I did a search on this forum prior to starting this new thread, and I found another subject thread entitled "
Jet vs Prop" The problem is most of this discussion was centered around larger yacht systems.
I seek to get a discussion going of smaller units as utilized on vessels in the 10-30' foot range. I've included a couple of quotes from that other subject thread that might be applicable to these smaller units:
Quote:
| Originally Posted by Capt J Aside from the shallower draft as mentioned earlier, jets have two major disadvantages to props.
1. Jets are usually around 40% less efficient at putting the power to the water compared to props
2. Low speed manueverability, you really have to pour on the power to maneuver the boat in docking situations.... |
Quote:
| Originally Posted by YachtForums don't know how I've missed this thread! Too many thing to do around here I guess. Capt. J... welcome aboard! I'm curious, how did you arrive at the stats of jet pumps being 40% less efficient than conventional props? |
Quote:
| Originally Posted by YachtForums I didn't want to make one of our latest new members (Capt. J) uncomfortable by challenging his post. I'm sure he was just relaying information he gathered elsewhere. However, nothing could be further from the truth. Properly designed jet-pumps are absolute models of concentrated thrust. Quite simply, a ducted prop is more efficient than a non-ducted (or shrouded) prop, unless ofcouse... the drag created by the shroud outweighs the benefits of encapsulation. This isn't the case with a jet-pump on boats.
This being said... I have to clarify one thing. Application is critical!!! There are applications better for pumps and application better for props. In depends entirely on the operating parameters of the craft. An offshore race boat would be the wrong application for a pump, just like a Bravo drive would be the wrong application for yacht |
Quote:
| Originally Posted by Capt J ...Unless a jet is spinning at the speed it is designed to run at, it is very in-efficient below that because the housing is designed for a certain amount of water to flow through it and the size of the nozzle is designed according to that. Another reason a 9' waverunner with 155hp only does 65 mph. Another reason a 70hp outboard is re-rated around 40hp when a jet lower unit is installed. |
Quote:
| Originally Posted by YachtForums You'll notice as speed and RPM's increased, the efficiency of the pump increased exponentially and quickly caught up to the propped hull. The pump was only just beginning to come into its spectrum. To me, this indicated the pump may not have been the best choice for the operating speeds of this hull. That doesn't mean it's a less efficient propulsion system.
Also, referencing the outboard reduction... you need to examine the jet-drives that are adapted to these lower units. They are centrifugal pumps. There are no similarities between this and what we're discussing. |
Quote:
| Originally Posted by Capt J The reason the jets are not as efficient at lower speeds is because they have to be designed to achieve 2350 rpms at full throttle and the impellor has to have the correct pitch to allow the engine to achieve full throttle rpm, also the pump nozzle has to be large enough for that volume of water. Therefore at cruise rpm 1950, the jet is not nearly as efficient as a propellor. A jet works great with a turbine because a turbine is designed to spin a certain rpm all of the time. When a pump is combined with a diesel engine it is not an efficient system because of the nature of the diesel engine. |
Quote:
| Originally Posted by YachtForums 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. |
This initial posting is already longer than I originally planned so I will continue the discussion in a separate posting on a particular item I viewed at the Miami show, the JetPac