Confirmation on my calculations

Hi All,
I am a complete newbie to boat building and am trying to desing my own little fishing pontoon platform.
According to my research, 1 cubic foot of water is 62.5 lbs.
If I have a pontoon that is 12" x 9" x 90" = 5.625 cubic feet. x 2 pontoons = 11.25 cubic feet x 62.5 = 703lbs of displacement.
Now, obviously I don't want to have the deck underwater, in fact I would like to be 3 to 5 " above the water so, that means I am actually reducing the height of the usable volume of the pontoon to say 8" or say 66%.
703 * .66 = 468 lbs.
I think my gear and I would weigh about 250, with a little room for lunch, and my boat about 150. That leaves me with 68 lbs of room to be where I want to be.

Now, the big question is, am I nuts?

Thanks, I am going to go have a beer and ponder my design.
Khaulenn

 

And if you happen to get a BIG fish??

 

I usually aim for 100% reserve buoyancy with sealed hulls. If I determined total displacement was going to be 400lb I would design for total volume no less than 800lb. If you want to sit low on the water then it is a mater of shaping the hulls accordingly. Ideally the pontoon would have sufficient buoyancy so that you can stand at any location on the deck without getting feet wet or it rolling over. You can limit the deck area if necessary.

If you had unsealed hulls then you need considerably more freeboard so waves do not slosh in.

A sketch would help us better understand what you are planning. You do not say much about its intended use. Will it be moored or do you want to power it in some way?

Rick W

 

Here's a thought-suppose you wanted- or happened to -stand on one side of your pontoon. Then all your weight would be depending on one pontoon to hold it up. You mentioned your boat. Are you planning to haul or launch your boat off one side of the pontoon? Even more weight on one side. Hence the need for more buoyancy than you proposed. Otherwise your method is sound.

 

Khauleen,

I have no idea what happened there but

lets start over.

To answer your question, yes you may

be nuts, but your calculations look fine.

Don't forget your pontoon weight.

I like the idea of double buoyancy.

Have a good look at commercially available

rigs. It's always easier to modify an existing

design than to struggle through evolving

your own.

Good fishing, Tom.

 

description of boat

I am just going to put a slightly raised seat on it with a couple of oar locks and the row around a few small lakes around here and do a little fishing.

Picture a float tube with the seat out of the water.

Hmmm, I never thought about walking around on it. But there won't be any room to do that any way, the deck will only be about 3'6" wide by about 5' long, with the chair in the middle. Just something to fish on.
Oh yeah, what about 2" ridgid foam insulation inside the pontoon. Do you have to worry about mould? or rot? and if so how do protect against it?

Thanks,
Khaulenn

 

I should have said pontoon, not boat.

 

You need enough reserve buoyancy so you can board it without the deck going under. If you allow 100% it is normally enough. That means one pontoon will float you so you can stand right at the side. If you move to bow or stern on the side then it is likely to go under but as long as you are aware of this then it should not be a problem.

Rigid polystyrene foam is good unless it is exposed to hydrocarbons as these will dissolve it. If the sheets are able to move relative to each other then, over a long period, they will abrade. I would glue them in place with a good expanding polyurethane glue inside the shell. Polyurethane glue is good for timber boat building as well. It does not dissolve the polystyrene foam. I use this stuff:
http://www.vise.com.au/

Rick W

 

For ease-of-doing, I'd consider jsut getting a nice 2-part, liquid, flotation foam. Build your pontoon, then, before you seal it up, just measure, mix, pour, wait... once the foam fills the pontoon & cures, seal it up & float away...might cost a little more, but the ease of use, and knowing that it's developed to withstand hydrocarbons/water exposure/etc. yeilds a little peace-of-mind.

Oh yeah, and it's usually gonna weigh around 2lbs/sqft, so you'll want to figure that into your buoyancy figures. Also, what country are you in? (if it's the US, there are LOADS of excessive gov't regulations to consider)

 

If you haven't started construction yet, have a look at some of the plans available at Duckworks. To look is free and you might get some ideas you can use for reference. Mark Gumprecht has built a boat that looks very much like what you have described, except with more capacity and freeboard.
http://www.duckworksbbs.com/plans/gumprecht/cat8/index.htm

 

That is a very nice little boat. Would be very easy to row and very stable. Weigh very little as well with heaps of freeboard. It would be my choice for something to fish from.

Rick