jet foot

[Octopus]

Hello guys, 1 year of using this site, 1st post. New to jet engines and did some searching here with some good post. However, I want to test this 18hp (output) in actual open water, the foot is about 7-9" below bottom, should i go for a test? 14ft aluminum semi V.

 

[tnriverluver]

That motor will have almost zero propullsion like that. Jets produce there power pushing water against air, not water against water. Use a spray nozzle on a hose as an example. Hold it loosely in the air and it will fly around. Stick it in a bucket of water and almost nothing.

 

[bulldog]

What he said^^^^^^^. That needs to be a lot higher to go anywhere. You could make sure the water pump works and all that but don't plan on going anywhere too fast.

 

[S&MFISH]

Jets produce there power pushing water against air, not water against water.

I'm sorry,but I will disagree with that assumption. The propulsion is made by the force of the jet stream against the water,not the air. You trim your jet nozzle out of the water and hit the throttle you are going - NOWHERE. The main problem that I see with the setup is that it defeats the whole purpose of having the jet- to get the lower end out of the water. There will be so much drag that ,as Bulldog said,you won't go very fast.

 

[Kevin Turner]

Newton's Third Law of Motion - "Every action has an equal and opposite reaction". With respect, think of the your jet much like a rocket engine. It moves the opposite direction of the created thrust, regardless (to an extent) of it's environment. EG: Under water, above water, in air or an vacuum.

As an example, if you throw your fishing buddy off the stern ('cause he's drivin' you nutz )the action of you tossn' him reward, will move the boat forward....

 

[S&MFISH]

My question now is.... Why even have the jet nozzle in the water?

 

[user 6672]

It will work that way but it is not it's intended setup. You will lose a lot of power and hole shot due to drag.

Make a jack plate for now for testing purposes and set the leading edge of the foot to the bottom of the boat.

Throw that on a 12'


Good luck..

 

[Kevin Turner]

"I want to test this 18hp (output) in actual open water"

TO be clear, are you wanting to test the combo boat/motor package? Or, verify the condition of the OB motor/jet then re-install your 8hp prop?

 

[tnriverluver]

There is a reason you see jetboats throwing rooster tails when running top speed. Jet in the air produces the most thrust. This is not for show but they do look cool LOL. Jet boats are generally set up to push against the surface tension of the water but not the water down below surface if that makes any sense.

 

[Octopus]

Great stuff fellows, I feel it is unanimous, I need the motor sitting higher to get the experience of its performance. Less drag, Newton's 3rd and overall proper install. I will do so this week and test it. Thanks.

 

[Kevin Turner]

Quick note: Smaller pumps are more susceptible to aeration, consider removing about 2' of the pictured center keel. (that is if you're planning to leave the jet on this hull)

 

[RivrLivn]

Lots of good info at https://www.outboardjets.com, I understand they make the jet units for all the manufacturers.

Here is a quote from their site on the height of the pump:

7. How high exactly do you have to raise the transom?

The height is approximately six inches higher than with a propeller. It varies between different motors and boats. When the jet drive is on the motor, set the height so that the leading edge of the intake is flush with the boat bottom. Then test the boat and adjust the height, if necessary, as high as possible without air entering the pump and causing cavitation (slippage).

 

[tnriverluver]

Looking at the transom of your boat I believe your best and probably only option is the put a jackplate on it. That would solve nearly all your problems. The jackplate is going to need to have a lot of adjustablility however.

 

[Octopus]

Smaller pumps are more susceptible to aeration, consider removing about 2' of the pictured center keel. (that is if you're planning to leave the jet on this hull)

The keel fades away to flat and now with the engine jacked up, the foot is at 12" away from the base, hope that helps.
I think I'm ready for the test run, is there any other suggestions before I take this out? Other than that the feed back was great and I thank all of you for them.

 

[semojetman]

After you run it, you may want to put a piece of aluminum running from the bottom of the boat to just above the shoe.

If you run it and need it, you will know why pretty quick. Lol.

 

[shallowminedid]

its funny how people say shooting a jet in air will propel u. its like a fire hose, shoot it straight in the air u wont go anywhere shoot into a wall ull feel the pressure. jet blast should be just underwater if its out of the water ull go no where quick. rooster tails are gay if u ask me the just lose power and are made by pretty much trimming up too deep underwater ob jets DO NOT WANT roostertails

 

[user 6672]

its funny how people say shooting a jet in air will propel u. its like a fire hose, shoot it straight in the air u wont go anywhere shoot into a wall ull feel the pressure. jet blast should be just underwater if its out of the water ull go no where quick. rooster tails are gay if u ask me the just lose power and are made by pretty much trimming up too deep underwater ob jets DO NOT WANT roostertails

You ever hang onto a wide open fire hose? it will propell you.. wall or not..

[youtube]X9rAfMPoVrw[/youtube]

I can trim up and throw a monster rooster and still get forward motion..

 

[Kevin Turner]

Guyz-
Under, at, or above water doesn't change the laws of physics. Jet powered crafts are able to accelerate due to the fact they exit water in the opposite direction of which they want to go.

In general, race hulls shoot high roosters to get bow lift, not b/c it makes more power. Exiting underwater reduces flow b/c the exiting thrust must push the existing water out of the way, much harder than pushing air.

There's more, but time is short...

 

[user 6672]

Guyz-
Under, at, or above water doesn't change the laws of physics. Jet powered crafts are able to accelerate due to the fact they exit water in the opposite direction of which they want to go.

In general, race hulls shoot high roosters to get bow lift, not b/c it makes more power. Exiting underwater reduces flow b/c the exiting thrust must push the existing water out of the way, much harder than pushing air.

There's more, but time is short...

Pfftt... like ima take your word for it...

 

[user 6672]

Seems to be hauling @$$.. even kicking it up in the air..

[youtube]l41kvqiB6Og[/youtube]