Episode 26 Wobbly Prop

The first problem to sort out on Bolero was its wobbly prop. It was an issue that had been pointed out by the previous owner and the Surveyor in the pre-purchase survey. The prop (or propellor) was OK as long as I did not try to take it about 3 or 4 knots, at which point it would try to shake the transom to pieces.

It was an older style flappy-folding prop. Folding props will give you about half a knot over fixed-blade props because their blades fold together when not in use, this streamlining the flow of water and reducing turbulence. Also, they will not spin the shaft when sailing without the motor, which could reduce wear in the seals. The flappy-folders have blades that fold independently on a pivot pin, meaning that one could be up and the other down, as shown in the photo below.

However, folding props wear out, like all props, and can become unbalanced. When they get out of balance, the eccentric forces shake the back of the boat. Not only is this uncomfortable, it will actually damage seals and bearings. When we removed the prop, we found that it had destroyed the cutlass bearing, which is the part sticking out from under the boat just in front of the prop.

Old flappy-folder prop

Because I liked the idea of an extra half-knot, I decided to replace the flappy-folder with a new geared-folding prop. These newer designs have sprocket-teeth on the blades so that they fold symmetrically. They open up with the centripetal force of the spinning shaft, and close under the pressure of water as the boat travels forward. They work in reverse, because the centripetal forces are enough to open the blades against the water pressure, which tries to close them.

There is quite a science to fitting the right prop, including boat size, engine gearing and prop size. I took the advice of David O'Shea at Speed Propulsion and got a Brunton Varifold 2-blade prop, 14 inch diameter. As Murphy would have it, the one-inch drive shaft was a tad too short for the new taper. Apparently, there is now a US standard and a European Standard, but no one manufactures to the old Australian dimensions any more. So, I needed a new drive shaft as well. The prop itself was about $1760, and the labour, shaft, cutlass bearing and assorted bits came to another $1500, or thereabouts.

I got a little worried that the old prop had cracked the hull round the base of the cutlass bearing (where the protrusion meets the hull), but the only cracks I found were cosmetic, due to minor delamination between the GRP hull and bronze cutlass housing. 

I also got a little worried when we put the boat back in the water, and the new seal started to dribble. Left unattended it would have meant a sunk boat. You need to "burp" the seals after slipping by squeezing the air out, then they become dripless. Once I had done this, by means of considerable contortions over the engine block to get to the seal behind, it kept the water out nicely. It seems that one of the tradies had done something similar earlier, because a couple of the electrical wire connections to the engine had been knocked loose. I got them fixed up with some soldering and new spade connections - my first attempt at diesel engine electrics.

New Brunton Varifold Prop