# building a charity raft race catamaran

Discussion in 'Boat Design' started by filthyoar, Jun 30, 2014.

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### filthyoarNew Member

Hi all,

I've come across this site a few times when researching for a boat I'm building, so thought I'd come o here for some advice....

first of all some background. I've entered a charity raft race with my work and took on the responsibility of designing the boat. I watched a few youtube videos, amd dived straight in.

I settled on a stitch and glue construction, a two hulled catamaran made from 8ft by 4ft plywood. Each hull is made from two 8ft x 2ft bits of ply, with a flat bottom cut to fit. The hulls are now built, but still need glueing together.

I'll be using epoxy resin/wood flour fillets, with fibreglass tape over the top with more epoxy. I'll be making 3 wooden square frames within each hull to reinforce it, and 3 crossbars attached to the frames to connect the two hulls.

my questions are, what have I missed?!

And how much do people think my raft will support? My caclulations seem to suggest it could take well over 2000lbs, which seems like a lot!

Any help/advice would be much appreciated.

2. Joined: Jan 2006
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Filthyoar; you have not given us enough information to make much of a judgement. The bottom which I presume is the two foot wide part has some shape one would suppose. If the shapes is pointed at both ends then a certain factor, called prismatic coefficient, is equivalent to one number or other. If the aft end of the bottom is squared off then a different coefficient is in play. In fact the degree of curvature may vary somewhat and the coefficient will be affected. All that is [part of the calculations that predict the extent of flotation that the structure will support.

To make matters even more sticky, the curvature of the bottom when viewed in profile also affects the prismatic. Bottom line is that you will need to furnish the dimensions of the box like structure at even intervals beginning at the front and progressing toward the back. With that information we could predict with reasonable accuracy, how much the structure will support.

A salute to you for becoming involved in a charity event.

If the object of the event is to race toward some destination, then potential speed is to be sought. A much superior design for that purpose is a long skinny hull. Sixteen feet by one foot wide will be far easier to propel and much faster than an eight foot by two foot design.

Keep in touch, you will get lots of advice here.

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### rwatsonSenior Member

FI - you are from the UK, so forget this lbs stuff.

Figure out the volume of the planned hull first.

Without having any measurements, and assuming straight sides.

16ft long = 4.8 metres.

Assume sides are .25 metres high

Assume .5 metres wide

without allowing for pointed bows

thats .6 cubic metres

A cubic metre weighs about tonne, or 1000 kilos, and a hull of 1 m2 displaces that weight of water

so a .6 m2 sized hull could support 600 kilos (1300 lbs) if submerged to its edges, so for safety, a maximum of 300 kilos (600 pounds) would be ok.

With pointy bows, less than that.

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### filthyoarNew Member

Thanks for your replies!

I've attached a photo of the two hulls. I'm 3 evenings in at the moment, so not watertight, but starting to resemble a boat at least! The plan is for two people to sit within each hull on top of a frame.

They are 2.2m long along the bottom, and 2.4m along the top as the front slopes back slightly. At it's widest point each hull is about 60cm, and they are about 60cm deep.

I expect the whole thing will weigh less than 50kg, and the crew weight will be about 320kg, so need to know if its likely to support us!

I thought of calculating the volume by working it out for a cylinder with the same circumference, but not sure how far off that would be?!

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5. Joined: Aug 2007
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### rwatsonSenior Member

About .56 cubic meters or 560 kilo displacement per hull on those measurements.

This is where CAD comes in handy for volumes that are not regular geometric shapes.

Or, you could have pretended they were two triangles ( half base x height )

1.2 M x .6 x .6

=1.2*0.3*0.6 for half a hull

x 2 for front and back half of each hull

= .432 cubic metres ( close enough )

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### rwatsonSenior Member

PS = you have made quite a slow hull. Each hull is only about 8 ft long, - you would have been better to make each hull 18 feet long, about .4 metres wide, and you only need .25 metres deep to have an amazingly faster boat that supports 500 kilos as well ,

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### filthyoarNew Member

Thanks very much for your help. It looks like the boat shouldn't have any problems supporting us all. And I might need extra weight to increase stability!

Although not the fastest design, the alternative was a timber frame on top of a load of barrels, so I hope it will be quicker than that!

We also have the constraints of transport and cost. The two halves were possible using cheap 8ftx4ft plywood, and will fit in the back of a small van to be connected by the river.

just got to seal them up now!

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### rwatsonSenior Member

You shouldnt need any extra weight.

If you joined them next to each other, you have a beam of 1.2 metres, plenty for 4 people on a catamaran.

If you separated the hulls by say .25 m, then the stability would be increased by quite a lot.

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