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Thread: True hp through a jet?

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    Member DanielApplin's Avatar
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    Post True hp through a jet?

    So outboard motors are advertised to have 30 percent less hp with a jet vs prop and inboards are 10-15 percent, correct me if I'm wrong. My question is why will a 150 hp yamaha with a prop step and haul significantly more weight than a 200hp optimax inboard jet or a 302 in a similar hull. Why does the hp produced through a jet seem to be way less than is advertised. Why will a 225 on a 7000 pound ocean boat step more weight than a boat with a 280-300 hp jet including the weight of the boat. It seems to me that either prop hp is completely different than jet hp due to efficiency and therefore you cannot compare the two side by side, or jets are advertised to be much stronger than they really are?
    I have owned a few inboard jet boats and knowing their advertised hp I know for a fact that with a prop with the same advertised hp I would be able tell a huge difference.
    Let me know what you think. Would a 150 prop step more weight than a 225 with a jet and if so why if the 225 has more hp (158)

    thanks
    dan

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    First off, not sure have you have a valid comparison because each different brand of boat has a different hull shape and just as important, weight.
    Have you ever actually took a boat off a trailer and weighed it?

    My question is why will a 150 hp yamaha with a prop step and haul significantly more weight than a 200hp optimax inboard jet ?

    A 200 hp Opti with a jet has about 140 HP at the jet. Doubt if you can tell the difference with 10 hp. How much more weight are you claiming you can haul with the Yamaha 150? In the exact same boat? Other factors to consider are impellars type and wear. Have you actually swapped out a Yamaha with an Opti jet and tried the exact same run on the same boat?

    Just are just to many variables to debate this with you. weight, style, impellar wear, how the boat was loaded, etc.

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    Member DRIFTER_016's Avatar
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    Though they are close HP wise they are not when you consider torque.
    It's that torque that gets you moving not so much HP.
    The difference in torque is caused by the difference in blade surface area. The jet impeller moves a lot less water per revolution than a larger propeller with it's larger size.

    FYI. I'm not an engineer but I did stay in a Holiday Inn Express last night!!!!

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    Member Yukoner's Avatar
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    The jet is creating/operating on a pressure differential, while the prop is "propelling" through the water. Thats why you can have "load" props vs. "speed" props and much options in between. A good illustration of this is low speed control with an OB jet vs. a prop. No comparison. In a jet, all the low RPMs are pretty much wasted, not so a prop. You could throw the RPM vs MPH #s for both (say for accuracy's sake a 150 Yam 4s prop and jet) on a graph, and the prop would be much more linear while the jet wold have a significant bump where it goes from pretty uch no speed to on step.
    As far as whether a 150 prop would step more than a 225 jet, I would say emphatically yes. But with the proviso that much depends on the hull its mounted to.
    Any thoughts gents? Am I on Meth??
    Whers Glen when you need him eh?
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    Member DanielApplin's Avatar
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    Quote Originally Posted by DRIFTER_016 View Post
    Though they are close HP wise they are not when you consider torque.
    It's that torque that gets you moving not so much HP.
    The difference in torque is caused by the difference in blade surface area. The jet impeller moves a lot less water per revolution than a larger propeller with it's larger size.

    FYI. I'm not an engineer but I did stay in a Holiday Inn Express last night!!!!
    Ok so this makes sense so I guess they cannot be compared side by side just a difference in torque

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    Member DanielApplin's Avatar
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    Quote Originally Posted by Yukoner View Post
    The jet is creating/operating on a pressure differential, while the prop is "propelling" through the water. Thats why you can have "load" props vs. "speed" props and much options in between. A good illustration of this is low speed control with an OB jet vs. a prop. No comparison. In a jet, all the low RPMs are pretty much wasted, not so a prop. You could throw the RPM vs MPH #s for both (say for accuracy's sake a 150 Yam 4s prop and jet) on a graph, and the prop would be much more linear while the jet wold have a significant bump where it goes from pretty uch no speed to on step.
    As far as whether a 150 prop would step more than a 225 jet, I would say emphatically yes. But with the proviso that much depends on the hull its mounted to.
    Any thoughts gents? Am I on Meth??
    Whers Glen when you need him eh?
    this makes sense I am always impressed with the high end speed that jets can produce but always unimpressed with the weight hauling abilities.
    Thanks, so once on step a jet might feel like it has the appropriate advertised hp

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    Member Yukoner's Avatar
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    Quote Originally Posted by DanielApplin View Post
    this makes sense I am always impressed with the high end speed that jets can produce but always unimpressed with the weight hauling abilities.
    Thanks, so once on step a jet might feel like it has the appropriate advertised hp
    Jets can certainly make some high speeds, but compared to a prop, it takes more HP. Much of those speeds is also due to less drag in the water from a leg.
    I'm not sure if it feels like they have more HP once on step. It's like a plane. Once in the air, you can throttle right back for cruise, same with a boat, it's all about lift. It just takes lots of HP to get on step or in the air.
    boat speed with jets is also dependent on pump design. Hamilton 212s are a volume pump and can haul more than a pump design like an American Turbine. I've been very impressed with the weight hauling capabilities of my 212 pump. I have seen first hand that my 20' Wooldridge with the 212 hauls much more than a 20' T -Jet with the same engine and a AT 309.
    this is all just my observations, there are tons of guys on here that know much more than I, but they are all off doing something fun.
    Never wrestle with a pig.
    you both get dirty;
    the Pig likes it.

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    Member DanielApplin's Avatar
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    How much hp does your 5.7 have with a jet?

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    Member DanielApplin's Avatar
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    Quote Originally Posted by Money Pit View Post
    First off, not sure have you have a valid comparison because each different brand of boat has a different hull shape and just as important, weight.
    Have you ever actually took a boat off a trailer and weighed it?

    My question is why will a 150 hp yamaha with a prop step and haul significantly more weight than a 200hp optimax inboard jet ?

    A 200 hp Opti with a jet has about 140 HP at the jet. Doubt if you can tell the difference with 10 hp. How much more weight are you claiming you can haul with the Yamaha 150? In the exact same boat? Other factors to consider are impellars type and wear. Have you actually swapped out a Yamaha with an Opti jet and tried the exact same run on the same boat?

    Just are just to many variables to debate this with you. weight, style, impellar wear, how the boat was loaded, etc.
    I know there are many variables and I am just curious as to why the jet hp rating still feels weaker than the equivalent hp with a prop. I have never taken a boat off the trailer and weighed it, I'm just going off of what the boat is advertised to weigh, such as a 26 foot ocean boat vs a 21 foot shallow water jet boat. The jet boat will have almost as Much advertised jet hp as a much bigger heavier boat with a prop. I think my questions have been answered here to as why the prop will feel stronger with less hp.
    So let's say we have to identical boats with identical advertised hp one being a jet one being a prop, the prop will feel stronger so the hp doesn't mean much when it comes to weight hauling abilities it's other factors.

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    I think there are different "skew" ? impellers available for Jet owners that may change top speed vs hole shot. I know this was an option for my Jetcraft and then Mercury started using High Skew impellers in all there latest 200HP sportjets..

    Here is an old post..

    http://forums.outdoorsdirectory.com/...-Come-Standard

    M.

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