Gunboat vs Atlantic on Sailing Anarchy

[Sand crab]

There is a current interesting and some times heated thread on the Sailing Anarchy forum about the relative merits of the Atlantic 55/57 vs the Gunboat 62/66. What is the really interesting part is there are numerous posts by Peter Johnstone the creator of the Gunboats as well as comments by Ian Farrier. There are also those really cool vids of the Atlantics and GBs that I can't seem to get enough of. It's a good read. BOB
Chris White speaks up - Sailing Anarchy Forums

[David M]

In reference to the first post..

A hull lifting out of the water for a catamaran is the point of maximum righting moment. Any vessels overall stability is the area with positive GZ under the stability curve.

Typical stability curve from a monohull.....


Typical stability curve for a catamaran...



The area for a catamaran is shifted left meaning the strongest righting moment starts occurring sooner than a for a monohull. This does not mean a catamaran is more or less stable, it just means its maximum ability to right itself starts occurring sooner.

The premise of the OP in the SA thread is wrong...you still have righting moment after the vessel has reached maximum GZ. Monohulls must heel more before reaching their maximum GZ.

[Cotemar]

Sand crab,
I love threads like that. Chris & Peter just design for two different sailors, so its nice to see them banter about why they designed boats the way they did.

[tsmwebb]

Originally Posted by David M

In reference to the first post..

A hull lifting out of the water for a catamaran is the point of maximum righting moment.

...

The premise of the OP in the SA thread is wrong...you still have righting moment after the vessel has reached maximum GZ. Monohulls must heel more before reaching their maximum GZ.

I beg to differ. The premise is correct. At some point very near to the hull lifting completely out of the water the max RM of a typical catamaran will be attained. Cats tend to reach max RM at relatively low heel angles. With cats it is important to note that the heeling moment from wind will tend to decrease less quickly than the RM decreases beyond the point of max RM. So, typically a cat sailed to its max rm will capsize if the crew does not take some action. This seems like sailing on the edge to me and is quite different than the keel yacht experience at max RM.

Some numbers can be found in the Deakin paper. They are discussing both tris and cats. Cats generally reach RM at lower heel angles than tris so this applies even more to them.

The wind heeling moment reduces with heel angle at a lower rate than the righting
moment of a typical multihull. Their range of stability normally is about 60 to 80
degrees, and the wind heeling moment remains significant at 90 degrees. The wind
heeling moment is therefore greater than the righting moment at large angles of heel,
even at low wind speeds, and this fact may have implications for multihulls heeled to
large angles by wave action. In this respect multihulls differ from ballasted monohulls
which, when at 90 degrees, have positive righting moments but negligible wind heeling
moments.

Tom.

[tsmwebb]

Originally Posted by Cotemar

Sand crab,
I love threads like that. Chris & Peter just design for two different sailors, so its nice to see them banter about why they designed boats the way they did.

For the record I don't think CW participated in the discussion. Some quotes selected from his site were used to motivate rebuttals mostly from PJ but the result was hardly a balanced or complete discussion IMO.

Tom.

[Sand crab]

Originally Posted by tsmwebb

For the record I don't think CW participated in the discussion. Some quotes selected from his site were used to motivate rebuttals mostly from PJ but the result was hardly a balanced or complete discussion IMO.

Tom.

I agree. Maybe CWs absence was a prudent decision. You know, not to stir the pot. BTW, I am not taking sides. BOB