M15 sail plan consequences

Discussion about the boats Jerry Montgomery created: sailing, maintenance, repair and renovation.
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bill
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M15 sail plan consequences

#1 Post by bill » Fri Dec 21, 2012 9:31 am

Hey Jerry,

Obviously this isn't your highest priority but I have a pretty serious interest in adapting some sort of unstayed rig to my rather new M15. Crazy idea but I can't let it go. The direction the S15 is heading is reinforcing this notion.

The thing I'm hung up on is that I'd like to move the COE aft a little but I'm wondering if you could quantify the consequences of such a move. I guess I'd be sacrificing helm neutrality and maybe more that I don't understand.

I also have to study on foredeck reinforcement but that's a problem for another day.

Have fun,
Bill

bill
Posts: 17
Joined: Tue Mar 20, 2012 11:06 am

Illustration of above

#2 Post by bill » Fri Dec 21, 2012 10:37 am

Here's the M15 with a Beetle Cat rig for illustration purposes.......at least if I can add a picture:
The attachment M15Cat.jpg is no longer available
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M15Cat.jpg

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Jerry
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#3 Post by Jerry » Thu Jan 03, 2013 4:58 pm

Why would you want to move the CE aft? It would add weather helm and the 15 is about right as it is.

Since you'll be stepping the mast in the bottom of the hull you won't really have much load on the deck; no compression load, but some sheer loading. If you could make a plate that is thru-bolted to a similar plate under the deck, that should do it. You'll need to work up a foolproof seal for the balsa core; maybe weld a ring of the proper size, the depth of the thickness of the deck (3/8 or 7/16"?), to make a flange.

Good luck, an interesting project.
--
Jerry Montgomery - Designer of the Sage 17
Founder - Montgomery Marine Products (M-dinks, M15, M17 and M23)
Montgomery Rigging - http://jerrymontgomery.org/

bill
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#4 Post by bill » Fri Jan 04, 2013 7:28 am

Jerry wrote:Why would you want to move the CE aft? It would add weather helm and the 15 is about right as it is.
I was having trouble designing a sail shape of 120 sqft that didn't move the CE or have too long a mast, but in the meantime I devised a gunter that I think would work pretty well and be a good compromise.

In choosing an unstayed mast section, using Kinney-Skene, I've calculated a 30 degree righting moment around 800 ft lbs for the M15, does that seem fair?

According to my dumb guy cantilever physics I could safely get away with a 2.25 inch aluminum tube with 1/8 wall thickness to carry 120 sqft. I could also use a 3 inch 1/16 tube but that's awful expensive and I assume that would not be exactly aero ideal.

I haven't figured out what I need for boom and yard, both around 13 feet, any guesses?

Bill

ps I've really enjoyed all the momentum around the S15, I've never really watched something like it take shape.

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#5 Post by Jerry » Fri Jan 04, 2013 3:35 pm

Nothing wrong with a gunter, except how do you reef it w/o going on the foredeck?

If you use an aluminum pipe for a mast, unsupported, it will do most of the bending near the deck. A carbon mast would be tapered and eliminate this problem, and reduce weight and windage up high.

Consider making it yourself. About 50% woven cloth and 50% unidirectional is close enough to ideal- that would give you about 75% linear and 25% for hoop strength. I don't know what is ideal; it seems to vary with who you talk to, but I have no doubt that 3-1 is close enough for Govt. work.
--
Jerry Montgomery - Designer of the Sage 17
Founder - Montgomery Marine Products (M-dinks, M15, M17 and M23)
Montgomery Rigging - http://jerrymontgomery.org/

bill
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Joined: Tue Mar 20, 2012 11:06 am

#6 Post by bill » Fri Jan 04, 2013 10:18 pm

Reefing...that's easy, I'll just wait and see what you do with the S15.

I had considered a CF mast for another project, it intrigues the hell out of me. My Hobie AI has a CF mast and a furling main, pretty cool. In my perfect world I'd adapt the furling main to this project, but the four sided sail looks way better to my eyes on the M boat.

Do you have any resources you'd be willing to point me at for spar design decisions you respect? The carbon info I have is pretty random and ****py, not exactly science.

The last time I talked myself out of making a CF mast it was because I read somewhere that wall thickness ought to be about the same as an aluminum mast (which sounds like an amazing oversimplification) but my own calculations are way more encouraging. I think I just have to build a sample and break it.
Jerry wrote:Nothing wrong with a gunter, except how do you reef it w/o going on the foredeck?

If you use an aluminum pipe for a mast, unsupported, it will do most of the bending near the deck. A carbon mast would be tapered and eliminate this problem, and reduce weight and windage up high.

Consider making it yourself. About 50% woven cloth and 50% unidirectional is close enough to ideal- that would give you about 75% linear and 25% for hoop strength. I don't know what is ideal; it seems to vary with who you talk to, but I have no doubt that 3-1 is close enough for Govt. work.

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#7 Post by Jerry » Sun Jan 06, 2013 6:29 pm

I dont claim to know what I'm talking about, but I think you'd have to either compression mold or vacuum mold the mast. I racked my little brain about how to mold a mast in two pieces (pt and stbd) and I think it could e done but it would take some serious tooling work. If I were to get serious about it I'd spend some time on Google and see what I could dig up regarding what others are doing and hopefully eliminate part of the learning curve.

The carbon surfskiis that I've designed and done much of the engineering on essentially become tubes; the deck is highly crowned and the hull is pretty semicircular. We 'deck" them in the molds; the hull and deck molds are keyed together and bedded in an interior flange. They almost never fail, and you wouldn't believe the beating they can take. They weigh less than 25 lbs and the really good guys race them inter-island in HI, and it's nothing to race across San Francisco Bay. I think the same method would work for a foil-shaped sailboat mast, but the cost would be really hi- very labor intensive. An alumimun mast would be way cheaper, way tougher, and probably as effective. Getting rid of the shrouds would create other problems.

If you do go for alauminum, be sure that it's 6061 T6
--
Jerry Montgomery - Designer of the Sage 17
Founder - Montgomery Marine Products (M-dinks, M15, M17 and M23)
Montgomery Rigging - http://jerrymontgomery.org/

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