Re: Best new 10hp (displacement issues)
The Tohatsu has a 2.08:1 gear ratio. This means the engine turns 2.08 complete revolutions to every 1 revolution of the propeller. Or in other words, the engine rotates 748 degrees to every 360 degree rotation of the prop. The engine will have a mechanical advantage of 2.08 to 1 over the prop.<br /><br />The Mercury has a 2.0:1 gear ratio. So the engine rotates twice to every prop rotation, and so on.<br /><br />Both engines run out at WOT max 6,000 rpm. So, the engine turns 6,000 times each minute.<br /><br />Using the gear ratios....for every 6,000 revolutions of the Tohatsu, the prop rotates 2,885 times. Because 6,000/2.08 = 2,885.<br /><br />And the Merc prop rotates 3,000 times per minute because 6,000/2.0 = 3,000<br /><br />So in one hour the Tohatsu will rotate the prop 173,100 times. Because 2,885 revolutions per minute X 60 minutes = 173,100 revolutions/hour.<br /><br />Likewise, the Merc will rotate the prop 180,000 times in one hour because 3,000 revolutions per minute X 60 minutes = 180,000.<br /><br />Now, assuming each outboard uses the exact same prop pitch on the exact same boat, we can determine what the speed difference is.<br /><br />Lets assume both these outboards are running the same 7" pitch prop (just for example). This means that each time the prop rotates the prop will advance through the water 7". For calculations we will assume prop slip through the water is zero.<br /><br />So in one hour the Tohatsu prop will advance through the water 100,975 feet, because 173,100 revolutions X 7" all divided by 12" per foot = 100,975 feet. There are 5,280 feet in one mile, so dividing 100,975 by 5,280 =
19.12 miles per hour.<br /><br />Now for the Merc at 180,000 revolutions per hour you get
19.89 miles per hour, because 180,000 revolutions per hour X 7" divided by 12" per foot, divided by 5,280 feet per mile = 19.89 miles per hour.<br /><br />There you have it. Not a big difference with these little props. But it looks like the Tohatsu has about 35% less displacement, so we could probably assume it makes less horsepower at the crankshaft and therefore it would have a harder time providing power to a bigger prop, especially getting on step or at slower speeds, say in a rough sea. This is partly why it has a larger gear ratio (geared lower)<br /><br />Just for fun we could plug in a 7" pitch prop for the Tohatsu and a 10" prop for the Merc. Now the speed difference is 19.12 mph for the Tohatsu to 28.41 mph for the Merc. As you can see, speed is greatly affected by the pitch of the prop, and of course the ability of the outboard to turn the prop at high revolution. <br /><br />So why don't we all run geared down 9.8 Tohatsu's with 22" props? Mainly becuase the little Tohatsu wouldn't be able to supply enough rpm to the prop to maintain hp. Hp can not be measured, only calculated as it is a direct function of torque and rpm. Although through gearing the torque would be there, it wouldn't be there at any significant rpm to make the hp needed.<br /><br />Obviously an outboard that can turn the largest pitch prop with a gear ratio closest to 1:1 and maintain high rpm will be the most efficient and provide the best speed.<br /><br />Considering the boat you have, and the fact weight and cost are important to you, I would go with the Tohatsu. They're ok outboards. The bigger Merc might be more than you need. The weight difference and lower cost might out weigh the benefits of a little speed.<br /><br />
