T&T: "Ideal" engine speed

[LRZeitlin at aol.com]

When I first started messing around in boats soon after Columbus sailed the 
Atlantic, the prevailing philosophy was to "overprop" all displacement type 
motorboats. This allowed you to run the boat at higher speed with lower engine 
RPM's and simultaneously prevent overrevving the engine.
   
Today, engine manufacturers do not recommend this approach.   
They recommend that the running gear be set up, (engine, reduction gear, 
and prop) so that the engine develops maximum recommended RPM.

A lot of this change in philosophy is due to changes in the design of diesel 
engines over the last 30 years. Older small diesels were naturally aspirated 
and designed to deliver maximum torque in the meddle of their power curve. Most 
were developed for tractors or small trucks and were basically "luggers." 
Overpropping did little damage and promoted fuel economy at cruising speeds. In 
fact some manufacturers (Ford Lehman) recommended overpropping as a way of 
limiting engine speeds. On one of my boats, the old Volvo MB10 engine peaks at 
2000 RPM and delivers maximum torque at 1400 RPM. It also uses a direct drive, no 
reduction ratio needed. So the "conventional wisdom" was correct for the 
engines of the time.

Modern diesels have a much higher power to weight ratio, obtained by higher 
RPM and turbocharging. Torque comes closer to the top of the RPM range. Many 
modern diesels can be run continuously at 3600 RPM. Running the engine at higher 
speeds and using a higher reduction ratio gearbox subjects the engine to less 
torque. Crankshafts, rods, etc., need to be less sturdy and can be lighter 
than in older engines of the same power.

You will get different recommendations for the optimum engine speed and prop 
specifications depending on whether you talk to engine manufacturers or prop 
manufacturers. Engine makers want to be certain that you can reach maximum RPM 
so that you can't blame them if performance fails to reach your expectations. 
Prop makers want you to use the largest, slowest turning prop that will fit, 
to achieve maximum efficiency. 

Here is my personal recommendation. Since most recreational trawlers have a 
surplus of power for normal cruising, I would select a prop that would give you 
cruising speed at about 75% RPM. This should let the engine operate in a 
desirable range but not subject to the wear of operating near maximum RPM. For a 
40 ft. displacement trawler with an engine that peaks at 2800 rpm, this would 
indicate a cruising speed of 7.5 kts. at 2100 rpm. 

Years of fooling around with sports cars convinced me that the lower the 
piston speed, the longer an engine lasts. This, of course, is just an opinion. 
I've never had an engine fail because it was running too slow. On the other hand, 
I've broken crankshafts, spun bearings, and had connecting rods smash through 
the side of the block by running engines too fast.

Larry Z


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