PK78 Plywood Weight and Fiberglass Cloth

[JLL]

I am about to start building a PK78 and I have a couple of questions. I will use the boat for a dingy for a 30 foot sail boat (and fish from it also). I will mostly motor or row, but eventually plan to put together the sailing package. I have previously built a D-15, so I have some experience (and hope this project will go better).

1) I want the boat to be as light as possible. I would really like the boat to come close to 60 lbs when finished, as called for in the plans. I know very little about different types of plywood. I have read all the messages on this board… but I am still a little confused. I would like a simple answer. What kind of plywood should I use? Where can I get it? I will order the plywood or do whatever it takes to get the boat as light as called for in the plans.

2) On the D-15 I previously built, I have noticed that the areas which were covered with fiberglass cloth have held up significantly (night and day) better than areas simply epoxied but not fiberglassed. I have enough 6 oz. cloth left over to cover the bottom and part of the sides of the PK78. First, how much weight will it add to the finished boat to cover the bottom and sides in 6 oz. cloth? Second, if I do cover the bottom and the sides, do I still need to tape the outside seems? Third, does anyone think it would make a difference if I covered the bottom and sides?

Any and all comments and help are greatly appreciated. Thank you.

[jacquesmm]

The lightest 6 mm (1/4") plywood is Okume. Lauan is the same weigth but not marine.
You could also use 4 mm aircraft plywood but that is very, very expensive.
Gain weight by spreading epoxy the best you can, parcimoniously small fillets etc.
More weight gain = foam sandwich frames and seat tops -> expensive again.

Fiberglass the bottom? The weight of 6 oz cloth is 6 oz per yard
at 40% glass -> 15 oz/yard = less than 2 lbs total.

You should tape the seams anyway.

[Rick]

Hey, JLL, for what it's worth, I not only covered the bottom of my PK78 with 6-oz cloth (a friend had some left over from his drift boat project), I accidentally put two layers of tape on each side of every seam (I had been helping my friend with his drift boat, and that's what his plans called for...).

I used Okume BS6566 for the sides and bottom, and 3/8" Lauan for the transoms and seats. I think I could beat my boat with a baseball bat without hurting it, and I can still carry it alone. It's awkward, of course, but not very heavy.

Mine is a sailing version with a daggerboard trunk and double-thickness front seat, too. I haven't weighed it yet, but it is not bad.

Have fun,

Rick Tyler