Sketchy information - what model yacht is it? Not knowing the drive train brings up the possibility of failure of one or some or all of the below;

coupling
gearbox
keyways in the shafting including at the propeller

Suggest contact DH Porter and give them your full drive train details and the full factual details of the 3 instances - if they cannot help they will be able to put you onto a company that should be able to.

www.dhporter.com.au/services.html

Hi (again) Brisa
The cockpit gear selector is usually connected to the gearbox lever by a cable.
Sometimes called a Morse Cable.
This cable has an outer sheath and an inner flexible rod/cable that something like a brake cable on a bike.
The outer cable is secured at both ends with a saddle (a u shaped clip) at each end.
Sometimes these clips or the holding screws fail and the whole cable can move instead of just the inner.
This causes symptoms similar to what you have described.
Check behind the cockpit gear selector and the fitting onto the gearbox leaver that the cable hasn't come adrift.
Could be an easy fix. Here's hoping




Gary

Brisa said..
When we went out in the boat today, We had 3 instances where the engine was on at full throttle but the boat wouldn't move at all, We need to get this checked and sorted. Would you please be able to recommend a good boat mechanic. Brisa is moored at Woodford Bay. We were thinking of calling Woolwich Marina but we are not sure if they also have boat mechanics. Any information will be much appreciated. Thanks in advance.

Did the boat move at all not counting these 3 instances? It's possible this Northshore 27 with a Bukh has a saildrive and the propeller has fallen off. Go take a swim and check.

If you have a hydraulic gearbox I would try changing the transmission fluid.

It is just come into my head what the issue might be.............sorry if it is............

Folding props are geared together so that when the shaft turns they spin outwards in unison and still in gear and drive the boat forward - if fwd gear is selected. The water force on the blades is forward and the blades are held at a suitable angle to the shaft so that they thrust aft effectively.

For reverse gear they still spin outwards and remain in gear but the water force on the blades is aft.

So a clear difference in prop loading and with age and wear issues it could be that the gearing on the prop castings has worn and when in fwd gear at the higher throttle ranges the blades fall out of mesh and angle forward and don't thrust aft as they should.

I had a yacht with a saildrive of this make and vintage and got used to the maximum fwd revs that could be applied before there was an audible knock and different vibration feel and the prop blades somehow stopped thrusting the boat fwd to anywhere near what they used to. Had to go back to idle and start again gradually bringing the revs up but not too much so as to keep the blades in mesh.

See similar discussions here;

www.cruisersforum.com/forums/f54/folding-prop-and-sd20-neutral-or-reverse-147832.html

www.ab-marine.com/folding/gori/gori-2-blade/

www.cruisersforum.com/forums/f114/volvo-saildrive-problem-not-engaging-forward-nor-in-reverse-143219-2.html

www.kiwiprops.co.nz/cms/index.php/resources-general/lost-propeller

Brisa said..
Thanks everybody. We will have a look at the cable. Hopefully it is an easy fix. The boat did move but it took a while for it to start moving forward and it doesn't seem to be as fast as it was at full throttle. It seems to work immediately in reverse. We will post more once figure out what is causing this issue.

Hi, do you by any chance have a hurth gearbox? Symptoms sound so painfully similar to my experience.

Thanks for the feedback and close out on this, good to know.