bouyancy problems

i'm making a small 12' trolling motor only, boat. the pontoons are foam filled. i had to add a 37"x 96"x 10" plastic coated foam block (about 1000lbs more bouyancy) in the back 2/3's to increase the flotation. now, with 4 batteries and me in the boat, i've got about 4" between the water level and the deck. the front remains at about the water level. the boat is obviously too heavy and needs more flotation, as well as less weight. my question is, if you look at the pictures, especially the front, is the upswept, pointed shape of the pontoons so limited in their carrying capacity that no matter how much weight i remove i'm still going to suffer from the same problem? i want to be able to fish 2 people. i have limited space on either side of the trailer runners to add more foam.

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Just adding foam wont do it, you need larger toons or start removing wood

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Originally Posted by brushnjigs

Just adding foam wont do it, you need larger toons or start removing wood

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Yep, you need larger diameter toons so that there is greateer height. The plywood and 2x4's get heavy!! It would make a great duck boat as-is though!!

i definitely underestimated the weight of the lumber. bigger toons aren't in the cards. i'm wonderin' how good i can do by removing wood.

take the wood off and replace it with wire fencing or straps ????

can't quite picture what you mean by fencing or straps, rocket.

Too big and too heavy for the pontoons that you have. Narrow it down and lighten it up....use aluminum where ever you can.

i'm going to take the boxes off and see how much difference that makes before going the narrowing/aluminum route. i really want a wide platform if possible.

looks good if you can get ride high up 4 to 6 more inc it will be a killer toon go all the way and put small 5hp hand tiller on it did you make the toons for it

It's actually pretty easy to estimate how much boat you can float with pontoons.

As an example, a 1ft diameter pontoon 10ft long has a volume of about 8 cubic feet. For two pontoons you double that so you have a volume of 16 cubic feet for the pair.

Now say you want the pontoons to ride half in the water. That means you want to displace half the volume of the pontoons in water or 8 cubic feet. 8 cubic feet of water weighs 8 X 62.42796lbs or right at 500 lbs.

That means if everything on the boat weighs 500 lbs, including the pontoons and fishermen, you'll float with the pontoons half in and half out.

You can do the same calculation with your pontoons before you start changing things to try it out. Probably better to just measure the portion of the pontoons that are full diameter to give a little margin for safety.

If you aren't mathematically inclined, post your measurements and I (or somebody else) will figure what you have to work with.

Foam is only useful if the boat is sinking. Otherwise it is just weight that pulls the boat down. You want air in the pontoons.

And...I would move the two batteries to the front of the boat. Wire is lighter
than lead.

Good Luck!

Wow, way too go Catfan I bow to the Man That was really good

yes jimmy i did make the pontoons for it. filled them w/foam to stiffen them. catfan i did the bouyancy calculations. there's roughly 36 cu ft with toons and added foam, 2160lbs total carrying capcity. my problem came from not knowing the weight of the lumber. in my ignorance i convinced myself that a ton of carrying capacity would be way more than i would need. or, as you pointed out, 1080lbs at 1/2 capcaity. i filled the toons w/foam because i wasn't certain that a) the toons were strong enough; and b) that there wouldn't be pinhole leaks in the glass.
after sleeping on it i think i'm going to remove the front center transducer nose cone and move the rear flotation block up to the front 8' to see if that helps at all. as it turns out, the front of the toons have little to no carrying capacity. murphy's law is alive and well.

vol, the batteries are in the back of the boat. i have 6ga tinned wire running to a perko swich in the front from both sets of batteries. its been suggested that i remove 2 of the batteries. probably a good idea, but i'm somewhat paranoid about running out of juice. (again)

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Originally Posted by dwaw

after sleeping on it i think i'm going to remove the front center transducer nose cone and move the rear flotation block up to the front 8' to see if that helps at all. as it turns out, the front of the toons have little to no carrying capacity. murphy's law is alive and well.

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Moving foam forward will lower the front of the boat. It's just dead weight (albeit not much) unless it is in a spot that is otherwise filled with water. You want your foam evenly distributed so the boat floats reasonably level if you take water.

What about adding a third pontoon? Tri-toons are commercially made, and it would give you roughly 50% more capacity than you have now.

when the boat is in the water, the foam is in the water. when i move it forward, it will be moreso in the water, due to the fact that the front end is lower in the water than the back end. if need be i can add additional foam at the back of the moved-forward foam of the same dimensions. the current foam piece has a bouyancy roughly equal to the 2 pontoons together. one of the pictures, taken from the back, should show what i'm explaining. the front of the roughly 3'x8'x10" center foam piece is shaped in a forward-moving friendly manner. appreciate your imput.