Multihull microcruiser design (capsize approach, vol 2)

[magwas]

My takeaway of the previous post was that a proa with a floodable ama could be a solution to right after capsize, if the boat itself is either small, or built from material at least as dense as water. If built from wood, the amount of ballast to be put into the ama would feel inonvenient for me.

So here is another approach. My question is whether it is just crazy, or could actually work? If not, why?

It is a cat with a hullform which works both upward and upside down. The obvious questions are:

1. how to place the hatches to be usable in such a small boat and not sink it upside down?
2. how will we have a mast upside down?
3. same for rudder

For #1 my solution is to have a two-part hatch, the lower one starting above the waterline up to roughly the middle, the top one is from the top down to roughly the middle. They are sealed with rubber in the hinging side, fore and aft such that the rubber is fully stretched when the hatch is open, and tucked inside the boat when closed. (not modeled in the picture).

For #2 the masts are freestanding, bedded in a tube which is open at both the top and bottom of the boat, and the ends of the mast are in the form of the hull missing at those points. There are hauling lines to raise and lower them. This arrangement also allows to lower the mast to the water in foul weather, lovering the point of attack and acting like a drouge.

For #3 the rudder (not modeled) goes all the way up and down to the extents of the hull and have a horizontal edge to put the tiller on in both states.

Here is a model mainly concentrating on the hatch arrangement:

 

[Skip Johnson]

I vote for crazy. An alternate approach would be to have the beams end in real ring frames and then you could just rotate the hulls back upright, far easier perhaps than dealing with double ended masts.

 

[peterbike]

Skip, sounds interesting - although I do not follow/understand ?
Could you elaborate please ?

 

[montero]

@magwas I had a similar idea. Definitely the crazy category.

 

[Skip Johnson]

Skip, sounds interesting - although I do not follow/understand ?
Could you elaborate please ?

Assume hulls with a circular cross section between the beams. Beams ends would terminate with circular rings sized to fit in a slot in the circular hulls so that hulls can rotate 180 degrees. I was going to continue with explanation using port/starboard nomenclature but the imaginary crafts hulls switch port/starboard identities depending on which side is up...... This gets confusing even for a dedicated proafile. I'll whip something up in designspark this evening, I need some practice. A disclaimer; there was no intent for this to be a real solution but I thought it was an interesting thought experiment.

 

[Skip Johnson]

Here tis, rude and crude

 

[magwas]

@magwas I had a similar idea. Definitely the crazy category.

Why doesn't it work?

 

[montero]

In general concept is ok . After capsize without sails , you will open "bottom" hatches and close "deck" hatches .
Steering , sails controls , instruments should be 180 deg reversible . It will be complicated.
Another concern is low crossbeam-surface clerance but it could be solved with high enough hulls.
Hatches should be easy to open from outside yet very secure , it could be tricky.
For me masts and sails are biggest concern during capsize.
What if you will have some sail set .How wil you handle them underwater.
Maybe we need to differ capsize during sailng and storm surviving capsize.

 

[Robert Biegler]

Why doesn't it work?

Yours because
1) Flush hatches that are totally waterproof, and you will always have one set under water, are difficult to make.
2) The easiest part of what you will need to reverse after a capsize is rudders and daggerboards. Tillers and rudder linkages are already more complicated. The rig will have to be very heavy, because the loads of an unstayed mast are higher near the base. But you need both ends strong enough to act as the base. When you reverse the rig, you will need to remove sails, halyards, sheets, reefing lines. Will you dive to do so? How much drag is there from the attachment points that are underwater? What will you do about navigation lights? Antennas?

Skip's proposal I have seen somewhere before, though the beams were to be attached to a hinge in the deck. This scheme would still require that the unstayed masts swing forward enough to clear the hulls and each other as you rotate each hull in turn around the hinge, but at least there wouldn't be any hatches permanently underwater.

I have considered a pantographing catamaran with biplane rig. The important feature would be raking back the axes around which the beams rotate relative to the hulls. That would have two consequences: one, sail pressure would make the leeward hull rotate forward and the windward hull rotate back, just what you want for diagonal stability, and to get positive stagger on the biplane rig. Two, if you let the beams rotate until the hulls are side by side, they would be vertically offset from another. When capsized, and if the masts are intact and buoyant, that would make the boat heel, and it also would be narrow, and that might be enough to make it unstable. The question is whether hull flare might be enough that the boat goes beyond lying on its side.

What really puts me off this idea is that hulls and beams would move relative to each other during the normal operation of the boat, and sooner or later, crew would get squished in between. Of course, you could give up on the diagonal stability and positive stagger, and let the beams only move after a capsize.

What I like better is the notion of applying the same idea to a trimaran, only rake the hinge axes forward, so that if the boat is right side up, the amas move up as they swing back. Use different angles for port and starboard ama, so that one of them swings higher, one of them to a little above deck height, the other well above. After capsize, swing amas back, lift the main hull out of the water. But because one ama moves a larger vertical distance, the boat again ends up heeled and unstable, and will fall over onto side of the ama that is not so high above the deck. Unfold both amas to right the boat the rest of the way, first the upper, then the lower.

Or else, build a narrow catamaran, like the Gougeon G32, but with either a canting (and lifting) keel between the hulls, or one canting keel in the bilge of each hull, like a twin-hulled version of the Defline 19. If Imocas can be righted with a canting keel, so can a catamaran with similar beam to draft ratio and a heavy enough keel bulb.

 

[montero]

From how many feet of length of a cruising catamaran the chance of capsizing decreases to almost zero ?
I know the question is not very precise. Maybe you guys know of a case of the largest catamaran capsizing?

 

[redreuben]

Those hulls with fat midsection and fine ends will hobbyhorse like a mother f*cker

 

[montero]

Skip drew it to show the principle of operation, not as a design of the hull shape . I hope.

 

[Skip Johnson]

Skip drew it to show the principle of operation, not as a design of the hull shape . I hope.

Very, very true ;-) After the fact it occurs that you could in fact make a "reasonable" hull form work by incorporating a half elliptical section for the hull shape but you are still left with the issue of needing a reversible bridge deck . All on all it's an interesting thought experiment but.
1. How often is a 180 degree capsize an issue?
2. in my case the majority of the waters I operate in are too shallow to allow such a capsize.

 

[peterbike]

in my case the majority of the waters I operate in are too shallow to allow such a capsize.

Thanks to all who participated in this thought exercise.
The stumbling block for me are the mast/s. Then I thought once the masts have been done away with, maybe you could use a kite to get moving & then if you are going to use a kite ; do you need special shaped hulls ?
With an inverted cat/tri the deck/beams are going to create extra drag - but at least you would be moving ?
If you had lifting rudders & daggers, they could be used in the inverted position ?
Any idea how fast you would be able to move (with kite) inverted ?

 

[peterbike]

In answer to my own question ;
a. if no bridgedeck - then probably acceptable ?
b. with bridgedeck - probably pretty poorly. ?