Help with porpoising

Outboard designs up to 14'

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Doug G
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Location: Spring Hill, TN

Help with porpoising

Post by Doug G »

My TNT is porpoising very bad and I'm looking for suggestions on fixing the problem. I obviously have too much weight on the rear of the boat but can't really move things any further forward. I plan to add trim tabs but will this alone solve the problem? Anyone know how big they should be and how much angle should I start with?
Thanks in advance for any help,
Doug

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vupilot
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Re: Help with porpoising

Post by vupilot »

It looks like your tilt position is all the way down in the first hole in your pics. Did you try running it the second and third holes? If not Id try them and see if that helps before making any modifications but it probably will only lift the bow more and not help. You might try some experimenting to see if you can get enough weight forward. Perhaps simulate the weight of the battery moved up under the bow as far forward as possible and then maybe you could extend your cables and move the battery if that works.
You might also look into a hydrofoil which mounts to the outboards cavitation plate if you don't want to modify the boat itself. http://www.overtons.com/modperl/product ... PING_28985
If you put a level along the bottom of your boat from the transom forward along the keel is the bottom perfectly flat? Is the bottom edge of your transom rounded at all or a sharp corner?

Doug G
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Location: Spring Hill, TN

Re: Help with porpoising

Post by Doug G »

I ran the engine all the way down and figured raising it would make the problem worse. I think the bottom is flat and the bottom edge of the transom is just slightly radiused. The rear of the boat sits deep in the water and I am certain that it is too heavy in the rear. My problem is trying to shift weight, it is nearly impossible to move anything up front because the battery and fuel tank simply won't slide under the dash. I would have to cut a hatch into the deck to put anything up front. I hope I can solve the problem without doing that but will if that is the only option. I was thinking about putting a sand bag or two up front and adding trim tabs. I just have no idea how much weight and what size and angle to make the tabs. I might consider the hydrofoil also.

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BayouBengal
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Re: Help with porpoising

Post by BayouBengal »

Doug, that's a beautiful boat. I really like the paint scheme. Your boat is too pretty as is to cut a hatch up front. If you decide to move your fuel, I'd look at a setting up a fuel cap/fill valve option. You can use a small lawn tractor battery for your 12v supply, that will reduce the battery weight.

From your other post "I was able to get it up to 38 with me and my wife both leaning as far forward as possible. It was still accelerating but I had to back off because of the porpoising", it sounds like you have a real need for speed. Trim tabs will probably work to give you a stable ride, they will also slow you considerably. I don't know about the hydrofoil, but I expect with it you won't take as big of a hit on speed, but you also won't get as much stabilization.

Of course I could add what you don't want to hear; that you're pushing the envelope on the motor weight and horsepower that this design can handle, but you already know this.

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vupilot
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Re: Help with porpoising

Post by vupilot »

Yes too pretty to cut into anything. There are other battery options if you cant move the battery to get rid of some weight in the rear. You can get Lithium starting batteries they weight 1 lb. http://www.onesourcebattery.com/full-sp ... g-battery/

Doug G
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Re: Help with porpoising

Post by Doug G »

Thanks for the help guys. I am going to take a serious look at the battery and see if I can get something up front. I am also going to get a set of trim tabs on it. That will be all I do until I try it again. I might take along a few sand bags to try also but really don't like the idea of adding weight.
Thank You, Thank You!!!!

fergal butler
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Re: Help with porpoising

Post by fergal butler »

You can also get wedges for the bottom of the boat to help stop porpoising, you could make some of these from wood at different angles and try them out till you find ones that work.


Image

Doug G
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Re: Help with porpoising

Post by Doug G »

Thanks Butler! I didn't know about the wedges. I wonder how affective they are compared to trim tabs?

It turns out that the battery will slip under the dash. That is 40 lbs going from the rear to the first frame which is pretty far forward. I think that alone gets me headed in the right direction. The fuel tank will have to stay behind the seats but I will keep it as forward as possible.

John K
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Re: Help with porpoising

Post by John K »

Doug, I did wedges and smart trim tabs on my flying saucer. I made the wedges from oak to try them and then a friend made them with Aluminum. I never had a problem after adding both, but I did not add them one at a time. Just wanted to do everything to correct it and not have to do one more thing later.

Mike Sea
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Re: Help with porpoising

Post by Mike Sea »

I think the biggest disappointment a boat builder has is when he launches the boat for the first time and discovers that his pride and joy wants to porpoise. My boat is similar to the Flying Saucer but has a flat bottom and non-trip chines, the boat is 13ft 4 inches long and weights about 250 to 275 pounds, when I launched the boat it was a handling nightmare. The boat would porpoise in smooth water I had to run the motor at the first tilt pin hole to get the boat to handle half way decent and then if you hit a small wave the porpoising would start again.

I took a long look into what causes a boat to porpoise and determined that the three main factors that cause porpoising are too much weight in stern of boat, a flexing transom and believe it or not a boat bottom that is perfectly flat. When we build are boats intuitively we know that the boats bottom must be perfectly flat and we do everything in our power to make it flat. I believe that a slight hook in the bottom of a boat can significantly reduce the tendency of a boat to porpoise. What I did on my flat bottom runabout to stop porpoising was to braced the transom with 2 pieces of ¾ plywood and attached a aluminum plate 15 inches by 14 inches to the transom, then the plywood pieces were bolted to the plate, bottom of boat and last frame section(I know this was over kill). Next I apply two additional layers of fiberglass cloth across the width of bottom, starting about 8 inches from the transom and taking it up the transom about 2 or 3 inches (thus making a hook/wedge in the boat). One nice thing about making a fiberglass pad is if it doesn't work you can sand it off.
After applying the fixes the first time I ran the boat the porpoising was completely gone. I have attended a speed of 42.6 mph (GPS) at a 3 tilt pin position with a 1964 Merc 500 and no porpoising. Also in the back I had a 12 volt battery, 3 gallon gas tank and 170 lb driver and no porpoising.
When I add a passenger in the rear cockpit the boat will porpoise (too much weight in rear of boat).
Checkout out my boat at viewtopic.php?f=20&t=14659

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mrintense
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Re: Help with porpoising

Post by mrintense »

Mike Sea wrote: I took a long look into what causes a boat to porpoise and determined that the three main factors that cause porpoising are too much weight in stern of boat, a flexing transom and believe it or not a boat bottom that is perfectly flat.
Hi Mike,
I am not denying that your fixes worked. However, it seem counter intuitive based upon what we are told in the Building With Plywood book about keeping the bottom flat . I'm sure you wrestled with this as well.

Did you by chance try the other changes before modifying the bottom. Perhaps the stiffening of the transom was sufficient in itself?

Not trying to be a pain here, just curious about this subject. Since I haven't actually gotten to the point of launching, it is a worrisome thought. Granted, my boat will be quite a bit bigger and heavier and may not even exhibit these tendencies, but like you said, it can be a big disappointment. So if it does happen, it will be nice to know what other's have done to correct the problem.

Thanks for posting your experiences because we can all learn from this.
Carl

Crafting a classically styled Vera Cruise

Clipper Boating

Mike Sea
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Re: Help with porpoising

Post by Mike Sea »

Carl your not a pain, I fully expected some comments on my post. I still believe that when building the boat you should make it as flat as possible as mentioned on page 157 of the Boatbuilding With Plywood book, after you have a flat bottom then put the hook/wedge in boat.
I agree with you if I would have done one fix at a time we would know with one really worked. My gut feeling is that the transom fix solved some of but not all the porpoising problem and the fiberglass pad solved the rest and allowed the boat to run higher in the water and smoother. I can not tell you how many people have commented on how flat and smooth my boat runs.
Adding to my initial post it is very important that any additional fiberglass you add be smooth across the bottom of the boat or the boat will have the tendency to dive to one side or the other when turning.
Production boats are sometimes made with a hook in the bottom to improve the ride. In the 60's when I was OPC racing several people would remove the hook to make the boat go faster, sometimes this worked and sometimes it didn't. When it didn't the boats with the hook removed were very ill handling.

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mrintense
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Re: Help with porpoising

Post by mrintense »

Thanks for responding Mike,

one of the problems I face is having no previous experience with boating or building a boat. I know others here have faced the same issues. But that leaves me in a position of trusting in others and doing theoretical work until I can get to the point where the practical application can be tried out.

:)
Carl

Crafting a classically styled Vera Cruise

Clipper Boating

pamelalynne1
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Re: Help with porpoising

Post by pamelalynne1 »

I have the same problem with my Stiletto and it seems to be common to both these designs. I was able to move my two 6 gallon tanks up under the foredeck and I added some extra ballast and trimmed the motor all the way down. It has cut my top speed to about 37 mph and still porpoises a little above 32. The wedges Fergal mentioned may do the job for you. They have been used successfully on Jackson's classic Ace runabout. Smart tabs should also help. With trim tabs you will most likely be able to trim the motor up. What motor are you running? The weight of the motor could also be a factor. My 85HP Johnson weighs almost 300lbs.

Doug G
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Re: Help with porpoising

Post by Doug G »

The motor is definately a factor it is a 1985 30
Hp Johnson. I have moved the battery and have a hydrofoil I am working on installing now. If that doesn't fix it I will do trim tabs next.
Thanks for all the help.

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