Mystery Noises
In order to escape the nasty NE swell that had moved in, Rebecca and I left Cane Garden Bay early yesterday and headed westward, motoring along the coast of Tortola with light winds directly aft of us. Eddie, our autopilot, was steering the boat and the engines were running at a steady 2800 RPMs, resisting the swell which was attempting to push us towards the shore. It was a relatively gentle run with only a minor amount of surfing effecting our speed.
As usual, although the conditions were mild, we were paying close attention to the boat as we find that it talks to us continuously through its gauges, movements, smells and sounds. It was this awareness that caused us to notice a peculiar noise coming from the stern.
I should point out that there are always noises on a boat, especially when underway. The engines rumble, the boom creaks, and unused halyards can bang on the mast. Even the propellors make a “wurrring” sound when we’re under sail at a high rate of speed. Add to all this the sounds that the wind and waves generate as they have their effect on the boat. This was a new one though. This noise, which we would later describe as a groan, was intermittent so we didn’t think that it was engine related. We noted that it only seemed to occur when the larger of the waves, perhaps every 10th or so, pushed us along and the autopilot resisted its force to keep us on our heading. Taking turns, we each moved to the transom to listen. We opened the engine compartment door and again, we listened. Could it be the autopilot? Could it be the rudder, or perhaps the hull itself? Regardless, I didn’t like it, and immediately made some adjustments.
I don’t know what everyone else would have done in this situation but I quickly turned off the autopilot, reduced speed and headed away from shore at a right angle. Although this put us on a less comfortable heading, beam to the waves, the last thing I wanted was drama when we were close to a lee shore.
As luck would have it, once on this new heading, the sound failed to materialize again. In spite of that, we weren’t going to ignore it. We wondered, could there be something wrong with the rudder? Did we pick up another line? We couldn’t be sure until we dove on the boat and checked. The question was, where best to do that?
We initially thought that we’d go to Jost Van Dyke as that was where the boat was pointed on that heading, and we suspected that Great Harbor would be both calm and the water clear enough to see under the boat. Unfortunately, it was out of our way as our intention was to head towards Roadtown on the other side of Tortola. We decided then to change course again and make way to Soper’s Hole, hoping to find an available mooring to tie up to.
Twenty minutes or so later, without any additional drama or mystery noises, we motored into Soper’s and grabbed one of the few available balls. As soon we had the boat secure, I grabbed my mask and snorkel and jumped in the water. Note that this is the second time in as many visits that I have had to dive on the boat in Soper’s Hole, the last being to clear a line which had fouled our starboard prop. Fortunately, by this point in the day the sun was high enough that I had reasonable visibility in the water. What did I find during my inspection? Nothing. No, unlike the last time, there was no big line wrapped around the rudder or prop. Additionally, when I checked both rudders, neither felt as if it had any more play than the other. Where did the strange groaning sound come from? It remains a mystery, hopefully to be solved another day.
I’ll add my opinion to the flood of theories sure to come: Eddie straining against those bigger swells to keep One Love on course…
my guess would be that the larger waves/swells pushing you along were the cause, having changed the force acting on the props/drivelines/engines. Next time try changing your rpm to hear if the noise changes. Likely nothing to worry about but it wouldn’t hurt to make sure everything is secure from prop to engine.
Do each of the rudders move freely. A recurring problem mentioned on M4US is tight rudder bearings on some boats, Leopard I think but I’m not sure on that.
Mike
Our autopilot (Simrad hydraulic arm) makes a groaning/whining sound when it’s working really hard. Maybe that’s what you’re hearing?
Hmmmm,
What happens when the propeller tries to spin faster than the drive? Could this be the source of the groan? Did it sound like a diesel truck when engine braking?
(Diesel trucks actually only engine-brake if there is a purpose-built compression brake built into or added on to the engine, otherwise the engine acts as an air spring and doesn’t brake).
If your engines were accelerating due to surfing, or trying to accelerate, could that cause the noise? Possibly a groan through the air intake of the engine(s) as they accelerate?
I didn’t pick up whether you disengaged the autopilot and continued to steer along your original course for any amount of time in order to rule out the autopilot as the source of the noise.
I might not do this while close to a lee shore, but after disengaging the autopilot, I would next put the drives individually into neutral to see if the noise stopped.
After that, I might actually stop one or the other engine.
I’ve heard lots of hydraulic groans. Did it sound like an automotive power steering pump with air in the lines?
Is your steering hydraulic or cable?
Is your autopilot actuated by a hydraulic cylinder?
Is there a valve that could be opening or closing, or a pump that could be automatically coming on at certain pitches?
Are the saildrive units hydraulic, belt, or shaft?
Just a few thoughts/questions…
Dave W.
Given the circumstances you describe Mike, it sounds very likely that, with Eddie fighting the natural push of the swell to keep you on course, it may have put the boat into a yaw situation. This may have been enough to throw the props slightly out of sync momentarily resulting in a harmonic vibration. This would have been heard as a ‘groan’ and even possibly felt in the hull.
That’s my 2 cents worth, FWIW.
Thanks for the suggestion/guesses, guys. Until I can duplicate the conditions we may not know. The surf was by no means large so the auto pilot wasn’t working all that hard. Who knows though.
Hi Mike, Matt from Troubadour here. We met in St. Martin last year. I just wondered if your funny noise was related to the rudder hum I have experienced on various catamarans. On the Hobie Cat I used to own, we called it the “Happy Hum”. It happens on certain points of sail especially downwind when there is less sideways pressure on the rudders. If you Google “Hobie Cat rudder hum” you will see lots of posts in the forums. There is even a YouTube video where you can hear what it sounds like and yes It is sort of a moaning sound. I doubt it is an issue unless you were experiencing it for days on end.
Interesting. Perhaps. Thanks for sharing.