Are props the old fashioned propulsion system?

[jfm]

Re Questions of drag

Gludy:
Agree much of what you say. However, I dont think it's correct to say no appendage drag. There is very substantial drag with a jet, as water is pushed very fast through a grille, along a bent tube, past an impeller (and importantly past the impeller hub and driveshaft), then thru a nozzle. All that creates significant drag, and is the reason why you get less motive power from a jet for a given engine power, usually.

The lowest appendage drag is still achieved by surface drives, there's no way a jet can better the drag performance of a surface drive.

The other thing to bear in mind is that jets need the engines at the back of the boat, like V drives or stern drives. That's no prob imho, but Uk manufacturers in the 40' plus camp would need to alter their designs to get rid of lazarette and find something else to do under the saloon floor. Not a prob, but they would have to put their thinking caps on (mind you, the Sunseeker hattan 44 was like this, it had v drives. What was under the saloon floor btw?)

 

[kgi]

Re: Reverse with jets

yes you can stop quickly,BUT you will fill the cockpit up with water as well, in the military boats when we emergency stopped in the RIB type boats we put the wheel over as well to stop the boat being swamped, i also worked parttime on a commercial boat with twin jets, and when you "dumped the buckets" she stopped absolutely dead, the stern went down about three feet and the wake over took you and swamped the boat, yes very manouverable ,yes very safe, but when barnacles build up on the impelleryour thrust disappears rapidly, do not use them in shallow water because the tolerance between the impeller and the jet tube is critical andusing them in shallow water pulls mud and sand through the system and kills your tolerances lastly you will pick up plastic seaweed etc around the impeller and you will know its there because of the vibration and if its bad the loss of performance PS on the working boat we had corrosion probs ,it could have been our fault but there is a lot of dissimilar metals in the modern drives

 

[kgi]

one thing i forgot to add to my last mail is that some hull forms going downwind/wave have a tendency to slide around all over the place, due to there being nothing in the water to help it track in a straight line

 

[adarcy]

Gludy

I don't think it is as one sided as you seem to suppose.
I think jfm is exactly right about jets having plenty of added internal losses. The most efficient prop to transfer power to the water is supposed to be a single blader as I posted some time ago on the 4-blade prop discussion. Anyhow, a small whizzy prop in a tunnel is quite likely to be less efficient than a slower prop out in the water although I concede that shafts and outdrives have their own hydrodynamic penalties. But if a small fast prop is better than a bigger slower one why does every other installation use reduction gears? If a jet drive ahs gearing then it is not as simple a setup as you suggest.

The servicing may be simple as you say and I agree with many of your points about advantages for jets but surely you cannot be right about no fouling or weeding of the jet prop unless it sits out of the water at rest which I doubt very much

BTW I think the Manhatten 44 had v-drives with the engines in the space for the lazarette and some storage under the cabin floor.

Good discussion though.

Anthony