Crankshaft and prop rotation

[MapisM]

The statement that Deleted User got from Caterpillar UK re. port engine load (see this thread) got me thinking.
Aside from the inconsistency in that specific case, the general point is, would you agree that there is a higher load on port engine - AOTBE - because it spins a left hand prop, whilst the crankshaft spins the other way round?
There's a specific reason why I'm wondering, and it's related to single engine boats, but let's leave that aside for the moment.
I'm curious to hear your views first.
Ta!

 

[spannerman]

In a twin engined boat with handed props one gearbox runs in reverse to give the counter rotation.
This means that one box puts the power straight through with everything rotating the same way as the crank. The other box has to reverse its direction and this is usually done with planetary gears similar to the set up in an auto box on a car. The hydraulic clutches either couple together or brake different components to allow bits to spin or not and in the process giving rotation way way or the other, simply put.
Obviously putting the power through some extra gear trains saps a few horsepower. IMHO.

 

[aquapower]

In a twin installation the port gearbox is the one thats reversed so is having to turn more gears than the star one, this will take more energy.
I have raised this question with engine makers several times but always get the same answer that its perfectly normal to burn a few litres more on one, the bigger the engines the bigger the difference.

 

[DAKA]

The statement that Deleted User got from Caterpillar UK re. port engine load (see this thread) got me thinking.
Aside from the inconsistency in that specific case, the general point is, would you agree that there is a higher load on port engine - AOTBE - because it spins a left hand prop, whilst the crankshaft spins the other way round?
There's a specific reason why I'm wondering, and it's related to single engine boats, but let's leave that aside for the moment.
I'm curious to hear your views first.
Ta!

I was also curious.
Hurth ZF (as fitted to Volvo, Cummins etc) produce a range of gear boxes that have different ratios for Port and starboard.
Perhaps this is a deliberate attempt to counter the loss through reverse.

note the A / B ratios here http://www.simplicity-marine.com/ZF/ZF63/Technical.htm
there is a slight typo in the 2.5 range as they are also different.

Mikes boat has one of these twin disc type fitted but not sure of the model ?

 

[jfm]

the general point is, would you agree that there is a higher load on port engine - AOTBE - because it spins a left hand prop, whilst the crankshaft spins the other way round?

Yes. For the reasons spannerman explains - there is more friction in the gearbox that is reversing the rotation.

It can be a small amount in a good set up - between 1 and 2% according to the instruments on my boat

Some engines have been built in reverse rotation (volvopaul told me about some last weekend, can't remeber which, some Perkins maybe?) to deal with this but it isn't very economic to make the necessary mods (mirror image camshafts, etc)

 

[volvopaul]

Yes. For the reasons spannerman explains - there is more friction in the gearbox that is reversing the rotation.

It can be a small amount in a good set up - between 1 and 2% according to the instruments on my boat

Some engines have been built in reverse rotation (volvopaul told me about some last weekend, can't remeber which, some Perkins maybe?) to deal with this but it isn't very economic to make the necessary mods (mirror image camshafts, etc)

John, this was done in the days before reverse gearboxes were made small enough for pleasure boats, it was the perkins 6354 engine that was rotated anticlockwise viewed from the front, the borgwarner gearbox was also modified to work backwards, the difference being in the pump at the front of the box, it was rotated through 180 degrees to transfer the oil another way into the valve then to the clutch pack.

There was another issue with this bw box that when it was mated to engines over 200hp, mermaid were the culprits, it smashed the idler gear as it was in effect running in reverse to counter rotate the prop, the box wasnt designed for it and later on twin disc boxes were fitted.

Thread drift there but basically yes the power is sapped slightly, but if you ever see a zf irm280 strpped you can see how the drive is altered from fwd to reverse, it still drives through the same amount of gears to the output shaft.

So in later years of use if possible, swap the boxes over from engine to engine to match the wear on each clutch pack, bit like swapping the outdrives when the cones are wearing out, ive been asked to do that before rather than customers forking out for a new cone!

 

[MapisM]

It can be a small amount in a good set up - between 1 and 2% according to the instruments on my boat

Interesting, I also suspected that one additional gear step could only draw a very marginal amount of power.
I'd be interested to hear if anyone knows the comparable figure when the engine spins a Bravo outdrive, instead of a shaft.

But there was also a bit of a catch question in my OP (with apologies for that ):
In my books, the standard engines crankshaft spins CCW, which is the same rotation as the port prop.
So, what did Cat UK mean by "propeller will be driven against rotation"...?

 

[jfm]

In my books, the standard engines crankshaft spins CCW, which is the same rotation as the port prop.

A normal engine rotates CW, viewed from the front. CCW if you face the flywheel end. So yes, that's same as port prop (I think!)

I see your point therefore. FWIW, I agreed your view on the other thread - that the advice from so-called "experts" at Cat seemd quite wrong and I too wouldn't rely on their advice or even give it 5 mins of attention.

 

[MapisM]

CCW if you face the flywheel end. So yes, that's same as port prop (I think!)

Exactly. I didn't mention CCW "facing the flywheel" because as I understand that's the standard way to look at it (see engine rotation).

 

[aquapower]

A normal engine rotates CW, viewed from the front. CCW if you face the flywheel end. So yes, that's same as port prop (I think!)

Quite simple then, to turn port prop in same direction as engine you have to use 3 gears as opposed to the 2 on the star to turn prop in opposite direction to engine hence more energy required on port side.

 

[BartW]

I'd be interested to hear if anyone knows the comparable figure when the engine spins a Bravo outdrive, instead of a shaft.

I thought that principally in a Z-drive, the number of gears is the same in Fwd or Rev, I can not explain this in words,
but the picture below shows the part in the lower Unit that shift's Fwd or Backw to select the prop to run CW or CCW

this is diagram of a alpha drive, but I guess the principle is the same in Bravo or any other Z-drive (?)

anyway, I never discovered any consistent difference in load / power of the two drives of my actual boat (2 x VP SX drive), appart from other reasons, such as weight distribution on the boat, different trim, etc, ...
max RPM is exactly the same (ususally)

On the other hand, the 1 or 2% difference on shaft drives I guess is also not very good noticable in practice.

 

[jfm]

Quite simple then, to turn port prop in same direction as engine you have to use 3 gears as opposed to the 2 on the star to turn prop in opposite direction to engine hence more energy required on port side.

I think generally they are planetary gears, so many more than 2 or 3 cogs

 

[Trundlebug]

It doesn't necessarily follow that more gears are needed to reverse the normal direction.

Referring to the diagram shown above, all that's needed to reverse the prop direction is for the drive to be taken from the other side of the input gear, i.e. for the uppermost gear on the vertical shaft to take drive from the top of the input gear, rather than as shown, on the bottom of the input gear.

As has been said, it all depends on the type of gearbox, but there's no real intrinsic reason why this principle can't be used to maintain equal efficiency between props of different rotations. Especially as the engines have to be geared down to some degree anyway (unless you're talking about an old Bolinder on a narrowboat, where it's 1:1 because they only fire once a week!)

On my boat, the port engine if anything seems to use less fuel than the stbd, despite having 2 calorifiers to heat and reversing the drive direction. Haven't quite worked out why yet, unless it's that once it has finally warmed up it runs slightly hotter than the stbd engine and is more efficient as a result.

 

[Hurricane]

I have a question.

How is this all set up in the gearbox.
Is it a factory setting of something that is commisioned later.

I havent seen anything on the gearbox to change it over so it seems to me that the gearboxes would have been ordered from the factory accordingly.

 

[DAKA]

I have a question.

How is this all set up in the gearbox.
Is it a factory setting of something that is commisioned later.

I havent seen anything on the gearbox to change it over so it seems to me that the gearboxes would have been ordered from the factory accordingly.

Its the same gear box and you can change them without any modification from one side to another.
There is a lever on the top that your F/R gear lever either pushes or pulls.
So it is set up on your gear levers at your helm position.( which are also interchangeable.)

On the back of your gear box there will be a plate with serial number and the ratio which on many isnt straight forward
Yours will most likely be marketed as 1:2 gearbox but the plate will be more precise and may say

Ratio
A 1:1.98
B 1:2.10

A usually being forward
B usually being reverse

I am interested to learn just why there is a petty difference in the two gears and believe it may be to counter the effects of CW and CCW .

On a VP outdrive leg there is also a switch on the back that can be flicked to alter F/R.

 

[MapisM]

It doesn't necessarily follow that more gears are needed to reverse the normal direction.

Referring to the diagram shown above, all that's needed to reverse the prop direction is for the drive to be taken from the other side of the input gear, i.e. for the uppermost gear on the vertical shaft to take drive from the top of the input gear, rather than as shown, on the bottom of the input gear.

Precisely!!!
In fact, the diagram posted by Bart shows the Alpha, but a proper drive works exactly as you say - see diagram below: the upper gear for l/h rotation, and the lower for r/h (both marked as #20).
In fact, with Bravo drives there's just one version, and it's used for either port or stbd installations, simply switching the cable on the throttle. But I've never heard (also from some maniac guys in the US, who get mad for squeezing the last mph out of their speedboats) of any difference in power absorption depending on the rotation!

PS for BartW: actually Alpha and Bravo are very different. Aside from what you can see from the diagram, B has no water pump, and A can NOT be used either side just reversing the rotation: you need to change the whole lower part of an A and fit the L/H version, if you want to use it on port side.

 

[MapisM]

I am interested to learn just why there is a petty difference in the two gears and believe it may be to counter the effects of CW and CCW .

You're saying that as if it were normal.
Surely it isn't the case with my Cats - both 2:04.
I never thought to check that on any other boat, though.

 

[DAKA]

You're saying that as if it were normal.
Surely it isn't the case with my Cats - both 2:04.
I never thought to check that on any other boat, though.

I am asking and seeking answers rather than attempting to supply them.

I only know about Hurth ZF 63 which is most likely the most popular gearbox in use on this forum as it suits 200 hp-400hp up to 6000 rpm.

Now hurf ZF have as much experience with gear boxes as anyone and several gear boxes in their range including the popular 63 series have different ratios but I do not know about the whole range.

I am guessing that the different ratio is not a mistake and could be a deliberate design for one of two reasons

a) matched revs produce a damaging drumming vibration throughout the boat.

b) perhaps to overcome the forward/reverse gear differences ???????????


Now if HurthZF do not continue the ratio difference higher up the range perhaps that is due to the boxes being more commercial and not as often fitted as a pair , or perhaps not redesigned recently.

Just to double check......... who manufactures your gearbox and what ratios do they show in their specification, asking as my engine manufacturer who brands mine with their own name lists mine slightly wrong according to the gearbox manufacturer .

 

[aquapower]

The ZF 63 definatly runs an extra gear in reverse (port engine) thats why the port one can chatter at low revs in fwd gear,

The velvet drive boxes have the planetary gears as this is required to keep the output shaft at the same height as the input, however the drop centre boxes/down angle are different

 

[BartW]

In fact, the diagram posted by Bart shows the Alpha, but a proper drive works exactly as you say - see diagram below

Can I conclude that you consider the Alpha is not a proper drive ?
I agree as far as it is not a very technically advanced design, but nevertheless it does what its supposed to, and as far as I know it is reliable when it is not stressed with too high power. And there are a impressive number of them used around the globe
I wouldn't be surprised if it is the most used drive in boating world
ofcause thats no proove that it is a "proper drive"
Bravo drive, No personal experience myself, but can read some issues about them here on the forum every now and then

PS for BartW: actually Alpha and Bravo are very different.

yes I know, but the gear "principle" is more or less the same,
VP SX is more similar to Bravo, the gearing is in the upper unit (I believe)

but all this just as a side note, as the main topic of this thread is about shaftdrive(s)