Minimum TWA for cats

[shipofools]

Sniped from a article by John Shuttleworth.

"Total potential loss of windward ability due to Drag.
Theoretical calculations of the aero, and hydrodynamic drag of various types of multihull shows the following effect of keel efficiency, weight, sail area, and aerodynamic drag.

25% decrease in keel efficiency = total loss in tacking angle of 5 degrees.

Double hull weight = total loss in tacking angle of 6 degrees.

If we include the lift to drag factors of the sails.... 16% decrease in sail area = total loss in tacking angle of 4 degrees. 35% increase in aerodynamic parasitic drag = total loss in tacking angle of 6 degrees.

The total of all these factors is a loss in tacking angle of 21 degrees.

If we compare an open bridgedeck cat of type 6 to an older type of multihull, this is exactly the sort of difference in pointing ability we observe.

When compared to a design of type 1 or 4 the modern multihull is much more streamlined, about half the weight, has an efficient keel, wide width for high stability and sail carrying power, and larger rig. Where the older designs tacked through 100 degrees, a modern design will be sailing considerably faster and tacking through 80 degrees or less.

In order to achieve large volume in a cruising cat without increasing windage, the hull can be flared above the waterline. This flare can also be used to increase the reserve buoyancy forward, and to control the movement of the center of buoyancy as the boat heels. The same applies to the main hull of a trimaran."

link to full article - Heavy Weather Sailing Aritlce

[44'cruisingcat]

It's pretty simple: if you really must "improve" your TWA, keep sailing the same AWA, but slow the boat down.

For instance, in 15 kts true, at 32' apparent, a 9 knot boatspeed will have you tacking through a little over 100degrees. Slow down to 6kts and you'll tack through less than 90.

[yeloya]

The best I've seen in almost similar conditions (20 kts TW, calm water , max 10-15 cm waves)


Gunboat 64 : AWA 23 , making 11 kts SOG. When giving coule of degrees more, she would easily goes 13-14. VMG probably would be around 27-28 degrees)


Outremer 51 : AWA 27-28, making 8 kts. This boat has a carbon rigging and revolving mast. AWA 30-32, 9-9,5 kts.


Catana 47: AWA 27-28 , 7-8 kts, AWA 30-32, 8-9 kts.


I couldn't figure out the leeway as the distance was pretty short. In a chopy sea, these numbers would deteriorate significantly though.. I wouldn't therefore consider these numbers as a "usual" cruising speeds.


Cheers


Yeloya

[StuM]

Originally Posted by 44'cruisingcat

It's pretty simple: if you really must "improve" your TWA, keep sailing the same AWA, but slow the boat down.

For instance, in 15 kts true, at 32' apparent, a 9 knot boatspeed will have you tacking through a little over 100degrees. Slow down to 6kts and you'll tack through less than 90.

Giving 22 kts AWS in the first case and 20 kts AWS in the second.

Ignoring leeway ( or assuming it is the same in both cases), you have 9 knots @ 50° TWA v 6 knots @ 43° (?) TWA (based on "less than")
Resulting in a VMG to windward of 5.8kts @ 50° v 4.4 kts @ 45°

It's the old story on a cat - crack off a bit and the increased boat speed more than makes up for the loss of angle.

[44'cruisingcat]

Originally Posted by StuM

Giving 22 kts AWS in the first case and 20 kts AWS in the second.

Ignoring leeway ( or assuming it is the same in both cases), you have 9 knots @ 50° TWA v 6 knots @ 43° (?) TWA (based on "less than")
Resulting in a VMG to windward of 5.8kts @ 50° v 4.4 kts @ 45°

It's the old story on a cat - crack off a bit and the increased boat speed more than makes up for the loss of angle.

Yep, just trying to point out that (some) cat's can sail good apparent wind angles, it's their speed that makes their tacking angles look not so great. Sail at monohull speeds, and your track will look just as good as theirs.

But you'll take longer to get upwind.