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Boojum's Twin Keels
...form follows function
Copyright 2000 - 2004 Michael Kasten
The Design Requirements
This page is a brief introduction to the requirements set forth prior to the design of the 25' Tug yacht, Boojum. Please also see the article, An Overview of a Few Common Roll Reduction Strategies presented at this site.
Aside from the primary design requirement, "She's got to be Cute!" Boojum's other owner-imposed requirements necessitated the eventual keel configuration given to the boat. Here is a partial list of those requirements, many of which may at first seem to be in conflict with one another, but which in the end have been nicely resolved in the final design:
- A limitation on overall length for a passagemaking "trawler-yacht" of 25'
- A limitation on beam imposed by a requirement for trailering the vessel anywhere in the US without a permit
- That the vessel sit upright on the mud; on the deck of a ship; or on a flat bed trailer without requiring a cradle
- Full 6.5' standing headroom in the Pilot House
- That the vessel carry nearly all the comforts of home and no small amount of electronics
- That the vessel carry enough fuel for a 3,000 mi offshore passage (thus the fuel tankage size and location)
- That the vessel pass the full IMO offshore stability criteria (thus the ballast quantity and location)
- Excellent roll dampening (thus the overall size of the keels and the distributed masses within them)
- Enhanced directional stability in such a short-overall-length vessel (thus the long symmetrical foil shape; the drag given to the leading edge; and the lack of toe-in or toe-out)
- Some amount of performance under sail with the (albeit somewhat small) get-home sail rig
This is a partial list of the rather strict design requirements imposed by Boojum's owners, Charles and TC Vollum. After no small amount of research and no fewer than some 60 or more iterations of the hull model, the above requirements were best met with the eventual design configuration.
It should go without saying that another vessel design, starting with a different or less rigid set of requirements, would naturally have emerged with a rather different configuration. This is a good illustration of the task of Custom Boat Design: to take the stated design goals and bring about an integrated solution to the requests...
The Design Choices
Short of actual tank testing, Boojum's twin keels have been designed using the best information available. In that sense they are experimental. However as you will see, they have been carefully designed to fulfill the tasks required of them. Here are some of the issues which were considered, and the solutions which have been offered in the final design:
The twin keels on Boojum are absolutely huge in proportion to the size of the boat, and also by comparison to the usual concept of "bilge keels."
Boojum's twin keels have been provided for a variety of reasons. As described above, they are for the purpose of being able to sit upright on the mud, to sit upright on a flatbed trailer, or to sit on the deck of a ship or on a marine railway without a cradle. In places with large tidal ranges and shoal water estuaries the use of twin keels is justified on that basis alone.
The keels on Boojum will additionally provide stability and lateral resistance for sailing. The get-home sail rig is very small for the boat, so performance will not be stellar. Nevertheless, she should be able to make her way under sail for the purpose of self-rescue, especially with a good strong wind.
The twin keels hold Boojum's fuel, so help to lower the center of gravity and also help to spread out the vessel's masses.
The twin keels contain the vessel's ballast, for which purpose they could not have been made any smaller and still contain the required amount in the correct location. The ballast amount was determined by the requirements of adequate stability per the IMO offshore stability criteria. The ballast was divided equally between the two keels for the purpose of spreading out the overall mass of the vessel athwartships in order to increase the vessel's roll moment of inertia. Using this arrangement, the entire mass of both lumps of ballast will have to be put into motion whenever the vessel rolls.
The effect is similar to that of a gyroscope which has its mass distributed toward its perimeter. The effect is augmented by the entrained mass of sea water which must move along with the keels in their boundary layer.
Studies done by the US Navy have indicated that a roll reduction on the order of some 30% is possible with passive twin keels depending on the speed of the vessel, sea state, etc. The US Navy tests were done on rather larger vessels than Boojum. In proportion to vessel size the Navy vessel's "bilge keels" were much smaller.
On Boojum, the twin fins are relatively large and they extend farther from the vessel's roll center than do most "bilge keels" on power vessels. We are therefore expecting excellent results in terms of roll reduction.
Of course, the twin keels provide a much greater overall wetted surface, so will contribute to the overall drag of the vessel. However, in consideration of the possibility that the use of paravanes would contribute much greater drag, and that the paravanes will now be able to be smaller, and will need to be used far less often, we expect that the net result will be greater overall passagemaking efficiency. In addition, tank tests on other vessels having twin keels have shown some "wave-cancellation" effects provided by twin keels. In other words, there is a tendency for the displacement of the twin keels to help "fill" the 'midship wave trough at speed.
The relative benefits of this "wave cancellation effect" seem to depend on placement of the keels and their size.
Boojum's twin keels have been designed using a low drag, symmetrical foil shape. The foil chosen is in the NACA 0012 family for the sake of being able to sustain high angles of attack (as the hull rolls) without stalling, therefore creating less turbulence and less excess drag.
Flat plate "bilge keels" on the other hand, if given the same the same profile, would have nearly the same amount of wetted surface as a foil, but would have no grace whatever for moving through the water. A foil shape provides far less turbulence under way, so less energy is dissipated into the water. The foil shape also contributes some amount of lift, which will be of use for Boojum's "get-home" power: the sails.
Due to the twin keels, Boojum has greater wetted surface overall (the dominant drag factor at low S/L ratios) but as mentioned, there is also the potential for fairly effective wave cancellation, a benefit at higher S/L ratios (wavemaking being the dominant drag factor at higher S/L ratios). Being a very small vessel, and therefore having a low absolute speed potential, it is anticipated that Boojum will be driven at the upper S/L ranges quite frequently, and that the wave cancellation effects may prove to be a significant benefit.
The twin keels are located as far aft as possible without the vessel falling on her nose while on the mud. The twin keels have been aligned parallel to the centerline and have been swept back to a large angle. All of these features were chosen in order to minimize any potential steering anomalies.
Based on the evidence from prior vessels which have made use of twin keels, we expect that Boojum's twin keels will actually help the vessel track more steadily.
The Prediction
My hunch, after no small amount of research, conjuring, scheming, modeling and re-modeling the hull lines, is that the result will be excellent. Being quite experimental, whether these strategies have been successful will only be known for sure during sea trials, and after a good number of sea miles...
The Result
Boojum, now launched, has proven the above concepts admirably. In other words, the efforts expended have paid off, and the vessel behaves as intended. Tracking is excellent, and performance under power is as expected.
Performance under sail is also as expected, i.e. less than stellar...! This would be enhanced considerably by the addition of more sail area, particularly forward. This would be best achieved via the addition of a mast forward in order to improve balance.
Roll attenuation due to the presence of the bilge keels is of course impossible to quantify without having another vessel of equal proportions with a single keel... a difficult proposition at best... The subjective impression however is that roll decays very quickly.
Roll attenuation is augmented by the presence of a riding sail, and of course by paravanes. In combination, the result has been described as being comparable to the stability offered on a sailing vessel under a full press of sail. In other words, quite steady!
More Information...
For a discussion of some of the research sources used for Boojum's keels, and a discussion of roll reduction in general, please see my article, An Overview of a Few Common Roll Reduction Strategies. Also, please check out the 25' trawler, Boojum.
Please see the AVAILABLE BOAT PLANS web page.
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