John,
It sounds like some of the items that are priorities for me
wouldn't be a priority for you. I'd argue that any safety margin,
such as capsize recovery and reduction in heeling moment as you
heel, are as important for cruisers as for racers. But if you're
intentionally way undercanvassed, it could be a non-issue.
<<Fore and aft loads - good point. How much torsion is there
on the beams in a cruiser due to the water drag forces on the
windward hull in a rig to leeward>>
I don't imagine that drag adds much torsion. The larger issue,
probably in terms of several magnitudes, is that the rigs are going
to deliver a pitchpoling force that wants to drive the bows down,
and with the bows in the opposite hull, being driven up by flotation
and/or wave action, tons of of diagonal forces will need to be
transferred from one hull to the other.
I'm just a fan of designing forces out of the equation, and have
been since my engineering days in college. Thus, I like the
harryproa downspiral design: Eliminating torsional (forces in line)
and compression forces (unstayed mast) means less weight, which
means less sail, which means lower forces, which means a lighter
structure, which means less sail, and so forth.
At the same time, less carbon and epoxy mean a lower cost, as does
skipping additional structures such as crew pods.
But there's no reason to be dogmatic. If you don't care about the
weight, you don't care about the weight. It's not the end of the
world. There are plenty of catamarans and trimarans that deal with
larger forces using whatever additional structure is required.
While I can't see myself building an atlantic proa, I can definitely
see the allure of having both the sail controls *and* the sails in
the cockpit.
One question is whether or not moving the rigs to the ww hull, and
thereby incurring these larger forces and structural weight, is the
most efficient way of dealing with any tendency to luff.
I'd argue that spreading the rudders as far apart as is practical
would do the same thing more safely. And if you want additional
leeway prevention while reducing steering loads, throw in a central
daggerboard/leeboard that kicks up in both directions. I'm not sure
it's necessary if your rudders are deep enough, but it's certainly
an option. (unless you're almost always sailing in shallow water
that doesn't allow enough foil depth to get the lift you need).
I can't prove that, though. Any boats in the water are worth much
more than my theories, and I'd love to see an Atlantic design with
unstayed masts.
- Mike
On 11/10/2010 1:19 PM, John wrote:
Thanks for the reply Mike. May I continue with the conversation:
I am not sure just how great is the capsize problem for a cruiser. Multihull cruiser capsizes are extremely rare - when a cruiser goes over it is usually because the crew are pushing it too far, i.e. racing. Agreed that rig to windward adds a hazard but on top of a very small risk I think.
Reduced sheeting options is a good point. This would be helped using a windward pod for the crew but does not give as many options as rig to leeward.
Increased windage as you heel I agree - adds to the problem at just the wrong moment! Again this might be a small risk with a weighty cruiser.
A bulkier hull at the mast position would add weight, yes. How much can the support for the mast be taken by the beam itself in a schooner rig in which the masts are aligned with the beams?
Accommodation loss - maybe single berths where the masts are in a schooner rig - or the heads maybe. So even a schooner rig would compromise accommodation.
Fore and aft loads - good point. How much torsion is there on the beams in a cruiser due to the water drag forces on the windward hull in a rig to leeward? Probably less than the reverse but still not inconsiderable?
A benefit of rig to windward would be the balancing of the luffing component of the rig being off-set by the bearing off component due to the drag of the leeward hull? This aspect would be an attraction as it helps to solve a number of problems.
--- In harryproa@yahoogroups.com.au, Mike Crawford <mcrawf@...> wrote:
>
>
> I'd say there are several disadvantages.
>
> - As you point out, masts can't help prevent a capsize. Once you're
> over, you're over. With the rig(s) in the lw hull, you might have the
> chance to pop back up if things get pushed too far.
>
> - The shorter ww hull provides fewer options for sheeting angles.
> With rigs in the lw hull, you can sheet out further to the bows and/or
> back to the ww hull.
>
> - As you fly a hull, the sails raise higher into the air, getting more
> wind high up, and adding their area to the area of the tramps/deck
> that's catching wind. With rigs in the lw hull, the sails lower as the
> boat flies a hull, and the tramps/deck blanket the rigs.
>
> - The ww hull has to be a lot stronger to handle the structural loads,
> adding weight to the boat.
>
> - The ww hull would also lose some accommodation due to the masts.//
>
> - The fore/aft rig loads and fore/aft hull loads would be on different
> hulls, requiring the beams to carry a lot of torsion/wracking loads,
> requiring stronger beams and attachment points, adding weight to the
> boat. With rigs in the lw hull, the wind and water loads all line up on
> the same axis.
>
> Mike