Yamaha24ft said..
I have a Yanmar Diesel engine inboard 8hp.


Now 1 month later the same issue but this time there is no seaweed on the prop but when I rev high and put it in gear (forward or reverse) the prop spins (in water) but the boat doesn't move. The prop doesn't seem to slip and I have no idea what the issue could be.

Any thoughts and assistance appreciated :)

If you put the engine in gear, can you stop the shaft by holding it? That will tell you if the gearbox is at fault. It is possible for the geartrain to fail but the prop continue to turn due to internal friction. If you can hold it and easily stop it from turning, that's your problem.

Yara said..
Or perhaps a little safer, with the engine stopped and in gear, see if you can turn the prop shaft by hand.

And engine de-compressed!

A few things need to be clarified here.
Is it a YSE 8 or a 1GM10?
How old is it?
Is there oil in the gearbox?
Did you verify that the prop was rotating or that the propshaft was rotating?
If it was the prop, how did you verify (IE; taking an dive while engine running)?
Was there any wake?
Did you see the propeller action when revs were applied?
What happened when revs were applied? Did the prop stop rotating then?

I have a YSE 8 that will push the boat forward at idle, so if your prop was rotating and the boat not moving there is something very strange going on.
My first suspicion is your clutch. That is why I inquired about age and gearbox oil level.
I can supply a workshop manual for the YSE with details of the clutch mechanism if that is the model that we are dealing with here.

gary

I just noticed that your name is "Yamaha24ft"
That would indicate a 1GM10.
here is a manual.
www.manualslib.com/manual/791391/Yanmar-1gm10.html

Thanks everyone for the prompt replies and thank you for the links. It's a YSE8 though, I should have probably mentioned that at the top :)

I did go for a dive and checked the propeller (save distance) when in gear and it rotated and accelerated with increased throttle but I haven't checked yet if I can stop it - might opt for the engine off in gear rotation check, don't wanna get too close to a rotating propeller :)

I will check on the weekend and report back here. Thanks for all the tips. Much appreciated as always :)