what will the differantial do whilst charging down the river?

Discussion in 'DIY Marinizing' started by Volksliner, Jan 8, 2005.

  1. Volksliner
    Joined: Jan 2005
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    Location: Ghent, Belgium

    Volksliner Junior Member

    Hello Fellow Buckaneers,

    I've come to beg for help.
    I'm planning to build a small powerboat from scratch and need to regularly compare my thoughts and ramblings to real world fysics and expertise.
    The thing I'd like to build is slightly based on the mighty v8 powered, flatbottomed, v-driven hotboat. You know, some 18ft long, only protruding a few inches obove the waterline and burdened with horsepower.
    Since I live in Belgium, Europe where we tend to think smaller, and since I'm also an aircooled vw nut, I'd like to shrink those headboats a little bit and install an aircooled flat four into it.

    Since I kind of know what the boat should look like, I'd like to start of with the engine and drive, and design the hull around this later on.

    I know some, if not a lot, of you are opposed against installing a car engine into a boat, and sometimes I can even understand why. So I'd like to point out that this boat would only be used for fun on small rivers and canals, making short blasts into plane and then stop, turn and blast again. Fooling around a bit.

    VW engines are easy and quite cheap to rebuild, and you can get performance parts as easy as stock parts. They are aircooled so no need to marinate the cooling system, and a lot of spark avoiding goodies are available of the shelf for the buggy people who enter the woods. Torque is already quite good given the small engine, and can still be enhanced.

    Where I'm a bit lost at the moment is the transmission. I'd like to stick to a V-drive but a marine gearbox is so damn' expensive and second hand v-drives are quite uncomon over here. Also I would like to stick to my vw-theme. So I was thinking of using the stock gearbox, either manual or semi-automatic, rebuild this so I'm only left with stern and astern. Then I could put those industrial thingies on that change the shaft direction, I'd bolt them on instead of the outputshaft either side of the gearbox in a 10° angle. This way I'd have some sort of twin prop v-drive. Only what will the differantial do whilst charging down the river.

    Have you guys got any thoughts on this other than I shouldn't use the car engine, if so I'd love to hear them.

    Thanks for reading this short novella,
    Tim Wolff

    I had posted this message in design also but this seems to be the more apropriate place for posting it. Sorry, Won't happen again.
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    D'ARTOIS Senior Member


    You want to build a boat around an engine - a floating (just) piece that may carry your engine as fast as it will run down the kanaal van Gent naar Terneuzen.
    In case you need a Vgear (koppeling) they are around here in Holland; I know a few 2nd hand shops where you can get the, however still expensive.
    In your case, I would abandon the idea of a regular transmission- propellor train and instead thinking of an airscrew-propulsion alike those Everglade boats; in that particular case you don't have to worry about a lot of things and I am sure that the audience here may exactly tell you how to make such a device.

    Idea or not? :idea:
  3. Volksliner
    Joined: Jan 2005
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    Volksliner Junior Member


    Thanks for the advice. Earlier on whilst still thinking over the use off a vw motor I get the www. And guess what I dug up...


    As I mentioned earlier on, I'd like to stick to the v-drive option, so maybe you could send me the adresses of some of those shops. that way I could at least put a pricetag on secondhand stuff.

    If this one works out, maybe later I'd like to try somethings along the lines of the airboat.

  4. Volksliner
    Joined: Jan 2005
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    Volksliner Junior Member

    gear ratio


    I've looked up on the vw gearbox. It seems the gearratio at the outputflange of the diff. Is around 5 to 1.
    And as those industrial thingamajingies have only gearratios as low as 1 to 1, at least the ones I've found, and I use an engine that can only pull 4000rpm comfortably, I'd be stuck with only 1000 rpm at the prop.
    Seems a bit to low for a planing hull, I think.
    But then again I could be wrong, I hope ;)

    And I'm still wondering wether I should put a difflock on the gearbox, or would the differential benefit the driving of the boat, as it does when driving a car?
  5. PowerTech
    Joined: Feb 2005
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    PowerTech Senior Member

    What i would do.I take that back What could be done :) .Mount the motor/transaxle asembly in the hull side ways so the shafts point front and back.Use a I.R.S transaxle so you can stop one axle from spining which would be hard and uglyer to do on a swing axle.take a steal plate and bolt it to the forward facing out put flange and to the transmition housing,that will lock that side up giving you some sirious overdrive through the spider gears.In forth gear that shaft will zing.Then use the CV joint to give you the shaft angle you want add a coustom shaft through a stuffing box with a choper on the back and it just might work.good luck :idea:
  6. Volksliner
    Joined: Jan 2005
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    Volksliner Junior Member

    thanks for the reply Carlwun.

    some people already advised me in the same direction, they didn't think about the overdrive though. It would be a great idea, if only the engine would be stuck in an engine bay with a lid on it.

    Me i'd like to build along the lines of a flatbottom raceboat, with the engine sitting on top as the main feature of the boat, that way it be better looking if the engine faces it's normal direction.

    I could maybe look for a different gearbox that I could fit in the vw engine, so the output shaft faces backwards, the other one blocked of, the way you suggested it.

    Greets, Tim
  7. Geert-Jan
    Joined: Jan 2005
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    Geert-Jan Geert-Jan

    Blocking one side of a differential

    If you block one of the output sides of a differential, the other axle will spin twice as fast!
    If you use an automotive gearbox, you need some sort of oil cooler.
    I like the project, please keep posting updates.
  8. Volksliner
    Joined: Jan 2005
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    Location: Ghent, Belgium

    Volksliner Junior Member

    My problem with the gearbox and differential is fixed. Only not really, it's more that i've put aside the idea.

    Just this week I located a good as new Hurth V-drive which will suit my project just fine. It's the hbw 150 v, and this will take up to 75 hp, if used with an oilcooler, which wil suffice for testing and fooling around on the canals at ghent city. I can have it for 600 euro's which would compare to like 450 dollars I guess. which is like a quart of the new price.

    I'll keep you posted geert-jan, probably on a new thread.
    Thanks for the input

  9. SAE140

    SAE140 Guest

    Bit late on this thread (!) - only just joined ...

    The problem with locking one axle and driving the other is that the differential gearing will be subjected to constant rotation - but it ain't designed for that, and will probably only have a short life. Much better to weld-up the differential and only use one shaft.

    In passing - the other big problem with using auto engines + boxes for marine use is that the clutch is in the wrong place. Ideally it needs to be between propshaft and gearbox, rather than between gearbox and engine.

    But - it's still a good cost-saver, as long as you're aware of the limitations.

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