RonS

So has anyone ever tested different drive oils and compared hp loss??? or compared synthetic to conventional?

Rage

Actually HP is the rate of doing work. The HP losses in the drive is purely do to friction which is converted directly to heat. That heat is why for higher HP engines the drives require drive showers to help remove that heat so as to survive. Lower HP engines do not require drive showers though it sure does not hurt. The HP losses in a drive at any point in time is a function of how much HP is being transferred through that drive or a percent of that HP being transfered. It is generrally assumed that this percentage of frictional loss is basically fixed thought that does not have to be the case. Different drive designs can have different drive HP loss characteristics.

In general as Steve mentioned the main interest about drive HP losses in this site is in regard its effect on the remaining HP available at the prop shaft at WOT for top boat speed. Also of related interest is to be able to accurately compare / corrolate an engine's bench dyno test data/ratings with that from a prop shaft dyno test as obtained through the drive.

I take the 25-30HP Bravo1 drive loss to be associated with motors in the 500HP - 525HP range. I would not expect a 1500HP Stirling to only experience 25-30HP loss through a Bravo 1 if that drive could in fact survive the torque of that motor.....not.

If I am in error on the 25-30hp Bravo 1 drive loss being associated with the 500HP - 525HP range motors please advise.

tomcat

Rule of thumb: one set of gears is 98% efficient (2% loss due to friction). For ease of calculation I assume this loss is constant, it does not increase with RPM or load. But pumping losses, as in wet sump gears flinging oil around, are not constant, they increase faster than RPM (the percentage loss increases with RPM). And if the load on the gears exceeds a certain contact stress, or proper lubrication conditions go away, then friction at the gear teeth may increase and the percentage loss increases to more than 2%.

So a Bravo drive handling 500 HP and delivering 470 HP to the prop is 94% efficient, but push it beyond that 500 HP @ 5200 RPM and it might be only 90% efficient. Convert to dry sump, change oil viscosity and treat the gears to reduce surface roughness and you get some of this back.

To answer the original question, you are close enough if you just assume a 6% loss at all RPM up to 5200. So if you are cruising at a speed that requires 300 HP at the prop, the engine must deliver 318 HP at the crank. The loss is 18 HP. At 200 prop HP the loss is 12 HP etc.etc.

P.S. The HP loss through a Bravo drive at 1500 HP is 1500 HP.

ECeptor

What about losses at 5,500rpm? 6,000rpm?

Steve Zuckerman

Ron,
When I was drag racing, the gearcase guru David Steckbauer (formerly of Mercury Racing) and I had several lengthy conversations on this matter.
David's trickest piece is a small 4 1/4" V6 (for the MercXR150) gearcase machined down internally to accept the larger billet bearing carrier and gears for the racing 2.5s, normally in a 4 3/4 housing like the Sportmaster. This may seem trivial, but in spec class racing where every little detail could not be overlooked, this is a really trick item.
It had been rumored that removing the seldom used reverse gear could free up as much as 10/15 horsepower, in these cases. I asked David if this was an urban/racer myth or true.
I could give up reverse easily on a race boat for 15 "free" HP.
He said maybe 1-2 horsepower on a 300 horsepower motor, but I doubt if you will ever feel it or measure it (ET/MPH), because there are so many other variables that have a greater effect on HP, like weather and tuning.
This research was going on at Mercury during the heyday of factory racing (F-1, Mod-VP).
We went on to lubricants. He said this where Mercury was able to pick up discernable HP increases. He said using lightweight synthetics, with high shear strength, netted as much as 10 HP at high RPM (in this case 9000+) and reduced operaing temps considerably, which helped the small volume speedmaster live through a 50 lap circle race on an F-1.
Since drag racing is high horsepower (I was making 600+), but short duration, most of serious hitters adopted this theory.
Some guys were actually running transmission fluid. I used a thin weight Alisyn synthetic in mine.
David says people often confuse (high) viscosity with protection. With petroleum based lubes that is mostly true. He says with synthetics it isn't. They did Timken bearing test on all kinds of oils, petro and synthetic, and the the synthetics, were much better overall, had higher shear strengths, and allowed a decrease in viscosity, which freed up horsepower.
Obviously, Mercury carried this even a step further in their racing inboard drives, by dry sumping the #6 drive, and using synthetic oils.
Regards,
Steve

Rage

Thanks.

Yes, at 1500HP the drive losses are no longer converted to heat but instead converted to the acceleration of the molten metal that was the drive into space.

Escape Velocity

I was told by an engine builder that the difference between my engine's horsepower on the dyno and what I'll actually see at the propshaft is a fixed number of about 25-30 HP at WOT, regardless of the horsepower of the engine. You simply subtract that amount of horsepower.

The reason that makes sense to me is that you can think of the drive as a "black box" with a set of gears, bearings, universal joints and a particular lubricant. At every RPM you turn the input shaft, there will be a corresponding amount of resistance that will have to be overcome by the driving force of the motor. The drive doesn't care if it's got a 50 HP electric motor or a 1000 HP blower motor turning the input shaft - the internal resistance of the drive is going to be the same at any given RPM.

Tomcat's example of using a percentage reduction for a single engine situation works. However, using a percentage reduction in horsepower from one engine to the next could be misleading. Calculating the percentage reduction in horsepower in those two examples, subtracting 30 HP from 50 HP leaves 20 HP at the propshaft for a 60% loss, and in the second example, you'd only see a 3% loss. That's why I don't think it's helpful to try to use percentages to predict the difference between crankshaft horsepower and propshaft horsepower. It's more accurate to subtract a fixed number.

I was also told that a particular Factory 1 race boat striving for a kilo record used the standard Bravo drive because the horsepower loss was noticeably less than for a Bravo XR drive with straight-cut gears.

tomcat

What you say sounds logical for just the "rolling" resistance of the gear box, and probably to wet sump "pumping" losses as well, but I don't believe it applies to losses at the gear teeth.

If you are spinning the input shaft at 5200 RPM with a 50 HP electric motor, then you are spinning the prop in air or spinning a very small prop, and there is next to no load on the propshaft. Under these conditions it will not take 30 HP to spin the drive since much of the loss comes from the gear teeth having to carry a load.

If you are spinning the engine at 5200 RPM with 1000 HP, then the prop is putting a huge load in the gears and the loss will be higher. And with 1000 HP you are definitely exceeding the design of the gear, so more deformation of the teeth under load, more contact area, more friction and higher percentage losses. A loss of 6+% or 60+ HP seems reasonable in this case.

The higher losses on the XR straight cut gears supports this argument. They are stronger due to more tooth contact area, but more contact means more area for friction to happen.

I admit to having reached the limit of my meager knowledge on this subject, and would appreciate a better explanation from someone who really knows this stuff.

Escape Velocity

Tomcat, you've got to be right about the increased frictional losses created by loading the gears. That means there will be variation in internal resistance based on how the gears are being loaded. Somewhere, someone has tested this!

Rage

In listening to the gear case GURUs I heard that
the XR gears are more ductile and tougher to better tolerate the higher torques where as the standard X gears are much harder and therefore also more brittle and less tollerant of higher torque loadings. In the end this I again have heard results in higher wear rate of the XR gears than the X gears wear because the XR gears are softer and the X gears are harder. Higher gear wear rate may (or may not) also be indicative to higher friction losses with the XR gears.