Porpoising help

Outboard designs up to 14'

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Mojo
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Porpoising help

Postby Mojo » Wed Sep 03, 2014 9:32 pm

Hi Folks,
I'm after some advice. I have a newly launched Flying Saucer. The engine is now run in and I can use full throttle. During the run in phase, it seemed pretty balanced, but now as I have access to more power porpoising has become more of an issue. I've kept moving weight forward to try and balance this out, but it is now at the point where we are too heavy in the front, but the porpoising is still present.

I've checked the hull. The rear 3ft is flat except for the aft 3", which needs to be filled about 1/8" to be straight with the rest of the bottom. Could this be causing the problem?

Cheers
Mojo

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vupilot
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Re: Porpoising help

Postby vupilot » Thu Sep 04, 2014 2:13 am

Also, you may have already done this but check that you have a sharp edge where the transom and bottom meet. You don't want that edge too rounded over. What tilt position are you running the motor in? I'd fill the 1/8" void and test again.

cusoak
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Re: Porpoising help

Postby cusoak » Thu Sep 04, 2014 3:46 am

As Chris said most of the time that is caused by the fact that the front end is to when running at full throttle. I would ck to make sure that you have the motor as close to the transom as possible.
What hp motor are you running and do you know what speed you might be at when it starts to porpoise
May be you are not getting up on plain.
Jeff

miyot
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Re: Porpoising help

Postby miyot » Thu Sep 04, 2014 5:39 am

Sorry to hear your problem as it may affect me as well. I'm also building the FS. I agree with vupilot. An 1/8 inch problem right at the transom may well be the cause. At least you won't have to turn the boat over to fix it. I would sand back to the glass and fair this area with epoxy and some glass strips laid in for strength and quick build up. Then grind back making sure you have a crisp square corner or trailing edge and nice flat bottom.

Keep us updated on your fix and the results. You will get it right. Just some fine tweaking. Is the boat porpoising at cruise or just top speed?
Dave

John K
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Re: Porpoising help

Postby John K » Thu Sep 04, 2014 10:19 am

My FS did the same thing. I made a triangular shaped shim that was 1/2 inch at the top and the length of the motor mount. The shim as also as wide as the motor mount, about 1 inch. The shim I built was about 10 inches long. I installed it on the both motor mounts. That gave the motor 5 degrees more movement towards the transom which helps drive the bow down.

I also installed smart tabs on the transom to take the bounce out of the boat. http://www.nauticusinc.com/smart_tabs.htm I used a 30 lb shock absorber on them. I think they recommended a 20 lb. Shop around as the price varies a lot.

Mojo
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Re: Porpoising help

Postby Mojo » Thu Sep 04, 2014 6:25 pm

Thanks for the advice, I'll keep tinkering away.
The boat is good at cruise speeds, but the porpoising starts becoming a problem over about 28mph.

I went for a play today and found I could pretty much stop it by trimming down and adding about 50lbs to the front, there was also about 5 gallons of fuel up front as well. I don't really want to go adding weight though, plus it goes faster when the engine is trimmed up a little to put the thrust inline with the water level.

I'll fair the aft end of the hull and see how it performs from there. I'll keep you posted.

Cheers
Mojo

Reado
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Re: Porpoising help

Postby Reado » Thu Sep 04, 2014 7:06 pm

Lots of good advice here. I built a few Minimax hydro's back when. Notorious porpoising!
I incorporated some ofl the solutions suggested here with success. Especially having a dead flat planing surface.
I would add one more suggestion. Make sure the cavitation plate on the outboard motor is lined up with the bottom of the transom. Sort of like an extension of the bottom. Adjusting the motor height will achieve this. Adjust motor angle (as mentioned) to help get that nose down.
We used to use plywood tabs and jury-rigged turnbuckles! Probably not a good idea on a FS!!

Keep the faith...porpoising can be conquered!

Mike Sea
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Location: North Canton, Ohio

Re: Porpoising help

Postby Mike Sea » Sat Sep 06, 2014 9:17 am

Hi Mojo
I built a boat very similar to the Flying Saucer but with a flat racing bottom and non- trip chimes. I was running a motor that weigh about 115 lbs and had about 40 hp. On the third pin the boat porpoised very bad and on the 4th pin position it was almost uncontrollable but the speed picked up 2 mph over the 3rd position. I was able to stop the porpoising my running the motor tight against the transom but the boat lost almost 5 mph in speed and I didn't like the way it handled. I was able to stop the porpoising and get a fast and good handling boat by making some changes that I know may not agree with present theory. The first thing I did was fill in the voids in the transom with white oak in hopes of adding strength, it didn't help. Next thing I tried was bracing the transom by installing a U shaped aliminum 3/16 in plate that covered the transom from the floor to the top. The alimunim plate was then bolted to 3/4 in plywood panels that were also bolted to the floor and the the next frame member. With this over kill I had no movement of the transom. My thinking was ever time the transom moved the motor had to go with it and as the motor moved the porpoising got worst. The next thing I did was install a taper in the bottom of the boat about 6 inches forward of the transom. The tapered pad is about 1/16 to 1/8 high and is highest at transom and was installed the width of the bottom with one layer of fibre glass cloth and resin. With these fixes the porpoising was gone when running in the 4th pin position but I did not like that much boat out of the water so I normally run the 3 pin. The boat is currently running a 1964 Mercury 50 hp at 43 mph, that weighs 160 lbs with no porpoising. Many people comment on how good my boat looks at speed check it out at download/file.php?id=8465&mode=view
Mike

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vupilot
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Re: Porpoising help

Postby vupilot » Sat Sep 06, 2014 10:29 am

The Utility I bought earlier this year has a bottom that is not perfectly flat. I get a slight porpoise only when running near top speed and more pronounced when running solo. I've heard for a few years a very simple mod to help a slight case of porpoise was to add a small wedge or shim to the bottom of the boat. I've heard good results from this from one other builder here so I took this as an opportunity to experiment with them.

What I did was take two door/window shims and epoxy them together then cut the length down and round over the nose. I then encapsulated and installed it to the bottom with epoxy and painted it. Just tested it this morning and ran wide open with no porpoise. I think for slight imperfections resulting in intermittent porpoise this is a great, easy fix
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John K
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Re: Porpoising help

Postby John K » Sat Sep 06, 2014 10:53 am

that is cheap way to fix the problem, vupilot.

Flash
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Re: Porpoising help

Postby Flash » Mon Sep 29, 2014 8:29 pm

Hi Mojo,

Just wondering how you're are doing with the porpoising problem.

Did you say that you have a bow tank plus 50 lbs. in the bow?
Flash
Yellow Submarine II - Flying Saucer
Summer Wind - Zip

Mojo
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Re: Porpoising help

Postby Mojo » Tue Sep 30, 2014 1:29 am

The problem I find is that the trim and balance is greatly altered when adding passengers. As expected for a small boat the difference between having 180lbs of passengers vs 800lbs changes things a lot.

With a full 12gallon tank of fuel in the bow, just me in the front seat (180lbs) and the outboard trimmed down about 4 degrees I can hit full throttle speed of 32mph without porpoising. I can get 33mph with the outboard trimmed to 0 degrees, but the boat porpoises.

With 800lbs worth of passengers, 6 gallons of fuel I get porpoising over 25mph. This is even with the trim tabs.

I'm going to fill the aft end of the bottom to get a flat surface. I think this should help, but ultimately I suspect there is always going to be porpoising at top end speeds with a full load of people.

I'm still experimenting with the smart tabs, but don't think they're the cure. With just me in the boat the don't seem to reduce porpoising and the boat is light enough to get on to the plane early.
With a full load I suspect they do make a useful difference in getting on the plane.

I'm still experimenting with the various setup options and weigh distributions. I also hope to get the bottom flat before too long. I'll post more information as I make progress.

Cheers
Mojo

Flash
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Location: Eagan, MN

Re: Porpoising help

Postby Flash » Tue Sep 30, 2014 5:16 am

Thanks Mojo,

I finished fairing the Yellow Submarine II last Sat. and started touching up the encapsulation yesterday. So, I'll be watching your progress on this problem closely.

Good luck,
Flash
Yellow Submarine II - Flying Saucer
Summer Wind - Zip

John K
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Re: Porpoising help

Postby John K » Fri Oct 03, 2014 11:36 am

Mojo, You can try heavier shock absorbers on your smart tabs. Don't know if you already tried that. I am really shocked that you have not found the right combination.

Smart tabs and motor angle did it for me no matter how much weight was in the boat.

Just thought about it. Did you make yours the twin cockpit? I made mine as a single cockpit and may have more weight upfront than yours.

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hzchevy
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Re: Porpoising help

Postby hzchevy » Tue Oct 14, 2014 11:53 pm

I had a huge problem with porpoising on my PeeWee. Largely due to not having enough weight in the front. Even with me mounting the fuel tank up front and moving the seat forward it still occurred. Lead weights helped alot but slowed me down. My solution was I made trim tabs and put a wedge between the transom and motor for more angel. Now LIL INSANE is a pleasure to ride in.
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