Rudder & centerboard profiles

[cs]

Does anyone have anything on rudder & centerboard profiles to reduce cavitation? Something given in maybe percentages of thickness and tapers to leading and trailing edges?

Chad

[Thorne]

Now someone with over 15k posts should know about the search function...

;0 )

Try this link, it is what I direct folks to with these questions (and use myself, too) -

http://www.philsfoils.com/designTips.html

[cs]

Yeah but 10K of those are probably about football. ;)


I looked at that site and few others. Is there anything that is not so much black magic?

Chad

[Thorne]

Well, try the white robe and Rowan wand, lose the Ev-il Laugh, and Hey Presto! White Magic!

Most folks seem to go by a ratio of the width of the CB and rudder, the former usually being narrower in thickness than the latter. You pick a spot 1/3 of the way back on the foil and leave that thick, then taper the front to about half overall thickness, the back to as small as you can get it.

What sort of boat, size foils, sailing speeds, and cavitation issues????

For the replacement CB on my dory skiff, I just marked the foil off in equal increments, then planed/beltsanded the shape -

http://www.luckhardt.com/newcb1.jpg

http://www.luckhardt.com/newcb2.jpg

http://www.luckhardt.com/newcb5.jpg

[cs]

Thanks, that is what I was looking for, something simple and direct.

I'm working on the Meerkat design drawings and need to do a profile drawing of the swing keel (I call it that in lieu of centerboard because of the size) and the rudder.

Chad

[Thorne]

Yeah, I'd love to know the point at which a weighted CB becomes a swing keel...perhaps when a winch is required to raise it?

Good Luck!

[cs]

Not sure, but this one comes in at about 225 lbs.

Chad

[John E Hardiman]

Yeah, I'd love to know the point at which a weighted CB becomes a swing keel...perhaps when a winch is required to raise it?

Good Luck!

It is a matter of deadwood, not weight. A vessel that has a swing keel has a "normal" keel with a foil that swings out of it (i.e. a part of the keel swings). A centerboarder has very little deadwood and the board resided in a trunk inside the hull (i.e a trunk in the center of the cabin/hold).

Or so says I...;)

Edit, BTW Chad, any fair symetric section shape of 8-12% thickness with the maximum thickness at ~30% aft, elipitical leading edge, and a square trailing edge of ~1/8" will do fine. You will never be able to produce any real measurable difference in performance in open water conditions.

[Thorne]

Well, my (unmentionable material) San Juan 21 had what Clark Mfg called a 'swing keel' -- I think it was around 500lbs. But the boat was quite flat underneath with a slight V-hull forward, no deadwood or plastic...

;0 )

I like MY new definition better: Swing Keel = a weighted foil pivoting in a sealed case that a wench needs a winch to raise...

[cs]

I started calling it a swing keel after looking at my Balboa. It has what is called a swing keel. It is a massive but it doesn't retract inside a box, but rather is proud of the hull when stowed.

Here is a photo is with the swing keel stowed.

http://farm1.static.flickr.com/241/518721180_762285e626_b.jpg

As you can see on this next image The Meerkat swing keel does in fact come up isnide the cabin in a case. I still think that it is a swing keel because of the weight of it is part of the ballest and not just for windward tracking.

http://farm1.static.flickr.com/114/278487183_f2149a95f9_b.jpg

Chad

[cs]

I'm going to say that a swing keel should be defined as a swinging portion of the keel that acts as ballest and helps with windward tracking.

A centerboard on the other hand is a foil that is weighted enough to keep it submerged to help with windward tracking.

Chad