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Old 12-12-2009, 23:42   #1
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Strange Noise From Transom

So we go out Friday at 4 PM and load up the boat for an afternoon sail. We motor for about 30 seconds to get the sails up and head off on a close reach. Pretty soon the boat is heeled over about 30 degrees and I notice that does a good job spilling the crap from the aft bilge into the forward bilge and then into the cabin floor.

I clean up the water and come back up on deck where the crew is smiling for the first time since about May. They have spilled two beers (a crime punishable bykeelhauling on my boat) and are clutching strings and tillers and getting bugs in their teeth.

I then notice a very strange whooshing/gurgling noise from the back of the boat. We have a squared off transom and at first I suspected "excessive air" coming out of the exhaust system but then realized the engine was off.

Then because it was quite warm I thought that the 5 gallon diesel tank might have heated up and as the air expanded it was venting. I opened the aft locker and confirmed the tank was OK.

My trimmer is somewhat of a gas bag but between trying to hold the genny string and using a fresh beer to clean the bugs out of his teeth I reckon the gas noise wasn't him.

I looked over the transom to try and identify what was up but I had a hard time seeing below the waterline because the water was completely churned up like a washing machine. I got really concerned then because with all that churn below the waterline I realized the air making that wooshing/gurgling must be escaping below the waterline!

Normally a cool and expert skipper I was now worried that if enough air escaped from the cabin the differential pressure might cause the deck to collapse. I considered opening the forward hatch but was worried the the water coming over the bow might make it into the boat and we'd have another problem.

I decided to stick it out and thankfully nothing dramatic happened. We did about 15 kilometers in a little over a couple of hours so that leaking air was having a positive impact on thrust maybe?

The next day we sailed again with the same results. Still perplexed I got my Blackberry GPS fired up and saw we were doing 12 kmh (My BB gps is "knotically" challenged). I converted that and saw we were doing 6.5 knots and my theory now is that the saildrive gill holes must be venting badly to propel the boat that fast.

Today I am going to do some more testing.

If the saildrive theroy doesn't hold up, does anyone have any idea what would make a 2500kg, 26 foot boat boat go 6.5kts with whooshing/gurgling noises at the transom?

Oh, if it's any help someone in the bar said that the 25 knot winds this weekend resulting in everyone getting bugs in their teeth is caused by something called the Northeast Monsoon. I told him he was fulla bull...
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Old 13-12-2009, 02:11   #2
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OK what the heck ...its late and this is my last post so I'll bite..

I think this is a trick question and your fishing for atta boys....

So I will just go ahead a slap you on the back...commend you heartely for finally getting your boat past the 4 knot speed barrier you have failed to break to date and welcome you to the Rooster Tail Club...all of whom are members relish the gurgling washing machine wooshing phenomena every chance we get. Hows that!

See ya on the other side of the moon..off to bed
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Old 13-12-2009, 05:53   #3
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Ahhh shucks!!

I hope this is not your subtle way of letting us know that you have fitted some "skunk works", boundary layer, air injection thru the cockpit drains, science to your ride..
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Old 13-12-2009, 06:27   #4
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Quote:
Originally Posted by Ex-Calif View Post
..............
If the saildrive theroy doesn't hold up, does anyone have any idea what would make a 2500kg, 26 foot boat boat go 6.5kts with whooshing/gurgling noises at the transom?

Oh, if it's any help someone in the bar said that the 25 knot winds this weekend resulting in everyone getting bugs in their teeth is caused by something called the Northeast Monsoon. I told him he was fulla bull...
Pretty sure you will have to dump the saildrive theory and you might have to apologize to that fella in the bar.

You will find that the NE Monsoon IS bringing in the bugs, they are getting stuck in everyone's teeth, everyone spits them out (mainly in the water), rinses with beer. After passing through the body, the excess beer is also deposited in the water and along with the now dead bugs, the water resembles a washing machine full of bugs and beer residues.

You go sailing in the same water with the Monsoon up your clacker, the dead bugs build up on the under-section mainly near the transom and act as vortex generators - and before you know it, you have whooshing/gurgling noises at the transom.

The proof is simple, take away the Monsoon and I bet the funny noises also go away
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Old 13-12-2009, 08:52   #5
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Pretty soon the boat is heeled over about 30 degrees ...
Thirty-degree heel with an outboard rudder? My guess is that your rudder was only half submerged, and that the top half was acting as an airfoil, especially given that you'd have had serious weather helm at that point. There may have been a bit of cavitation at the top part of the rudder, and that created the noise you were hearing.

Funny things happen when non-planing sailboats get into semi-displacement mode when overpowered. Most will squat at the transom, and some will actually throw rooster tails. Next time it happens, put your hand on the rudder itself, and see whether you can feel what's going on.
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Old 13-12-2009, 08:58   #6
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Thirty degrees heel.. Think I'll pass

Paige
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Old 13-12-2009, 12:13   #7
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Ex-Cal,
Are your strings really sheets?
Good on you for getting real speed out of your boat.
regards,
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Old 13-12-2009, 12:44   #8
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Hi Dan,
Rudder aeration is my best guess.

I used to hear a similar sound on my Hobie 18 when I was really heeled over. Air would get sucked down by a vortice on the low pressure side of the foil.

Was there anything wrapped on the upper leading edge of your rudder like a plastic shopping bag disturbing the laminar flow encouraging air to flow down? Did the rudder feel only partially effective when you heard the sound?
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Old 13-12-2009, 16:13   #9
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Yes my post was somewhat tongue in "chic" - It was an awesome weekend of sailing. Just thought you northerners with your shrink wrapped boats on the hard needed something to cry about - LOL.

Yes 30 degrees of heel was too much and we considered shorting sail but in the lulls we were OK. The boat is very well behaved and was still very balanced at speed and heel. At that point we are steering with the main traveller pretty much. Slight weather helm which is good for when the tillerman drops his beer, thus dropping the tiller to save the beer (my crew has their priorities set)

We also had the J24 Nationals this weekend and we went out to watch some of the windward lewards. It was a small turnout and we were most interested in how the less experienced crews were gooing to handle the conditions. OK we were looking for a train wreck.. Fortunately everyone kept their cool and did well. There was only one exciting spinnaker drop for which we gave full style marks.

The cats were out in force and they were having races as well. One unfortunate team went off the beach, inexplicably didn't power up and drifted sideways into the dock. The rudder snagged on a dock mooring line and the poor crew got flattened onto the dock. Thankfully no one was hurt as the boat went over and the crew fell out. Could have been nasty if someone landed on the dock.

Yesterday we only did a short 2 hour twilight. There was rain during the day and a mid level overcast when we went out. The gurgling noises had reduced remarkably so I think it might have self cured...
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Old 13-12-2009, 17:16   #10
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I write this with humility and all due respect.
You were doing 6.5 knots all right, over ground (current) but not through water, and the gurgling sound is either an attached obstruction, or there is something loose in your lower unit.
BTW, I know of a Lightning that "hums" because of a defect in the centerboard.
Again, Respectfully,

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Old 13-12-2009, 21:49   #11
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lack of due care and attention

It would take a while before I was able to trust a crew that dropped their beer
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Old 14-12-2009, 05:40   #12
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It would take a while before I was able to trust a crew that dropped their beer
A while - I don't think so - try never.
Its one hand for the beer and one for everything else.
Still they do things different in Asia, even the ex Americans - all part of the diversity of cruising I guess.
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Old 14-12-2009, 07:45   #13
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Sunday was a much lighter day but I left an unopened beer on the coachroof, the helmsman called for a tack, I masterfully tacked the genny and heard the mainsheet man shout, "Beer overboard!"

Oh, the shame. The skipper not stowing his beer before a tack.

The good news was I immediately jumped into action and took command. Moved the mainsheet man to the genny and told the ex-tillerman to be the lookout.

It was a good thing that beer was wearing a life vest but damn, that was my favorite bikini-photo beer coozy and I was darned if we weren't going to pick it up. It was a small target but floating nicely.

We made a direct return and approached from downwind. I didn't slow enough and the bow wake pushed it away. We slowed enough but the wind caught the boat and we drifted further away. FAIL!

Made another pass and decided to stop on the upwind side. Good call. Boat stopping with the beer floating just off the quarter. 5 seconds later we drifted down and rescued the coozy.

Good chance for practice but I'm still embarassed for losing the beer overboard in the first place. Bad for crew discipline.
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Old 14-12-2009, 11:03   #14
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LOL ..You put a smile on my face this morning EX-Cal ..Thanks for sharing !...


So did you use a quick stop..figure 8 or what?...

Snowing Here today.
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Old 14-12-2009, 12:31   #15
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I do love the Maxi 77s spent many an hour out on them in Gothenburg while in Uni.

Yes its worth it to practice your BOB ofter for when the time arises.
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