Jet boat vs. V-Drive/ I/O ENGINE BUILD DIFFERENCE
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Jet boat vs. V-Drive/ I/O ENGINE BUILD DIFFERENCE

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    T&Y
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    Default Jet boat vs. V-Drive/ I/O ENGINE BUILD DIFFERENCE

    All right so I know this is a noob question but don't really care.

    Is there a difference in how you would build a Jet Boat engine vs. something for a V-Drive or other I/O setup? Not talking about drag boats or anything, just a simple family cruiser.

    I'm talking in real general terms here as far as are you looking for more HP in a Jet Boat engine given the type of drive or does it even matter? For example could you take a basic Merc 454 330hp and run it in a Jet then pull it and put it in a Prop driven boat (same hull just different drive) and see the exact same type of handling, response, etc???? and if not what would make the difference?

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    Wet
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    Quote Originally Posted by T&Y View Post
    All right so I know this is a noob question but don't really care.

    Is there a difference in how you would build a Jet Boat engine vs. something for a V-Drive or other I/O setup? Not talking about drag boats or anything, just a simple family cruiser.

    I'm talking in real general terms here as far as are you looking for more HP in a Jet Boat engine given the type of drive or does it even matter? For example could you take a basic Merc 454 330hp and run it in a Jet then pull it and put it in a Prop driven boat (same hull just different drive) and see the exact same type of handling, response, etc???? and if not what would make the difference?
    Yes. A jet is going to require more HP to achieve the same numbers vs prop.
    Yes.
    Handling, NO. Have you driven each and are you really asking this?
    Response, NO. Prop variance, slip, size, etc. Jet cavitation, cut. Hole shot or long legs?
    Just gave it to you.

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    T&Y
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    Quote Originally Posted by Wet Dream View Post
    Yes. A jet is going to require more HP to achieve the same numbers vs prop.
    Yes.
    Handling, NO. Have you driven each and are you really asking this?
    Response, NO. Prop variance, slip, size, etc. Jet cavitation, cut. Hole shot or long legs?
    Just gave it to you.
    More HP was more or less what I figured to make the same overall speed. Speed is not really my concern. Yes I've driven both of them and no I wasn't refering to handling in terms of how it planes or turns.

    I'm not an engine guy and have never done any major mods to one so it's an area I really don't know too much about. I guess I'm really trying to figure out if when you consider what it is going in (jet vs prop) would you be putting different combo's of cam, valves, etc. or is it simply get the HP rating your looking for a drive it?

    All that and I am extremly bored today and pretty much this post is an example of a random thought that I couldn't answer with a simple search.

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    Quote Originally Posted by T&Y View Post
    Is there a difference in how you would build a Jet Boat engine vs. something for a V-Drive or other I/O setup? Not talking about drag boats or anything, just a simple family cruiser....real general terms here...
    Generally, it is possible to build one engine that could serve the purposes of either jet or v-drive for a family cruiser boat. Oil pan and pickup would likely need to be changed but other than the engine could be built indentically for the family cruiser(s) application in question.
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    Senior Member GT Jets's Avatar
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    My uncle explained this exact scenerio to me a very long time ago....

    The prop vs. jet debate is an old one...

    The best explanation I have ever heard is pretty simple...Take the words "Prop" and "Jet" and think about an airplane (different, but same).

    To make a prop plane go, you need to be able to spin a prop to a point at which it will generate enough thrust to pull the plane to a speed where it will gain enough "ground speed" to make enough lift to fly the plane....

    The propeller is an extremely efficient way to move enough air to get this done, need more thrust, the prop gets bigger, more speed (velocity) the pitch gets steeper....Pretty simple, not to mention predictable (pitch is identified in inches or feet per revolution).

    Now take the jet, it needs to accomplish the same outcome, but is doing this with a concentrated outlet of thrust at an extreme velocity (several times that which of a prop) and with an extreme pressure differential (a propeller is not a very good producer of pressure diff), the inefficiencies come in where the HP requirement is plotted on a curve, the prop fan curve (if you will), will come out fairly linear (straight line) so they can be predicted and calculated really easily (surface area, rotational speed, and pitch can be put into a formula and thus CFM, thrust and Brake HP can all be figured out) and the torque requirement is fairly linear with HP...

    The jet on the other hand, if put on a fan curve is a pretty extreme curve, the faster she spins the more pressure differential (the propeller makes virtually none), the higher the velocity the more brake HP it absorbs while the torque requirement keeps increasing. The torque required to keep the rotational speed up never falls off, but keeps going up, somewhat backwards to the prop.

    This is why everyone keeps talking about torque when talking about a jet boat motor build.

    Sorry to get so long winded with the word vomit, but I never forgot that analogy, which to me means it must be a good one.

    So in short, the jet boat motor will work fairly well in the prop boat, but more than likely not the other way around....I/O's for example use automotive type cam grinds (for the most part). A car motor will run for about an hour in a jet boat.....

    Hope this was helpful...I just finished off a bottle of Gentleman Jack and am ready for some serious eyelid performance checks.

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    Quote Originally Posted by GT Jets View Post
    My uncle explained this exact scenerio to me a very long time ago....

    . The torque required to keep the rotational speed up never falls off, but keeps going up, somewhat backwards to the prop.

    This is why everyone keeps talking about torque when talking about a jet boat motor build.



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    Default Power Curve

    Building a universal application engine has it's drawbacks. Primarily the power/torque curve. A jetboat will limit RPMs as the pump reaches terminal flow. A prop will not. So an engine built to have the power "all in" at a target RPM, let's say 5500, for a jet, would nose over in a prop boat. A target RPM for a prop may be in the 6500-7000 range to have it "all in", and a jet drive wouldn't allow the engine to get to that "happy place"......I'm speaking about a bit more performance orientated engine, not usually found in the family cruiser. You have to remember that performance is related to efficiency, and efficiency is related to economy. Economy is related to breaking the bank account for a weekend's gas, so there are $everal rea$on$ to design an engine for a specific purpose/drive to get the most bang for the buck.......Ray
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    Quote Originally Posted by Moneypit View Post
    . So an engine built to have the power "all in" at a target RPM, let's say 5500, for a jet, would nose over in a prop boat. A target RPM for a prop may be in the 6500-7000 range to have it "all in", and a jet drive wouldn't allow the engine to get to that "happy place"......I'm speaking about a bit more performance orientated engine, not usually found in the family cruiser. .......Ray
    In this case he did state I/O. An I/O will be done at 6k maybe 6,400. I think you still want to build an engine with a broad trq curve that for a typical cruiser is making peak trq and hp at roughly 1,000 rpm split.

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    Senior Member CK7684's Avatar
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    A big fat broad torque curve is always a good thing

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    You could build them the same for a "simple family cruiser" If each boat they are going in is a simple family cruiser type of hull.

    But what will happen is the engine buyer will complain about the performance. Because each type boat is different.

    All 3 have should have different RPM ranges and idling characteristics. How thay handle around a dock is important to a stern drive boat and not even considered for the majority of v-drives for example.
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    Quote Originally Posted by Sleeper CP View Post
    In this case he did state I/O. An I/O will be done at 6k maybe 6,400. I think you still want to build an engine with a broad trq curve that for a typical cruiser is making peak trq and hp at roughly 1,000 rpm split.

    S CP
    I'm pretty sure a typical Merc stern drive rev limiter will hit at 5,400ish RPM
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    Quote Originally Posted by Infomaniac View Post
    I'm pretty sure a typical Merc stern drive rev limiter will hit at 5,400ish RPM
    I guess I should have read his post 3 times. Jet, I/O or V-drive. I guess all three depend on the operating range. The jet and I/O a little closer with the v-drive engine being built a bit different ?

    What's the limit on a Bravo-One ? I thought more than 5,400

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    Quote Originally Posted by Infomaniac View Post
    How thay handle around a dock is important to a stern drive boat and not even considered for the majority of v-drives for example.
    This is a big issue on a stern drive...bravos don't like to shift much above 900rpms.

    Add a big prop (to go with your big HP), and it becomes a whole other problem. With carbs, a large cam, or blower (even with a #6 transmission), when you drop it in gear, it gets old having to feather it so it doesn't die.

    Ever try to time your shift on blower motors at the bottom of the surge so you don't hurt your Bravo? With lots of other boats around close quarters?

    Dock manners are important on a a stern drive...unless you go EFI.
    Last edited by Havasu Hangin; 12-02-2009 at 10:20 AM.
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    As far as I know most Merc stern drives are rev limited at 5,400ish RPM. But it's the engine not the drive. They are re-programmed etc and given more RPM. The drive is good for quite a few more than the engine is rev limited for.
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