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Drive problem

I have an Isuzu 35 engine coupled to a PRM120D gearbox via a centraflex shaft coupling. When selecting forward drive the contact seems quite heavy and is not smooth like on other boats I have owned. This was picked up during a recent survey and as a result a new drive plate was fitted with little or no effect. My boat is 6 years old. Any ideas please.

Asked by: Mike Jordan  | 4.43pm, Thursday 3 November

WW says:

In this installation the propeller thrust is being taken by the gearbox, without an extra thrust block between the propeller shaft and the gearbox.
When forward gear is selected, the propeller pushes towards the boat, with the opposite happening in reverse. It sounds as though the thrust bearing in the gearbox is worn, with the forward shaft pressure moving the clutch plates. So when forward gear is engaged the propeller thrust engages the clutch quicker than it would normally engage giving a jerk or thump. Reverse is used much less so that works as designed. The other possibility is that the clutch plates are worn, but this seems unlikely in a six year old boat.
The solution is to recondition the gearbox, but the real challenge is how to stop it happening again. It is worth checking your engine mounts are not badly worn. The best solution is to fit an external thrust block to take the thrust off the gearbox. These often have CV joints to allow for any misalignment of the engine to the propeller shaft. Although you already have a flexible coupling, this might not be sufficient if your installation is poorly aligned.

Rupert Smedley  | 12.07PM, Friday 4 November

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