Simple Question: Why a Proa?

Click for rotating view! (253K) At sea it quickly becomes apparent that no other single factor contributes more to safety, speed, and comfort than waterline length.

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)."
August 1999, Emails to the Multihull Mail List (MhMl) from Joseph Oster


What keeps it from tipping over?
Stability Curve
Stability Curve Stability Curve

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.

Marshal Isles Proas (270K)
Marshal Isles Proas (270K)

The history of "ancient" proas, however, developed in the Pacific islands from paddling outrigger canoes, shows precisely the opposite path of development and the reasons are simple and clear:

  1. As the sail is loaded up, regardless of which hull the rig is on (because it makes little difference), weight is transferred to the leeward hull, which must carry more of the total displacement, up to 100%. If the heavier hull is to windward (WTW), then both hulls need to be big enough to carry substantial displacement, one at rest and in light air, the other powered up under sail. That suggests hulls of equal volume for WTW proas, like Cheers, and catamarans.

    Cheers
     
    If the heavier hull is to leeward (WTL), then it alone must be able to carry the total boat weight at speed while the windward lifting "outrigger" carries only 1/3rd or less of total boat displacement. This suggests unequal hulls, a low windage hull to windward, and that the leeward hull on a WTL proa will be larger and better able to carry weight than a WTW proa of the same displacement.
     

  2. photo by Gary Dierking
    Marshall Islands Proa
  3. With the heavier hull to windward (WTW), it never comes close to flying a hull, so two hulls are always being dragged through the water. This creates weather helm and increases wetted surface because both hulls are displacing substantial volume.

    A WTL proa, on the other hand, starts with the majority of its displacement already in the bigger leeward hull, and is designed to shift up to 100% onto that same hull, as the lighter windward hull lifts. One hull instead of two gives better surface to volume ratio, less surface area and drag, reduced weather helm. Even when sailed conservatively, at 50% of maximum stability, still far from flying the ama, there is just ~12.5% of total boat displacement being dragged through the water in that windward hull.

    NOTE: crew movement on small, light proas can significantly affect WTL vs. WTW.
     


  4. Hawaiian Sailing Canoes
  5. When pressed for maximum speed, the greater lateral stability of the WTW proa (the ability to resist tipping over sideways) actually becomes a vulnerability, shared with extreme racing catamarans and trimarans. When it exceeds the longitudinal stability of the leeward hull, the boat will bury the leeward bow and pitchpole before lifting the windward hull.
     
    By contrast, the WTL Pacific proa has greater longitudinal stability than lateral stability, so before it pitchpoles, it rolls gently onto the leeward pod, like the "safety amas" on Hawaiian sailing canoes.
     


  6. angular momentum
  7. Connecting beam and rig loads will be greater on WTW proas when ~50% displacement per hull or more may be common under sail vs. only 25% or less on the WTL small hull. In part, this is due to greater moment of inertia of the heavier windward hull, which adds stress in all directions by "wanting to move" independently of the leeward hull. It makes the boat heel over less quickly in a gust, which increases beam and rig loads. WTL proas are more like monohulls in this regard, as they absorb gusts by rolling safely (warning the crew!). Less distance between hulls (shorter beams) helps reduce stress but, of course, reduces righting moment too.
     
    Also, imagine two proas sailing such that the WTL proa flies its outrigger (25% of boat displacement); when the WTW proa lifts 25% of its displacement off its windward hull, it will still be dragging more than that in the water, to weather.
     

Summary:

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:

  • On a cruising Pacific proa, flying the small hull when there is a leeward pod is not as dangerous or as easy as it may seem; the pod is very forgiving and angular momentum goes up in larger boats so they rotate more slowly in response to wind gusts.
  • When both hulls must be capable of carrying 50% displacement or more, the catamaran has a list of advantages and has proven to be the best path for quickly carrying the most load in the least length. Cruising catamarans never fly a hull, they don't need more than 50% displacement in each hull and they don't get safer or faster by adding extra WTW.
  • The Pacific proa (WTL) gets advantages in speed and seaworthiness (longitudinal stability) from the extra length of its larger leeward hull and less massive, "just enough weight", small hull to windward - without those factors, the gains are lost. Flying a hull isn't necessary and is easily avoided, but if being able to do so in ideal conditions doesn't excite you, don't get a proa!

How does a Proa compare to a Catamaran?
See separate page for discussion of Proa compared to Catamaran
Click for rotating view! (253K)

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Also: Jim Antrim, Naval Architect

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