10 ft FG/ply boat question

[Rich M]

I've recently stepped-up from 8 ft boats to a 10 ft boat. It's made out of inexpensive 1/4 inch ply and glassed with 7-1/2 oz tape (double taped) on inside & outside seams, 7-1/2 oz cloth inside and out. Mid grade Ployester resin.

Here is the boat - http://www.youtube.com/watch?v=blumcB0x2Is

(The 15 hp was used for the test run only - while I wait for my dad to bring my 8 hp down from VT. I also have a 6 hp but it is heavier than the 8 hp. The boat will be used with a 3, 4, 6 or 8 hp in the future. The 3 won't plane it, the 4 is very heavy...)

I had a couple of questions regarding FG strength.

Does it matter if the FG tape goes on before or after the FG material?

Would increasing the FG from 7-1/2 E cloth to 8.9 oz satin weave E cloth make much of a difference in strength on the outside?

If the 8.9 satin isn't going to do much, how about using S cloth instead?

I'm wondering about epoxy on the next build.

My boat building finances have done dried up and a possible move in the near future have my boat building days very numbered. I'd like to get the most out of what may be my last boat build for a long time.

Thanks!

[marshmat]

Hi Rich,

It doesn't really matter whether you lay the fibreglass tape into wet resin, or work resin into dry tape. As long as the fibreglass ends up saturated, the fibreglass will be happy. For it to stick to the wood, though, you need a continuous chemical bond from the resin that soaked into the wood, right through to the resin in the fibreglass. This means wetting out the wood with resin, then wetting it out again as soon as the first bit starts to kick off, then getting the fibreglass on before the resin in the wood is too hard to bond to.

On these small plywood boats, the outer fibreglass skin usually isn't adding a huge amount of strength or stiffness. It's there to keep water from getting into the plywood, to protect against abrasion, to keep teredo worms out, and to prevent the wood from checking. Heavier fibreglass often won't make much of a difference.

What will make a big difference, in both ease of construction and longevity, is switching to epoxy instead of polyester. Yes, they cost a lot more per gallon, but on a 10-footer you're not using a whole lot of resin to start with. Switching to epoxy is like a dream- no more checking the thermometer before counting out tiny little drops of catalyst, no more worrying about whether it's really sticking properly to the plywood, no more attacking the thing with a paint scraper after it stays gelled for a week without curing, a much less nasty smell, and a stronger and longer-lived product to top it all off.

[Rich M]

Thanks, Matt. I'll relax the thought of heavier cloth on the outside.

My first question wasn't worded correctly.

Is it okay to put the FG tape over cloth that's already been put down, but the resin is still wet? Is it as strong as putting FG tape on and then putting the cloth?

My guess is that the strength would be the same, as long as they were put down at the same time so the adhesion would be the same - with no real chance for delamination (fiberglass from fiberglass) since there was only one resin to cure.

Does this sound about right?

[JEM]

What you're doing is sometimes called "wet on wet". You save some time in that you don't have to wait for anything to cure, sand/scuff the surface, and apply the next layer.

You get a little better chance of avoiding delamination because the layers cure in the same wet resin at the same time.

Be careful thought, that dang fiberglass slides around easy and might pull out of place.

[Rich M]

Thanks, JEM.

I've experienced that cloth moving and sure it isn't easy to get everything back the way it should be.

My experience with 4 oz cloth made me move up to 7-1/2 just so it woulnd't move nearly as much...

I got a little nervous when someone told me that I should be putting the tape down first - which I had been doing but had been getting a lot more bubbles under the FG cloth. I switched to FG cloth first and tape second and things got much nicer in fiberglassing land.

[lewisboats]

Generally tape is put on the seams first then the hull is glassed for some abrasion resistance but more importantly (with fir plywood) for checking resistance. The tape is sufficient to give the joint strength when backed up with a fillet and tape on the inside and the cloth is usually lighter. Most times the cloth isn't necessary as a structural element if you have properly filleted and taped seams.

[JEM]

The tape usually has a woven edge to it to keep it from unraveling. If you laid that down and tried covering it with fiberglass cloth without trimming off that woven edge, the cloth may not have been able to lay down properly and created an air gap.

When using tape by itself, you have to sand/smooth down that edge and feather it to the hull. If you can lay glass over the tape while everything is still wet, then you can avoid some of that... if the tapes woven edge doesn't get you.

I've trimming off that woven edge and then applying the tape, but it can get messy.