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  #1  
Old 10-05-2007, 08:07 AM
bart streb bart streb is offline
Bart
 
Join Date: Oct 2007
Rep: 10 Posts: 25
Location: North Carolina
keel conversion - fixed to lifting

Hello,
I have a SanJuan21 with a fixed keel, not swing keel and am considering a winter project. Only a few fixed keels were made in the 70's. I think they basically slapped a SanJuan23or24 keel onto these 21' boats. I am adding a picture.
The design challenges are:

Length width and thickness of the keel
NACA foil section of keel
Bulb or lead daggerboard tip
contstruction of keel
placement of the keel on the hull

The current keel is about 5' wide at the top, 2'8" long and about 4.5" thick. it seems massive for a 17' waterline and 1400lb boat. I am hoping to get a little more performance out of a more up to date, daggerboard foil. This is an old school swept back keel.

What I hope to gain is, shallower ramp launching ability, beaching ability, and a small increase in performance.
What I will have to deal with is:
Another mechanical apparatus and loss of some interior space. I think the pros outweigh the cons as most of our ramps in Eastern NC are very shallow, too shallow for this boat. Now the Lakes are dried up in the Central Part of the state from the drought.
I am proficient in working with wood, glass and can work with lead. I have most of the raw materials and would like to keep the cost under $500.
Any thoughts or help would be greatly appreciated as I put this together in my head. Help me put it on paper!
Bart
Attached Thumbnails
keel conversion - fixed to lifting-sanjuan.jpg  
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  #2  
Old 10-07-2007, 11:09 PM
bart streb bart streb is offline
Bart
 
Join Date: Oct 2007
Rep: 10 Posts: 25
Location: North Carolina
Due to hull constraints, it looks like the keel will be 22" wide, and 46" long with 30" hanging out the bottom. The Naca 12 section looks about right. It should be 2.5" thick. I'll use plywood and glass for the keel, and either put a 200lb bulb on the end, or form a plug to match the keel out of lead and bolt it on. I think I would get more effect from pouring a bulb, I could add more weight at the tip.
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  #3  
Old 10-08-2007, 09:02 AM
Brent B Brent B is offline
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Join Date: Nov 2005
Rep: 10 Posts: 14
Location: Coopersburg, PA
Look at Holder-20

The Holder-20 is a reasonably similarly sized boat with a bulbed lifting (straight up) keel. If you can find one to look at, it may help you design your own. The part in the hull is tapered so it wedges in place when lowered.

As to fore/aft location, consider the present sailing characteristics. If helm balance is good, try to put the center-of-effort on the new keel in the same place as the old one. Otherwise, place it to correct current balance problem.

NACA0012 may be a bit thin.

Brent
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  #4  
Old 10-08-2007, 10:43 AM
bart streb bart streb is offline
Bart
 
Join Date: Oct 2007
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Location: North Carolina
Thanks Brent, I wish There was a Holder around here. I think an Ultimate 20 is similar too.
The current keel is very wide at the top (41"), but sweeps back at a steep angle. Not sure where the CE would be, but can take some measurements and guestimate. She is very balanced. Wish I could make the keel longer but would have to hack a slot in the deck for it to exit at top. I really need the keel to retract as far as possible, only the bulb out the bottom, to ease ramp launching.
The NACA 12 is pretty slim. Do you have another suggestion for a section?
Bart
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  #5  
Old 10-09-2007, 10:50 AM
Brent B Brent B is offline
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Join Date: Nov 2005
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Location: Coopersburg, PA
Proposed keel too small

Your proposed keel with 22"x30" exposed comes to 4.6 sq ft area. A reasonable keel area to aim for would be 3.5% of the sail area. With genoa, your sail area is around 200 sq ft, so you want a keel foil area around 7.5 sq ft. With a good foil shape, and the end plate effect of the bulb, you could go a bit lower in keel area. Light air performance will suffer if you go much smaller.

A thicker foil is recommended when you have a small keel area (to reduce stalling).

I suggest you look further into the practicality of of the proposed conversion, and do some real calculations.

Brent
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  #6  
Old 10-09-2007, 11:42 AM
bart streb bart streb is offline
Bart
 
Join Date: Oct 2007
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Location: North Carolina
Thanks Brent, your suggestions are what I was looking for. I have to decide wether to cut a slot in the cabin top (which has only a 10% chance of happening) to make the blade longer or make the foil wider, which starts to get cumbersome in the small cabin. Going thicker is not a problem. I guess a naca 0015?? I am currently only a student of naca profiles and trying to climb the curve.
The boat may not even use a genoa. In superlight winds 1-3knts, when I have raced her, she ghosts along really nice with the jib, the flow stays attached longer, and she is faster than most other boats larger than her or boats using genoas. If she excels in any condition, it is that wind range.
The practicality of the conversion has been weighed and it is worth pursuing. She will be easily ramp launched, and beachable. I want to be able to raise the blade without using the mast or rigging.
Does a NACA 0015 seem reasonable?
Thanks.
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  #7  
Old 10-10-2007, 09:01 PM
bart streb bart streb is offline
Bart
 
Join Date: Oct 2007
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Location: North Carolina
The NACA 0015 would be 3.5" for a 22" wide keel. Looks chubby on paper, but I guess if I have to go shorter (30-38" long), then I might have to go thicker, like the 0015?
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  #8  
Old 10-11-2007, 10:59 AM
Paul B Paul B is offline
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Quote:
Originally Posted by bart streb View Post
The NACA 0015 would be 3.5" for a 22" wide keel. Looks chubby on paper, but I guess if I have to go shorter (30-38" long), then I might have to go thicker, like the 0015?
For this type of dagger keel even 12% is very thick.
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  #9  
Old 10-11-2007, 01:06 PM
kitedancer kitedancer is offline
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Quote:
For this type of dagger keel even 12% is very thick.
Why ?
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  #10  
Old 10-11-2007, 08:02 PM
bart streb bart streb is offline
Bart
 
Join Date: Oct 2007
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Location: North Carolina
The 0012 is 5.7cm wide (2.25")
The 0015 is 8.9cm wide (3.5")

On paper, it looks like a huge difference. I have the full size template plotted out.
Hull speed on this boat is only 5.5 knots. Which foil would give the best lift for the drag? (humble opinions welcome!)
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  #11  
Old 10-11-2007, 10:33 PM
markdrela markdrela is offline
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Location: MIT Aero & Astro
Quote:
Originally Posted by Brent B View Post
A thicker foil is recommended when you have a small keel area (to reduce stalling).
Not in this case. In the NACA 00xx family at around Re=1M, the 0012 or 0013 have the largest maximum lift. But there isn't much difference in max lift between the 0010 and 0015 in any case.

But there are a number of other considerations besides keel profile drag and CLmax.
One such consideration is corner separation from the keel/hull intersection, which almost invariably gets worse with thicker airfoils. This pushes you towards the 0012 or 0011.

It's not clear if you're constrained by max draft. If not, then increasing the keel span will almost surely give a much bigger benefit than any airfoil tweaking. More span will reduce the induced drag, and will also reduce leeway. Leeway is also reduced with more chord, but not as much as with more span.
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  #12  
Old 10-11-2007, 11:01 PM
bart streb bart streb is offline
Bart
 
Join Date: Oct 2007
Rep: 10 Posts: 25
Location: North Carolina
Thanks,
I have some constraints to max draft. I would have to chop a slot in the deck to get a longer keel to retract into and out of the cabin when raised, or have keel hanging out of the bottom of the boat on the trailer, which makes ramp launching more difficult as the boat sits higher up. Neither of those choices are something I want to deal with. I need enough keel in the boat when it is at it's lowest point to support the forces on the keel under sail (at least a foot in the trunk??). More span (over 22") puts the keel trunk in your way in the cabin. So it looks like a maximum of 38" of draft down, that leaves 4" hanging out when up, and 22" of width. That makes 5.5 sq ft of board, and still only 2.9% of sail area (190 sq ft main/jib), short of the 3.5% desired.
thanks again.
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  #13  
Old 10-14-2007, 08:05 PM
bart streb bart streb is offline
Bart
 
Join Date: Oct 2007
Rep: 10 Posts: 25
Location: North Carolina
thanks guys, I will have to go with a chord of 24" and a span of 38" (limited b y boat design) and use the NACA 0015 template. I have to get started at some point. If this seems unreasonable, let me know!
Thanks,
Bart
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  #14  
Old 10-23-2007, 04:48 PM
bart streb bart streb is offline
Bart
 
Join Date: Oct 2007
Rep: 10 Posts: 25
Location: North Carolina
For this keel that will "live" in a world that flows by it at 2-6 knots 95% of the time, would a NACA 64A010, 0012 or 0015 be of best efficiency?(I promise I have been studying, but there just doesn't seem to be a good answer). I will add a 200-300 lb bulb, but will tackle that later. It will have an end plate effect.
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  #15  
Old 10-24-2007, 11:51 AM
Paul B Paul B is offline
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Have you considered the 0010-34?

You may want to take a close look at that one.
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