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cargo ships and tank-ers are economically attractive. The bigger the vessel the higher the transport effi-ciency. If we scaled

Summer Moon II up to 900 feet LOA super-tanker size, she would displace 80,500 tons on an 800-foot water-line. Because the wa-terline is so long, we would only need to drive this supertanker

Summer Moon II at an SL ratio of 1 be-cause—with such a long waterline— this still gives 28 knots. (Slower is more efficient, and 28 knots is faster than nor-mal for cargo transport.) Power would be around 55,000 hp. The resulting trans-port efficiency: 289! Yes, simply by scaling Sum-mer Moon II up to super giant size. (Naturally, Sum-mer Moon II isn’t the right propor-tions for a super-tanker, but it’s the principle that counts here.) Of course, compare any vessels of the same size (displacement) and we’re right back to the three underlying efficiency drivers: go-ing slower, longer more sender hulls, efficient propulsion package.

Summer Moon II is both a relatively large boat and is long slen-der, and light, and is fitted with a deep re-

duction gear for a large-diameter, slo turning prop. With 5,840 gallons of di sel, she has a rang 14 knots of 2,000 miles. At 9 knots,

Summer Moon II is non-stop transpacif capable, with a ran of 5,500 nautical miles, plus a 10% r serve.

The Problem with Dockage and Stor Fees One of the unf tunate proble with long slen hulls is that al-most the entir boating indust charges for bo based on leng not on their re size, which is their displace-ment. If only dockage and s age was calcu lated based o displacement tons not lengt feet. Using ton all of our nor ized example boats from 45 67 feet LOA, would pay the same storage fees. Instead—as things are—the mu more economical longer slender boat are penalized by pa ing higher storage to their greater len This is a real sham and something tha the boating industr ought to address to encourage more ef cient boats.

Tons may be too a stract a number to Iron Kyle —45-ft. Tug Yacht

Peregrine —45-ft. Ultra-Shoal Motorcruiser

Narrow Boats Do Not Mean

Tender Boats

One of the common misconceptions about slender hulls is that they are necessar-ily tender and deep rollers. In fact, this was a difficulty experienced aboard the early, slender offshore powerboats of the 1907-era Bermuda race we discussed last issue. It absolutely does not have to be that way and shouldn’t be if the de-signer knows his or her business.

The belief that narrow boats are rollers is so pervasive that the builder of one of the boats we’ve used for an example here simply couldn’t imagine that that ves-sel would be workable. He was astonished when the boat not only floated exactly on its lines but was moderately stiff as well.

The key is that the stability characteristics have to be worked out from the early stages in design so that roll time in seconds is equal to between 1 and 1.1 times the beam in meters, or a bit less. Roll time is governed by metacentric height (GM), which in turn in controlled by the moment of inertia of the waterplane and the relationship of the vertical center of gravity (VCG) to the waterplane. These are standard naval-architecture calculations. (See the December 2007 issue of The Masthead , for complete details on these calculations.) There’s no reason not to get roll characteristics right on every design, slender or beamy.

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