Prop rotation and gearbox question.
Discussion
I have a long keel sailboat with the propshaft coming out of the port side of the keel.
It's a 43 year old boat which I'm restoring, and I have a brand new Nanni Diesel with a Twin Disc Technodrive (TMC40P)gearbox.
Unfotunately the output flange of gearbox rotates clockwise in ahead and I need my prop to rotate ant-clockwise ahead, due to the offset shaft, and the effects of propwalk. (I think!)
The forward ratio is 2.00 and reverse is 2.13. I there any reason why I can't run the box in reverse, for forward drive of my boat? I will be re-propping anyway.
Can someone confirm I've got my head around the propwalk thing correctly? Everytime I think about it, I come up with a different answer!
Cheers,
Mick.
It's a 43 year old boat which I'm restoring, and I have a brand new Nanni Diesel with a Twin Disc Technodrive (TMC40P)gearbox.
Unfotunately the output flange of gearbox rotates clockwise in ahead and I need my prop to rotate ant-clockwise ahead, due to the offset shaft, and the effects of propwalk. (I think!)
The forward ratio is 2.00 and reverse is 2.13. I there any reason why I can't run the box in reverse, for forward drive of my boat? I will be re-propping anyway.
Can someone confirm I've got my head around the propwalk thing correctly? Everytime I think about it, I come up with a different answer!
Cheers,
Mick.
Some marine gearboxes will run either way depending on engine rotation, BW72 as an exmaple, but on those you set the oil pump accordingly. So fwd is defined as both input and output shafts running the same way, regardless of whether that is clock or anticlockwise.
You need to check if that is the case with yours.
Most will not take as much power/revs in reverse, BW72 a good example, manual says no more than 1000rpm in reverse.
Not sure I can really comment on your prop walk question, try the PBO forum section on YBW.com
You need to check if that is the case with yours.
Most will not take as much power/revs in reverse, BW72 a good example, manual says no more than 1000rpm in reverse.
Not sure I can really comment on your prop walk question, try the PBO forum section on YBW.com
mickrick said:
Unfotunately the output flange of gearbox rotates clockwise in ahead and I need my prop to rotate ant-clockwise ahead, due to the offset shaft, and the effects of propwalk. (I think!)
Can someone confirm I've got my head around the propwalk thing correctly? Everytime I think about it, I come up with a different answer!
Is 'propwalk' what I've heard as the 'paddlewheel effect', ie stern pulls to one side in reverse? If so it will go one way or the other, changing the gearbox won't stop it.Can someone confirm I've got my head around the propwalk thing correctly? Everytime I think about it, I come up with a different answer!
My Freeman (petrol engine, small prop) had a shocking pull to starboard in reverse, really bad. However a very similar boat with diesel engine (larger slower prop) had none that I could tell.
Thanks for the replies.
Huntsman, I doubt my gearbox will have an oil pump as it's tiny, weighs less than 9 kilo's and holds 0.2 litres of oil. I'm asuming it's just an oil bath jobbie.
I'm thinking if it has straight cut gears it shouldn't make a difference, but my concern is if it has helical gears.
I suppose the best thing is to e-mail Technodrive directly, the trouble is, it's Italy....
Simpo two, yes it's the paddlewheel effect, the bottom blade of the prop on a counter-clockwise rotating prop will push the stern to port. As you say it doesn't make much difference once you know which way it is, if you have a prop on the centerline of your hull...there's the propblem! My prop is to port of the center line.
Searider, the prop will be turning clockwise in ahead, which will push the stern to starboard as you say, but with my prop being on port side, this will also push the boat to starboard, so I think my boat will be going sideways!
But when I read you reply, what you say makes sense! It's all very confusung!
Drinking my coffee this morning, I thought I had my head around it again....seems not!
You can see the shaft in this picture, and I believe the old prop is in fact counter-clockwise rotating in ahead.
Huntsman, I doubt my gearbox will have an oil pump as it's tiny, weighs less than 9 kilo's and holds 0.2 litres of oil. I'm asuming it's just an oil bath jobbie.
I'm thinking if it has straight cut gears it shouldn't make a difference, but my concern is if it has helical gears.
I suppose the best thing is to e-mail Technodrive directly, the trouble is, it's Italy....
Simpo two, yes it's the paddlewheel effect, the bottom blade of the prop on a counter-clockwise rotating prop will push the stern to port. As you say it doesn't make much difference once you know which way it is, if you have a prop on the centerline of your hull...there's the propblem! My prop is to port of the center line.
Searider, the prop will be turning clockwise in ahead, which will push the stern to starboard as you say, but with my prop being on port side, this will also push the boat to starboard, so I think my boat will be going sideways!
But when I read you reply, what you say makes sense! It's all very confusung!
Drinking my coffee this morning, I thought I had my head around it again....seems not!
You can see the shaft in this picture, and I believe the old prop is in fact counter-clockwise rotating in ahead.
Edited by mickrick on Thursday 15th September 07:51
Although my stern pulled like a b
d in reverse (until the keel got a grip as speed increased), forward was perfect; millimetric precision at all times. But then of course you have a rudder helping you!
It's amazing that this effect hasn't been engineered out somehow, eg by some kind of deflector on the hull. It made slow speed handling a real pain... into the wind on starboard side, great. With the wind and on the port - no chance because the moment you try to take the speed off it's bye bye stern, unless you possibly 'handbrake' it in like a nautical Russell Swift.
d in reverse (until the keel got a grip as speed increased), forward was perfect; millimetric precision at all times. But then of course you have a rudder helping you!It's amazing that this effect hasn't been engineered out somehow, eg by some kind of deflector on the hull. It made slow speed handling a real pain... into the wind on starboard side, great. With the wind and on the port - no chance because the moment you try to take the speed off it's bye bye stern, unless you possibly 'handbrake' it in like a nautical Russell Swift.
Hmmm, propwalk. I guess it's one of those nesessary evils, but when you learn to live with it, it can sometimes be just as much of a help than a hindrance. (a bit like tides really!) However my boat is a fin keel, not a long keel, and I do understand that makes things a lot more tricky.
I once spent about an hour just buggering about in Portsmouth harbour seeing what she would and would not do. It is very satisfying turning 360 almost on the spot, never moving the rudder from one position and doing it all on the throttle.
I once spent about an hour just buggering about in Portsmouth harbour seeing what she would and would not do. It is very satisfying turning 360 almost on the spot, never moving the rudder from one position and doing it all on the throttle.
Propwalk I don't have a problem with, in fact going astern, once you're moving, the rudder takes over anyway, and I always knock it out of gear once I have some way on untill I need some more speed, then give it another Kick to keep the momentum up.
What I don't want is propwalk the wrong way with an offset prop.
What I don't want is propwalk the wrong way with an offset prop.
I would have thought you could ignore propwalk in that set up Mick. Propwalk only occurs at all if you have a shaft angle and that photo shows you have very little.
The reason propwalk occurs is that as the shaft angle increases the distance travelled by the blades through the water differs from upside to downside as you are going through at an angle. Like a big italic X.
X
As for the twindisc, I have a pair of those running on the Detroits, one forward, the other reverse but of course they are the same ratio either way or I would spend the day going round in circles.
The reason propwalk occurs is that as the shaft angle increases the distance travelled by the blades through the water differs from upside to downside as you are going through at an angle. Like a big italic X.
X
As for the twindisc, I have a pair of those running on the Detroits, one forward, the other reverse but of course they are the same ratio either way or I would spend the day going round in circles.
What make? I'll give you a clue...read my first post...
Personaly I rate Yanmars, we had the 6 cylinder jobbies in the RIB's on the big green tug I worked on. Those where in fact Toyota engines. (The 4 cylinder Yamaha engines are also Toyota.)
BUT, the ancillaries rust if you don't keep salt water away from them. I had a potentialy nast incident with one where there was an oil leak from below the sump. Turned out to be the dipstick tube which was connected to a banjo joint on the bottom of the sump had rusted through. Fortunately I spotted it before we lost all the oil.
The reason I chose the Nanni was because all the ancillaries I needed to get to, when it was installed where in the right place. It also happened to be the cheapest, probably the first and last time in my life that has happened to me!
All the other 2 cylinder engines in the same power range where in fact the same engine, including the Yanmar, a Kubota mini digger engine! Brilliant, Paddy proof!
Going back to my original confusion...it seems I was very confused, as after going down to my boat this evening, and sitting under it for half an hour turning the old prop and muttering to myself, it turns out (excuse the pun!) that the original prop (Maybe? remember it's 43 years old) turns clockwise in ahead! So I'm assuming lady luck has smiled on me once more!
Personaly I rate Yanmars, we had the 6 cylinder jobbies in the RIB's on the big green tug I worked on. Those where in fact Toyota engines. (The 4 cylinder Yamaha engines are also Toyota.)
BUT, the ancillaries rust if you don't keep salt water away from them. I had a potentialy nast incident with one where there was an oil leak from below the sump. Turned out to be the dipstick tube which was connected to a banjo joint on the bottom of the sump had rusted through. Fortunately I spotted it before we lost all the oil.
The reason I chose the Nanni was because all the ancillaries I needed to get to, when it was installed where in the right place. It also happened to be the cheapest, probably the first and last time in my life that has happened to me!
All the other 2 cylinder engines in the same power range where in fact the same engine, including the Yanmar, a Kubota mini digger engine! Brilliant, Paddy proof!
Going back to my original confusion...it seems I was very confused, as after going down to my boat this evening, and sitting under it for half an hour turning the old prop and muttering to myself, it turns out (excuse the pun!) that the original prop (Maybe? remember it's 43 years old) turns clockwise in ahead! So I'm assuming lady luck has smiled on me once more!
Hi Motorvator, thanks for your post. Propwalk astern doesn't bother me too much, I just want to avoid a boat which goes sideways ahead without weather helm!
I think I'm worrying over nada. It seems the Bukh DV10 I took out was the same rotation as the one going back in.
I think I'll assume it was O.K. before I bought the little shed 6 years ago. (She was out of the water for 4 years before I found her) Although the Bukh wasn't original.
As an aside, I'll give her the epoxy treatment before she goes back in, as I think she'll be dry now.
I think I'm worrying over nada. It seems the Bukh DV10 I took out was the same rotation as the one going back in.
I think I'll assume it was O.K. before I bought the little shed 6 years ago. (She was out of the water for 4 years before I found her) Although the Bukh wasn't original.
As an aside, I'll give her the epoxy treatment before she goes back in, as I think she'll be dry now.
mickrick said:
Hi Motorvator, thanks for your post. Propwalk astern doesn't bother me too much, I just want to avoid a boat which goes sideways ahead without weather helm!
I think I'm worrying over nada. It seems the Bukh DV10 I took out was the same rotation as the one going back in.
I think I'll assume it was O.K. before I bought the little shed 6 years ago. (She was out of the water for 4 years before I found her) Although the Bukh wasn't original.
As an aside, I'll give her the epoxy treatment before she goes back in, as I think she'll be dry now.
That's OK Mick, propwalk of course occurs both ahead and astern, just that astern generally seems the worst due to a lack of propwash over the rudder. Again not as relevant for you with that long keel and rudder ahead of the prop other than she will not be that responsive in close quarter manoevres.I think I'm worrying over nada. It seems the Bukh DV10 I took out was the same rotation as the one going back in.
I think I'll assume it was O.K. before I bought the little shed 6 years ago. (She was out of the water for 4 years before I found her) Although the Bukh wasn't original.
As an aside, I'll give her the epoxy treatment before she goes back in, as I think she'll be dry now.
You should also remember that the chaps who built your boat weren't idiots and I'd bet they probably introduced a slight sideways angle to the shaft to take care of the offset.
I reckon you should worry more about the assymetric rigging you will require for windy power to take care of the additional drag on one side of the keel due to the non folding prop. I would suggest moving the mast stepping approx 2" to port to start with and take it from there.
mickrick said:
BUT, the ancillaries rust if you don't keep salt water away from them.
No salt for me, just osmosis!mickrick said:
The reason I chose the Nanni was because all the ancillaries I needed to get to, when it was installed where in the right place.
Fair enough. In my case only Yanmars are listed as options; whether this is just because he gets a good deal, or because they have a good salesman in that area I don't know. It would be nice to do the research and say 'Actually no I want a Nanni please, and with the correct number of cylinders' but it may be that the engine bay/propshaft etc have been designed around Yanmars. Dunno.I suppose my experience of Yanmars is a bit negative as I once hired a small narrowboat with one. It sounded wrong (not chug chug chug), once started backwards with exhaust smoke pouring out of the air intake, and in reverse the boat was totally uncontrollable, even with way on. I think it had an offset propshaft which didn't help.
I wonder if they could marinise the Ford/Peugeot 2.7 twin turbo from my S-Type...? Do you think that would get a bathtub on the plane Mr M?!
A little bit over the top perhaps, but I remember reading in one of the yachting comics about the restoration of an old pilot cutter and they had got around the cranky handling of the offset prop by fitting one on the other side(folding props natch)! They used hydraulic transmission so the powerpack could be placed somewhere out of the way; it turned(slowly) on a sixpence.
MOTORVATOR said:
A somewhat incomplete question Mr S. How many people in said bathtub?
But to preempt your next question - It's generally best to fit them at the tap end
Hence the phrase 'opening the taps'!But to preempt your next question - It's generally best to fit them at the tap end
And yes, this one will actually have taps, not a little pumpy lever thing. Luxury!
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