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[Nige52]

If you were to put a Graupner speed 600 race into a boat conventionally, with a propshaft, prop and brass rudder assembly and it went at 'X' speed. What would the difference, if any, would there be if into that same hull, you fitted the exact same motor, but with a Graupner water jet drive? Would the boat be as fast? Faster? Slower? Whatcha reckon?
Cheers
Nige 

[dreadnought72]

One the one hand, a rotating prop throws some water sideways - not all the thrust is directed aft. Therefore I'd suggest that a shrouded prop (such as you'd find in a waterjet) would produce more thrust-per-watt = higher speed.

On the other hand, shrouding anything will increase the wetted area and the drag = lower speed.

So take your pick.     

Andy, voting for half a finger on the first hand.

[BarryM]

Have a look at this article:

http://www.camarc.com/Reference%20Docs/waterjets%20vs%20propellers.pdf

Barry M

[Martin (Admin)]

I think the same motor won't work as well, you'll need a higher speed version.

Cons:
Jet drives run at higher speeds and are therefore power hungry.
Reverse is mechanical and a bit faffing about.
No slow speed control.
Noisy

Pros:
They will run in very shallow water.
Good top end speed.
Spectacular!

[Nige52]

Thank you for the comments so far 
The boat concerned is a brushed race boat so reverse is of no importance. The course will be the standard Club 500 M shaped one so manoeuvrability is important, I was just wondering if two identical boats were fitted with identical motors, but one is shaft driven with a rudder, and the other has the water jet fitted, which one would have the most acceleration/top speed. Hmmmm 

[Norseman]

Hi

Sorry if I'm stating the obvious but surely the only way to know definitivley is for someone to actually do it.

Regards Norseman

[Nige52]

I would imagine many people already have in the past, so they would know which is quicker, hopefully someone will see this thread who knows the score 

[Norseman]

Bearing in mind I'm new to this. Would you also have to experiment with the water jet position /set up before you made a comparison, or is there only one way to set it up?

Norseman

[Martin (Admin)]

I like surface piercing propellers!  

[Talisman]

My experience,
In 'real' ribs i found that props were better in almost all situations.
Especially so if the boat was likely to ventilate and also in inshore work the stones / gravel lifted made short work of the impeller (whereas outboards could be lifted - obviously shaft driven could not.
Perhaps some more info on the type of craft may lead to a more conclusive recommendation.
Regards,
Kim