I have an Adams 12m launched in 1983. It has an Autostream 3B S series feathering prop on a shaft from a Yanmar 3HM motor driving through a KBW10E transmission.

The prop (from before I bought the boat) is overpitched meaning the motor wont do more than 1800rpm under load going forward. Motor specs are max continuous rpm of 3200.

The prop has pitch adjustment for forward and reverse. I understand the Yanmar crank rotates counter-clockwise viewed from the front and so clockwise viewed from the stern/flywheel.

Am I correct in thinking the transmission when forward is selected, reverses the motor's direction of rotation?

It is a pretty simple transmission constant mesh box with an input shaft and an output shaft. There is an intermediate shaft for reverse only. So in forward gear, the input cog drives the output cog directly hence reversing the direction of rotation. When reverse is selected, the input drives the intermediate cog which then drives the output, so the output shaft rotates in the same direction as the input shaft.

Is there anyone with knowledge of this engine/transmission combination who can confirm this or set me straight?

Reason I ask is that if the prop shaft rotation is counter-clockwise in forward gear (viewed from behind the prop looking forward), then as I understand it, the prop should be left handed. Receipt says it was supplied right handed. This can be corrected, but might go some way to explaining why pitch is so out of whack.

Thanks in anticipation

is it not possible to view the prop shaft / shaft coupling ?

A couple of questions. Does the vessel reach hull speed at the 1800 rpm ? & does the engine rev to 3200 rpm, wide open throttle while in neutral?

The KBW10E uses an idler gear for reverse, so reverse prop ration will be the same direction as that of the engine, while forward will be opposite direction. The 3HM engine is very similar to the GM series, which rotate anti-clockwise viewed from aft (ie looking forward).

Cheers, Graeme

woko said..
A couple of questions. Does the vessel reach hull speed at the 1800 rpm ? & does the engine rev to 3200 rpm, wide open throttle while in neutral?

reaches an indicated 6.8 knots, but with a fair bit of black smoke. Engine revs to well over 2000 without load (and without smoke), Not sure it gets to 3200 - another observation to make when I have the chance. Opinion of the surveyor when I bought the boat was that prop was likely overpitched, Yanmar certified mechanic thought the same after doing a basic service on the engine.

Jolene said..
I think that gearbox should be running in a clockwise direction in forward gear ,,,viewed standing behind the prop looking forward. And I'm sure that gearbox has different ratios for forward and reverse .
Reverse rotation is slower than forward at any given engine rpm. The slower rotation will determine which gear is reverse, hence the correct direction of the prop rotation

Thanks, you first statement chimes with what Graeme has also said, and what I have now gleaned from a manual for the motor (about the direction of crank rotation). I am not sure how much different the ratios are between forward and reverse, and I'm also not confident I could reliably tell by eye the difference in shaft rotating speed in the two directions. Complicated by being unsure the load from the prop would be the same in both directions, given separate pitch adjustment for forward and reverse.

woko said..
If the prop doesn't need dismantling to adjust it should be able to be done in the water. Another thing to consider is prop diameter. I recently cleaned a mates prop in the water and it made a huge difference

Thanks. She's on a buoy in a bay which catches some swell. I am not confident I could do the adjustments working underwater with a spanner and an allen key with the boat moving with the chop/swell. Perhaps on a calm day. The prop is 16 inch and low revs/black smoke occur when the prop is clean.

Just to follow up, in case anyone with similar issue comes across this thread. the problem is pitch. The feathering prop rotates clockwise for forward. The engine will do 3200 in neutral. The prop was clean and freshly propspeeded.Following the auto stream manual and adjusting the prop by 4.5 turns out, of the forward adjustment screw (manual states as rule of thumb decreasing pitch by half turn of the adjustment screw should increase engine rom by 150) if anything, made the problems worse. With the boat back out of the water after that attempt, prop was adjusted by eye for pitch, forward and backward to have much less pitch in both directions. This resulted in the adjustment screws being at the extreme of their range. After the de-pitching by eye, the engine happily revs to 2700 under load going forward (and would probably make it further), is not smoking and labouring when it does this and readily achieves 7.6 knots boat speed without being maxed out (was in an 8 knot limit channel.)

Somehow the self feathering prop has been seriously out of whack, such that it still isn't right when adjusted to the ends of the ranges of the adjustment screws. Next step is to remove and rebuild the prop.

Yes I will follow up after the prop has been dismantled. Not at all sure how it ended up so wrongly pitched. I certainly don't think it came that way when it was first installed several owners back in 2002. When the prop was first bought the pitch was 10 inches on a 16 inch prop. Somewhere along the way it has got seriously out of whack. I am not sure how, as it was that way when I bought the boat. But I do know the previous owner caught a rope in the prop to the point where it pulled the shaft skeg almost out of the hull. But the prop shows no external, visible signs of damage, and speaking to the shipwright who did the repairs he said the prop wasn't altered as part of the repair work, though of course it was removed and reinstalled. In any case, will report whatever can be gleaned from the disassembly and inspection in a few weeks.

Jolene said..

Quixotic said..
Yes I will follow up after the prop has been dismantled. Not at all sure how it ended up so wrongly pitched. I certainly don't think it came that way when it was first installed several owners back in 2002. When the prop was first bought the pitch was 10 inches on a 16 inch prop. Somewhere along the way it has got seriously out of whack. I am not sure how, as it was that way when I bought the boat. But I do know the previous owner caught a rope in the prop to the point where it pulled the shaft skeg almost out of the hull. But the prop shows no external, visible signs of damage, and speaking to the shipwright who did the repairs he said the prop wasn't altered as part of the repair work, though of course it was removed and reinstalled. In any case, will report whatever can be gleaned from the disassembly and inspection in a few weeks.

Your prop may have been reinstalled for left hand rotation instead of right hand (clockwise) when it was last removed from the shaft.

Yes, I've been harbouring that suspicion for a while, hence my earlier questions in the thread (when away from my boat) about the crank rotation on my Yanmar 3HM, and the effect of the gearbox in reversing that, when in forward gear and so on. I don't yet know enough about the way Autostreams are put together to know whether/how it might be possible to reassemble a right handed prop to operate as a left handed prop, but I'm working on it...

Jolene said..
If I recall correctly the pitch range is different for reverse . That range is swapped from left to right rotation by advancing or retarding a number of gear teeth on the hub when the propeller blades are in a known position,, hence the left and right timing marks on the hub. Basically by the sound of it the relationship between your stops and your blade angle is out , whether it's rotation confusion or just something that was overlooked when the prop was last assembled.

Thanks. I am certain the relationship of the blades to the stops is fundamentally wrong, as the stops have had to be set at their absolute limit to get close to a decent pitch. You can see what I mean from this piccy. Ignore the position of blades as they are not on either stop in this piccy. It's clearly a crazy amount of adjustment. Hence the need to pull the prop off, dismantle and try to figure out whats wrong with the way it's assembled.

By the way, per the discussion of the gearbox earlier in the thread, the forward ratio on mine is 2.14 and reverse is 2.50.

Based on my amateur calculations, for a pitch of 10 inches, the blade angle needs to be 11.5 degrees, which at engine revs of 3200 and shaft speed of 1495 rpm, with a16 inch prop will generate 6.9 knots of boat speed at 44% slip. 3200 is the max continuous rpm for the 3HM. The slip's a guess, and given with the adjustment shown below, the boat has got close to 8 knots, I'm guessing the slip in practice is less, even tho' a chap at Austral Props suggested it'd be closer to 50% or more. My boat is an Adams, which are generally easily driven, so I suspect the slip is less than for some others.


Below is a cutaway from the Autostream Manual. The stops limit the rotation of the prop body on the shaft, which in turn limits the extent to which the blade sprockets (and so the blades) get turned by the body sprocket. This makes me all but certain the blades are one or more teeth (probably more than 1) out of alignment relative to the body sprocket. If so, this can be remedied by disassembling the prop and rotating each blade by a tooth at a time, until I get them as near as possible to the right pitch (11.5 degrees) while having the pitch adjustment screws in the middle of their range. That will be the coarse adjustment as it has to be done in whole teeth, then the screws can be used to achieve the fine adjustment.

I'm inclined to have the pitch the same forward and backwards, to the extent that's possible, as the the lower ratio of the gearbox in reverse will soften this a bit. It is nice to be able to pull up quick by hitting reverse, if necessary. A side benefit of all this is de-pitching should also reduce the amount of prop walk (it's been operating more like a paddle wheel at the pitch it's had!).

Jolene said..
I have my auto stream pitched quite fine in reverse,,, maybe a bit too fine. It's a bit long in the tooth when pulling the boat up. It keeps prop walk to a minimum but that's also a bit of a disadvantage as i mostly dock to port in reverse. My gear box is also a reduction in reverse so a bit more bite from the prop wouldn't hurt. Every time I go under to give the prop a clean I think about adjusting it.

Best not to rush into these things...