Shoal draft vs. deep keel

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Mark F. Arena

Check out these equations:

Here goes: shoal keel=too much slippage bulb keel=too much drag fin keel=too much depth Now, check the list, and eliminate the two you care LEAST about. Does this answer your question? M
 
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Alan

Different

It's a different prespective. I'd like to know how it handles against the traditional bulb wing and the fin? If you have any info, please lets know. I wonder about the excess drag due to the doubling of the wetted surface area?
 
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Colin

bilge keel performance

performance is almost identical to the shoal keel.Pointing is similar.Only the 306,326 & 356 have this keel option here in the UK. The two keels together with the rudder allow the boat to dry out upright. The two keels are not parallel, they are slightly toe-in at front.As the boat heels this means the top keel tries to dive down whilst the bottom keel becomes more vertical.Stability is good. Drawbacks are that keels are iron, at high angles of heel there used to be a tendency for the uppermost keel to break the surface and cause slapping but the new hunters are a modern design keel which doesnt seem to do this. Advantages..u can dry out upright...also means if u go aground accidentally..as you are upright and can sit out comfortably in the cockpit you can make it seem to the world that you dried out on purpose. Also great for quick work on the hull at low tide over here tidal range can be pretty big which is why these 'bilge-keelers' have always been popular and retain greater resale value than the fin and shoal keelers. Colin
 
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Alan

Bilge Keel

It appears to be a great tradeoff. I'm wondering how the rudder, which in the US, is a foam cored design can support the weight of the stern? I'm sure I wouldn't want my boat supported in the stern by its rudder even if it was long enough. P.S. To Robert, I just remembered I have photos of the new keel when it was delivered and as it was installed. The final weight of the deep fin is exactly the same as the old bulb wing. It's CG is 1ft lower making the boat stiffer, and the local PHRF fleet changed my handicap by 6 seconds, well worth the adjustment.
 
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Robert Dean

Keel Info

Alan, As you can tell, there is quite a bit of interest in your keel. We are all anxious to see your pictures. I am most interested in contacting MM and have a follow up discussion re: your keel design. If you have an order date or a POC at Mars Metal, it would be most helpfull. And I am still interested in a keel swap if any of the readers have an interest in my 4 ft 6 inch shoal draft wing keel for a HL 35. Robert
 
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Alan

Keel...

Robert, I'm in the process of finding the pics to download, but I'll tell you now that the deal I made with Mars included the return of my old keel.
 
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Steve Dion

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Dec 2, 1999
15,184
Hunter Vision-36 Rio Vista, CA.
What did the keel cost?

Alan: What did it cost you for the new keel. How much credit did they give you for the old one? I got a quote from them last year for a couple of bulbs to attach to the keel on our H'31 and it was about $1500-1800 for the bulbs and freight.
 
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Rodger

Fin Keel

I would be interested in swapping the SHoal Keel for a Fin also. Any info on cost? Design? From Mars metal. What is depth of the Fin? Less then 6ft?
 
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Alan

Keel

As I recall, the cost for my keel was $4800 including shipping and exchange, however that was in 1996. This is not including the marinas charge for the swap. The original plans for the keel put the draft at 6'10". I had them rework the numbers so that my draft came in at 5'10". If I had it to do over again I would go for the 6'10". Having said that, the performance difference was nothing short of remarkable. Leeway was reduced dramatically and pointing ability went up 5 degrees. Also acceleration out of tacks increased and the ultimate speed increased. For all practical purposes it became a different boat. On a reach with my #1 we can hit 8.5kts and with a spinnaker run and a good wind of 25kts we can hit 14.5kts. I have been able to run down and pass a J-27,Ericson39,and C&C40 which should not be possible in a stock boat.
 
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Robert Dean

A Keel Package

To Roger and All, Possibly MM would consider a package deal for say three or five Hunter 35/35.5 keels cast according to Alan's design info. I intend to call MM on Monday and see if their files still have the info on the HL35 keel cast in 1996 for Alan. If others interested in a HL35 keel would also call MM prehaps they will feel the interest level we have out here in Hunter world. They have a web page: marsmetal.com that has their phone number on it. Prehaps we can benefit from the dollar exhange rate of late A keel question ?? Why does the leading edge of the keel have such a large sweep ?? Kelp sweep ?? Robert
 
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Robert Dean

Swept Keels

To Alan and all, A swept keel is one that has the leading edge of the keel cantted back from the vertical. Keels live and work in a fluid. Water is a much more dense fluid than air. Airplanes have swept back wings because they cruise at higer speeds. If one looks at the Concorde it has a radical swept back wing (35 to 60 degrees) and it cruises at Mach 1 plus. Most commertical airliners have a wing sweep of about 38 degrees as they usually cruise at .78 Mach. A straight wing aircraft can only go so fast because of the drag rise on the leading edge of the wing as it speeds up. To counter this, designers sweep the wings back to lower the onset of the drag rise and thus increase the effective cruise speed allowed by the shape of the wing. Propeller aircraft do not cruise at high speeds (relative to the jet aircraft). To take advantage of the power of the jet engine jet aircraft cruise at much higher air speeds thus require wing sweep. Keels in water act exactly the same as wings in air. At the water speeds we travel (6 to 10 knots) there is a sweep from the vertical of the leading edge of the keel that is optium for the symetrical keel shape. The current crop of sport sail boats are appearing with keels that have straight up and down keels - and it could be surmised that these boats cruise (race) a tad faster than the family of Hunter boats. So, to go back to my basic question - why did Hunter sweep the leading edge of the keel ?? Repel kelp build up ?? If we start dealing with MM for a custome keel should we accept the Hunter leading edge sweep or request a more vertical (read efficient)leading edge of the keel (aka the sports boats) ?? Robert
 
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Rodger

FIN KEEL

Let me know what you find out from MM. I think I would do the swap if I can keep draft to 5'10" or less.
 
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Alan

Temptation!!

Wow!! Now you've got my juices going. I sell my 5'10" draft and buy a 6'10". I can only imagine what the boat will perform like.
 
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Alan

Hunter sweep

Robert, To answer your question about Hunter's keel sweep vs sport boat vertical keel leading edge. The sport boats do have a very small amount of sweep. The amount that can be taken is determined by the keels width and depth. On a very narrow keel, like sport boats have, the leading edge is almost vertical, but as the width increases the sweep angle must increase. Our boats have a deadwood that the keel attaches to. If this deadwood is to be used then the new keel must use that width. If you wish to use a narrower keel then the deadwood must be removed and then a more vertical leading edge can be used. However, this leads to more complications dealing with center of effort and center of gravity and righting moment not to mention the need to support the new keel. I opted for leaving the deadwood. To its advantage the wider keel accelerates better than the narrow. If you would be interested, there is an excellent book on the subject, Aero-Hydrodynamics of Sailing by C.A. Marchaj
 
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Robert Dean

Hunter Keels

To Alan and all, Called Mars Metal - they are closed until Monday. Alan, checked with Amazon.com and the book you recommended is available - but at a cost of $120. I'll check with the library. Question - why, as a keel becomes wider must the rake increase ?? Seems to me the only factor in the angle of sweep is the speed through the water. At least that is the way it works with aircraft wings Robert
 
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Alan

Hydrodynamics

I've had my copy for years, didn't know the price was that high. If you can get a look at it you will see that it is very technical but very complete. It helps if you have an engineering or mathematical background. Anyway, with respect to keel sweep, simply put, as the water builds up in front of the leading edge, it must force its way around the keel. If the keel has width, the build up of fluid has no place to go and begins to create drag. By sloping the leading edge, the excess bleeds off downward. With each successive layer there is more buildup, however, as the leading edge moves to the rear, the buildup is kept in check. As you can see, this slope will be a factor of speed, density of the fluid, width of keel and depth of keel. Aircraft do face the same problem, however the foil sections are not as wide, as a factor of cord length and the density of air is very much less than water. The wing of an aircraft needs to create lift in one direction only, whereas a keel needs to be able to create lift in two directions depending on which tack the boat is on. I hope that I am explaining well enough what is really a much more involved process.
 
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Robert Dean

Keel Sweep

To Alan and all, We seem to to be taking up a lot of space on this thread. I trust the other readers will bear with us as we thrash this out. I need to read the Aero-Hydro book by Marchaj to see what he has to say about keel sweep and span wise flow of a fluid. If I apply the pricipels of a wing in air to a keel in water - span wise flow of the fluid is not a good thing. Early jet aircraft had stall fences that followed the chord line of the wing to reduce span wise flow. Modern day jets have winglets to reduce span wise spill off (vortex) on the end of their swept wings (aka Gulfstream IV's, 747's etc.) Modern keels have winglets for - I would guess the same reason - control span wise spill off. While aircraft wings are asymetrical in shape to produce lift, boat keels are symetrical. I find it difficutl to believe that a symetrical boat keel produces lift. There is no way that a symetrical foil (same shape on both sides of the chord) can produce lift as it moves through a fluid, be it air or water or vodka. I know that this subject has received much coverage in many place in the past - read so in the Practical Sailor who quoted a noted author several months past. Enough said - do not intend to beat a dead horse. Have a good New Year. Robert
 
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Sam Kurtz

Close Robert

It has been a long long time since I was taught about Lifties and Thrusties but I think that wing shape is not the most crtical factor in it ablility to creat lift that the angle of attack is a greater factor. Go look at the wing cross section of the F-104 I used to fly. I am still amazed it would fly at all but boy did it. The other thing to consider is how do we fly straight and level inverted? The rest of this is all way beyond this old boy. The link is to a fun place for all you snowbound folks to visit until your marinas thaw.
 
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