My current Project-Pontoon Junk Raft.

[djwkd]

Hi everyone-
Just to let you all know about my most recent project-it's a 3' long junk Pontoon Raft made of plywood for the hulls and foam for the floatation.The deck will be made from scrap wood.

My Question-
The Draft is only 8".Will this be enough, considering that the foam will give extra buoyancy to it?


Thanks,Dominic.

[Petros]

We need more information to give you an answer:

What is your intended use? Is is a model, toy or human carrying, at 3 ft long it will not carry much, or is that the correct length?

What is the total weight, or payload weight?

what is your beam?

How is it powered, sail, paddle or motor?

What is your hull/deck configuration, monohull or catamaran?

[djwkd]

1-For use in still waters carrying humans.3' is the correct length(i'm not very tall lol)
2-weight shouldn't be more than about 160 Ibs (73 KG)
3-Beam is 3' also, (Its because of the limit of the plywood i have) but only 2' of that is of the hull.
4-It is powered with a paddle.
5-It is catamaran-there are two 1' Wide pontoons.

[robherc]

Well, if your pontoons are perfectly rectangular, at 1' square all the way through, you're only talking about a TOTAL of 372lbs displacement. You're also talking about only 186lbs buoyancy per pontoon, which means that you'll get your feet wet if you get too close to either side, and that's assuming you are on mirror-flat water!

I'd highly recommend going with slightly larger pontoons; or wearing a wet suit.

[djwkd]

Do the Bottles and foam not add extra buyoancy, or have you considered that in your calculations?

Thanks,Dominic.

[djwkd]

Oh-Just noticed-the name of the thread-The boat isnt a sailing boat, its not a Junk raft as in the sail rig, its junk as in scrap.
Hope this clarify's things.

[robherc]

The bottles and foam keep you from losing buoyancy when everything goes to hell, and the boat fills with water (I.E. hole in a pontoon).

In everyday use, though, they actually have weight to them, and thus slightly reduce buoyancy. The only way you could improve your buoyancy to .62lbs/cu.ft. would be to fill your pontoons with helium, or draw a vacuum in them...neither one is a very realistic approach, however.

My recommendation would be to go with (at MINIMUM) 4' long x 18" wide x 18" high pontoons (assuming you make them square-profiled). This would give you up to 558lbs (9 cu ft) buoyancy in each pontoon...that's probably still gonna make for a wet ride in any kind of waves, but at least you'll be staying ABOVE the water.

Myself, I don't plan on going out on anything smaller than my 9'4" x 4'10" cat. The "pontoons" on that one slope from 8' x 12" at the bottom, to 9'4" x 18" at the top. They give me about 15cu.ft./930lbs. of displacement/pontoon before I end up submerged, and yield about 11" of freeboard with only me on-board, so I can stay dry(er) on most protected/semi-protected inland lakes. If I decide to take that one out in the Gulf, I'll be expecting to come home wet! ;)

[robherc]

Just re-read your 1st post...if you're wanting to make the pontoons 3' long by 12" wide, and only 8" draft, you're only going to have 2 cubic feet, or 124lbs per pontoon...that's less buoyancy than some inflatable pool floats!

[djwkd]

Hmm...What i may do to save my back (from kneeling down marking ply wood again, including erasing current markings) is build another 2 pontoons of the same dimensions (although maybe just 2' long, i dont know) and attach them to the stern or bow of the raft.
It will also help me complete the project as i will delve into the building of the first two pontoons fairly quickly.
What do you think of this idea?

[robherc]

Add the Length x width x height (all measurements in inches) of all four pontoons together and divide the total by 1728. This will give you four total raft displacement in cuft; multiply that number by 62 and you have your displacement in pounds.
If you're happy getting wet while on your raft, you MIGHT be ok with as little as 400lbs (6.5cuft) displacement (though that'd be REALLY pushing it and the raft would want to flip over every time you got near the edge). If you want to stay relatively dry (i.e. don't get your feet wet every time you dip the paddle in the water, shift your weight, or encounter a wave that's more than 2" high) I'd recommend staying on the high side of 600lbs (9.7cuft) displacement.

In the end, it's your raft, your life, and your decision...I'm just telling you my opinion. Any way you go about it, I wouldn't dare take a new/experimental (like this raft) craft anywhere I wouldn't be able to easily swim to shore from, until I'd put it through some fairly extensive testing (including leaving it on the water for a few days to see if anything leaks).

Hope that was helpful,
Rob

[djwkd]

Thanks rob.I have decided that i will make the raft with 4 pontoons, as this will give 744lbs of displacement.

[robherc]

744lbs sounds pretty safe to me...show us some pictures once it's complete :)

[djwkd]

Ah...Due to my masterful Plywood Marking, the width of each pontoon will be 8" also.
I cant however, seem to get the calculations right on the new amount of buoyancy, as i keep getting 320lbs Per pontoon, which cannot be right.
Could anyone help me out?

Thanks,Dominic.

[robherc]

What are your overall dimensions for each pontoon now?

[djwkd]

3' long 8" deep and 8" wide.

[apex1]

3' long 8" deep and 8" wide.
Ahhh.....re we happy to use metric system...:D

[robherc]

36"x8"x8" = 2304in^3

2304/1728 = 1.333ft^3

1.333x62 = 82.667lbs displacement per pontoon... Are you a good swimmer?

To put it in perspective, 4 pontoons of that size would displace 330.667lbs while glueing 36 used milk jugs (5.5"x5.5"x10" each... but not all of that is buoyancy) to the bottom of your 3'x3' piece of ply (they'll fit..you MIGHT even be able to sqeeze 49 in there) would give you 324lbs of displacement (49 would give 441lbs displ.). Honestly, I'd go with the milk jugs for this raft...they're free and require less work to assemble...just smear glue on the bottom of the board (epoxy, polyester, or vinyl-ester will do), set the milk jugs on, and voila, raft (check out the "Free boat plans" thread for a cool "milk jug battleship").

[djwkd]

Right.I'll use the Two Bottoms of the first pontoons as sides for another..I'll have to plank out the bottom...
You got the same thing as me, but i thought that that was impossible to have such a reduction in Displacement.
So i'll have to plank out the bottom of the pontoons.

Thanks, Dominic.

[robherc]

N.P....good luck

[djwkd]

OK, so in the back yard i have an old cane shelf that is pretty big. What im thinking of doing is covering it with tarp and filling it with floatation. Then, n the top i will add a wooden deck. Doing this gives me 944lbs buoyancy without factoring in the weight of the bottles, or between 609lbs and 761lbs with factoring in the weight of them (between 40lbs - 50lbs per sqr foot), which is more than my previous idea.

Here is a pic of the thing i found a while back and left in my back yard, knowing it would come in handy for some project to do with boats.
http://www.freewebs.com/raftbuilders/photos/Fuji%20cam%2020.07.09%20016.jpg

[djwkd]

Progress can be tracked here ----
http://www.freewebs.com/raftbuilders/thebottlebasket.htm

[rwatson]

Ummm - how can I put this nicely, do you enjoy very cold water ?

Try a slightly different approach for safetys sake. Rather than fill the frame up with junk that will cause problems and be hard to control, get some sticks or bamboo about the same dimensions as the larger bits of wood in the garden frame, and fasten them across the 'bottom' of the U shape like a floor. ( use wire, nylone ties, clothesline string etc. to secure them in place)

Then run down to the local fabric shop, and buy enough cheap cotton fabric (thick and strong as you can afford), and cover the bottom and sides (on the outside) with the fabric, and stitch it tightly to the upper edges. You should have a quite taut covering all over the bottom and sides.

Finally, scam some housepaint (not water based) from your neighbours and paint it all over on the outside with as many coats as you can.

Tada - one lightweight, fairly usable Coracle (look it up on the net).

If money is really tight, you can cover the frame and floor on the outside with Papier mache. (newspaper and wallpaper glue) to about 1/4" thick. Paint that thoroughly.

The advantage of a hollow boat is - you can sit in it, less likely to tip over, and if it leaks, you will see the water coming in before you lose enough floatation to be dangerous.

[djwkd]

The advantage of a hollow boat is - you can sit in it, less likely to tip over, and if it leaks, you will see the water coming in before you lose enough floatation to be dangerous.

The advantage of a raft, however, is that if the water comes in, it doesnt matter because the floatation that is there keeps it afloat. Also, i only have to buy tarpaulin. I already have everything else. What i'm building here is on about a £10GBP budget. One day, all the 'traditionalist' mariners will start noticing that rafts are the way foward for budget 'boats'.
If it gets a hole in, all this thing needs is a patch of tarp getting stitched on.

On your (particularly valid) point of it tipping over, i have pondered fitting an outrigger type thing with stuff i have lying about the yard.