FS12 15hp porpoising

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pcity2
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FS12 15hp porpoising

Post by pcity2 »

Splashed my FS12 yesterday, I think she's too stern heavy, currently have yamaha f15 on. Anyone else have a 4 stroke 15hp on an fs12? Boat will plane off, but porpoises badly. I don't have a tiller extension yet, but working on setting that up so I can move to middle seat, I'm 165lbs wet. Is it really more of a weight distribution issue? I tried moving the motor up an 1-1/2" this morning, same result with the porpoising. Would a set of small trim tabs be overkill for boat this size? Or something simpler like adding a doel fin. Wondering what other's experiences have been on the fs12.

My other thought is going to a 2 stroke 9.9. Would like to be able to plane and get speed in the 20's.

Thanks!
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Reid
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Re: FS12 15hp porpoising

Post by Reid »

pcity2,

The FS12 looks really nice!
I think you definitely need to get a tiller extension so you put your weight forward. Also, how much fuel are you carrying and is the tank under the stern seat? Can the tank in the bow? This may help with weight distribution.
Bennett does make a set of self-leveling trim tabs for small boats. Here is the link on our site: https://boatbuildercentral.com/wp/prodd ... d=219-SLT6

I would try the weight distribution first.
Good luck!
-Reid
Someone asked me, if I were stranded on a desert island what book would I bring... "How to Build a Boat."
- Steven Wright

pcity2
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Re: FS12 15hp porpoising

Post by pcity2 »

Reid wrote: Thu Oct 10, 2019 11:56 am pcity2,

The FS12 looks really nice!
I think you definitely need to get a tiller extension so you put your weight forward. Also, how much fuel are you carrying and is the tank under the stern seat? Can the tank in the bow? This may help with weight distribution.
Bennett does make a set of self-leveling trim tabs for small boats. Here is the link on our site: https://boatbuildercentral.com/wp/prodd ... d=219-SLT6

I would try the weight distribution first.
Good luck!
-Reid
Thanks, 6 gal fuel tank under stern seat currently.

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OrangeQuest
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Re: FS12 15hp porpoising

Post by OrangeQuest »

Before you do to much more can you get a person to sit in the front?
"that it isn't just an ordinary sort of boat. Sometimes it's a Boat, and sometimes it's more of an Accident. It all depends." "Depends on what?" "On whether I'm on the top of it or underneath it."
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Re: FS12 15hp porpoising

Post by Fuzz »

Motor still looks to be a little too low. But moving weight forward is the ticket. This is a common problem with small boats. Take sand bags or anything else and add 150# in the front and give it a try. Or wait till you get the tiller extension and then run it from the middle seat.

pcity2
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Re: FS12 15hp porpoising

Post by pcity2 »

Thanks all, I'll try moving some weight forward, also think I'll try a doel fin, they're cheap enough, I can take off and keep around the shop if they don't help.

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Re: FS12 15hp porpoising

Post by jacquesmm »

Very nice boat but the porpoising is certainly dues to weight and weight distribution.
One of our FS12 prototypes was built by a bateau worker and he made her heavy. He used a 8 HP Honda and sitting in the stern, the boat porpoised. Once he rigged a tiller extension, the boat ran fine.
I still own another of those FS12 prototypes and rigged with an old 6HP and tiller extensions, she planes but no porpoising.

Your outboard is heavy but before you switch to another motor, spread the weights.
The most important weight to move is you, the crew. Get a tiller extension that allows you to sit on the mid bench.
Those extensions cost $ 40.00 online.
Next, move that gas tank forward.
If that does not take care of the problem, install a Doelfin or similar on the outboard. Or, install a pair of small fixed trim tabs on the transom: cut 6" slices of a L shape aluminum profile. The transom angle should be fine.

All that is for a one person boat. As soon as you have a passenger, the porpoising should stop.

Let us know how all that works.
Jacques Mertens - Designer
http://boatbuildercentral.com

pcity2
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Re: FS12 15hp porpoising

Post by pcity2 »

jacquesmm wrote: Thu Oct 10, 2019 3:03 pm Very nice boat but the porpoising is certainly dues to weight and weight distribution.
One of our FS12 prototypes was built by a bateau worker and he made her heavy. He used a 8 HP Honda and sitting in the stern, the boat porpoised. Once he rigged a tiller extension, the boat ran fine.
I still own another of those FS12 prototypes and rigged with an old 6HP and tiller extensions, she planes but no porpoising.

Your outboard is heavy but before you switch to another motor, spread the weights.
The most important weight to move is you, the crew. Get a tiller extension that allows you to sit on the mid bench.
Those extensions cost $ 40.00 online.
Next, move that gas tank forward.
If that does not take care of the problem, install a Doelfin or similar on the outboard. Or, install a pair of small fixed trim tabs on the transom: cut 6" slices of a L shape aluminum profile. The transom angle should be fine.

All that is for a one person boat. As soon as you have a passenger, the porpoising should stop.

Let us know how all that works.
Thanks Jacques! Had wife on mid seat and son up fwd in boat last night, still porpoising pretty bad. But lots of good suggestions, I grabbed a doelfin today and hope to try that this weekend as well as putting 100lb's or so of weight right up fwd in the bow locker. Also I'll get tiller ext when I can.

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Re: FS12 15hp porpoising

Post by ctbeutel »

I built a FS12 and have a Tohatsu 8hp 4-stroke carbed up to a 9.8. I am surprised you are not reving out with the 15. I had to put a slightly larger prop on mine to keep from over reving. My son is 125lbs and his buddy is about 110. The boat runs right at 22mph and get on plane quickly. I put a 3 gallon fuel tank up front. I took it out with a buddy of mine a few weeks ago. I'm 240 and he was about 165. The motor had to work a bit to get on plane but it did. We ran about 18 to 19. I was pretty impressed. Switching to the lighter 2-stroke and maybe going to a 3 gallon tank would help a lot. That is a lot of weight in the rear if you are going to be riding solo.
Owner of a FS18, completed FS12.

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Re: FS12 15hp porpoising

Post by jacquesmm »

Let us know how it works with the Doelfin but as you can see from the other testimonies, the boat should not porpoise.
Let's try the Doelfin plus a better weight distribution.
Jacques Mertens - Designer
http://boatbuildercentral.com

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