Why don't boats have gears?

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

  1. cbeckjr
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    cbeckjr New Member

    Alternate ideas?

    For months I have been wondering why don't I/Os have at least 2 forward speeds but had given up on the idea due to the complexity of it. But I wanted to ask 2 questions one of which is on the path of a torque multiplier and the other down a different path.

    1. Use a lock-up torque converter (no tranny) between the engine and outdrive with a big pitch prop. Unlocked during acceleration to allow the engine to rev and get the boat on plane, then lock up the converter during cruising to drop the engine rpms down for fuel economy. Sort of a 2 speed tranny without all the gears and parasitic losses they incur.

    2. Turbocharge your engine with a big pitch prop. Turbos really are mid range torque boosters so put on the big pitch prop and have an extra 100 lb/ft of torque to turn the big prop for similar low-end acceleration to that of a small pitch prop on a N/A motor but when you level out you would come off the boost and the motor would turn a much lower rpm.

    Not looking to hijack just for some feedback on those 2 alternatives to the gear idea.
     
  2. whitepointer23

    whitepointer23 Previous Member

    gears work well, been there done that. i have had a boat with a 1400 datsun motor and 5 speed gearbox. another one with a 221 falcon with 3 speed box. one with a ford consul and 4 speed. a friend of mine builds a lot of boats with autos which work well. he fits props to suit 2nd gear because ratio is close to reverse, i have been in these boats heaps of times, they change gears in auto just like a car.
     
  3. CDK
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    CDK retired engineer

    On a typical I/O the engine rests on the drive's inside bracket with the bell housing.
    You would need a torque converter with the proper geometry, I don't think it exists. The prop in the water already acts as a torque converter, manufacturers are not interested in using a cascade because there is no need for it.

    Your second idea implies that it would be advantageous to use the turbo during acceleration only. But that is not the case.
    The drop in rpm is not large enough to return to normal aspiration and there is no need for that because a turbo charged engine is more efficient so why not make use of the investment.

    The only short action acceleration aid I can think of is nitro injection. That you can use if the engine/prop match does not allow normal take off or if there is a high passenger load.
     
  4. Brian@BNE
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    Brian@BNE Senior Member

    Gears for cars are typically necessary to provide enough power at low speed or for high loads such as towing or going up hills.

    For boats the best way to achieve the analogous higher power or towing ability at low speed, and fuel economy at cruise speed, is with a CPP. It does the same job as a multispeed transmission does on a car.
     
  5. Squidly-Diddly
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    Squidly-Diddly Senior Member

    I bet that 600 CBR motor would blow up soon at 100hp.

    They just aren't meant for hours at 90% of full power and neither are car motors.

    Boat and airplane motors would be and need a whole other level of reliability for safety.

    I've known of ski-boats with Ford 351 and 3sp AT(C-4 or C-6 I can't remember) with Vee-drive(motor bass-ackwards). I do remember the owner mentioning it had some marine 'hole shot' shift kit and the thing was setup for pulling skiiers without regard to much else.

    Seemed much like a nice powerful smooth American v-8/at car.
     
  6. whitepointer23

    whitepointer23 Previous Member

    show me a cpp for a planing hull under 30 ft and how much it cost. because i can't find any .
     
  7. Aharon
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    Aharon Junior Member

    very good, wholehearted laugh, 10x! That was the "1-2 humour punches"
    ata boy!
     
  8. cbeckjr
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    cbeckjr New Member

    I think i follow on your converter answer but i was thinkin a small pitch prop allows for a lot of slip out of the hole to let your motor rev more freely but then you have to rev 1000 rpm higher than you want to maintain speed. A big pitch prop turns lower rpm at speed but is a dog out of the hole. A device that would allow even more slip out of the hole but then tighten up to drop your rpms at speed seems ideal to me. 500-1000 rpm over what a big prop would let the motor rev out of the hole and then lock it up when on plane to sustain speed at a low rpm. I know the ratio is the same once the motor gets to that delta so it's not a 2 stage multiplier like a 2 speed tranny would be but it would put the motor in a higher powerband to take advantage of more horsepower. just thinkin' out loud. Props are always a trade-off, just wondered if a TC would equalize things a bit. Of course an affordable CPP would probably be better. More slip out of the hole then more bite at speed.

    My Datsun with 305TPI and 3.23 gear turns low rpm on the highway but the stock torque converter (~1200rpm launch) was a dog out of the hole at the track. A 2500 stall converter (~2000rpm actual) would give much better 60ft times. Then I'd lock it up on the highway and still get great gas mileage.

    On your nitro injection did you mean nitro methane or nitrous? It made me think of what I told my buddy a few weeks ago. I thought about building a motor with TPI induction. The long runners would have good torque at 2k rpm to maintain speed with a big pitch prop. And then just put on a 75 shot of nitrous to get the skiers up when needed. :D
     
  9. DCockey
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    DCockey Senior Member

    Boats have PROPELLERS.

    Cars have TIRES.

    That is a fundamental difference.

    There is a direct, simple, constant ratio between the speed of the car and the rotational speed of its tires. This ratio is independent of the amount of torque being applied to the tires. (1) If the car isn't moving the tires are not rotating. In order to change the relationship between the speed of the car and the rotational speed of the engine a transmission/gearbox with multiple or variable ratios is needed.

    The relationship between the speed of a boat and the rotational speed of the propeller(s) is NOT constant. Rather for a given propeller it depends on:
    i) the torque applied to the propellor, which in turn depends on the engine characteristics, reduction gear ratio (if any) and throttle position.
    ii) the boat speed vs resistance characteristics
    It usually works out that a fixed propeller's speed vs load characteristics are a reasonably good match to an internal combustion engine's characteristics.

    .................
    (1) To be precise the ratio of the rotational speed of a tire to the speed of the vehicle changes slightly with the torque applied to the tire. This is a result of deformation of the tire though it is commonly refered to as "slip". Also, the effective tire diameter can increase very slightly with increasing speed due to centrifugal effects. These changes of the ratio for conventional road vehicles are on the order of a few percent or less.
     
  10. cbeckjr
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    cbeckjr New Member

    "Boats have PROPELLERS.

    Cars have TIRES."

    Well yea of course i get that. I'm not an engineer but not that stupid either. No need to yell lol. I'm askin' if allowing a motor to rev 1000 rpm higher than it normally would w/ a big pitch prop help 0-20 acceleration. Which you still haven't answered. While a prop is not constant like a tire, there is still a relationship between between the 2 isn't there? 19 pitch/200 ft/lbs vs 23 pitch/200 ft/lbs. Isn't that why you mix and match props on a boat according to desired outcome (top speed or holeshot)

    "The relationship between the speed of a boat and the rotational speed of the propeller(s) is NOT constant" True, but 2300 rpm on a 23 pitch is higher mph than 2300 rpm on a 19 pitch. While not constant like a tire, there is a difference. And it's the same difference on Tuesday as it was last Thursday. That's what I mean by constant (maybe bad choice of words). It's not some random ever changing outcome. It is somewhat fixed and repeatable.

    For the same weight, same pitch prop, same water density, blah blah blah, the acceleration is absolutely repeatable (maybe a better word than constant). But I'm asking if you were able to raise the engine's rpm with all else fixed (torque, pitch, etc) , would you see better 0-20 on a given prop or would you still need multiple ratios. Seemed like a yes or no kinda thing to me. :)

    Funny thing is I'm not the only one who compares prop pitch to gear ratios. Pulled this from a boating magazine article.

    "Think of a propeller as you would a car's axle ratio. The lower the ratio, the more pulling power from a standstill. The same is true with a prop. The lower the pitch, the better your hole-shot. However, this comes at a price: top speed.The lower pitch makes the engine reach maximum rpm at slower speeds.

    Conversely, a higher pitch will deliver greater top speeds, but slower acceleration. Be aware that lower-horsepower engines can bog down if fitted with a propeller with too high a pitch and diameter, and that can wear heavily on internal engine parts."

    Should I tell them they are nothing alike, or do you want to break it to him......
     
  11. DCockey
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    DCockey Senior Member

    cbeckjr, my posting above was not a response to you, even though it happened to follow you posting by 9 minutes. I started writing it before your post appeared.
     
  12. DCockey
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    DCockey Senior Member

    When I used "constant" in my note above, it was in the sense that in a car with a given tire size the ratio of tire rotational speed to road speed is the same at all speeds whether the car is accelerating, driving at constant speed, or decelerating. It's the same irregardless of the weight of the car, and whether the car is on level ground, going up a hill, down a hill, into a headwind or being helped along by a tailwind.

    In a given boat with a given propeller the ratio of propeller rotational speed to water speed varies depending on:
    - the speed of the boat throught the water
    - displacement and trim
    - accelerating, decelerating or constant speed
    - ambient wind
    - waves (if significant)

    Changing the diameter and pitch of a propeller on a boat will change how fast the boat accelerates and the top speed of a boat. In that sense it is like changing the gear ratio of a car. As others have noted there can be advantages to installing a multible/variable ratio reduction gear/transmission converter in a boat. But the magnitude of the advantages are considerably different than in a car.

    A car with a single ratio between engine speed and tire speed (no gear box/transmission or torque converter) would be very unpleasant to drive and either be very slow to accelerate and have limited ability to climb hills or have a low top speed.

    In contrast most boats with a single ratio do quite well.
     
  13. Brian@BNE
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    Brian@BNE Senior Member

    Michael Kasten's article is a reasonable starting point for cost comparison and suppliers although its a couple of years old. http://www.kastenmarine.com/CPprops.htm

    Richard's thread had some more suppliers, as well as some comments on cost also. http://www.boatdesign.net/forums/archive/t-30695.html If you are changing from petrol twins to single diesel then you'll need new shaft installation anyway, so don't just compare prop prices alone.
     
  14. whitepointer23

    whitepointer23 Previous Member

    thanks for the links. interesting reading.
     

  15. nukisen
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    nukisen Senior Member

    So then I suggest we build a boat with a variator. Then you dont have to chose the gear as the torque handle this by itself. Then you have always have a transmition from the engine with perfekt balance all the way from displaced speed to full planing speed. And when top speed is reached we have a boat running in perfekt torque regarding highest effektive motor rounds.
     
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