Multi speed marine gearbox.

[ianj99]

There would appear to be an opportunity for an enterprising gearbox manufacturer (so not a UK one then) to manufacture a two speed marine gearbox or twin ratio reduction drive.

You'd size and pitch the prop for the higher ratio (lower numerically) and then by operating an additonal gear lever a lower ratio can be selected.

In my case, for example, the prop is adjusted (its a Kiwi) for quiet low revving cruising speed of 6kts at 1800rpm with a 1.86:1 ratio. (= 970 shaft rpm). Max engine rpm is 2100 at the hull speed of just under 7kts in calm conditions.

But this clearly means the size and pitch are too big to permit maximum bhp rpm of about 3000 which could be required under arduous conditions.

Being able to engage a second ratio of say 2.2:1 means more engine and shaft rpm and hence more thrust available when needed. (1360 shaft rpm with 3000 engine rpm, equal to a theoretical speed of about 8.5kts. However the aim is not more speed, its more thrust)

I would think the cost would be less than an Autoprop.

 

[pvb]

A bit like the PRM StepDrive maybe?

 

[ianj99]

A bit like the PRM StepDrive maybe?

Yes, but one that can be fitted between shaft and gearbox flanges on any engine.

 

[ffiill]

In the days when people were poor I recall that Hartley Boat Plans NZ did suggest use of a conventional car petrol engine and car gearbox with addition of indirect cooling.

 

[ianj99]

In the days when people were poor I recall that Hartley Boat Plans NZ did suggest use of a conventional car petrol engine and car gearbox with addition of indirect cooling.

Yes, when I was a nipper, the old man and many of his friends with boats had just this setup. (petrol engines in those days)

 

[Cookee]

Sunseekers have them as an option/standard on certain models with trimax drives and they're made by ZF. they have quite a selection as well ......... scroll down a bit On this page

 

[pagoda]

Yes, but one that can be fitted between shaft and gearbox flanges on any engine.

That sounds uncannily like the venerable Laycock overdrive system which I had fitted to an MGB, and a Reliant Scimitar. Switchable under load - solenoid operated -pretty reliable all round. Fits between gearbox & driveshaft.
The ratios would need fixed of course- but the principle is much the same?

Graeme

 

[Cookee]

That sounds uncannily like the venerable Laycock overdrive system which I had fitted to an MGB, and a Reliant Scimitar. Switchable under load - solenoid operated -pretty reliable all round. Fits between gearbox & driveshaft.
The ratios would need fixed of course- but the principle is much the same?

Graeme

I think we must have posted at the same time!

 

[rotrax]

Yes, but one that can be fitted between shaft and gearbox flanges on any engine.

In the seventies when I was rushing about in a van full of race motorbikes trying to make a name for myself the thing to have on your Merc. transporter was a modified MGB overdrive unit fitted between the gearbox and the rear axle. A fine engineer and racer named Roger Moss modified MGB ones in Leicstershire to gear the old vans up a bit for better fuel consumption and cruising speed. It worked very well. A marine version would be worth a try. Good idea.

 

[Latestarter1]

A marine version would be worth a try. Good idea.

Sorry but utterly utterly pointless. Ignores the simple law that propellers move boats not the engines.

ZF went down the two speed gearbox route for planing vessels back in the 90s. Idea was good, as we had performance boats which struggled to get over the hump particularly when certain engines were used.

Using two speed concept you prop normally, ie to match propeller to rated engine power. The low ratio was also helpful in the marina by reducing speed at idle. Nail the throttle in low gear and you were over the hump then you shift into your 'normal' ratio.

For example Yanmar LY had poor turbo match and worked well with two speed box. However advent of electronic engines pretty much killed two speed boxes off as they had torque curves which made a dramatic improvement to out of the hole performance.

Likewise Step Drive was solution looking for a problem, Volvo had issues with their older '40' series engines having a torque curve like a sway back horse mainly due to being short stroke motors.

In a displacement hull turning propeller into an egg whisk will achieve nothing, remember it is always the propeller demanding power from the engine which at any lower rpm than WOT is always less than potential engine power.

Just take a look at the propeller law curve to say the exponent of 3, and you will see why concept is flawed.

 

[oldharry]

Sorry but utterly utterly pointless. Ignores the simple law that propellers move boats not the engines.

ZF went down the two speed gearbox route for planing vessels back in the 90s. Idea was good, as we had performance boats which struggled to get over the hump particularly when certain engines were used.

Using two speed concept you prop normally, ie to match propeller to rated engine power. The low ratio was also helpful in the marina by reducing speed at idle. Nail the throttle in low gear and you were over the hump then you shift into your 'normal' ratio.

For example Yanmar LY had poor turbo match and worked well with two speed box. However advent of electronic engines pretty much killed two speed boxes off as they had torque curves which made a dramatic improvement to out of the hole performance.

Likewise Step Drive was solution looking for a problem, Volvo had issues with their older '40' series engines having a torque curve like a sway back horse mainly due to being short stroke motors.

In a displacement hull turning propeller into an egg whisk will achieve nothing, remember it is always the propeller demanding power from the engine which at any lower rpm than WOT is always less than potential engine power.

Just take a look at the propeller law curve to say the exponent of 3, and you will see why concept is flawed.

Halberg Rassey fit their boats with a two speed box. Not being amongst the priviledged, and having only crewed on them, I dont know the details, but the HR36 with a Volvo engine certainly has a low speed setting for manoeuvring and a different ratio for cruising. It apparently gives significant fuel savings on a long engine thrash, as engine speed is kept low. Again I do not know what prop was fitted, except that it was a folding type on a saildrive. As far as I know this set up is a standard option.

 

[davidej]

Sorry but utterly utterly pointless. Ignores the simple law that propellers move boats not the engines.

ZF went down the two speed gearbox route for planing vessels back in the 90s. Idea was good, as we had performance boats which struggled to get over the hump particularly when certain engines were used.

Using two speed concept you prop normally, ie to match propeller to rated engine power. The low ratio was also helpful in the marina by reducing speed at idle. Nail the throttle in low gear and you were over the hump then you shift into your 'normal' ratio.

For example Yanmar LY had poor turbo match and worked well with two speed box. However advent of electronic engines pretty much killed two speed boxes off as they had torque curves which made a dramatic improvement to out of the hole performance.

Likewise Step Drive was solution looking for a problem, Volvo had issues with their older '40' series engines having a torque curve like a sway back horse mainly due to being short stroke motors.

In a displacement hull turning propeller into an egg whisk will achieve nothing, remember it is always the propeller demanding power from the engine which at any lower rpm than WOT is always less than potential engine power.

Just take a look at the propeller law curve to say the exponent of 3, and you will see why concept is flawed.

Makes sense.

What you want is a variable pitch prop.

Early aviators just had a two pitch prop - low for maximum acceleration for take-off and high for cruise. Now aircraft have constant speed props whose pitch is infinitely variable.

We have the Brunton Autoprop

 

[Tranona]

Halberg Rassey fit their boats with a two speed box. Not being amongst the priviledged, and having only crewed on them, I dont know the details, but the HR36 with a Volvo engine certainly has a low speed setting for manoeuvring and a different ratio for cruising. It apparently gives significant fuel savings on a long engine thrash, as engine speed is kept low. Again I do not know what prop was fitted, except that it was a folding type on a saildrive. As far as I know this set up is a standard option.

Think you will find that is the Gori 3 blade folding prop with "overdrive", set by going into neutral while motoring then going into forward to alter the pitch. Details on the Gori site www.gori-propellers.com Not a 2 speed gearbox. Other manufacturers are devloping the same, or similar, idea.

 

[ianj99]

Sorry but utterly utterly pointless. Ignores the simple law that propellers move boats not the engines.

ZF went down the two speed gearbox route for planing vessels back in the 90s. Idea was good, as we had performance boats which struggled to get over the hump particularly when certain engines were used.

Using two speed concept you prop normally, ie to match propeller to rated engine power. The low ratio was also helpful in the marina by reducing speed at idle. Nail the throttle in low gear and you were over the hump then you shift into your 'normal' ratio.

For example Yanmar LY had poor turbo match and worked well with two speed box. However advent of electronic engines pretty much killed two speed boxes off as they had torque curves which made a dramatic improvement to out of the hole performance.

Likewise Step Drive was solution looking for a problem, Volvo had issues with their older '40' series engines having a torque curve like a sway back horse mainly due to being short stroke motors.

In a displacement hull turning propeller into an egg whisk will achieve nothing, remember it is always the propeller demanding power from the engine which at any lower rpm than WOT is always less than potential engine power.

Just take a look at the propeller law curve to say the exponent of 3, and you will see why concept is flawed.

So propellers move boats - obviously, but I always thought it was the engine that drove the prop....
I'm talking about displacement boats and in relation to cruising speed under ideal and adverse conditions, where the thrust requirements are different to maintain the same speed.
I'm well aware of prop law and this is why without a variable pitch prop, there is a benefit from having 2 speeds.
You size the prop for the high gear for relaxed cruising and then to develop more thrust you need to let the engine revs increase by using a lower gear, which will increase prop revs.
By careful choice of prop size and gear ratios I'm confident the result would be effective.

 

[Latestarter1]

So propellers move boats - obviously, but I always thought it was the engine that drove the prop....
I'm talking about displacement boats and in relation to cruising speed under ideal and adverse conditions, where the thrust requirements are different to maintain the same speed.
I'm well aware of prop law and this is why without a variable pitch prop, there is a benefit from having 2 speeds.
You size the prop for the high gear for relaxed cruising and then to develop more thrust you need to let the engine revs increase by using a lower gear, which will increase prop revs.
By careful choice of prop size and gear ratios I'm confident the result would be effective.

No your assumptions are incorrect. You did not take a peek at engine spec sheet did you?

Think of the governor on a diesel engine as being akin to cruise control on a car. You set the rpm with throttle lever.

If we take say 75 Hp marine diesel engine 75 Hp @ 3,200 rpm with a fixed pitch propeller. Correctly propped vessel should be absorbing say 67 Hp at rated rpm, remember you can never safely have all the power diesel engine has to offer. Take a look at our mythical 75 Hp engine data sheet, we want to cruise at a nice comfortable and economical 2,400 rpm. If you look at the full load curve, engine still has the POTENTIAL to develop 70 Hp according to the full load curve, however if you look at the propeller law curve you will see that with our throttle lever still set at 2,400 rpm the propeller is demanding say 30 Hp........Where has the 40 Hp gone?? Simple because the propeller is not demanding it, the engine governor is not making the power.

On a displacement vessel the only place power demanded by the propeller produced comes close to absorbing all the power engine has to offer is at rated engine rpm. take a look at a propeller law curve and see how much power engine has in reserve. If you add an overdrive gear all you have is the potential to overload, and therefore wreck your mythical engine, for no purpose whatsoever.

 

[lw395]

No your assumptions are incorrect. You did not take a peek at engine spec sheet did you?

Think of the governor on a diesel engine as being akin to cruise control on a car. You set the rpm with throttle lever.

If we take say 75 Hp marine diesel engine 75 Hp @ 3,200 rpm with a fixed pitch propeller. Correctly propped vessel should be absorbing say 67 Hp at rated rpm, remember you can never safely have all the power diesel engine has to offer. Take a look at our mythical 75 Hp engine data sheet, we want to cruise at a nice comfortable and economical 2,400 rpm. If you look at the full load curve, engine still has the POTENTIAL to develop 70 Hp according to the full load curve, however if you look at the propeller law curve you will see that with our throttle lever still set at 2,400 rpm the propeller is demanding say 30 Hp........Where has the 40 Hp gone?? Simple because the propeller is not demanding it, the engine governor is not making the power.

On a displacement vessel the only place power demanded by the propeller produced comes close to absorbing all the power engine has to offer is at rated engine rpm. take a look at a propeller law curve and see how much power engine has in reserve. If you add an overdrive gear all you have is the potential to overload, and therefore wreck your mythical engine, for no purpose whatsoever.

That sounds like a lot of simplistic assumptions to me.
If you are driving a boat into waves, you will need a lot more power than a flat sea, so the prop must normally be pitched for some compromise.
If you wish the boat to accelerate it will need more thrust than to hold a steady speed.
If you want to tow something, or drive the boat off the mud, you need more thrust and less speed.
Many cruising yachts now are quite overpropped, which gives good fuel economy and a quieter life when the wind dies, but lacks thrust when push comes to shove. Also modern engines often don't have flat torque curves, it is often possible that in adverse conditions the engine won't get enough revs to produce full torque.
A two speed gearbox could help in many cases. A variable pitch prop may be more elegant though.

 

[oldharry]

May be right about the VP prop on the HR 36 - as I said, I am not one of the chosen few....

However, I still remain firmly in the two speed box camp unless you acheiuve the same result by fancy propping. Power output is function of engine speed. The effect is very pronounced on my 24 footer, with its 12hp engine: in smooth water, power and to spare, no problem acheiving waterline speed, with a shaft speed of 1500rpm. However in bad conditions, rough sea, and strong wind slowing the boat, the engine can not acheive full speed, dropping to around 1100 rpm, at which point the engine is only developing 8 -9hp. So just when you need the full power you cant get it as you cant change gear. Unless of course you go for a fancy VP prop which can be flattened enough to allow the engine to keep up full revs and therefore full power.

 

[ianj99]

No your assumptions are incorrect. You did not take a peek at engine spec sheet did you?

Think of the governor on a diesel engine as being akin to cruise control on a car. You set the rpm with throttle lever.

If we take say 75 Hp marine diesel engine 75 Hp @ 3,200 rpm with a fixed pitch propeller. Correctly propped vessel should be absorbing say 67 Hp at rated rpm, remember you can never safely have all the power diesel engine has to offer. Take a look at our mythical 75 Hp engine data sheet, we want to cruise at a nice comfortable and economical 2,400 rpm. If you look at the full load curve, engine still has the POTENTIAL to develop 70 Hp according to the full load curve, however if you look at the propeller law curve you will see that with our throttle lever still set at 2,400 rpm the propeller is demanding say 30 Hp........Where has the 40 Hp gone?? Simple because the propeller is not demanding it, the engine governor is not making the power.

On a displacement vessel the only place power demanded by the propeller produced comes close to absorbing all the power engine has to offer is at rated engine rpm. take a look at a propeller law curve and see how much power engine has in reserve. If you add an overdrive gear all you have is the potential to overload, and therefore wreck your mythical engine, for no purpose whatsoever.

Nope, you are still wrong as far as my own boat is concerned.
I have the perkins 4108 power curve and prop nomograph as well as Castlemarine's prop calc program for checking the results.

My boat needs about 22shp for cruising speed of 6kts at 1800rpm and as hull speed is about 7kts and approx 30shp at 2200rpm, this leaves about 10bhp & 1000rpm 'unused' and with the current prop size and pitch, in accessible since the engine is limited to 2200rpm by the prop.

Suppose I needed some extra thrust, the only way to get it without altering prop pitch is to use the same technique every road vehicle has used since they were invented. Namely torque multiplication produced by introducing a lower gear in the drive train. Engine rpm goes up and so does torque at the wheels goes up.

The prop law isn't strictly relevant, just as the weight of a vehicle and gradient of the hill isn't.

If you want more torque at the shaft, multiply engine torque with a lower gear. More torque = more thrust albeit with more engine rpm. In my case, as stated there is a good 1000rpm to 'play with'.

So there is a case for a second lower gear when more thrust is needed.
QED

 

[Latestarter1]

Nope, you are still wrong as far as my own boat is concerned.
I have the perkins 4108 power curve and prop nomograph as well as Castlemarine's prop calc program for checking the results.

My boat needs about 22shp for cruising speed of 6kts at 1800rpm and as hull speed is about 7kts and approx 30shp at 2200rpm, this leaves about 10bhp & 1000rpm 'unused' and with the current prop size and pitch, in accessible since the engine is limited to 2200rpm by the prop.

Suppose I needed some extra thrust, the only way to get it without altering prop pitch is to use the same technique every road vehicle has used since they were invented. Namely torque multiplication produced by introducing a lower gear in the drive train. Engine rpm goes up and so does torque at the wheels goes up.

The prop law isn't strictly relevant, just as the weight of a vehicle and gradient of the hill isn't.

If you want more torque at the shaft, multiply engine torque with a lower gear. More torque = more thrust albeit with more engine rpm. In my case, as stated there is a good 1000rpm to 'play with'.

So there is a case for a second lower gear when more thrust is needed.
QED

I completed my studies in Naval architechure in 1971, perhaps you have discovered that the way that propellers function has somehow changed over the years.

As to the comment 'prop law isn't strictly relevant' statement is simply nuts. This is ALL about propeller demand curve.

Having dug out my old Perkins stuff I realise why you have become all screwed up with your bright idea.

Perkins 4.108 has three ratings.

Continious 27 kW (36Hp) @ 3,000 rpm

Commercial 34 kW (45Hp) @ 3,600 rpm

Pleasure 36 kW (48Hp) @ 4,000 rpm

As to your estimates of requiring 22Hp my SWAG would be a displacement of around 9 tonnes.

You constantly refer to thrust, on the face of it you should have plenty. However your comment 'limited to 2200rpm by the prop' is the clue to your problem. Whoever came up with the plan to over-prop your vessel to limit WOT to 2,200 must have been on drugs. Why were they nuts, because the culprit was paying no heed to engine peak torque. Peak torque on the 4.108 is 83 lbs/ft @ 2,200 rpm thefore your engine is always responding propeller to propeller demand below peak torque, or put another way, with your set up you always have negative torque rise.

Vessel needs to be propped to achieve nearer 3,000 rpm in order for you to cruise a tad over 2,200 rpm. As the propeller is forced to work harder and demand power from your engine the torque rise has the effect of propping up your power curve. You are not allowing your engine to do what it was designed to do.

Suggest that you take time to digest my comments and to propely understand how propellers work purchase a copy of Propeller Handbook: The Complete Reference for Choosing, Installing and Understanding Boat Propellers By Dave Gerr SBN-10: 0071381767.

Cheaper than joining the Flat Earth Society.