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Old 16-02-2015, 11:58   #1
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Join Date: Aug 2014
Location: Katy, TX
Boat: Westsail 32'
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MD40 w/ Hurth HB100 - Prop Rotation?

Hello all,

I just bought a Westsail 32. She came with a recently-installed Universal 5432 M40 with a Hurth HB100 transmission. When we hauled the boat for the survey, Bud Taplin (surveyor) noted that the prop was LH rotation, but because of the engine and transmission configuration should be RH.

I currently have the boat in a yard, so I want to change the prop while drydocked. Can anyone confirm the proper rotation of this drive system? I looked for nameplate info on the engine/transmission but it has been removed or painted over.

The Universal manual says RH, but I read somewhere on the web that the HB100 reverses the engine rotation. Any thoughts?

Thanks!
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Old 16-02-2015, 20:42   #2
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Re: MD40 w/ Hurth HB100 - Prop Rotation?

The HBW 100 transmission is, as you know, equipped with forward and reverse rotation capability which Hurth refers to as position A and B respectively. Hurth recommends that position A be used as the normal "forward" position, because position A allows the power flow to travel directly from the input shaft to the output shaft and not use the extra idler gear which comes into play with the B position. Thus, position A provides the least power losses, least heat generation and is quieter.

As you mentioned when the transmission is operated in position A, the input shaft and the output shaft have opposite directions of rotation, and when operated in position B the engine crankshaft and the propeller shaft rotate the same direction. Because your boat is equipped with a LH rotation prop, it is using B position for forward travel. This is not necessarily bad, but Hurth recommends the use of a gear box cooler if position B is used continuously with more than 25kW (34hp).

It is possible to change the propeller to a RH rotation and adjust the shift linkage to accommodate, but you should be aware that the forward and reverse gears on this transmission have different reduction ratios. This will be of little consequence if you have the 2:1 ratio(really 1.79/1.86) but it could be significant if you have the 2.72:1 ratio. The 2.72:1 transmission has a 2.72 ratio in position A and 1.86 ratio in position B. This means that a direct swap of propeller size (pitch and diameter) won't be possible for the 2.72 ratio gear. Your prop supplier will have to calculate a new prop size, which isn't the end of the world.

Hope this info is helpful.
DougR
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Old 17-02-2015, 06:51   #3
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Location: Katy, TX
Boat: Westsail 32'
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Re: MD40 w/ Hurth HB100 - Prop Rotation?

Quote:
Originally Posted by DougR View Post
The HBW 100 transmission is, as you know, equipped with forward and reverse rotation capability which Hurth refers to as position A and B respectively. Hurth recommends that position A be used as the normal "forward" position, because position A allows the power flow to travel directly from the input shaft to the output shaft and not use the extra idler gear which comes into play with the B position. Thus, position A provides the least power losses, least heat generation and is quieter.

As you mentioned when the transmission is operated in position A, the input shaft and the output shaft have opposite directions of rotation, and when operated in position B the engine crankshaft and the propeller shaft rotate the same direction. Because your boat is equipped with a LH rotation prop, it is using B position for forward travel. This is not necessarily bad, but Hurth recommends the use of a gear box cooler if position B is used continuously with more than 25kW (34hp).

It is possible to change the propeller to a RH rotation and adjust the shift linkage to accommodate, but you should be aware that the forward and reverse gears on this transmission have different reduction ratios. This will be of little consequence if you have the 2:1 ratio(really 1.79/1.86) but it could be significant if you have the 2.72:1 ratio. The 2.72:1 transmission has a 2.72 ratio in position A and 1.86 ratio in position B. This means that a direct swap of propeller size (pitch and diameter) won't be possible for the 2.72 ratio gear. Your prop supplier will have to calculate a new prop size, which isn't the end of the world.

Hope this info is helpful.
DougR

Thanks DougR! That is very helpful indeed. I suppose it depends on the reduction ratio. I'll see if I can't find the nameplate info this weekend. It may be painted over. At any rate, I know I need a 16" prop, but the pitch is where I'm uncertain. The reversed linkage appears to be an easy fix as well.
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