Please Sanity Check My Catamaran Design

All those pictured catamarans are shown sailing a hull. What percentage of the time are they operating in that mode? For the rest of the time their two hulls probably aren’t so deeply immersed as with the single hull, so how deeply are their transoms immersed then, if at all?

 

Re-read post #42.
All the pictured cats are day sailors.
Crew weight shift easily shifts the transom from full out of the water to fully in the water.

I light air you wouldn't want the transom sunk. In heavy air you do want it.
So you are asking the wrong question.

The transom is sunk as much time as you the skipper want it to be, except in heavy air it probably is a safety issue if you don't have it submerged (cause that means the bow is buried deep which gives you little margin of error when a big gust comes).

 

It's a Decision 35, the ultra-light one design catamaran of the Swiss lakes. The class was created to replace the 40s (many of them former Formula 40s) after many of them were destroyed in a storm that struck them in the middle of a major race.

The D35 boats | D35 Trophy https://d35trophy.com/en/trophy/decision-35/

 

The amount of time they are flying a hull depends on the wind strength of the area in which they are sailing, so no one can give you a percentage.

Almost any cat stern will immerse when the boat is being pressed hard and more weight is being transferred to the lee hull, even if the weather hull is not flying. If the lee hull is out of the water when being pressed hard, then it would have to be a long way above the water in other conditions and that would be wasted length and weight. Even something as buoyant as the Lagoon 380 will push the lee hull down far enough for water to wash over the lower transom step(s) at times.

Sure, you don't want a vast, deeply immersed and wide transom in a big boat where you can't use crew trim to lift the stern in light as as Upchurch notes. Many people prefer enough rocker to ensure that the transom is at the waterline in light airs when the boat is moving slower and both hulls are sharing the load pretty equally, and that's got many advantages. We've also known for decades that at higher speeds a transom stern is faster even for narrow hulls (look at modern destroyers, for instance) and therefore designing the transom to be at the waterline in light airs and immerse when the boat is moving at speed and the lee hull is carrying more of the displacement is sensible.

Personally, I don't like deep transoms because I don't like the sound of the little "feather" wake they drag in light winds. But given the high performance of many cats with transoms that immerse, it is apparent that claiming that you should never choose that option is way over the top.

 

Knock yourselves out.
Submerged transoms give higher speeds? https://www.boatdesign.net/threads/submerged-transoms-give-higher-speeds.22541/page-2

 

Sigh. There wasn't a dragging transom in any of those pictures, not at those speeds. And, in light airs - every one of those sailors would be moving the weight forard to keep the stern pout of the water.

And not one of those hulls was a full displacement hull like what I spoke about.

 

Sure - thats why racing shells are the most efficient displacement hulls in the world, but you would know better.

 

I went back to check.
Watson did say "displacement" catamarans. I personally don't know what that means, when you compare "most" cats to displacement monohulls.

But typically, we are talking about two different things and arguing.
Waste of time.

But I appreciate seeing the Decision 35 pictures!

 

Watson said "No sailing catamaran of any type leaves a flat transom in the water. Its just so obvious how badly flat immersed transoms drag, and no one EVER should choose that option"..... no square transom is underwater on sailing cats !!!! Plenty of sugar scoop upper transoms - none in the water flow".

"Of any type", "no", "none" and "no one EVER" would seem to be pretty clear. It wasn't claimed that "no displacement sailing catamaran leaves a flat transom in the water" or anything like that, and like rdreuban I'm left wondering about what a "displacement" catamaran is and, since we are talking about a very short cat design, why the beach cats are not extremely relevant.

Rowing shells have different requirements. They operate at a fixed speed, in flat water, in a straight line, and without varying effective displacement. And it may be significant that no sailing craft with a canoe stern is really known for outstanding light-wind speed. International Canoes, for example, are at their worst in light winds despite their canoe stern, probably because the canoe stern represents extra wetted surface area.

As noted before, designers tried double ended (or effectively double ended) hulls in the Australis A Class, the early Quest series C Class, the Hobie 16 and other cats of the '60s. They then gave them up. The world's best small racing cat designers are not idiots, they use transom sterns because they are faster. Can one imagine the Cunninghams, for example, saying "hey, we tried the canoe stern for years but now let's put on transoms for our Little America's Cup winning cats because transoms are slower"?

Either one can believe people who have spent their working lives in this area, or people who have not and have no track record in it. That seems to be an easy choice.

 

Sailing cat hulls are all displacement, they exceed displacement speed because of the fineness of the LB ratio being above 8:1
Flat transoms are draggy but only at very low speeds.
Rowing shells operate at displacement speed and don’t have the power available to make a flat transom work.

 

From Guillaume Verdier:

 

I guess my point is, what operating condition is the design being optimized for? If its target mode is riding on one hull, then decisions like transom configuration and rocker and so forth should be made with that end in mind. I don’t have an opinion over whether the transom should be flat or whatever. I would expect that could be derived from empirical evidence that already exists.

 

Those are not short length/high displacement catamarans like the OP proposes

 

That is not a short length/high displacement catamaran; just the opposite.