9.5' foot dinghy lamination schedule

I have designed a small pram similar to, but slightly larger than the Manly Junior. Self bailing, composite construction, 45 sq. ft. main and 15 sq. ft. jib. I will build it in a basket mold. I will be using 1/4" foam core and epoxy, with a few pieces in marine plywood (bow, stern, rudder, dagger board, DB case). I have done a lot of research but because most sources say that I don't need to use matt with epoxy, I will end up with very thin skins. Three layers of 7.5 oz. E-glass on the outside of hull and deck surface, and two layers of 7.5 oz. E-glass on the inside of hull and underside of deck. The outer skin will end up being .039" (prior to paint) and the inner skin will be .026" (no paint) with the 1/4" core in between. Does this sound strong enough? Tough enough for a children's boat? Yes, there will be stringers for additional stiffness in flat areas.

 

Use ply instead of foam. One layer of 6 ounce inside and out will make it very substantial and it will not weigh much, if any, more than the foam cored scheme if you use Okumee or similar ply. It will use a lot less glass and resin too. It will be a stronger boat, with less building hassle. It might cost a little more for the panels but less for resin and glass. Do it right and the boat will last longer than your expectant lifespan.

You want proof? Layup a foam test panel about one foot square, Then layup a ply panel of the same size. Weigh them. Load them with test weights. for deflection and puncture characteristics. See who the winner is.

 

I just finished creating a test panel in foam. I will make one in Okoume. I have helped build a couple skiffs in marine ply. I am not opposed to wood. I wanted to make something very light for the sake of two small kids being able to handle. I also viewed this as a bite-size way of learning composite construction. I will put a pencil to both and compare wood and composite cost and weight. Thanks.

 

Mick; I am a nut about lightweight in my little boats. Every component weighs something and one of the worst offenders is stringers, chine logs, inwales and that sort of thing. A one inch square stringer of fir or some such is going to weigh about a quarter pound per foot by the time you get some glue and paint on it. A filleted joint with some six ounce glass tape (ala stitch and glue technique) is lighter than the joining stringer in places like chines and side decks.

Some of my boats have been glued lapstrake types so that I could create some stiffness with somewhat less framing. For small boats, even the paint adds an unwelcome amount of weight. I do paint the boats, even use epoxy saturation, but I resent the added weight.

Two dandy books that address the lightweight idea are Tom Hills: Ultralight Boat Building and Tom Firth Jones: Low Resistance Boats.

 

I found an article on the West System site that shows that with foam versus marine plywood, the foam composite is roughly twice the cost but half the weight.

 

Mick the article is worth considering of course, but what kind of ply were they comparing to foam and also what kind of foam. I have used Klegicell foam for some stuff and it works very nicely with glass sheathing. Of course there are several different densities and yes it is not cheap. My experiments with that kind of structure revealed that it weighed pretty near the same as a comparably stiff glass over ply. I might have used too much glass when I was fiddling with the method. Indeed my test panels were not much short of bullet proof.

There is a lot of messy, stinky, sticky, stuff involved but if you are to build the boat from flat panels, you can lay up composite planks or sheets on a smooth table (waxy formica maybe). The resulting glass finish can be good to excellent. Not only that but you can save some tedious scarphing were you to use ply.

If I were going to build a composite boat, I'd use at least three eigths or even half inch foam for high load areas like the bottom. The stiffness of the finished panels will be a function of the cube of the thickness. Roughly.....3/8 foam will allow the plank to be more than 3 times the stiffness of quarter inch foam. Half inch foam about eight times stiffer.

I have a kayak all laid out on my drawing board. As stated before I place a premium on light weight as long as it is sufficiently substantial. I reckon it is time for me to experiment some more with foam/GRP composites.

 

I may end up using 3/8" foam instead of 1/4". Here is a 3D model from DelftShip, and a drawing from CorelDraw. My design goal is for this larger pram, suitable for two children, to weight no more than a one child Optimist at 85lbs. Since a wood/epoxy Optimists weigh approx. 85lbs. I felt it necessary to go with foam.

 

I think that you are on the right track Mike. The drawing of your pram shows some sensible design features. The kids will surely like the boat.

 

I've been playing around with some composites also,just for comparison 12mm M80 foam with 1600 gsm one side 800 on the other infused is coming in around 460 grams a sq/ft.Similar to 1/4 merenti ply with 6 oz skins on ea side.

 

Boat is finished

My design goal was 85 pounds. The finished hull with attachment hardware is 79 pounds. We launch this weekend. While there was some amount of 3/8" marine ply at the bow, stern, daggerboard case and reinforcement along the centerline of the deck and one underlying stringer, the boat is mostly high density extruded polystyrene, fiberglass and epoxy. Believe it or not, the boat is rock solid and even seems overbuilt. I used a layer of 6 oz. plus 10 oz. on interior skins and 10 plus two layers of 6 oz. on the exterior of deck and hull. The boat is finished in Interlux Perfection. Spars are from Dwyer and sails are from SuperSailMakers.

 

Neat and pretty job. Well done, glad she's on weight target - hope the kids enjoy her.....

 

Looks great, sure you could have gone lighter but at expense of durability, have fun

Jeff.

 

Video of boat launch

https://youtu.be/BEi3hXutSso

 

Great !. Congratulations

 

Awesome... so an opti rig drops in too?, always thought that little tackers grow out of optis & the MJ style a better bet for building further skills.... maybe you can set a sprit for an assymetric spi too!

Jeff