Jet vs. Prop Efficiency

I have read a lot of threads and articles on jet drives, and their efficiency related to boats driven by propellers. I keep seeing claims that jet drive efficiency can be nearly as high or higher than that of propeller drives. The claim seems to be that the jets are more efficient at higher speeds, say 25 to 60 knots than propeller driven craft.

My experience is limited to small high performance boats, mainly low profile "West Coast" flat and semi-V bottom boats with V-drives and jet drives. I think in every case with a similar hull and similar engine, the V-drive outperformed the jet in both acceleration and top speed. The only exception to this might be the immediate hole shot, where the jet was able to get its engine in the powerband a bit sooner, but the jet was quickly overtaken by the V-drive. Also if you look at the times and trap speeds of drag boats, I don't recall ever seeing a jet boat even close to that of the V-drive (admittedly, I have not reviewed every record, but in general I believe this to be true). It is my understanding that a V-drive is not a particularly good example of an efficient propeller drive either, with surface and stern drives being more efficient. For the sake of argument, I'll define efficiency as (thrust less drive related drag)/(engine power input).

To make matters worse, if we factor in engine efficiency in the equation, the jet comes out even worse for low and medium speed cruising, since the power absorption of the jet pump goes up by the cube of the RPM. A jet driven boat needs more engine rpm than a propeller driven boat for the same speed, which reduces engine efficiency (more pumping losses for gasoline engines, and more friction losses for diesel and gasoline engines).

Nevertheless, jet drives still prevail in the unexpected vessels, such as ferries, where you would think efficiency, and fuel consumption would be extremely important.

What am I missing here? There appears to be a contradiction. Under what conditions will a jet driven boat outperform a similar propeller driven boat with the same power?

Michael

 

Everything you say is absolutely correct.
The only advantages are low drag at high speed, High acceleration
Highly maneuverable in slow speed compared to regular stern drive, low draft.
But they are inefficient at all but highest speeds, Expensive to maintain, low fuel economy, Low bollard pull compared to properly propped boat.
They are supposedly safer for swimmers, manatee and like. But my people with these boats think they are immune to swallow waters and run around in a dangerous manner. In other words they allow a person run boat in water swallower than they really should be running.

Another reason they use these on ferries has to do with engines and hp needed. It is not easy to mount 4 props under a cat hull. Four jets are split to 2 to a hull and engines are staggered in each hull. Check on fuel tank size of these ferries, then calculate distances they usually travel. They are anything but cheap on fuel.

 

I totally agree.
My 2 Berkeley jets will be for sale next months when I take them out to be replaced by tunnel drives. The engines that did 30+ knots while I had stern legs were hardly able to achieve 20 knots with jets.
The island where I live has a daily catamaran service to the mainland. They use stainless steel, hydraulically controlled jets, powered by 2200 HP turbines. Why I don't know. They have individual control for each nozzle, still they have tremendous difficulties to position the large ship with crosswinds.

 

Michel1, you seem to have fallen into the common logic trap that high speed requires high efficency and that cost effectiveness requires high efficency. Neither are the case.

For speed efficency issues see this thread with a very good diagram by RANCHI OTTO: http://www.boatdesign.net/forums/showthread.php?t=17003

As for why ferrys use them, it is important to understand why jet-GTs are a good tradeoff for the issues like econimics and weight/arrangements (no cost/weight of reduction gears). In transportation, the old addage of time is money is very true. Holding the schedule with a low weight, high volume, high value cargo is often more important than lowest net cost per passanger trip. Also total trip costs includes terminal idle time, as well as loading factor and trips per day, so you have to look at the whole picture. If you expend twice the fuel but make 3 times the trips with twice the passangers did you cover costs at a lower per passanger fare? Often, economics is a greater driver in ship design than efficency.

 

The catamaran ferry I mentioned travels for 2,5 hours picking up passengers at several harbors/islands, then lies in the mainland's harbor from 09.00 AM to 03.30 PM next to a sister ship that serves other islands. Economy is clearly no issue.
The ships, with aluminum hulls, have 300 seats, but the payload rarely reaches 50%. Maybe is was only the lower initial investment that made the company decide for jets.

 

From http://www.ultradynamics.com/sections/educational/why.asp


Safety - Water skiers, swimmers, recreational and commercial divers are safe from the threat of injury (or worse) from turning propellers. There is minimal potential for damage to jet-driven boats from hitting floating debris, and there are no propellers to foul on stray ropes or fishing lines. Jet-propelled boats simply ride over floats and lines attached to lobster and crab pots. Jet driven boats have expanded access to shallower waters, particularly in tidal areas. The potential for major drive damage from running aground, as with submerged propellers, is eliminated.

Maneuverability - Jet-driven boats have excellent maneuverability, even at low speeds, because of vectored (directed) thrust. Whatever a situation requires, the needed amount of thrust can be directed fore or aft, port or starboard. Thrust is applied smoothly by control of the reverse deflector, an integral component of the unit. This vectored thrust affords jet-driven boats very responsive steering, ideal for control in narrow waterways and avoiding other vessels. There is infinite control of fore and aft movement, even in severe wind and sea conditions.

Economy - Jet drives are easy on engines and transmissions. A jet drive, well matched to an engine and hull, allows the engine to operate at optimum cruise throttle settings, thus providing better fuel economy. In start-and -stop situations, (for example, water taxis, lobster and crab fishing boats), the engine is set at optimum throttle, the transmission is left in forward, and all stopping, holding and accelerating is smoothly controlled with the reserving deflector. Basic maintenance consists of monitoring bearing oil levels and checking for anode depletion. If service is required, many external parts can be owner-serviced in the water, without hauling the boat (such work can often be done on a beach or rack which is exposed at low tide). This is especially beneficial if inspection or service is needed miles from a full service marina. If operated in relatively clean water, many years of service may be expected from the encased impeller. End result: lower operating costs.

Comfort - Compared to propeller-driven systems, jet drives greatly reduce on-board noise and vibration levels. Jet drives rotate at higher speeds than propellers, and the impellers are encased in precision-crafted housings. Thus, vibration levels are much lower than with propeller-driven units. Considering that thrust is developed within the jet itself and is transmitted directly to the hull, engines may be fitted with flexible engine mounts to further reduce engine-generated.

Environment - Vessels with jet drives have much lower underwater noise signatures than vessels with propellers. In addition to being a relevant consideration for military applications, this in an especially significant factor where marine life and other environmental matters are concerned. Propeller-driven boats are restricted in several areas of Florida because of potential harm to fragile coral. Along with being safer for swimmers, the jet drive is also safer for marine life. The endangered West Indian manatee is a good example. Since 1976, more that 43% of manatee deaths were attributed to humans, most of them in boating related incidents, primarily from impact with underwater propellers. Manatees, if struck by jet-driven boats, are much less likely to be injured.

Other Applications - Jet driven vessels are ideal for use in towing or recovery operations because maximum thrust for any throttle setting is available, at even the lowest boat speed. Jet-driven boats are often able to get closer to the a grounded vessel to assist in the recovery. Jet drives have no equal in their ability to stop a planing boat. Approximately 50% of forward thrust may be utilized when the reversing deflector is deployed (it is essential to warn and prepare passengers and crew before performing an emergency stop).

While jet-driven watercraft are definitely the "wave" of the future, they may not be suitable for all applications. Propeller drives still have their place!

Jet drives should best be considered as alternatives to propellers, not necessarily as replacements. Jet drives have unique advantages for vessels operating in certain roles. Propeller-driven vessels have other attributes. Matching their attributes to the particular vessel's configuration and mission is advised.

Indeed, much is happening in the marine propulsion arena these days. The trend toward jets is real, primarily because of the various benefits over propeller drives as noted in this article. So, to summarize....Jet driven boats give you more maneuverability, tend to be more efficient, more economical, quieter, safer and more environmentally sound. You'll be seeing more Jet drives on the waves in the future!

Of these reasons, it is the reduced strain on the engines that is paramount.

Pericles

 

Pericles, your article originates from a highly biased source. What is presented here are theoretical qualities and wishful thinking.

Safety- When I started the port engine for the very first time, the jet immediately gobbled up my mooring line, wrapped it around it's shaft and pulled the boat's bow against the jetty. It took the better part of the day to remove the rope from the pump housing. I had to install intake grates to prevent such events, which further reduced the already poor efficiency. And even with the grates, the jets suck up ropes, fishing lines, wood and in shallow waters also large amounts of mud with stones in it. A prop can be cleaned in minutes, cleaning the jet involves diving, cutting and groping around in the dark.

Maneuverability- Jet-driven boats have extremely poor maneuverability, especially at low speeds, because there is only vectored thrust, no grip. Rudders under the nozzles are a marginal improvement only. Even with a twin installation, not hitting other vessels in a narrow waterway is pure luck. Steering response is so slow that I regularly went to the stern to check if the nozzles were still there.
The large catamaran I mentioned earlier performs just as badly as my 26 ft. cruiser.

Economy- Fuel economy is about half compared to props. Where you can cruise doing 8 knots at 1500 rpm with props, the jet-driven boat hardly moves at all. Optimum throttle setting is around 4000 rpm with the sea boiling around you and jetplane-like noise from the exhausts. Using low tide on a beach to do service is an option only if you turn the boat upside down or dig a tunnel to the intake holes. To remove the numerous clams, oysters and other shells from the impellers and pump housings, you must remove steering levers, nozzles, bowls and pull the impellers. The flood will surprise you long before everything is reassembled (with new gaskets).

Comfort- Exhausts are not through the propeller hub, but through the transom. Plus: more revs means more noise. Vibration from oysters using the impellers as a merry-go-round is felt throughout the boat because the pump housing transmits it directly to the hull.

Environment- There are 10 berths around the jetty. One has 3 ft deep crater-like holes, exposing bare rock and large stones. I keep my boat moored over it so people can't see what the jets have done to the seabed.

Other applications "Jet drives have no equal in their ability to stop a planing boat". True only if you don't count anchors, piers and other, lager boats. A Bombardier jet-ski owner told me he once used the reverse deflector for an emergency stop. Both arms broke off and the spoon was catapulted high in the air...

A survey in the nearest marine with well over 600 vessels shows 4 jet-powered objects, each one being a fun craft stowed on deck of a cruiser. There may be a future for jets, but we won't live to see it.

 

Well, you either like them or you don't. The ferries are always on the go, so shellfish have a hard time clinging on. There are such things as underwater exhausts. Both Hamilton http://www.hamiltonjet.co.nz/blue_arrow and Ultrajet http://www.ultradynamics.com/sections/control/index.aspoffer joy stick control.

You have experienced troubles with ropes, try removing steel wire from your props as a result of running over a poorly marked lobster pot at night. Yes, they shouldn't, but they do.

As with most things, it is the inappropriate use of an item that causes grief, like using an electric egg beater when suited and booted to go out for the evening. I should have used an apron and a fork.

BTW, you live in a beautiful part of the Med.

Pericles

 

Manueverability & Crosslinks

I know this factor to be one of the most quoted negatives I hear...poor manuerverablity at low speeds...no 'grip' to the water as with a prop.



I'm going to provide a few crosslinks to other discussions I've introduced on the subject of Jet Drives vs Props

Jet, Jet Pump, Waterjet, Jet Drive vs Prop
http://boatdesign.net/forums/showthread.php?t=11254

Jet Drive vs Prop
http://www.yachtforums.com/forums/technical-discussion/4470-jet-drive-vs-prop.html

 

I looked at the diagram (I had seen it before, too), and I still don't know why efficiency and speed aren't related. If I have 500 hp, and the drives have two different efficiencies (including underwater drag of the drive system), then I would expect the system with the greater efficiency to have higher overall net thrust, and a higher top speed. More of the engine horsepower is going towards moving the boat, and less toward heating water.

I also question that diagram, or perhaps I'm not reading it correctly. If we go back to my V-drive vs. Jet example, according to the diagram, the V-drive should be slower, because the conventional propeller prop efficiency shows a sharp decline after 20 knots, and crossing over with the jet drive at 50 knots. Yet, a 500 hp V-drive will typically do 90 or more mph, while the same jet will do just over 70. I would certainly like to know where this data was derived, because it doesn't seem to apply to small high performance boats.

Regarding cost effectiveness, I can see your point in terms of maintenance and other factors. Fuel efficiency, on the other hand, would again be directly related to the drive efficiency multiplied by the engine efficiency. Every person I have ever spoken to admits their jet boat uses more fuel than a propeller driven boat. You can also go to boattest.com and get a rough idea that this is true, although the samples are limited for jet propelled boats.

So my original question remains. What conditions will a jet drive be more efficient (or as efficient) as a propeller driven boat, or are the jet drive manufacturers blowing smoke when they say their drives are as efficient as propeller drives? I'd like to see a real world example.

Michael

 

Speed and efficency aren't related because you can always just throw more horsepower and less weight at the problem (i.e. the jet aircraft syndrome). What more speed with a jet drive? Go to 600 hp. Oh, whats that?...You want constant HP...ok, then a jet drive is faster because you can use a lightweight GT to have a vessel with 1/2 the displacement of the prop boat has to carry around a couple of large chunks of iron. What?...now you want constant displacement? Ok, now the jet boat has twice the range or carrying capacity, which makes it the better choice as the propboat cannot meet the transport capacity needed. OOOHHHH....you want everything (weight, HP, torque, SFC, fuel type, etc) artifically constant between the two propulsors. Which leads to your second comment.

First of all, a V-drive is an arrangement of prime mover to shafting. It is not a propulsor. And it seems that you are reading the figure wrong because it appears you do not understand the differences between a conventional propeller, a transcavatating one, and a supercavatating one. No conventional prop is going to do 90 mph at anywhere near 50% efficency, you need a supercavatating one or a surface piercer for that. The figure shows you that point clearly, so I don't understand your statements. Additionally, only very small and light weight vessels will ever approach 90mph on 500 hp and only then in very calm water. I believe that the data in the figure was done for a small patrol vessel (~20m, 35T IIRC) which has different needs than maximum speed.

You are correct here, and the figure shows, that on that one point, with all else being equal, the effiency of a properly designed and sized jet propulsor will always be less than a properly designed and sized open propeller due to the losses the jet suffers from wall friction. But, as I pointed out above, and as others have discussed, absolute propulsor efficency is not the only reason for chosing a given propulsor type.

You can never say that a prop is always better than a jet. What you can say is that for a given application and requirements, a propeller better fits the needs and requirements. The life of a Naval Architect is not simple decisions...get over it. The Fun is in getting to make the choices.

 

After reading the ongoing debate, I thought I might add about my experience with jet drives.

I have been following this thread, as I have an interest in using a small jet drive in a Yacht tender of say 14 feet or so, and I am gathering as much knowledge on the subject as possible. A jet drive is of interest, I know of its drawbacks, and also its good qualities. Years ago when I was a teen, Dad had a ski boat with a Berkley jet drive powered by an olds 455. I got to tell you. That boat provided us with more fun than a teen could ever ask for. You could almost bury the sheer on a hard turn; I have never seen a 20 foot boat that could turn so sharp. You could stop so fast and hard, that the nose would just about dip in the water. You never had to worry about chopping up one of your ski buddies; in addition, when approaching a downed skier, at slow speeds, its steering was very responsive. You could swing the aft end port or starboard almost instantly to position the rope however you decided. It would pull 12 skiers, but we never had a need for that. Did it suck gas? Probably I can’t remember it being a big issue. If I am remembering right it used about 10 gallons per hour. But consider this, there was no such thing as going slow, and very seldom a slow start. “Balls to the wall baby, watch this thing fly.” It would really jump out of the whole.

O-K, now I am in my forties, and I’m not quite as crazy; I still think of the virtues of the jet drive. I think there is potential for a small jet drive for a tender:
The ability to beach the tender without wrecking a prop.
The ability to launch in the surf and apply throttle almost immediately. (I’m sure there would have to be a strainer for sand)
The ability to maneuver around divers without worrying about cutting someone.
Not having to worry about someone stealing the out board. (Although you may have to worry more about them stealing the whole tender)
Lower center of gravity for extra security in ruff weather.
Tighter forward and reverse control when approaching and docking to a swim platform in heavy weather
Better protection against damage when you mess up, and the waves launch you onto the beach, or worse yet, roll you over.

I know there are negatives: parts availability, weight, fuel economy, complexity.

Some thoughts would be appreciated.

 

Tom,

I wouldn't hesitate to build a yacht tender with a jet drive. I think your main challenge would be packaging, if you are trying to keep it to 14 feet. The drive will stick out the back. Parts are readily available for jet drives. The jet drive itself is light, being made out of aluminum. Fuel economy shouldn't be a huge concern unless you plan on going long distances. Jet drives are quite simple, so no worries about complexity either. Depending on what engine you use, you'll need to be sure the engine matches the drive, so you get the engine operating in its target rpm for best horsepower. If you are going to leave it in salt water for long periods of time, drive corrosion could be a concern.

Michael

 

I don't see that it would be that hard to hold these variables constant. Some boat manufacturers build or have built the same basic hull in both V-drive and Jet, eg. Sanger and Hondo. The weight of a V-drive is about the same as a Jet. You put the same Chevy engine in it, and run some tests. The boat tests I recall seeing, and this was some time ago, the jet was always slower. That to me indicates the drive efficiency was lower for the jet.

I also saw a boat test where the manufacturer provided a sterndrive and V-drive version of the same boat (a 30'+ cruiser), with the same engines, and both had approximately the same weight. The results were revealing (the sterndrive was faster and had better fuel economy). It doesn't prove it to be the case all the time, nor would I want a sterndrive in a cruiser, but the difference was dramatic.

BTW, I would be interested in an explanation of transcavitating, and super-cavitating propellers if someone has one. Which category would a high speed (100 mph) V-drive or I/O fall under?

Michael

 

yes, I will need to do much studying, I also need to consider the design of the tender. Anything over 14 feet is getting pretty big for a tender. Could a jet from a Jet sky be used? I am not looking for breath taking speed; the tender will be a true tender, for taking people and supplies back to the mother ship in all types of weather. Power is probably a bigger concern, along with light weight. I want to be able to beach it, and get back through the surf.

I don't want to hijack this thread, I had some related comments that have lead to these questions. I can ask this question in a new thread.