Which plywood dinghy?

Kelpie said:
One thing I'd like to get a better answer to is just how important it is to have a dinghy light enough to carry up a beach.
Some cruisers appear to have big dinghies with console steering and a 25hp on the back, and a lot of these people seem to be retired couples who are clearly never going to pick that thing up. I presume they are using dinghy docks or wading/swimming ashore.
Maybe the big dinghy is used where a dock is available, and a lightweight alternative for times when you will need to carry the tender up a beach.

My ideal- I think- would be a 9-10ft alu hulled RIB weighing around 35kg, with a Tohatsu 9.8 (26kg), giving a total of 61kg plus oars, fuel, etc. I think this is about manageable for the two of us, especially if it's a RIB that can be dragged without damage. And there is the option of leaving the motor behind, or using our 3.5hp (13kg) for those times that it really matters.

The only real downside I can see to this type of boat is the cost- at least £2500 new, and they are not common secondhand. Hence looking into building a plywood boat to do the same job, which I think I can do for around £500.

FYI the planned cruising area is Atlantic circuit and we would, one day, love to get as far as the Pacific.
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I would guess that the ability to carry the tender is less important in the Caribbean than it is in Cornwall, due to the tides. Wading out to an anchored tender is unappealing in a muddy Solent harbour in October, compared to warmer climes.

But, tropical sun will be quite unkind to a painted plywood dinghy?
 
lw395 said:
I would guess that the ability to carry the tender is less important in the Caribbean than it is in Cornwall, due to the tides. Wading out to an anchored tender is unappealing in a muddy Solent harbour in October, compared to warmer climes.

But, tropical sun will be quite unkind to a painted plywood dinghy?
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There are cruisers with ply dinghies but they are normally epoxy ply to give them better durability. We have friends who built a very nice dinghy from 4mm ply and epoxy/glass. It is a noticeably heavier than our grp/carbon dinghy that is two foot longer. Ours is 12’5” long but nesting so only 7’ on deck. Their ply dinghy is 10’ but only uses a 5hp that it was designed for. To get things in perspective our dinghy weighs 54kg and we use a 15hp two stroke.
There are some build details that are important when using a hard dinghy. The fendering on our hull is sanitation hose. Its nice and soft and none marking. The transom is reinforced with carbon and glass so it can take the bigger engine. The floor is reinforced with carbon stringers. Planing with three people in a chop with a large engine puts plenty of force on the dinghy. It needs to be built for the job. You can have strong and suitable for three people to plane but not light. I cant imagine a lighter dinghy than mine that can plane with three people and has built in buoyancy. Rule of thumb is 5hp per person if you want to plane.
Most people use ribs but they are expensive. If you use a rib a lot they wont last for ever. Chaps help keep the tube in good condition. Hyperlon os the only material that lasts in the sun. PVC won't last more than a couple of years as the UV/heat kills the glue. Our dinghy cost more than a rib when it was new but it is now 26years old!
 
I'm new to this, so will need to learn about the different construction methods.
The dinghies I am considering are designed to be built in 4mm ply sheathed in epoxy and glass. So does that make them ply epoxy?
 
Epoxy coating does two things, first it gives you a very durable waterproof coating which is of course good but it also increases the strength and stiffness of the sheet considerably so your 4mm become 6-8 mm. The glass just adds to the strength aspect
 
If it's going to be covered in glass and epoxy anyway, is there merit in using a different material in place of the marine ply? Thinking of something lower density but thicker, maybe a foam of some sort?
 
Kelpie said:
If it's going to be covered in glass and epoxy anyway, is there merit in using a different material in place of the marine ply? Thinking of something lower density but thicker, maybe a foam of some sort?
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You could make up glass and epoxy panels or slightly heavier glass and polyester panels instead of plywood. I cant remember the weight of cloth on my dinghy hull but there is just two layers on the hull sides and four layers on the bottom of the hull. The buoyancy chambers and bulkheads are foam core panels so stiff and light.
If you do make one, don't forget to put a perspex window panel in the bottom so you can check your anchor and survey seabeds. Mine is about 300x300mm square. Mine is 8mm thick.
 
I was thinking of using poplar ply as it's much lighter than marine ply. My thinking being that if water gets into a glass covered piece of ply then it's going to rot regardless of what what grade the ply is, seeing as the water can't get out. But maybe a foam panel would make more sense.
 
A resin coated ply panel of equivalent strength to a GRP panel is lighter so there may be little to gain from that route. On consideration when selecting a non marine ply is the core quality. If voids are permissable then strength may be compromised. A key point during construction is that all surfaces must be coated including the cut edges.
 
maxi77 said:
A resin coated ply panel of equivalent strength to a GRP panel is lighter so there may be little to gain from that route. On consideration when selecting a non marine ply is the core quality. If voids are permissable then strength may be compromised. A key point during construction is that all surfaces must be coated including the cut edges.
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I suspect that depends how you are defining 'strength'.
Ply will be stiffer.
GRP will probably take a greater point load before damage.
IMHO epoxy coated ply is a fine way to build a fairly heavy racing dinghy, but way too vulnerable to choose for a tender.
 
lw395 said:
I suspect that depends how you are defining 'strength'.
Ply will be stiffer.
GRP will probably take a greater point load before damage.
IMHO epoxy coated ply is a fine way to build a fairly heavy racing dinghy, but way too vulnerable to choose for a tender.
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Still if you are limited by weight then ply is still likely to be the best option unless you have a good GRP design facility
 
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