mark1882
Well-Known Member
I am thinking of fitting a bow thruster on my Westerly Oceanlord, has anyone had or got experience of the Jet Thruster as opposed the conventional tunnel thruster.
Thanks
Thanks
Jet Thruster
- Thread starter mark1882
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PCUK
Well-Known Member
They don't work well. Vetus made them for a short while and then dropped them.
sarabande
Well-Known Member
I think these are a newer design than the Vetus ones.
Worth asking the maker for a reference owner near your area ?
EDIT
I've just looked at their videos.
http://www.jetthrusters.com/video_files.html
Interesting that air seems to be entrained in the output stream,. and I was wondering if that was to exploit a little known phenomenon about injected air increasing thrust in tunnel propulsion. ((Air injected under pressure, and the bubbles expand, creating more water flow.) Or perhaps a cavitation problem, in which case nil effect overall.
Worth asking the maker for a reference owner near your area ?
EDIT
I've just looked at their videos.
http://www.jetthrusters.com/video_files.html
Interesting that air seems to be entrained in the output stream,. and I was wondering if that was to exploit a little known phenomenon about injected air increasing thrust in tunnel propulsion. ((Air injected under pressure, and the bubbles expand, creating more water flow.) Or perhaps a cavitation problem, in which case nil effect overall.
Last edited:
Bobc
Well-Known Member
I have to say that when I first saw them I thought they looked like a fabulous idea.
Seems to work ok here
Seems to work ok here
pvb
Well-Known Member
But it does look like a fairly calm day.
sarabande
Well-Known Member
they quote 120KgF - whatever that is ! - as the power of the big units.
It's the equivalent humph you would need to lift 90Kg off the floor and hold it in mid air.
It's the equivalent humph you would need to lift 90Kg off the floor and hold it in mid air.
Bodach na mara
Well-Known Member
Where do you get the 90 kg from Saraband? 120 kgF ("kilograams force") is the force of gravity on a 120 kg mass.
From what I remember of water-jet theory and experiments with a hosepipe, the available force would be greatly increased if the discharge was above the water level. The jet force is equal to the momentum per second in the jet and if this is in air, the velocity (and thus momentum) is much higher. It seems counter-intuitive, but try it out with a hose and a tank of water. With the discharge of the hose in air, the force of recoil is greater than if the hose end is dipped into water.
From what I remember of water-jet theory and experiments with a hosepipe, the available force would be greatly increased if the discharge was above the water level. The jet force is equal to the momentum per second in the jet and if this is in air, the velocity (and thus momentum) is much higher. It seems counter-intuitive, but try it out with a hose and a tank of water. With the discharge of the hose in air, the force of recoil is greater than if the hose end is dipped into water.
Bobc
Well-Known Member
All you want it to do is hold the boat against a bit of breeze on the bow as an aid to parking, not push it along at 5kts.
KevB
Well-Known Member
Not sure I'd want the hassle of all those extra pipes and skin fittings to keep an eye on. Can't see any benefit over standard thruster, only drawbacks.
sarabande
Well-Known Member
Doh ! Their website says "90Kgf (120 KgF)" and I took the lower number. Any it's quite a push. Imagine pushing a boat's bow round with enough force to lift 2 cwt!
pvb
Well-Known Member
That figure is for their biggest model, which has a massive 14kW motor and requires 24v power. A better comparison would be to look at a typical bowthruster's power rating and see whether the jet thruster has more thrust. My 37ft boat has a SidePower SE60, a fairly typical 3kW thruster which produces 73kgf at 12v. The corresponding 3kW Jet Thruster, the JT-30, only produces 30kgf thrust - less than half the SE60's. On this basis, the Jet Thruster concept is obviously less efficient than conventional bowthrusters and needs much more powerful motors (with associated extra cabling and batteries) to produce comparable results. Having said that, I can see that the concept has some value for stern thruster installations.
William_H
Well-Known Member
The thrust of any propulsion system is dependent on coupling the horsepower of the motor to the water. This coupling depends on the speed of the thing being propelled.
So aircraft boat or bow thruster the principle remains the same. For low or zero speed as in a thruster a large diameter prop with fine pitch is the best way to couple the power to the water. Jet coupling using a small amount of water at high speed can be geat for jet boats travelling at high speed but is very inefficient at zero speed.
It is no different to the fact that we still build prop driven planes for short field low speed performance and jets are at their best at high speed and high altitude.
So the only/big advantage of the jet thrusterr is the small sized outlets and inlets. The efficency for pushing a bow around as shown is very poor.
So yes I would consider fitting jet thruster if I ever needed a thruster but I would consider an engine driven pump with clutch and would fit stern and bow outlets all above water level. All in theory at least good luck olewill
So aircraft boat or bow thruster the principle remains the same. For low or zero speed as in a thruster a large diameter prop with fine pitch is the best way to couple the power to the water. Jet coupling using a small amount of water at high speed can be geat for jet boats travelling at high speed but is very inefficient at zero speed.
It is no different to the fact that we still build prop driven planes for short field low speed performance and jets are at their best at high speed and high altitude.
So the only/big advantage of the jet thrusterr is the small sized outlets and inlets. The efficency for pushing a bow around as shown is very poor.
So yes I would consider fitting jet thruster if I ever needed a thruster but I would consider an engine driven pump with clutch and would fit stern and bow outlets all above water level. All in theory at least good luck olewill
Rivers & creeks
Well-Known Member
OOI - why above the waterline?
johnphilip
Well-Known Member
See #7
Elessar
Well-Known Member
True. But for a bowthruster force is only half the story. You need torque, which is the force times distance. Ie how far away from the keel the thruster is.
A tunnel thruster has to ideally be a tunnel diameter below the water, and in a modern yacht that can mean the tunnel is a long way aft. If the jet can be significantly further forward, it may be a better bowthruster even if it produces less force.
Chalker
Well-Known Member
So why is my main prop system set up below the waterline instead of above?
pvb
Well-Known Member
Modern yachts tend to have fairly fine bows, and the need for seacocks on the jet thruster outlets means they need around 350mm minimum width inside the boat, so they can't go too far forward. The jet thruster outlets also need to be about 100mm below the waterline. Looking at my boat, I reckon that jet thrusters might increase the distance from the centre of the keel by about 20%, not really enough to make a big difference if the jet thruster force is only half that of a bowthruster. Oh, and I'd need to lose the forward water tank.
Bodach na mara
Well-Known Member
Because it does not operate as a jet. I believe that some experiments were done using a prop below the water with a scoop astern of it raising the discharged water and discharging it above water level. I cannot trace them however. When I first heard about the improvement if the discharge was above water, I did not believe it and disputed it with the lecturer. I was told to go away and do the hose experiment.
Elessar
Well-Known Member
Interesting and every install is differnt. The distance times force is important to remember though, same if planning a conventional install a less powerful thruster in a smaller tunnel may go further forward and be better in some cases.
With the jet Id consider a 90 degree fitting on the inlet (same metal) , ( maybe even glass over it ) and put the seacock on that facing aft. That may help in some cases.
Fendant
Well-Known Member
I had a close look at the Jet Thruster as well:
a) I could not find despite postings in two German Forums users who had installed this System and who would share their experiences
( either there aren`t any, or they do not have anything positive to Report )
b) My insurance told me that I hd to close ALL through hull valves when leaving the boat
I went then for a Sleipner Exturn instead which works perfectly, quiet, no cavitation, lot of thrust
a) I could not find despite postings in two German Forums users who had installed this System and who would share their experiences
( either there aren`t any, or they do not have anything positive to Report )
b) My insurance told me that I hd to close ALL through hull valves when leaving the boat
I went then for a Sleipner Exturn instead which works perfectly, quiet, no cavitation, lot of thrust