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Underwater Profiles

I own a 31-foot yawl with a cruising fin keel and a skeg-hung rudder, but I am now looking at larger boats capable of handling ocean crossings. The tougher, more stable designs all seem to be full keel. While I want a safe boat, I don't want to give up good sailing. Can you tell me the different sailing characteristics of the many underwater profiles?

Tom Wood responds:
A lot of sailors seem to get hung up on this issue. Underbody configurations are important to sailing characteristics, and are normally blended with other design criteria in an almost baffling array of possible combinations. So let's just take the very basics—even then we tread on dangerous ground as for every statement there are four or five exceptions.

Keels    The longer a keel is, you'll find that:
    1. The more spread out the ballast can be, making for an easier motion.
    2. The more adaptable she is for grounding and hauling out.
    3. The better she will track, especially downwind, but also making her turn and  maneuver more slowly.
    4. The more wetted surface she will have, making her slower in light airs.
    5. The more displacement she will likely have, giving her more storage space inside and giving a slower roll period, but making her accelerate more slowly and limit her feel of being "lively" to sail.

Rudders    You'll find the following to be true regarding rudders:
 1. The farther aft a rudder is, the more leverage it has to turn the boat quickly.
    2. The more vertical a rudder is, the more effective it is at turning the boat.
    3. The more detached from the turbulence of the keel a rudder is, the faster the boat will react to helm movement.

So take the two possible extremes: On one hand, we can have a very light displacement boat with a deep, narrow foil of a keel and another deep, narrow foil of a free-standing spade rudder mounted way aft. Her characteristics would be a fast accelerating, lively, and highly maneuverable craft with limited storage and tankage. She would likely be a handful to steer a straight course in a seaway and have a quick, snappy motion. In short, such a boat would be a fantastic racer or daysailer, or possibly a fast coastal cruiser, that would tire a crew out quickly and not be able to carry much in the way of stores. 

At the opposite extreme, is the full-keeled, apple-bowed hull with no cutaway to her forefoot, and with the rudder set on a 45-degree angle to the aft end of the keel. She would be ponderous in light air, not very maneuverable in harbor, and would not speed up or slow down very quickly, but she would hold a course long enough to duck below and pour a cup of coffee and have a slow rolling motion that is easy on the crew. Such a boat wouldn't be affected much by another ton of water, fuel, or ship's stores.

Is it any wonder why the hard-core, long-distance cruisers opt for the latter while the performance group chooses the former? The real problem comes due to the fact that there is every possible shade and nuance of hull shape along the continuum between these two extremes—Brewer bites, cutaway forefoots, keel fins of every possible length, long fins in combination with skeg or spade rudders, and short keels with attached rudders.

My recommendation would be for you to sail as many as you can, and choose the compromise that best suits the type of sailing you plan to do, and the "feel" you like. Perhaps you'll find that you simply need to buy a larger version of your 31 footer. Good luck.


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