Normal prop noise?

[Tweegs]

First let me state that we are still extremely new to sailing. The boat is an Irwin 39 with a fixed, 3 blade prop.

We were out this past weekend, our instructor had over 30 years of race sailing experience. Cruising along with apparent wind speeds of 9-12 knots and making roughly 6.5 knots, he turned to me and said: “Is that the prop I hear?”

Sure enough, there was a whirring noise that I hadn’t noticed before, and probably wouldn’t have noticed if it hadn’t been pointed out to me. Not overly loud in the cockpit, but I did open the engine room access to verify. There, naturally, it was much more noticeable, and was indeed prop noise. Just noise, the shaft seemed to be spinning true, no apparent vibrations.

The fact that our instructor questioned me about it suggests that it’s not something every boat experiences, the amount of noise, that is. Perhaps he is more used to sailing on boats with folding props? He showed no concern once the noise was identified, but now, after a few days, I can’t help thinking about it.

So, those of you with fixed blade props, can you routinely hear the prop spinning while under sail?

Call it paranoia on my part, last thing I want to do is lose the prop, so I’ll be giving everything above water a good looking at here in the next day or two, and we’ll anchor off next time out and have a look below water. I’ll be looking at anything a rookie like me would consider obvious, but would appreciate some advice on checking things less obvious.

What would be considered a “normal” amount of noise is irrelevant, we all hear things differently and it would be too hard to convey in text anyway. For now, I’m going to assume something is bad until I can prove otherwise.

[Zanshin]

On some engines/transmissions one is allowed to put the drive into reverse in order to stop the prop freewheeling. It used to be recommend for Yanmar engines with Kanzaki transmissions, now Yanmar has issued a note stating that the gear should remain in neutral.

There have been a couple of studies regarding drag in a freewheeling prop vs. a fixed one and the results are mixed.

Do you know the make of your transmission?

[scraph]

I would say it is normal. I experience it with my own (2-blade fixed prop). The screw turns upon reaching sufficient speed and I can hear a slight hum and feel it in the deck. The real concern is whether the output section of the transmission has sufficient lubrication when the engine isn't running (transmissions generally provide their own lubrication flow via an internal mechanical pump) to prevent damage to the transmission from the spinning prop.

You might consult the manual for your specific transmission on whether you should shift to reverse or remain in neutral while under sail. However, the question of whether a free spinning prop will hum ... yes, it will.

[Tweegs]

Ummm,…no… I don’t know the make of the trans
But I’ll find out soon.

I do know that it is a 1980 with the Yanmar 3QM30 engine, it is the original (less than 700 hours on it). Lots in the original log book about replacement parts, the owner was quite dedicated to filling out the maintenance history. No mention of ever repowering it, so I assume the trans is original also. He seemed to be a stickler for periodic maintenance…good for me. Always hired the work done, tons of receipts to show for it.

Talked to a few other sailboat owners who all also concur that it is probably normal. Just the same, I figure it won’t hurt none to give it a good looking over, if I’m not careful, I might learn something.

Wish I could give a little more detail, but the engine manual is on the boat, about an hour from me here.

I expect that by poking around, I’ll find a little play in the prop shaft. In and out, up and down, side to side. I also expect a little gear lash…(or are most prop shafts chain driven?) …forward and reverse. I’ll report the findings to see if anyone thinks they are excessive.

Engine oil is OK, but I don’t remember seeing anyplace to check the trans fluid, I’ll look harder.

[Zanshin]

Next time you sail at speed and hear the noise, put the transmission in reverse and see if the sound stops, or go below and look at the stuffing box area to see if your shaft is turning.