Thursday, 16 February 2012

An Answer

Two days ago I wrote about the location of the bow thruster in the tube.  Since then I have written to the supplier regarding my proposed location and asking whether there will be any issues.  I want to position the thruster towards one end of the tube thereby improving the available unrestricted storage space. 

Top view of the bow thruster locker

I want to locate the bow thruster to the left of the weed hatch [A] whereas the boatyard suggest it go in the middle of the tube [B].  I suspect there may be sufficient room above the thruster to fit the two 12v batteries.  This would then give us the entire area to the right of the weed hatch for storage.

The bow thruster supplier’s reply to my query was that my proposed location would work but there might be a very slight difference in the amount of thrust at opposing ends of the tube.  I’m not surprised by their comments.  Although I’m no hydrologist, I consider the basic principles of the movement of water to be very similar to electricity.  I know a little about distributing electricity.  Voltage is the pressure applied to push the electricity down the wire.  Current is the amount (volume) of water being moved.  Resistance is the restriction on the movement of current (volume of water) caused by the size of the wire (diameter of the pipe) Obviously the further you want to push the current (volume of water) the greater the resistance.  To overcome this you must either increase the required voltage (pressure), or reduced the resistance by having a larger wire (or pipe).  Power is the voltage (pressure) multiplied by the current (volume of water).

The size of the tube isn’t going to change.  The amount of pressure produced by the thruster impeller isn’t going to change.  So we will have more resistance at the long end of the tube and consequentially get less power at that side of the boat.  But the tube is only 4ft long so I consider the loss of power at the furthest end will be minimal.


Alf said...

But surely if there is resistance pushing the water to the "long" side there will be an equal resistance pulling the water from the long side ?

Tom and Jan said...

A good point. This would mean the power at either end of the tube would be equal!

David said...

Hi Tom

Our bow thruster is a quarter of the way along a 5' tube and can't say there is a deal of difference, but I am wondering if there is room on the left to work without welding blind and risking a poor seal.


Tom and Jan said...

Hi David,
Apparently there is no welding. Three holes are drilled into the top of the tube. One larger than the others, which is for the thruster shaft. The other two are for the mounting bolts.
The prop and gearbox is fitted from inside the tube and the motor from the locker.

nb piston broke said...

dont forget the bow thruster WEED HATCH Paul

Tom and Jan said...

The weed hatch has already been installed between A & B in the photo!
I want to install the thruster to the left of it.

John/Waimaru said...


Just a thought, as we do not have a bow thruster, and therefore have no experience of problems that may arise.

I know photographs can be misleading, but assuming the impellor is mounted at right angles to the tube, does that not make the forward edge very close to the side of the hull in your preferred position. If that is the case does it not make it vulnerable to damage from striking underwater obstructions?

Tom and Jan said...

Hello John,
In the position I would like it the impeller will be approximately 6" from the end of the tube and protected against a direct hit from a foreign object. Each end of the tube has a grate over it to prevent large objects from entering.
I suspect the greatest hazard is a piece of rope or rag. In which case I can access the leading edge of the impeller from the end of the tube. If it has come from the opposite direction I can get at it from the weed hatch.

nb piston broke said...

Im going to SLAP me wrists If you dont hear from me again Tom the man in the white coat has come to collect me

Tom and Jan said...

Paul, LOL :-)