performance of sailing boats having a Schelling keel

[Grigrigrigoris]

I would appreciate comments regarding the advantages and disadvantages of sailing boats having the Schelling keel. Thanks

[Jeff_H]

I believe that you are referring to a Scheel keel. A scheel keel is a specialized form of a bulb keel that mitigates some of the negatives of a somewhat decreased draft. Generally Scheel keels seems to be applied mostly to shallow fin keels.

To begin with shallow fin keels are a tough animal to classify. They have much of the drag of a full keel with little of the efficiency of a full depth fin keel. A lot can be done to improve a shallow fin. One way is to add a bulb. A bulb is a cast metal ballast attachment added to the bottom of the keel. They concentrate the ballast lower providing greater stability and sail carrying ability than a simple shallow keel. Traditionally bulbs were torpedo or teardrop shaped. They have been re-contoured to provide some hydrodynamic properties. A scheel keel is one form of a keel that has been reconfigured to improve hydrodynamic performance by reducing the tip vortex. In other words, shallow keels need to be longer horizontally than a deeper fin in order to get enough area to prevent leeway. This means that a shallow longer fin would generate more tip vortex and more drag than a deeper keel. The top of the scheel keel bulb creates a surface to turn the water aft and prevent it from slipping over the tip of the keel thereby reducing tip vortex. This does not come free since a bulb increases frontal area and surface area while further reducing the span of the keel. A scheel keel will permit a reduction in draft in the 15% to 20% range with a minimal reducton in performance. The rounded bottom of a scheel keel also makes them easier to unstick when aground than more traditional long keels or wing keels.

Jeff

[RichH]

JeffH - Nice reply on the scheel keel.

Would you kindly list or recommend a source or reference for keel shapes. I''m aware of the NACA series but need to reshape (and verify) the profile of poorly designed cut-away full keel.
For the ''easy work'' sections if started to profile to the NACA 64A010 shape for the leading edge; but, the original ''knuckle'' section is too broad and I cant cut away to attain the 010 shape (without resulting in huge bulb extension like a super-tanker to get a 010 shape at the knuckle). The original profile is a large flat forward surface at the knuckle .... and "that aint good". Its a Ty37 Perry design but the yard took lots of ''liberty'' with the orig. drawings and screwed it up when constructing the molds, etc. Thanx.

[Jeff_H]

I would contact Bob Perry directly. He really knows this design and is far more knowledgable about these things than I will ever be. That said, my first take is that the Tayana 37''s have so much wetted surface and tip length that altering the keel sectional properties to reduce drag is bit like putting lipstick on a gorilla. The realtities of these boats are that they offer good sailing ability for a full keeled boat, (a statement not all that different in implication than the old punchline, "You don''t sweat much for a fat girl'') and that minor changes in keel section won''t improve thier preformance very much at all.

Respectfully,
Jeff

[Grigrigrigoris]

Hi Jeff,

Thanks, you are, as almost always, right. I meant a Scheel keel, I just misunderstood the name. I heard that Mr. Scheel is an American naval designer. You mentioned all the advantages of this keel. However if it has so many advantages why only very few boat designs include this keel? Is it too expensive? What about drifting when the boat is sailing close haul? So far I have only seen (here in Europe) a Moody 38, which is no longer in production, with this keel.

[Jeff_H]

The shape of a genuine Scheel keel was quite sophisticated. It was designed to minimize drag while increasing performance and so careful shaping was essential. If I am not mistaken, Henry Scheel who developed the Scheel keel was an American yacht designer who was associated with S&S at one point in his carreer.

The reason that Scheel Keels are not used more frequently probably results from several factors. The actual design of the Scheel keel was very specific and was patented and so had to be licenced with a fee paid to actually use a genuine Scheel Keel.

But probably more significantly, like almost anything else in yacht design, the shape of a keel bulb is really the product of balancing a set of conflicting criteria. The Scheel keel seems to be biased a bit more towards minimizing drag which comes at the price of a comparatibly limited increase in stability and leeway reduction performance. Different designers have chosen to push the envelope more towards adding greater stability and reduced leeway at the price of higher drag. When you look at modern bulb keels (which after all is what a scheel keel really is) there are wide variations in the sectional and profile characteristics each with its own set of advantages and liabilities and as each designer presses the envelope in the direction that makes sense for that individual design, there is less motivation to use a design as specific shaped in the manner of a genuine Scheel keel would be.

I want to emphasize that on a lift to drag basis nothing is as efficient as a simple fin keel perhaps with winglets or a very small bulb. It is only the desire for shallower draft either for pragmatic reasons or because of a racing rule that pushes designers to explore alternatives like the Scheel keel. While the Scheel keel is definitely an improvement over the typical shoal draft keel without augmentation, it will not go to weather as well as a simple fin with greater depth.

Respectfully,
Jeff

[Silmaril]

One of the best examples of a Scheel Keel was the S&S designed Tartan 33 from the early 80''s. This was a really neat boat. 7/8ths rig with a Scheel keel (deep fin option) it was a breeze to single hand (I did a couple of single handed trips from Westport CT to Block Island) on one. The boat was nicely balanced. It suffered a bit close hauled, but on all other point of sail it was the equal to it''s deep finned cousins.

S&S also designed the Tartan 42 during the same period with the Scheel keel. Not as nicely balanced, it was sort of a big Tartan 37 with a smallish aft cabin. It was a masthead rig and just did not seem to have the legs that the 33 had. But for a shoal draft cruiser, it was pretty nice. You can find them in the mid $90''s on yachtworld.

Like Jeff mentioned, the boat needed to be a complete design to take full advantage of the Scheel. The Tartans of that era were fairly well constructed, and seem to have held up well 20+ years later.

[Grigrigrigoris]

Dear sainet members,

I appreciate very much your excellent comments. Thank you very much.

[sailnow]

I have a 37 morgan heritage 1976 and it has a 7'' draft via a fin keel. I sail the Florida west coast and am very limited in the channels and marinas due to the deep keel. I want to lop off 1'' of the lead keel. This would be a 6'' horizontal cut and the maximum thickness would be about 4". Can anyone offer ramifications of the reduction in keel and the best method how to accomplish the reduction. It has been suggested to use a torch and it would melt like butter. Thanks, jim

[Denr]

A guy in my boat yard used a chain saw, true I watched him do it! He then bolted on a two-piece bulb, I believe that the hardware came from Mars in Canada.