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post #54 of Old 01-03-2013
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Re: Blue Jacket 40 (new racer/cruiser)

Originally Posted by zz4gta View Post
+1, makes me wonder if the 'experts' have ever sailed a bene First 40.7.

There's a lot more to it than just the numbers Bryce is posting.
Could be.. performance boats tend to carry more sail. On the other hand.. stiffness is not the design criteria for sail area.. rather RM is pretty much the design criteria for sail area. However, to say that a more stiff boat requires more sail area is absurd. Rather a less stiff boat requires more sail to achieve the same drive.

If we look at the Lagoon 380 which has similar weight and LWL as the First, it only has 883 square foot of sail. However, no doubt it is considerably stiffer, yet carries less sail. So stiffness clearly is not the design criteria for sail area.

Originally Posted by PCP View Post
I will not even try to discuss that. Anybody that does not see that is an absurd statement will not deserve my time or effort.


Anyone who thinks otherwise deserves my attention. However, you can easily prove me wrong by producing the RM diagrams for a First 40 and a Catalina 40. It would be interesting to see who is absurd.

From a yacht design book:
"The true reason for giving a crusing yacht more beam: In the first place more initial stability... It is very desirable for comfort that a crusing yacht should be kept as upright as possible.."

For those that don't know the term "initial stability" translates to stiffness at low angles of heel. The end result is that cruising boats are more stiff than performance boats at low angles of heel.

So then the question becomes, why are performance boats more tender at low angle of heel. It seems the performance boat manufacturers do not subscribe to the same design philosophy that Paulo is expecting.

The reason is quite simple, the design goal of a performance boat at low angle of heel is the reduction of drag, not high stiffness.

This intuitively makes sense because low angle of heel with high speed occurs during down wind sailing. Or similar in light wind. You can only get high boat speed in light wing with minimal drag. Minimal drag is light weight and minimal wetted surface. So as expected, performance boats are light weight.

When sailing broad reach with a performance boat, heeling is required to create righting moment. So stiffness only increases as a means to create righting moment at some heeling angle.

So we can have two different boats, Catalina 40 with high initial stiffness to reduce initial heeling angle. Then the First 40 with low initial stiffness for reduced drag at low angles of heel. Then both boats producing the same heeling moment at 15 degrees, where the First may be more stiff.

Probably at 25 degrees both boats have the same stiffness. Whenever we discuss stiffness, we need to be careful how we present it. Given this scenario which boat is more stiff?

Last edited by BryceGTX; 01-03-2013 at 07:07 AM.
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