Porpoising

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FLYonWALL9
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Porpoising

Post by FLYonWALL9 » Sun Jul 02, 2006 11:18 pm

I know its due to little weight in the bow. Short of
adding more weight in the bow should I

A. Move the tilt of the motor out
B. Move it in
C. Try a hydrofoil



timoub007
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Post by timoub007 » Sun Jul 02, 2006 11:29 pm

Scott,

Trim in and it will bring the bow down. When you trim out it moves the direction of force from the motor. The prop is always trying to be parallel with the surface of the water. Thus it puts leverage on the transom and boat to change the angle of the boat.

A hydrofoil might help by providing more surface area for the motor to apply that leverage with. It works in some cases, but not in all.

In an ideal world, the ventilation plate should be just out of the water when you're up on plane and trimmed out. This would mean that the hydrofoil would also be out of the water when running WFO. All I can say is try it or you won't know for sure.

Tim

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macfiveo
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Post by macfiveo » Mon Jul 03, 2006 4:25 pm

I'd agree to trim it in a bit, Scott. That may hamper your speed a tad, and maybe moving a little weight forward would permit you to still trim the engine out.

Seems like most of our Bateau boat builders eventually get the advice to put more weight forward. Perhaps it's because we are such substantial fellows..... :lol:

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Hunch
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Post by Hunch » Mon Jul 03, 2006 5:12 pm

macfiveo wrote: Seems like most of our Bateau boat builders eventually get the advice to put more weight forward. Perhaps it's because we are such substantial fellows..... :lol:

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I resembler that remark fat boy.

It's fairly easy to move some weight forward. Coolers, tools, spare parts, anchore, extra beer, the mother-in-law, etc.

I need to trim my xf20 by shifting weight forward when I run with a full live well (35gal) or I just keep the throttle back a tad.


The sweet spot on every boat is different.

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Post by stickystuff » Mon Jul 03, 2006 9:06 pm

Here is a little tip for every one. The most important thing you want to do is find the center of gravity of the hull. Once the hull is assembled and epoxied together lay a long 2X 4 underneath the hull crossways and find the balancing point. A 2" pipe or whatever you have will work as long as you can balance the hull on it. Don't go useing a 6 X 6 or something wide like that. Keep it narrow and balance the hull. This is your center of gravity. Now you have an idea where to mount your center consol, batteries, fuel tanks etc. First thing you want is your consol at least 2/3 the length of the consol in front of your C/G. A lot of people want the consol further back, batteries in the stern, livewells in the stern etc. You have engine weight, jackplates if you use one etc. All this adds up and you have a stern heavy boat. It will be harder to get up on plane and when you do, thus porpoising. Think about it. A little prethought and laying out will give you a stronger and better performing hull. Hope this helps some.
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FLYonWALL9
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Post by FLYonWALL9 » Tue Jul 04, 2006 12:02 am

Helps lots ya'll thanks. Before when it was a center console it was a bit different I didn't sit on the back deck so that put my fat arse another 2ft forward and the console about foot forward. So, now I'm more towards the stern. I'm thinking I may build a battery box or tray behind the front cockpit bulkhead and put my trolling battery and cranking battery. These batteries are the largest ones you can buy. Thats really the only thing I can do to more weight forward. I already have my fuel tank livewell and bow storage. It rides much better with more HP, and the weight gain there wasn't but like 15lbs. So, I think I am going to do two things, move the batteries, and add a hydrofoil. At this point it couldn't hurt.

Sticky, that tip with the pipe is one of the best I have heard here in a long time. Thanks for that, I will surely use it on my next boat when I can find the room to build it.

Oh one other thing if moving the batteries and adding a fin doesn't fix it does anyone have real life results of the SMART TABS?

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Post by JerryF » Mon Jul 10, 2006 11:32 am

I tried different hydrofoil designs and motor heights as well as motor trim positions to see how they affected running angle and hole shot with a lot of aft weight.

My advice is to try a hydrofoil. They are cheap and easy to install (if you don't mind drilling holes in your outboard AV plate). I like the Sting-Ray best because of it's curved top surface and the bulk of the foil is behind the engine instead of out to the side (cosmetic difference). Big foils work better than little foils but any will improve porposing.

Motor height is critical with a foil. They work best (max lift) if the AV plate is slightly below the bottom of the hull (underwater at speed). Above the bottom and you don't get much help except for the hole shot. At the bottom line, it may give full lift or it may cavitate.

Motor trim is important too. Foils should be parallel to the bottom of the boat for min drag / max lift. Trim in and you get more lift but a lot more drag too. Trim out and the foil will try to dive instead of lift. If you have power trim on your outboard, you can change the foil angle of attack underway for different ride characteristics.

I found that a good foil setup can make a quicker hole shot, stop the porposing, make the hull ride level and improve steering and handling at speed but it will knock a mph or two off the top end.

One other thing about a foil vs tabs. When you raise the motor to beach the boat, the foil raises up too.
JerryF
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Post by jacquesmm » Tue Jul 11, 2006 10:35 am

Lots of good advice but also try to spread the weights. Porpoising is a pendular movement. Increase the amplitude by spreading the weights.
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FLYonWALL9
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Post by FLYonWALL9 » Tue Jul 11, 2006 12:48 pm

Thanks tons guys (sorry for the pun :D ). I purchased a
fin, that I'm sure will help. I also am trying to come up
with a design for a battery box to move my batts near
the bow but still in the cockpit area. I just have to figure
how to make them water proof. I think moving that 80lb
or so will help a bit. I know it will make it sit level at rest
which it leans now a bit.

So, I have a few things to try when I get well enough to
work.

Oh one last thing I am on the search for a fishing padna
who is more like 5ft2, 110lbs rather than my normal
padna that is 6ft3. 210 :doh:

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Post by maxgsx » Tue Jul 11, 2006 1:38 pm

Might be off the mark here but if one has loads of weight at the stern and tries to ballance that with weight in the bow, won't the boat want to pivot about the middle ??

My river cruiser had big water tanks forward and aft and used to plunge and rise over wakes when they were full. I am only talking of a speed of about 6 knots here so I don't know what effect planing speeds have.

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