A new propeller

I wrote this some time ago but realised I hadn’t published it so here goes. This is about how I selected my new propeller (last year).

Having bought the reconditioned engine (Perkins 4.108) with a TMP12000 reduction gearbox (2.1) from John in Benderloch (Scotland) I have discovered that it is fitted with left-hand output shaft. This is a problem as the propeller originally fitted to Anne Marie was for a right hand gear box. If I don’t change it I will be going backwards to go forwards and go forwards to go backwards. Not very intuitive and definitely bad for the gear box thrust bearings as they are not designed to work that way for long periods.

This is not a good idea.

So I needed a new propeller and this requires me to know certain facts about the existing one.

1. The DIAMETER of the ‘swept’ area. This is the distance from the tip of the blade to the centre of the shaft x 2.

2. The NUMBER OF BLADES. In this case there are three.

3. Because the propeller fits on a tapered shaft I need the SHAFT DIAMETER at the boat side and the shaft diameter at the rudder side. In this case the larger one is 30 mm and the smaller one 20 mm.

4. The THICKNESS of the propeller at its centre (hub size) along the shaft. In our case this is 3 inches.

5. Finally we need the PITCH. This is a strange concept (for me anyway). Imagine screwing a nut along a bole. As you turn the nut it moves along the bolt and the propeller is no different. The Pitch of the nut & bolt is the distance the nut travels along the bolt in one complete revolution. The pitch of the propeller is the distance the propeller would travel on one revolution if it were moving through the water with no resistance or drag. To measure this on a big propeller you really need to have the propeller off the boat and on a flat surface.

Now fortunately I have taken the propeller off the boat and removed it from the prop shaft. I have attached a short video of how I calculated the pitch because it is easier to see than to explain but effectively you do this.

Firstly the three blades of the propller are identical so it doesn’t matter which blade you use to do the measurements as they each give the same answer.

Lay the propeller flat on a sheet of paper. Mark the centre point and the point either side of one of the blades at its widest point. This should be the same distance for each measurement from the centre point.

Measure the height of each blade at its top-most point and its lower-most point when lying flight flat on a tabletop. The tells you how far that blade will move when rotated the width of that specific blade.

You need to know how far the blade will move between these two points and that’s a simple measurement from one side to the other on the flat surface. This is because you need to calculate the angle that each blade moves from one side of blade to the other when it is rotating.

In Anne-Marie’s case the propeller angle is 53°. Now circle is 360° therefore it’s a simple matter to know if you divide Trevor and 60 x 53 you get a little over six times. On the basis that you know this you can multiply that figure (six) by the height of the front and back element of the blade (40 Or 1 1/2 inches). This gives a figure of 10.

Therefore the dimensions of the propeller 18 diameter with a 10 pitch.