Aft water tank resonance

[High Current]

Has this ever happened to anyone else?

We anchored in a nice cove last night.  Bedded down in the aft berth.  Shortly thereafter some small (very small) waves started to come up from abeam of where the wind was pointing us.  We were awakened by a loud "thwap thwap" against the hull, even a bit of a banging noise, about the speed of a clock ticking.  After letting the dinghy further out (it had come up and was kissing us) and securing the wheel, I finally realized it was coming from under my feet, so I opened the lazarette and observed water sloshing back and forth violently in the tank.  Running the faucet for a minute or so didn't fix it, and we didn't want to drain the tank, so we ended up moving to the V-berth.  Good thing the kids weren't aboard!

So...was this just a freak thing or is it a fairly common occurrence?  It's my first season with the boat so I'm still discovering its personality.

[Jon W]

There are no baffles in the tank. As the water level in the tank lowers, the water can get pretty noisy as it sloshes around. It can happen while sailing also.

[waughoo]

I have noticed it under sail a number of times but never at anchor.

[scgunner]

High Current,

It's not a freak thing. You can't eliminate the noise completely unless you're willing to run with a dry aft tank (I'm not). Couple things you can do to minimize sloshing. First keep the tank topped up to the very tip top until you need it. Second when your forward tank is empty run the water from the rear tank to fill the front tank then start using what's left in the rear tank (around 15 gallons) until empty. The tank is only quiet when it's full or empty. But when almost empty it's quieter. When it's 1/2 to 2/3 full there's plenty of water and plenty of room for it to slosh around in and that's when it's loudest.

[Noah]

High Current: what year is your boat? MK I or MK II? Engine info, etc.  It helps if you put your boat's info in your signature.

[Ron Hill]

High C : What kind of transom???  Hull #???  Standard, Suger scoop, Walk thru ???

A thought

[High Current]

Thanks for the replies everyone!  I love the suggestion about using the starboard tank first and draining the aft into it.  Funny though: this all started because I noticed the boat listing more after I used the forward tank first and thought to myself: "aha, if I mainly use the aft one, the weight will stay balanced."  Trading one problem for another...I think the listing is more manageable.

Added boat info to my profile.  It's a 1990 mk 1.5 walk-through, #1050.

[Jim Hardesty]

waves started to come up from abeam of where the wind was pointing us.  We were awakened by a loud "thwap thwap" against the hull, even a bit of a banging noise,

Are you sure that it wasn't the waves hitting the stern counter?  Think that's the right term, where the hull rises at a slight angle aft.  That can be really loud from the aft berth.  Some people in our club hang pool noodles under the stern to help.  I sleep in the v-berth, rarely notice it.
Jim

[Ron Hill]

High : I agree with Jim. I suspect that what you were hearing was the slapping of the waves against the underside hull of the transom.

Fresh Water Management - when we're on an extended trip I always use water out of the starboard tank.  When it "spits" and is empty I can (gravity) refill from the aft tank by just turning aft tank ON for 15 minutes and then turn OFF the aft tank. then I know how much water we still have onboard.

Infact I only fill the starboard tank in the spring and the rest of the year I just gravity refill it from the aft tank.  When we go to a marina I then only have to refill the aft tank to have 100% fresh water!!

A few thoughts