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timgoz
04-12-2018, 07:17 AM
If I were to use foam or honeycomb core for my pilothouse roof would veneer be suitable for sealing the end grain. Design calls for 3 layers of 1/4 inch ply which would then be glassed and epoxied. I am planning on goin increasing cabin/pilothouse length one to two feet so want to keep topside wieght down. Am using the boxkeel option so that should help compansate for the extra windage and reduce leeway. Thanks.

Peerie Maa
04-12-2018, 09:27 AM
I would use a solid strip rather than a veneer That way you can finish it with a nice roll edge.

David G
04-12-2018, 09:46 AM
I would use plywood for the field, glassed top and bottom. You don't mention the boat - will there be camber to the roof? With a cored panel, the strength will come from the fg/epoxy skins. The core is simply there to separate those skins - creating a beam. Remember to use as thick a core as is practical. Beam stiffness/strength increases geometrically, so doubling the thickness of the beam will increase the stiffness 8 times.

My personal approach is to assume that wherever on a boat you don'd want water to go... it will. So I don't like balsa cores, nor open-cell foam cores.

mmd
04-12-2018, 10:21 AM
What Nick said...

timgoz
04-12-2018, 12:02 PM
There is camber. Design is the Redwing 26.

David G
04-12-2018, 12:06 PM
Camber bakes in some strength. If it were flat, the thickness would be critical. With the camber... not nearly so much.

But, honestly, with that design, the amount of structure down low, and the box-keel version... I think growing the length of the cabin top 1'-2' is not much to be concerned about. Perhaps Michael, who's an actual designer, will comment.

Peerie Maa
04-12-2018, 12:58 PM
http://www.cmdboats.com/images/rw26boxkeel.jpg

What David said, 6 to 12sq foot of 3/4 ply is not going to make that much difference. About 3 - 6% of the hull weight, less with the engine and all of your kit.

master of nun
04-12-2018, 03:30 PM
Seems like a lot of work for little gain
If you're concerned, use a lighter weight ply like Bruynzeel for that part of the build.

Sent from my SM-G935F using Tapatalk

mmd
04-12-2018, 03:48 PM
Unless you are installing a chin-up bar or a 6-man cannister-type liferaft on the roof, there should be no problem with extending the roof aft 24". Maybe fit a 2-1/2" x 3/4" extension of the cabin-top stringer to reinforce the aft corners, 'cause that is where folks will hang on when boarding and disembarking.

Regarding 'to core or not to core', I would adhere to the original, all ply, scantlings unless you want to do the core for the challenge. You won't gain any significant weight savings in the end, and putting a proper edge molding on the cambered ends will be tres fussy. But sometimes doing fussy is an end in itself.

timgoz
04-12-2018, 08:48 PM
Yes, all true. Ballpark figure approx 50sq ft extra. And no, my first big build will be enough of a challenge w/o me adding any unnecessary challenges. Great advice!

Peerie Maa
04-13-2018, 04:38 AM
Yes, all true. Ballpark figure approx 50sq ft extra. And no, my first big build will be enough of a challenge w/o me adding any unnecessary challenges. Great advice!

50 sq foot? The boat is only 8ft 6" beam
(Redwing 26

Specifications:
Length: 26'-0"
Beam: 8'-6"), so the cabin top is 6 or 7 max, that is only 7 x 2 =14 sq foot additional area.
However, I Just checked back, you are extending the entire pilot house not just the roof? Probably still less than 50, but some will have less effect on weight aloft as the weight of the new sides will only act at half height.

timgoz
04-13-2018, 08:58 AM
I am including the area of the extra cabin side material in the 50ft. Sorry, I should have specified that.

MN Dave
04-13-2018, 09:35 AM
would veneer be suitable for sealing the end grain
Yes, but veneer is more difficult to handle than thin plywood. Since the skins will need enough strength to walk on (someone will), 1/8 inch plywood over 1/2 inch core will give nearly the same stiffness as 3/4 inch plywood. Closing out the edges with solid wood will make it easy to finish.

A core can save considerable weight. As David said, the strength is in the skins and the stiffness increases as the cube of the thickness. What is not obvious is that plywood is almost as strong and stiff as fiberglass, so a thin layer of glass doesn't add a lot to plywood.

I punched a few numbers into a spreadsheet just for grins. The link give the density of the balsa as 9.5 lb/ft3 and I took a stab at 40 for the plywood. It looks like you can cut the weight in half, and you don't want any extra weight up high. It will be more work, but since I'm not doing it, it doesn't bother me a bit.
Edit: Added labels in the spreadsheet for clarification.

<tbody>
https://www.fibreglast.com/product/end-grain-balsa/Vacuum_Bagging_Sandwich_Core


t
b
d
A
=t/b*d*A


inch
in/ft
lb/ft3
ft2
pounds


0.5 balsa

12
9.5
50
19.8


0.25 (2x1/8 ply)

12
40
50
41.7





total
61.5









t
b
d
A
=t/b*d*A


inch
in/ft
lb/ft3
ft2
pounds



0.75 (3x1/4 ply)

12

40

50
125.0

</tbody>

timgoz
04-13-2018, 03:22 PM
Thanks

timgoz
04-14-2018, 05:45 PM
50ft was high estimate. 42ft is much closer.