do some major build up considering the amount of surface area that would have to be worked on.

Raising the motor a hole is something I will probably try doing first!

Thanks to all for their input on this subject!!


Sal DiMercurio posted 03-06-2004 10:43 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Far to many people make the mistake of putting all the weight of the boat on the bunks.
Doing this will cause a hook to form in your bottom.
The bunks are just supposed to ballance the boat, so it wont tilt to one side or the other, it's the keel rollers that the weight of the boat belongs on.
The strongest part of the boat, is the keel.
northwhaler posted 03-07-2004 08:31 AM ET (US)     Profile for northwhaler  Send Email to northwhaler     

My boat came on a EZ Loader bunk trailer, with no keel support, brand new from the dealer. Let me guess...he sold me the wrong trailer. (Which I guess shouldn't come as a shock to me.)


jimh posted 03-07-2004 09:24 AM ET (US)     Profile for jimh  Send Email to jimh     
The argument about proper trailer rigging is an old one, and each side has its supporters. In the Reference section there is a set of lengthy articles about trailer topics. See:

If you have a 1/32-inch variation in the hull's surface located three feet ahead of the transom, I doubt that is affecting much of anything.

It is interesting to see that there is a difference of opinion on the effect of hull rocker on porpoising--for some it is the cause and for others it is the cure. I would not know which is more correct, as I have not actually investigated this myself. One way to resolve the conflict would be for people who have hulls that seem prone to porpoising to check their hull surface near the transom and report if they found any sign of rocker or not.

Or conversely, people whose hulls have not been prone to porpoising could examine their hulls and see if they had any rocker or not.

One phenomenon that I have observed is that in a given sea state, it is possible to get some porpoising from any hull if you have the engine trim set incorrectly. No hull is totally immune to porpoising, but clearly some hulls are much more sensitive to trim and weight distribution and will go into the porpoise like oscillation very easily.

Moe posted 03-07-2004 12:10 PM ET (US)     Profile for Moe  Send Email to Moe     
I'm not saying it's impossible, but I, and another 150 Sport owner, have _tried_ to make the boat porpoise without success. That's with the weight and thrust of the four-stroke BigFoot EFI on the transom.

I can trim it up just beyond the point of max rpm, and as the engine speed drops, the bow comes up far enough to obscure forward vision, but stays up there. Trimmed up just beyond that, the prop blows out.

Some of that resistance to porpoising is in no doubt due to Whaler mounting the 60 HP BigFoor motor so the AV plate is up at the bottom of the transom. With the same hull design, I'd bet the 170 Montauk is just as resistant to porpoising, but from what I've seen, Whaler mounts the 90 four-stroke with the AV plate about an inch below the transom.

I just checked our hull with a 4' carpenter's level and it is dead flat in the 4' forward of the transom, no hook, no rocker.


Sal DiMercurio posted 03-07-2004 12:35 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Moe, mounting an engine with it's antivent plate 1" "BELOW" the bottom is nuts.
Do they give any explanation as to why?
That sounds like just plain not caring where they mount it, as many dealers [ not all ] do.
Moe posted 03-07-2004 01:11 PM ET (US)     Profile for Moe  Send Email to Moe     
I guess I should qualify that statement, Sal. That's ONE observation of a four-stroke 170 at a boat show. I was looking at the smaller Whalers specifically to see where the motors were mounted. A 40HP 2S on a 130 and a 90HP 4S on a 170 were both mounted one hole up from the center hole, putting the AV plate below the hull. A 60HP 2S on a 150, as well as my 60HP 4S on our 150, were mounted in the center hole.

If I had to guess as to why lower on the 170, it would be to ensure the water pickup stayed submerged in chop, and better their odds against having to pay a warranty claim.


JohnJ80 posted 03-07-2004 09:10 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
Sal is the guy with the answer on this one IMHO.

1. Move the motor up as high as you can get it without the prop blowing out and without losing you cooling stream (I think you should be able to go almost all the way up with that hydrolift on it). Moving the motor up and down, even one hole, has a huge effect on ride and performance.

2. Then add tabs. They will most assuredly cure the porpoising problem and you will get a much better ride when you learn how to use them.

mounting the motor with the av plate 1" below the hull would be painful in performance and ride (like awful) not to mention highly inefficient.

My bet is when you get this tweaked, it will be like a whole new boat.


jimh posted 03-08-2004 08:31 AM ET (US)     Profile for jimh  Send Email to jimh     
I am putting down Moe's data as follows:

No rocker ---> no porpoising

northwhaler posted 03-08-2004 10:24 AM ET (US)     Profile for northwhaler  Send Email to northwhaler     
Looks like trim tabs are the way to go. But I think I will wait to install them until after I try raising the motor a hole or two.

It would have been nice to find a 20' Dauntless or 20' Ventura owner that isn't having a porpoising problem to find how their boat is powered, if they run tabs, etc.

My motor is already mounted over an inch above the bottom of the boat. Unlike other outboards that I have read about on the site, the Optimax doesn't have a slot for the bottom bolts. There are holes for all four bolts, so I can't just loosen the bottom bolts...they have to come all the way out. Should make for an interesting "backyard mechanic" session.

Since the lakes around here are still under about 2' of ice, will be a while before I can test the results, but I will report back what I find out. (With the way our winter is going, probably around June or so!!!) Hopefully sooner.

Thanks to everyone for your input.

northwhaler posted 05-27-2004 08:31 AM ET (US)     Profile for northwhaler  Send Email to northwhaler     
Hello All,

I finally had a chance to test the effect of raising my engine to the third hole, and see if that would have any effect on the porpoising problem I wrote about a few months ago. (Pulled the boat over to Door County about a month ago, but with 30 - 35 knot winds, wasn't able to really open her up on that outing.)

Yesterdays results were great! As others have written in the past about the effects of raising the engine, it's like having a whole new boat.

I now have the ability trim the engine and the porpoising problem appears to be gone. I'm turning a Vengeance prop with a pitch of 17 on my 200 Optimax. Top end was 47 MPH on my GPS, at 5600 RPM. I couldn't get the prop to slip even in hard turns. Didn't do any scientific measurements of time to plane, but it's quick, and the boat will plane at 19 MPH. I tested it in 48 degree water (for you warm weather guys...yes that temp is correct...)winds were about 10-12 knots, in about a one foot chop. Both into the wind, and following seas, the boat handled great. One person on board, about 40 gallons of fuel, and a 8 hp Yamaha kicker on the stern as well, mounted on a Tanner mount. I had previously installed a Turbolift, and that was on the engine for the trial run as well.

My thanks to all who responded to my earlier request for help on this. Simply amazing what 3/4" of lift will do for the performance of a boat!

I'm now trying to talk the local Merc dealer into getting a Mirage 15 pitch on a trial basis. I'm thinking that the extra diameter will offset some of the pitch difference, and bring my WOT right up to the 5750 maximum spec.

If your experiencing some porpoising...raise that engine!!


jimh posted 05-27-2004 08:48 AM ET (US)     Profile for jimh  Send Email to jimh     
Don--Many thanks go to you for locating this thread, reviving it, and appending the information on the results. Following up like this greatly increases the value of the information in the thread.

I found your results to be very interesting. Perhaps others who report a tendency for their boat to porpoise can be guided by these observations.

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