Join Date: Feb 2000
Location: Annapolis, Md
Thanked 216 Times in 167 Posts
Rep Power: 10
In a general sense, there is nothing currently available that is more efficient in terms of lift to drag than a deep fin keel. For a given draft and an equal weight, a bulb keel will get the weight of the keel lower than a fin keel, but that comes at the price of increased drag. Designers using bulb keels are making a trade off between greater stability vs. decreased lift and increased drag. Properly designed bulbs are shaped to provide an end plate for the keel and reduce keel vortex, which to a small degree offsets the decreased lift and increased drag.
Few, if any, so called wing keels provide any lift at all and at best they are a glorified bulb keel, which like a well designed bulb, provide some end plate effect and reduce keel vortex (although their own wider spans may produce tip vortexes equal to or even larger than keel's tip vortex.) Wing keels, especially the low aspect wings used on production boats, are extremely hard to design so that they have a proper incident angle and since the angle of attack relative to the water flow greatly affects drag, and also and since the incident changes dramatically in waves, despite the marketing claims to the contrary, in practice, the kinds of low aspect ratio wing keels used on production boats tend to have enormous drag compared to deep fin versions of the same design (or even bulb keel versions) As a result performance oriented production boats are just about doing away with the option of the type of wing keels that were popular just a few years ago.
There was a question about the Naval Academy study. This was a study that was undertaken a couple years ago. The Naval Academy was getting ready to move onto its next generation of 44-foot cutters that are used for training purposes. As it turns out the NA44's really get abused. They apparently are in collisions once a week on average, and are rung aground with fair regularity (sometimes as a training drill). Since the NA44's are also raced, there was some concern about performance as well.
The study involved a rather large hull model of the proposed new NA44. There were a variety of model keels constructed that could be switched out as part of the testing program. The bottom of the testing tank was lined with a wedge of sand chosen for its consistency and linear behavior. The model boat was towed at a constant speed and force into the wedge of sand and the forces of impact and depth of penetration precisely measured. Then the boat was extracted and the force of extraction measured. They also measured rotation forces as well, since rotation is often a component of extraction.
The experiment was repeated with a number of different wing, bulb, fin, and long keel configurations as well as at various heel angles. If I remember correctly the results were then calibrated by towing full sized 44's with mock-up keel bottoms into the section of Bay bottom with speed and tow load made as constant as possible. Impact loads and withdrawal loads were then measured.
What was observed was that the bulb keel went the least distance into the bottom, and had the least extraction loads. It was found that the bulb also had the lowest rotation forces, pivoting easily on its rounded bottom. Fins came next in terms of ease of extraction. Fins and bulbs had near equal straight back extractions but fins lost out because its narrow frontal area allowed it to penetrate deeper and so made rotation much more difficult. Wing keels were a distance loser in this study, which is consistent with the in the field observations of the TowBoatUS operator that I mentioned.