PDRacer Sailboat - Measuring Tacking Angle

Speed over Distance record & Measuring Tacking Angle
by Tim Cleary
7-28-06 record: 2.5 mph over .69 miles

Although 96 degrees F this afternoon, there was a good 8 mph breeze that dropped down only occasionally. I went out to the local lake to take a measure of the pointing ability of the remodelled hull #59.

Rather than setting out buoys, I made the assumption that my starboard tack pointing angle was approximately equal to my port tack pointing angle. By going on a starboard close-haul tack for a certain distance and then going on a port close-haul tack for the same distance, the cosine of the pointing angle becomes the distance to return to the starting point divided by the total distance of the equal-length tacks.

So I did a trial by starting just off the shore near a large stump and began a close-haul starboard tack. When I got near a wind shadow caused by a point of land, I noted that the GPS read .22 miles and came about on a close-haul port tack. I kept to it until I went another .22 miles and the GPS read .44 miles. I then went on a dead run straight back to the stump.

The GPS read .69 miles making the return distance .25 miles. So .25 divided by .44 is the cosine of my pointing angle. Doing a google.com search for: acos(.25/.44) in degrees returns 55.4 degrees, my pointing angle. Twice that, 110.8 degrees, is my tacking angle.

The trigonometry works as long as the tacks are of equal length. So I did another trial of four tacks, each of .1 mile distance. I did a port close-haul tack for .1 mile, a starboard close-haul tack for .1 mile, a port for .1 mile and a starboard for .1 mile, then a straight run back to the stump. Total distance was .61 miles so a google.com search for acos(.21/.40) in degrees returns my pointing angle, 58.3 degrees. Twice that is 116.6 degrees, so that's my tacking angle for that trial. An average tacking angle over both trials is 113.7 degrees.

The almost finished remodelled hull #59 on shore after second trial. The water looks red because I stirred up a lot of red mud wading around.

The bottom is 5.2 mm lauan. All other panels are 1/8". The bow, side, and stern air chambers are connected and ventilated by 2 deck plate hatches. The volume of the unified air chamber is 7.2 cubic feet. Weight of hull is 75 lbs.

The trial was a run of .69 miles with an average speed of 2.5 mph, which sets a new Speed Over Distance (.5-2 Miles) record.

Tim C.

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