Jet drive mounting configuration

[nbehlman]

Could someone explain to me the pros and cons of mounting a jet drive either recessed into the transom or mounted flush?

For example, jet skis always have the impeller and stator in a cavity, with only the steering nozzle extending beyond the hull.

http://images03.olx.com/ui/10/52/26/...thing-Else.jpg

I find that most jet boats don't have the recess feature and simply mount to the transom

http://woodyboater.com/wp-content/up.../jet-drive.jpg

Is there a benefit to recessing the drive? If it is mounted to the transom, it buys me some more arrangeable space.

Also, I am in the process of designing my hull, and I am concerned that I can't get the jet drive mounted low enough. The waterline passes more or less through the drive shaft. This means I will have an air gap in the intake when i start up the engine. I have attached a diagram. Should I be concerned about this?

[baeckmo]

A minimum recommendation for "start-up" water level is that the pump must be submerged to shaft level. There is a point in having the inlet as far aft as possible, since any bottom surface aft of the inlet will generate a new boundary layer, with the corresponding additional losses. Beyond that, the outboard versus inboard mounting is more a question of mechanichal design of the jet unit.

[nbehlman]

That's good information, thank you. I think I will mount the jet directly to the transom. That will buy me a little more space in the boat, and should shift my CG aft (if only slightly) and maybe help keep the jet further be low the waterline.

[jim lee]

The faster you want to go, the further back you mount the pump. This is the mantra of the drag boat types. The intake needs a solid column of water at speed. The best place for that, with the least amount of hull dragging, is at the very back.

-jim lee

[Ad Hoc]

In the case of the jetski, recessed, the jet/nozzle etc is being protected by the hull extensions either side. It therefore provides a simple "safety" feature of the "exposed" and accessible transom of personal water craft. If it did not have the recess, the nozzle is vulnerable to any Tom, Dick and Harry getting a bash from it when falling off.

It also is one way of going faster too. Since if the recess where not there, yet everything remaining in the same location/position,the hull length would be less. So think of it as "adding" a bit of hull, ie increasing the LWL.

It also helps to add buoyancy aft, if you have a trim problem too.

Horses for courses