http://www.tadroberts.ca/ Access key="1"Tad Robertsyacht design and naval architecture

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Yellow Cedar, 38' Classic Motor Yacht

Construction: cold-molded
LOA: 38' 0"
LWL: 37' 11"
Beam: 10' 0"
Draft: 2' 8"
Top Speed: 9 knots
Power: Perkins M30 28 Horse Power @ 3600 RPM

Extended Comments

From “ Power Boat Design: Form and Fuction”: Yellow Cedar is a minimum powered cruiser seemingly escaped from an earlier time. But for this type of boat the vertical stem and stern are functional and not a retro styling attempt. In her role as a liveaboard home for a retired couple the vertical ends provide the most interior space for her length, because livable volume is dependant on waterline length. Yes, she would look lovely with a fantail stern or maybe double-ended. But that would ruin the roomy cockpit/backporch that the wide transom provides, it will also pick up buoyancy quickly as it sinks, limiting trim problems.

Long, light boats are easy to push; with proper hullform easy to push fast, or in this case easy to push at moderate speed with very little power. Though possibly a candidate for a hybrid electric/diesel generator drive, I've stuck to the conventional and drawn in a Perkins M30 that produces 28 HP at 3600 RPM. But what about our 500 lbs per horsepower, doesn't Yellow Cedar, weighing 15000 lbs, need 30 HP? Yellow Cedar is a minimum power cruiser with a very light D/L, and if we back off the speed a little things look pretty good. For a S/L of 1.2 or 7.5 knots we only need 21 HP, which is 2200 RPM on the engine. Or if we drop down to a S/L of 1.1 or 6.8 knots, we only need 16.5 HP, which is about 1900 RPM, ideal cruising speed with fuel consumption of about .9 Gal/Hr.

In 1928 Yellow Cedar would have been equipped with a 8 HP four cycle, gasoline marine engine. "Those old engines produced real horses, not the ponies we get from these new things" is a statement often heard around the docks. The answer is the law of torque (twisting power). Torque = 5252 / HP RPM

Maximum RPM for the old 8 HP engine was 600, producing 70 lb ft of torque. The modern engine puts out 28 SHP at 3600 RPM, 40 lb ft of torque. But we fit a reduction gear of 2.4:1 and get a maximum of 98 lb ft of torque. This will turn a 16" by 13" three bladed propeller. The old engine was direct drive with no reduction gear, and turned a 18" by 13" two bladed prop. This would give Yellow Cedar a speed of about 7 knots in calm water, with no reserve power for adverse tide or weather. It was a different time, old timers spent many hours hiding behind a point waiting for the tide to turn or the wind to go down.

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