Stripping with mahogany

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parteam
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Joined: Fri Jun 19, 2009 9:30 pm

Stripping with mahogany

Post by parteam »

I am going to build a canoe this summer and have been offered mahogany to use for stripping. What are the pros and cons for using this wood in this way? Thanks for your help.
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Gary
Posts: 19
Joined: Mon Dec 05, 2005 9:46 am

Post by Gary »

A mahogany canoe would be beautiful, but it would be heavy. Perhaps using 3/16" thick strips would save weight. I would use the mahogany for accent strips, and keep the rest for furniture.
Rick
Posts: 727
Joined: Thu May 27, 2004 9:23 am
Location: Bancroft, Ontario

Post by Rick »

With a wood as dark as mahogany, light-toned glue lines could spoil the beauty of the wood... it might be good to experiment with some tests to see if any thicker areas of glue are visible after sanding. There is a wood glue available with a dark toner added to it which should help minimize this.

There are several species of mahogany on the market and they might be different in their properties. The boards that I have used for trim and paddles have been fairly light in weight. while providing a nice dark brown color.

A problem in using mahogany instead of softer cedar could be the extra time needed to sand the strips smooth, especially on the inside. Maybe using narrower strips could help with that.

My choice would be to build the entire hull out of the same kind of wood, to allow the beauty of the wood to stand on it's own... my opinion only. Good luck and may your time spent sanding be worth it.

:wink
parteam
Posts: 2
Joined: Fri Jun 19, 2009 9:30 pm

Post by parteam »

Thanks for your feedback. This is great help. Using mahogany may be our direction. So we should consider using 3/16" thickness to save some weight, and look for a glue that is darker in color to blend in with the mahogany. Sanding time may be an issue that we should also plan for. I was thinking of using a light wood for an accent stripe and am intrigued that it might be better to use one wood.

Are there any concerns about the mahogany being too difficult to bend or shape? Again, your feedback is helpful. Thanks!
Big Woody
Posts: 71
Joined: Tue Apr 07, 2009 9:18 pm

Post by Big Woody »

I used a mahogany board to make strips for the bottom of my Hiawatha. My board was only 14 foot long so the shorter strips weren't long enough to be full length except on the bottom.
Image

I think having the heavier wood on the bottom may add a tiny mite of stability. I used Elmer's probond interior/exterior glue. It dries a sort of translucent orange. If you keep the glue lines thin you don't see them because the color of the wood shows through the glue.
The mahogany was not nearly as flexible as the softer cedar, but it worked well on the flatter portion of the bottom where I used it. It would probably only give a problem around the bilges. It is much harder to sand away than the softer woods I used. I liked the color of the board I used, and the uniformity, but the growth rings were invisible, which makes for little visual interest up close.

As someone who makes sandwich panel devices for my living I don't see much advantage to using a thinner core material for weight savings. First look at the numbers on this chart.
Image
As you can see if you click on the chart to get a bigger view, the stiffness goes up immensely with a small increase in thickness. The core in the chart is a virtually weightless aluminum honeycomb, so the weight comparison is slightly different, but your wood, if it is cedar, is five times less dense than glass and epoxy. If you thin the core, to get the same stiffness you would have to add a far heavier amount of glass and epoxy to get that stiffness back. If you're OK having a less stiff boat you could save more weight by keeping a thick core and going with a lighter weight glass cloth and the less epoxy required to fill it. In reality fiber based composites are far stronger in tension than compression. You can pull a lot of weight with a rope, but they're not much good for pushing on. when flexing a sandwich panel one skin is in tension and the other is in compression, while the core is in both shear and compression. Like a chain is only as strong as its weakest link, so also a sandwich panel with fiber based skins will invariably hold up on the tension side while suffering a compression side failure triggering core failure to allow folding of the sandwich material. Since the core and the one skin both take compression loading, a thicker core not only helps take the load but also places the compressed skin further down the lever from the opposing skin giving it greater mechanical advantage to resist failure. That is why a tiny bit of thickness can add so much stiffness with out adding hardly any weight. If your core gets too thin then you no longer have a sandwich panel, just a fiberglass canoe with a weaker internal wood layer which will fail first. In addition with thicker strips there is more room to sand out mistakes without sanding things too thin. So it is more forgiving.

Feel free to use the mahogany, and use full thickness strips. Your canoe will just be a little heavier and stronger. If you wanted the world's lightest canoe it couldn't be a typical beautiful stripper, you wouldn't be using any fiberglass and the only wood core you might consider would be end grain balsa. That's my engineering opinion.
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Denis
Posts: 313
Joined: Tue May 11, 2004 8:11 am
Location: Lakefield, Ontario

yep!!

Post by Denis »

Big Woody <---- yes to what he said.

thinner strips fair down thinner esp on bilge areas. Also makes boats more prone to oil canning (ie flexing in the middle). I suppose a super careful person with a load in canoe wouldn't be bothered by it though.

To take out the flex one must add another layer of glass so now how has one saved weight by using thinner strips?

Denis from Lakefield Ontario
AlanWS
Posts: 209
Joined: Thu Mar 17, 2005 4:30 pm
Location: Shorewood, WI

Post by AlanWS »

Mahogany won't sand as easily as cedar, but it should scrape more smoothly. You might be amazed at how quickly and well a simple card scraper works instead of a sander.
Alan
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