Winter Repairs, Part 1

[jdoorly]

Just wanted to underscore GL's point about leaving the trailing edge a flat of 1/8"- This is actually better, hydrodynamically speaking, than a tapered point.

[GreenLake]

Best is to make that flat 1/8" just a little bit asymmetric, said to help in shedding vortexes.

[K.C. Walker]

I wish I had known about the eighth inch flat at the trailing edge when I did the fairing on my centerboard a couple of years ago. I do get better performance with the more foil shape but at high speed the board does hum now with the sharp trailing edge. If I raise the board a little bit the hum goes away, so it's not that big a deal. One of these days it will be time to do another yard launch to inspect and touch up the bottom. I will take care of that then.

[GreenLake]

I used to get not just hum. but really bad vibration with the factory board -- usually downwind at speed. I did fill and fair many/most of the small dings and defects early on, w/o trying to alter the shape of the edges. That made no difference. Later, after doing my "cheap" profile upgrade, with flat 1/8" the vibration went away. I did not purposefully try to introduce an asymmetry, but one may well have crept in, because I didn't use any sanding jigs, and when sanding freehand it's easy to give the final edge an angle other than 90°. Actually, I did the "reprofiling" twice. First time, I careened my boat and worked w/ the CB attached to the boat. That was OK for the edges, but the "underside" was tricky to work on. I also tried to restore the gelcoat, and that didn't turn out the way I had hoped. But the results, when tried in the water, were encouraging, nevertheless. When I repainted the hull a few years later, I took the CB out completely, and did a more extensive job of filling fine cracks in the original gel coat. The initial job essentially served as proof of concept, establishing that there was a positive return from doing some work short of re-building a perfect foil, so I re-did the work more thoroughly and under better conditions and painting the CB with the same paint as the hull.

[TIM WEBB]

GL, why would you have the board down going downwind?

[GreenLake]

When I forget to pull it up

[GreenLake]

Point is, I'd get the vibrations only when going fast, so probably something like a reach, even though I "remember" it as going downwind. Tricky thing, that, memory.

[Salty Dog]

I would think down wind would allow the CB to move back and forth. While any othe point of sail the CB would be forced to one side or the other stabilzing it.

[TIM WEBB]

Ah, good point ...

[GreenLake]

@salty dog. You are correct in that a boat sailing dead downwind (DDW) does not make any leeway. However, that does not mean that there are no sideways forces on the CB. When you move a foil through the water it leaves a trail of vortexes. When each separates it gives a push to the CB (sideways). When they alternate, you get vibration. You can reduce the effect by careful shaping of the trailing edge.

Leeway is just a few degrees, so even on other points of sail, the flow parallel to the CB dominates. But there's a small angle of attack, and that's enough of an asymmetry to create lift. However, if your board is shedding vortexes you could still get a vibration, I think. On sails you can get them upwind (think of the leach fluttering if not set properly).

Finally, if you partially raise your CB, you get some other effect, which is that the trailing edge is no longer at right angles to the flow (neither is the leading edge, so the whole CB does the swept-back wing thing).

OK, now we are getting into aero-hydrodynamics, a fascinating subject, but not really something that fits under "Winter Repairs, Part 1".

[Salty Dog]

What is the deffinition of Leeway? Because I would have though a boat going down wind would be making 100% leeway. as in Leward being down wind.
I'm sure I'm looking at it wrong and I should change my thinking . Not that this has any thing to do with the subject at hand.

Thanks SD

[Salty Dog]

I looked it up........Never mind

[GreenLake]

If you look it up, it gets complicated

The leeway angle does get to zero going DDW, but you can argue about the rate. DDW sailing is essentially drifting under sail, so I see why it could make sense to think of 100% leeway. But for the general case (any angle) it's easier to deal with only the sideways drift component (which by vector addition to the forward motion gives you the leeway angle). Seen that way you have no observable leeway going DDW.

Anyway, that's the sense I used the term. (I admit that it may not be all that useful a definition for when you are trying to understand different types of scenarios like an underpowered boat motoring upwind into a gale and drifting backwards -- you's day that that boat is making leeway, but not sideways.)

[jdoorly]

For those not 'looking it up', leeway is the difference between where you are aiming and where you are going.

[GreenLake]

For something like a DS going upwind that leeway angle would be on the order of, say 5°. Perhaps even a bit more. The trick is that this angle isn't directly visible (except if you measure the direction of the wake you are leaving behind your boat). Therefore, when you are pointing too high, you can fool yourself about where you are going, because you don't notice the increase in leeway when you are pinching.