Best wood for laminated coach roof frames?

[Uncle Duke]

I'm coming up, hopefully later this month, on having to laminate a bunch of frames for my coach roof (and some deck, also). I'll want to end up at about 1" square, about 8' end to end, with about a 3" rise at the center.
If it matters, coach roof will be 2 layers of 6mm ply with dynel, etc.
Is there a consensus on what wood to use and/or what schedule of laminates?
Spruce? Mahogany? Other?

[Canoeyawl]

I would not bother to laminate to achieve a crown of only 3 inches in 8 feet…just bend them, any wood will do. It is considered good practice to keep the construction as light as possible above the deck.
But - you must remember Bubba jumping down from the dock with a case of beer under each arm and landing on your coach roof! “Hoo-Rah - lets go sailing!”

[Bob Cleek]

Ditto what Canoeyawl said. Is this a new build or are you replacing them? A one inch by one inch eight foot beam seems rather light for such an application, even with ply overlay. What was there before or what do the plans call for?

[Jay Greer]

Laminated frames will be the strongest. The ply you are using, when arched and fastened at the edges will impart strength as well. Bent frames will be most flexible. Sawn frames will be the least hassel and nearly as strong as laminated. Fir, white oak, live oak mahogany, spruce, ash, all will work. Pick your favorite for rot resistance.
JG

[Uncle Duke]

Actually, upon review of the drawings, the arch will be about 4.5", not 3", over the whole 8’ length. But I don't imagine that difference matters much.
New build, sort of, Bob. I'm replacing the deck and cabinhouse entire to remove existing problems (from previous owners lack of maintenance) and to reconfigure (back from the previous owners idea of “hey! let’s just paste a really ugly pilothouse on it!”).
Current is 1” x 1” laminated beams with about 1/2” cold-molded ply overlay. It seems quite sufficiently strong - while the laminated beams add some strength, most of the strength is in the curved skin, with the beams being a nice “in-place” form.
[Added info: current has the beams every 24", new will have them every 16". If that reassures anyone, besides me, about strength issues]
My decision to laminate was based on that current method, as well as a thought (perhaps incorrect) that this would be the easiest way to get consistent shape without worring about springback or spending time nervously running 20 planks through a bandsaw, hoping not to screw up.
Having said that, sawn might really be the easiest, I guess - thanks, Jay.

[ 11-09-2005, 07:34 AM: Message edited by: Uncle Duke ]

[TimothyB]

1" square with 8' end to end? Hmm...that's pretty small.

Personally I'd go with hardwood for maximum strength. I'd choose black locust or white oak. I'd use maximum 3/8" laminates. You'd want it in lengths in excess of 9 feet. I'd imagine mahagony would work as well, as long as it was good quality and not fast grown plantation stuff. Then again, I like the idea of my deck being as strong as my hull smile.gif

I just thought.. I wonder how much it would cost to take a squared piece of wood to a veneer plant and have them slice it up for you?

[Uncle Duke]

Thanks, all.
I see a trend here of replies from you experienced folks that 1' x 1" seems small, so I'll assume that you're right. I planned for that size on the basis that that is there now and seems to have worked OK. But given that I'm replacing all of them, maybe I'll just move up a step or two in size = 1" x 1.5" or something. If I make them taller (rather than wider) strength would be improved - I'd have to then raise the cabin sides a little to maintain headroom under the beams, but that's not a big deal - another 0.5" won't kill me.
Gracias!

[ 11-09-2005, 10:45 AM: Message edited by: Uncle Duke ]

[Bruce Hooke]

Hmmm...it seems to me that thinking about the size and spacing of these beams without considering the thickness of the plywood on top is missing half the picture. Remember, it is possible to build a cabin top without any beams. I do not have enough engineering knowledge to analyze the existing design to determine how much load it could withstand (and how much load it should be able to withstand), but I am wondering if those who are objecting to the size of the beams have taken into consideration the relatively thick layer of plywood on top? I'm not saying the current design is insufficient, I'm just saying that we need to be careful here before jumping to any conclusions.

[George Roberts]

Uncle Duke ---

I would build as close to existing as possible.

1"x1" at 24" with 1/2" ply seems reasonable.

[Gary E]

I'll want to end up at about 1" square, about 8' end to end, with about a 3" rise at the center.
If it matters, coach roof will be 2 layers of 6mm ply with dynel, etc.
The 8 ft is the beam dimention? the 3" is the rize in the center... and you think those little beams are good enuf?... Maybe, but dont plan on ever standing on it.

Before you do this for real, make one and set it up outside on the driveway, then ask a litle kid riding by on his bike to come help you for a sec. Have him stand on it in the middle... DO NOT ASK FAT ALBERT,,,just a skinny kid will do...

[ 11-09-2005, 12:37 PM: Message edited by: Gary E ]

[Bruce Hooke]

Gary,

Did you overlook the fact that he is basically replicating an existing design that appears to be strong enough?

[Gary E]

I did not overlook that... why is it being replaced? not strong/ sturdy/ ridgid enuf???

If ya want to STAND on it...test the beam...

Of course if your a lightweight, then maybe you can "get away" with skimpy ...

[Uncle Duke]

Gary,
This is replication of existing structure, which has been informally tested by having myself (205 lbs), my running buddy (195 lbs) and another friend (?? lbs, but more than me...) walking around the existing coach roof. My wife, inside, saw no deflection or heard any odd noises.
Replacing because the previous owner cut into it to make "improvements", like a stupid pilothouse made out of what looks like 1/4" luan, and propane storage compartments of much the same. By the time you remove those (which really has to be done) you might as well take it all off. My original concession to adding strength was to move the beams to 16" centers instead of 24".
"Gougeon Bros on Wooden Boat Construction" indicates that 1/2" molded ply should be just fine without any beams at all (as long as there is some curvature), and I tend to believe them. So having that with beams seems OK to me.
Still - open for suggestions, as always.

[Bruce Hooke]

Originally posted by Uncle Duke:
"Gougeon Bros on Wooden Boat Construction" indicates that 1/2" molded ply should be just fine without any beams at all (as long as there is some curvature), and I tend to believe them.I suspected this might be the case and I am inclined to believe them too...

[Gary E]

Maybe heavy enuf for you... if so. fine..

I have a Fly Bridge on my Sportfish, and there is NO FRICKIN WAY that construction would be strong enuf for ME...

Seems to me so many peole come here with questions they already know the answer to...
Go for it...

[Jay Greer]

A last comment on sawn beams here. If a consistant pattern is called for, rough cutting on the band saw followed by a pattern tracing clean up on a spindle shaper or with a pattern tracing bit in a router will do the trick in jig time and a lot faster than lamination. However if the beams are laid out to a parobolic arc, as they should be for added strength, the center line will flatten as the ends are nipped to match the narrowing of a deck house that follows the curve of the deck. If a consistant height of the center line is desired, then the beams will need lofting on each station to ensure the centers don't flatten out.
JG

[Bob Cleek]

If you want to be sure, check Herreshoff's, Nevin's or Lloyd's scantling requirements. You will find these in Skene's Elements of Yacht Design or Pardey's Classic Yacht Construction, as well as elsewhere. There will be lists of dimensions and proportions that will save you the risk of hit and miss guesstimate engineering. It will also avoid an endless dispute about whether or not it is strong enough BEFORE you build it.

[Uncle Duke]

Gary noted:
Seems to me so many peole come here with questions they already know the answer to... Gary, my original question had to do with what woods might be recommended and what schedule (how many layers at what thickness) would be best. The discussion about strength issues is a tangent. An interesting tangent, at least to me, but a tangent nonetheless.
For what it's worth, I'll probably end up with mahogany just because I can get it pretty easily. Locust is a great idea, but I have no idea where I would get enough clear locust in those lengths.

[Bruce Hooke]

Getting back to your original question...while you can use fairly thick laminates when you are making such a gentle bend, you will experience less springback if you use thin laminates. That said, since much of the strength is in the plywood you should be able to deal with a certain amount of springback by simply pressing the beam against the plywood and screwing it or otherwise attaching it. You also should not get that much springback on such gentle curves anyway.

On calculating the beam size, I don't think the sources Cleek mentioned are going to be that helpful when it comes to a situation where most of the strength is in the plywood. Dave Gerr's book, The Elements of Boat Strength might have the necessary information, but if the Gougeon Brother's book provides sufficient detail I see no reason not to trust that as your source.

[Bob Cleek]

Yes, The Elements of Boat Strength would be an excellent resource. As for tangents, I think what people are trying to tell you by raising other issues is that you need to do some more research before starting to saw. The potential problem they are seeing isn't the type of wood used for the beams.

[Uncle Duke]

Bob, et alia,
Sorry if it seemed that I was unclear. I am not ignoring any of the 'tangent' issues. I take them very seriously, and I really appreciate the helpful opinions of all involved. I thought that my comments along the way already indicated that, but if not - "Thanks to all. I'm not ignoring the suggestions".....

[paladin]

O.K. Now that all the experts have checked in...I'm just a poor slob that's built a boat....so..
The beam ain't worth laminating unless you have six or more layers on it....so 1 inch thick I would go for actually go for 6 each 3/16th inch layers...only an eigth over... it should be about 35-40 percent stronger than a sawn 1 inch frame..mahogany is nice, fir will work just as well....epoxy the fir once before trying to sand it or you will have a gazillion little furry b its all over the place......then your plywood overlay......Then I can walk on it....If the boat goes upside down the overhead becomes the bottom, and I like my bottoms strong.....