View Full Version : Jet power vs Prop for sailboats?
05-24-2003, 09:10 AM
At risk of offending the die-hard traditionalists in the Forum, I raise the following question: Has there been any discussion or consideration given to using the hydro-jet method of propulsion for auxiliary power in sailboats? I would think the mechanism could be engineered so as to provide very little drag when compared to a prop. Maneuvering at very low speeds would be a challenge to be surmounted, but I have great faith in the creative resources of our Forumites.
Is there such thing as "jet walk".
05-24-2003, 09:16 AM
An interesting question. I think of water jets as being used almost exclusively on high power planing craft, but I don't know about the engineering questions involved in lower power outputs. It'll be interesting to hear what people say.
05-24-2003, 10:11 AM
Somewhere not long ago, perhaps as a link on this Forum, there was a link to a site mentioning a jet powered canoe for sale!
05-24-2003, 11:35 AM
Won't the great big hole in the bottom of the boat have a lot of drag when sailing?
I think that jet drives are less efficient than props. Consequently, for a given amount of propulsion, you'll need a bigger engine.
Long ago (middle 1960's) my uncle Floyd had a waterjet outboard, about five HP IIRC. It worked well on the transom of a twelve-foot aluminum fishing boat, and indeed did not have problems with going through weeds.
05-24-2003, 02:15 PM
I think the additional expense of the jetdrive components would exceed the benefits, but perhaps it depends on what size boat you are thinking of. Is drag from a prop on a sailboat really that much of a problem? A folding propellor ona cruising design would be alot cheaper than a jet drive unit.
There is a 38' dual jet powerboat (with bow thruster) moored in our harbor. The owner seems to have a great deal of difficulty maneuvering at slow speeds. His attempts to pick up his mooring remind us of those little black and white magnetic Scottie dog plastic toys which repel each other. He'll approach the mooring and the wash from his jets or bow thruster will push it away, so he circles around or backs off and has the same problem time and again.
I've heard another story, which could easily be apocryphal, that Hinckley HAD to develop and offer the JetStick control mechanism for their jet boats because the coordination required by the average owner to manage the jets, thruster, helm and throttles while mooring or docking was overwhelming.
[ 05-24-2003, 02:17 PM: Message edited by: rbgarr ]
05-24-2003, 06:06 PM
a sailboat is traditionally a full displacement hull...waterjets like planing hulls...not displacement types....
05-24-2003, 06:10 PM
I have only run a jet boat one time and if I rember corectly it had almost no thrust in reverse
not a good thing when trying to dock a heavy sailboat. smile.gif ....Phil
05-24-2003, 09:32 PM
There are a couple Hinkley jet boats down by the head of the harbor in Camden. A good spectator sport is to watch the Wayfarer yard crew try to dock the things in their slips in the spring smile.gif (Then, they won't move again for the entire summer, but that's another topic....) They are a barrel of fun scooting around at high speed (if you can justify half a million for a toy), but are extremely squirrely around a dock, due mostly to their lack of any kind of latteral resistance.
Jet drives are great if you're going over 40 knots, have a shallow hull, or operate in an area where there is an abundance of objects that get wrapped up in props. Otherwise, it's hard to justify the expense. Mounting one in a sailboat hull would present quite a few engineering problems (read: more expense), depending on the hull. The nozzle would need to be on the centerline and at or just above the waterline, which would mean that between the intake and nozzle you would need to jog around the rudder post, keel, or whatever else is in the way. The nozzle and bucket would need a flat, vertical (or close to it) surface near the waterline to be effective, which would eliminate most sailboat hull designs, or at least those that look good. I doubt the intake for the drive, when sailing with the motor off, would present much less drag than your average feathering prop, without engineering some kind of door or something. Finally, you have all this junk (nozzle and bucket) hanging off your hull that you need to hide or leave looking mechanical and ugly. Systems like Volvo SailDrive, if you have a hull that could mount one, present almost no drag as they are, and it would be hard to improve on that (SailDrive: vertical shaft through the hull, 90 degree joint at prop hub, folding prop, all faired and foil-shaped).
For what it's worth, no, there isn't any 'jet walk.' It does, however, take a few practice rounds to wrap your head around how the system maneuvers. Once you get used to it, you can really do some amazing things. The thing with jets is that you can leave the engine running at high revs and use the bucket for forward, neutral, and reverse, and really whip the boat around. Takes some nerve doing that in close proximity, though.
Did know a guy that had a jet-powered inflatable tender for his boat, and it used some other sytem than a bucket for reverse (I think it may have just reversed the drive, something that would be exceptionally inefficient). This was probably something like Phil used: It had diddly squat for backing power. Coming to a dock was very much like playing bumper boats. Bucket systems like I've played with can stop a boat from full speed in a boat length. Hold on tight! :D
[ 05-24-2003, 11:53 PM: Message edited by: JeffH ]
05-27-2003, 10:21 AM
Most jet drives are not very efficinet at slow speeds. A jet drive of the tractor type, high volume/slow speed units developed for the fishing industry in the Pacific northwest, might work in certain transome designs with inboard rudder. In general the thrust directors of all jet drives are pretty much in the way and it's true that the hole would create some turbulence when sailing.
Personally, I find it hard to envision an approach much better than controlable pitch of the hundestead or sabb types - gives you the right pitch whether beplonking along in a calm or punching straight into 45kt head wind or towing a disabled friend.
05-27-2003, 10:31 AM
There was at least one Dovekie equipped with an inboard water-jet drive by Edey & Duff. It used the powerhead from an outboard coupled with a jet nozzle assembly, all mounted under the stern seat.
It was expensive, but did preserve the ultra-shoal draught of those boats.
05-27-2003, 03:08 PM
I can honestly say i wouldn't recomend a jet drive for a sail boat. Here in the northwest there are a lot of "jet sleds" that are mainly welded aluminum boats used for fishing. They are mainly designed to run shallow water. But as said before they are very ineffecient. With a outboard jet you loose 1/3 of the rated horsepower of the outboard motor. I'm not sure of the change on an inboard but, it should be about the same. All the jet sleds i've fished out of are terriable in reverse, the impeller keeps going in forward and a small bucket is lowered over the nozzel and forces the water in the other direction. They are also (as stated before) not a lot of fun to dock. A jet (outboard at least) is always in gear, the impeller is always turning (there are a few exceptions though) so you always have some forward mementum from a jet drive. As for going over or through weeds they still have there problems, weeds, sticks etc. will plug up the intake grate and you there for won't get the performance required for planeing hulls (this wouldn't matter so much for a sailboat) but with a planeing hull this can be a problem. Sometimes you get lucky and can just shut the motor off and the weeds will fall from the grate (lack of suction) but if the weeds are sucked up past the grate and in/near the impeller you have to kick the motor up (outboard in this case), go for a little swim, or limp to shore. Many people think this isn't to big of a deal until they try crossing the Columbia River bar and suck some kelp into the intake great. The last thing you want to do is shut down your motor crossing the bar and pick out some weeds! Don't get me wrong, i love jet sleds but, they have there place as do jet powered motors. I will also say all my experiences with jet powered motors have been on jet sleds in the 16'-24' range, so some of my info may not apply to what is in mind here, but i thought this might be of a little help in decideing on what to do. Hope this helps smile.gif
05-27-2003, 07:41 PM
If a guy with a 38' dual jets AND bow thruster has problems docking his boat, I'd think that says more about his abilities as a sailor than his propulsion system.
The passenger catamarans around here all have jets. They certainly don't seem to have any problems with maneuverability; they turn around their own axis, and never seem to have any problems docking even in high winds. And they don't have bow thrusters, either. Of course, the crew seems to know what they're doing, too.
I once did some minor diving work in the navy, on an MTB that had been retrofitted with jets for research purposes. The crew said the maneuverability was amazing compared to conventional screws. They also said they could stop real fast, because the jets can be slammed into reverse at full throttle with no danger of busting the gearbox, twisting the shaft or even tearing the engine off its mounts.
My only experience with jets on smaller boats are offshore MOB boats with a single jet. They primarily have jets for safety purposes. Never had any problems maneuvering it, either IIRC.
The thing to remember about jets is that they need power to turn efficiently. Laying off the throttle when trying to make a tight turn will only make the turn wider. Instinctively pulling back the throttle when making an emergency maneuver could lead to disaster, as happened with a passenger catamaran north of Bergen around 1990. The captain, discovering that he had missed a light, turned hard to port because he knew he had rocks straight ahead. He also instinctively backed off the throttle. The boat ran into the rocks, killing several passengers and injuring many more. If the captain had made the turn with full throttle instead, the boat would probably have made the turn safely.
05-27-2003, 08:08 PM
The power-on-the-throttle/steerage connection you make is an important one. Jetskis accidents are often related to the very loss of control you mention:
A jetskier notices (often very late) he's on a high-speed collision course, backs off the throttle, turns the 'handlebars' (now without thrust for steerage power) and coasts (sometimes at continued high speed) right in the direction he doesn't want to go. The correct maneuver is to throw the 'ski into reverse (the nearest equivalent to braking available) or into as tight a turn as possible with the throttle on. Or intentionally capsize the jetski, if worse comes to worst. Many jetskiers (or skippers of jet powered boats, like the owner of the 38 footer I mentioned) involved in accidents or losses of maneuverability haven't learned the new skills necessary... more's the pity.
05-28-2003, 10:53 AM
good point about backing off the throttle. In tight situations that is one of the worst things you can do.
05-28-2003, 06:45 PM
I just thought I'd elaborate a little more on the causes of the accident I mentioned above, as I have been told by my father-in-law who's a first mate on one of the passenger catamarans serving the coast north of Bergen.
The "Seacat" was southbound for Bergen from the Sognefjord on the dark evening of November 4th, 1991. As the boat entered Mj°mnaosen, a sound separating two larger islands, it was probably at its cruising speed of around 35 knots. The captain was watching a lighthouse on the shore. He was in the green sector of the light, and would turn to port as soon as he entered the white sector.
The problem was that the light had a very long period of darkness between its flashes. That had never posed any problem to the traditional coastal fleet, steaming along at perhaps fifteen knots max. But the Seacat made more than twice that speed.
As the boat entered the white sector, the first white flash was, by chance, hidden from the captain's view by a window post. When he saw the next flash, he immediately knew they had gone too far and began his turn while unfortunately also pulling back on the throttle. The Seacat hit the rocks at high speed, killing two and injuring seventy passengers and crew. Fortunately, the boat was driven well onto dry land and did not sink.
The accident made clear the need to seriously upgrade the marking system along the waterways used by these fast passenger boats, and it has now been upgraded considerably
05-29-2003, 10:33 AM
It's an interesting idea. The maneuverability probably would be a plus, assuming there was a directional thruster that could be pointed in any direction. Doubt there would be sufficient thrust for a displacement hull, though. The reason is that at the slow speeds of a sailboat, the prop is working like an aircraft propeller, generating lift at the blades which provides the power to the hull., A jet drive is more of a reaction-type affair, which depends on the boat reacting to the mass of the water being expelled. It's analogous to the disc area of a helicopter blade--which needs to be large to generate the total lift, unlike a small-diameter jet that works best at high velocity.
06-04-2003, 05:24 AM
I have been considering using a high volume centrifugal pump, such as a Honda engine driven trash pump, to provide primary propulsion for my seabird yawl. I believe the most important question, which I have not yet found an answer to, is that of efficiency. If a typical sailboat (i.e. 2 bladed) propeller is no more efficient than a typical centrifugal pump, I think it's only a matter of horsepower and proper sizing of the waterjet piping. I was intrigued to find this link: http://www.schottel.de/english/p_pump_jet/index2.htm
This is a more high tech solution than I need, but the principle is the same. One way to settle this would be to rent a 10 HP engine driven pump and rig up some rudimentary plumbing to test the concept. If it drives my boat at hull speed than the efficiency is comparable. Furthermore, if you have a big leak, all you need to do is reroute the pump suction and, voila, dry bilge! If anyone is still interested in this topic, I would like to exchange thoughts on it.
06-04-2003, 12:09 PM
Before you waste time and money take your pump down to the dock. Hold the discharge hose in the water and feel the force. Now hold a 2hp ouboard in the water and feel the force. hmmm
06-04-2003, 09:46 PM
I have done more research on this and here's what I have found:
1. Pump-jet propulsion is typically MORE EFFICIENT than external propellers in a speed range that starts at more than quadruple my boats hull speed. So I could not justify this arrangement on the grounds of efficiency (under power) alone.
2. Using a simple equation (which is actually used for rocket science) from my physics text book I can easily calculate the gross thrust produced by a given pump (neglecting headloss in the pipes, more on this later). The equation is: Thrust = |Ve * dM/dt| where Ve is exhaust speed and dM/dt is the rate of mass exiting the nozzle. It is worth noting that the waterjet does not need to be in the water, hence it produces NO drag under sail or under power.
3. It is very difficult to find a direct correlation between engine horsepower and peak thrust, but so far I have concluded I'd need a pump of about 18 HP to equal the thrust of a 10 HP outboard (about what I need for hull speed). I am still working on refining this.
4. The thrust provided by a pump is essentially instantaneous and not hampered by cavitation, say, when you're about to hit the dock and throw it into "back full emergency".
5. Keeping the length of piping to a minimum is paramount to overall efficiency under power, not to mention cost, complexity and potential flooding.
6. Some research has been done by the Navy in directing the thrust to minimize hull drag, boosting the efficiency of this arrangement for slow (displacement) boats like mine.
7. The pump-jet system is impervious to choppy seas which seriously degraded the capability of my last outboard. This situation and maneuvering in close quarters are when I feel I most need to rely on my engine, and in both cases the pumpjet wins hands down.
8. I could go on, but I'll stop with the obvious: it can't be fouled by a stray rope. Can I say rope or is that not salty enough? Stray line.
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