Prop wobble

[stranded]

I have a Gorilla 3 blade prop on a shaft through a p bracket. I noticed a wobble the year before last and had the yard check it out - new shaft seal, cutless bearing, alignment checked, prop fold checked. Driving away from the relaunch it was much, much better.

A few days later we set off for France but it was immediately clear that the wobble was back. It is particularly bad in overdrive - feels as if the stern of the boat is actually kicking sideways with each prop revolution. There is a sweet spot around 1850 revs where it is still present but considerably reduced. And it is intermittent in that it occasionally runs smoothly when I put it into gear but then if I put it in neutral and then into gear again the wobble is back.

We muddled through the summer -it was a great way to reduce engine hours. assume the most likely cause is that the prop opening mechanism is buggered. It is 16 years old now. It went back to Gori for a refurb 5 years ago - cost about a grand and I am not inclined to repeat that.

I am putting that out there just in case I am leaping to wrong conclusions. But mostly I am hoping to get a bit of guidance on refitting the original, fixed prop to make sure I have correctly identified the problem before ordering a new folder/featherer - don’t think I’d go with Gori again - the overdrive works great but engaging it has always been a mechanically clunky process which I don’t like.

So, I found the original prop in the bottom of a locker. It had a big bronze cone taped to it, but nothing else. Googling how to make the change I am reassured that it should be straightforward but that I must ensure all the nuts and washers go on in the right order. Thing is I don’t have them at all, never mind in order (unless I can use the Gori ones, but that doesn’t seem a given). Is there a standard arrangement that I can use, or do I need to try to contact the prop maker if I can trace them - likely to be Austrian or German?

Thanks for any and all advice and musings as ever.

 

[NormanS]

Fitting the original prop should be straight forward. The shaft will have a keyway on the tapered part. You will need a parallel key (often referred to as a "feather"). Hopefully this feather is still in use on the shaft. There is usually some mechanism to prevent the cone nut from coming slack. Sometimes a tab washer, sometimes a split pin, sometimes a grub screw. Having a look at the cone nut may give you a better idea of how it was prevented from coming loose.

 

[Tranona]

Love your autocorrect that turned Gori into Gorilla!. The fixed prop should go straight on as doubt the taper, keyway and thread has been changed. The cone is probably the lock nut but without seeing it difficult to know the exact arrangement. Normally a fixed prop would have either a locking washer with a tab located in the keyway slot of the propeller and plain nut that is then locked by lifting another tab on the washer; or a castellated with a split pin through a hole in the shaft. However I expect the Gori has its own arrangement that probably uses a Nord Lock washer
nord-lock.com/en-gb/nord-lock/?gad_source=1&gclid=EAIaIQobChMIzM7Ime6TgwMVtI9QBh0GqANqEAAYASAAEgLgzvD_BwE
If there is no obvious way of locking the cone you may be able to use a Nord Lock with that.

As for a replacement suggest a Featherstream as the best value and arguably best performance. One of the big advantages is that you can have the pitch set at different level from each other which generally improves reverse performance and for some installations is essential as the reduction with many gearboxes is different forward and reverse. Darglow will set the prop to exactly match your installation.
darglow.co.uk/featherstream/ photo of one on my boat, the second that I have had

Hope this helps

 

[stranded]

Thanks chaps - very helpful as ever.

Tranona - read in an old Yachting Monthly test that the Featherstream can sometimes be a bit reluctant to engage in reverse. Is this an issue you have experienced? If so, is it just a matter of technique, or is it a bit of a lottery?

 

[Tranona]

No problems. That was a very early model in the test and was probably not set up quite right. Have not heard anybody complaining about that, In fact most comments from owners are positive about reverse. Mine engages instantly and is very smooth partly because it has roughly 1" less pitch than in forward (reduction ratios are the same) so does not get that surge you often get with a fixed prop so is more progressive in its response to the control lever. Prop walk tends to be less pronounced and also more progressive.

 

[Sandy]

Have you checked your P bracket?

I had a 'rumble' at certain revs and it turned out the P bracket was moving by about 2 mm at the shaft end!

EDIT: 2 mm not 20 mm, that would have been a nightmare!

 

[stranded]

Hi Sandy. Darthaven gave it an end to end check last winter and didn’t find anything apparent. That, and the fact that it is intermittent, leads me to think it is something to do with the prop unfolding which isn’t picked up in an out of the water test. Putting the old prop on to see if it disappears while out of the water anyway seems the cheapest way of testing that theory - I’ll sea trial it and decide if the sail speed penalty or spinning prop noise is too irritating, in which case I’ll lift it again and swap it for a new prop - Tranona has put featherstream in pole position - otherwise manage til next winter. But of course if the problem continues it’s going to banging my head against a reinforced bulkhead time…

 

[stranded]

Have just received a message from Darglow in response to my request for a quote for a Featherstream prop. In it they say

“Yanmar 4JH3-TE (75HP@3800RPM) We would cap this output to run a larger prop size as per original prop sizing.”

Don’t understand what this means. Somehow reducing the power of the engine? Or that fitting a larger prop would itself limit the maximum power, or revs? The inference is that this is what the OE prop did. Why?

 

[Tranona]

Yes, he is suggesting a bit of "over propping" to reduce the maximum rpm and therefore the cruising rpm. The boat is probably over powered and does not need to use full output from the engine to achieve hull speed. A larger prop either diameter or pitch - but I guess he means diameter is better for motoring and as the blades feather, no penalty under sail. We did exactly the same with my project. The boat needs 26hp for hull speed and the engine has 29hp. So we went for a bigger reduction ratio (2.5 rather than 2) and a 17" diameter rather than 16" and pitched it to max at 3300rpm rather than 3600rpm. This brings cruising (5.5 knots) revs down from 2400rpm to 2200rpm. All still within the engines designed operating range. Big advantage for my boat is to get the blades out into clearer water plus the 17" blades have substantially larger blade area.

While the numbers will be different for your boat, the principles are the same.

 

[scottie]

Have just received a message from Darglow in response to my request for a quote for a Featherstream prop. In it they say

“Yanmar 4JH3-TE (75HP@3800RPM) We would cap this output to run a larger prop size as per original prop sizing.”

Don’t understand what this means. Somehow reducing the power of the engine? Or that fitting a larger prop would itself limit the maximum power, or revs? The inference is that this is what the OE prop did. Why?

Ask them to explain what they mean

 

[stranded]

Yes, he is suggesting a bit of "over propping" to reduce the maximum rpm and therefore the cruising rpm. The boat is probably over powered and does not need to use full output from the engine to achieve hull speed. A larger prop either diameter or pitch - but I guess he means diameter is better for motoring and as the blades feather, no penalty under sail. We did exactly the same with my project. The boat needs 26hp for hull speed and the engine has 29hp. So we went for a bigger reduction ratio (2.5 rather than 2) and a 17" diameter rather than 16" and pitched it to max at 3300rpm rather than 3600rpm. This brings cruising (5.5 knots) revs down from 2400rpm to 2200rpm. All still within the engines designed operating range. Big advantage for my boat is to get the blades out into clearer water plus the 17" blades have substantially larger blade area.

While the numbers will be different for your boat, the principles are the same.

Thanks Tranona - that makes sense. He has also given (insignificantly different) prices for a 21” 3 blade or a 20” 4 blade. Are there clear advantages with one over the other?

 

[stranded]

Ask them to explain what they mean

Thank you.

 

[Tranona]

Thanks Tranona - that makes sense. He has also given (insignificantly different) prices for a 21” 3 blade or a 20” 4 blade. Are there clear advantages with one over the other?

I don't have any direct experience of the 4 blade. Surprised that there is not much difference in price. Advantage is again larger blade area so suspect better motoring in adverse conditions. Talk to Chris and ask which is better for your boat. The smaller diameter might be an advantage if you have a problem with prop clearance, but don't think you will - usually more of an issue with apertures like on my boat.

 

[stranded]

Thanks, I’ll talk to them. Couple of hundred quid difference. So it’s “ouch” either way!