Thread: Alaska GIS plywood Issue

Welcome!

I guess that it depends much on what you prefer: light weight or a bit more durability.

My GIS called GISwerk is all 6 mm plywood. I would personally not be happy to use 4 mm for the bulkheads. Especially the middle one is taking a bit of strain during the assembly (also because it is a fair bit larger than the front bulkheads and thus the timber gives relatively less support (i.e. the middle frame is more bendy than the smaller frames at the front).

9 mm bottom would feel more solid. I can feel a bit of "oil-canning" in my 6 mm bottom, though I am not worried by it a bit. A 9 mm bottom will feel a lot stiffer at a small weight penalty.

To put things in perspective, just looking at the plywood based on 5 sheets, we are talking about a 10% increase in weight. Of course the timber, epoxy, paint, fittings, etc. add a lot to the total weight. I would estimate the total weight penalty perhaps at around 5%. Not really something to worry about, especially if you will trailer the boat. And a lot of people tend to build on the heavy side so carefull use of epoxy and light weight timber may still result in a lighter boat than most.

Just my 2c.

Best regards,

Joost

I did bulkhead 1 and 2 from 6 mm ply, but bulkheads 3 (the middle one Joost refers to) and 4 are from 4 mm ply. I haven't had any problems, everything feels very solid, although my bulkhead 3 has no cut-outs in it.

I think bulkhead 4 in 4 mm ply is absolutely no problem.

I think bulkhead 3 in 4 mm ply with cut-outs as per the plans is probably OK as far as the overall structure goes, but the thin ply edges of those cut-outs might be vulnerable to a kick splitting the edge.

I'd be nervous going to 4 mm ply for bulkheads 1 and 2 as they are part of the structure supporting the mast.

I'd consider the rear seat top done in 4 mm ply, if you doubled the number of stringers that support it, it would be as stiff as the original, though of course more vulnerable to a point load from someone in high heels or cowboy boots leaping up and down on it. I think Mik shows a single WRC stringer on the centerline supporting the rear seat, add another one either side so the span is halved and I think it would feel solid.

Best to check what Mik has to say.

Ian

I used 6mm all around. I did notice that the strain on BH3 was enough that notching the topmost frame to accommodate the centercase caused it to flex. My solution was to clamp a temporary brace along the aft face to straighten it up while the center case was installed. I don't feel like 4mm would tolerated that treatment well. Even my BH4 showed some flex as I was fitting the aft tank top. Maybe my frame stock was not as stiff as the WRC called; I used a light Eastern White Pine.

I guess I'm advocating 9mm bottom?


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And welcome to the forum!


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I think it could be done with some of the internal structure in 4 mm ply provided you were careful and put extra battens across those panels on the inside to compensate for the lower stiffness of the thinner ply. They made the older Moth dinghys out of some very thin ply, so I guess with care and understanding of all the issues it could be done.

By the way, difference in weight 6 mm Okoume ply to 9 mm Okoume ply on the bottom panel is about 4 kg extra in weight.

Ian

Originally Posted by davlafont

I used 6mm all around. I did notice that the strain on BH3 was enough that notching the topmost frame to accommodate the centercase caused it to flex. My solution was to clamp a temporary brace along the aft face to straighten it up while the center case was installed. I don't feel like 4mm would tolerated that treatment well. Even my BH4 showed some flex as I was fitting the aft tank top. Maybe my frame stock was not as stiff as the WRC called; I used a light Eastern White Pine.

I guess I'm advocating 9mm bottom?


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I think it's better to cut the notch in the top rear corner of the centerboard case rather than in the topmost frame of BH3 so you don't weaken that bulkhead in any case. Can't remember what the plans said, if anything.

Ian

A single layer of 12 ounce (400 cgm) biax on one side of a 4 mm okoume sheet, will make it about as stiff as a 6 mm sheet. A layer of 8 ounce (300 CGM) biax, if you can find it, on each side will make 4 mm slightly stiffer and stronger than a 6 mm sheet. The same is true of thicker sheets (different schedule of course). This is a technique I use often in bottom panels, where boat rocker may require a thin sheet, but you want the strength and stiffness of a thicker sheet, that can't make the bend easily.

Originally Posted by PAR

A single layer of 12 ounce (400 cgm) biax on one side of a 4 mm okoume sheet, will make it about as stiff as a 6 mm sheet. A layer of 8 ounce (300 CGM) biax, if you can find it, on each side will make 4 mm slightly stiffer and stronger than a 6 mm sheet. The same is true of thicker sheets (different schedule of course). This is a technique I use often in bottom panels, where boat rocker may require a thin sheet, but you want the strength and stiffness of a thicker sheet, that can't make the bend easily.

How would a 4mm okoumee sheet reinforced with a layer of 12 ounce glass compare weight wise to a 6 mm sheet and a 9 mm sheet with these sheets just being epoxy coated?

I am curious to understand this for potential future builds.

Originally Posted by IanHowick

I think it's better to cut the notch in the top rear corner of the centerboard case rather than in the topmost frame of BH3 so you don't weaken that bulkhead in any case.

I concur. Do this ^^ everybody.

Assuming you'll be encapsulating the 4 mm panel anyway, the fabric and wet out goo, will be slightly lighter than a 6 mm sheet, which (assumed) is also encapsulated.

I decided to go with the 3/8" bottom. The boat will be a little heavier maybe. I don't have any WRC but have plenty of very nice local Sitka Spruce for frames etc. I appreciate everyone's informative replies.

You have plenty of Sitka Spruce!!!

I am jealous to the core. It is like gold here and probably harder to find.