reduction gears

[rubino]

My engine gearbox has two reduction gears : 2,63:1 and 1,95:1. Which one is for forward and which one is for backward ? ...and why ?
I am quite confused ! Thanks a lot.
Rubino

[tkeithlu]

Which one is straight through and which one is routed through the second shaft? Which one has the shift lever forward and which one aft? In automobiles you could expect the 2.63 to be reverse, but a transmission specialist on the forum could tell me and you that reverse has a lower ratio because props are inefficient in reverse. Hopefully a pro will speak up very soon, and we'll both learn something.

[DougR]

Transmissions such as the one you describe are frequently found on low h.p. engines where there is normally only one engine in the boat. Some Hurth(ZF) and Kansaki gears come to mind...

The wide gap in gear ratios is the result of the power path thru the gearbox. In one rotation the power moves directly from the input shaft to the output shaft across only one gear interface, and in the other rotation the power has to cross from the input shaft to an idler shaft and then from the idler to the output shaft, crossing two gear interfaces.

Usually the ratio with the least amount of reduction is designated the forward rotation, 1.95:1 in your case, for two reasons. First, the power is only crossing one gear interface making for less loss and higher efficiency, and secondly by using only the two main shafts in the gearbox, the power path is more robust.

When the rotation with the most reduction is used (reverse rotation), the power has to cross two gear interfaces, therefore absorbing more h.p. and being less efficient. Also, almost always the idler shaft is not as robustly built as the as the primary shafts and therefore unable to carry as much power continuously.

In the case of the Hurth gears, if the rotation with the more reduction is selected as the "forward" rotation, the allowable power capacity of the transmission is reduced to assure suitable service life. The same is probably true for the Kansaki gears as well.


In larger transmissions which might be found in twin engine power boats, the transmissions are designed differently so that both forward and reverse rotations have the same or very close reduction ratios, and the port and stb. transmissions would be run in opposite rotation with no problems.

Hope this helps.

DougR

[tkeithlu]

Doug, that's a wonderfully clear essay on the topic. Thank you.

[RaymondR]

I wondered about this for years until I disassembled the gearbox on my 3GM30. It a while ago and my recollection is a bit hazy but I think I decided that the reason was because both shaft assemblies assemble from the same end of the box and would not fit if the gearing was designed for the same ratios forward and backward.

[valhalla360]

Short follow up on Doug's excellent explanation:

It's common to run in forward at 50-100% of max power output and to do so for hours at a time.

It's rare to run in reverse at more than 20-30% of max power output and then usually only for a few seconds at a time.

Result: Reverse doesn't need to be as robust and efficiency in reverse is largely irrelevant.

[rubino]

Thank you Doug, your essay is clear and makes a lot of sense. .....but I am still confused because the Volvo engine D1 30 manual states that the propeller preferred rotation is right hand and the gearbox is 2,63:1 x righthand and 1,95: 1 x lefthand (????). Please keep helping me !
Rubino

[tkeithlu]

Be very careful here. Engines are "handed" from looking at the front; a right-handed engine turns clockwise as viewed from the front. Propellers are handed from the rear; a right-handed propeller turns clockwise as viewed from the transom.

Your two ratios are for right and left turning, because one is reverse gear.

Painful experience. I paid $1,200 to have my DD 4-53 converted to right hand, thinking that went with a right-handed propeller. Had to buy a left-handed prop as well. Just a little surprise for the next owner of my boat the first time he/she tries to dock it.

[FPNC]

There is the opportunity for you to get some really good advice and information on this site. You must help us help you. At the very least, you need to tell us what model gear that you have. Unless it has fallen off, the model number can be found on a tag at the side or top of the transmission gear case.

[kenbo]

If it's a Kanzaki/Yanmar gearbox 2.63 is forward and 1.95 is reverse

[Wotname]

Originally Posted by tkeithlu

Be very careful here. Engines are "handed" from looking at the front; a right-handed engine turns clockwise as viewed from the front. Propellers are handed from the rear; a right-handed propeller turns clockwise as viewed from the transom........

Be extra careful as the above statement is certainly not universally true.

Crankshaft rotation for the Yanmar GM and HM series (and I imagine many other Yanmar series) is determined when viewing from the stern.

And FWIW, the cylinders number from the flywheel.

From the service manual:

[model 10]

You view an engine from the rear and American rotor blades turn the correct direction, clockwise. Foreign helicopter blades go backwards.

[Wotname]

Originally Posted by Ecos

.... and American rotor blades turn the correct direction, clockwise. Foreign helicopter blades go backwards.

Viewed from where?

[model 10]

Originally Posted by Wotname

Viewed from where?

The drive end, like looking up at the rotor blades.
In the case of a boat going fwd, that would be from behind a RH prop?

[DougR]

Originally Posted by rubino

Thank you Doug, your essay is clear and makes a lot of sense. .....but I am still confused because the Volvo engine D1 30 manual states that the propeller preferred rotation is right hand and the gearbox is 2,63:1 x righthand and 1,95: 1 x lefthand (????). Please keep helping me !
Rubino

Rubino,
Sorry, I have mispoken when I stated that usually the lower amount of reduction is the forward gear. This was based on the old Hurth gears where the HBW 150 forward ratios were 1.56:1, 1.88:1, and 2.63:1, and the reverse ratio for all three was 1.95:1. All of the forward ratios which were offered were the "A" position and the power only crossed one gear face. The reverse ratio of 1.95:1 was the "B" position and the power crossed two gear meshes.

When the power path crosses only one gear face, the result is that the input shaft and output shaft end up rotating in the opposite direction to each other. So, if the engine rotates counter clockwise when viewed from the rear, the output shaft and prop shaft will rotate clockwise. Which gives you a RH propeller.

Back in the mid to late 90s Volvo Penta stopped production of their own transmissions and saildrives and made a marketing agreement with ZF to private label gearboxes for VP, and to develop a saildrive as well. The gearbox which you have, the MS15, was the outgrowth of the Hurth HBW 150 with a few modifications that VP wanted. They changed the 1.88:1 ratio to 2.14:1 as that's better for displacement boats, and they changed the output flange to match the older VP flange dimensions. VP doesn't offer the 1.56:1 ratio as its not much good for sailboats.

So VP offers two ratios on the gear, 2.14:1 and 2.63:1. Both of these ratios have the power going directly from the input shaft, across one gear face to the output shaft, and that means it maximizes efficiency. It also changes the rotation of the output shaft relative to the input shaft, and that's where the right hand prop recommendation from VP comes from.

Your recommended RH prop with the 2.63:1 ratio is correct.

DougR