Tuesday, July 23, 2013

Fat Guy in a Little Boat - Light Air Speed

We all have seen them (or I should say "us" now), these 220+ big guys that seem to be able to win in any conditions, low side drifters to heavy air hike-fests. It's easy to understand how the fat guy weight helps in a blow. They can get more hike with less effort. The boat stays more upright. But I believe it is a bit more than that. Weight alone is not what works. Someone 190 and fit would beat 220 and fat any day.

And the reverse is also true. Yes a 120-pound skinny person has an advantage in light air, a big one. But we still see these fat guys coming out on top. How?

I have my theories.

But first let's talk about what EVERYONE should be doing to be fast in light air.

In Florida at the Midwinters, I did very well when the wind was over 10. When the wind got light, I started to drop back in speed, straight out boat speed.

I had done the basics. I thought I had the setup right.
  • A new sail
  • The right sail shape
  • Trimmed tight but not hooking the leach
  • Just a hint of shelf in the outhaul (Z-Max)
  • No cunningham, no vang
  • Traveller dead center
  • Boat healed pefectly to make the board well vertical
  • Weight forward
  • Minimum rudder movement
  • Constant body movement to maintain the angle
  • Intense level of concentration - don't lose momentum
It wasn't enough.

I sent an SOS e-mail to E Hood, Ted Keller, and Andy Burdick among others. Besides losing 50 pounds, what do I need to do to get my speed up in drifters? I got many standard answers that I expected and already knew. You can read the tuning guide to get most of those answers. But the conversation helped me sort some things out mentally.

1. Adjust your board angle

One response on follow-up conversation was interesting, and in working through the physics I believe it was the most important of all responses, and it was almost off hand.

Adjust your board angle/depth.

Like most people, I tied knots in my board lines that didn't need to be untied until the lines had to be replaced. Down was down, up was up. Yes, downwind I use just a peak of board but we are not talking about downwind here.

I can't lower my boards on shore. So two weeks ago when I finally got to do the first hull maintenance in years, I tipped over the boat on shore and I checked the angles. Low and behold they were swept back a bit, not down far enough.

So what would that do? Why would it make a difference?

There are two foils in the water. The rudder and board. The rudder is fixed in position but rotates left to right. The board is fixed left to right but can sweep fore and aft. Between the two of these you "balance" the lateral or sideways forces underwater. Think of it like this. With no sails, you hook onto the side of the  boat with a rope and tow it sideways with a motorboat. The perfect balance point would be where the rope was attached to the boat and it towed perpendicular to the direction of travel. That is the center of lateral resistance. If you move the rope forward, the bow points up toward the direction of travel. If you move the rope back the stern points up and the bow dips down. If you rotate the rudder you move that resistance point. If you sweep the board you also move the point.

Of course it is more complicated. With sails up and the boat moving forward this center of lateral resistance moves forward. The foils start "lifting" and all kinds of other factors are involved. But let's keep the concept simple, just think in terms of a center of lateral resistance.

If the boat is designed perfectly, tuned perfectly, and sailing at exactly the perfect angles, you can remove your rudder and sail in a straight line. As a skipper you feel this as neutral helm. The center of lataeral resistance is perfectly in the middle of the board. Lasers do this as a practice drill. You can "steer" the boat by sail trim, body weight movement, angle of heal changes, etc. (In reality the weight of the tiller on a healed up laser compensates for the design flaws by inducing added rudder forces.) In MC-Scows you can also steer by  moving the board fore and aft by anjusting the angel or depth (board up a little or down a little). This changes where the center of lateral resistance is and moves the bow up or down a little.

If you have a baord that is not down far enough, you have moved the center back a bit. This will make the bow want to come down a bit. To compensate, you imperceptibly steer up a little. Now this is the equilibrium point. But the foils may not be as effective underwater as the should be. The rudder is slightly angled to bring the bow up. This creates drag. But more importantly that angle changes the direction the boat ravels through the water affecting the angle of attack of the sails to the wind. It also induces drag in the board. All these are tiny angles. But all combined it may mean the difference of pointing that axtra 2-3 degeees up or having that extra 1% of boat speed. And those differences will mean a clean start or getting buried by the guy properly tuned.

So what's the right angle?

Beats the hell out of me. All I can do is share my findings so far.

Of course it depends on wind speed, sail choice, mast rake angle, and many other factors. After I waxed my hull, I adjusted the board depth. I actually got out a square and squared the leading edge to the hull. But the hull has a rocker point, basically the front half is flat, the back half flat, and the mid point rounded. So that was a stupid idea. So I eyeballed it and set them up to look as close to perpendicular to the water line as I could. I figured this was MAX DOWN for any condition. Any more down and you get into forward swept foils with all kinds of physics problems only compensated for with computer controls. (Plus the board can break the board well when it hits it.) From this depth I can raise them slightly to test. The change in angle from what I had was significant, maybe 10-15 degrees! I was nowhere near vertical. I'll bet I moved my center a couple inches forward. It was a major move.

In Saratoga the conditions were perfect to test the depth and angles. I sailed race 1 and 2 at this new vertical setting. Both races were light. There were a few times that I was near enough other boats to get a guage on speed and point. Honestly I could not find any time where I was slower upwind. I can't say for sure but I think it made a dramatic difference in my speed and point. Very dramatic.

In race 3 I played with it a bit to see if it mattered. I'd drop the board then pull it back up a little bit. I think it hurt. But of course these aren't laboratory conditions.

What about heavy air?

When you over trim, or vang on, you do a couple things. The leech tension pulls the tip of the mast back. The adjusted angle of the sail creates a different lateral force point. Your angle of heal may change. All these things tend to move the center of lateral force back a little bit. You get some weather helm. Sweeping the board back a bit balances the boat again. How much is right? I still have to play. It depends on how you sail, how much you weight, where you sit, if you hike out lots or a little, etc. It turns out my heavy air sweet spo was where the knots were when I bought the boat, swept back some.

I only was able to test that a couple times in one race though. More testing is needed.

I think you don't need to adjust the angle if you adjust the rig pulling the mast forward some. But that is all mental theory to me right now.

2. Lose weight

I weighed 225 in FLorida and 210 here. I also tossed crap out of the boat that I didn't need, maybe another 5 pounds. And I changed my clothing to lighter materials, and didn't take as much water onto the course, maybe another 3 pounds. I also maticulously dried out the boat and prevented even a little water from getting in, maybe another pound or two. That's maybe a 25 pound difference, 15 of which was through diet.

Why does this make a difference?

We all have experienced someone very light coasting past us down wind. It still happened to me at Saratoga. But a couple things were distinctly different. I was more mobile, I could bend at the waist a bit more, I needed less water since there was a little less fat guy sweat, and I felt better. But the boat also sails better. The acceleration is slightly better. There is slightly less wetted surface. There is slightly less weight in the back, better balance.

Were these two things enough of a change to make me win? Maybe.

Since the regatta I have sailed a number of club races. I've played some more with board angle. But I have on my crap sail. So I think I have made some progress, but I can't be sure.

If anyone else has insight, I'd love to hear about it.

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