A new prop? verify your transmission ratios

[sailingdeacon]

My 1987 E34 with a transmission serial no. of HWB 100-2.0R actually is 1.8:1 not 2:1 as one would think. This correction allowed me to order a properly sized prop rather than 1" too large. Always measure the engine rpms vs the shaft rpms unless you know for certain.

 

[newgringo]

Ditto! Had same experience with an HBW 100. Prop guy said my measured inwater test data did not make sense. He did me a favor not just sending me a prop. After sending him corrected rpm, ratio and boat speed data he hit the nail on the head.

 

[Rocinante33]

I have an opposite situation. My trans, a Hurth HBW 50, has failed & needs replacement. I have always thought that my boat was under-propped. It would only cruise at about 5 knots at 2500 rpm & max was about 5.7 kts. with a clean hbottom. So if I replace my trans with an HBW 100 (now called ZF10, I believe), presumably going from a 2:1 to a 1.8:1, my performance should inprove, right? Since it is a reduction ratio, the prop will turn faster, if I am thinking of it correctly.:thinker:

BTW, the trans now slips so I am unable to check the gear ratio on it in a conventional manner, as mentioned on other posts.

 

[newgringo]

On Gearboxes and Props

Keith,
A couple of comments:
1. Maxing out at only 5 kts at 2500 rpm sounds like you are over propped. Generally the Universal diesels need to turn up about 3000 rpm to get full hp out and thus get max boat speed (say 6.5 to 7.0 kts for an E33). Or maybe your engine is not up to snuff if you are propped correctly?
2. The ZF10 will fit in place of an HBW 50 but, and this is important, the ZF10 is a longer transmission. That means the prop shaft will have to slide aft about 1.5 inches. That was done on my boat. Now my prop is pretty far aft of the strut and I don't have room to install a modern dripless stuffing box either. I recommend you reconsider installing an ZF10. OBTW mine is a 1.8:1 unit.
3. Generally props on these displacement hulls are more efficient with a bigger diameter when turned slower as would be the case with a 2.0:1 ratio vs a 1.8:1 ratio. I believe you are better off with the 2.0:1 ratio.
4. Maybe you can disconnect the prop shaft (to remove all turning resistance) and check the ratio by turning the engine by hand in FWD gear?
Fun stuff.

 

[Rocinante33]

Keith,

1. Maxing out at only 5 kts at 2500 rpm sounds like you are over propped. Generally the Universal diesels need to turn up about 3000 rpm to get full hp out and thus get max boat speed (say 6.5 to 7.0 kts for an E33).

Fun stuff.

Jerry,

The manual for the M25 Universal (in the Specs & Docs section of this site) says that max is 3000 RPM. Further states that cruising RPM should be about 80% of max. I have always used 2600 as my cruising rpm, but 3000 pushes the boat better. The engine very easily revs to 3000 (moving thru water at under 6 kts) & I don't think it would if over propped. I am leary of cruising at 3000 with respect to the manual calling it max, though. Does anyone else regularly run the engine at 3000 RPM, even for hours at a time?

I will take some measurements to see if I can accomodate the extra 1 .5" for the bigger trans. The HBW 50 is no longer manufactured and locating one might be tough.

Fun stuff, indeed!

 

[treilley]

I would stay with the HBW50 as that is what is specced for your engine/boat combo. I have the same combo on my 35-5 and it pushes the boat easily over 6kts at 2600rpm. If I push the motor to 3000-3200, I do not get much more speed but the engine is deffinately working harder. More noise and higher temp.

 

[newgringo]

I agree, cruising rpm should not be the max full power rpm. It is just an indicator that the prop pitch and diameter match the boat and engine.
Getting at Keiths slow boat speed another clue popped out. He says the engine easily turns up to 3000 rpm but the boat speed is still too slow. Maybe the boat us UNDER propped where the engine would govern out at 3000 rpm and not be fully loaded. Perhaps feeding all the performance data (full rpm, ratio, pitch and diameter, boat WL etc) to a prop guy would be informative?
I also stand corrected. The ZF10 vs. ZF5 in my book shows 24 mm longer, about 0.94 inches. Not 1.5 as I mentioned earlier.
Hey, I know a guy on this site who may have a good useable ZF10 1.8:1 available. Contact "Hodo".