how it works...

[fish2keel]

Hey guys,

I got some questions for you jet guys! When it comes to outboard jets how exactly does the jet pump work with the outboard itself? Also why exactly are jets so bad on fuel and have a horsepower loss on outboards? Just trying to figure out the ins and outs of outboard jets and how they work mechanically

Thanks guys!

f2k

 

[Ranchero50]

An outboard jet is just a big centrifical shell over a screw impellor. A lot of the loss is because the outboat setup has the impeller mounted vertically vs. horizontally on an inboard and water doesn't like to change direction.

On an inboard the hull is the pickup and uses hydrodynamic's to draw the water up into the inlet into the face of the impellor which is horizontal. The impellor pushes the water against straightening fins and out through the pump nozzle and steering nozzle.

On an outboard the water turns 90` and gets sucked vertically up into the impellor, then turns again into a snailshell housing that gets the water aimed out the back again, through the pump nozzle and steering nozzle.

Because the impellor isn't in the water stream like a prop it's not as efficient. Turning and changing the form of the water column uses a lot of energy, energy that is lost efficiency pump vs. prop.

There's a ton more stuff involved but that's the water flow side.
Jamie

 

[fish2keel]

Wow jamie thats awesome!

Im just trying to figure everything out with these things. There has to be a way to make the motor either more efficient fuel wise or atleast a better way to get a better horsepower closer to a prop set up.

If the impeller is vertical on a prop why cant you put it horizontally on a prop or just turn it horizontally on a jet on the outboard?

Sorry if these are newbie questions....not alot of people run jets down here in the south so just trying to figure it all out

f2k

 

[RPjet]

https://fishandboat.com/anglerboater/2011ab/vol80num4_julyaug/05invsout.pdf

 

[fish2keel]

That helps alot!

So anyone got a picture of the impeller on the shaft of an outboard? Just trying to figure out how it is mounted on the shaft

 

[Ranchero50]

The jet lower has the bearing and seal package, the shaft is just simply keyed with threads on the end. The impellor has a telfon spacer that wraps around the shaft where the brass key is and you have a series of spacers to set the clearance between the impellor and liner.

 

[fish2keel]

Oh i see now! Well hmm.....Im just throw ideas of how to make them more efficient. There has to be a way to make the outboard jets be more efficient such as some how coming up with a system to make the impeller more efficient or turning it on a gear system

Thoughts?

 

[Lil' Blue Rude]

There's a few mods you can do tothe pump assembley. Sharpen and polish the impeller, stainless are best in my opinion. You can remove the casting flaws from the inside of the pump and smooth everthing up. You can bevel the bottom of the liner so the edge is even with the bottom of the impellers leading edge. You can sharpen the leading edge of the greats and bevel the leading edge of hte shoe.That's just a few things that have been done to improve the performance of the pump. I think one of the big reasons that there haven't been any other outboard jet pumps designed is because of the cost and complexity that would be involved in it. You could always make some kind of and line system like the inboards but it would alot bigger and and the cost would go up with the complexity of it and you might not have the reliablility of the standard outboard jet pump with little to no performance gain over it. I think it's a limited market so there hasn't been any major ddevelopments in the designs. Could be wrong. All I know is I love them and wouldn't own anything else.

 

[RivrLivn]

Outboard power head mated with gearing to an axial flow pump = Mercury SportJet

https://www.mercurymarine.com/engines/jets/optimax-sport-jet/

This is what the River Pro boats uses.

Some manufactures have even used it in a "outback" design so it takes up very little interior space.
https://www.motionmarine.com/outback.asp

 

[fish2keel]

Wow that mercury is amazing! I like that alot!