Re: Outboard Torgue Output
Hi Scott. You can simply figure the Torque yourself by doing a little math. I'll try to explain it best I can. <br /><br />There is a basic formula derived by Mr. James Watt that includes all the constants and unit conversions you need. He made it easy for us to calculate HP using measurements of Torque, so you can use a relationship of HP to get the Torque...<br /><br />But first you have to decide if you're looking for Torque at the
engine or at the
prop. The maximum HP published will be at the
prop. But the maximum RPM where that HP occurs will be at the
engine.<br /><br />So you have to figure in the gear reduction. Most outboard manufacturers publish these gear ratios.<br /><br />For example if the 115 HP has a gear reduction of 13:28 you would divide the 28 turns of the engine by the 13 turns of the prop to get a 2.15 reduction (you could also count gear teeth).<br /><br />With an outboard, you can treat the gear reduction directly as a
Torque multiplier and
RPM reducer at the prop.<br /><br />As an example, with a gear reduction of 2.15 we can find the acutal
prop rpm at an engine RPM of 5500. 5500 RPM/2.15 =
2558 RPM at the prop.<br /><br />Now using Watt's help, we can find the Torque at the
prop... <br /><br />
HP = (Torque X RPM)/5252<br /><br />So...for an outboard producing 115 HP at max prop speed of 2558 RPM...<br /><br />115 HP = (Torque X 2558 RPM)/5252<br /><br />(115 X 5252)/2558 = Torque =
236 ftlbs at the prop.<br /><br />You know the gear reduction increased the Torque to the prop and decreased its RPM, so we can use the same gear reduction to calculate actual
engine Torque.<br /><br />With a Torque of
236 ftlbs at the prop and a gear reduction of
2.15, the engine will have a Torque of 236 ftlbs/2.15, or
109.7 ftlbs.<br /><br />You can double check yourself by using Watt again. 115 HP = [(109.7 ftlbs X 2.15) X 2558 RPM]/5252. Yep, it works.<br /><br />Remember, these Torque number are only at
max RPM. The outboard will make Torque throughout its entire RPM range. So you really need to look at the entire Torque curve over the RPM range to get an idea of what the outboard can do. Oh, yea...not all outboards make their maximum HP at maximum RPM. For example, Honda's 115 HP runs up at 6000 RPM, but makes its maximum HP at 5500 RPM.<br /><br />If this still doesn't help, post the gear ratios and operating RPM for all those Merc's and I'll calculate it for you.
