|Simple Question: Why a Proa?|
The unique weight-distribution characteristics of the Pacific proa with the main hull to leeward and
smaller, lighter hull to weather, give it the
longest possible waterline for any given weight and rig!
Compared to similar catamarans, trimarans, and monohulls, the Pacific proa configuration stretches the same materials, rig and associated costs into a sailboat that is 64% longer!!!
The typical Pacific proa rig would be considered conservative in sail area for other boats the same length, yet is adequate for routine high speeds, occasionally on a single hull flying the small ama to weather.
For example, this 21 meter (69 feet) design has accommodation, displacement, sail area and cost very similar to the
Venezia 42, Catana 43 or F-41 catamarans.
For blue water cruising, the extra twenty-seven feet of waterline(!) on the main hull of the Pacific proa handles 75% to 100% of vessel displacement with ease, resulting in a swifter and more seaworthy sailing yacht.
"You need a precise amount of righting moment for a given rig and wind conditions and a precise amount of power to drive a given hull shape at a given speed. Cruising catamarans normally have loads of extra righting moment to spare. It makes more sense to me to reduce the size, windage and weight of the weather hull to the minimum actually necessary while extending the length of the leeward (main) hull for higher length/beam ratio and prismatic coefficient (required to compensate for the double ended proa hull which is a distinct departure from most cruising cats that have broad, beamy transoms to reduce pitching and carry the displacement)."
|What keeps it from tipping over?|
It takes some skill and effort to fly the ama, it doesn't usually happen by accident. Careful steering and sail trim are then essential, of course. Even so, distractions or wind gusts can and will happen. Suffering a knockdown when sailing at speed isn't such a big deal as the boat settles easily into a position of increasing stability between 45 and 60 degrees of heel. Simply easing the mainsheet drops the ama back in the water.
45 degrees of heel
The "leeward pod" is designed to prevent the yacht from heeling over too far, providing additional buoyancy beyond thirty-five degrees of heel. If the yacht is knocked over to a sixty degree angle of heel, it becomes as difficult to tip further as it originally was when the ama was in the water, so soon falls back "on its feet"!
The height of the pod is a critical factor in maximizing stability because at high degrees of heel, the pod becomes the leeward hull (CB) while the boat's center of gravity (CG, ~7.5 tons) hangs out to windward (~7 feet?), below the mast - ~104K ft-lbs of reserve righting moment.
In other words, the closer the proa comes to 90 degrees of heel, the more critical is the horizontal position of CG, relative to the pod's CB. The greater that separation (at 90 degrees), the greater the safety factor. This absolutely depends on a low CG relative to the pod's CB when the boat is horizontal.
A graceful pod shape that settles into the water easily, surfs well and carries full displacement at maximum heeling angles(!) are some of the details.
|Why not keep the heavy hull to windward?|
That approach was used by Dick Newick on Cheers and advocated by Joseph Norwood and others.
Marshall Islands Proa
Hawaiian Sailing Canoes
Having "weight to leeward" (WTL) in the big hull and ~30% displacement or less in the windward hull ("flying hull", outrigger or "log") is precisely what defines a "Pacific proa" and gives it certain specific characteristics that are not obtained when 50% or more of the weight is in the windward hull (WTW). So much so that it's not accurate to claim the same benefits for both WTW and WTL proas.
The higher righting moment of a WTW proa is useful only in heavy air and then only to the point the leeward bow isn't driven under. Water ballast can add righting moment to a WTL proa when needed, and then only as much as needed. A fair comparison between WTL vs. WTW proas would use boats of the same weight/cost, including rig, and measure performance over a wide range of conditions. “Same weight/cost” also implies building methods and designs suitable and proven for offshore conditions.
Some believe very firmly that a cruising multihull should never fly a hull and for a proa, that means carrying the heavy hull to windward. There are several answers to this:
|How does a Proa compare to a Catamaran?|
|See separate page for discussion of Proa compared to Catamaran|
|Also: Jim Antrim, Naval Architect|
Links to other Proa Web Sites