Lead with rotating masts??

[patrik111]

Hello All,

I would like to have some understanding on what a rotating mast (alloy tornado mast) would do to the helm compared to a non-rotating mast?

I have a hull and I have a mast, the position of the keel is fixed, and now I wonder where I would place a rotating mast in comparison with the non-rotating original mast.

6m LWL
The geometrical center of effort for the original setup rig is at 3,03m from the stern,
The geometrical center of lateral resistance is at 3,27m for the boat.
Where should I aim to put the center of effort if I instead as outlined above have a rotating mast?

The reason for asking is that the picture for the tornado, the measuerments are as outlined below
CE 2,28
CRL 2,36
As could be seen this distance is significantly smaller.
Also there is an indication along these line in Bethwaites' High performance sailing. (p209) where he states that the boat developed lee helm when fitted with wing masts as the actual CE moved forward compared to a non-rotating mast.



In very short, should I change lead?

Kind regards

Patrik Elfving

[gggGuest]

Over the time period Bethwaite mentions it became increasingly fashionable to have more of the lateral resistance taken up by the rudder. I suspect that was far more of a factor.

To my mind folk get wildly overexcited about clr. Provided the boat isn't actually generating lee helm it seems far less critical than traditional wisdom has it. Provided the tiller is on the centreline does it really matter whether the rudder is taking up 5% or 45% of the task of resisting leeway? That influences optimum sizes of each foil of course, but that's another matter.

Many folk get confused between how hard the tiller is pulling on your hand, which is just a measure of the amount of side load on the rudder, and is neither a good thing nor a bad thing, as opposed to the amount the tiller has to be offset from directly fore and aft to keep the boat tracking in a straight line, which is a bad thing, as its a braking force, but which is far more affected by heel angle and hull shape than the exact rig position...

The best cure for weather helm is usually to let the sheets out a bit and keep the boat flat...

[Eric Sponberg]

Yes, a rotating mast, properly designed for less drag, will have the equivalent of moving the CE forward, creating more lee helm. To compensate, you have to shift the rig aft. You can do this my moving the mast aft, or you can make the mainsail bigger by adding area along the leach. How much of a shift to make is anybody's guess. It appears from your numbers that you actually have negative lead, i.e. the CE is aft of the CLR, which is unusual. The difference is 0.24 meters = 240 mm = about 9.4". I think a safe bet would be to double that distance = 480 mm maximum, or somewhere in between that limit and what you have.

I hope that helps.

Eric

[patrik111]

The mainsail is a Tornado sport sail, see line drawing. With a larger jib. (ca 8m2)

The tornado I know is a well behaved and balanced boat, so something is fishy here. I would suspect maybe my method.

To find the CLR and CE, I took the geometrical center of the sailplan, and the geometrical center of the centerboard and keel respectively. (This would differ only slightly from using 25% from leading edge of keel fin as proposed by Larson&Eliasson ).

For a 49er, using the same method I get a CE at 1,64m from the stern, and the CLR at 2,20m from the stern.

To compare apples and apples, would you find the same for the centers when studying the attached line drawings?





M60SailPlan.pdf

Kind regards

[Eric Sponberg]

Regardless of what is "proper" in the conventional sense, lets look at the three boats. The M60 is a conventional sportboat. Without doing actual calculations, but merely visualizing where the CE and CLR are, the CE appears to be vertically right over the center of the keel blade. On the Tornado, the same applies; to me, the CE appears directly over the keel, perhaps more toward the leading edge of the keel than the trailing edge. On the 49er, which is a much lighter and faster boat, and which sails at apparent wind angles that are smaller overall, the CE is decidedly aft of the daggerboard. The increased efficiency of the rig (more lift for less drag) causes the driving force to point more forward. To compensate for balance, the sail area has to shift aft, which apparently the designers of the 49er have decided works best. So that shows the trend, which is what I have seen on my wingmasted monohulls--as rig efficiency increases, you want to move the rig geometrical CE aft to maintain sailing balance.

Eric

[CT249]

Practical experience in Tasars (which has the same underwater shape as the boat that Frank developed his wing on) and Taipans (a 16' cat weighing 95kg all up and with a 28' high wingmast of 8" (IIRC) chord) shows that there seems to be no discernible change in weather helm whether the wingmast is rotated or not.

A former national champ's Tasar mast would regularly pop out of rotation on port tack (a maddening problem we could never trace or solve) and from the front o the boat I could never feel anything, nor did the skipper call helm as a problem. On my own Tasar we've had occasional rotation issues and I've never noticed the helm issue..... the boat does feel bad, but in the same way as it does when the main is having issues because the cunningham hasn't been pulled on, or the outhaul's slipped. It's nothing dramatic.

In Taipans, one former national champ used to take all the rotation out of the mast in a big breeze, while others (like a world A Class , F18 and Tornado champ IIRC) increase the rotation. Surely if a rotating mast had such a dramatic effect on helm, you could not have such widely diverging mast trim on a class that tends to be meticulous about weather helm and rudder angle.


PS Eric about "On the 49er, which is a much lighter and faster boat, and which sails at apparent wind angles that are smaller overall, the CE is decidedly aft of the daggerboard. The increased efficiency of the rig (more lift for less drag) causes the driving force to point more forward."

Sorry, but there is no way that a 49er is a faster boat overall than the Tornado, and there's little chance that it's a more effective rig. The Tornado is rated at about 20% quicker. For comparison, the 49er is rated almost the same speed as an Yvonne, a ply 20' cat designed in about 1956. The 16' Taipan rates about 15% quicker than the 49er, even without a kite, and the Taipan's rig is fairly similar to that of the big T.

I'm fairly sure that you'll also find that the 9er sails at wider apparent angles downwind. The cats sail higher angles at faster speed, which is why they have smaller, flatter kites. Hanging something like a 14 kite, which is fairly close to a 9er kite, just doesn't work, even on something like an F16.

[patrik111]

Having now done the same excercies with 420,470,29er,10Rater, I have come to the conclusion that lead for keel boats does not fully apply to boats that are intended to sail completely flat as the lead rules of thumb likely relates to the moving out of CE by heel.

If this assumption or theory is wrong, please correct me.

Thanks for the good input.

BR
Patrik