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  #11  
Old 04-11-2014
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Re: Rigging question

Quote:
Originally Posted by zz4gta View Post
Please provide some data to back up that statement.
Without going into the engineering this is a quote from Eric Sponberg who is one of the premier composite mast engineers in the world, a forensic mast expert and a pretty well known NA otherwise.

Quote:
Originally Posted by Eric Sponberg View Post
...whether a mast is stepped on deck or on the keel is immaterial to the material used to build it. A keel-stepped mast is always stronger than a deck-stepped mast, on the order of +50%, because of the fixity provided by the deck partners. Therefore, if it is inherently stronger, it may also be a slightly smaller cross-section, and therefore lighter, than a deck-stepped mast. Although, a true detailed comparison may need to be made because the keel-stepped mast is longer.


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Old 04-11-2014
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Re: Rigging question

I think 50 is quite a stretch

the point about failure is the same however...if all lowers are snapped then the mast will buckle no matter if its keel stepped or deck stepped

I do prefer keel stepped though...
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Re: Rigging question

ps composite as in carbon fiber?
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Re: Rigging question

Carbon fiber, aluminum and fiberglass. I don't want to get into someone else's vc, but he has done a lot of pretty well known boats and is the go to guy for free standing/unstayed masts. The Freedom and Nunsuch lines are his (I believe). He is a pretty well regarded NA who specializes in mast designs.

Is his 50% number exact? No probably not it was a response to the relative strength of deck vs keel stepped, not intended to be an exact number. Wether it is 47% stronger, or 52% stronger, doesn't matter much for general purposes, but if one of the premier mast designers in the world says it's roughly 50% stronger, absent strong evidence to the contrary that's the number I am going with.
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Re: Rigging question

gotcha unstayed is a whole nother shebang...you are absolutely right there and quite a different subject than standard rig talk

but for a simple keel stepped versus deckstepped on a stayed mast and boat id say there is very little difference in stregth in complete working rig...

like mentioned before stronger or not a keel stepped mast will buckle just as easily if unsupported when losing lower or intermediates on a double spreader rig as on a deck stepped mast

totally different animales here

not to mention nonsuch and unstayed rigs use round an tapered masts like windsurf masts
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Re: Rigging question

Christian,

Assuming everything else is the same, a keel steep gains the lateral support of the partners, switching the entire design from a cantilever to a stayed beam. Deck stepped masts are much much weaker. Of course a mast only need to be strong enough, which is why in practice both work fine, and have different trade offs.

But if you keep the same extrusion, and the same rigging, a deck step will be substantially weaker than keel step.
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Re: Rigging question

stumble while I agree on some points with you it means little when you have a failure that will make you lose tension on one side hence buckling the mast

the mast will go down if this happens keel stepped or not

where it fails changes

and I also dont beleive that deck stepped masts(the mast itself) is much much weaker

that is a false statement

the partners like you say add stiffness and keel stepped masts act much more in unison with the whole boat if you will especially if tangs are used at the deck entrance to andf from the mast to other points of the hull

I guess we see things a bit differently

again I prefer keel stepped....I also think keel stepped masts suffer less from oil canning hulls versus deck stepped

especially if the support beam on said boats has failed or sagged or detached from the underside of the deck mast step which is common on many old boats

anywhoo

cheers

oh and just to play devils advocate deck step masts do have their pros...tabernacling them is one and also in the off chance you lose both lowers and uppers on side by mistake, failure, etc it is possible that the mast simply lean over into the water and have much less damage

when a keel step mast fails damage usually is catastrophic.

ever see a hobie mast fail?

it just plops over to the side and bam into the water tie it up again and your good to go....

wish it were that easy on bigger boats
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Last edited by christian.hess; 04-11-2014 at 07:20 PM.
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Old 04-14-2014
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Re: Rigging question

How much additional support do you think the partners add? Lets say there's 7' below decks, you're really only getting 7' above decks. I don't consider that 50% more. Also the support would be less the longer the stick is.

Consider the source, was that article you quoted defending the use of unstayed/freestanding rig design? Or was it a direct comparison between stayed deck stepped and keel stepped rigs?

I don't know for sure, but I'm having a real hard time wrapping my brain around how the partners of a modern rig adds 50% to the strength of a stayed mast. Sorry, one selected quote isn't going to sell me on that idea.
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Re: Rigging question

Thanks for the input on the repair. I am working on it and not sailing in meantime :-(

The mast is deck stepped and too long to come down in the slip. So my plan at this point is c-clamp the tang back into position and replace all 5 rivets on topside of tang (above spreader). Only two of the five appear broken but all will be drilled and replaced. I will then have rigging checked before sailing again.
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