Pointing ability?

[Hardhead]

I've frequently heard it said that full keel boats, and catamarans, do not point as high as other designs. I've never actually heard why though, from a physics sort of perspective.

It would seem full keel boats and catamaran hulls would almost track a course like they were on rails, and assuming the sail sheeting angle is close enough to the center line, can anyone explain why they don't seem to point as high?

I might be the only one this is not obvious to--

[Montanan]

Your question is specific to pointing ability and keel, but there are so many factors as to performance of pointing ability. Always compromises.
And then there is the difference between VMG and point to a close course, trading off of speed of travel and angle of travel.

The Lift-to-Drag ratio of the sail(s) and the Lift-to-drag ratio of the underwater form.
Speed of course shifts apparent wind and pointing ability.
Sail shape and type.
Foil shape and surface.
Keel and skeg as to leeward slip.
Hull shape

An endless discussion and very vessel specific.

Just remember, Gentlemen don't sail to weather. They instead wait for weather to shift in their favor. I.e., it becomes a wind thing and selecting the best route to your desired destination.

And when one adds in current, water being the second flow not just air, pointing ability becomes an issue of both air and water flow.

References:

http://https://www.yachtingmonthly.c...formance-54322

[Hardhead]

Thanks Montanan. Your answer covers it well. It seems many influences just adding up that trend that way.

I love the look of older full keel boats in particular, but never actually sailed one to judge the relative trade-offs. General pointing ability is something I've often heard with that design.

[thomm225]

Originally Posted by Hardhead

Thanks Montanan. Your answer covers it well. It seems many influences just adding up that trend that way.

I love the look of older full keel boats in particular, but never actually sailed one to judge the relative trade-offs. General pointing ability is something I've often heard with that design.

Older full keel boats say 27' -30' simply put will not point.........unless you call 60-70 degrees off the wind pointing

I had a couple racing beach cats though that would point about 30-35 degrees off the wind

[Montanan]

Hardhead.

Most vessels can point directly into the wind, it is just a matter of turning on the engine.

A favorable attribute of a comparatively wide body catamaran is that one can equip such with a moveable forestay, just shift the tack to leeward and you have effectively tacked the hull towards windward. Think of the tack of the stay being on a traveller. You will lose speed because of the increase vector of angle of the forward force but will gain pointing ability. Useful for gaining flexibility of navigation towards the wind, but no real gain in velocity made good towards wind. But you can hold a course very close to the wind useful for navigating narrow channels, making distance from leeward shore, or clearing a point.
I have rarely seen such feature but it sure does work and is obvious once one understands such mechanism and the physics involved. It does require modification from standard practice of a fixed centerline tack of the forestay sail configuration.

With a catamaran one could equip the vessel with two masts and use forestayed sails only and then shift both forestays on a curved traveler track [like an inverted self tacking rig]. The practice is to bring your boats and sails to a close haul and then tack the boat hull while keeping that favored close haul angle of the foresails by pulling the forestay to windward. The sails see a rather similar apparent wind angle, albeit the apparent wind shifts a bit to lee as the boat slows it forward progress against the wind as the force vector moves to be less usefully applied toward forward propulsion and more towards the lee.

[Dave_S]

Duplicated post

[Dave_S]

Originally Posted by Hardhead

I've frequently heard it said that full keel boats, and catamarans, do not point as high as other designs. I've never actually heard why though, from a physics sort of perspective.

It would seem full keel boats and catamaran hulls would almost track a course like they were on rails, and assuming the sail sheeting angle is close enough to the center line, can anyone explain why they don't seem to point as high?

I might be the only one this is not obvious to--

Just an observation and a great deal of porely thought out guesswork working backwards to support the observation.

Every week I go out racing, we are not talking serious stuff just a few hours around the islands followed by a couple of beers type racing. No motors, no autohelm.

The multihulls consist of mid performance cruisers to flyweight trimarans and everything in-between, some with dagger boards some with keels they all seem to track similarly across the ground (within a few degrees) but the fast boats are going faster (doh) so their apparent wind angle must be a significantly lower number but they are pretty much tracing the same course.

So I'm pretty sure it all comes down to drag, the same sails at the same angle of attack with the same apparent wind will produce the same power in the same direction regardless of what boat they are attached to so it must come down to the boat.

Maybe

If your boat slides nicely through the water the same power from the sails pulls the boat along faster which increases the aws which allows you to run flatter sails which allows you to point the sail closer to the wind while maintaining attahed airflow. If your boat has more drag underwater and can't go as fast you will have less aws then you need more curve in the sail to produce the hp. More curvature means you have to bear away from the apparent wind to keep the air flow attached.

IF the amount you need to bear away from the apparent to compensate for the lower speed is evenly proportional to the change in speed then the twa should be the same for both boats. If the twa is the same the cog should be the same +/- current.

[Paul L]

Think of the foil created by a deep thin keel and spade rudder vs the foil created by a full keel and a barndoor rudder. Now take these two different foil designs and put them as the wings and tail of an airplane. Which one do you think will actually be able to take off and stay airborne?

[Cheechako]

Airplanes dont use spade rudders. :>)

[Sailmonkey]

Originally Posted by Hardhead

Thanks Montanan. Your answer covers it well. It seems many influences just adding up that trend that way.

I love the look of older full keel boats in particular, but never actually sailed one to judge the relative trade-offs. General pointing ability is something I've often heard with that design.

For a relative look at how a small full keel boat fares in an upwind race, go to the 0:54 mark in this video. Pay attention to the relative tracks of the other (exclusively fin keel) that whipped us around the course.

https://www.youtube.com/watch?v=qk8VRBpANyA

Keep in mind that all of us were college dingy sailors and two of us sail foiling catamarans when we're not on the lead-mine, we know how to tweak, trim and balance.

[Cheechako]

Originally Posted by Sailmonkey

For a relative look at how a small full keel boat fares in an upwind race, go to the 0:54 mark in this video. Pay attention to the relative tracks of the other (exclusively fin keel) that whipped us around the course.

Keep in mind that all of us were college dingy sailors and two of us sail foiling catamarans when we're not on the lead-mine, we know how to tweak, trim and balance.

Good demonstration, Been there, done that!

[Hardhead]

Everyone - Thanks for all the great replies.


Sailmonkey - that's a great video. I guess I now know why the expression "Gentlemen do not sail to weather" has been around so long... they miss the post race cocktail party.