Water jet in a (relatively) shallow draft boat

Probably a newbie question but is it ok to install a water jet on a boat that has such a shallow draft in the aft that it would leave part of the tunnel over the WL?

 

Water jets have been installed such that all of the moving parts were several feet above the water surface. This may not work with all possible configurations, you are going to have to make sure that the jet you use can handle having so much air in it, and you need to pay attention to reducing the inflow of air through the intake because your propulsion system will provide a great deal more thrust throwing water back than it will air.

 

Most jet units will prime ok with the shaft center line at WL level.

 

Assuming the pump only absorbs water, the amount of water propelled back will be the same as if the jet were totally submerged. Therefore the water "linear momentum" (mass of water multiplied by its speed) will be the same. According to that, the thrust of the jet on the boat would be the same.
The truth is that I am not very convinced. I suppose, somehow, the jet loses effectiveness. Some opinions, please?.

 

Baeckmo has answered your question about the priming, so that seems not likely to give trouble. What might give problems is a hull not designed primariliy for jet propulsion. If you have a very shallow vee aft, you really don't want any kind of noticeable forefoot.

 

It would appear that you mean that the intake pipe is above the WL.
As compared to a tunnel?

You do not want any air in the intake as the thrust will decrease quickly.

 

I think he means the bore of the unit is only half full of water at rest. The intake is underwater.

 

Thank you for all input, duly appreciated! The intakes are fully below WL. As far as an "educated" amateur can tell the vessel is within the parameters for jet drive but I was not sure about effects of the tunnels' vertical positioning and couldn't really find info about it on the jet drive manufacturers' sites. Otherwise the design manuals are great! The aim is to achieve reasonable efficiency also at lower speeds (<20 kn.) and thus use a larger diameter jet, which brings the original question. Thanks again, perhaps I'll subject the whole design for review when it's done... or not

 

I'd like to add that for shallow water operation it is wise to use a jet having a reverse deflector that directs the reverse flow in one stream each side of the inlet opening. That will reduce the "stirring-up" of bottom debris into the jet inlet.

 

Back in the 70's Boeing was building hydrofoils that used water jets that when flying, the nozzle was many feet above the water. They were pretty efficient. As long as your intake and impeller are below the waterline it should work ok. I've seen lots of jet boats here in the Northwest that are like yours.