QThe Lister LH150 hydraulic gearbox on the ST2 engine in my 53ft boat works perfectly in reverse and neutral, but in forward it slips on higher revs. I’ve adjusted the bolt inside the box to give a 2mm gap. It runs at about 4mph with no slipping (but it does labour a little producing some black smoke); however, if you give it more revs the prop doesn’t turn any faster.
I have changed the propeller from a 17x13 to a 16x12 and this helped, but I still think it is over-propped because of the black smoke. I do not have a reduction box. Do I need to look at clutch mechanism, or is it still slightly over-propped causing slip?
GARY, via email
ATONY REPLIES… My prop on a 54ft boat with a 2.5:1 gear reduction and a 35hp engine is something like a 14x12. As a Lister LH150 with no reduction box fitted gives a direct drive (1:1) and the ST2 produces about 20hp, I would say your boat is over-propped. That explains the smoke but not the slip.
The LH150 uses oil pressure to give neutral and astern, while ahead is engaged by the removal of oil pressure from the ahead piston, allowing a set of strong springs in a hub behind the ahead cone clutch to engage it. There are two possibilities. The first one is that the circlip has fallen off the clutch extension that holds the clutch into the spring hub. But more likely it is old age and the clutch needs replacing. You should also check the free length of the springs in the hub when the box is stripped.
The free play on the adjustment gets less as the cone wears and, just like on old cars, this can cause slipping. The 2mm gap should be okay but when you next have the top off the box, make it a bit wider.
I would sort out the slipping and worry about the prop later. A prop specialist might re-pitch the prop for less than a new one.