69967RE: Re: [bolger] Booms strength
- Nov 17, 2013
This doesn't help,but I have just built a 3" diameter birdsmouth spar for the boom of my 200sqft balanced lug for my Chebacco 25. Not tapered at the forward end, tapered to 2" at the aft end. 15'6" long with a 3' solid plug at the forward end. My theory is that the forward end is where the maximum bending moments are and the aft end is where I want it to be lightest.
My only feedback on this is that I can sit and bounce on the middle with the ends supported (I'm about 75kg) and I can easily lift it in one hand but I wouldn't want it any heavier. I am going to build something lighter for the 15' yard. I have no idea if it will work though - the boat isn't in the water yet.
---In firstname.lastname@example.org, <Leighpilot@...> wrote:Hi John,Thanks for the insights. I'm figuring on having a purchase at the boom to haul down on the luff , even if I have to beef up the throat and tack to take the strain. I'll likely lace the foot to the boom and will use bolts or s/s pop rivets to hold the fittings. It's just that this is such a long boom. It is two and a half feet longer than the boom on Galatea and she's 40ft. Still it's only controlling 200 sq ft so it needn't be too heavy.I'll see what other suggestions come from the great Interweb thingy.
Leigh Ross484 464 1575 C610 624 1730 H
On Nov 16, 2013, at 14:12, "John Trussell" <jtrussell2@...> wrote:
I have owned several balanced lug boats. In order to achieve any windward performance, it is necessary to maintain a tight luff. This is accomplished by applying tension to a downhaul attached to the boom. If the sail is loose footed, the stress of the downhaul tends to bend the boom and make the sail fuller. Consequently, the boom needs to be stiff and on small boats, I use a plank boom. Rigging any boom involves some cost and labor. While I have no experience with Martha Janes, I would hate to go to the expense of building a boom and finding it was unsatisfactory. Therefore, I would be inclined to use the stiffest tubing I could find. If you look around, you might find a section of an old aluminum mast which would work.
You might also want to consider using stainless pop rivets rather than aluminum for attaching hardware.
I'm busy refitting a Martha Jane and need to replace the boom. It's about 17ft and I'm likely to make it from aluminum tube, simpler and lighter than making a fir boom. So do I need to use Schedule 40 pipe or will tube be stiff enough? If so what diameter would one use? I'm thinking about 2 1/2 to 3 inch should be fine.
484 464 1575 C
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