Keeping Centerboard Lowered Downwind

[bjmoose]

For last night's racing, in addition to wind we had quite powerful gusts and shifty conditions (storm front moving through.)

Previously, I've observed that if I leave the centerboard in the "down" position when I turn downwind (i.e. I don't raise it) the boat is more stable and doesn't rock as much from side to side.

Since the fastest way to the bottom mark involves not capsizing, this seems like a pretty good idea to me in conditions that challenge the limits of my ability.

But I was talking to one of the most experienced laser racers afterwards, and he said that in windy conditions downwind, it's possible for the laser to "trip" over its centerboard, and that raising it therefore promotes control and makes wipeouts less likely. However he wasn't able to give me a very good explanation of how/why this occurs, and I should further mention that the only time I passed this same sailor all night was when he deathrolled and wiped out on one downwind leg.

I searched this forum and found nothing.

Tell me more of this "tripping." Why do we raise the daggerboard on the downwind leg? Is it just to reduce drag? Or are there other reasons?

 

[Chainsaw]

If i have experienced "tripping" then I don't know it as "tripping"...no...that didn't come out right

But in my experience, raising the centre board is like putting your foot on the acelerator when running. The difference between not enough up and too much down is quiet easy to find if you make the adjustment while you are acelerating. It also reduces weather helm in gusts. And obviously the faster you go the less likely you are to get a case of the wobbles - as long as wind strength allows.

The only instance I wil lower the board for downwind is in large waves and light winds. In those conditions you can actually feel the boat being pushed off course by the wave quite dramatically and any ability to surf or steer is reduced.

Apparently the top metre of water in a seaway moves faster than the water below it. This seems to be more important in big boats on big waves with shallow keels but the effect is felt in our lasers too.

 

[Deimos]

Imagine you are pushing your centre board through the water holding it at the top (in the normal motion, rounded edge moving forwards). Then, continuing to move in the same direction you start to rotate the board a bit (i.e. so it is slightly side on to the direction of movement) - the bottom of the board will start to rise (actually it is being pushed back but because it is held at the top it will try to rotate, bottom lifting). When it is fitted in a boat this will tip the boat towards the direction of motion (which is now actually slightly sideways.

Of course the boat's direction of movement will soon change to "catch-up" so the CB is now in line with the new direction of movement, but normally your turn continues, so more sideways force on the CB as it starts to move flat side to the direction of movement, tipping the boat, etc. -> what is called "tripping over".

However, I'm terrible at these types of explanations so the above might make no sense.

Think of a hull and CB being dragged through the water bow first - all goes smoothly. Now think of it being dragged through the water sideways (beam on and the dragging not doing any tipping). The CB will cause the hull to tip in the direction of movement i.e. sideways. Same thing happens when you turn. Of course a turn will not normally change to full sideways movement through the water but the effect is the same but not as pronounced. If going fast the tipping motion is greater.

(My understanding and pathetic explanation anyway - others might be able to explain either the real reason and/or in language that makes it understandable).

Ian

 

[Dave Janson]

nice explanation

 

[abenn]

I haven't head any problems with tripping over the board while going downwind - I only heard about it with relation to gybing. So as you rapidly change direction and there is a bit of sideways movement, the lower blade will have more side resistance and create a torque that can heel you to leeward - ie tripping over.
Straight down wind, a lower board will give more drag but as long as you're essentially going in a straight line, you shouldn't trip over it. And a lower board will help in strong winds to stop the death roll, as the boat tries to roll, the lower board will again have higher sideways resistance and slow the rolling. If you feel the boat is unsteady in strong winds tap the board down, as you say staying upright can often gain you a few places. Then again, we could start on sailing by the lee to get stability and keeping the speed up - but thats off topic...