Cabin top rafters

[cs]

The cabin top of the weekender calls for the rafters to be of 1-1/2" clear Fir, Mahogany, or spruce.

I was wanting to use clear 4/4 white oak. Do you think this will be okay or do I need to double it up?

The rafters are 4' +/- long and about 21" apart.

Chad

[captain's gig]

[ 07-03-2005, 08:58 PM: Message edited by: captain's gig ]

[WFK]

cabin beams.......
Chad, It would be best to keep the weight down any chance you get, I'd stay away from the oak, and if it was me I'd be tempted to laminate for strength.

[Gary E]

Is not the oak stronger? I am guessing it is, and would use it. As far as the additional weight?.. common whatcha talking about the weight of a case of beer? ...

MORE BEEF... STRONGER STRUCTURE... LONGER LASTING

[Ken Hutchins]

Just keep in mind that if you ever roll the boat over the top becomes the bottom.

[Bob Cleek]

I suppose there's an engineering manual around that will tell you the equivalent strengths of different dimensions. However, if you want to use oak, there's no problem with that. It's a bit heavier, is all. Now, is there some camber to the roof? I assume so. I'm also assuming that your 4/4 oak is plank stock.

Okay, so they say 1.5" instead of the 1" you have. You can do a couple of things. Your 4/4 probably needs to be finished, or you wouldn't be calling it 4/4. After you dress it, you're going to have maybe 7/8 or 3/4 at best. Take two 3/4 pieces and laminated them side by side with epoxy. Cut your overhead beams out of that.

Or... you can stick with the 1" stock, plane it as little as you can get away with, and place the beams closer together than the scantlings call for. You do the arithmetic, but figure if your beam is 2/3 of what they call for, then run the spaces between the beams 2/3 of what the 1.5 scantling calls for. That should do it.

Or, you can buy the 1.5 stuff and do it like the designer called for. It can't make that much difference in the cost of the whole boat, right?

[cs]

Okay, thanks for the replies and the e-mails. And yes oh wise one they are beams and not rafters. Guess I've been building houses to long. ;)

I will use the white oak and laminate the 4/4 to get a finish thickness of 1-1/2 (as called for), despite the weight issue. As addressed in an e-mail, I'm not worried about the direct sunlight, as it will be under the cabin top. The weight I don't think is an issue, because there is only 2 beams.

Chad

I want to finish these clear and that is part of the reason I'm staying with white oak.

[Jack Heinlen]

Hm. Seat of the pants, and there are references and ways to figure it out, I think that inch oak is going to have at least the huevos of inch and a half spruce, one of the specified woods. That the designer has such a range, from spruce to mahogany, says it's probably overbuilt. If worried you could beef up the molded dimension a bit. It's not like headroom is a real issue.

I'd use the oak without laminating to get the specified thickness.

[mmd]

White oak hasa modulus of rupture about 15-20% greater in bending than Douglas Fir or White Spruce. You can reduce the molded depth of the beams by a quarter-inch to 1-1/4".

[Jack Heinlen]

The issue is the sided dimension. As I read Chad, the plans call for 6/4 stock of spruce or fir or mahogany. Can he get away with 4/4 oak? Again, seat of the pants I think he can, and if worried could beef up the molded dimension by say a quarter. Laying out the beams to follow any sweep would be good.

Am I missing something? Reading stupid? It happens.

[Frank Wentzel]

If I recall correctly, beam deflection varies with the cube of the beam thickness. That would mean that a 1" beam is only 44.4% as stiff as a 1-1/2" beam. If you reduce the beam thickness to 1" the new wood must be 2.25 times stiffer than the specified wood for equal performance.

/// Frank ///

[ 07-06-2005, 04:20 PM: Message edited by: Frank Wentzel ]

[Gary E]

The weight I don't think is an issue, because there is only 2 beams.
TWO as in 1...2...?
TWO BEAMS??? make them from anything ya got... plywood, oak, laminated, whatever, glued together with whatever...uhhh, maybe not ,,, that other thread might object...

Sheesh, ya coulda had that job dun by the time it took to ask the question...

[ 07-06-2005, 04:34 PM: Message edited by: Gary E ]

[Bob Cleek]

LOL... Gary's pretty much right! I had visions of an eight foot long cabin top!

[cs]

Yep I could have had them done by now if I could've had more of a chance to get in the shop. As it is I've laid out one set. I've I need the other set I will lay it out and laminate them together.

I tried to contact the Stevensons, but the e-mail link I had didn't work.

Chad

[cs]

Finally heard from Mike Stevenson (the designer)


I think you could get away with the single 1" white oak. It would probably look better with the 1 1/2" thickness, but the other would likely work IF there aren't splits or grain problems running across the beam.

Mike
Since he says that I could "get away with" 1" I think I will go with the double thicknes. "Get away with" does not sound like a shining endorsement.

Chad

[Jack Heinlen]

Double them, as you like. It's your boat. I don't think it necessary.

[cs]

I cut one rafter out last night. I will look at and study it some to see if it needs to be doubled.

Chad