R21EC (2012) Fuel Tank Vibration

[shackinballard]

There is a related forum concerning the fuel vent and sloshing for the 21, but my concern is a bit different than losing fuel out the vent. We recently did a short cruise to Matia Island in the San Juans. After we hit the rack, our mooring ball and tide/current situation put us perpendicular to the rollers (at times) which were a foot high, maybe less. I woke in the night thinking something was hitting the boat, perhaps the mooring ball or a log. In the first light I realized it was fuel in the tank. We had felt this slamming before, but this specific situation was much more acute. It's not an exaggeration to say it felt like a bowling ball was loose in the aft compartment causing a bang and a vibration we felt in the V-berth. Besides a night of little sleep, my concern is the amount of repeated lateral forces the tank is enduring. Granted, I believe our position against the rollers and the level of the tank (about two-thirds full) was a perfect set-up to create an acute bang and associated vibration.

I am hoping there is some way to get baffles in the tank. The related forum suggests whiffle balls in the tank. That solution doesn't sound great to me. I am wondering about inserting some tubing or other obstacle to keep the fuel from slamming the sides and top (the fuel slamming the top seems to generate the noise and force). Is there another tank that might work, hoping there is an easier fix. I can't seem to find other forums with this 'issue' which is a bit surprising. It made for a long night. If I was in a better situation for a stern anchor I would have given it a try. I can also try keeping the tanks absolutely full but that doesn't seem practical. Any help or ideas appreciates.

cheers,
M/V Darby

 

[abcandjrc]

A problem with blow-molded tanks in general. And there is no good solution. (I agree that stuffing things into the tank might help but has several downsides). Having a fab shop build a custom tank out of aluminum would do the trick but unless you own the shop it is not a viable economic choice.

One thought about keeping the tank full is that you will need a stopper for the vent or you might be pumping fuel out on each roll. See gallery2.php?g2_itemId=28091 for my solution to that. My reasons were different but it would also fit to reduce spitting in this case.

Your comment about a stern anchor triggered a thought. Obviously that may or may not be acceptable in a mooring field. But at times, to keep us from sailing about the anchor, we have led the anchor rode outside the rails and tied it off on the stern cleat. It changes the motion significantly. It might help, if not cure the problem, in a situation as you describe. Sometimes it also improves the air flow through the boat. We quite often used to fish in that configuration since the cockpit did not swing as wide and as fast. People will look at you with a strange expression on their faces, but you might see them try it next time out.

 

[RProffer]

The first night I anchored out on this boat something strange happened. I set the anchor with 7:1 and all was well. I hit the sack and I started hearing water slapping on the hull. I could not go to sleep. I got up and found the anchor rode was not ahead of my boat, but instead the anchor was to the stern. I pulled the anchor and reset it, and the same thing happened. The boat did a 180 to the anchor rode and the water was slapping the transom - very noisy. I do not know what was causing that reversal. I have not seen that happen before or since. That night I had to move to another cove to get a good nights sleep. Good thing there are a lot of coves here in the Chesapeake.

The point I am making in this post is I have not had a problem with my R21-EC with fuel making a sound in my tank. But I did have a problems with sounds coming from the transom with water slapping it.

 

[shackinballard]

Thanks for the information. I think a stopper for the vent is a good idea (not just for the noise).

At one point, I thought the thwack I was hearing was waves against the bottom of the boat, but a good thing about the tank is you can see the fuel. On the morning I describe, at first light I was able to see the fuel hitting the top and making the noise.