fountain 47 sc

Has anyone changed a 47 Fountain to jet drives?

Dkahnjob

Why????

35 fountain

Good question

carcrash

I looked into this in detail. The deal is that jet drives really do work better than props when using diesel power and going between about 30 and 40 knots. And that's it. In any other condition, props, and in particular outdrives or surface drives like Arnesons, beat the daylights out of jet drives.

So you'll see jet drives on diesel powered high speed ferry catamarans. They are perfect for that application. Those boats just run the diesel engines 'flat out' (really, at a specific but high RPM) all the time, and teh boat just goes at whatever speed that happens to be based on sea conditions.

With jet engines, you are simply setting your fuel burn, the amount of work the engines are doing, how hard the engines are pushing the boat.

But jet drives suck on a typical recreation boat. For example, in big seas, I need to be very specific about the speed I am going: I need to fit into the swells. So speed matters, and with props, I can actually control my speed.

Also, as you go faster, the jet pump just can't maintain any efficiency. They are only efficient in that speed range because they are gaining from not having ectra junk under the boat. As speed goes up, they lose, because they are making force by throwing water -- actually moving the mass of water -- rather than by sweeping a wing -- the prop blade -- through the water. With the pump, you must keep moving more water. With the prop, you are not moving water, you are just making a pressure differential, and that pressure differential increases by the square of the prop speed (RPM). Throwing water is no better than linear.

So the faster you want to go, the more substantially better props are than jets. Double the speed, and props are 4 times better. Triple the speed, nine times. Four times the speed, 16 times. So at 120 MPH instead of 30 mph, .... you can do the math.

carcrash

Its really not that simple. They are the most fuel efficient propulsion systems for high speed ferries. That is why they dominate that market.

But for sportboats, their sweet spot is so narrow as to be usually the wrong choice.

Rik

I don't think efficiency is neither the strong point nor the reason they are used on ferries.

A lot of things come down to tradition and mis conceptions.

Arneson has a lot of ferry applications and they are more efficient than the jet pumps both in the slower speeds and the higher speeds.

But, tradition and mis conceptions dictate what builders use more than efficiency.

Example: Sport fishers are only inboard powered with key'd shaft boats. There are better alternatives but they will never change this tradition. Consumers are accustomed to this and its jus the way it is.

As for the larger ferries, jet pumps are the only alternative at this time. Arneson's biggest current model is good for 16,500 lbs of torque. A 5,000 hp diesel with a 2:1 is over the rating of the Arneson, so jets are the only current alternative.

In 2008' we will again have a drive rated at 40,000 lbs of input torque so that will open the larger markets.

I bid against the pumps all the time. I hear the spill see their #'s and I beat them all the time. Once I get a manufacture to see the facts and they try the Arnesons they see how bad and inefficient the pumps really are.

Shallow water is no friend of the Jet Pumps either as the debris and the bottom (sand, rock, mud) take the wear ring tolerances out of the pump and things go from bad to worst really fast.

Let's not even talk of the maintainance of the jets. Just run over a trash can!