|
|
|
 |
A floating object has part of its body under water and part above water. The part above the water is subject to the pressure of the wind and so the object tends to move in the direction wind pushes it. The part below water resists this movement. The name of the game in sailboat design is to use some scientific magic to make the wind power and water resistance move the boat to where you want it to go.
As you probably know by now, a sail curved like an airplane wing is used above the water to do its magic to convert as much of the wind force as possible into forward motion. So in this paper we will not go into the details of this trick. Instead we will focus on the underwater fin; the necessary evil that makes the sail capable of doing its thing.
We call the keel "evil" because: a) It makes it necessary to operate a sailboat in deep water instead of the shallows we would like to be able to sail in. b) It adds weight to the boat which in turn means that it takes more wind power to move it. Most of us would like to be able to sail even when there is no wind. The lighter the boat,the closer we can come to this wish. c) It slows the boat down by increasing the amount of boat's surface area that comes in contact with water. This "Wetted Surface" is resistance and, again, more wind power is needed to overcome it. Sooner or later, we all want to move a little faster. d) It ups the cost and upkeep dramatically, in ways you may not immediately think of; things like bottom painting, repairs from grounding, higher handling and storage costs, and so on. e) It decreases safety since most keel boats cannot carry enough flotation to make them unsinkable.
The words "design" and "compromise" go hand in hand. Nowhere is this more evident than in sailboat design. The more area a keel has, the more efficient it is in keeping the boat from sliding sideways. But the more the area, the greater the wetted surface so the more the resistance. The "form" of the fin area is another conflicting factor. Consider two boats with the same fin area, but one has its area in the form of a long and shallow keel for less draft. The other has the same area fin but in the form of a narrow, deep keel for greater speed, at the expense of greater draft. A compromise between these two extremes is the wing keel; a fin with small perpendicular fins on either side. These keels usually have greater draft than long shoal type keels but less draft than deep fin keels. Their lack of depth is made up for by the fact that when the boat heels, the wing tilts downward, thus increasing the overall effective keel depth. But adding these wings seems to increase performance more than what might be expected, so we have to look into the "why's" of keel shaping.
Why does the deeper keel, and now the shallower keel with wings, work better than a keel that is not as deep but yet has the same total area? It is not hard to understand when you consider the fact that, when the boat moves through the water, the pressure on one side of its keel is different than the pressure on the other side. This difference in pressure helps the boat slow down its sideways slippage. But water wants to even out its pressure so it tends to want to flow under the keel and up the lower pressure side of the keel to neutralize the pressure differ-ential. The deeper the keel, the greater the distance the water has to travel to cause this cancellation of the difference in pressure on the keel so the more efficiently the keel performs. Wings on the bottom of keels have this same effect of making it more difficult for the water to flow around to the low pressure side. But the story does not stop here. In addition to area and depth, the cross sectional shape of the fin affects its performance. And here is where we get into some radical new thinking. Up to now it was thought the keel should have a shape similar to the sail (airplane wing). But now we think we had this backwards and the fin's cross sectional shape should be turned around. This obverse reasoning is: Reversing the con-conventional fin shape increases the area available for pressure surface and decreases the area that results in drag caused by the flow of water breaking away from the fin. See drawings below.
There is one more consideration we should be aware of before we get into choosing our proper keel. The fact is a boat does not need a keel when it is being pushed by the wind in the direction you happen to want to go. So the ideal fin disappears when we do not need it and reappears when we do. Furthermore, the amount of keel we need also varies with speed. That is because the faster a boat goes through the water, the more efficient the keel becomes so the smaller it has to be to do the job. You can "see" this better in a fast jet plane with its small wings and a slow glider with its very large wings. You may also have noticed, when you sit in a commercial plane looking out the window, that the pilot expands the wing area when he is slowing down to land.
Some boats ignore this need for a variable fin area. Others use center-boards (fins that completely disappear inside the boat). And others use combination keel/centerboards that become smaller by having the board go into the keel.
Now that we are smart, we have to decide which is the best kind of fin for our particular needs. So let's review the various types:
Swing Keels: A swing keel is essentially a centerboard that has a lot of weight to it. They perform well and allow floating the boat in shallower waters when they are cranked up. Those that go inside of the boat reduce wetted surface when going with the wind but take up valuable cabin room. Those that only crank up to the bottom of the boat do not reduce wetted surface. Most manufacturers are no longer producing boats with swing keels because of the mechanical problems that go with having to raise a heavy, ballasted fin.
Shoal Keels: These are shallow draft fixed fins that run along a long length of the boat's bottom. Shoal draft keel boats are less costly to build and less maneuverable to sail. This type keel is mostly found on inexpensive small entry boats and on some larger cruising boats.
Centerboards: These are relatively light weight fins that can be lifted into a trunk inside the boat. They are mostly used on small racing boats since they can be varied in area all the way down to zero. There is a slight move towards using this type of fin on very large boats but only a few manufacturers are trying this at the moment.
Deep Fin Keels: These are the best type of keels for upwind performance. Most larger racing sailboats use them. Their obvious drawback is the amount of water they require. Their secondary drawback is that the area of the fin cannot be controlled.
Combination Keel/Centerboards: The boat has a shallow draft fixed keel with a centerboard that pivots into a trunk, inside the keel, instead of going up into the cabin space. Considering all the pros and cons of fin design, for both performance and for use purposes, all magazine articles on fin types say this is the best way for most buyers to go, even though the combination K/C is the most expensive type of fin to manufacture.
Wing Keels: These are somewhat deeper shoal keels but have horizontal extensions added onto each side. This recent idea has the wanted effect of decreasing draft without sacrificing performance as much as the shoal draft keels do. But they are not for everybody. Although almost every manufacturer has gotten into this act, they do have drawbacks: If you hit bottom they tend to dig in and act as a terrific anchor. You can sit or lay there 'til the next tidal cycle. You cannot use wind or crew to tilt the boat to decrease draft and float off because tilting this boat with wings increases its draft! So no escape this way. They draw more water than variable fin boats nor are they as easily beached or trailed and launched. Their fixed, wetted surface exceeds that of a same size centerboard or combination keel/centerboard boat, slowing them a bit. They generally are of cast iron and must be more carefully maintained.
Daggerboards: This is a fin that is variable in area by moving up and down, instead of pivoting back. For sheer racing they are probably the best way to go. For the relaxed sailor they can be sheer disaster. Their trunk divides the cabin. If you hit bottom something has to give. They are mostly found on very small boats. Dealers, who handle boats having daggerboard, tell us they simply carry a stock of replacement boards.
HOW THE RHODES 22 DOES IT. From the beginning, the more expensive combination keel/centerboard was elected to be the fin for the Rhodes design. During all these years it has performed respectfully. Although it provided respectable speed for a trailerable/cruiser design, pointing ability was only acceptable; i.e., it tended to slip more than boats with larger keels. Recent rethinking of sailboat keel design prompted complete reshaping of the Rhode's 22 board, trunk and cap to develop the backward logic of the Diamondboard.
Based on theory you have read in these notes, three areas of fin design were re-addressed. The ground rules: Improve sailing performance without the handicap of production cost considerations. If only one of the three theoretical approaches proved valid, the performance improvement would be worth the effort and expense. If two worked, so much the better. And if all three were on track, the results would be spectacular.
Recent model Rhodes have combination Keel/Diamondboards and testimonials have been positive. It is easy to sense improvement. It is difficult to measure it. Sailboat design is still partly art. If you have an artistic bend, you will be pleased by the feel and performance of this new hidden asset of the Rhodes 22.
One design goal: to provide an extension to more of the shoal keel length (than that provided by typically narrow centerboards) to reduce pressure neutralization of the keel part of the combination. A return to earlier sailboat designs, where lowered boards dropped out along a longer run of the keel bottom than today's narrow boards, did this. At the same time a secondary benefit slipped in: The re-shaping of that part of the board, that remains inside the keel, now closes off the slot opening along the bottom of the keel. In narrow board boats this opening creates turbulence that racers try to reduce with flexible tabs over the slot.
The second design improvement goal: to increase the total fin area. The shape of the Diamondboard, and its unique interior cap, increases total keel area over the prior conventional centerboard outline. Not only has the new design met this goal but it spun off another side effect benefit. While the new Diamondboard increases pointing ability with greater area, at the same time it decreases draft for expanded sailing horizons. Long, thin centerboards do just the reverse.
And finally, we wanted to increase the coefficient of efficiency of the board itself. We turned the board around, back to front, and reshaped it. The new board has an asymmetrical diamond cross section that results in a decreased "break-away" drag area, while the area for critical side pressure is greatly increased.
The results are truly impressive. Come join us for a demonstration sail on the new Rhodes 22 and share the thrill of great performance.
