The cross-section of the keel on my shoal-draft H31 has a shape I call "hollow-ground," like a knife. The cord is full forward, then curves in as it goes aft, and, about mid-keel, straightens out from there to the aft end. Does anyone know why the keel is so-designed? Why not a long airfoil curve from front to back? Is there supposed to be some advantage to a "hollow-ground" keel? Because I race my boat, this is a critical question for me. I'm looking for learned answers. Thanx
- Status
- Not open for further replies.
Probably difficult to get an answer from Hunter.
Dave: First thing I do not believe that a shoal keel boat is necessarily design to be a "racer". I do not think you could get a definitive answer from Hunter because I doubt that the designer of your model are there any longer. You also realize that you have a cruising boat vs something like a Santa Cruz which would have a VERY deep keel 7'-9' with a very slim cross section and a very large bulb. Take a look at this site and it may give you some insite. http://www.vacantisw.com/KeelDesign.htm
Dave: First thing I do not believe that a shoal keel boat is necessarily design to be a "racer". I do not think you could get a definitive answer from Hunter because I doubt that the designer of your model are there any longer. You also realize that you have a cruising boat vs something like a Santa Cruz which would have a VERY deep keel 7'-9' with a very slim cross section and a very large bulb. Take a look at this site and it may give you some insite. http://www.vacantisw.com/KeelDesign.htm
T
Tom Madura
Mine's the same
Hi Dave. I don't have an answer for you, but I found your post interesting for two reasons: I have an '85 shoal draft 31 that I race in JAM class, so I'm interested in seeing some responses; and secondly, I bought my boat in Sandusky three years ago! Steve is right about one thing - any shoal draft boat is no "racing machine" (doesn't point well), but that is accounted for in the handicap system; although I don't think my boat ever sails anywhere near her PHRF rating. I think that's mainly due to the sails and some to my relative inexperience. (I've been sailing for a long time, and crewed on other racing boats, but this is only my second season skippering my own boat racing.) BTW Steve, you're obviously not familiar with Lake Erie - a racing boat with a 9 ft. fin keel wouldn't even be launchable at most marinas here! You ocean guys have all the water! Tom Madura s/v Good Vices
Hi Dave. I don't have an answer for you, but I found your post interesting for two reasons: I have an '85 shoal draft 31 that I race in JAM class, so I'm interested in seeing some responses; and secondly, I bought my boat in Sandusky three years ago! Steve is right about one thing - any shoal draft boat is no "racing machine" (doesn't point well), but that is accounted for in the handicap system; although I don't think my boat ever sails anywhere near her PHRF rating. I think that's mainly due to the sails and some to my relative inexperience. (I've been sailing for a long time, and crewed on other racing boats, but this is only my second season skippering my own boat racing.) BTW Steve, you're obviously not familiar with Lake Erie - a racing boat with a 9 ft. fin keel wouldn't even be launchable at most marinas here! You ocean guys have all the water! Tom Madura s/v Good Vices
9ft keels don't fit many sailing grounds.
Tom: This was only to point out what a race/cruising boat keel looks like. Our harbor would have a difficult time with anything over 6ft., so I am aware of your point. There has to be a reason that why cruising boats (all that I know about) have a very broad cross section while these racers and racer/cruisers have these very narrow cross sections with large bulbs or wings. This is evident regardless of the depth of the keel. My point being, why don't boats like a Hunter, Catalina, Beneteau etc have a keel that looks like an Americas cup boat with only a 5ft draught? Some of our engineering types may have input on this subject.
Tom: This was only to point out what a race/cruising boat keel looks like. Our harbor would have a difficult time with anything over 6ft., so I am aware of your point. There has to be a reason that why cruising boats (all that I know about) have a very broad cross section while these racers and racer/cruisers have these very narrow cross sections with large bulbs or wings. This is evident regardless of the depth of the keel. My point being, why don't boats like a Hunter, Catalina, Beneteau etc have a keel that looks like an Americas cup boat with only a 5ft draught? Some of our engineering types may have input on this subject.
T
Tom Madura
2 more cents worth
I admit that I don't understand the rational behind all the various keel shapes, but as a physicist who has read a few books on boat design I can comment in general terms. Boats are designed to be most efficient within a fairly narrow range of heel angle, typically 15-20 degrees. In high winds cruisers achieve this by reducing sail area (reefing, etc.) With more weight further below the waterline, a racing boat can carry more sail without exceeding its efficient heel angle. In general deeper is better (faster). Thinner is better because of less drag and less turbulence around the aft edge of the keel. However, thinner is weaker unless it's reinforced or uses some exotic material. So thin and deep is best for racing, but more expensive to build. The "wings" and bulbs are there to reduce turbulence and add weight without making it too deep. Cruising keels are always a compromise to keep them shallow but still efficient, but also relatively cheap to build. Tom Madura
I admit that I don't understand the rational behind all the various keel shapes, but as a physicist who has read a few books on boat design I can comment in general terms. Boats are designed to be most efficient within a fairly narrow range of heel angle, typically 15-20 degrees. In high winds cruisers achieve this by reducing sail area (reefing, etc.) With more weight further below the waterline, a racing boat can carry more sail without exceeding its efficient heel angle. In general deeper is better (faster). Thinner is better because of less drag and less turbulence around the aft edge of the keel. However, thinner is weaker unless it's reinforced or uses some exotic material. So thin and deep is best for racing, but more expensive to build. The "wings" and bulbs are there to reduce turbulence and add weight without making it too deep. Cruising keels are always a compromise to keep them shallow but still efficient, but also relatively cheap to build. Tom Madura
D
Doug T.
Complications
Makes sense that the designers design for a narrow range of heel angles. My Sabre owner's manual specificly mentions that. On advanced racing boats, they are starting to use canting keels -- the keel tilts to windward so that there is more weight on the windward side and thus reduces the angle of heel. However when the keel is angled like that, it must really screw up the lift calculations -- it's more horizontal than it should be for optimum lift. Anyone have any insights or knowledge on how they reconcile that??
Makes sense that the designers design for a narrow range of heel angles. My Sabre owner's manual specificly mentions that. On advanced racing boats, they are starting to use canting keels -- the keel tilts to windward so that there is more weight on the windward side and thus reduces the angle of heel. However when the keel is angled like that, it must really screw up the lift calculations -- it's more horizontal than it should be for optimum lift. Anyone have any insights or knowledge on how they reconcile that??
D
Dave Teal
To fair, or not to fair?
The only reason I can come up with for the shape of my keel is to reduce weight aft. As it is, at dock my boat sits down by the stern. This is ok, however, as the bow drives down in 7 or more knots of wind. Now, the burning issue: will the keel be more efficient if it is faired out? (I accept that a shoal draft isn't going to perform like a deep fin, but if I can squeeze another quarter-tenth of a knot out of it, I'll be that much closer to first place!) Tom (and anyone else who might race an H31), I'd like to exchange racing knowledge. I'm competitive against most boats with JAM ratings close to mine. Since you're in Erie and I'm in Sandusky, it's unlikely that we would be "facing off." Shoot me an e-mail.
The only reason I can come up with for the shape of my keel is to reduce weight aft. As it is, at dock my boat sits down by the stern. This is ok, however, as the bow drives down in 7 or more knots of wind. Now, the burning issue: will the keel be more efficient if it is faired out? (I accept that a shoal draft isn't going to perform like a deep fin, but if I can squeeze another quarter-tenth of a knot out of it, I'll be that much closer to first place!) Tom (and anyone else who might race an H31), I'd like to exchange racing knowledge. I'm competitive against most boats with JAM ratings close to mine. Since you're in Erie and I'm in Sandusky, it's unlikely that we would be "facing off." Shoot me an e-mail.
- Status
- Not open for further replies.