Jet questions

[Eodman]

Could you tell me when using a jet system does the nozzle have to be in the water, at water level, half in half out?

Just curious, wondering what would be the best hull shape! Would a cat be a possible choice?(duals maybe!!!!!)

[Diegoboy]

It should be completely out of the water, from what I was told by several diff people.

[Quicksilver]

was going to say the opposite. The higher up, the harder it is to prime the pump. Mine's completely submerged at rest.
pics are of the same boat, I just repainted it all yellow

[Diegoboy]

...at rest, but what about at speed?

[Quicksilver]

well, not completely out of the water. See there's a balance, between least amount of wetted surface and the jet output position. getting to push water instead of shooting it out over the water. The nozzle should be angled just slightly down for good acceleration. If it's pushing on the surface, it'll have slower acceleration but more top end(at the right angle).

I'm not saying my boat is the perfect boat, but it pushes at maximum, but runs pretty wet. This is why I'm messing with my battery configuration and trimming things off the hull that I dont need, like trim tabs and my aluminum plate that goes to the tip of the nozzle, I cut it down flush with the transom today.

I guess it's not the best answer, but it's not black and white, you need to go on instinct, what do you want more, a boat with more power and more priming ability or lighter and maybe a faster top end, but hard to prime and slower acceleration?
complicated

some pics of the mods I did(center of gravity was about where the brush cooling is, now it's by the motor mount)

[Diegoboy]

I searched the net, found 50% shows the outlet above water, and the other 50% shows the outlet submerged.
I can't find a definite answer to this one.
All the submerged outlets had small nozzles (like on a jet ski) I dunno if that means anything or not.

[Quicksilver]

do you have example pics? I'm trying to grasp this debate.

[Diegoboy]

Darn, I closed the windows already when I gave up...
Looking again, I'll post some when I find'em again. (few minutes)

[Diegoboy]

Here are some...

[Quicksilver]

well that helps, in the first pic all are submerged at rest, and only the outboard is going to have the jet stream out of the water on plane. The second pic submerged all the time(probably a displacment hull), the third pic is a hydrofoil and as I've seen the models at least dont go fast at all, when comparing to yours or mine. The thrust gets lost by the time it hits the water. The fourth again an outboard,

One thing about these outboards, theyre not true jets, or at least are not up to par with a normal axial pump. The outboard are designed that way out of necessity than function. It just wouldnt work the normal way. on topic the outboard placement standing off the transom and having the intake flush with or even above the bottom of the hull causes the jet stream to go through the air first , losing velocity, like the hydrofoil. I got into quite a debate with an full scale boat designer and this goes along the lines of what he said.

The whole basis of jet pumps and speed is all about jet stream velocity and mass. Those are the only factors really limiting a jet boats speed. The longer the stream is out of the water the more time gravity and wind resistance get to slow down the stream. So effectively the shorter the stream the better when planing, however angling the stream down can only help so much, if it's angled to far than you're using some thrust to lift the stern and puch the bow down. angling up, will decrease wetted surface by trimming the bow up, but you could loose thrust by forcing the stream up. It's all about force vectors, when talking about this.

also weight is the easiest aspect to help with speed, the lighter the faster the boat will go, unless lightening the hull causes the drive to be out of the water

boy now I'm making people think, including myself, that was not easy to write, been out of college way too long.

I'm going to try to explain this with pictures, might be a little while

[Quicksilver]

there, soak that in hope it makes sense.
and as a disclaimer, I'm on pain meds for my back, so if I made an error, correct me.
edit; also The arrows are force vectors, so the length is the strength of that force, except exit angles, those are.......angles

[AtlanticChallenger]

The jets should always be submerged. When I was figuring out the balance for my Challenger, there were times when the jets weren't even completely out of the water (just partially), where at WOT I wasn't even seeing any thrust at all until I forced the rear down. Always keep a jet well fed.

[Ocean Racer]

DIEGO BOY[/QUOTE]:All the submerged outlets had small nozzles (like on a jet ski) I dunno if that means anything or not.[/QUOTE]

OK heres a example that might help ansewer this Q. Ok picture a hose ok the end of the hose is a jet nozzel. Now what happens when you put your finger on the end of the hose? It incrases the preasure and increases the rate of speed in which th water comes out at correct. So if you have a smaller nozlze you increase the speed of the water coming out of the jet. So if you have a large nozzle just like a hose with your finger off it just falls out with very little speed. From what ive heard the size of nozzle doesnt increses or decrease the amount of water coming out of the nozzle BUT it can increses or decrease the speed in which it comes out at.

Thank you Discovery channel