John Atkin

Most of our auxiliary sailing yacht designs are heavy displacement, and for maximum efficiency should be fitted with relatively large, slow turning, narrow blade propeller wheels. In many cases suitable reduction gears are necessary, presently more than in years past, when slow turning marine engines were common. Sailing men want their yachts to perform their best under sail. Providing a narrow, two or three blade propeller is installed well abaft the stern (or propeller) post and this member is properly faired off, there will be a reasonably good flow of water and efficiency obtained. The propeller shaft should be of adequate diameter. I do not feel propeller wheels should be installed with only minimal space between the wheel's hub and cutless bearing, or outside stuffing box. This will create excessive cavitation in addition to decreasing the ability of the auxiliary engine to provide proper handling in docking, etc. A most essential requisite under a11 similar situations.


The same general observations apply to moderately to very heavy displacement power boats, the selection of propeller mean width ratio (M.W.R.) will greatly influence efficiency. The determination of a proper engine, reduction gear, propeller wheel efficiency is a complex matter. Despite claims to the contrary, the determination of the most efficient propeller wheel is not an "exact science." Even the "experts" calculate propeller wheels with a strong element of "Guesstimates" to crank into a crystal ball! While theory may be a snap -- actual performance is quite another matter.


A deep, heavy displacement hull can move through the water within the natural law called "speed-length ratio." Non-planing hulls are limited in speed by their waterline length. A hull with a longer waterline can go faster than a short one. The square root of the designed waterline times a factor ranging from 1.4 to about 1.25 will quickly determine the greatest hull speed to be expected -- despite the amount of power installed. A modified, semi-displacement hull, capable of reducing the wetted surface of her underbody is a more complex matter -- accurate calculation of the hull speed a more difficult task. True planing hulls which are able to lift their hull weight (displacement) vertically are quite another matter.


Marine engines must work very hard. A marine propeller's diameter has an enormous effect upon the power to turn it. A 1 inch change of diameter has more effect upon the wheel's absorption than a 1 inch change in pitch. Assume that to turn a 10 inch diameter propeller on a given hull requires 25 h.p. To turn a similar 12 in. diameter wheel the same r.D.m.s require almost 70 h.p. Allow about 10% of the propeller diameter between the blade tips and nearest obstruction.


Virtually all, so called "marine engines" marketed" today are automobile, truck or industrial blocks "marinized" by various companies. There are, fortunately, some high quality gasoline and diesel engines available based on this practice. There are, as well, Scandinavian, Oriental and European engines -- countries who are producing true, 100% marine engines -- as well as "marinized" versions -- which provide reliable service. Engines ranging from 5 h.p. to 350 h.p., which are those which will be required in the utility boats, power cruisers and auxiliaries included in this catalog. The propeller is a submerged pump which pushes a powerful stream of water. Reaction of the gush of water drives the hull ahead or astern. There is slippage and lost motion involved in getting the hull to accelerate.


Not too long ago, a typical 90 to 110 cu. in. displacement engine developed about 25 to 31 h.p., respectively, turning about 2,000 r.p.m. The propeller load torque curve of these engines is very closely related to the r.p.m.s indicated. In practice, some consideration had to be given the matter of wheel selected, as the engines never turned the recommended, nor calculated, wheel to anticipated revolutions.


However, in currently produced much higher speed gasoline engines, the indicated horsepower is often most overrated. A typical V eight engine, of about 330 cu. in. displacement, rated at 240 h.p. at 4,4000, will indicate its peak propeller torque at about 3,000 r.p.m. where it develops approximately 140 h.p. Thus, in actuality, it only produces about 66 to 70% of its rated horsepower -- for marine use most engines, of this nature are designed, when used in yachts, to obtain greatest efficiency at some two-thirds of the r.p.m.s indicated.


Several small diesel engines of about 100 cu. in cylinder displacement may rate at 22 h.p., at 1,800 r.p.m., in practice develop 20 h.p. at the top of their propeller torque curve, when turning 1,600 r.p.m. Which is not so bad. More typically, most present diesels of about 100 cu. in. rated at 30 h.p. at 3,000 r.p.m. win actually develop less than 20 continuous h.p. when turning 2,200 r.p.m., or at the peak of the torque curve.


You will find, in exploring the question of suitable power, that there are, rather unfortunately, many kinds of "horses" -- or horsepower. A most confusing dilemma. Diesels, in general, produce more productive (or true) horsepower than the present day, high-turning "marinized" gasoline block.


When the term h.p. is used, it is generally meant the actual power which can be developed on a testing stand -- thus "brake horsepower." However, "continuous horsepower" related to the torque curve is an almost essential consideration.


High piston speeds are undesirable from an engine longevity standpoint, the lower the piston speed the longer the engine will run without overhaul.


Seldom do you find a gasoline engine with a compression ratio of higher than 9.8 to 1. Diesels, however, are designed with ratios as high as 20 to 1. One adverse effect is that the lower horsepower, one and two cylinder diesels vibrate rather badly at idling and lower speeds. Flexible mountings are normally used as well as flexible shaft couplings. Higher horsepower diesels run with far less vibration, but both high and low speed diesels are inherently far more noisy than a comparable gasoline engine.


Keep in mind that most of the diesel engines we are concerned with are about 6 inches greater in height from the crankshaft to the top of the engine than a similar gasoline engine. A great many of the currently produced engines, both gasoline and diesel, have hanger bolt centers of 22-1/2 inches. There are, of course, many exceptions to this, as well as other critical engine dimensions, and an installation drawing must be obtained of the engine considered for power.


Reference texts, as Conrad Miller's Small Boat Engines published by Sheridan House, his Wonderful World of Boats written for the Columbian Bronze Corporation, The Commonsense of Yacht Design by the late L. Francis Herreshoff and texts provided me by the Westlawn School of Yacht Design, at Stamford, Conn., were used in preparing the above observations. Such material, written by knowledgeable authorities, should be in all boat builders' libraries.