Boom Design for Int 505

Discussion in 'Sailboats' started by Russell_Miller, May 8, 2009.

  1. Russell_Miller
    Joined: May 2009
    Posts: 4
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Rhode Island

    Russell_Miller New Member


    I sail International 505's. Its a single trapeze two man dinghy. The class recently made a rules change to not restrict materials in spar production (except masts, they're still aluminum). I'm in college and my partner in the boat isn't rich, so we can't quite afford the upgrade.

    Just completed a course on mechanics of composites and am considering applying my knowledge to the design and construction of a boom and some spin poles for the old battle axe.

    What I hope you all can help me with is two fold.

    1) After the requisite research here and elsewhere I have not really found a solid source on boom design except for a forum started by Brian Eiland where a pdf document was posted by a frenchman. Vague I know....

    Do any of you have a reference you could direct me too? The issue here is calculating leech and vang loads so I have a number to put into the engineering analysis. I can design the spar theoretically, but I don't want to go through a big iterative process of not making it strong enough and breaking it because I didn't begin with a good assumption of the applied load.

    Along the same lines, what are the commonly accepted "design standards" that apply. Please don't say that if it breaks that I should build it stronger, I know that. What would help here is some standard deflection (which should be small) or safety factor to apply which accounts for an ultimate limit state (like broaching and trying to ease the main into the water....)

    2) Can you recommend a laminate schedule that you've found to work well empirically? I've contemplated just making a laminate which will mimic the stiffness of my aluminum section and this is plan B, but my concern is that I won't properly account for buckling or local stresses due to fittings. If any of you have homebrewed your own booms or spar sections and can give me some advice on design details with fittings into composite tubes, that'd be sweet.

    Thanks alot for your time.
  2. Chris Ostlind

    Chris Ostlind Previous Member

    Hi Russ,

    You may want to direct this really great query over to the Dinghy Anarchy Forum at Sailing Anarchy. There are a huge number of really active 505 sailors over there who regularly toss this kind of subject around.

    In fact, right now, there is a fairly good sized thread cooking along on creating a 505 rigging and tuning database. If they haven't already tossed some of the info you need into the mix, I'd be willing to bet that your questions would get a quick response.
  3. bistros

    bistros Previous Member

    Russell is one of the participants in that thread! It may be worth a call to Larry Tuttle at Waterrat asking for help - explain you are student and you are interested in the design and engineering aspects, not just purchasing one of his products. Bet he'd help. Lots of people like Jesse Falsone and the Team Tuesday crew probably all have firm ideas about this.

    In absence of theoretical calculation, it would be interesting to collect data and find out exactly what the loads are.

    Linear load cells and Analog to Digital conversion isn't really tough, so it would be cool to actually place load cells on the leech, vang, outhaul etc. I bet you could write an iPod/EEEPC/whatever app to watch load cell input on a USB connector. Google USB A-D input!.

    I would think the loads would be quite variable on 505s - everybody has different ideas regarding bridles, center versus stern sheeting, travellers etc. The loads would pretty much be rig-specific. One of the things about a class like the 505 is that nothing is the same from boat to boat - Waterrats are different from Rondars, different hulls have completely different stiffness and rig tensions. I can look at a row of 15 505s at my club with very little to no similarity in rigging from one to the next.

    I think it would be tough to expect generic design formula to handle a situation as variable as 505 rigging.

    Sometimes when theory is thin, you've got to start with data and work backwards to create equations that fit the reality.

  4. Chris Ostlind

    Chris Ostlind Previous Member

    Well, that's what I get for not following the SA thread and simply reading the Topic heading.

    Oh, well.

  5. Russell_Miller
    Joined: May 2009
    Posts: 4
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Rhode Island

    Russell_Miller New Member

    Right on fellas

    SA is more of a habit than a useful resource. There are times it has good information, but usually just full of clowns spouting their tops off.

    I'm sure Jesse has an opinion, and I reckon it'll be to spend the money on a professional. Larry is a genius boat builder, but I know he doesn't want to waste his time on a kid that won't spend money with him (he freaks out over emails larger than 20k)

    I think the leech and vang loads for the boat would be similar as the sails aren't changing that much. Agreed, the details of it all will be vastly different from boat to boat, but preliminary design (wall thickness, layup to achieve a given stiffness, etc...)will be very close to the same. If not, how could a vendor sell a standard boom section?

    I haven't played much with load cells, but will do the homework to see if I can sort it out. Its worth it to me to do this right instead of trying to cut corners and wasting my money in the end.
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