Differing revs in fwd and reverse

[Warby12]

Hi, my boat is behaving strangely and it could well be a slipping transmission but I want to be sure before we have to replace it

Perkins 4108 and a transmission which is unidentifiable.

• In neutral it can happily rev past 25,000.
• In reverse full throttle gives 19,400 revs and
• fwd only 17,400 revs.

Last time we tried motoring we could only get 2.5 knots fwd out of it.

The prop is a J-Prop feathering prop which has adjustable pitch
My question is what could cause different engine speeds in fwd and reverse? Sipping transmission I guess but the speed is very consistent not wavering up and down.

Anyone?