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11242011, 12:31 AM  #24  
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11242011, 01:54 AM  #26 
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It's not so much the horsepower, but the torque that matters. However, since the horsepower and torque output of a specific engine are pretty much tied together, it makes little difference.
Torque is measured in ft lbs for a reason. It is a measure of rotational force, and this rotational force is transferred into a translational force when the spinning propeller of the engine pushes against the water. The propeller will create a force on the water and (by Newton's Third Law), the water will create a counterforce on the boat. This is what pushes the boat forward. Torque is additive, and only the NET torque in a particular direction matters, so one 100 hp motor going north is the same as two 50 hp motors going north, is the same as one 150 hp motor going north with one 50 hp motor going south, is the same as one 70.71 hp motor going northeast and one 70.71 hp motor going northwest. This is all assuming that the motors are operating at an equal power output the whole time, are indeed identical, and the motions of one aren't affecting anything about the other.
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11242011, 03:26 AM  #28 
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11242011, 03:32 AM  #29 
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I've done a couple of boat projects and I want to chime in with something here. It's not just about weight and combined horsepower, but two propellers each driven by it's own motor is going to be more capable than one similar sized propeller powered by a twice as powerful motor. Think of it like stuffing 500 hp in a car with some 195 tires...

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11242011, 03:36 AM  #30 
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11242011, 09:57 AM  #31  
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+1, specifically, aerospace engineering. They will teach you the difference in various factors of having one engine plane, twoenginesplane, or fourenginesplane. Not to mention the gravitational force derived from weight of the plane, drag coefficient derived from aerodynamics of the plane.
In your case of a boat, you will study fluiddynamics and the drag coefficient of the boat. Please take your friend back to school. Quote:
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11242011, 01:34 PM  #32  
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11242011, 10:12 PM  #33 
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I own 2 boats and both are twin engined, just because they have totals of 540 and 770hp, does not mean they will act like they do.
having 2 engines is more for torque and outright power as well as hull shot. a single 600hp engine in the dead center of the boat will be able to lift the boat higher out of the water resulting in more speed. so a single engine can actually be quicker in some cases depending on application. this is why some of the bigger formulas and dozi boats have offset motors, they get the drives closer together resulting in getting the drives closer to center on the boat, and acting closer to being more like a single engine. But it's still not 100% also keep in mind, a second motor adds between 100 and 1000lbs to the boat, slowing the boat down rotation of the props also has an effect, will one of the props be counter rotatating? long story short, 600 horsepower is 600 horsepower no matter how you look at it or put it together eg, 6x100hp or whatever, but it's how it handles the power that makes all the difference. does this help? I can try and clarify more. 
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11242011, 10:15 PM  #34  
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just like how a car engine can be 300hp, but at the wheels it's 270 due to loss. counter rotating drives are pretty uncommon on boats however. 

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11252011, 01:28 AM  #35  
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11262011, 01:53 AM  #37 
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This is my second time reading this thread. Up until just now I was really confused as to how 2,300hp equaled either 600 or 450 ponies
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11262011, 01:47 PM  #38 
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12022011, 01:29 AM  #39 
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if your talking at the crank than they would equal 600hp but at the prop they would come close to 600 but not exactly. A 600hp outboard factors in drivetrain loss once as twin 300hp outboards factor in that same loss twice resulting in slightly less final output HP

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12022011, 01:27 PM  #40  
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besides, let's say it's a 10% loss on the 600hp (it will be less then that but i'm making this easy for people) the 600 would lose 60hp the drives on the 300hp engines will have a similar if not the same loss, so it would be 30 per engine which will add up to 60hp again, so you're in the exact same situation again. so unless the OP specifies that the 600hp engine is power at the crank and the 2 300hp motors are at the prop, then there will be no real differences. 

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12022011, 01:31 PM  #42 
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12022011, 03:33 PM  #43 
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Not sure how this thread got to drivetrain loss. The question was referring to the engines only; plus, boat drivetrains are much simpler. 300 + 300 = 600

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