Positioning of centerboard when solo / heavy winds

[PG]

I am probably getting this wrong, but I remember reading somewhere (this forum?) that when sailing solo or in heavy winds one should put the centerboard only about half way down. Today I finally tried it. I was solo, with winds of around 10 mph, perhaps gusts of up to 15 mph. Maybe I'm imagining things, but it felt as though the boat was not heeling as much and I had more control. When the boat was heeling at the max, I could lean way out and see that the board resembled more of a traditional keel shape than a fin pointed straight down.
I have two questions: would it be safe to try this in 15 to 20 mph winds, or does that increase the risk of having a sudden blowdow? Given that the boat seems to perform better with the board halfway down, under what circumstances would one put the board all the way down?

[kkearns]

Someone once gave me the same advice with respect to a Flying Scot I owned for years. The rationale was that with the board half to three quarters down, the boat would not be so inclined to "trip" over itself in a gust ... I believe the person said that this placement of the board would allow the boat to "slide" laterally slightly in the heavy wind or in sudden gusts.

While I followed the advice at times (and I sail solo a lot), I was never totally convinced of the logic. On the Scot, like the DS, the board does not add much balast below the water line (it is not a heavy board and does not really act like a keel), so in theory dropping board all the way will not add a lot stability in the form of true ballast. Still, it always felt to me like I lost some control, and certainly lost some pointing ability with the board only partially extended. I suppose I am of the opinion that control of the helm in heavy weather is supremely important.

My DS1 has a lever operated board and the board seems to be either fully extended or fully up ... no in between. I find the DS to be an incredibly stable boat. I feel best with the board fully extended.

Kevin

[GreenLake]

The board does not add much balast below the water line (it is not a heavy board and does not really act like a keel

Correct, the contribution of an unweighted centerboard is not that of adding a righting moment. Instead it provides a sideways force that is opposed to the sideways force of the sail, allowing you to point.

These two opposing sideways forces are what sets up the heeling moment. (A pair of opposing forces, that don't act on the same spot of an object, create a moment, which, unless it's opposed, will start to turn it).

To keep your DS upright, you create an opposing moment of two vertical forces, the boyancy and the weight. By hiking, you move the spot where these two forces act further appart, increasing the righting moment (without changing weight or buoyancy).

Raising your CB, will decrease the sideways force it generates. That reduces its contribution to the heeling moment. So you heel less, or can tolerate higher winds at the same angle of heel.

But something still has to oppose the sideways force of your sails, or your boat would forever accelerate leewards. That's done by the hull. It starts to drift (slip) leewards at a steady rate, wich creates a sideways force from friction. The hull slips just enough to create a force needed to balance the sail force: more in gusts, less in lulls.

So you still have the same opposing (sideways) forces, but since the hull is closer to the sails, the offset distance between these forces is less, and therefore the heeling moment is reduced, reducing your risk of capsize.

Nothing is free: you lose pointing ability and your leeway increases as well. In addition, because the center board rotates back, as it is raised, you are changing the fore and aft alignment of the opposing sideways forces, creating or increasing a moment that tries to turn the boat away from the wind.

Where you had weather helm before, you might get neutral or lee helm instead, which might appear like a loss of control. As was pointed out in the thread on sailing with main alone, this side effect of raising the CB is useful to counteract the loss of the sideways force when the jib is taken down, restoring some balance.

Whether raising the CB and trading pointing capability for reduced risk of capsize or improved helm balance (mainsail only) is a beneficial tradeoff depends on the circumstances. But in my view theres no question that it is useful to know that the tradeoff is possible and how to achieve it when needed.

[phil]

As I spend most of my time cruising, I have weighted my CB to 60+#. This was done uniformally across the board by the addition of lead shot to the hollow interior. I have yet to try this and I wonder what your thoughts are regarding this modification. (I retained the orig. CB so the boat can be returned to it's original configuration.

[GreenLake]

When you hike out, your own center of gravity is perhaps 3.5ft from the centerline. With a bit of heel, the center of buoyancy is some distance on the other side of the boat, so you have perhaps 4-5ft of lever arm. Compared to something like half the mast height for the lever arm at which the wind (and water) forces act to heel your boat.

That's pretty effective.

In contrast, the CB is just too close to the hull to give you enough leverage. At small angles of heel, the distance between the weight in the CB and the center of buoyancy is perhaps a foot or less (all rough estimates). So, the effect of weighting your CB should be equivalent to putting a 15 # weight in your pocket while you are hiking.

In other words, given the size of the sails, my expectation would be that you see limited benefit of this weight at normal angles of heel. (Let me know if you find otherwise.)

It's useful to sit down and draw a cross section (to scale) of your boat, inclined to 30 degrees. You can then mark the spots where all the forces act. I did that some time ago when I was first learning the concepts. Here's the sketch.


[click to enlarge]

[PG]

Thanks for the insight, which makes sense. I did notice that it took me several more tacks than usual to get back to the other end of the lake. Still, I agree that partially raising the CB can be a very useful strategy. I can imagine situations where it's better to tack a few extra times than to risk capsizing. Next time I'll try putting the CB in a couple of intermediate positions to measure the trade-offs of pointing and heeling. One final semi-related note; I also read that if one loses or breaks a rudder, the proper strategy is to raise the CB in order to make forward progress without rounding up. That seems consistent with the aforementioned points.

[dannyb9]

when overpowered to windward you have choices. if you have too much wind you can just let the boat heel, so that you are spilling wind from the sail and spilling lateral resistance of the centerboard and rudder. this technique has proven to be inefficient. keeping the hull close to level is very important for efficiency to windward in most cb racing classes. so your choices are to flatten or reduce sail area(leverage), reduce cb area/depth(leverage) or add leverage (hike harder, trapeze out, get a crewmember to hike etc.) in my experience its faster (more efficient) to reduce leverage including raising the cb somewhat than it is to allow excessive heeling.

[seandwyer]

Greenlake,

You don't look anything like I had imagined - but thanks for the illustration!

[GreenLake]

That's my avatar!

BTW, The location of the CG dot in the diagram is my best gues of how much the center of gravity moves based on crew weight and position. As shown, he's not even hiking yet, just sitting on the side.

[GreenLake]

when overpowered to windward you have choices... in my experience its faster (more efficient) to reduce leverage including raising the cb somewhat than it is to allow excessive heeling.

Thanks, that's worthwhile to remember.

BTW, I wanted to show the effect of heeling on the shift of the center of buoyancy, that's why I picked the particular angle for the diagram (just in case anyone mistakenly thinks I'm recommending it as ideal).

[talbot]

This was a useful thread. Thanks.
One issue that was mentioned but not followed through was how to hold the board in intermediate positions. I have seen lever-operated DS-I's that have added uphaul and downhaul pennants and cleats to their levers. DS II's with a single cleat that alternates between pennants just need to add a second cleat. I put two cam cleats neat the aft edge of the trunk, so I can operate the CB uphaul and downhaul from the helm while sailing solo.