Re: Raising Centerboard

Andrew Sallee (ajsallee@engin.umich.edu)
Sat, 24 Apr 1999 13:49:29 -0400

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West Wight Potter Website at URL
http://www.lesbois.com/wwpotter/
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Gordon,
Just a little correction...
You're right about calculating depth, but for moving the CENTER of effort,
you need to figure out how far back the center of the area of the board
moves, not how far back the tip moves. This is actually a fairly
complicated calculation because of the way the amount of board 'hiding'
within the hull changes. You need to know how high the pivot is in addition
to the profile of the board. As a quick estimate, just use half of the sin,
since you're looking at the middle of the board. So it moves back ~12%.

Andy
Beatrix, P19-854

-----Original Message-----
From: Gordon <hlg@pacbell.net>
To: wwpotter@tscnet.com <wwpotter@tscnet.com>
Date: Saturday, April 17, 1999 12:26 PM
Subject: Re: Raising Centerboard

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> West Wight Potter Website at URL
> http://www.lesbois.com/wwpotter/
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>Great post, Dave. Thanks for doing the math. It does appear that 10 or 15
>degrees aft could be effective in reducing weather helm without affecting
>stability significantly.
>
>Harry
>
>>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
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