Now a bit of proa promotion...
bgytr (post # 65) asked:
what purpose would there be for building a proa as opposed to a symmetric catamaran for an ocean-going vessel? Is there an operational advantage?
there is in theory a bigger bang for the buck. As someone else pointed out; loads and stresses are important in the cost of the boat. For a catamaran, the compression loads of the mast mean that your main beam must be stronger: more complex structures, more material, longer to build, so more expensive...
A pacific proa has virtually no compression load. If you have shrouds to windward to the ama, you almost achieve perfect tensigrity; everything is in compression or tension (not totally true for the mast). The aerodynamic forces heel the mast, that pull on the shroud that lift the ama, that generate righting moment by having its weight so much to windward... The loads on the beams are minimal.
Because the loads are lower, you can build lighter; or for a given displacement, a longer mainhull, which should lead to higher sustained average speed...
Now what if you are caught aback? You actually tacked, and the wind is coming from the wrong direction. Your structure (and your rigging) must be strong enough to take that "one time load", for sure, or your sheeting system / sail plan set up ensures that you immediately/automatically depower everything in that case.
Regarding the operational advantages/disadvantages...
It is definitely different than on other boats. Your boom(s) swing(s) close to 180° from one tack to the other when close hauled...
You don't make "sharp" 90° turns for tacking; you can have a really long, narrow mainhull with no rocker that tracks like a train.
You can completely depower the rig on ALL point of sail: just let the sheet go! On the leeside, there is no shrouds in the way; you can let the mainsail weather vane as long as you wish. This is also an advantage when something is wrong and you need to "park it". If you have a dagger board in the ama (like Russel Brown's boats for instance), with the board down and the main sheet released, you end up in a "beam to the wind" position with the boat going to nowhere. As long as you are not in breaking seas, that's ok....
So for given displacement, you can end up with a longer boat (better average performance), accomodations will somewhat suffer, a bit in volume, but more so in layout (long narrow hull), unless you go with a Harryproa.
Gaia's Dream after modification:
You can see a shunt there:
That is a the second worst shunting video on the web! Not a very good "proa promotion". 13 minutes into https://www.youtube....tyqI2aJlo#t=825 shows how it should be done. This is the 2 ton 50' harryproa designed 12 years ago to take blind and disabled people sailing in Holland. It has the rudders Laurent referred to, which have since been much improved. Earlier in the video there are some slower shunts, but they are all first time sailors, apart from the skipper.
Beams: You must design the beams for other loads. Including: the ww hull hitting the water after an emergency sheet dump from a near capsize, hitting something solid with one hull, getting caught aback and the "immediately/automatically depower" not working, twisting loads from cross seas, weight on the bridge deck/trampoline, the large shock loads from a partially stayed mast and the also large bending loads from a daggerboard in the windward hull. Once these loads are taken into consideration, a beam with a stayed mast weighs near enough the same as one without. On a pacific proa, the caught aback loads involve lifting the hull with 75% of the boat's weight, so they must be stronger than a harry's which only lifts 60%.
A stayed beam and a partially stayed mast will provide all the worst features of both.
Rockerless hulls are an original harry feature and are better than the conventional rockered proa hulls, but they are not ideal for various reasons and we have made some adjustments, while retaining the ease of build and most of the tracking capability.
A well designed proa lies ahull with the windward hull to windward without a daggerboard and all the strife that occurs when it hits something hard.
Most of the operational benefits described apply to an unstayed mast on any boat. Areas where a proa is different to a symmetric cat are:
1) Because you shunt, you do not have to pass through the eye of the wind. Hence it is a much more controlled operation than tacking or gybing. It is far safer as you can change your mind at any time and do not have to worry about hitting waves and getting caught in irons during a tack or surfing out of control during a gybe. These are heavy weather safety considerations.Shunting is a little slower than tacking, considerably slower than gybing.
2) Apart from sailing down wind, a shunt will put you on your reverse course. This means you get back to a man overboard in seconds rather than minutes. The unstayed mast then allows you to stop on a dime to pick him up.
3) You don't need to change sides. The crew is always on the drier, more comfortable windward side, away from the rig, with better visibility and shelter. This only applies to a harry. Other proas sit the crew to leeward, not sure why.
4) For a given length, a proa will be lighter (ref the 2 ton 50'ter in the video and the half ton 50' racing one in my previous post), so the loads are lower. This makes everything much easier to handle. eg, the race proa only requires one #16 single speed winch.
5) Headsails are a pain as they have to be removed each time you shunt. Harrys get round this with the balanced rig on the above boat, mainsail only schooners or very large mainsails on the racers. These work as the unstayed mast bends in gusts, but the mast engineering to achieve this at the right windspeed is not trivial.
Other than these, a proa is sailed the same as a conventional cat.