4. October 2022

Submerged job

When we bought Mabul a few months ago, we knew of some damage and problems, some we had already fixed, but one major repair was still pending….the propeller had to be taken off – under water.

Sea Trial

It all started with unusual vibrations. We felt them every time we motored at any speed. The first time I had heard and felt the unusual vibrations was during the “sea trial”, i.e. the test drive before the purchase. Even a layman can immediately tell that this can’t be correct.

The noise? No problem, according to the two at sea trial

The previous owner already pointed out to me that the cutless bearing (ball bearing that supports the drive shaft under water) is brand new and the motor has been realigned to the shaft. He attributed the noise and vibration to an incorrect prop pitch (angle of attack of the blades), but none of that was a problem. Oh yes and I should grease the prop before I splash Mabul.

At that time I did not understand a word and relied on the expert. He followed the above reasoning, so in the end I was not really smarter, but “reassured”….
So greasing the propeller properly and dealing with the pitch is what you have to do, was my conclusion at the time.

Back on the hard

So far so good. The propeller itself is what I now know to be a Bruntson H5 Autoprop. It has three blades that can rotate independently of each other. This allows the blades to turn in the direction of travel while sailing, thus causing hardly any water resistance. Another advantage is its principle variable pitch. It cannot be adjusted or changed. Conventional, fixed propellers either rotate while sailing (and thus everything up to the clutch in the gearbox) or the propeller stands still when engaged. Neither are great options.

Perfect access to prop on the hard

Anyways, I grease the prop with a grease gun while Mabul is still on the hard. That the individual blades no longer have the best bearings, I notice even then. I don’t notice any play between the shaft and the cutless bearing, also it is true that this is new and the motor is realigned. Well then, what can go wrong, Mabul is ready for the water.


After Mabul is back in the water, I immediately hear and feel those “lovely” noises again. Now I know, greasing was not enough, and since the engine and gearbox purr like a kitten, well more like a fat tomcat, the source of the vibrations must be the propeller.

The problem is not really acute at anchor and also kind of inconvenient. So I always put the propeller problem on the bottom of the list and hope that everything would take care of itself if we just moved the boat a little further with the engine. The thing will run itself in… No, it won’t…

As our planned trip to Carriacou gets closer and closer, the propeller problem becomes acute. Who knows if we will return to the south of Grenada, with its good marina infrastructure, once we have started north.
The first step to solving the problem is simple: clean it up!

The overgrown prop at anchor near Grand Anse, Grenada

After weeks on anchorage with only a few trips, an amazingly diverse coral reef has formed on the underwater hull and also the prop. If I want to know what’s going on, this has to go. Friends lend us a diving bottle – we had bought two bottles from the local fishermen, but they leak…- so we can free the boat and the prop from its coral reef and algae. After half a diving bottle of compressed air and a lot of scratching under water, the outline and the bare metal of the propeller is visible again. Surprisingly I notice already during the cleaning that I can not only turn one of the blades, but also tilt it a little bit! Not good, or already, let’s see.

The Plan

After a short research it’s clear, I probably just have to tighten a screw inside and I’m rid of the tilting, and hopefully with it the vibrations. But to be able to do that, the propeller must be in the dry. Lifting Mabul back ashore for repair because of this is not an option, as it is far too expensive. I have already heard some happy hour stories of underwater repairs, also of changing the propeller somewhere off shore in bad conditions.
Doable then, especially in calm water on anchorage. And what exactly is the difference if I clean underwater with a scraper or loosen screws? In the evening I still tinker with the execution, so that I can start the operation the next day at daylight: I should tie up the prop so that it doesn’t accidentally sink into the depths of Woburn Bay. I also had to secure the tools and in general I just shouldn’t lose anything. So I prepare a bucket to which I tie all the tools, a lead weight comes for the balance still pure and a rope at the top, with which I want to position the bucket directly below the prop, in the hope that dropped screws are caught there.

Diving in a different way

Fortunately, I had the opportunity to learn scuba diving very early aged 13. At that time I spent some school vacations with my family on a powerboat in the Meds and I went underwater with my father, who taught me how to dive. All my life I have been a diver, although with some breaks. This helps me now to concentrate on the activity under water, I completely fade out the diving itself.

I bring the unscrewed zinc anode to the surface

So I dive with my bucket in the morning to the propeller. I can unscrew the zinc anode, clamping screw and lock nut as planned. One at a time, Karin takes the parts from Mabul and hands me more tools, like two cable ties, to fix the key on the shaft once the propeller is loosened. Now the propeller sits tied to our mainsheet actually loose on shaft, you just have to pull it off.
At this point, what should have been a simple task becomes once again a challenge.
The prop sits on a tapered fit and was pressed onto it by the locknut during the last assembly. In other words, even though everything is loose, the prop won’t move a millimeter no matter how hard I pull on it.

Tinkering, welding, screwing….

Back on the surface, I wonder what to do. A proper workshop would now have a puller, the best would be the special puller available from the prop manufacturer. Sounds complicated at first, but quickly it becomes clear, the special puller can be simply rebuilt from a plate with a few matching holes and a welded nut. So I draw a sketch, pack a suitable stainless steel plate and dinghy over to Clarke’s Court to have the welder drill the holes and weld on the nut.

Making the puller during the lunch break for a few beers

With the right tool, everything then goes very quickly. The prop comes off the shaft in a controlled manner, I can fix the key on the shaft at my leisure and finally lift the prop out of the water onto Mabul.

The patient is on board

Now the actual repair can begin. Fortunately, there is a complete service kit on board, with all the special tools needed. After an hour, the loose blade is properly tensioned again and the prop is reassembled. I can no longer detect any play and thoroughly lubricate all blades with the grease gun.

All is well again?

I dive under Mabul one last time and put the prop on the shaft, everything slips, 20 minutes later the prop is firmly seated on the shaft and all underwater tools are back on Mabul.

The last dive to put everything back together again

We start the engine and let the prop rotate forward and backward alternately on anchorage. The vibrations are almost completely gone. What a difference! But don’t get too excited yet. The real test comes the next morning when we leave Woburn Bay for Carriacou. Only then will the engine and prop be running under real continuous load.

Whether the repair was really successful and how our trip to Carriacou went, you can read in my following blog….and already in our BoatCast “Von Fischen, Unterwasserreparaturen und der schlimmsten Nacht auf Mabul”.

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