Raising Speed & Efficiency in Jet Boats

Discussion in 'Hydrodynamics and Aerodynamics' started by Phancy, Sep 25, 2013.

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PhancyJunior Member

I am starting this thread so people can find it easier with a title search, there is another description under my thread of 'Jet Boat Brakes'.

Boats hopefully travel at the junction between air and water.

A hull on plane tries to slide along the top of the water with minimum contact to the water.
How an inboard jet boat rides in the water is affected by the suction at the pump inlet.
Let’s assume your jet pump takes 2000 GPM from under your boat and squirts it out behind at full throttle and has no drop scoop to increase the inlet pressure.
We have 2 different conditions that apply here, ‘FULL THROTTLE SITTING STILL’ and ’FULL THROTTLE AT PLANNING SPEED’.

FULL THROTTLE SITTING STILL
You will have a slight negative pressure in the inlet of the pump causing water to set up flow patterns under the boat, this flow does not cause the craft to be pulled down.

FULL THROTTLE AT PLANNING SPEED
The flow patterns do not have time to set up.
The slight negative pressure in the inlet has increased to a high negative pressure for 2 reasons: Bernoulli's principle and change in velocity.
A drop scoop or shoe is added to get rid of both of these reasons; the frontal area of the scoop is sized for the speed of the boat.
But there is still the same amount of negative pressure or suction under the boat, it has moved behind the scoop.
This negative pressure tapers of to the water line of the boat at plane and pulls the boat down into the water.
Add a MURPHY behind your drop scoop, this is a piece that is the same width and depth as your drop scoop and extends all the way to the stern of the craft (note; get rid of ride plate).
Now instead of pulling the boat down into the water, air is pulled into the void leaving only the scoop in the water for drag.
Before your hull moving forward was pushing water into the inlet of the pump and pushing it aside, making a wake.
The big advantage of adding a MURPHY is your boat is not making a big wake at speed, less water moved at same velocity = less energy used.
The extra energy that is now available can now be applied to speed.
This will not help you outboard pumpers; because of the minimal area of your pump and you boat is already on full plane.

How to get a MURPHY; make one yourself in your garage, shop or kitchen.

Sometimes you need to examine what you are told and not treat everything you read as gospel, isn’t nice to know that the world is not flat?

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PhancyJunior Member

After boaters have tested the Murphy and find that it works, the next step would be to make it adjustable.

Raising and lowering the bottom of the Murphy and scoop using the difference in ram pressure between the scoop and outside the scoop comes to mind.

The larger area of the scoop will cause the boat to come up on plane faster, the Murphy will have to be increased also.

My boat had an oversized Murphy, lots of drag on smooth water moderate speed increase.

In real rough water the boat had a large speed increase and a very rough and scary ride.
The only reason I can think of is loss of drag between wave tops, but scoop was filling up at top of wave so was thrust would continue for some time after leaving the wave top.

Last edited: Oct 1, 2013
3. Joined: Sep 2013
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PhancyJunior Member

If you have not thought of it already, the Murphy might not have to be adjustable.

Take a jet boat using 2000 gallons per minute through the pump.
With a maximum speed of 35 mph without a Murphy installed.

At the following speeds with a Murphy that is 12 square inches frontal area this is the results:
Below 35 mph hull drag is predominant but reduced, and Murphy drag is negleable {because of low pressure at inlet).
At 35 mph the hull drag has been reduced as much as possible and the Murphy starts to become the more predominant drag factor, but still very low.
If your boat could originally go 35 mph without the Murphy, and you have gotten rid of a lot of your drag, then you can go faster or reduce throttle for economy.
But at speeds above 35 mph your Murphy becomes more and more of a drag, till you reach a maxim speed.

Now we take the original boat and put a half size Murphy on it, 6 square inches.
With a 12 in wide inlet, a ½ inch drop is not very much.
At the 6 square inches this is the results:
At 35 mph hull drag has been reduced by 50% and there is no drag from the Murphy.
As you get closer and closer to 70 mph, your hull drag has been reduced further until a maximum speed has been reached.
A larger effective inlet area for the Murphy works better in rough water.
So for every hull, engine, pump and usage configuration there will be a sweet spot.
I don’t know how to figure this out except by testing with different sizes for the Murphy.

Still make the back part adjustable for boat trim.

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PhancyJunior Member

An object going through a fluid, the velocity of the fluid is increased to move to the area behind the object, unless the object is near the edge or surface of the fluid.

With a foil flying close to the fluid, increased lift is experienced (not part of this thread).

With a foil under but near the surface or a strut piercing the surface, the object pushes the water into the air to some extent.
This is a waist of energy and should be avoided if possible.
If it cannot be avoided, then don’t use more energy to put the fluid back to its original position again.
Use the fluid with less viscosity to replace what you moved in this case the air, saving energy.

That is why a hydroplane uses a supper cavitation propeller.
Which is not cavitation at all, but air induction.
Cavitation: as I know it is when a propeller or impeller reduce pressure in water and steam bubbles form and when those steam bubbles move to higher pressure area they collapse
The collapsing of the steam bubbles is what eats holes in your propeller.

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tom kaneSenior Member

I do not believe that water jets suck the water from under the boat but they do displace the water from under the boat and atmospheric pressure pushes in to replace the displaced water and air. Vacuums (low pressure) do not suck but atmospheric pressure pushes in to replace displaced air.

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Mr EfficiencySenior Member

Any mention of the word "suction" would have met with a clip over the ear from my high school science teacher.

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