jet boat efficiency

From the perspective of someone who never read such an article, or ever calculated anything about thrust of a jet boat for that matter, I would be a little suspect about a statement which says the "most efficient operation of a jet drive for a boat occur(s) when the exit velocity of the water jet (is) twice the velocity of the boat."

A given mass flow rate of water produces a given thrust. e.g. F = ma is still at work, as in all things of that sort. Power = force X velocity (get the units right). Maximum efficiency occurs when the pump engine is matched with the amount of water being thrust out of the nozzle and has little to do with speeds (although for a given pump curve, max. eff. may occur at 2:1 ratio).

I can believe that such a relationship probably exists, but being the person I am, I would want to test that a bit more.....

You also probably need to know your "hull-speed", which is very hull type and hull length dependant....

Adding air bubbles along the hull under water to reduce the friction of the boat, may also change the statement you once read, as well as reducing fuel/power needed for a given speed....

The type of coating and the surface finish of your boat will also make some changes!

Upon thinking about this, I feel that the statement as made is probably false.....sorry!

It was probably made by one person about one boat type with one hull type and one engine type, then of course it may be true.....

In my many years of owning and operating jet boats, I can offer the following:

Jet boats are fun!

Twin engines make more fun than one.

Screw the efficiency stuff. You're wasting precious time. Get out on the water and HAVE FUN!!!

My guess is that it is a fair approximation.

Sitting in the water with no forward motion, the efficiency of the jet pump would be low because it is not being "fed" water at the inlet. If the boat were travelling at the exit water velocity, the efficiency would be zero because the jet could not increase the water velocity to produce thrust. Somewhere in between it would max out. Thrust is a product of amount of water X change in velocity (delta MV). If supply of water or change in velocity approaches zero, the product approaches zero.

If I understand the mechanics of the Jet-ski boat in some simple way, you suck water in from under the boat, increase its speed dramatically by blowing it thru a much smaller hole than it came in thru, with a poweful motor/impeller system.

This difference in speed between the sucked in and the blown out water is caused by the power of the motor/impeller......and the size of the orifice.....which must be matched to the power of the engine and the efficiency of the impeller and the weight/drag of the boat.....as the boat starts to move, the water is probably (I have never checked) being scooped up in some manner to help with efficiency, I would guess.....at some point, the power of the jet is balanced by the weight/drag of the boat and you go no faster.....

It sounds like a fun way to drive.....I'll try it out in Antalya later this week.....

If you assume that the engine draws water in at speed v1 and expells it at speed v2, then in one second it takes in a mass of water m=rho*A*v1, where rho is the density of water and A is the area of the intake. It accelerates this mass by amount (v2-v1).

F = ma: Force (thrust) = rho*A*v1*(v2-v1) = rho*A*(v1*v2-v1^2). At the maximum, the derivitive of F with respect to v1 = 0, i.e., rho*A*(V2-2*V1)=0. This occurs when v2 = 2*v1.

As has been pointed out, the overall efficiency of the jetski depends on drag as well as thrust. The drag is a complicated function of speed as it increases as the square of speed but is also a function of the surface area of the boat immersed in the water. At high speeds this surface area decreases as the boat rides on top of the water.

Thanks. That is EXACTLY what I was looking for. (rufus, fka guest)

Now I can size the pump/orfice for maximum eff.

I have redesigned a jet boat propellor system DiDion Jet from New Zealand back in the 80s to a full ringed propellor style. The resultant effect was to reduce the nylon stator ring wear caused by the sand errosion and propellor tip rounding off. As the propellor was now 1/4" smaller in diameter, the initial thought was the loss of thrust. Trial and error proved the thrust was not lost but actually increased as the engine now peaked from up from former 4300rpm to 4,950rpm. I then redesigned the discharge port to accomplish the same effect as the TF 30 turbojet engine that powers the F111C Royal Australian Air Force fighter bomber. Notice:- In effect, if the discharge nozzle does not include external air intake into the exhaust gases, the outside air would actually flow back up the tail pipe of the engine. The thrust is created inside the engine and not by the speed of the escaping burnt gases and so the necessary 'Blowin Doors' to take ambient air into the tailpipe. Other turbine engines just use closing nozzles to increase the exhaust gases airflow to stop resurgent air backflowing up the tailpipe.

Using this technology, my redesign to diverge the water also increades the boat speed by 15% using the same engine rpm on the same type of day. The actual water velocity was as close as possible to the boat speed to maximum the thrust at full speed. The resultant fuel comsumption also dropped and was far more efficient. Note the boat was used in racing sprints and was never headed in two years. GO Skippy!

As others mentioned, for a particular boat and nozzle system, the statement may be true.

Many thanks, you have demonstrated my thoughts in a most practical manner.

Is the boat in question to be seen somewhere on a website?

Do you have pictures for us?