Engine + Jet = Hp lose?

[LVjetboy]

I have no problem starting sh*t.
I think you can loose both power and efficiency depending on drive choice. So American Eagles question's just fine. Efficiency is generally in terms of power loss, or power out / power in. So if you loose one you loose the other.
Also, power is defined as the work done per unit time, so power can be expressed as a function of jet pump thrust and velocity. If you know your pump thrust and hull speed you can calculate power out. Then if you know engine power to the impeller you got a ball-park efficiency. If I remember right.
Of course there's no hard numbers but I'd say in general, pumps are less efficient than props.
I believe the reason is water friction drag and viscous losses within the pump. Intake, bowl housing and nozzle interior surfaces. We'll say the impeller's about like a prop so a wash, although the blade shrouding and wear ring adds drag. Now that's a theory of mine and not something I'm stating as fact. But I'm leaning that way. And it is a fact that water drag is a huge loss as speed goes up. Guess how fast water travels through a pump? A lot faster than water past the lower end of an i/o.
Why do cars go way faster than boats with the same power? Because water drag is huge compared to air drag at speed. The same principle I think applies to losses depending on drive type. Water drag kills efficiency.
That said, with enough massaging, a jet boat pump can perform quite well against a prop. How many props out there do this: 100 mph with 650 hp?
jer

[Blown 472]

Originally posted by LVjetboy
I have no problem starting sh*t.
I think you can loose both power and efficiency depending on drive choice. So American Eagles question's just fine. Efficiency is generally in terms of power loss, or power in / power out. So if you loose one you loose the other.
Also, power is defined as the work done per unit time, so power can be expressed as a function of jet pump thrust and velocity. If you know your pump thrust and hull speed you can calculate power out. Then if you know engine power to the impeller you got a ball-park efficiency. If I remember right.
Of course there's no hard numbers but I'd say in general, pumps are less efficient than props.
I believe the reason is water friction drag and viscous losses within the pump. Intake, bowl housing and nozzle interior surfaces. We'll say the impeller's about like a prop so a wash, although the blade shrouding and wear ring adds drag. Now that's a theory of mine and not something I'm stating as fact. But I'm leaning that way. And it is a fact that water drag is a huge loss as speed goes up. Guess how fast water travels through a pump? A lot faster than water past the lower end of an i/o.
Why do cars go way faster than boats with the same power? Because water drag is huge compared to air drag at speed. The same principle I think applies to losses depending on drive type. Water drag kills efficiency.
That said, with enough massaging, a jet boat pump can perform quite well against a prop. How many props out there do this: 100 mph with 650 hp?
jer
Hummm hows bout a hydrostream with half the power, ie a 300 hp merc. You dont get out much do you Jer?

[miketsouth]

If it wasnt for the bricks the Jetpump boat would rival any wakka.
mikeT
Hey Jer :wink:

[LVjetboy]

That's an outboard Blown, even though props, they're in a whole 'nother power to weight catagory
jer

[canuck1]

Originally posted by Blown 472
Hummm hows bout a hydrostream with half the power, ie a 300 hp merc. You dont get out much do you Jer?
Yep that same motor in a jet is rated at 240 hp

[moneysucker]

I have always been told that 30% is lost through the pump.

[King Kuracz]

Maybe this is too simplistic but try it on for size............
Engine dynos at 600HP at 5200 RPM.
Mount it into a hull with a fresh jet drive with the impeller selected to allow running at 5200. After going thru all the setup tricks (shoe, rideplate yada yada) the boat runs a top speed of 78 MPH.
Add or remove weight to equalize the difference in weight between the two drives.
Remount the same engine in the same hull with a Bravo drive, apply all aplicable tuning tips, prop and gear selection to allow the same 5200RPM and get a best of 88 MPH.
You would say that is a difference in HP???
Sure sounds more like a difference in efficiency to me.
Hey, what the hell do I know?

[1979tarva]

heres what i see its the same or similar to a drag car as far as there are diff gear ratios in diffs. so if im correct with a custom prop and gear setup or the same with a pump I think you could get the same speed or whatever with the same hp. its all in the setup.
does it really matter as long as your having fun and going fast?

[Snowboat]

If you run in water that is at least three feet deep, run a V drive. Efficiencey, parasitics, HP, RWHP, they're faster.

[MudPumper]

Maybe Im wrong, but I'm gonna say that a Jet Pump is essentially the same as the water pump in the engine dyno running 1 to 1. So if the engine puts out 500hp on the dyno, it is putting out 500hp at the impeller. Shaft Hp on an outdrive is going to be different because of the loss of hp through the gears and what not. The difference is efficiency. Props are more efficient than jets period.