V-Drives and power loss???

[Bubba1235]

Does anyone know off the top of their head what sort of power loss is incurred using a V-Drive. (All gear sets have some loss.) Example, say I am pushing it with 300 HP, what sort of HP do you get at the prop?

(Never owned a V-Drive and am looking at one that needs a repower.)

 

[arks]

Re: V-Drives and power loss???

Compared to a standard straight dive, the V-drive rotation has to change direction 180 degrees. As I recall, this accounts for a 10-15% power loss at the prop. That 300 HP suddenly becomes 260 - 270.
The advantage (if that's what you want to call it) of the V-drive is that the engine can be placed closer to the rear of the boat, saving space in the cabin.

 

[QC]

Re: V-Drives and power loss???

I don't think V-drives are that inefficient. I believe an I/O eats 20 - 25 bhp and they are less efficient as far as the gear-train is concerned (not the propulsion itself where I/Os are waaaay more efficient).

Also, it is not simply a percentage loss as it is the load on the gear set, fluid etc. and those things are sorta fixed by RPM . . . very dynamic discussion, buuuuut, changing a V-drive powered boat to anything else would be a huge undertaking. I would bet it loses less than 15 bhp when all is said and done.

 

[arks]

Re: V-Drives and power loss???

You guys got me curious about this, so I did some searching. I could't find any direct comparisons between straight-shaft and V-drive, but I did discovered that QC is right about V-drive versus sterndrive. I found this embedded in a "Boating" magazine article:

"V-drives still get a bad rap for what they used to be ? gear-mashing consumers of power. Older designs consisted of a transmission attached to an engine's bell housing by a short shaft. This shaft was fitted with a universal joint that connected it to the input shaft of the V-drive. This meshed with the output gear on the end of the propshaft. The design was highly inefficient. But today's integrated V-drive units, such as the ZF units on our Amberjacks, have the transmission and gears in the same housing and let a lot more power reach the prop.

Theoretically, a stern drive, which redirects the power through two right angles, should be less efficient than a V-drive, which reroutes the power only once. A stern drive connects to the engine's flywheel with a short splined shaft that has a universal joint at its end. This lets you raise or lower the drive. Within the drive there is an upper horizontal shaft that has gears linked with those on a vertical shaft. Then the gears at the bottom of the vertical shaft connect to the horizontal propeller shaft. A stern drive loses about 13 percent of its power compared to 8 percent for the V-drive, according to Daniel Clarkston, director of product engineering at Mercury Marine's MerCruiser division."

What these 2 paragraphs DON'T take into account is the difference in exterior drag between the two systems. The inboard drive (V or straight) will always have more drag than the sterndrive. The article confirmed it by testing 2 boats with identical power but different drives. The sterndrive boat was faster than the V drive by almost 14 MPH. Wow, that's huge.

 

[QC]

Re: V-Drives and power loss???

The inboard drive (V or straight) will always have more drag than the sterndrive. The article confirmed it by testing 2 boats with identical power but different drives. The sterndrive boat was faster than the V drive by almost 14 MPH. Wow, that's huge.

Yup, and that's what this was supposed mean . . .

(not the propulsion itself where I/Os are waaaay more efficient).

 

[HT32BSX115]

Re: V-Drives and power loss???

My 454 Bravo III is rated at 300 propshaft HP (Bravo I II III) I know the engine is rated around 330 crankshaft hp or so. So that's about a 10% loss thru the drive.

I would be willing to bet that a V-drive is less than 1/2 that loss.

The other loss in a V-drive is the thrust vector not being parallel to the bottom of the boat .....but I don't think it's all that significant.

 

[DJ]

Re: V-Drives and power loss???

The other loss in a V-drive is the thrust vector not being parallel to the bottom of the boat .....but I don't think it's all that significant.

That's where the V-Drive starts to lose ground. Don't forget, you're dragging around a shaft, supports and a rudder back there too.

 

[chiefalen]

Re: V-Drives and power loss???

Where the v-drive is more desirable is in a boat that says in the water and used primarily in commercial applications.

When it is impractical to pull the boat out of the water for work on the drive: ie cruise ships, sail boats, and large yachts.

And the rudder doesn't help ether dragging it behind the boat.

Stern drives also turn in a smaller circle, thus more desirable for water skiing,fishing and such.

One more thing stern drives are easier to trailer, a big plus.

So a fishing boat 100+ with a diesel lets say will have a screw, and a 17-40 will have a stern drive.

PS notice i didn't mention the dark side ( outboards ) I whispered it.

 

[QC]

Re: V-Drives and power loss???

Annnnd, the angle of attack matters too. Just think how much speed you gain with your trim button . . .