A glassy smooth Careel Bay beckoned me to do some speed trials. Cos so far there’s always been a bit of wind and it wasn’t clear just how much the wind was contributing to, or slowing her speed.
So I dropped the mooring and started taking notes as she wove out through the moorings.
1630 watts = 4.5 knots
4600 watts = 6.2 knots – full throttle
Strange… in earlier tests I was confident that she was topping out at about 6.5 knots so this is odd to see maximum speed a little less than I expected. Perhaps those readings were wind assisted. Anyway, back to it. I’m using the stop watch on my iPod Touch to time how long it takes to come to a full stop. And then I can do the same test late with the standard prop. Back to neutral and then to full power in reverse.
and the watts drop off to around 700. What was that? Can’t see any obstruction to the prop. Try forward – also in forward the maximum load is about 700 watts.
I suspect the shear pin may have broken. Now I’m worried the prop might fall off, so I throttle back to about 100 watts and she’s going along at 1.5 knots. I haven’t gone far from the mooring, the day is still calm, so I’ll limp back ok at this speed.
Sure enough the ends of the pin have sheared off as designed and have laid against the shaft in such a way that they still grab a prop a little. 700 watts worth actually.
Speculating about the maximum speed being reduced, and remembering having hit some big jellyfish last weekend, its likely that the prop has been slipping just that little ever since the jelly fish encounter. One of them really got hit with the prop and the motor almost stalled, and it was probably then that the pin sheared.
So its rather clever of me to have decided to do this test today in perfect calm, and discover the problem. Otherwise I may not have noticed until the chips were down and I really needed it. I now think the clunk I heard was the pin ends swapping around as the load reversed on the prop.
Its nice that the nut didn’t come undone with the free spinning of the prop on the shaft. And drop the prop into the sea.