Re: 30-40% HP loss for a jet drive? Why?
I don't think there would be much suck downards action either but I did find this interesting so I did a bit of reading and this is what I found from a few different articles.
The biggest net loss in Jet drives is due to the backpreasure in the tube after the impellor. So the sucking up is not that big, but the pushing it through is difficult.
I read an article of a patent that was done to design a turbo charged marine engine whereby the expelled turbo charged air was force injected behind the impellor and this reduced the backpreasure to the point wherby you get nearly identical net horsepower of a propellor driven system.
Here is a bit of a primer on how I understand horsepower.
Horsepower is a calculation based on RPM's and Torque. It has nothing to do really with the available power from the engine. It is just a derivitave. It can be used to calculate speed with gear ratios and drag co-efficients. and the vice versa can be done, you can calculate accuratly with cars the horsepower, based on top speed.
Torque is a measurable force that the engine is able to exert in a twisting motion
I think the calculation for hp is something like
(Tourque * RPM) / 5250 or something close the that
Eventually the torque an engine is able to produce stops increasing. Thats when you stop accelerating or your acceleration slow down.
Different engine designs are able to produce increasing torque at higher revs some at lower revs. The lower revs are good off the line and the higher revs you increase torque the faster your top end is.
Shortly after your torque stops increasing your Horsepower drops and you can't go faster.
Torque is simply a measurement of the twisting force of the engine at a given RPM. If you bolted a rod to the flywheel and hung a weight to it there is another little formula to determine torque in pounds per feet by how much weight the engine can lift. I don't know exactly how they manage to measure this but thats not my job.
A dyno uses a fluid coupling to determine the HP and tourque curve of the wheels of a car or prop of a boat. The water coupling acts as a fluid brake. The preasure exerted on the brake is what is used to determine the BHP or brake Horse Power. The fluid coupling/pump is powered byt the wheels of the car or the prop of a boat.
BHP is Dyno measured brake horsepower and
HP is calculated via the calculation and torque measurements
What does this mean for Jet Drives? Well not a heck of alot but I had some time to kill at work.
If you wanted to increase your horsepower for a jet drive you could eliminate the lenght of the tube after the impellor or increase its diamter to decrease the backpreasure, but without enough back preasure the impellor would just cavitate and self destruct(I think).
That turbo thing I mentioned above must try to find a balance of backpreasure and efficiency.
Eitherway you lose HP with a jet drive but increase other advantages of driving the boat.
