It's important to stress here that what I'm going to talk about applies only
to the P15 with it's pivoting centerboard - raising the daggerboard on a P-19
is an entirely different matter and may have unfortunate results.
Because the P15 centerboard rotates around a pivot, in the process of moving
it from the fully lowered position to the fully raised position it will
scribe 1/4 of a circle or 90 degrees. For now, let's say that fully lowered
is 0 degrees and fully raised is 90 degrees.
As it turns out, on a P15, trigonometry can be our friend. The depth of the
centerboard will vary as the cosine of the number of degrees that we "raise"
the board. The distance moved aft by the centerboard will vary as the sine of
the same angle. I think it's safe to assume that the center of mass (and any
change in the righting moment) of the board is moving up with the cosine.
Soooo, heres a little table which should help us understand why "raising" the
board slightly can change the way the boat handles without imposing
significant danger:
Angle Cosine Sine
0 deg 1.0 0.0
5 deg .996 .087
10 deg .985 .174
15 deg. .966 .259
20 deg .939 .342
30 deg .866 .500
45 deg .707 .707
60 deg. .500 .866
75 deg. .259 .966
90 deg. 0.0 1.0
We can multiply each of these entries by 100 to get a percent change in
height with angle - So "raising" the board by 15 degrees, as I suggested
before, actually leaves the board at 96% of it's original depth while moving
it back 26%. 10 degrees would maintain 98.5% depth while moving the board
back by 17%. Going back 30 degrees leaves 86% of the board down while moving
it back by 50% of it's length.
At 45 degrees things become even (71% depth, 71%aft), the change in depth the
same as the change in fore/aft position and after that the relationship
reverses and we are then quickly lifting the board out of the water.
I hope it's obvious here that while raising the board a little is okay (in my
opinion, anyway), raising it more than, say, 30 degrees will have diminishing
returns in balancing the helm and will start to seriously compromise the
ability of the boat to "self-right". Coming from a background of sailing
dinghies with unballasted boards which are frequently raised all or part way
may make me more cavalier with this than is prudent, so it is probably better
that I suggest that readers of this not interpret it as advice or
recommendation. If there is any question in your mind, leave the board all
the way down.
Best regards to all,
Dave Kautz
P-15 #1632
Palo Alto, CA