Michael Storer Boat Design

PD RACER FAQ

Performance measurement - Pointing Angle and Leeway

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One of the questions on everyone's lips was just how well the carefully shaped foils would help the OZ PD Racers to windward.  How much the performance would be improved.

In the end we were not set up to make these measurements - but incase any of you are ... here are the usual methods.

Another way of getting an approximate idea is to sail alongside a reasonably well known type of sailing dinghy.  Generally a mid sized boat in non trapezing conditions and see what happens.

Generally a Fireball, 470 or other reasonably performing dinghy will be tacking through 90 degrees.



Hi Andrew,

Pointing angle is not toooooo hard to work out, but leeway has been a bugbear since it was first invented!
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Tacking Angle and Pointing Angle

For pointing angle you need some sort of compass and flattish water and a comfortable sailing breeze - maybe 10 knots (it's important weves shouldn't be slowing the boat - see my list of tacking angles below and how rough water cuts the pointing ability of a yacht.).

Sail on one tack with good upwind speed, then tack and proceed on the other tack with good upwind speed.  Note the difference in angles - the difference is your tacking angle.

Simply halve the tacking angle to get your pointing angle.

Do this several times - the number that the results group around is your pointing angle.

The only problem is that there is an angle where the boat is sailing at its best to windward.  Point too high and the boat slows up too much, point too low and you're not heading in the right direction.

Very Roughly Speaking Tacking angles for different boats Efficient keelboat in flat water - 70 degrees
Efficient keelboat in rough water - approx 85 degrees
Conventional sailing dinghy (efficient foils and sails) - 90 degrees
Tahiti Ketch - 120 degrees
Square Rigged Ship - 140 degrees.

My feeling is that the Puddleduck with efficient sail and foils would tack through about 95 or 100 degrees but will rapidly deteriorate as the water gets rough.
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Leeway

is much more tricky. Generally in yachts it takes some time to work out.

It also varies with different wind and sea conditions and how you sail
the boat.

In small boat racing we are generally only worried about relative leeway.  If your boat makes more leeway than the competition there is something wrong with your boat or your sailing technique.  This is trialled by sailing alongside similar boats but not so that one boat obstructs the others wind (normally the second boat is about 30 or 40 feet to leeward and slightly ahead.

It relates strongly to pointing - good windward performance is pointing as high as you can without losing too much speed.  If you lose too much speed by pointing too high the boat makes much more leeway - so you end up losing more sideways than you gain by pointing high.

If you go the other way and don't point high enough you are as above - making less leeway but heading in the wrong direction.

Anyway to work out leeway you need to do quite a few trials to get a feeling of what is happening.

The crude way of working it out is to sail upwind and look at your wake behind the boat.  The angle is your leeway - this gives you a feel but it is notoriously inaccurate.

Another way is to sail alongside a boat in the list above - if you sail alongside a laser for a few minutes pointing in the same direction and see where you end up relative to it.

The proper way to get a figure requires some navigation or a GPS.

Head off from a known point heading upwind and hold a compass course once you are settled in.

Finish the trial after 5 minutes and record the finishing point.

On a chart or graph paper mark the start point, mark the finish point and mark the steering angle from the start point.  The angle is your leeway.

The easiest way to work it out is to have a sophisticated GPS linked to a flux gate compass which is firmly attached to the boat.  It can record heading angle (where you are steering vs where you are going) and work out the discrepancy which is made up of a mix of leeway and current.

It takes quite a few trials to work it out. Again smooth water, no tide and consistent breeze.

My feeling is that most important way is to compare against other PDRs, but the most important for ball park would be to compare against a good racing dinghy - laser or something else that is being reasonably well sailed

Tacking angle part 2

Pointing angle = angle to the wind = 45 degrees approximately

When you tack the total angle is 45 degrees to be head to wind then another 45 degrees to be on the new tack.

U have gone through a total of 90 degrees = tacking angle.

MIK

Best Regards
Michael Storer




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