Question for all gearheads

[sailaway78]

Here's the situation. I was heading into Biloxi, in the east channel. It is only a little over a mile long, and fairly narrow. Skinny water on both sides. I have a thunderstorm pounding me pretty good. 30+ knots and a very nasty following sea. About half way up the channel, the wind shifts, and picks up. Gusts approaching 60 knots, dead on the beam. The boat is pitching with the following seas, and heeling over 35 or 40 degrees to port. Wallowing real bad. I'm managing pretty good, but it's work. All of a sudden something goes off, sounds like a AK47, on full auto. Very loud. And sounds like it's coming from under the boat. My mind is trying to solve two problems at once. What is wrong, and what am I going to do if something breaks. There is no way I can get anything done in this channel before I get blown out of it, and find the bottom. I throttled back to about 1500, the frequency of the noise reduced as the RPM came down, but was just as loud. Then after a lifetime, it just faded away. Real time less than a minute. About 20 minutes later the exact same thing. Again after less than a minute it faded away. And before I got in, one more time. As soon as I got behind Deer Island, into calmer water, it was gone for good. I checked everything I could think of. Oil pressure was a steady 55lbs, water temp @ 135. I thought maybe something had shaken loose and was hitting the coupler. NADA. Today I took her out and played around in some fairly snotty conditions, but could not make it repeat, but I couldn't duplicate the conditions. I can find absolutely nothing wrong, anywhere. Any of you gearheads have any ideas. I have a theory floating around in my heat, but am going to wait and see if anyong else has the same idea.

 

[Bob J.]

With the rough conditions you encountered, any chance you picked up something on your shaft, like a tree limb?

 

[Ross]

I expect when you haul the boat you will find heavy fish line wrapped on the shaft and some bruising on the hull where a sinker beat on the hull until it either broke off or wrapped too tight to reach.

 

[Stu Jackson]

Sounds like you may have to go over and swim to check it out down under.

You mean you were out in that snot without zeehag?!?

IIRC, you DID fix your rudder, right?

 

[kenn]

First impulse is to suggest a loose item that got somehow in between your coupling and thew hull or supports, and was gradually machined down? Look for shavings or bits near the coupling...

Ross's idea is probably the more likely. Time for a snorkel-dive...

 

[Stu Jackson]

...thew hull...

Threw me for a loop.

Threw a shoe...

 

[Bill Roosa]

Since the oil pressure was ok during all this I'd rule out the engine. Engines also don't heal themselves. Once they start making noises like those they just get louder till everything goes quiet.
So something banging around under the boat and the frequency is in sync with the motor RPM has to be the prop/shaft. Can't for the life of me figure out how it would stop and then start back up though without stopping the prop first. Even then it does not make much sense.

 

[sailaway78]

OK Heres my theory

I have not yet been under the boat, so nothing under there is ruled out. I did use some heavy reverse docking, so it is possible that something on the prop or shaft, could have unwound itself. However, it seems unlikely that it would quit completely as soon as I got into calmer water. There is nothing that has been close to the coupler, there is absolutely no shiny spots on the coupler, and there are no shavings or liittle pieces down in there.
What I think happened, and I know this is a liittle far out, but I think that somehow the exhaust got in exactly the right conditions, and burped all the water out of the muffler. What I was hearing was unmuffled exhaust. You divers/snorkelers know how you blow the water out of a snorkel. Thats what I'm thinking. One of my reasons for thinking this is that the noise did not quit suddenly. It just kind of faded away, each time. Each time it came on suddenly. My exhaust is above the water line, so if the water burps out of the muffler, straight exhaust noise. The boat was moving pretty around pretty violently, and maybe somehow the water in the muffler sloshed to one end and got blown entirely out. Possiible, hell, I don't know. But at this time I can't think of anything else.

 

[Benny17441]

I concur with your theory as I have experienced such noise from a boat with above the waterline exaust in rough waters. It sounds like the short burst of a machine gun or an air hammer. Noises are hard to describe and I can understand how it is easy to reach a conclusion of something banging on the hull but the noise I refer to is nothing at all like that. As to the reason for the noise I really never gave it much thought but figured the water had come up with force and pushed up the exaust only to be expelled in a long gurgle seconds latter. I would not worry about and would not even consider getting wet to have a look if the noise is gone. The best I can describe the sound is like a long and deep burp.

 

[Ted]

Questions for all gear heads

Do you happen to have a folding or feathering prop? Something may have interfered with the blades which may have prevented one of them from opening fully. It's happened to me when my folding prop got fouled with seaweed. The seaweed gradually worked its way free as the noise progressively diminished.

 

[Don12364]

The only time my H376 has made loud, violent, frightening, unexplained noises from underneath it turned out to be that my prop shaft was not centred in the log tube at the inside end.
When I used a certain rpm and power level either the engine could vibrate or the shaft could whirl and strike the inside of the log.
This made a serious amount of noise and instilled mortal terror into us!

One needs to remember that there is about 700 lbs of thrust at max power and this is trying to compress the propshaft so it would like to go into Euler strut type bending.
This thrust is supposed? to pass straight through the engine mountings and so not cause any misalignment.
In my case this loud banging only occurred when butting into a head sea at nearly full power - just like in your case.

To prove this I took out the bolts between coupling and gearbox flange and slid the coupling and shaft back 1/8" so the locating boss on the coupling was out of the gearbox flange recess. Then I pressed firmly on to the coupling, both up and down and side to side whilst noting the distance it moved before the shaft came into contact with the inside of the log tube. This distance was measured against the gearbox flange using a dial calliper. In my case the engine had been moved or sagged on its mounts so there was only 1/4" movement down but 3/4" up.
The end of the log tube is about 2" ID and the shaft is 1" dia so one would expect the shaft to run with 1/2" clearance all round.
Realignment of the engine solved the problem.

A possible simulation would be to securely moor the boat alongside, engage ahead gear and slowly wind up the power to max. Then rapidly raise and lower the power. This might simulate the conditions except that the boat's movement would not be present.

Another and better way would be to dry out on scrubbing piles (or lift out) and slide the rubber hose off the log and view the situation.

However you might not see this effect again until the next bash to windward under engine.
Either way it needs resolving though.

 

[kloudie1]

Question For Gear Heads

Interested to know what ya find under the boat.. My guess is a part of crab pot line?? or some of that super strong fishing braid ... or nothing at all since it now is gone.. ?? I dunno..
Donalex, Our normal tidal range here is around 6-8 inches. Not so good for drying out.

 

[Tim R.]

I had nearly the exact same symptoms on my Yanmar. It urned out that the alternator belt developed a worn spot. Every time that small worn part went over the pully, the belt got longer and slapped, making a noise like a thrown bearing. It would change and go away based on engine speed. It also developed literally overnight.

Check the cheap and easy stuff first.

 

[Wanderer II]

Yes, I've similar experiences and when the noise varies relative to shaft r.p.m., as Bill Roosa so rightly commented it's a 99.9% chance that the stern drive has become fouled with a loose end of rope or net flogging the 'hollow rear end'. Had it been a failed P bracket bearing or something similar that had failed, as soon as the propeller was put under load the 'prop-walk' would have caused the prop-shaft to rotate eccentrically within the failed bearing's support.

 

[tommays]

You have diesel on silly putty mounts with a shaft going through a log with perhaps 1/8 " clearance and the whole thing just got the ride of its life

It is NOT that hard to picture the shaft bumping on the log form all that stuff going on

 

[serenisea]

Check strut and cutless bearing

Here's the situation. I was heading into Biloxi, in the east channel. It is only a little over a mile long, and fairly narrow. Skinny water on both sides. I have a thunderstorm pounding me pretty good. 30+ knots and a very nasty following sea. About half way up the channel, the wind shifts, and picks up. Gusts approaching 60 knots, dead on the beam. The boat is pitching with the following seas, and heeling over 35 or 40 degrees to port. Wallowing real bad. I'm managing pretty good, but it's work. All of a sudden something goes off, sounds like a AK47, on full auto. Very loud. And sounds like it's coming from under the boat. My mind is trying to solve two problems at once. What is wrong, and what am I going to do if something breaks. There is no way I can get anything done in this channel before I get blown out of it, and find the bottom. I throttled back to about 1500, the frequency of the noise reduced as the RPM came down, but was just as loud. Then after a lifetime, it just faded away. Real time less than a minute. About 20 minutes later the exact same thing. Again after less than a minute it faded away. And before I got in, one more time. As soon as I got behind Deer Island, into calmer water, it was gone for good. I checked everything I could think of. Oil pressure was a steady 55lbs, water temp @ 135. I thought maybe something had shaken loose and was hitting the coupler. NADA. Today I took her out and played around in some fairly snotty conditions, but could not make it repeat, but I couldn't duplicate the conditions. I can find absolutely nothing wrong, anywhere. Any of you gearheads have any ideas. I have a theory floating around in my heat, but am going to wait and see if anyong else has the same idea.

Our sailing school had a charter boat that the skipper ran up on an old submerged barge. The barge was mostly just a skeleton, since it was sunk there after WW2. He did not crash the hull, but the strut got clipped by a piece of the submerged barge frame that was sticking up. It made a similar rat-tat-tat sound spinning, probably knocking against the hull. Good luck.

 

[zeehag]

stu--i am on other coast--in my own boat enjoying mexico, pacific side, riviera .....
phillip--did you check in your bilge--could something have been knocking around in there??
dayumm--that isnt a great place to have a problem... hope it finds easily....

 

[sailaway78]

This morning

Have been crawling all over this boat this morning. I can find nothing, absolutely nothing out of the ordinary. I do not have a folding prop, which was a good suggestion. I cannot rule out the shaft somehow flexing and hitting the shaft log, but can rule out bent. Not going to get under there till I get back home. Looking at the forecast, gonna go Thursday. I.m about 8 hours out on a good day, so feel comfortable doing that. Going to keep looking and thinking, but so far all ideas are either proven wrong or unproven. But keep them coming.

 

[Stu Jackson]

The record was skipping on the turntable?

 

[dparilla]

I blame zombies.