17' pram - lighter scantlings?

[lncc63]

Awhile ago I was reading the thread on the 17' pram. I like the design, actually I also like the 19' but the 9 mm ply always bothered me.

Don't get me wrong Morten, I trust your judgement but maybe like other designers you have another set of scantlings which you don't publish as not to confuse browsing customers. So do you have say scantlings that use 6mm ply instead of 9mm?

[Boatplans.dk]

The discussion of what plywood thickness is preferred is an interesting but complex discussion. Whether you use one or another thickness is basically a matter of personal preferences.

If the calculation shows that a scantling of 9 mm [3/8”] plywood is needed, you can choose to use 6 mm [1/4”] plywood and add the missing plywood as glass fabric and epoxy. That way you sort of compensate the missing plywood with glass fabric.

Now you won’t need 3 mm [1/8”] glass laminate to compensate 3 mm [1/8”] plywood. The mechanical properties of the glass laminate are better than plywood so less thickness can do. Unfortunately the glass laminate has a higher weight than plywood, so when taking the weight into consideration there won’t be much difference if any at all.

Now is the time where the personal preferences come in. I prefer to work with plywood instead of glass fabric (glass tape is alright). I find there is much too much grinding when working with glass fabric, and it is basically difficult to grind glass. Therefore whenever it is possible I choose thicker plywood. It is not always possible due to the curvature of the hull, but that is analyzed and evaluated in every single case.

So Louis, back to your question, is it possible to use 6 mm [1/4”] plywood instead of 9 mm [3/8”]? Yes, as long as you compensate the missing thickness with some glass fabric. I will of course always be able to help you choosing the right laminate and thickness for the project you choose.

Regarding the 17’ Norwegian pram I can assure you won’t get any problems with the 9 mm [3/8”] plywood. Long before the glass fiber and epoxy was invented, there has been build similar boats with 25 mm [1”] spruce planks, so bending 9 mm [3/8”] plywood will not be a problem.

[lncc63]

Thank you Morten. It is good to know you'll be there when I am ready to build.

Actually, using 9mm is better for me from a cost point of view than using 6mm with glass and lots of epoxy. I'd probably still put epoxy and fiberglass on the bottom for abrasion protection.

My next question is what would you specify for waterproofing here in the tropics? I assume the epoxy quantity in your BOM is only for the seams and does not include sealing. From my little experience, I think rot is a big problem here because of the heat and humidity so encapsulation is very desirable if not a necessity. On the other hand, I think our fishermen get away with thinned down regular epoxy and plain old enamel paint.

[Boatplans.dk]

I think waterproofing the plywood is important no matter if your boat is used in the tropics or in some cooler waters.

I always recommend sealing the plywood with epoxy and the amount of epoxy in the BOM contains a certain part for sealing. It is however difficult to predict how much epoxy will be used for that part, since the quality of plywood is an important factor for how much epoxy resin the plywood will absorb.

Beside the epoxy you should use some kind of paint or varnish to protect the epoxy from the sunlight. The UV radiation from the sun decomposes epoxy, so if you use clear varnish it is important that it has a UV filter.

So I think it sounds like a good strategy your fishermen are using, except that I would not recommend thinning the epoxy.