I've pondered the situation with a cat with the mast in one hull, trying to visualize what happens to stability / capsize resistance. Using the unpaid bill model again, but this time, the vertical center of effort is the top corner. The load resisting initial heeling is suspended beneath the corner opposite the corner where the load is applied in the HP model, thus the length of the vertical arm divided by the length of the horizontal arm divided by the length of the vertical arm multiplied by the weight of the hull gives the force required to lift the hull. So if the vertical center of effort is twice the horizontal distance, then your resistance to heeling is 50% of the weight of the opposite hull.... more or less.
On the other tack, we have a pretty similar situation initially. The lever arm is the same horizontal distance from the pivot axis to the mast base. You are lifting the entire weight of the hull from the base of the mast, but you still are pivoting it about an axis the same distance out. The difference being what was originally pointed out, he center of effort goes up as the boat begins to heel. This means that the boat must be sailed more conservatively on the tack where the mast is to the windward.
This is a good argument for the biplane rig. Masts are shorter, and the windward mast can be reefed down to the degree you are worried about wind capsize, which should not be the case except in very rare situations.
So how much of a liability is a single mast setup on one hull of a non shunting catamaran in real world sailing??? I would say that if someone sails the 13' beam 30' Iroquois catamaran safely as a "blue water cruiser", the 30' long 19' beam single mast X-Cat30 is probably perfectly safe when sailed properly. It's normally only in racing that people even approach flying a hull...........
For your enjoyment, I suggest reading this entertaining article in the May '68 edition of Sports Illustrated, titled Hey Ho and Up She Rises The author is extremely hostile to multihulls in general, but that was the general mood in those days. Things have come a long way since those days. The multihull movement has exploded, and many have proven that multihulls are if anything MORE seaworthy than monohulls. When a ballasted monohull is knocked down or capsizes, the probability of it ending up on the bottom are extremely high. When a multihull capsizes, it cannot normally be righted without outside assistance.......with very few exceptions. It's ability to give before the wind and waves instead of standing on a deep keel and taking a battering seems to be a huge asset. Note the number of boats that went to the bottom during the ill fated Fastnet '79 disaster, and in comparison look at the Queen's Birthday race 15 years later in '94. Not a single multihull was capsized and none would have been lost, except one where the skipper insisted that the rescue ship ram and sink her.... presumably for insurance reasons. Rescue in the Pacific.
I tend to obsess on what can go wrong, and make preparations to prevent things from going wrong. In contrast, many people simply assume that everything will go well. Those people often live from one crisis to the next, while I never seem to walk into a crisis. They say that if you are planning to go to sea, you should not read about marine disasters, sinkings, capsize, drowning, etc....... I take the opposite tack. The more I know about the anatomy of these events, the comfortable I am with that environment, and my preparedness, and it would seem, the less likely I am to end up in a crisis situation.
H.W.
On 06/07/2018 07:25 AM, Doug Haines doha720@yahoo.co.uk [harryproa] wrote:
I think the atlantic proa is going to be the worse case scenario of this heeling versus force of wind on sails situation.
Sails are not blocked at all by hull.Whereas hp sails are covered as hull comes up. So not as much boat being hit by wind.
Bjorn:
For a cat take any unpaid bill laying on your desk in an unopened envelope ;-). Draw a diagonal line from corner to corner. Stand it upright on the short side of the envelope, with the long side pointing up. The long side is the mast. The diagonal line is an imaginary shroud leading all the way from the mast head to the pivot axis. Now pivot the envelope about that corner, and note that the masthead continues to rise relative to the surface of the desk (ocean), until the shroud (diagonal line) is vertical. Now flip the envelope over so the diagonal line cannot be seen for the HP. Now the envelope is stood up again, and the one corner is the pivot axis of the hull with the mast, and the long side above that is the mast. Pivot on the corner again, and you will see that the mast head will immediately begin moving downward slightly as soon as you begin to pivot.
This of course ONLY works with unopened unpaid bills, past due notices, collection agency letters, or court summonses ;-)
H.W.
Rick, you wrote (in another thread):" An interesting feature of the harryproa configuration with regard to heeling is that the heeling moment reduces as the boat heels. A cat with the mast in the middle or on both hulls has initial increase in heeling moment when one of the hull lifts. "
Why does the heeling moment reduce on the harryproa, but increase on the cat?
Posted by: StoneTool <owly@ttc-cmc.net>
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