Design Considerations and Details
The Pacific proa's accommodation space consists of half the length of the large leeward hull plus the overhangs on each side - the leeward "pod" and "bridge deck" to weather.
To facilitate the primary design goals of speed and seaworthiness offshore, and to handle the extra displacement caused by lifting the small hull, the clearance above the water for the bridge deck
is greater on a WTL Pacific proa than a typical equivalent cruising catamaran.
At the same time, overall height above the water of the enclosed space cannot be increased over the
equivalent cruising catamaran without causing excess windage. These two factors limit the height of the central pilot house roof (at its edges, the top of the windows) to just barely five feet above the bridge deck, the same height that is commonly found on 40' catamarans and adequate for seating. Full standing head room is available only in the main hull.
The galley (in yellow), salon and steering station is located in the center of the main hull where pitching motion is at an absolute minimum.
- Bridge deck / crossbeam clearance is intended to be 3' to 3.5' (easy at the ama end), with some allowance for less near the main hull, but knowing that it will sink ~6" when the ama flies.
- 15" (two 7.5" steps) up from the center galley sole to the bridge deck floor, where the salon seats are located. This difference puts people at eye level with each other, both seated and standing, which is also optimized for the height and position of the surrounding windows.
- Height Overall - the top of the salon roof - has crept up to 9'3" above the waterline in my current model. Will look like a VW beetle if it goes much higher, besides reducing performance.
- Mast Placement - Current model (May, 2018) has it at ~17 degrees to windward from center-line (7.5'), roughly the same relative position as ["proven proas"]. On the roof, not down in the cockpit.
- Displacement is ~7 to 10 tons max. with ~25% on the ama at rest (and hanging in the air when flying!).
From the beginning, there has been tension between two different ways of thinking about the crossbeams.
My "V1" model had beams that are distinct from the pod/shell's leading edge.
All the other versions until 2014, including those at the top of this page, have the beams integral to the leading edge of the pod/shell, per this Antrim concept sketch, which was for a preliminary, relatively cramped racing version, not (yet) a "cruiser":
Sketch by Jim Antrim, 1996
Then a series of sketches done over the last four years (2014..2018), referred to by the Rhino renderings on this page, reverted to the idea of the beams being separate from the shape and length of the pod/shell.
Spring Equinox, March, 2016
The issue with the beams comes down to this question: Can a person pass through them in the main hull with standing headroom?
If that answer is "yes", beams to windward are possible while allowing graceful access forward, then they can be placed closer together and the pod/shell shape less affected by them. That would be GREAT!
If that answer is "no", then having them integral to the leading edge is the only solution, along with pushing them as far forward as possible, with their impassable bulkhead, which produces constraints of its own.
NOTES TO SELF, April/May, 2018
w/ CAD drawings
Questions that keep me from knowing how to finish a credible, detailed model of this 30 year old vision, despite the excellent CAD tools. I just don't have the building or engineering experience to answer them myself.
1) How tall does the 8" thick crossbeam need to be at its outboard end? Currently 16" at 28.5' from main hull center-line, where it is 37" tall.
- a) How much of the trailing edge fairing can be cut away inside the pod/shell area at the main hull?
- b) About that curve?
2) How thick is the bridge deck floor? As drawn in proa_2018_Apr28a.3dm, I have 9.5" under the leeward salon seat and 8" under the windward seat (visible on the 'interior::extend to bottom' layer). This is in accordance with the 15" two-step difference expected between galley floor and salon floor.
3) What holds up the mast?
- a) A transverse beam under the salon table (and mast), between the main hull and the bench seat beam to windward?
- b) A 30' fore/aft truss beam under the mast, inside the pod/shell, 7.5' to windward? Either of these could affect the span remaining for the pod/shell floor, and interior space utilization.
4) How thick is that "bench seat beam" (truss) to windward? It is now 19.5" high at the center, including the seat. ~24' long at its current position, ~14' to windward of main hull center-line. Long due to wide beam separation.
If allowed to follow the cut away pod/shell surface, it would be at a comfortable height for supporting winches at the center. Two concerns, though an uncovered truss structure handles both:
- a) windage
- b) water drainage - if pooped/swamped or otherwise swept by an ocean wave, it must all drain away FAST!
5) Egress - Absurd that I haven't got a clear answer for this, but it's like using the windward cockpit for a main entrance on a Chris White or Gunboat catamaran - without headroom.
- a) Considering that there is a hatch over each of the four berths, one main "hatch" to the cockpit from the salon/galley should be enough. Except the height restriction at this length and the mast location both interfere with a single, central entrance.
- b) Two entrances, so one or the other is always aft, protected from weather. But only ~5' to 6' of head room for 7' inboard until you can step down to galley floor level from the bridge deck. Awkward!
- c) Single entrance, not central...?
- d) hatch details!
Proa compared to Catamaran