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Old 17-05-2014, 17:55   #16
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Re: Left Handed Prop

I haven't checked the trans against the
manual, and now I'm 1000 miles from the
boat. I only get up there every couple
of months.

That would be cool if fwd and rev were
both 1:1, but it's probably not. I couldn't
find the manual. It's somewhere in all the
stuff that was on the boat. We're still in
the process of cleaning. I didn't get any
numbers off the trans while I was there.

Is running in what should be reverse, while
going forward for long periods of time hard
on the trans?

I guess I have lots of investigating to do.

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Old 17-05-2014, 18:16   #17
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Re: Left Handed Prop

Hurth HB Transmission Manual - PDF but I am leery of downloading from these websites, since it may be viral in nature. anytime something is for free.......

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Old 17-05-2014, 20:04   #18
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Re: Left Handed Prop

Originally Posted by cappy208 View Post
Do you have the manual? Or can you find one? It should have a diagram of Fwd and Rev. I would bet dollars to donuts that yours is simply being used backwards. The solution would be to get the proper rotation prop. But before you go for a haul out, ensure the facts of the F. N. R. Lever on the transmission. See which way the lever actually lays when it is in F. Compare that to the manual.

An interesting aside, Most 30 to 40' yacht transmissions are not actually reduction gear, but simply F N R controls. Sailboats because of their relatively low speed and very low HP usually do have reduction in the gear. Especially reverse. Or the little engine would stall when put into reverse

Next time you check out powerboats note the phrase 'direct drive' when the transmission is discussed. That's a straight 1 to 1 right out the back of the engine to the shaft. If that were your case, then there would indeed be no difference between F and R. But I suspect your transmission is geared desperately for F and D.
G'Day all,

For yachts of this size, it is likely that this is a Hurth HBW 100 gear.From their manual, there are three models, HBW-100 1.5,-2 and -3. The ratios for these are:
100-1.5 F 1.48:1, R 1.86:1
100-2 F 1.79:1, R 1.86:1
100-3 F 2.72:1, R 1.86:1

Again, from the manual:
"All transmission sizes are available for right hand and for left hand rotation of the input shaft, the direction of rotation being specified as seen by an observer facing the input shaft. In position A the engine shaft and the propeller shaft rotate in OPPOSITE directions. In position B, they rotate in the same direction".

So, it is only in the 100-3 model that there is a great difference in the F and R reduction ratios, and in the 100-1.5 the usual difference is inverted, ie less reduction in reverse gear than in forward. Weird...

Anyhow, it sounds like you need to do some research and find out what model you have. Mine has a big plate attached to the top of the gearcase... easy to see. Dunno about yours!


Jim and Ann
s/v Insatiable back in MBTBC marina, waiting for next eye jobs to be done
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Old 17-05-2014, 21:08   #19
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Re: Left Handed Prop

Out of curiosity looked into it a little more, but couldn't tell absolutely that you could run in reverse for forward with no ill effects. A call to any reputable Hurth dealer could tell you though. But after looking at the parts breakdown both engagement
modes (clutches) have the same type and number of plates (discs), which would suggest that the torque capacities are the same. This gear is immersion (splash) lubricated, so direction of rotation should have no bearing on wear. As far as adjustment, other than linkage, there is none. There are no bands, as in an automatic transmission; engagement pressure on the clutch packs is (apparently) supplied by the rotation of the shifting mechanism itself (I didn't go too far into it).

The Hurth gear that is most directly applicable to a Universal M25 Is an HBW50. This gear has either a (roughly) 2:1 or 2.7:1 A position, and a 1.86:1 B position.

So if you're running a gear in reverse with a prop that was speced to run at say the A position of 2.7:1 and you're actually running at 1.86:1, that's what, 30 percent less forward than reverse? But if your gear is a 2:1 and you're running 1.86:1 that's only 5 percent and might not be so obvious.

And that leaves (providing the 'forward' clutch pack is good) the questions "Do I get my existing prop repitched (and is there enough room in the prop) for the 1.86:1 (B position) ratio", and "Will my reverse speed at the new pitch be manageable/acceptable?"

From your last post it sounds like you've time to figure it out so maybe make sure the gear is sound and work from there. Some info on the Hurth HB gears is here: 977.075.tif 1624x2310 x 2

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Old 17-05-2014, 23:15   #20
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Re: Left Handed Prop


That is great info. Thanks a bunch! As you say
it can get confusing and weird.


That's fantastic. I agree that the info I need is
probably in that manual. I downloaded it. Thanks
a whole lot!

I really appreciate the time people have put into
figuring out my dilemma. When I get back to the
boat, I'll post what I find out.
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Old 17-05-2014, 23:52   #21
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Re: Left Handed Prop

Okay. It's getting close to midnight here and
I am actually having fun reading the Hurth
transmission manual. Can't put it down. Learning
a lot.

So, best guess is that there is no necessarily
right or wrong forward or reverse direction.
That reasoning is from the manual using "A" and
"B" for running one way or the other, not "forward"
and "reverse".

Also, as Jim noticed, the ratios are all over the
board. Some, the "A" ratio is higher and some
the "B" ratio is higher. It seems you would just
choose that tranny that most closely meets
your needs.

I'm also guessing that with different ratios,
the torque available at different speeds is
different, so you make a decision based on
that also. Higher torque, high speed or higher
torque, low speed? Seems one would have
to try different prop pitches and different
handedness to come to any conclusions.
Though I think someone must have calculated these
all out and that information would be valuable
to someone like me trying to figure out what
to do. I wonder where that info is?

Thanks again. Knowledge is power...diesel power.

You can't buy happiness, but you can buy ribeye.
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