Greenheart
Well-Known Member
Indeed. By the way...quite a warm pro & anti discussion about bow-thrusters, is going on right now in the Yachting Monthly Scuttlebutt.
why are bow thrusters so noisy ?
- Thread starter sarabande
- Start date
BoyBlue49
Well-Known Member
Simples
The reason your bow thruster was noisy is because it was mounted too near the water line and was sucking air in with the water. The noise you heard was the air/water being pumped through.
A bow thruster needs to be mounted ideally 6" below the water line to be silent. many modern boats are too shallow in the bow to allow this.
To achieve sufficient depth they are sometimes mounted further back which reduces the turning moment and is therefore less efficient.
The reason your bow thruster was noisy is because it was mounted too near the water line and was sucking air in with the water. The noise you heard was the air/water being pumped through.
A bow thruster needs to be mounted ideally 6" below the water line to be silent. many modern boats are too shallow in the bow to allow this.
To achieve sufficient depth they are sometimes mounted further back which reduces the turning moment and is therefore less efficient.
Fire99
Well-Known Member
Another point is that I'll hold my hands up and say that on more that one occasion, i've made a hash of mooring my boat. OK I always urge on the side of careful and always have plenty of fenders etc but it hasn't stopped that unexpected gust of wind or me forgetting something.
An extra tool in the box is never a bad thing.
An extra tool in the box is never a bad thing.
dslittle
Well-Known Member
I have sailed for many years. My current boat is the first one on which I have had the added luxury of a bow thruster. I use it almost everytime I moor up. I did the RTIR on a chartered boat that didn't gave a bow thruster and managed to moor it. There was a time when I navigated the Swedish Archipelago on a yacht that didn't even have a depth gauge but make great use of my current one. Advances in sailing equipment are there to be used...
markdj
Well-Known Member
You are all wrong [ducks for cover] 
The reason some (practically all) bow thrusters are noisy is due to a poor installation, many times from the factory.
Turbulence in the water is causing the noise and that turbulence is caused by having sharp edges on the two ends of the bow thruster tube.
Bow thrusters need to be installed with 1. a minimum of the radius of the tube below the water 2. with a radius at the edge of the tube to be 10% of the diameter.
This feathering gives up to 20% greater thrust as the water flows faster and straighter through the tube.
If you can get the hold of a Side Power install manual, it's all there in black and white - why leading british power boat manufacturers continue to install the thruster tubes with hard (sharp) edges can only be down to cost...
The reason some (practically all) bow thrusters are noisy is due to a poor installation, many times from the factory.
Turbulence in the water is causing the noise and that turbulence is caused by having sharp edges on the two ends of the bow thruster tube.
Bow thrusters need to be installed with 1. a minimum of the radius of the tube below the water 2. with a radius at the edge of the tube to be 10% of the diameter.
This feathering gives up to 20% greater thrust as the water flows faster and straighter through the tube.
If you can get the hold of a Side Power install manual, it's all there in black and white - why leading british power boat manufacturers continue to install the thruster tubes with hard (sharp) edges can only be down to cost...
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Csail
N/A
no idea on noise but i can't understand why boats smaller than mine use them but i cope fine.
Delfin
Well-Known Member
Good question. I have hydraulics on Delfin and they are not silent, but very quiet. I don't know what the electrics are so obnoxious, but they are.
D
Deleted member 36384
Guest
Is the radius at the end of the end of the tube an external curve (bell mouth) or internal narrowing (nozzle)?
SHUG
Well-Known Member
Whatever happened to the water-jet bow thruster idea??
markdj
Well-Known Member
Radius should be applied at both ends of the tube, bell mouth ends as you describe.
mitiempo
Well-Known Member
It is a warning that a large boat is trying to dock and you should get the fenders out.
colingr
Well-Known Member
Certainly the case in Yarmouth on Saturday. We were sat on the balcony overlooking the harbour and a couple of big flybridges came in making a proper pigs of rafting up. Despite much thruster use they still needed the berthing masters help to get along side 
I suppose it make everyone look at them so that they can bask in the envious stares of the unwashed. Mostly though it drew attention to their inability to handle their boats
I suppose it make everyone look at them so that they can bask in the envious stares of the unwashed. Mostly though it drew attention to their inability to handle their boats
pheran
Well-Known Member
You are all wrong [ducks for cover]
The reason some (practically all) bow thrusters are noisy is due to a poor installation, many times from the factory.
Turbulence in the water is causing the noise and that turbulence is caused by having sharp edges on the two ends of the bow thruster tube.
Bow thrusters need to be installed with 1. a minimum of the radius of the tube below the water 2. with a radius at the edge of the tube to be 10% of the diameter.
This feathering gives up to 20% greater thrust as the water flows faster and straighter through the tube.
If you can get the hold of a Side Power install manual, it's all there in black and white - why leading british power boat manufacturers continue to install the thruster tubes with hard (sharp) edges can only be down to cost...
Hooray - the right answer at last. Nothing more to be said.
Greenheart
Well-Known Member
Not quite convinced, yet.
I can see how a substantially bevelled edge on the in/out holes would cut the noise of water rushing through. But...the reduced radius at the opening...it makes the thruster into a...jet? With the tapered outlet increasing the jet's pace?
But, doesn't the similarly tapered inlet on the other side, prevent the jet from working any better than if the 'tube' were all of the same diameter?
I can see how a substantially bevelled edge on the in/out holes would cut the noise of water rushing through. But...the reduced radius at the opening...it makes the thruster into a...jet? With the tapered outlet increasing the jet's pace?
But, doesn't the similarly tapered inlet on the other side, prevent the jet from working any better than if the 'tube' were all of the same diameter?
Ex-SolentBoy
Well-Known Member
I looked into this for my new boat. It's a great idea and very quiet.
The problem is that it required an ENORMOUS inlet (hole in the hull) that was bigger than anything for the engine so I ditched the idea.
Ex-SolentBoy
Well-Known Member
You are all wrong [ducks for cover]
The reason some (practically all) bow thrusters are noisy is due to a poor installation, many times from the factory.
Turbulence in the water is causing the noise and that turbulence is caused by having sharp edges on the two ends of the bow thruster tube.
Bow thrusters need to be installed with 1. a minimum of the radius of the tube below the water 2. with a radius at the edge of the tube to be 10% of the diameter.
This feathering gives up to 20% greater thrust as the water flows faster and straighter through the tube.
If you can get the hold of a Side Power install manual, it's all there in black and white - why leading british power boat manufacturers continue to install the thruster tubes with hard (sharp) edges can only be down to cost...
We installed our sidepower with the tunnel exactly as their manual said.
Its still noisy. The problem is cavitation.
Submarine propellers arent in tunnels and look how much they worry about cavitation.
Did you see that programme on the BBC where they showed how the noise that waves make (not when they hit the shingle) is the sound of the bubbles bursting?
ianabc
Well-Known Member
So how many kN or HP needed for a water jet..
A water jet nozzle could be quite small and could be installed on both sides of bow and stern.
How powerful a motor would be required to pump water to the nozzles?
A water jet nozzle could be quite small and could be installed on both sides of bow and stern.
How powerful a motor would be required to pump water to the nozzles?
Ex-SolentBoy
Well-Known Member
See my previous. Willdo supplied this. the problem is the volume of water required at the inlet.
markdj
Well-Known Member
There is no tapering at the inlets, it is feathering on the outside of the tube all around...
I can confirm that this installation is very quiet and extremely powerful, fitted to a oceanis mid cockpit yacht.
Don't you just love the shape of that hole ?
I can confirm that this installation is very quiet and extremely powerful, fitted to a oceanis mid cockpit yacht.
Don't you just love the shape of that hole ?
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Monique
Well-Known Member
Mine is noisy because it annoys those people who do not need them and like to pontificate about it...
Given my berth, it would be virtually impossible to moor if not for the BT under certain conditions. However, I use it as little as possible.. because I like to keep my boat handling skills sharp.
"As he struts away proudly.... "
Given my berth, it would be virtually impossible to moor if not for the BT under certain conditions. However, I use it as little as possible.. because I like to keep my boat handling skills sharp.
"As he struts away proudly.... "