Flow rate through cutlass bearing with rope cutter

[winsbury]

Having once been caught up on a lobster pot ( or possibly a small submarine judging by how firmly it was attached to the seabed ) I am not in a hurry to have it happen again so wish to fit a rope cutter. My problem is that the distance between the forward side of the prop hub and aft end of the P bracket cutless bearing is not exactly generous and all rope cutter designs will reduce this space dramatically. The cutless bearing is lubricated by water flow so I presume there is some clever calculation that states the minimum distance aft of a cutless bearing that must be kept clear of obstruction to allow adequate water flow. I also imagine such a calculation would include variables for shaft diameter and maximum revs.

Does anyone have a table / formula for this ?

PS: Apologies for the spelling of cutless in the title but can't seem to edit it now.

PPS: An US website suggests a flow rate of 2 (US) gallons per inch diameter of shaft but makes no reference as to how this is achieved in practice.

 

[Tranona]

Having once been caught up on a lobster pot ( or possibly a small submarine judging by how firmly it was attached to the seabed ) I am not in a hurry to have it happen again so wish to fit a rope cutter. My problem is that the distance between the forward side of the prop hub and aft end of the P bracket cutless bearing is not exactly generous and all rope cutter designs will reduce this space dramatically. The cutless bearing is lubricated by water flow so I presume there is some clever calculation that states the minimum distance aft of a cutless bearing that must be kept clear of obstruction to allow adequate water flow. I also imagine such a calculation would include variables for shaft diameter and maximum revs.

Does anyone have a table / formula for this ?

PS: Apologies for the spelling of cutless in the title but can't seem to edit it now.

PPS: An US website suggests a flow rate of 2 (US) gallons per inch diameter of shaft but makes no reference as to how this is achieved in practice.

Not a problem. Look on www.ropestripper.com and you will see that you can fit a spacer between the gearbox and the coupling to move the shaft back. Available in 5mm increments so that you can maintain the correct gap of around 15mm. Very common arrangement and works perfectly well.

 

[winsbury]

Thanks, yes a spacer would increase the amount of space available and solve the fitting issue and your comment of 15mm sounds about right to my ear and is also about what I currently have.

There are other threads that cite anything from 2mm to 20mm being adequate. The engineer geek in me wanted to know if there is a more scientific way to work out the optimum distance knowing initially that too long and the shaft will be insufficiently supported and can vibrate, too short and water flow through the bearing is impeded, either of which leads to premature bearing wear; somewhere in between is nirvana. However in a moment of clarity I realise the full engineering answer has the potential for hundreds of pages of very complex maths for fluid dynamics, resonance and material deformation and all with such a level of uncertainty that even after hours on a supercomputer the answer will probably be 15mm+- 2mm.

So, ... 15mm it is then.

 

[Tranona]

It is not that critics!. Remember you also have also for the shaft to move forward and back as you go in and out of gear as well, which can be 3mm either way. Also a P bracket has good flow being open at both ends.

 

[vyv_cox]

My experience with a clearance of 20 mm or maybe more with a 1 inch shaft is that some rotordynamics problems can result. The propeller, especially a folding or feathering one, has a large mass and needs to be supported by the bearing as close as is practical. My prop shaft hits the stern tube when motoring in lively sea states, which is theoretically impossible as I have an Aquadrive. It can only be happening because the shaft is deflected. I need to shorten the shaft a little to reduce my excessive clearance.