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  #1  
Old 04-09-2010, 03:52 AM
SuperPiper SuperPiper is offline
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Winged Keel

I sail a micro-cruiser with a 300-lb dagger board.

I have been considering adding about another 100 lbs of ballast to the bottom of the keel. The aspect ratio of the existing keel is terrible at about 1.0 and it has no taper (it's square when completely extended).

Instead of a straight extension or a bulb, I am attracted to a wing as per Eric Sponberg: starting at the keel's maximum thickness and extending beyond the trailing edge to form a "beaver tail".

The wing may provide an endplate effect and reduce the induced drag of the stubby little keel. When the boat heels, it will probably act to reduce leeway too.

So, what section should be used?

With the boat in a heeled position, would the wing be more effective if it was asymmetrical (like the dagger boards on a canting keel boat)? Flat on top, cambered on the bottom?

Good advice always accepted.
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  #2  
Old 04-09-2010, 08:02 AM
bistros bistros is offline
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There is no way to give you a definitive answer without a lot more information. Yes, you could be reducing the induced drag - at the risk of increasing the form drag. If the designer of your boat is still around, and if he works with the right tools he may be able to provide you with an answer (that will still have to be field verified!).

Otherwise, it may just be one of those things you have to try and see.

Another point of view is that a 5-10% increase in performance in optimal conditions may not be significant enough to warrant all the work involved. Kind of like adding a aerodynamic trunk lid spoiler to a Yugo.

If you want to proceed:

First, I'd establish a baseline set of numbers for the boat in it's current configuration - windspeed, point of sail, angle to wind, boat speed, velocity made good (VMG), rig settings as well as your subjective impressions. Do this across a range of conditions. A Velocitek GPS can be your friend here.

Then you can experiment and compare results. Frequently academic predictions are trumped by field observed performance, so be prepared to go back to the drawing board a few times.

You may find it worthwhile to talk with Steve Killing as well. He lives sort of near Midland / Alliston area - and given your location "North of Lake O", he may be close enough to consult with. Search for him on the web and see if he seems suitable.

--
Bill S. in Ottawa
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  #3  
Old 04-09-2010, 10:53 AM
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Doug Lord Doug Lord is online now
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Quote:
Originally Posted by SuperPiper View Post
I sail a micro-cruiser with a 300-lb dagger board.

I have been considering adding about another 100 lbs of ballast to the bottom of the keel. The aspect ratio of the existing keel is terrible at about 1.0 and it has no taper (it's square when completely extended).

Instead of a straight extension or a bulb, I am attracted to a wing as per Eric Sponberg: starting at the keel's maximum thickness and extending beyond the trailing edge to form a "beaver tail".

The wing may provide an endplate effect and reduce the induced drag of the stubby little keel. When the boat heels, it will probably act to reduce leeway too.

So, what section should be used?

With the boat in a heeled position, would the wing be more effective if it was asymmetrical (like the dagger boards on a canting keel boat)? Flat on top, cambered on the bottom?

Good advice always accepted.
-------------------
I'd ask Eric, if I were you-if you haven't already. Also, another member here-Milton Thrasher -added an endplate to the keel of his Irwin 23-you might contact him. I think he had good results with it. I used an endplate consisting
of wide long piece of iron bolted to the bottom of the keel(and well shaped and faired) on my 20' motorsailer. The "plate" was about 8" wider than the keel and stretched behind the keel about 18'' in a beavertail shape. The most noticeable effect from it was its dampening of the motion of the boat when anchored or tied up in waves. She behaved like a much larger boat. The boat went well to windward despite the large pilothouse...

Here are pictures of Milts winged keel: http://www.boatdesign.net/gallery/sh.../5/ppuser/3587 Go to "Members List" and search "Milton Thrasher".
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  #4  
Old 04-09-2010, 04:42 PM
SuperPiper SuperPiper is offline
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The purpose of the additional ballast is to improve the boat's stability. Installing the ballast as a wing is incidental.

I'm trying to jack up the boat's AVS and STIX numbers so that I may qualify for an offshore race.
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  #5  
Old 04-09-2010, 05:20 PM
bistros bistros is offline
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Quote:
Originally Posted by SuperPiper View Post
The purpose of the additional ballast is to improve the boat's stability. Installing the ballast as a wing is incidental.

I'm trying to jack up the boat's AVS and STIX numbers so that I may qualify for an offshore race.
That's a different ball game! The way the first post was written it appeared the point was something else entirely. Sorry about the incorrect interpretation.

Given the need to make sure adequate and correctly located buoyancy is a major factor in this, knowing the boat design is pretty critical. Adding ballast has to be done without compromising the boat's recovery ability.

Can you let people know the design name, builder and designer? This would be critical before a NA could give an opinion on the probability of successfully increasing stability index.

--
Bill in Ottawa
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  #6  
Old 04-11-2010, 03:29 AM
SuperPiper SuperPiper is offline
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I don't know where I got this, but I'm considering a NACA 5313 with the camber turned down.

When bobbing along at 3 knots, it's not likely to create any significant lift.

But when sailing at 6 knots and heeled to 22 degrees in a bow-down trim, the wing would start to resemble a dagger board. It would be creating lift to windward AND increasing righting moment due to the downward component of this lift.

The extra mass will factor well in improving AVS and STIX calculations.

I don't think boat particulars matter. This would be true for any craft.

What section would you use?
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Last edited by SuperPiper : 04-11-2010 at 04:13 AM. Reason: Added Diagram
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  #7  
Old 04-13-2010, 06:52 AM
SuperPiper SuperPiper is offline
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What are the details of a Scheel keel?

Do any of you have a good link?
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  #8  
Old 04-13-2010, 07:16 AM
LyndonJ LyndonJ is offline
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The scheel died a bit of a death , it was extensively tank tested a few years back and the lift drag volume trade offs were worse all around when compared with a standard foil shape. From memory Ted Brewer was involved in the testing.
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  #9  
Old 04-15-2010, 06:38 AM
SuperPiper SuperPiper is offline
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Here is a photo lifted from the Marskeel site. I've seen this style of keel bulb on recent racer-cruisers.

Is there a name for this keel? What are the details for this?
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  #10  
Old 04-15-2010, 07:39 PM
LyndonJ LyndonJ is offline
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Keel design is covered in Pier Gutelle's tome on yacht design I think he looks at a variety of shapes.
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