BuiltWithNOF
The Ideal Motorsailer

Health Care Reform, Insurance Reform, Quality

Roger Strube MD / Roger H Strube MD

Roger H Strube, MD

I love bulldozers. They are so aesthetically pure and true to their function. They are modern mechanical marvels that are the most efficient way to push dirt out of the way as they move from one point to another. They are very heavy with large engines and massive structure built to plow the earth out of the way as they move through it. I especially love them because they are configured like catamarans rather than motorcycles. All trawlers and most large powerboats are like the motorcycle bulldozers of the sea, except (to my eye) for the aesthetic purity. With their large bow waves and 4 to 6 foot hump of water dragging at their sterns, they are the water pressers of the sea plowing uphill, with less fuel economy. Their large sportier cousins, the Sea Rays, convertibles and sport fisherman of our water world are faster, and burn even more fuel bringing the image of stampeding white elephants to mind. At anchor or at a dock they rock and roll violently with the wake caused by similar passing water pressers.

I wasn’t always this cynical about large, inefficient sea going bulldozers. My second sailboat was a Taiwan built Sea Tiger 41 ketch with wooden spars and teak as far as the eye could see inside and out. At 15 ton with it’s Perkins 4108 it would press the water at 7 knots pushing an impressive bow wave and sucking a remarkable stern wake. Maintenance was a real “pleasure”. I got over it. Converted to multihull sailboats for the speed, comfort, stability, space and safety. 

We are now considering a sail assisted catamaran powerboat. Aesthetically it will look more like a “Star Wars” or “Star Trek” transporter or fighter than a conventional vessel. I am not interested in the larger catamaran power boats that have been made to look like trawlers. I am not interested in heavy, narrow, overpowered large catamaran runabouts that are designed to look like a Grady White or Down East fishing boat and built using yesterday’s heavy technology. The boat must have the necessary living space and carrying capacity while being light enough to be powered with small, light diesel or electric (solar) engines.

Because any speed over 15 Knots in a sea way tends to be uncomfortable and the speed limit on most of the ICW in 25 Knots, a vessel capable of cruising efficiently at about 15 Knots with a top end of 25 Knots seem to be ideal. The torque of twin Beta Marine 75HP diesels could work well as would a pair of Suzuki 115HP outboards. A catamaran with very narrow low drag or wave piercing hulls makes most sense for us. Long term live aboard facilities with additional accommodations for the occasional guest couple is a requirement. Very high fuel economy is a must as we face ever-increasing cost of gasoline and diesel. This is the reason for the “sailor” part of the motorsailor. The boat must be as near zero maintenance meaning no wood anywhere in construction. The boat must be all GRP with polyurethane core. I believe this package can be constructed in a vessel of 35’ to 40’ weighing less than 7,500 pounds. The boat lift I had installed will hold a 16’ wide by 7,500 pound boat. Most slips will moor a 16’ wide vessel.

The following formula will give a reasonable approximation of speed achievable for a semi-displacement hull form based on the length in meters, weight/displacement in tons, and horsepower. The resulting speed calculation is derived from the square root of the computation of these parameters. It is intuitively obvious without the formula that a long light hull with high horsepower will go very fast. The commuter craft from the early 20th century are a testament to this concept.

If you are interested in testing this formula on several vessels it is a simple matter to set up a small spreadsheet using Excel or Lotus. The values are placed in a table 7 columns by as many rows as you need to test multiple vessels. In the first row label the columns:

A1 = Vessel I.D. (Name/Model);       B1 = Length in Feet;
C1 = Length in Meters;              D1 = Total Horsepower;
E1 = Weight in Pounds;              F1 = Weight in Tons;
G1 = Predicted Speed.

In the second row enter the values or formulas. For example:

A2 = Vessel (e.g., “Kurt Hughes 35”;    B2 = Length in Feet (e.g., 35);
C2 = Length in Meters (enter formula “=(B2*12)/39” and copy down entire row);
D2 = Horsepower number (e.g., Kurt Hughes 35 = 200);
E2 = Weight in pounds (no formula - Hughes 35 = 5,000);
F2 = Weight in Tons (enter formula “=E2/2000” and copy down entire row, then over write F2 cell with Kurt Hughes 35 design weight, 2.5 Tons);
G2 = The formula “=SQRT((C2*D2)/F2)” -- copy the G2 cell down the entire row.

You will see that the predicted speed of the Kurt Hughes 35 using 200 HP and a weight of 2.5 Tons is 29.4 Knots. Now you may enter any boat design and their relevant parameters in the following rows and their predicted speed (close enough for amateurs) will appear in column G.

Excel

A

B

C

D

E

F

G

Formula

(Alpha)

#

=(B2*12)/39

#

#

=E2
/2000

=SQRT((C2*D2)/F2

1

Vessel I.D.

Length in Feet

Length in Mtrs

H.P.

Weight #

Tons

Predicted Speed

2

Hughes 35

35

10.8

200

5,000

2.5

29.4

3

Skeeter 33

33

10.2

60

2,400

1.2

22.5

For a cruising sailor it may be more productive to work the formula backwards, starting with a target top end speed and the length required to float the living space desirable for your cruising style. Then test the formula to determine the horsepower necessary. Length, width, configuration, accommodations and construction philosophy will determine the weight of the vessel. Faster means more structural loading means stronger (heavier) construction. Faster requires bigger engines that add weight and require stronger (heavier) structural support. Heavier vessels with larger engines burn more fuel requiring larger fuel tanks filled with more pounds of fuel. Generally, fuel economy is inversely proportional to weight and horsepower. Long thin hulls track better than barges so a wing sail assist will work on most points of sail off the wind.

The pictures below are for reference only simply to provide a flavor for the design concepts outlined above.

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