Advice on boat building

[zoso]

Hi, new member, I've been reading this site for a while and you guys seem to be quite on to it so here goes.

I'm an aussie, planning my first boat project and I've found a nice little design I like for a 16' mini-cruiser using stitch and glue to build it. I plan to just sail around the local rivers and lakes for the time being but as I get more exerience sailing and gear for it I would ideally like to take the boat short distances offsore for fishing trips and such. I plan to make it more of a foam sandwich construction rather than just epoxy/glass/ply, using the plywood panels essentially to hold the shape while I apply layers of foam and exoxy/glass. The plans call for 6mm ply but I was thinking of using 3mm ply, layer of epoxy with fibreglass mat, layer of foam 2mm or 3mm on both the inside and outside of the ply, then more glass to finish it up.

My question is this, given that I will be completely encapsulating the ply inside fibreglass and epoxy, keeping it far removed from the weather, and that the ply is being used to hold that shape, deriving strength from the layers of fibreglass, can I get away with using ordinary household grade plywood instead of marine ply? Does marine ply even come in 3mm thickness?? I am also planning on using closed cell PVC foam and getting it as a roll of packing material, rather than paying through the nose for the exact same product labelled "marine", is this going to be a problem also?? is there actually any difference in the marine grade material??

Also I would like to know if epoxy resin will be enough to ensure the foam will adhere adequately to the plywood and fibreglass without de-lamination issues further down the track. Is there a special glue used in the lamination process that is superior to epoxy??

The boat I'm planning on building is this one:

http://www.bateau.com/proddetail.php?prod=AD16&cat=14

Thanks in advance

[Raggi_Thor]

There is no need for the plywood in the middle of your laminate. It will do no work. If you want to spend the extra time and money on foam and glass, make the panels of foam and glass it on one side. Then assemble the boat and glass the other side. I am sure you will get more detailed advice from the fibreglass experts here :-)

[zoso]

Originally Posted by Raggi_Thor

There is no need for the plywood in the middle of your laminate. It will do no work. If you want to spend the extra time and money on foam and glass, make the panels of foam and glass it on one side. Then assemble the boat and glass the other side. I am sure you will get more detailed advice from the fibreglass experts here :-)

The plan was that the ply would be a stiffer material to use in the stitch and glue method, giving the structure some rigidity while I build it up, I dont want to have to build a complex frame or mould for this, just a simple basket frame as stitch and glue calls for.

So it is possible to just substitute the 6mm ply for foam (presumably thicker)?

and again... will foam sold as packing material be adequate or is there some special thing I'm missing that makes "marine" foam different??

[nero]

Since this is your first boat project. Build this boat like the plans and instructions state. Sail it. Have fun. When the time comes that you plan to do some saltwater fun, build a bigger, costal cruiser.

Most likely your needs and idea of a boat will have changed.

For a boat of this size and style, (personaly) I would build with the cheapest, materials that you can work with. Epoxy and glass sheathing will keep it strong for several years.

[zoso]

Originally Posted by nero

Since this is your first boat project. Build this boat like the plans and instructions state. Sail it. Have fun. When the time comes that you plan to do some saltwater fun, build a bigger, costal cruiser.

Most likely your needs and idea of a boat will have changed.

I really don't want to have anything bigger, for garage reasons and towing firstly, but mostly because of budget, I'm only a young fella. This boat will take me over the bar and out fishing one day when I'm confident, the designers claim its perfectly capable of such a task... and because I'm just that much into the fishing

Originally Posted by nero

For a boat of this size and style, (personaly) I would build with the cheapest, materials that you can work with. Epoxy and glass sheathing will keep it strong for several years.

This is exactly why I was planning on using foam, ply is not cheap, foam (if packing grade stuff is good enough) I'm sure is far far cheaper.

[Raggi_Thor]

The packing grade foam is probably not useful. Have you tried to stretch it? Does it have any strength at all? Will epoxy stick to it?
It's better to use cheap plywood. 6mm plywodd doens't cost much more than 3mm. Glass and epoxy is quite expensive.
If you want to save money, use as little glass as possible, maybe just in the bottom.

[nero]

When using foam as a core, you will use more epoxy and fiberglass than if you use plywood as the core. Make your calculations for cost of material for both ways. Then step back and look at a bigger picture. Find your balance between time, money, results.

A plywood boat will take more abuse than a cheap foam core.

What is your target budget for this boat?

[gonzo]

The plywood, as a core is very important. It makes the laminate stiffer by separating the laminates. Think of it as the web on an I beam. It does the same work. I suggest you stick to the designer's specifications. It is quite complex to make changes. Also, they would void any warranties. Will anyone telling you to make changes be responsible for the results?

[Raggi_Thor]

I meant that plywood in the center, between layers of foam is wasting weight and money. Either go for the lowest cost with only plywood or the more expensive and maybe slightly lighter sandwich with a proper core and biaxial glass.

[JR-Shine]

Like mentioned above, the foam cored boat will cost more to build and be more work (fairing the extra glass). The AD16 is one of our designs A foam cored boat will not be much lighter anyway.

There is better way of doing this is you must. I built one of our designs (ph16) using plain sheet Divinycell (you should be able to find it down under). I used the same building technique and sequence. H80 will take a bend similar to plywood. You will need to use a lot more glass than what is called for in the plywood version however.

Of course you could also just build the boat as designed and save time, money, and your sanity.

[zoso]

Originally Posted by JR-Shine

Like mentioned above, the foam cored boat will cost more to build and be more work (fairing the extra glass). The AD16 is one of our designs A foam cored boat will not be much lighter anyway.

There is better way of doing this is you must. I built one of our designs (ph16) using plain sheet Divinycell (you should be able to find it down under). I used the same building technique and sequence. H80 will take a bend similar to plywood. You will need to use a lot more glass than what is called for in the plywood version however.

Of course you could also just build the boat as designed and save time, money, and your sanity.

Ok, thanks for all the input. I really am chasing the cheapest way to make a strong boat but I also want to add as much bouyancy as possible for obvious reasons. I guess there are provisions in the design to add bouyancy in places other than the hull and deck?

For budget I was thinking around $3k (aussie), but I've done a lot of work with old cars and a general rule of thumb when working out a budget for these sorts of projects is to calculate costs then double it for the real figure

[gonzo]

The cheapest way would be to build a plywood boat with only fiberglass tape on the seams.

[nero]

Like gonzo said.

Mabye roll on 2 coats of epoxy and paint it. If you fall in love with the boat and have more money later, you could strip the paint and add a layer of glass/epoxy. Or perhaps add a layer of glass/epoxy on the outside only before painting.

Try to find a dealer for EPON resins. This could cut your epoxy costs in half or better. Perhaps by your small amount of glass needed from someone building a bigger project. Make sure the sizing is compatible with epoxy.

Your fun time is just beggining. smile

[Bergalia]

Originally Posted by gonzo

The cheapest way would be to build a plywood boat with only fiberglass tape on the seams.

Sound advice Gonzo. And nice to see you in a better 'frame' of mind

[jacquesmm]

Why don't you ask the designer? He responds in a few hours at his message board:
http://www.bateau2.com/index.php

OK, I'm the designer.

Short answer: don't do it.

Longer answer: don't do it because foam sandwich is not a valid option in such a small size. It will be heavier than the plywood shown on the plans.
If you insist, limit the damage by using for example Divinycell H45 with one layer 1208 each side.

More:
- that little boat is sensitive to excessive weight in the topsides, another reason not to do it.
- it's expensive and labor intensive.
- I designed my 1st foam sandwich boat in 1976 and know the material very well, better than most. It's not for small 16' cruising sailboats.