Why don't boats have gears?

Discussion in 'DIY Marinizing' started by jakefrith, Oct 28, 2008.

  1. jakefrith
    Joined: Oct 2008
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    jakefrith New Member

    I was thinking about using a CBR 600 motorcycle engine in a boat. This led me to look at the torque which is less than an outboard motor of similar power (100hp). Although I don't have figures to hand I seem to recall the CBR was 40 odd ft/ lbs @ 10,500 rpm ish and an outboard was more like 80-90 ft/lbs @ 6000 rpm ish.

    However, the bike has the benefit of 6 gears and a clutch, which allow (in the lower load context of a motorcycle) furious acelleration from back wheel not moving at all to back wheel moving at 150 mph. It also would take it about an hour to use up a full tank of fuel (14 litres) flat out at 150mph if you could find a banked track to do it on.

    Now, my last 2 stroke outboard engined 100hp boat flat out used a litre of fuel every 2 minutes or so. The above flat out motorcycle scenario equates to twice the fuel economy, a litre used about every 4 minutes. They are both at full throttle and full load. I am guessing the reason for this is that the marine engine is considerably larger than 599 ccs, (and that's also how it gets its extra torque).

    To what extent would 6 gears (which can very rapidly and clutchlessly be changed up due to a sequential gearbox) mean that a motorcycle engined boat make up for its lack of torquey flexibility?

    Would the motorcycle engine be more fuel efficient at different speed ranges, due to the ability of being able to keep the engine spinning at its most efficient rev range whatever speed the boat is going?

    Have boats not got gears because the medium of water has traditionally allowed designers to get away without having them (you can still get going from standstill due to prop slipping, but prop of planing boat is designed to work most efficiently at point where engine is producing max power, so until it's got to max speed it's inefficient) Cars and motorcycles have always needed gears to pull away from standstill as they are pushing against an imovable object. Or could it be about cost of manufacture, ease of servicing, reliability in the marine environment when a boat engine will work after a fashion elbeit less economically with only one gear?

    In short - Why don't boats have gears?
    ...And would a 100hp cbr 600 engine and 6 speed gearbox and sterndrive power a small 2 person 14 foot speedboat to a similar extent that a (more torqey but only one gear) 100 hp outboard motor would? (the two systems would be similar weight)
    If it could power it as well, surely the fuel economy would be so much better?
    My fag packet calculations are for flat out, but imagine the joy of redlining it in 3rd or 4th to get on the plane, then being able to shunt it down into 6th for economical lower revs cruising, still upon the plane. Surely the savings would be even greater in this type of scenario? (Cbr in bike form nipping around town and motorway faster than any car, but not completely murdering motor returns circa 60mpg)
    Jake
     
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  2. Ike
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    Ike Senior Member

    boats do have gears. It's called a reduction gear. A prop does not turn at anywhere near the speed of the engine. If it did it would cavitate badly. It is simply not efficient to have gears like in a car.
     
  3. jakefrith
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    jakefrith New Member

    Thanks for your reply. I know boats have a reduction gear. My question is why they don't have more of them like cars and motorcycles. A range of gears and the ability to select between them on the move so that a smaller engine can be used than would otherwise be necessary on a conventional marine instalation with only one forward gear.
     
  4. Fanie
    Joined: Oct 2007
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    Fanie Fanie

    I think a gearbox where you can actually change to a different ratio to the prop could work very well. I have a 115 on the boat and there are many times I wished I had a faster prop on it.

    To get on the plane you can use a low ratio where you have lots of power, then once on the plane, just get the prop to rotate faster and the revs could drop some.

    Should work even better on a downhill :D

    No but seriously, I think it could work quite well. You won't need a six speed box though. Just reverse, neutral and forward :D

    OK OK, Serious seriously, good idea. I think 3 ratio's forward would be very nice.
     
  5. StianM
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    StianM Senior Member

    I think the formula1 offshore boats have gears.
     
  6. Lt. Holden
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    Lt. Holden Senior Member

    Jake,
    I have always wondered the same thing! Take a lightweight higher revving motor like your Honda and use the transmission to control the motor rpm's while maintaining a relatively constant (for a given boat speed) prop rpm? What about in a jet boat application; where there is even less torque required?

    Once you have the boat out of the hole and on plane, isn't the actual torque requirement less than that required to get on plane? So theoretically couldn't you go to an overdrive type gear to reduce motor rpm's and still maintain the impeller rpm to stay on plane?

    What about with a lightweight higher revving diesel like a Kubota with a torque-multiplying transmission? It seems there would be some real benefits possible?
     
  7. Fanie
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    Fanie Fanie

    60 % ratio, you have 40 % more torque for slow speeds like towing or using lots of power to get on a slow plane. Use when all twenty family members are on the boat.

    1 : 1 for like your boats ratio is currently, nice middle of the road, use for conditions less favourable for high speed.

    135% ratio, 35% more speed at the same revs. Conditions are favourable for speeding, the chicks are watching so let rip.
     
  8. daiquiri
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    daiquiri Engineering and Design

    Boats can have transmissions with gears, usualy with 2 ratios, though some very high performance racers have more than 2. We are talking about boats like some Cigarette models, or the offsore racers which use 4 speed gears. You can find more here, for example:
    http://www.weismann.net/offshore.html (4-speed gears)
    http://boostpower.com (3-speed gears)

    Then there's a Volvo Penta's Stepdrive (2 speeds) for pleasure boats:
    http://www.stepdrive.co.uk/

    It is not a widespread solution because of drawbacks like costs, complexity, increased probability of failures, weight etc. but technically it can and has been done.
     
  9. Stumble
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    Stumble Senior Member

    I am thinking that there are a few reasons why this wouldn't work as well on a boat as on a car.

    1) the amount of torque required to keep a boat up and moving would prevent shifting to a much higher gear. Maybe you could drop a few hundred RPM from max, but not the 5k like in a car.

    2) Props start to cavitate beyond a certain speed, so to use a higher gear to increase top end speed would require a variable pitch prop that could be changed while running.

    3) The modest possible gains in performance would likely be offset by the cost and weight of the transmission that would have to be lugged around all the time.

    At least these are the problems, but I wouldn't argue with someone if they can show me where I am missing someone.
     
  10. daiquiri
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    daiquiri Engineering and Design

    One important consideration: a boat is not a car. :D
    That phrase is not mine - it's been said by Antonio Soccol, a renowned italian boat designer and technical journalist. I like that phrase because it is so true and still too often ignored.

    Cars operate in a highly variable range of situations. A car needs to continually change it's working regime due to it's surrounding environment - traffic lights, queues, crossings, speed limits, accellerations and decellerations, high speed at highways, low speed at city roads, climbing up the hills, descending down again, towing the trailer, etc. etc. It is a continuos change in RPMS, power and torque requirements, which would be impossible to obtain (and would soon destroy the engine) wihout a multi-gear transmission drives.

    So first you have to ask yourself how do you intend to use your boat. If you will operate your boat in a way similar to the car in the example above (apart climbing up the hills :D ), then you might take into consideration a transmission with more than 1 gear ratio.

    A 90% (or more) of boats are made to travel most of the time at a certain optimal speed (optimal in a sense of fuel consumption), will from time to time accellerate shortly to it's max speed and will troll slowly or go reverse when maneuvering in a port. All things which can be done pretty well without a complication of a multi-gear transmission.

    Every transmission is a source of energy losses, so the less you install the less power you will steal from the prop. Commercial ships, bulk carriers and tankers, for example, which are designed to deliver their load with minor fuel consumption possible, have their props mounted directly to their slow engine's shaft - 1:1 gear ratio and very high efficiency.

    A 2-speed drive can be useful for a planing boat during the transition from displacement to planing mode if you have props optimized for high speed, but its benefits have to be evaluated against the increase in weight and the need for an additional space in the engine room, besides the mechanical complexity.

    By the way, remember that things you don't install will surely not break. ;)

    High performance racing boats need multi-gear transmitions because they have a combination of super-cavitating props optimized for very high speeds (which perform poorly during the low speed, low rpm accelleration) and engines which deliver the power at high rpms. Since they need to maximize accellerations too (if they want to win the race :) ), the use of a multi-speed gearbox is obviously justified in their case.
     
  11. Mykul
    Joined: Jul 2007
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    Mykul junior member

    I have seen a few Nissan car commercials on TV demonstrating their variable ratio gear box. It sounds to me like a similar set up to what a snow mobile has. Wouldn’t that be more beneficial than a regular set of automotive gears?
     
  12. StianM
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    StianM Senior Member

    If I'm not mistaken the CVT is 5% less energy efficient than a normal gearbox.
     
  13. hmattos
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    hmattos Senior Member

    If you look at the Volvo Penta sterndrives made for larger boats in Europe, there is an option of having a two speed gearbox.
    The main advantages are fuel efficiency and slow speed handling.
    Without the 2 speed the creep speed is too high for docking and the engine revs at cruise are too high for economy.

    We - explorermarine.co.uk - make high speed RIBs with outboards, so I have never used one myself.
    HM
     
  14. Guest625101138

    Guest625101138 Previous Member

    Your question on the gears is a good one and there have been some fair answers so far. Fundamentally the water does not have traffic lights or big hills. There can be some benefit with gears to optimise the engine operation in adverse conditions such as strong winds and heavy seas but typically props can be well matched to the engine. Most boat owners have experimented with props to find the best compromise. If they had gears they would not have to fiddle with props so much.

    I am doubtful about the motocycle and outboard comparison on fuel economy though. You need to take examples from current designs and style of engine. Also do not forget you some how have to cool the motorcycle engine. It will not be in a nice breeze as set up on bike - cooling pumps or fan will take power. The claimed fuel burn for the bike of roughly 160g/kWh is better than most diesels give.

    I wonder whether your bike at 150mph really needs 100HP. Cyclists are now getting over 80mph and they generate about 1.2HP. Power goes up with cube of velocity so a well faired cycle could do 150mph with around 10HP. So what is happening with the other 90HP purportedly available from your engine.

    You need to ensure you are comparing horses with horses and do not believe all the sales literature. Without looking at it in a bit more detail you could be very disappointed if you lined up your cycle engine powered boat against a 100HP outboard even if you had the slight advantage of gears.

    Rick W.
     

  15. Mykul
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    Mykul junior member

    But to compare terrestrial vehicles to marine; wouldn't a marine gear box be similar to a differential, rather than a transmission. In which case the prop itself would be the transmission. An adjustable pitch prop would be a better solution than more gears, or am I wrong on that.
     
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