Cabin top beams and Deck beams

by Patrick
(Florida,USA)

I am building a 29' stitch and glue cruising sailboat and I plan on having traditional looking deck beams and cabin top beams laminated from mahogany and covered with 3 layers of 1/4" ply laminated together.

The cabin is approx. 5' wide by 10' long and I'm not sure what dimensions I should make the deck beams for proper strength.

1 1/2"wide by 1 1/4"thick perhaps?

Where can I look for guide lines?

Comments for Cabin top beams and Deck beams

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Good Info
by: Patrick

Thanks for your thoughts.

I am laminating the decks and cabin top from 3 layers of 1/4" ply as you suggest.

I suppose that I just like the traditional look of beams in my boat.

Perhaps I can scale back on the thickness to have the look but save some weight.

Thanks,
Patrick




A better, more appropate way . . .
by: Anonymous

I think a "stick built" approach on a taped seam build, is the wrong way to go, particularly using Pardey's half a century old techniques.

A 3/4" laminate of 1/4" plywood, if done over a mold, jig or buck will be more then sufficient without beams, assuming reasonable fillets and sufficient "tabbing" fetch.
In fact, this sounds well more then necessary and places a fair bit of weight high up, which generally isn't good.
1/2" would be acceptable on all but an offshore, deep water cruiser.

Grab a copy of Dave Geer's book "Elements of Boat Strength" and use the plywood composite techniques and scantling rules contained therein.

You'll be employing the same building method and engineering as the rest of the boat and you will not have beams to bang you head against too.

I'd probably use a slightly different approach and an inner skin of 1/4" laminated to a moderately high density foam, with an outer skin of 1/4" plywood bonded on.

This will form a self insulating, composite structure, much lighter then the stick built method previously described and it'll have a smooth underside.

I'd take it a step further and use a bead board, with the beads facing down on the inside, to give it a nice look.

Of course this would be sheathed externally, with a light weight cloth for abrasion resistance.

A perimeter frame of 1 by stock, some companionway carlins and hard points for any hardware that bolts to the cabin roof (hand rails, cleats, blocks, etc.), would finish it off.

I build cabins and decks like this fairly regularly and it's faster, lighter and usually stronger then stick built, plus it's in keeping with the yacht's current build method.

Bets Regards,

PAR




Deck Beams
by: Patrick

Thanks Mike. Your info is a great help.

I will try and keep this forum posted on my progress.




Scantlings Rules
by: Mike

Hi Patrick,

***C2Add.shtml***For my cabin top beams when rebuilding "Mignonne" I got the information from that superb book by Larry Pardey, "Details of Classic Boat Construction".

The figures are based, I believe, on "Nevin's Scantlings Rules for Wooden Yachts".

If you use the figures as a starting point, I don't think you can go far wrong.

Below is my interpretation of the info.

For 'ordinary or 'auxiliary' beams the recommended spacing between beams is 250 millimetre which is approximately 10 inch, and the recommended size for each beam is 30mm (approx) 1.2 inch by 45mm (approx) 1.8 inch (I've rounded the conversions up).

As you are laminating them (which apparently adds to the strength) I assume that you are going to camber the deck/cabin top, which will also add to the strength.

The recommended camber for your 5 foot would mean a center rise of about 2 1/2 to 3 inches.

For 'king beams', any beams which support a hatch or any opening, mast partners etc increase the beam size by half again.

Short beams, eg those going from the hull side to a carline should also be half again larger.

And don't forget the hanging knees, not just to hold the beam in place but to also maintain the angle between beam and cabin side.

So using the above as a staring point your 1 1/2" by 1 1/4" beams a foot apart would be more than enough.

I hope this helps,

All the best with your boat building,

And please do let us know how you are getting on with her.




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