<<why did both BMW-Oracle and Alinghi have bermudan rigs in
the last america's cup?>>
That's a good question.
My guess is that one runs into materials limitations at that
size. A wood mast in some ways is superior to other materials;
spruce as just about an infiinite life cycle in terms of fatigue.
But after a certain point wood is too heavy for a given strength.
Then it goes to aluminum. Then carbon.
It's hard to imagine the stresses in a 180' unstayed mast, and
harder still to think of what the mast bearings would be doing under
loads generated by that mast. The vaka is pretty short as well, so
there wouldn't be a whole lot of mast bury.
Perhaps it's tough to scale past a certain point.
Also, there's no shortage of expert crew on an America's cup maxi,
so some of the benefits of the unarig would be less important.
That rigid wing sail could also come into play. It's wonderfully
efficient, and even outsailed Alinghi upwind without the help a
jib. But such a sail won't work in every wind speed, and it's
definitely not easily reefed. The only way to add extra sail area,
or take it away, would be to furl and unfurl headsails.
<<not trying to be sarcastic or difficult here. really am just
curious to understand how it all works.>>
No sarcasm inferred. It's a tough question and everyone has an
opinion.
It still comes down to what you're doing and why you're doing it.
Let's say that you want to compete solo in the transpac. Due to
large swells, mast height really makes the difference between
grabbing some useful wind and getting none. Being able to throw a
lot of area up high, with an unstayed mast that will bend off in
gusts, and which would easily depower with a single sheet release,
would be very important to a solo competitor who wants to place
well.
The una also lets you skip the jib boom, get rid of worries about
headstay tension, running backstays, mast compression, and
eliminates the cost and complexity of a second sail.
But if you want a shorter mast, lower COE, and the lower sheet
loads and smoother jibes of a semi-balanced sail, an easyrig starts
to shine.
And if you have a team of fit experts who know just which strings
to pull and when, a stayed bermuda rig has a lot to offer.
<<sure would be nice to have a two boat campaign to figure it
all out.>>
Or... One boat with two masts!
I think that both Doug and Rob have proas that can even switch
between una, easyrig, and schooner rigs. Provided they would want
to, of course.
Doug will even sell you his for a reasonable price.
- Mike
bjarthur123 wrote:
i understand that it's about L/D upwind, and L downwind.
i've been told, here and elsewhere, that L/D is better for a unarig than main+jib. can't say i have a firm intuitive grasp as to why though.
given these bits as true, why did both BMW-Oracle and Alinghi have bermudan rigs in the last america's cup? the only rule was 90 ft long to my knowledge. would they have been faster with unarigs?
my guess is that the mast is heavier aloft on a stayed unarig b/c it's longer for the same righting moment. better it seems to put sails on both the aft and fore sides of the mast to maximize the sail area to mast weight ratio.
no intuition here either though, just data from what designers have done in races without rules. participants in "the race" all had bermudans as well, except for team phillips. cat boats are extraordinarily rare. why?
not trying to be sarcastic or difficult here. really am just curious to understand how it all works.
on an unstayed rig it might all be different. the added compression from the forestay calls for a heavier mast. fore boom adds weight. is it enough to offset the benefits? i dunno.
sure would be nice to have a two boat campaign to figure it all out.
ben
--- In harryproa@yahoogroups.com.au, Malcolm Phillips <malcolmdphillips@...> wrote:
> I would not assume that the slot effect is 1+1=3.
> The slot effect is a high lift device, but induced drag is increased far
> more than lift.
> For max speed a high L/D ratio (of sail plus boat structure) is more
> important than high lift.
> Its a cost trade-off. The slot effect gets cheaper lift.