Finally getting to know the quirks of my BMC 1.5.

However, maximum speed through the water is a staggering 2.43 knots.

Engine starts easily and ticks over around 700 rpm. When the gear is engaged, revs drop to 500 and will not increase. Speed through the water is tranquil!

First theory is weed round the prop. I deployed the underwater camera and SWMBO had a brief glance at the prop before the camera leaked. (Dismantled with great difficulty and dried out the interior and then gave it a dose of contact cleaner. Surprisingly it recovered).

I am still on the Sharpness Canal and the weed, stink and the presence of the odd floating (animal) corpse are not enticing me in for another dive.

Lifting the sole, I have access to the prop shaft. With the small leverage available, I cannot turn it.

The possibilities seem to be that the prop is fouled, the cutlass bearing has swollen or the hull shape has changed after first launch.

The shaft is connected to the gearbox via an Aquadrive so any hull movement should be taken care of.

The prop was very free when on the hard so I can't imagine it has changed too much.

The remaining (and hopefully easily fixed) possibility is a fouled prop.

But why 500 rpm and no more? Could it be to do with the power/revs relationship in the engine? I.e. at 500 rpm there is insufficient power to speed up to a greater torque figure.

I will move (tranquilly!) to a clearer part of the canal at the weekend and don the dive kit.

P.s. The engine normally starts pretty easily and I think my previous problems were down to insufficient battery ump. But the other day, it would not start. The reason was that the stop valve had not closed completely. A useful tip for others perhaps.