Why don't boats have gears like cars?

[Tranona]

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.

You can have exactly what you want - see post #17 - without the complication of a gearbox. Buy either a Bruntons Autoprop which essentially has infinitely variable "automatic gears" or a Gori "overdrive" prop that has effectively two gears.

 

[Lon nan Gruagach]

I think its because, on a boat, you are firstly not too bothered about 0- to 60 in 3 seconds and that if you select a gear that is too high, you dont stall the engine. Combine that with the prop having an idel speed which is matched to the engine ideal rpm (the engine burning more or less fuel as required) and that cruising speed is reasonably constant, the extra breakable bits of a multi speed box just isnt worth the cost or hassle.
Where sail is concerned, if you have enough wind to significantly affect boat speed then you probably wont be using the engine.

 

[wongaero]

I'd be concerned the load would always be a bit too high to justify going up a gear - like the experience of going on the York-Leeds trains (which I assume are diesel-electric) where the engine constantly sounds like it's nearly ready for second gear but never quite gets there. So you sit there thinking 'oo, nearly, here it comes, ohhh, no we're slowing down again'

 

[RAI]

Some inland waterways craft gearboxes have trolling valve that allows the propeller to be turned very slowly and avoids the need to go in and out of forward gear when manoeuvring in locks and such. Conversely, a buddy with a speedy boat has the problem that tick-over on one engine alone exceeds the canal speed limit.

 

[gordmac]

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

It seems reasonable that an engine would have slightly more losses at 3100 RPM than 1450 RPM but to use 25% more fuel seems a lot more losses. Maybe no wonder VW had to fiddle their emissions!

 

[ex-Gladys]

I'd be concerned the load would always be a bit too high to justify going up a gear - like the experience of going on the York-Leeds trains (which I assume are diesel-electric) where the engine constantly sounds like it's nearly ready for second gear but never quite gets there. So you sit there thinking 'oo, nearly, here it comes, ohhh, no we're slowing down again'

Diesel electric trains have a couple of approaches; the older dc generator technology uses stages of field diversion, reducing current through the field coils, and therefore the back EMF to allow the traction motors to rotate faster

More modern alternator based stock like the 125 uses a continuous control system which I don't understand....

 

[MGresty]

This is a very sensible question despite the scathing remarks above. I have a Bavaria sailboat with static revs about 1k and when the forward gear is JUST engaged, without opening the throttle, the boat speed is too fast to 'creep' when negotiating a complex harbor. I have to keep disengaging gear and coasting. A creep gear would be really useful.

 

[Tranona]

This is a very sensible question despite the scathing remarks above. I have a Bavaria sailboat with static revs about 1k and when the forward gear is JUST engaged, without opening the throttle, the boat speed is too fast to 'creep' when negotiating a complex harbor. I have to keep disengaging gear and coasting. A creep gear would be really useful.

The tickover is set too high. It should be 750-800 to avoid the problem you describe. If it has the standard prop size the boat should barely move when you engage gear and only move when you open the control.

 

[ex-Gladys]

Engaging and disengaging is quite normal if you have a Brunton autoprop, which effectively "changes up". Quite easy with a clean boat to see 4 kts.

 

[[70521]]

Actually, yes. My Audi manual says that you should have changed from 1st to 2nd by the time you have done a car's length.

Goodness! What speed are you doing in that time?

 

[Skylark]

On a vehicle engine loading is LINEAR

Not sure that I understand this, can you please explain?

Road load on a vehicle is dominated by pushing through air, or so I thought? It's been many years since I've seen the curve but I thought that the drag based upon frontal area x coefficient (hence the old 1980s phrase that the Volvo has a drag coefficient of a house brick) approaches exponential. Numbers that stick in my mind are that a car needs something like 25 bhp to do 70 mph and 100 bhp to do 100 mph. Far from linear.

 

[GHA]

Would gears make them more efficient and use less fuel depending on sea state?

The gori which tranona mentioned does just this, I've done a couple of deliveries on boats with them fitted. Flat oily calm put in overdrive and motor for ages at lower revs using less diesel. Being able to adjust the pitch of the kiwi prop sounded interesting as well, set it a bit higher for ocean passages, as you'll only be motoring in flat calm and keen on saving diesel.

 

[westernman]

Road load on a vehicle is dominated by pushing through air, or so I thought?

Not for a fully loaded articulated lorry going up a 1 in 5 incline.

 

[MGresty]

Many many thanks for your criticism of my posting which described my Bavaria leaping forwards when just engaging gear. I just bought the boat and had the local chantier revise the motor. He set it above 1000 which I presumed was OK never having entertained the idea of a diesel before! After your message I resorted to the Volvo handbook; found that tickover should be 800 and adjusted the motor myself. I was very pleased to find that the boat now creeps so thanks again. However, such a low tickover renders the actual point of engaging the gear insensitive so that it is difficult to know if one is 'in' or not.

Morale of story: check everything on work done for you.

Nevertheless you are perfectly correct. Let me know if you are ever passing by La Camargue and would like a little navigation.