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Note that this larger-propeller-diameter gain applies for boats in the semi-planing speed range and below. At high-planing speed, smaller diameter and higher pitch (given sufficient blade area to absorb thrust) are more efficient.

Better Seaboats As good as long slen-der hulls are at being efficient they offer still another advantage— they are better sea-boats. Long slender hulls can be driven faster in more elevated sea states than wider shorter craft. Slamming and pounding are much re-duced, which in turn makes for greater comfort, better crew performance, and lower loads on the hull, machinery, and gear. This critical consid-eration is often overlooked in evaluating the advantage of slender hulls. It shouldn’t be.

Cabin Layouts in Slender Hulls One of the drawbacks to slen-der hulls is working in com-fortable accommodations. You can see the arrangement of the very slender Ironheart is greatly controlled by the limited beam. Though there are sub-stantial gains to be had in efficiency going this thin—as you can see from the perform-ance tables (previous issue)—you can still gain considerable ad-vantages from even moderately slender hulls. Imagine and

Peregrine are two ex-amples. They have

quite comfortable generally what we think of as “normal arrangement plans

In larger boats, it’s easier to work in a commodations in quite sender hulls. drawings of Summ Moon II , show just comfortable the ar-rangement can be i an 82-footer, with length-to-beam rati of 4.5:1.

Larger Boats are More Efficient In order to keep th boat comparison tables fro growing too large and com plex, I did not include a nor malized Summer Moon II . This, however gives us an o portunity to look at anothe aspect of transport efficien Simply being larger makes greater transport efficiency

Summer Moon II is 82 ft. – in. LOA, 72 ft. – 11 in. DWL 17 ft. – 0 in. beam, 16 ft. – in. BWL, and 137,400 lb. displacement. This gives a ratio of 158 and a length-t beam ratio of 4.5 on the w terline. Maximum hull spee is 14.6 knots (SL 1.71), dri

by a single 720-hp diesel. At 12-knot cruise transport eff ciency is 15.5—hig than any of the oth normalized boats a any speed. Even if modified to run se planing at 16.9 kn transport efficiency would still be 6.5— again higher than a of the other normal ized boat s (see tabl previous issue).

This is the reason larger and larger

Ironheart —66-ft. Voyaging Motorcruiser

Imagine —57-ft. Voyaging Motorcruiser

The Motorsailing Option

It wouldn’t do to talk about efficiency and fuel economy without considering the motorsailing option. Ironheart ’s two masts (see drawing, previous issue) are intended for dinghy launching, steadying sails, flopper stoppers and paravanes, and for real motorsailing. The short rig that can be set will allow Ironheart to sail moderately well from a close reach on down. With some sail up, the engine can be throttled well back to achieve the same speed as without the wind-power boost. Fuel savings and increased range can be immense.

Another advantage to being able to motorsail is that— should the engine shut down completely offshore—you can still make progress and maneuver under sail alone. You’ll be able to take care of yourself rather than having to call for help.

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