Flywheel Hp. vs. Prop Hp.

[Hooty]

Another post prompted this one.<br />I know an old o/b wrench that sez the 10-15% horsepower loss figure we use between crankshaft and propshaft is b.s. He sites as an example,a 150hp engine vs. a 200hp engine powering the same l/u. At 10% loss the 150hp engine is only going to loose 15hp,but the bigger engine is going to loose 20hp? <br />He claims that the larger engines power loss through the drive train is more in the 3-5% range.<br />Anybody else done any dyno work?<br /><br />c/6<br /><br />Hooty

 

[JB]

Re: Flywheel Hp. vs. Prop Hp.

I agree with your wrench buddy.<br /><br />Spinning a gearcase is pretty much the same job in engines over about 70hp. I estimate that it is going to take about 10-15hp.

 

[Dhadley]

Re: Flywheel Hp. vs. Prop Hp.

The loss is going to be a number not a %. A V6 gearcase takes X ammount of power to turn it, like any gearbox whether its an outboard or a B/W transmission. <br /><br />If it takes X ammount of hp to turn a 14:26 (1.86:1) V6 gearcase a given rpm, it will require the same hp no matter what powerhead you put on it. <br /><br />You can change gear ratio to change that number too, but dont forget about the life of the gears.<br /><br />JB, you are pretty close with your estimate over 70 hp given stock lower units.

 

[Beaux]

Re: Flywheel Hp. vs. Prop Hp.

this makes sense....I read the same post that gave you this question Hooty. If you read the post by Capt. ken, you saw where he pointed out that the 48hp model came out instead of the 50. Doing the math, that means a drop of 4% for that motor. I assume that this is also the truth for the 88 vs. 90 hp which would be a 2.22% drop in rating. Although the differences in horsepower can be compensated for and tinkered with by changing the rpm measured at and also the ambient temp/fuel type/etc. The 3-5 % average seems more likely than 10-15%. This is not to contradict Dhad, he is completely right about the algebra...... I am commenting only on the commonly used percentage way of viewing the difference.....

 

[Forktail]

Re: Flywheel Hp. vs. Prop Hp.

The 10-15% and 10-15 hp numbers seem high to me. The only true way to answer this would be to dyno the outboard at the flywheel and at the prop, taking the difference. Most likely every engine (even similar models) would be slightly different.<br /><br />With that said, there are some accepted mechanical standards, formulas, and constants which we can use to get a general idea.<br /><br />The shaft horsepower after the gearbox can be found by multiplying the engine's brake horsepower (BHP) X .97 (constant). I believe the marine industry uses and accepts the .97 constant as a 3% loss.<br /><br />First, find the BHP by measuring the engine's torque (flywheel or PTO) at a particular rpm and using a constant. For example an engine is measured to produce 110 ftlbs of torque at 5500 rpm. So: 5500 rpm X 110 ftlbs/ 5252 (a constant) = 115 BHP.<br /><br />Multiplying that 115 BHP X .97 we get a shaft horsepower (SHP) after the gearbox of 112 SHP.<br /><br />But outboards generally have bearings between the gear reduction and the propeller. We can use the number of bearings and the gearbox reduction ratio to find the % of power loss due to the bearings and then the shaft horsepower at the propeller.<br /><br />So, lets assume there are 2 shaft bearings after the gear reduction. The % of power loss due to the shaft bearings is the number of bearings X .015 (constant). Again, I believe this constant is accepted by the marine industry as a 1.5% loss per bearing.<br /><br />2 X .015 X 100% = 3% power loss due to shaft bearings.<br /><br />Now we can find the shaft horsepower at the propeller. The SHP at the gearbox X the power loss due to shaft bearings = prop hp.<br /><br />So, 112 SHP X (100-3%)/100 = 108.6 SHP at propeller.<br /><br />And 115 BHP - 108.6 SHP@prop divided by 115 BHP X100% = 5.6% HP loss. This is quite a bit different than the 10-15% numbers. Although considering the many outboard designs, hp's, etc, each outboard will be different.<br /><br />We can also figure at what rpm the propeller is turning by taking the engine rpm divided by the gear reduction.<br /><br />An ouboard making max power at 5500 rpm with a gear reduction of 2.15 would have a propeller turning at 2558 rpm because 5500/2.15 = 2558.<br /><br />Using this rpm and the propeller shaft hp, we can find the torque at the prop. 5252 X 108.6/2558 = 223 ftlbs. More than double the engines PTO torque output.<br /><br />Using these calculations with a little information about the boat itself, we can find things like displacement speed formulas, required prop pitch and diameters, prop slip, and Bollard thrust.<br /><br />Hope this helps.

 

[Dhadley]

Re: Flywheel Hp. vs. Prop Hp.

The 48 vs 50 and 88 vs 90 take advantage of the 10% rule from the NMMA. They are exactly the same motor but the 28, 48 and 88 notes a change in equipment and / or styling. Non oil injected, hood styling etc. The first year or so of the 88 even had tnt as an option. <br /><br />Clear as mud, huh?