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Stringer Material

Posted: Tue Aug 19, 2008 1:38 pm
by Mike R
Hello everyone - I am about a month into my Riveria build and need some advice. I have all of my frames complete and started to work on the building form and stringers. I just can't decide what wood to use for the stringers. I have enough Mahogany to use or I could laminate some 3/4" Doug fir. I did find Southern Yellow Pine in 2x6's that were kiln dried. I can get lengths that will eliminate the need for scarfing or gluing up. Since they aren't treated, the weight is not too bad. These pieces are straight and true, but do have knots.

Would the SYP work or do I need to just bite the bullet and go with the DF? I want a quality boat with good quality material. Thanks for the advice.

BTW, the best pieces of equipment I now own is a 14" bandsaw and a 9" disk /6" belt sander!

Posted: Tue Aug 19, 2008 2:06 pm
by kens
Mahogany is the best wood. Doug Fir would be next best. The best for stringers is mahogany, since it is not really a 'hard' wood. this allows the stringers to absorb some engine vibration. Oak and SYP are so stiff they transmit vibes rather than absorb any. Dont use a solid lumber either, use laminations if the stringer is much over 1".
It could look cool with Mahogany/Fir laminate.
Mahogany is just the best wood all the way around.

Posted: Tue Aug 19, 2008 4:46 pm
by Bob Perkins
I used Mahogany, laminated and scarfed the laminations. http://im1.shutterfly.com/procgtaserv/4 ... 2TFi5bt2Pg

I had two 3/4" laminations and scarfed them to get the 17' length. Then glued the two 3/4' halves together to make the stringers. No matter what wood you use - the laminations will be stronger and mode stable than non-laminated wood.

Posted: Tue Aug 19, 2008 4:59 pm
by Mike R
You know, I hadn't thought of mixing the two woods and doing mahogany and fir together. That would look cool. I like the idea of going all mahogany too. I appreciate the feedback.

Posted: Tue Aug 19, 2008 7:08 pm
by Bob Perkins
hmm. I don't think it would be a good idea to mix species. You could end up with a giant laminated beam shaped like a banana. The motor stringers will most likely not be seen once your interior is in anyway.

Good Luck
Bob

Posted: Tue Aug 19, 2008 7:57 pm
by Bill Edmundson
I'm laminated and scarfted DF. I think... Oh, Hell! I'm happy!

Bill

Posted: Wed Aug 20, 2008 6:06 am
by Mike R
I quit worrying about it and picked up some DF last night. I think that will be the best option.

Thanks for the help.

Stringer Material

Posted: Thu Aug 21, 2008 7:37 am
by bob smith
What about engineered lumber like Paralam, LVS or LSL ?

Posted: Thu Aug 21, 2008 1:04 pm
by Bill E
Most engineered wood like LVL (laminated veneer lumber, a version of Parallam) is neither intended nor fabricated for wet applications. At least with LVL material, I'd hesitate to count on it holding fasteners as well that are applied into its edge vs. the face. LVL is essentially long plywood without the alternating grain -- it's all running lengthwise on the board.

I know this isn't a true "wet" application, but there will be times when you have varying amounts of water in the bilge, and you don't want these members soaking it up. They're usually made with a waxy coating that I expect would prevent you from even sealing or encapsulating them well with epoxy. The wax is intended to provide short-term weather resistance during (house) construction.

LSL is laminated strand lumber, roughly similar to OSB, but in lumber format vs. sheet format. Again, less than ideal for edge fasteners. I have just used a pressure-treated version of this material where the outer edge of an exposed porch had to carry some serious loads, but it was 6x14" in cross-section. A bit big for the boats we're looking at.

There is a tried & true product that was one of the earlier engineered lumber products, known as glu-lam. You'll often see these as stacks of 2x6 lumber, face-glued into larger beams. They can be custom-built in curves, arches, etc. and are common in churches and larger gazebos/pavilions. They're available in exterior grades, obviously. Downside, they're probably only available commercially in larger dimensions, like what you'd put across a garage ceiling to carry the joists above.

All that said, the real benefit of almost all of these engineered products is that they're made up of several layers of smaller pieces, stacked either flat (glulam) or on edge (LVL). Any local stresses or defects (knots, splits, etc.) in one layer are limited only to that layer and are balanced and compensated for by the other layers. It's also something you can fabricate pretty easily yourself, as folks have mentioned. Gluing up three 3/4" layers face-to-face gives you a much more stable end product than a single 2-1/4" piece of wood.

Also, while you might need to be more careful mixing species in thick layers, I'd be far less concerned inserting a thin layer of another wood for contrast. Expansion/contraction tables by temperature and moisture content shouldn't be hard to find, and picking two species with similar properties would alleviate worries. Even so, a thin layer (I'd say 1/8" or less) will absord a lot more differential movement than a thick layer. Consider most plywood with grain alternating at 90 degrees between layers. In thick layers, this is a huge no-no, but thin layers will handle it and actually increase strength and/or stiffness.

If you have a balanced construction, I'd also not worry about it turning into a "banana" with changes in temp/humidity. By this, I mean for example, a center layer of oak with matching layers of mahogany on both sides. I would never just put oak on one face and mahogany on the other face. If everything is sandwiched symmetrically, you should be fine.

As for contrasting species, sailboat tiller handles are often done in contrasting colors (mahogany & ash?), although they're supposed to look like a banana!

My $0.02,

-wte