WATERJETS: When to Use, Pros, and Cons

water jets are fun but are they efficient [Music] water jets made a big splash and now we see them on many high-performance craft there are one of the preferred options this prevalence of active installations it prompts us to search for other applications hey everybody's doing it there must be something to it can we put water jets on a slow river boat or on official vests or on a fishing vessel that's at 20 knots this video is going to focus on the merits of water jets their pros and cons with focus on the most important factor efficiency let's start with some basics what makes water jets special they work different from a normal propeller they work by pumping a lot of water through a closed duct the acceleration of this water creates a forward thrust for propulsion the figure on your screen shows a typical arrangement for a water jet suck the water in at the bottom the impeller adds power and the nozzle accelerates the water out the back that is a completely different operation from propellers a propeller tries to minimize the velocity change and focuses on creating a pure pressure difference but a water jet intentionally increases that velocity change that change in momentum is what creates the thrust multiply that thrust by a huge flow rate and you get propulsion this different paradigm leads just several differentiating features the four big ones to highlight the action happens in a closed duct not on the open water it is designed as a pump not a propeller you as a result you get higher RPMs on the impeller and you also have one of the other elements which are Staters put it in behind the propeller to recover those rotational losses yes but is it efficient well possibly a water jet is essentially a pump inside a very short pipe pumps work differently than propellers they don't show the same limits pump efficiencies around 90 percent or more are regularly attainable in contrast conventional propellers stop at around 60 to 72 percent if that's a huge difference but water jet efficiency involves more than just the pump a host of other factors produced the efficiency and you can see the long list on your screen all of those have to be accounted for but the really big one the one that makes the big deciding difference is the velocity ratio the largest factor in water jet efficiency is the ratio of velocity between the outlet and the inlet this velocity ratio strongly influences the total efficiency because water jets depend on high flow rates for efficient operation without any water acceleration the jet just fails to produce thrust but if we force too much acceleration the efficiency drops off fast DMS actually tried to contact a manufacturer and obtain typical values for the velocity ratio they rightly refused to disclose that information because that really is a critical trade secret it's the magic that makes the jet work in fairness to water jets we also need to remember that there are other drag elements required for propeller propulsion it's not an apples-to-apples comparison so let's talk about the oranges that are the propeller remember that the propeller has extra drag you have the shaft for the propeller drag from that the brackets holding the shaft that's also extra drag and you have the rudder all of those appendages because a water jet does not need a rudder it sits flush inside the hull with just one outlet and at speed that exit nozzle is completely clear of the water this reduces the total resistance on the vessel but as we saw from the last slide velocity ratios clearly show that efficient operation depends on picking the right jet for the right speed really it all comes down to speed at higher speeds water jets show better efficiency and they are the clear winners but at lower speeds they struggle to create enough momentum change with that limited water flow rate DMS created a simple comparison using the same water jet for six different peak speeds in practical terms this would be using the same a jet on six different types of vessels the peak speed of 60 knots that represents a sleek light patrol vessel designed to cut right through the waves at the other end peak speed of 10 knots that applies to some slow lumbering barge pushing along a heavy load do you must compare the different speeds like this because it's more instructive to assume a constant power and vary the peak vessel speeds water jet efficiency hinges on speed the graph shows how water jet efficiency drops drastically at lower speeds above 30 knots were doing great water jets offer a better option over open water propellers below 20 knots propellers were the clear winner the range of 20 to 30 knots of that remains a little uncertain propellers have a few tricks that they can use to remain competitive in that range and water jet efficiency may not be that good it depends on selecting the right model and remember they don't make infinite variation on jet models either option has merits in this speed range depending on your circumstances and wouldn't you know it as luck has it the speed range of 20 to 30 knots is also a very popular set of design speeds for many vessels picking the wrong propulsor here can drastically alter your fuel consumption for these applications dms offers careful design review in comparison for propulsion options in this range we can help you consider the pros and cons for your specific application talking about speed it's not just one speed remember to consider your operational profile when selecting your propulsor don't select a water jet due to its efficiency at high speed if you're going to spend 90% of the time trawling along at low speeds take a look at some possible examples in this case here for a water jet speed profile you'll be spending 90% of your time at high speeds but say if it's reversed and you're spending 90% of your time at low speed go with a propeller if you've got something more complicated in the middle well that's a case where you need to hire an engineer to create a fair assessment DMS can work with you to balance out all of the operational needs and Maxim your propulsive efficiency we shouldn't forget the maneuvering advantages sometimes we accept a lower efficiency out of water jets just due to their massive benefits for maneuvering unlike conventional propellers maneuvering works by controlling the direction of your water jet thrust your steering force links to engine RPMs not ship speed that's really important for docking imagine slowly drifting up to a dock with full steering control something a rudder will never achieve reverse thrust on water jets also promises greater control on a conventional propeller to reverse the thrust you need to first slow down the engine RPMs switch it to neutral switch it to reverse and then rev up again on our water jet just drop the bucket the bucket swings over the outlet and the water jet redirects the thrust into reverse no need to reverse gear or change engine RPMs this offers extremely quick reaction times for crash stops but wait it gets better some water jets include a neutral thrust position as well the reversing bucket partially covers the outlet sending half of the stream in Reverse and half and forward engine RPMs are no longer tied to your thrust you can have your engine running at full power have this per bucket in half partial position and you'll still be putting out neutral thrust going nowhere you gain the option to keep your engine at full speed for strong thrust control and only use a fraction of that one approaching the dock but immediately switch it to full thrust if you need it these extra options allow very fine maneuvering for a range of situations and they are one of the reasons that I would actually recommend a water jet for some applications even if that jet was less efficient than a propeller you know water jets are fun they give you great manoeuvring control and they promise much higher efficiency at high speeds but that flexibility comes with the price of more subtle limits on performance it's not quite so obvious anymore about how to pick the right jet efficiency rapidly drops off if you install the wrong water jet or if you use it at the wrong ship speed used incorrectly water jets perform worse than propellers the critical speeds of 20 to 30 knots are the really difficult part that's the transition from propellers to water jets being your best option DMS can help you decide on that right option and achieve all the promises of water jets without sacrificing efficiency thanks very much I'm Nick the naval architect thanks for watching don't forget to click that like button and subscribe for more videos and did you know that we produce more than just videos at DMS check out our website to find more articles free downloads and other help with ship design we offer a host of engineering services for budgets large and small so check us out to see if we can make your next project easier

Channel: DMS | Marine Consultant

Views: 355,921

Keywords: naval architect, ship design, engineer, marine, ship science, marine engineer, marine engineering, Waterjet, jet pump, jet drive, kamewa, thrustmaster, hamiltonjet, Wartsila, Wärtsilä, ship waterjet, yacht waterjet, water jet, ship water jet, hamilton jet

Id: fNNYUq5M-z4

Length: 10min 4sec (604 seconds)

Published: Mon Feb 25 2019