New builder question: Glassing inside/outside of boat

First post, as I build my first boat: A Honky Dory (16' Mackenzie drift boat).

Montana River Boats' plans call for 1/4" side panels and a 3/8" bottom. Both the interior and exterior are then to be glassed.

Here's my question:

Because I switched from building Spira's 14' Riverman to the Honky Dory after buying wood, I'm working with 1/2" ply for both the sides and the bottom. Can I therefore get away with just glassing the exterior and epoxying the interior?

I don't like the idea of having to glass the interior sides of the panels before stitching, as recommended by MRB and given the thickness of the 1/2" ply, it doesn't seem necessary. (Spira doesn't seem to recommend it for the 14' Riverman, I can't imagine that the larger size of the Honky Dory make it that much less ridge, but you all tell me.)

Thanks in advance,
Sean

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Permalink Reply by Rick Newman on July 12, 2016 at 8:57am

You may find difficulty in bending the 1/2 ply for the sides. If you add fiberglass to the inside it will become even more difficult, perhaps not possible without cracking. What kind of plywood are you using? Fir, Sapele, etc? Have you already scarfed the plywood? Is there another use you could put the 1/2" ply to?

Part of the best features of McKenzie drift boats is their manueverability which is a combination of the low weight and design. The old rule of thumb for a 4 x 8 sheet fir plywood is each 1/4" of thickness weighs 25 pounds. So you are basically adding q fair amount of weight. You will pay once to change the thickness but every time you use the boat you will pay the weight penalty.

The addition of fiberlass and epoxy resin can do several things for your boat. First of all it adds durability in a couple of ways. Dependent upon what weight fiberglass cloth, what type of fiberglass cloth, be it S-Glass or E-Glass, the type of resin you use, Resin Research or West System or System Three you can affect the strength resilience, durability and impact resistance of the finished boat.

A lamination of fiberglass and epoxy on the outside of the sides is crucial to the durability. The addition of epoxy and fiberglass to the inside of your plywood adds resistance to impacts as it doesn't like to stretch hence reducing the ability of an object to penetrate your boat. If you use the search feature on our site you can do some additional research on all these subjects. Most of them have been written about and discussed.

Sealing the plywood with the epoxy can also prevent or at least reduce the intrusion of water to the interior of you plywood which encourages rot. It is imperative to repair damage to plywood to prevent a short life span. Obviously you will need to cover your fiberglass and epoxy with a ultraviolet resistant coating, varnish, a polyurethane finish or paint. Again all these have been discussed at length so you can do further research here on the site. If you need help let us know. We enjoy sharing the knowledge our experience has provided us.

Rick Newman

Permalink Reply by Sean Larkin on July 12, 2016 at 9:33am

Thanks, Rick. I just cut my first scarf on a piece of 1/2" Okoume. (Slightly stressful but awesome first experience, even after a bunch of test cuts on common ply....)

I guess I could use the 1/2" I have for the floor and then buy two 1/4" sheets for the side panels. Having extra 1/2" laying around will hopefully just create an excuse to build a second boat down the road. ;)

I appreciate your help navigating this. I've done some research, but there are just so many takes on strength vs. weight vs. cost. I'd not thought about the potential challenge of flexing 1/2" side panels.

Quick follow-up: Is it significantly more difficult to glass the interior of a hull after stitching, compared to glassing the interior sides of panels before hand?

Cheers,
Sean Larkin

Permalink Reply by Rick Newman on July 12, 2016 at 10:07am

Are you doing stitch and glue or framed? It is easier on a stitch and glue but possible on both. I did the inner panels of my boat with 6 oz S-Glass. They can be punctured but it takes more force. The 1/4" will bend when with a lamination of cloth and epoxy on it. Take a look at Brad Dimock's test of Resin Research. You should be able to search for him and look at what he found out. He made relatively crush proof TP!

I found the link for you. Check it out.

http://fretwaterlines.blogspot.com/2011/12/demo-derby.html

Rick

Permalink Reply by Sean Larkin on July 13, 2016 at 8:20pm

That's helpful, Rick. Thank you!

Permalink Reply by Eric Loudenslager on July 12, 2016 at 3:16pm
You should post this question at the MRB forum site for a broader perspective on stitch and glue construction and the honky dory design specifically.

A couple thoughts. It is doubtful that you can get half inch ply to bend around the honey dory strong back. Check for sure with Sandy at MRB for his opinion. I believe others have tried, and I don't recall hearing any success.

There is no need to glass the inside if using 3/8 or 1/2 ply. It does add strength and stiffness if using 1/4 ply. Jason Cajun ar Montana Boat Builders constructs his river boats with 3/8 ply and does not glad the inside.

If you are going to glass the inside it is usually done to a flat panel, before putting it on a strong back. Glassing inside the boat on a vertical panel is more difficult. The outside is glassed after installing the panel because the glass would be in tension and make bending the panel difficult. The glass on the inside is in compression and bends more easily.

In the long-run plywood is not a very expensive component of stitch and glue - epoxy and glass and fittings are where the major expense is. I have a 1/4 side, 3/8 bottom s&g and love it. You may want to give serious consideration to using 1/4 sides.
Permalink Reply by Sean Larkin on July 13, 2016 at 8:20pm

Good advice, Eric. Thank you!

Permalink Reply by Phillip Westendorf on July 22, 2016 at 6:58pm

Sean,

Your question, "Is it significantly more difficult to glass the interior of a hull after stitching, compared to glassing the interior sides of panels before hand?"  

The simple answer is NO!  I built a 16 Ft. Cedar Strip Canoe prior to building my DB.  It was built on a strongback with temporary frames and after the outside was sanded fair and glassed (a single sheet 60" wide by 16+ ft. long) it was removed from the strongback and flipped over.   I then sanded it fair on the inside and then glassed the inside with 6 oz S-Cloth.   The stem ends were a little tricky but not a real problem. 

This was my first glassing experience and was time consuming but not that difficult.  With the S & G hull you can do the bottom and sides separate and just overlap the glass at the joints, actually adding the additional overlapping of the glass in the joints will add and extra measure of strength.

The final result will be a strong, stable hull!

G'Luck,

Dorf

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