if this pic shows, it is a better one to illustrate cabin

Howard, I'd like to oblige with a sketch, but my pic copying assistant with her equipment is away at the mo, so let me` see if I can load this pic of the Pahi ama and cabin to help explain.

If the`pic shows, you will see a dodger screen that will be facing to weather. The original Pahi has no cabin, only a long dodger almost the full length of the w/ward hull. Reason why this hull is the ama is because Micronesian star compass navigation conceives the canoe bow facing the rising star horizon and the opposite bow facing the setting star horizon, with the ama being the 'facing side' of the craft and this being to w/ward since it is a shunter.

Now assume we remove the dodger along with one pointy end, which is replaced with a transom stern and rudder, The companionway will have to be in the after part of the cabin (as per usual).This leaves us with bulkheads forward and aft, ending midway between the two hulls, with a fore and aft or longitudinally aligned bulkhead connecting them. The mainmast tabernacle is placed on the inside corner of the cabin (foreward) and is braced by struts to the bulkheads at 45 deg sideways and aft.There needs to be a beefy connecting beam at deck level as well.This provides a rest for a slatted deck on the other side of the craft, alongside the cabin, creating something like half of the structure on the Woods cat you describe.Sure there is less accomodation space, but there is less windage as well.The forward bulkhead could have a fairing panel as part of the connect beam structure.The foremast is now an A structure on the centreline (like the mainmast) rather than stepped in-hull, as suggested before.

Rigged for shunting (as the finished Pahi will be) the masts lean over to the centreline of the lee hull, where they are stayed to the ends, whilst shrouds are taken to a bridle and chainplates at the midship point of the cabin side.... where there is now a access hole while building or fitting out.

Anonymous wrote:

What about a stayed mast?

Howard,

from time to time, I have also been pondering this subject. One thing is how to support a freestanding mast. That may in many cases be solved, somehow. The other challenge is to find some sort of rule-of thumb as for the strength of the mast.

A third question is how to make an automatic sheet-release or sheet-brake device to avoid capsizing or overstressing the mast.

One can quite easily find the max righting moment of a cat by multiplying its displacement with half the beam. However, the resulting righting moment is so far beyond that of a monohull, that it would require an overheavy mast (aluminium), or an over-expensive one (carbon).

Then there is the question of simply staying the mast. There are two arguments against that on a monohull:

·         Since the shrouds has to be from the mast top, the shroud angles will be very fine and the unsupported pole (mast) under compression will be very long. In other words, the shrouds will add stress to, rather than relieve it from the mast.

·         Shrouds are in the way when we want to square out the sail for downwind sailing.

However, on a wide beam catamaran the shroud base will be so wide that the compression stress may be reduced enough to make it work. A bit serious engineering has to be done, but a cat is a result of serious engineering, anyway. A pair of shrouds, set well forward, and a pair of aft shrouds, set as far aft as is practical, is all that is needed. One may even support the mast at the partners and step to reduce the problem with compression loads (reducing the ‘compression length’).

As for sailing downwind, a cat may just as well be broad-reached instead of running straight before (faster). It may therefore not be such a big disadvantage with those aft shrouds (Btw. forget about running backstays)

As for the matter of making a sheet-release devise. I think the simplest and most reliable would be to make a sort of sheet brake or elastic devise, set up to ease the sheet as the load exceeds a preset limit. This should be easy enough to make and adjust.

From the safe position of my armchair  -

cheers,
Arne

Arne, your grasp of  nuanced stay and running shroud use tallies with My experience on a beamy and stiff monohull, where reducing mast height and weight aloft was of primary importance to the stability system, because draught was minimal and CG so close to CB that self righting ability depended very much on the rig being able to float and also to rotate very quickly when lifting to vertical.

Reaching a multihull compromise where the rig need not be designed to survive making contact with the water in knock-down, should be helpful in keeping weight down, without being getting overly costly.

Reducing headstay and shroud tension at the same time as allowing some mast flex by using most of Junk rig  design elements ( all besides squaring on all points)  makes this a possibility.

The ability to rapidly shorten sail and set just enough sail to meet conditions fits, with sailing to avoid heeling( like carrying sail to meet the gusts ) rather than rely on the rig to cope on it's own when over canvassed..... taken to the extreme we then get to the point of it floating when in knockdown and a multi that is light enough to be flipped about and over by the same wind, as well as be too damned expensive - the vicious circle of consequence.

Escape from this circle has been tried using bilateral asymmetry, but still gets caught in the spiral unless water ballast is used, which still bumps up the cost compared to a floating platform that stays down on the surface and hardly heels. or is likely to be blown away.

Such a craft could be rigged with enough flex and give in the rig, to take advantage of Junk design.

Probably starting with transom hung rudders.

Anonymous wrote:

Anonymous wrote:     Fused sheets or slip clutches are clearly called for, also mast failure is preferable to capsize I would think when far out to sea.   The mast itself should be the ultimate "fuse" to protect the life of the crew.

                                                        H.W.

Huh, ya, that would be a reasonable thought. However, assuming a mast that will fail just before a capsize when new, at what point will it fail when two years old or ten... Would it be possible to design a master with a replaceable "break point" for this purpose instead? That is a weak point in the mast. And in such a case what are the chances that only the break point would be damaged in a dismasting, That the mast would float, that the mast would not foul in an unsafe way. I guess that just not capsizing would still be a win anyway. Making the mast to break in a point that allows easiest jury rigging would be my choice... aside from designing a mast to break at all just feels... wrong.

With regard to a single mast in one hull if a cat, I am guessing that there would be some twisting across the whole boat with a tendency to turn toward the unpowered hull. I know with a sidecar on a motorcycle, the sidecar wheel is towed in for better steering. I would think that towing in the unpowered hull might be preferable to extra rudder as well. However that would not be possible (or at least less solid in a shunting design. Maybe this is not an issue with the length of the hulls.

 Good points......... I'm thinking of my experience with machinery.    I like to have a design failure point in systems.  That doesn't mean a "weak" point, just a first point of failure.  Assuming something is going to fail, what will be the easiest to fix, or cause the least expensive damage, etc.   I remember a number of years ago I was working with a farmer on a Harsh brand mixer box.  It kept breaking chains.  My advice to him was to install 4 master links equally spaced around the chain, and file each one slightly to make it the weak link, so if a chain broke, it was a cheap fast fix... just remove the clip and replace the master link.  Instead he wanted to use a heavier chain that wouldn't break, and against my advice went to an 80 chain from a 60.......  that solved the chain problem, but then the result was driveshaft failures..... again I advised the weak link solution, but he went to a heavier driveshaft instead..... that solved the driveshaft problem, but then he tore out the gearbox...... a $1500 gear box instead of a $2 master link!!!  He finally understood what I had been trying to tell  him all along, though at that point I was able to get him to purchase an expensive slip clutch, which he had been unwilling to spend the money for in the beginning.   We kept the heavy chain and driveshaft, and installed a slip clutch instead.   That worked, but he still had to get out of the truck and unplug  things.  

         Keeping the rig fairly conservative, or using good judgment and sailing it conservatively, really should go a long way toward preventing disasters.

                                                                             H.W.

Anonymous wrote:     Fused sheets or slip clutches are clearly called for, also mast failure is preferable to capsize I would think when far out to sea.   The mast itself should be the ultimate "fuse" to protect the life of the crew.

                                                        H.W.

Huh, ya, that would be a reasonable thought. However, assuming a mast that will fail just before a capsize when new, at what point will it fail when two years old or ten... Would it be possible to design a master with a replaceable "break point" for this purpose instead? That is a weak point in the mast. And in such a case what are the chances that only the break point would be damaged in a dismasting, That the mast would float, that the mast would not foul in an unsafe way. I guess that just not capsizing would still be a win anyway. Making the mast to break in a point that allows easiest jury rigging would be my choice... aside from designing a mast to break at all just feels... wrong.

With regard to a single mast in one hull if a cat, I am guessing that there would be some twisting across the whole boat with a tendency to turn toward the unpowered hull. I know with a sidecar on a motorcycle, the sidecar wheel is towed in for better steering. I would think that towing in the unpowered hull might be preferable to extra rudder as well. However that would not be possible (or at least less solid in a shunting design. Maybe this is not an issue with the length of the hulls.