Building a raft..we need help!

Hi all, wonder if I could pick the brains of some boat builders. We are building a raft for a charity raft race. raft needs to hold 10 adults and will be paddled down a river for approx. 6.5 miles. Some parts of the river can be shallow and there is a possibility of grounding said raft.
we have made a start on a very basic construction of plastic barrels (3 each side one in middle)and pallets and 2x4 wood - see attached picture. We now need to attach the barrels although they do slot into design this isn't obv enough. ideas weve come up with include rope, ratchet straps, duct or gaffa tape (although concern if this will last getting wet) and scres (worries re leaks and sinking). WE are a group of vets and nurses so none of us know much about building or boats. we would love some advice and any flaws you can spot on the current design. Thanks for your help!

 

I wouldn't use tape. If you know anyone in the warehousing business see if you can borrow their strapping tool. The stuff that looks like this. Of course ratchet straps would work too.

 

Is the middle barrel a good or bad idea?

 

Ratchet straps are probably best, nice and wide. Old fire hose or even garden hose will work as well. Loop it over the top of your frame and screw fasten on the far side.

If those are 35 gal barrels each one will support 390 pounds in fresh water. But that is totally sunk, and you only want them immersed about half-way or less. But ten adults at 185 pounds each is 1850 and your floatation (7 bls * 195 LBS) is only 1365. Don't make the frame or deck very heavy.

The center barrel is draggy but looks like you need it. Some fairing between the barrels or fitting them tight (end to end) together would be faster as well.

 

barrel probably needed

You mention risk of grounding - more bouyancy = less draft

Those look like the standard 225 liter barrels

3 on each side and 1 in the middle = 7

if floating at half depth you're on about 770 kilo of bouyancy

Weigh your crew add predicted weight of raft and decide.

I have a 'tweak' for you - goto your auto store and buy a tyre valve per barrel plus the fitting tool

drill a suitable hole near the cap and you can pull those into place maybe with some 5200 or silicone. replace the cap and you can now put 3-5 psi into each barrel - much better structurally.

 

Here's what I see, FWIW:
-A rectangle shape like a hollow river barge would have the least draft and slide easier when grounded, though solid would be better so it doesn't swamp.
-How much is the total weight (10 adults + cargo, seating, supplies etc.)and how will it be distributed evenly? You will need to have enough barrels so they float less than 50% submerged at full load. A rough calculation can be done to make sure you have enough barrels, add extra ones for possible punctures/leaks and better draft.
-Your single strut 2X4 center area looks weak to resist twisting and shock forces with 10 people, 4X4 would be better or metal tube. Bolts would be the best fasteners, or maybe lag screws with your design, nails not strong enough if that is what you are using.
-Ratchet straps will work if the barrels are air tight and sit in a notched area, but pallets add weight without much strength. I have built a similar 2 man with 3/4" pipe metal frame with inflatable pontoons many years ago. You need the push back from the sealed air inside to avoid crushing and may have to tighten tension of the straps in cold water.

Hope this helps.

PC

 

I suggest the barrels be turned sideways.

 

Hi all, thanks for the advice so far. We are carrying no real supplies, were going to attach some plastic seats but they are lightweight. Yes they are 200l plastic barrels. Wonder if we would be better adding in another barrel, you see we have a spare, so 3 either side and 2 in the middle. Ee have some old metal scafold poles..wd they be of use to strenthen snd prevent twisting?

 

Just some ideas, FWIW.

PC

 

Can you get more barrels? More the better.

 

The best raft I have paddled was made of two layers of refrigeration panel. EPS that comes sandwiched between two thin layers of sheet-metal.

As I recall it was about 5.5m long 1.2m beam and consisted of two sheets each 100mm thick. We glued and screwed them together and mitred the bow to get rid of the bluff face. The panels were recovered from a demolished freezer building on a construction site.
It had a draft of about 100mm or was half immersed.

Four crew two at front side by side, and two at the back. We bolted a barrel aft and one mid-ships to use as a back rest and cut holes in the top to store beer and rotten fruit and eggs for ammunition.
It was hands down faster than anything built with barrels, next time I would add a skeg to stop it being blown sideways.

Have fun

 

Your raft construction looks just fine! The barrels are "captured" between the fore and aft 2 x 4 s.
The centre barrel adds buoyancy but does nothing for stability and also adds drag, so may I suggest using four barrels per side.
Two ratchet straps per barrel - from Poundland possibly.

Edit - just checked, Poundland don't do ratchet straps but they sell "heavy duty" rope.

 

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Hi all thanks for all your help...in response we have managed to borrow a strapping device and someone who knows how to use it and will be strapping barrels on, we also have a few ratchet straps. We will add another barrel in the middle and some more suppport down the middle, then we need to make it look like an ark! Will post more pictures when we have some progress

 

Hi all
Thanks for your help. We used the strapping device as suggesred and added more barrels 9 in total. Just completed the race, didnt sink and raft held together very well despite hitting several rocks on the way down. Didnt get anywhere in speed stakes but won a prize for our sponsorship collection.thanks again
Elisa