Jet vs. Prop

Good points and I agree with you, except...

When pumps enter their designed operating spectrum, they will excel... to a certain threshold. After this point, they will become less efficient due to aeration and lacking the vacuum to bring water up to the intake gullet at higher speeds. As speed increases and the hull developes more lift, (riding higher on the water), pumps are prone to inducing more air, resulting in a reduction in vacuum. This is also compounded by water trying to by-pass the intake gullet.

This has been my point all along... it's not that jet pumps are less efficient. They are actually more efficient in many applications, especially when the craft will be operating at or near their optimum output.

I can tell you this.... there ARE configurations that are more efficient for pumps than what we see commercially, for example surface piercing pumps. These are pumps that are placed partially beneath the hull's surface. In this scenario, water doesn't have to make an abrupt turn up to the impeller (via the intake gullet), but rather... water encounters the impeller directly as it passes under the hull.

Couple of pages of relevant info.
http://www.wartsila.com/en,shippower,0,tbd,28068895862326984,601447187262570,,1600.htm

http://www.rodriquez.it/fastferry/tmv84.php

I really like seeing the side by side comparison on the Azimuts. Thank you Kevin for remembering that article and sharing it. It has been a few years since anyone has posted anything on this thread, I am hoping to revive it...

I was thinking about that little cavitation problem that jets encounter at higher revs. Unless I am missing something this is because of cavitation and because of when the hull rises out of the water air can make its way into the gullet. Does this apply to larger yachts, say the 100'-150' range? I could see this happening if the yacht was traveling very fast or in very rough seas, but in normal conditions would air getting under the hull and into the pump be a concern?

Some of what I have heard as far as surface drives go is that they are designed to run only half submerged so if the entire prop is submerged you are overloading your engine. So in rough seas and at slow speeds when the drives can be more than half submerged you can potentially overload your engine. Jets protect your engine from this, correct?


Another jet vs. prop argument I was thinking about was underwater debris. I dont think I need to mention the damage that has been done to props, but what about jets? Other than sucking up sand do they tend to get damaged by other debris?

As a closing comment, am I the only one who is nervous when jumping off a swimming platform on a boat equipped with surface drives?

Dan