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Author Topic:   1988Montauk and porpoising at high speed
llewellyn posted 09-13-2001 09:57 AM ET (US)   Profile for llewellyn   Send Email to llewellyn  
The 2002 Saltwater 2 stroke 90hp Mercury(303lbs, plus one battery starboard stern area) is really nice on my boat, fast, reasonably economical and with good convenience features. Problem is porpoising at high speed requiring what seems to be too much trim in. Would raising the engine a set of holes help eliminate the problem? Do you all have any other suggestions, please? Thanks, Lew
Bigshot posted 09-13-2001 10:07 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Before you do anything, raise the engine and check the prop. You might have a "bow lifting" prop. You can run a doel fin but in my opinion, completely unnecessary on a 17' with a 90(for high speed porpoising). If engine too low, it can cause that.
JBCornwell posted 09-13-2001 10:12 AM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
The hydrofoil will help, Llewellyn. If you move your battery to the console, that will help more. I had to do both to my Montauk when I put a Suzi DF70(338#) on it.

Good luck.
Red sky at night. . .
JB :)

acseatsri posted 09-13-2001 10:42 AM ET (US)     Profile for acseatsri  Send Email to acseatsri     
Could someone explain how a "bow lifting prop" works? I just don't see how this would be accomplished, unless it was based on impeded water flow from around the lower unit.
Bigshot posted 09-13-2001 12:35 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Props are designed in different fashions. Most props cause bow lift(we want that), some cause stern lift(ass-heavy or bow light boats). Surface piercing props usually raise the stern so the blades will get above the surface. That is why you do not see them on conventional boats without jack plates, etc. Bass boats use props that give extreme bow lift. How do they do it, different rakes and trails but I do not know the formulas.
Soho posted 09-13-2001 01:44 PM ET (US)     Profile for Soho  Send Email to Soho     
Okay, while I think that I know what porpoising is - your bow sort of undulates up and down - could someone please clarify what exactly this condition is. I presume it has nothing to do with the TV show Flipper or hunting actual Porpoises......



Bigshot posted 09-13-2001 02:22 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Basically it is the bow rising and dropping in a methodic sequence. Kind of nauseating. Can be slight or violent in nature. Usually in between. Would happen in calmer water, rough water would break the action.
Bigshot posted 09-13-2001 02:26 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Well that did not really help since you know what it is. Ok it is usually where your trim angle on the engine(no matter how tucked in it seems) is too high. This is due to many factors, most common being weight too far aft. The bow is thrust upwards and then physics and hull configuration bring it back down, sometimes very quickly. The faster you go the less it usually porpoises due to wind under the hull and the way most boats can handle or even perform better trimmed out at higher speeds. The slower you go(on plane) the less trim you want. Hope that helps a bit better.
Soho posted 09-13-2001 02:28 PM ET (US)     Profile for Soho  Send Email to Soho     
Thanks for the confirmation. Yep - I have had that happen in my boat before, and in calm water as indicated; I just never appreciated that it was due to mechanical reasons - I usually attributed it to a gentle swell or something..... Guess you learn something every day, at least I do here.
FISHNFF posted 09-14-2001 02:08 AM ET (US)     Profile for FISHNFF  Send Email to FISHNFF     
My Montauk will porpoise if I have my bow trolling motor and platform mounted. I can see how this weight forward would counteract the motor trying to push it up. The problem is worse with my 90 4S (386lbs.+) than with my 75 2S (303lbs.). I raised my motor to the third hole up with no help porpoising, but better hole shot. I think the DoelFin is not even in the water at plane, but is seems to keep water around the prop, preventing ventilation/cavitation?
llewellyn posted 09-14-2001 12:41 PM ET (US)     Profile for llewellyn  Send Email to llewellyn     
Many thanks, guys. I'll let you know if/when
I get it fixed.
llewellyn posted 09-14-2001 12:47 PM ET (US)     Profile for llewellyn  Send Email to llewellyn     
Now I'm wondering if the prop I got w/boat is not right for it...a stainless "Merc." that was 19 pitch and modified to 21, maybe also changing "rake and trails"? Thanks again, Lew
lhg posted 09-14-2001 03:26 PM ET (US)     Profile for lhg    
The Merc 90 is a great engine for the Montauk. My guess is that you are simply running with too much trim out.

As far as I know, Mercury does not make a 19" SS prop for that engine. With a boat that runs in the mid to upper forties, you should be running a Merc Laser, Trophy Plus or High Five propeller. (Laser would be my choice)
They come in pitches of 20" or 22" for your application, about $365 each. Engine could be mounted in 2nd or 3rd bolt hole up.

A bow lifting prop like a Laser will give you a better ride, better top speed, and superior holding in rough water. Bow lifting is actually accomplished by the action of the prop blades which push down on the transom.
I run these props myself, and they are highly recommended.

However, trim angle and weight distribution is the main cause of porpoising, not prop design.

Finally, there is much information on prop design elsewhere in this Forum.

llewellyn posted 09-15-2001 03:00 PM ET (US)     Profile for llewellyn  Send Email to llewellyn     
Thanks, Larry. I'll check it out. Lew
Whalerdan posted 09-16-2001 03:33 AM ET (US)     Profile for Whalerdan  Send Email to Whalerdan     
You say in your original post that it's fast. How fast? If you are in the mid 40's, I really don't see what the problem is with tilting the motor in if it alleviates the porpoising. My boat will sometimes do this depending on the weight and if I'm going up wind. But even when I tilt it down (to what seems to be extream) it's still going close to 45mph.
llewellyn posted 09-24-2001 04:01 PM ET (US)     Profile for llewellyn  Send Email to llewellyn     
A Laser II with 22" pitch performed fine, without porpoising, about 5200rpm WOT, and this used one is available from my dealer. He'll also take the unknown prop I have on trade. Larry, others too with Merc/Laser/Montauk experience, please; do you see an advantage to getting a new 20" pitch Laser II? Perhaps with more hours on this new engine more rpm would be possible with the used 22"?
Bigshot posted 09-24-2001 04:38 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
what is redline? If 5500 you will be fine with the 22" if higher I would go with a 20"
JBCornwell posted 09-24-2001 07:19 PM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
Say, Bigshot (or anyone else who knows).
My Suzi DF70 (338lb) is on a 4" setback on my '80 Montauk. Dealer says he had to because of holes not matching up. I think that has the effect of adding another 50lb or so to my stern. I moved the battery to the console but still get a lot of porpoising before reaching redline (5800@ gps 38mph) if I trim out enough to get there. If I trim in to stop the porpoising I can't get past about 5300-5400 at WOT.

I have a SE Sport hydrofoil on it and the cav plate at the keel level. 18"SS OEM prop.

I have an opportunity to trade down to an Evinrude version of the DF50, which is just over 100lb lighter (235) and reported to run a Montauk to low 30s at WOT. I would also get some cash.

Which is my best way to get high-plane (no porpoising)economy at 30 to say, 33mph:
Add trim tabs?
Raise engine and inch or so?
Go to the lighter 50 Hp engine?
Settle for running the DF70 at 4500 to 5000?
Go to a 17" prop?

Thanks for experienced opinions or even s.w.a.g.s.
Red sky at night. . .
JB :)

Bigshot posted 09-25-2001 09:33 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
For every 6 inches you go aft, you can raise a motor 1 inch. I would raise yours 2. That doel fin is slowing you down when it drags. Raise the motor and keep it. 100lbs does not meen doo-doo once on plane.
llewellyn posted 09-25-2001 02:32 PM ET (US)     Profile for llewellyn  Send Email to llewellyn     
Redline is 5500, Bigshot. Thanks. Traded for $100. I do like my dealer. Thanks to you all.Lew
lhg posted 09-25-2001 06:17 PM ET (US)     Profile for lhg    
Porpoising is the result of only two things!
Weight distribution in the boat at a given engine trim angle. To eliminate you only have to do the same two things. Trim in, or move weight forward.

In your situation, assuming the engine is at reasonable operating height for 70 HP (anti-cavitation plate running about 1/2" above water flow off transom) you will have to go to a lower pitch prop. But before doing that, try getting rid of the aftermarket engine fin. It's drag will really slow you down. If RPM's are still too low at non-porpoising trim angles, you need a lower pitch prop.

JAC posted 09-26-2001 12:04 PM ET (US)     Profile for JAC  Send Email to JAC     
The 16' 7" hull has always had this problem with the weight and power of modern motors. Talk with anyone who has used a Honda 4 stroke.
All the fixes noted will improve this problem but to corrrect it requires cupping the hull along the transom. I have done this on the pad area others have done this along the entire transom with similar results. It works.
The OEM hull is flat to slightly rounded across the transom, causing a turbulance behind that sucks the boat down into the water. With weight or high speed this condition is magnified. Cupping directs the water down , eliminating this turbulence with respect to the transom. Eliminating the symtom, porpoising.
Rather than explain the modification process, you don't want to do this w/o experiance, go to your local glass shop or performance marine shop, they can fix this problem for you. Modifications of less than 1/8" will uswally suffice.
Whalerdan posted 09-27-2001 02:10 AM ET (US)     Profile for Whalerdan  Send Email to Whalerdan     
Man that sounds drastic! I still don't see what's wrong with tilting the motor down until the porposing stops. For crying out loud I have an old 115 Evinrude on the back of mine, and if I can trim out the porpoising I would thing anyone can. I have never not been able to stop it. You might lose a mile an hour or two in top speed, but big deal, it's alot better than redesigning your hull!
JAC posted 09-27-2001 10:40 AM ET (US)     Profile for JAC  Send Email to JAC     
WHalerdan, cupping is a very simple proceedure that a good shop can do in an hour. The difference is weather you want to fix the problem or eleviate the symptom.
Whalerdan posted 09-27-2001 11:28 AM ET (US)     Profile for Whalerdan  Send Email to Whalerdan     
You got any pictures of a cupped hull?
JAC posted 09-27-2001 11:47 AM ET (US)     Profile for JAC  Send Email to JAC     
Sorry Dan, I sold that boat last year but there is very little to see. The last 3" or so of the hull, just in front of the transom is slightly indented. Once the gelcoat is replaced, you can feel the cup, but it is difficult to see.
This slight modification is enough to turn the water flow down, avoiding the turbulance issue.
If you would like more info, or to see this yourself, visit any performance marine shop. This is a common modification to change boat performance.
lhg posted 09-27-2001 12:56 PM ET (US)     Profile for lhg    
I'm sorry to be so blunt, but I believe this "cupping" advice is just bad information regarding a BW hull. Why isn't this built into the hull of this 40 year old, highly successful, design? Or most other boat hulls for that matter? I ran a 305lb engine, along with an 80 lb auxiliary, battery in the stern corner, on mine for years, with no porpoising problems that couldn't be corrected by proper engine trimming. And the boat would still do 55mph. (Merc 115)

I would never consider grinding out a "cup in the bottom of one my Whaler hulls! In the old days they used to call this a "hooked hull", damage that occurred from improper support of the transom, causing a concave shape in the hull bottom at the transom. Hooked hulls were generally considered serious damage. Why do it intentionally? If it's used in racing, so be it, but hardly seems appropriate on a Montauk.

If anybody here is considering taking this advice, I would recommend contacting the factory engineers first. I think all sides of a discussion have to be explored.

bigz posted 09-27-2001 01:15 PM ET (US)     Profile for bigz    
Let's see.... Larry is 100% correct... either trim in or redistribute the weight period.

Lew states which I don't understand "high speed requiring to much trim in" -- I don't think there is such a thing as to much....... that is you trim depending on the conditions until you eliminate the unwanted proposing that's all.

Appears that Laser II prop might do the trick.

Actually JAC, we are talking about a tried and true modern Merc here (hmm well I guess modern)! Though I haven't run across many people using a Honda on a Montauk ( don't think there are many anyway -- relative term many) the idea of modifying the hull seems like maybe someone picked up the wrong pack of smokes ;)

JAC posted 09-27-2001 03:35 PM ET (US)     Profile for JAC  Send Email to JAC     
Based on these comments, I believe many of you have no experiance with this technique and so consider it wrong...your opinion.
The reason this hull performs as it does is because it is a 40 year old design. Fluid simulation and the todays materials were not available then. Motors weighed less and drove less actual horse power. All factors not considered in the original design.
Many of us are pushing this design beyond its intended specs, so modifcations can compensate.
Many of today's production hulls use this cupping technique, but since it is so minor a feature, it is rairly noticed by the buyer.
This modification was passed on to me by a long time Whaler dealer and I have had experiance with two other Whaler dealers since then who have used this to improve the performance of this hull.
Good luck with what ever you decide to do. Jim
acseatsri posted 09-27-2001 04:09 PM ET (US)     Profile for acseatsri  Send Email to acseatsri     
When I was a kid, I remember we had "wedges" on the bottom of the wood boat to provide lift. I think trim tabs are the easiest way to accomplish what you want. All you're doing is providing more lift on the transom, which drives the bow down.
simonmeridew posted 09-27-2001 07:23 PM ET (US)     Profile for simonmeridew  Send Email to simonmeridew     
This is a very interesting series of posts. Perhaps the "cupped" or "hooked" stern is more theoretical than practical for most of us, but I appreciate everyone being civil and courteous in replying. It never hurts to hear someone else's opinion; occasionally I learn something.
JBCornwell posted 09-27-2001 07:42 PM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
I don't think I ever learned anything important from someone I agreed with at the beginning.

Red sky at night. . .
JB :)

gunnelgrabber posted 09-27-2001 08:02 PM ET (US)     Profile for gunnelgrabber  Send Email to gunnelgrabber     
very interesting... following along with j.b....if all of the kings advisors agree with him, he really can eliminate them as he really has no need of them, right?....but i'd disagree carefully, as to keep my head..."shall we gather at the river"?....lm
bigz posted 09-28-2001 05:25 AM ET (US)     Profile for bigz    
Jim (JAC)not sure wrong or right or in between.

I would appreciate the dealers you mention tech folks names so that I might get a little more insight on the why and where for of accomplishing this modification (in fact it intrigues me, so that today when I speak with a friend who is a fiberglass expert and also a go fast freak will question him on the concept and what is entailed to accomplish it --- hopefully I can explain it so he has a clue of what I am talking about) -- maybe Whaler would like to know and understand about this "improvement" -- for future production -- does seem like a minor mold modification could accomplish this "benefit" --- that's if it is functionally deemed necessary based on the your "newer" outboard premise.

Must say on the surface doesn't make sense to go to that cost and effort for basically a fishing boat semi-tri-hull -- which as noted has performed pretty darn well for many decades with its intended purpose mind -- actually a set of shocks might be a better modification on it and the 13 ---- just kidding Tom

Clark Roberts posted 09-28-2001 07:07 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Interesting that the classic 13 hull is designed with a hook and twin skegs! This hook is one of the reasons the 13 performs well with low hp. Also, it's one of the reasons the boat leaps, thrashed, chine walks, etc. beginning at speeds around 40 mph! Before I modified that Montauk, Llewellin, I would consider trim tabs. BTW, as you may recall, I ran a 17 with a 3cyl 90 merc for years and never had a problem... also the prop of choice for me was a 20"pitch Merc "High Five" which gave speeds of around 50mph... That 17 hull of yours is so clean and pretty that , personally, I wouldn't modify anything!!!!! Just my thoughts! Happy Whalin'... Clark... The Old Man and the Sea
Bigshot posted 10-01-2001 05:19 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
clark beat me to it. 13's have hooks, my 20 Hydra-Sport has such a big hook I have to wedge the cement blocks when I set it up for the winter. My Baja has a long hook that if you want to increase top speed, you remove it, not add it. Hooks are basically a cheap fix for what some might consider a poor design change. They also keep the bow down on light boats which lessen the blowover rates(ie-why they are on 13's). What I mean by that is my Hydra has it because the boat porpoised so bad they added it into the next years design, they should have added trim tabs as standard equipment instead. Which is cheaper? The whaler does not need a hook unless you have a bad weight distribution or maybe you have some other hull irregularity that causes the opposite of a hook. I have owned 3 17's and never had a porpoise problem even with a 115hp Johnson.
lhg posted 10-01-2001 05:45 PM ET (US)     Profile for lhg    
Well, I keep learning things here. I had no idea that the original 13 was designed with an intentional "hook".
acseatsri posted 10-01-2001 05:49 PM ET (US)     Profile for acseatsri  Send Email to acseatsri     
Intentional hook? I thought it was from sitting on a trailer for 30+ years! When they designed the hull in 1958, I doubt if anyone ever heard of hook. It was probably just a mistake in the mold!
jimh posted 10-02-2001 01:47 AM ET (US)     Profile for jimh  Send Email to jimh     
And people ask me, "What the heck do you talk about on these Whaler Rendezvous for a whole weekend..."

Quite amazing all the info that comes out on these boats!

TampaTom posted 10-02-2001 09:30 AM ET (US)     Profile for TampaTom  Send Email to TampaTom     
In ref to bow lifting props, "Cup added to a prop blade controls what the prop does for the boat. Cup on the trailing edge controls stern lift and aids to some degree hole shot. Cup on the blade tips controls bow lift and speed. Cup is not a generic item, it has to be designed to the application and what works for boat and motor X will not necessarily work for boat and motor Y."

More on setup can be found at ""

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