Pump idea

[2strokeforever]

I was doing some thinking on how to get more low end thrust without giving up top end.
When starting off towing or doubling my ski dosent have enough torque to get into powerband for 20 or 30 feet until I have some speed.
It also feels like it could handle more load when at speed.

My idea is to have a bypass nozzle on the pump that lets water flow out an additional nozzle approximately 2 square inches.
This would only open at low pump pressure.
That extra area gives more thrust while using less power, the using less power is important because it wil let the engine rev up to where it's really making power.

The math I did says it should nearly double thrust at low speed.

My only concern is intake cavitation, has anyone here increased nozzle size by 15 to 20 percent?

 

[WFO Speedracer]

Larger diameter exit nozzle always equals more thrust and it is always at the expense of top speed

 

[kevbo]

You will tend to feel more low speed thrust than a loss in top speed.. It sounds like you have too much impeller load. Bore the nozzle as much as practical then repitch the impeller. I know a variable nozzle was made and tested on a Seadoo 30 years ago. It was reported that the performance change was noticeable but not huge.

 

[2strokeforever]

I forgot the key idea is the extra nozzle closes once it hits a certain pressure

 

[WFO Speedracer]

Cool idea but it will be difficult to implement as many people with CATS trim systems have found out that pump has a lot of pressure that you will be working against to close the nozzle.

 

[Storbeck]

You probably don't need anywhere near that large of a size change. Not sure what ski/nozzle you currently have but going from an 83mm nozzle to an 85mm nozzle, for example, is a .4 square inch change and that makes quite a bit of difference. An 88mm nozzle is about one square inch more area than 83mm and that's a YUGE difference.

Cool idea. No idea how it will work in reality.

Relatively easy to do an initial prototype just build a nozzle with the bypass but don't bother with the shut off, test how it works at low speed. I suspect the geometry of this will really matter and it will be more complicated than just adding some bypass area, but there is one way to find out.

 

[wmazz]

First get a half decent tach and measure peak rpm at 50 yards.
Calculate where your engine makes peak HP using the pipe length
and exh port duration.

If you have an extra 200+ rpm increase the pitch 1mm on the LE of the impeller,
for increased acceleration.

If you have an extra 400+ rpm increase the pitch 1mm on the TE of the impeller,
for more top speed.

Always reevaluate your rpm because less rpm than peak hp leads to
detonation and wot seizures.

Bill M.