Eric---
Wrongo. An outboard does not produce a rolling effect on a boat. The engine would have to be horizontal to the boat's axis to do that. Torque acts around the axis of the rotating parts of an engine. On an outboard, the rotating parts are vertical, not horizontal. So when this twisting force is applied to the transom, it will yaw the boat, not roll it. Take the cover off a multi-cylinder outboard and look at how the spark plugs are arranged. They're in a line going straight up ( or two lines if it's a V-engine). The pistons are horizontal but pistons do not produce torque, the rotating power of the crankshaft does. And the crankshaft is vertical (as is the driveline). So the effect on the boat is as if you were above it, reached straight down and grabbed the transom in the middle, and rotated your hand. The boat movement this produces is yaw not roll.
This should be a no brainer to visualize. When you rev the engine of a car, the car leans (rolls) to the side away from the direction of the crankshaft rotation. The engine in a car is mounted horizontally which is why the direction of roll is ninety degrees to the ground. Now take that car and hang it vertically on a rope tied to the front bumper and rev the engine. The car will rotate the same direction as it did before, but since it's vertical, the direction of roll will be parallel to the ground. If the ground was a boat, and the back of the vertical car was fastened to the back of the boat, the torque would try to pivot the boat not roll it. This is yaw.
If I open the throttle on our 17' Arima fishing boat which is powered by a 90 hp, three-cylinder Yamaha, the boat turns a bit to one side. This is the result of the torque from the engine trying to twist the transom. It is not the result of anything the propeller is doing, because the slide (yaw) is actually in the opposite direction than prop walk would be if there was enough prop walk to make any difference (there isn't).
And you need to learn to read a bit better. I was not comparing a boat with two 240 hp engines to a boat with one 120 hp engine, nor was I comparing a boat with one 240 hp engine to a boat with two 120 hp engines. I was comparing a boat with two 120 hp engines to a boat with one 120 hp engine. Same model boat in each case, same model engine.
Anyway, your basic premise is totally wrong. I don't know how the hell you came up with the notion that a single 240 hp engine will have the same maintenance costs as a boat with two 120 hp engines. The very fact you have two engines doubles many of the maintentnace costs right off the bat. You have to buy twice as many filters, twice as many belts, twice as much (or nearly twice as much) oil, service twice as many injectors, replace twice as many heat exchangers, plus the labor costs to service the engines will be double because it takes twice as much time to service two engines as it does one.
-- Edited by Marin at 17:17, 2008-01-25
Wrongo. An outboard does not produce a rolling effect on a boat. The engine would have to be horizontal to the boat's axis to do that. Torque acts around the axis of the rotating parts of an engine. On an outboard, the rotating parts are vertical, not horizontal. So when this twisting force is applied to the transom, it will yaw the boat, not roll it. Take the cover off a multi-cylinder outboard and look at how the spark plugs are arranged. They're in a line going straight up ( or two lines if it's a V-engine). The pistons are horizontal but pistons do not produce torque, the rotating power of the crankshaft does. And the crankshaft is vertical (as is the driveline). So the effect on the boat is as if you were above it, reached straight down and grabbed the transom in the middle, and rotated your hand. The boat movement this produces is yaw not roll.
This should be a no brainer to visualize. When you rev the engine of a car, the car leans (rolls) to the side away from the direction of the crankshaft rotation. The engine in a car is mounted horizontally which is why the direction of roll is ninety degrees to the ground. Now take that car and hang it vertically on a rope tied to the front bumper and rev the engine. The car will rotate the same direction as it did before, but since it's vertical, the direction of roll will be parallel to the ground. If the ground was a boat, and the back of the vertical car was fastened to the back of the boat, the torque would try to pivot the boat not roll it. This is yaw.
If I open the throttle on our 17' Arima fishing boat which is powered by a 90 hp, three-cylinder Yamaha, the boat turns a bit to one side. This is the result of the torque from the engine trying to twist the transom. It is not the result of anything the propeller is doing, because the slide (yaw) is actually in the opposite direction than prop walk would be if there was enough prop walk to make any difference (there isn't).
And you need to learn to read a bit better. I was not comparing a boat with two 240 hp engines to a boat with one 120 hp engine, nor was I comparing a boat with one 240 hp engine to a boat with two 120 hp engines. I was comparing a boat with two 120 hp engines to a boat with one 120 hp engine. Same model boat in each case, same model engine.
Anyway, your basic premise is totally wrong. I don't know how the hell you came up with the notion that a single 240 hp engine will have the same maintenance costs as a boat with two 120 hp engines. The very fact you have two engines doubles many of the maintentnace costs right off the bat. You have to buy twice as many filters, twice as many belts, twice as much (or nearly twice as much) oil, service twice as many injectors, replace twice as many heat exchangers, plus the labor costs to service the engines will be double because it takes twice as much time to service two engines as it does one.
-- Edited by Marin at 17:17, 2008-01-25