Center of effort of the A class catamaran sail

(If this has been discussed before, please forgive and link?) Why does it work so far aft? Even on the classic low riders. The helm stands on or near the daggerboard / front beam upwind, and the CE of the sail looks significantly aft of that point.

Thanks

Paul

 

Hi Paul,
You need to think in 3D.
Picture viewing from above. Draw the force vector on the sail and the leeward keel.
Upright you will see that the mast needs to be a lot further back due to the lateral distance between the keel and sails

 

I can see that downwind, but upwind? Wingmasts tend to pull CE a bit forward, but that much? Henry Bossett and I had a conversation about this few years ago, around how much did I know ( not much), that left me mystified, but that started out concerning an una IC sail I was having him build, and drifted into other realms. I was left with the impression that moving CE aft was an ongoing feast, based on trial and error. Contenders have also done essentially the same thing, only with mast rake. I suppose it could be argued that the ‘real’ CLR could be somewhere between the DB & rudder, but the CLR of hulls is a real thing too, at least in my experience. And then there are cats with jibs, and the main is in a similar longitudinal position.

 

Just draw it.

 

Well that was unhelpful.
If he could have drawn it he probably wouldn't have to have asked the question.

 

I was the only one to reply. I explained where his thoughts were wrong. If he spends 5min trying to draw the vectors then it will become clear.
Why don’t you help before criticizing?
I think I was more helpful than you.

 

I did.

Windsurfing gave me some, granted, intuitive understanding of the dynamics, and whether D1, D2, or longboards, CLR of the hull, not the appendages, seemed to dictate sail (CE) position. From your comments, I’m thinking I don’t understand force vectors of various sails, esp sails that relax leach tension under duress?

 

The reason the CE is pushed aft has nothing to do with the lateral distance between board cl and sail cl.
The reason is that A class hulls are very narrow. When you are reaching, the vector of the sail force is far forward.
This depresses the bow. If you don't want to stuff the bow in the water, you need to get more bouyancy farther forward, and crew weight farther aft.
If you move the mast aft it gives you maximum bow bouyancy, so you can drive the boat harder, something you would want in competition.
The relative position of the sail CE and the hull/ board/ rudder CR stays the same with 2 exceptions. First is the point of sail (tacking, reaching, running) and the change in immersion of the bow.
Sticking the bow into the water means the hull has more effect on CR at the bow.

None of this has anything to do with the question of how does the angle of total force of the sail/ mast change.

You don't need one drawing of the force vectors, you need at least 3. And as a part of defining the force vectors, you need to define sail trim position for each. Form that you need sail force vectors for each sail angle of attack to the wind.

 

The wider the hull spacing the further back the mast needs to be when flying a hull or the upwind daggerboard is raised

 

Thanks Fastwave.

Somehow I was hoping for a discussion something more like this

Where is CLR on foil? | Windsurfing Forums, page 1 - Seabreeze https://www.seabreeze.com.au/forums/Windsurfing/Foiling/Where-is-CLR-on-foil-?page=1

since the setup as far as mast location for a low rider A Class seems much the same as the
Setup for a foiling A class.

I do wonder what the RIB is doing for dynamic balances in your example, as well as centers of weight…..

 

Fastwave,

Nice picture, but under those conditions the boat would be falling off the wind.
The line of action of the sails and the hull have to be in line or you are going to be turning.
Angle of attack of the sail or foil has a great deal to do with where the line of the force is.
With your main along the centerline you are not going to get much progress, anywhere.

You continue to ignore that the forces changing radically with the direction you are sailing.

 

Paul,
As above-mentionned, it is very helpful to draw what it known as a "free body diagram" where forces & moments are balanced for a constant speed (gamma=0, no acceleration)
You can assume that the weight/buoyancy issue is independent of Sail CE/ Centerboard+rudder blade COE
Crew position on the gunwhale (not sure it is the correct word) is balanced by the hull buoyancy for a A-Classic & the Foils lift for a foiler (centerboard foil+ rudder T-foil of course).
You have to consider the moment of the rig too.
Lateral forces from the rig are balanced the same way by the centerboard + rudder or centerboard foil + rudder blade (not T-foil) whether it is a floater or a foiler.
Just draw it & the light will come
Cheers

 

Arrow Catamaran foil centered between hulls