Why don't boats have gears like cars?

[Latestarter1]

Yes, one gear is enough to cover the needed speed range, but having more gears would save fuel at lower speed.
The prop is usually designed so that the engine can just reach the max power RPM. At this operating point you will utilize the rated maximum power of the engine. If the max speed is for example 8 knots, and you slow down to 6 kn, then the required torque to push the hull through the water will be much less. As a result the engine will output a torque which is much below the maximum available torque at that RPM, which means non-optimum fuel economy. (= consumed diesel/produced mechanical work) Having the possibility to switch to a bigger gear would reduce the RPM further and thus increase the torque taken from the engine to be closer to the max torque at that RPM. (the prop torque would remain the same)
Please google “Brake Specific Fuel Consumption Maps” to see how the fuel economy depends on the RPM and the torque.

Simply not true, somebody else who has their automotive head on and does not understand the basics of marine propulsion and the fact that propellers move boats, engines merely respond to propeller demand. OP was brave enough to ask the question, which has exposed others who wrongly think they understand.

Bsfc is a characteristic of an engines full load curve. In a boat a diesel engine responds to the propeller demand curve NOT the full load curve.

Let me provide an example: Perkins M135C hax maximum rated power of 125Hp @ 2,600 rpm, correctly propped 100% of power absorbed at rated speed.

Nobody rushes about pedal to the metal, nice cruising speed for this engine is 2,000 rpm. Potential power at this rpm on full load curve is 108 Hp. However if we assume displacement vessel with say 2.8 exponent propeller will be demanding 60 Hp, cruise a little slower say 1,600 rpm, potential power is 93 Hp but propeller is demanding 30 Hp. Where is the missing horsepower? The engine is not producing it, throttle is setting RPM, governor merely adjusts fueling to hold desired rpm. Fuel burn relates to power demanded not potential power.

Diesel engine consumption is always on the propeller demand cure for a given exponent and never on the full load curve.

 

[Woodlouse]

My boat has gears, one forwards and one reverse, there's even a reduction in the gear box too. The reasons for not having lots of forward and reverse gears have been explained but simply you'd need a variable pitch prop in order to make the most of the different speeds it would spin at in different gears. a variable pitch prop on its own will give you almost exactly the same advantage given the small range of speeds the average boat travels at.

 

[KINGFISHER 9]

Some of them do ... snag is they tend to be extremely fast and very very expensive .... also with Arneson drives I think.

 

[grumpygit]

Simply not true, somebody else who has their automotive head on and does not understand the basics of marine propulsion and the fact that propellers move boats, engines merely respond to propeller demand. OP was brave enough to ask the question, which has exposed others who wrongly think they understand.

Bsfc is a characteristic of an engines full load curve. In a boat a diesel engine responds to the propeller demand curve NOT the full load curve.

Let me provide an example: Perkins M135C hax maximum rated power of 125Hp @ 2,600 rpm, correctly propped 100% of power absorbed at rated speed.

Nobody rushes about pedal to the metal, nice cruising speed for this engine is 2,000 rpm. Potential power at this rpm on full load curve is 108 Hp. However if we assume displacement vessel with say 2.8 exponent propeller will be demanding 60 Hp, cruise a little slower say 1,600 rpm, potential power is 93 Hp but propeller is demanding 30 Hp. Where is the missing horsepower? The engine is not producing it, throttle is setting RPM, governor merely adjusts fueling to hold desired rpm. Fuel burn relates to power demanded not potential power.

Diesel engine consumption is always on the propeller demand cure for a given exponent and never on the full load curve.

I agree with what you are suggesting to a point, Engine specs are well documented but the engine manufacturer does know know what the unit is going in to and the technology is therefore choosing the correct prop as in size, pitch, no of blades for chosen engine, gearbox ratio and the boat design.
I was just wondering where you got your data from for the Perkins M135 from i.e. web page or experience or other? This is the engine in my yacht and I would be absolutely gutted if l had to run the engine at 2000rpm to get a "nice cruising speed", these revs moves my circa 25t of boat through the water and get me just about hull cruising speed of around 8.8 kts in flattish conditions.

 

[Nostrodamus]

I have to say I have found this thread fascinating and with some excellent replies.

 

[Heckler]

bb

Look, I know this is probably a stupid question and there is a easy answer but I wondered why boats don't have gears like cars?
Would gears on boats make a difference in different seas, ie choppy, following seas or into the sea?

Just trying to satisfy by curiosity?

Number one the speed range is small, from 0 to 8kts? Plus we have the equivalent of an automatic gearbox! The prop is like the fluid flywheel on an auto car. It isnt a direct drive to the medium.
S

 

[Spi D]

Marketed as Torqueshift or Power2.

Look see:

 

[Capt. Clueless]

When I were a young'n, I always fancied building a tiny speed boat with single seat, dome cover and driven behind the seat by a 650 Norton engine & gearbox. I pictured myself virtually flying over the waves as I went through the gears.....I never built it.

 

[grumpygit]

Look, I know this is probably a stupid question and there is a easy answer but I wondered why boats don't have gears like cars?
Would gears on boats make a difference in different seas, ie choppy, following seas or into the sea?

Just trying to satisfy by curiosity?

Marine gearboxes are just about bomb proof, a fixed blade propeller constant and all are proved, tested, mostly very reliable and fairly reasonable in cost to repair or replace . . . . the KISS rule does apply here.
If you wish for something different the price would probably through the roof and if you look hard enough there will be someone out there making all sorts of claims so to relieve you of loads of cash. More sailing and less motoring saves money.

 

[syTaika]

Bsfc is a characteristic of an engines full load curve. In a boat a diesel engine responds to the propeller demand curve NOT the full load curve.

Let me provide an example: Perkins M135C hax maximum rated power of 125Hp @ 2,600 rpm, correctly propped 100% of power absorbed at rated speed.

Nobody rushes about pedal to the metal, nice cruising speed for this engine is 2,000 rpm. Potential power at this rpm on full load curve is 108 Hp. However if we assume displacement vessel with say 2.8 exponent propeller will be demanding 60 Hp, cruise a little slower say 1,600 rpm, potential power is 93 Hp but propeller is demanding 30 Hp. Where is the missing horsepower? The engine is not producing it, throttle is setting RPM, governor merely adjusts fueling to hold desired rpm. Fuel burn relates to power demanded not potential power.

Diesel engine consumption is always on the propeller demand cure for a given exponent and never on the full load curve.

Bsfc shows the full load characteristics, but also the fuel efficiency at any operating point (combination of torque and rpm). From the bsfc map for your engine you can see that the fuel consumption will be different if the propeller takes 60 hp at 2,000 rpm, at 1600 rpm or at the lowest rpm having the potential power of 60 hp.

 

[wully1]

Was alongside a motor boat many, many years ago that had a standard truck engine and four speed gearbox complete with truck gear lever.
it was funny to hear it leave harbour as it changed up through the gears...owner said he only really needed fourth gear and it worked OK as long as he didn't use reverse as it was too low geared and was not much use.
I seem to remember hearing of some companies converting road going gear boxes to run the same fourth gear ratios in reverse for marine application?

 

[Latestarter1]

I agree with what you are suggesting to a point, Engine specs are well documented but the engine manufacturer does know know what the unit is going in to and the technology is therefore choosing the correct prop as in size, pitch, no of blades for chosen engine, gearbox ratio and the boat design.
I was just wondering where you got your data from for the Perkins M135 from i.e. web page or experience or other? This is the engine in my yacht and I would be absolutely gutted if l had to run the engine at 2000rpm to get a "nice cruising speed", these revs moves my circa 25t of boat through the water and get me just about hull cruising speed of around 8.8 kts in flattish conditions.

Engine model used to merely to illustrate a point with real verifiable data, Perkins Sabre publication 533/9/94. Manufacturer specifies that motor must reach rated engine with chosen transmission/propeller, which is a good point, no manufacturer would permit a transmission which prevented engine from reaching rated speed in any condition, therefore no installation sign off, no warranty. Actually makes any discussion around gears somewhat irrelevant.

Whilst I was quoting #'s illustrating a point from document I have to hand, however 2,000 rpm was around the cruise rpm of the specific wind farm vessel engine was installed in, however one cannot argue with your personal experience but that is the nature of boats.

 

[Latestarter1]

Bsfc shows the full load characteristics, but also the fuel efficiency at any operating point (combination of torque and rpm). From the bsfc map for your engine you can see that the fuel consumption will be different if the propeller takes 60 hp at 2,000 rpm, at 1600 rpm or at the lowest rpm having the potential power of 60 hp.

Apologies however I am obviously too dumb to follow your logic..........

Going back to the original engine model quoted Perkins Sabre M135C:

2,600 rpm Full load Bsfc 281 g/kWhr 31.2 l/hr Propeller law Bsfc 281 g/kWhr 31.2 L/hr
2,000 rpm Full load Bsfc 255 g/kWhr 25.0 l/hr Propeller law Bsfc 247 g/kWhr 12.5 l/hr
1,600 rpm Full load Bsfc 259 g/kWhr 21.0 l/hr Propeller law Bsfc 279 g/kWh 7.2 l/hr

Assumptions Propeller Law Curve to 2.8 Exponent.
Brake Mean Effective Pressure 719kPa

 

[syTaika]

Apologies however I am obviously too dumb to follow your logic..........

Sorry for not being able to communicate clearly. English is not my mother language. I try again using an example.
Let's assume that my boat has a diesel engine TDI 1,9 from VW. I will reach the full speed at 3750 RPM. The propeller takes and the engine provides 88 hp. (Max rated power from the engine) Then I slow down to a convenient cruising speed and the propeller needs only let's say 40 hp. If I have a gearbox or a variable pitch prop, I can adjust the RPM of the engine while keeping the boat speed, prop RPM and the engine hp constant. This example engine can provide 40 hp starting from 1450 RPM. The fuel consumption at 1450 RPM and 40 hp would be 200 g/kWh. At 2500RPM/40hp it would be 230 g/kWh and at 3100RPM/40hp 250 g/kWh.
Generally speaking for any load level, the fuel efficiency is the highest while the engine runs at the lowest possible RPM providing this power. For diesel the difference is not as big as with gasoline engines, because only the amount of fuel is adjusted and not the amount of air.
The bsfc map I was using as an example can be found on this page:
http://ecomodder.com/wiki/index.php/Brake_Specific_Fuel_Consumption_(BSFC)_Maps

 

[Latestarter1]

Sorry for not being able to communicate clearly. English is not my mother language. I try again using an example.
Let's assume that my boat has a diesel engine TDI 1,9 from VW. I will reach the full speed at 3750 RPM. The propeller takes and the engine provides 88 hp. (Max rated power from the engine) Then I slow down to a convenient cruising speed and the propeller needs only let's say 40 hp. If I have a gearbox or a variable pitch prop, I can adjust the RPM of the engine while keeping the boat speed, prop RPM and the engine hp constant. This example engine can provide 40 hp starting from 1450 RPM. The fuel consumption at 1450 RPM and 40 hp would be 200 g/kWh. At 2500RPM/40hp it would be 230 g/kWh and at 3100RPM/40hp 250 g/kWh.
Generally speaking for any load level, the fuel efficiency is the highest while the engine runs at the lowest possible RPM providing this power. For diesel the difference is not as big as with gasoline engines, because only the amount of fuel is adjusted and not the amount of air.
The bsfc map I was using as an example can be found on this page:
http://ecomodder.com/wiki/index.php/Brake_Specific_Fuel_Consumption_(BSFC)_Maps

Yes I hear what you are saying however your suggestion is totally implausible let me explain why.

When I used to spec engine transmissions for a trucks rules were simple gear the vehicle with say 1,900 rpm engine for cruising on the speed limiter at say 1,500 rpm 56 mph. Best point on a turbocharged diesel engine i.e Bsfc (sweet spot) is from peak torque, between say 1,200 rpm, 1,500 rpm. A good well specced engine will have a bundle of torque rise therefore as the truck climbs a gradient the torque rise acts as a cushion enabling it to hold road speed until a gear shift is required when engine speed drops below peak torque and a downshift is required.

How is the above different from a boat? On a vehicle engine loading is LINEAR , when transmitting power through a propeller the loading is subject the propeller exponent.

In your case how would you know % engine load to enable correct pitch to be applied? If we had electronic engine one could be constantly looking at % engine load say 80% and constantly adjusting pitch maintain that % engine load, once you get to 100% load engine may be 150% overloaded, it can no longer tell you and cylinder temperatures going bonkers. One could drive the boat on a an exhaust pyrometer instead, but what an absolute song and dance. As any high speed diesel engine manufacturers will never approve any transmission which has potential to overload their engine we are arguing semantics and why I failed to understand the original proposition.

The only times I have come across constantly controllable pitch propellers is behind large medium speed 500/600 rpm B&W's of the 1930's running at constant peed or similar.

 

[Nostrodamus]

I have got to say this thread has been fascinating but it is now getting way beyond my comprehension. There are obviously people on here who have fantastic knowledge and arguments but to a lay person or the village idiot like myself I am getting lost.
I understand that the consensus is that gears are not needed because the engine and prop only need to attain plaining speed and you cannot go beyond that.
Would some form of gearing make the boat more economical on fuel though and would it assist in different seas ie a following sea or going into a short chop where boat speed is dramatically reduced.

 

[westernman]

The only times I have come across constantly controllable pitch propellers is behind large medium speed 500/600 rpm B&W's of the 1930's running at constant peed or similar.

These ones are suitable for small boats from about 30HP to 300HP:-

http://www.frydenbosabb.no/bilder/filer/fsm/HVP-GEARS.pdf

 

[charles_reed]

Look, I know this is probably a stupid question and there is a easy answer but I wondered why boats don't have gears like cars?
Would gears on boats make a difference in different seas, ie choppy, following seas or into the sea?

Just trying to satisfy by curiosity?

No Hills?

In fact all marine engines have a gearbox - not only offering forward, neutral and reverse but also a permanent low gear, allowing engine repm to be x2 -x5 the speed of the propshaft. Unfortunately, with the majority of installations of a fixed pich prop, this only works ideally at a certain engine speed and resistance.
Hence the development of variable pitch props (like the Hunstedt), variable-set-pitch props (Kiwi or Variprop) or self-pitching props (Autoprop).
The 2nd part of this might answer some of the questions (and misapprehensions) of posters
https://en.wikipedia.org/wiki/Variable-pitch_propeller

 

[TOKOLOSHI]

Many years ago a guy had a ferro yacht with a lorry engine and it still had the gear box fitted! Quite interesting to see him press the clutch and engage gear with long gear stick! I recall he had removed some of the gears from the box.

 

[Latestarter1]

These ones are suitable for small boats from about 30HP to 300HP:-

http://www.frydenbosabb.no/bilder/filer/fsm/HVP-GEARS.pdf

Yes I am aware of these transmissions, however unlike the premise by poster who wants to increase propeller load at engine sweet spot these transmissions are designed with opposite purpose for vessels with highly variable displacement.

Cummins old distributor in Norway Nogva had trouble with a customer who wanted to spec up fishing vessel to achieve rated engine speed 'light load' and not when returning from fishing grounds heavy displacement. Operator claimed that they would reduce pitch on return voyage however no manufacturer will allow possibility to overload an engine. Simple rule with variable displacement vessels is the same, engine transmission match MUST be allowed to reach rated speed at maximum rated speed.

In reality you can only actually use about 80% of maximum rated engine power and that is not marketing spec sheet 25 degree test fuel advertised power either.