Speaking of refrigeration...

[S/V AMITY]

     Had a chat with Clive at Sea Frost concerning the Sea Frost BD refrigeration on Amity.  The unit is equiped with the water cooling boost option which was never hooked up when the unit was installed by the previous owner.  Only cooling is with the fan. The compressor unit is in the port cockpit sail locker and is subject to ambient engine heat when we're under power.  Not good.

    My question to Clive was if he saw any problem with using one of Amity's fresh water tanks for the Sea Frost cooling water loop BTU dump.  Since one side of the tank is comprised of the hull skin, that side would fairly effectively bleed the heat from the reefer cooling water through the hull skin thence overboard.  Plan will be to feed tank water to the compressor box from the bottom of the tank.  Return (warm) water will be fed back into the tank at the top.  He saw no problems so that's a new project on the list.  If this works it should reduce the daily refrigeration amp load somewhat and eliminate the need for another seacock and seawater strainer.  Since fresh water will be the cooling medium, the reefer will not suffer any corrosion or blockages  in the heat exchanger.

[SVJourney]

I looked at this as well not so long ago.  I think you would have to reconfigure the outlet from the tank so your faucets would run dry before the tank empties.  Like maybe 10 gals must remain in tank at all times?  I wish I had the math and skills to do the thermodynamic calcs as it is an interesting and elegant solution.

[S/V AMITY]

  If I can reach the outlet in the port tank I can put a 2" or 3" standpipe on it.  On the other hand, just to keep it simple I can leave it as is and manage the water so we use the stbd. and fwd. tanks first.  If the that port tank runs dry we'll be out of water so that'll tell me to shut off the cooling loop pump.

  The pump Sea Frost uses has a magnetically coupled impeller which Clive said will announce itself dry with a clattering noise but no harm to the pump.  Since the compressor will be using the fan in addition to the water loop, if the water loop drops out we still have refrigeration.

  I don't have the math skills either but I do know the additional BTU dumping will be fairly considerable since water is a much more effective coolant than air... one of the reasons we no longer see VW's with direct air-cooled engines and the P-51 was such a great plane.

[SVJourney]

Yeah, now that you mention it, if you used that tank last and it ran dry, you would have bigger problems than a defrosting freezer.  Something I never thought about.   

[S/V AMITY]

  The tank low level alarm is called a wife.  You didn't hear that from me...

[SVJourney]

LOL, yeah, noted and witnessed.

[S/V AMITY]

Way, way, way off topic:

  Wayne, did you fit a windvane to the transom of the 365? Can't tell from your pictures. If so, how did you work around the inflatable on the davits?

[SVJourney]

Nope, we did autopilots.  Dirk's Evening Ebb is the only 365 I know of who cruised with a vane.  He did a Hydrovane and was very happy with it.  He did the dink on deck.  I can't see a way to put both dingy on davits with a wind vane on the back.

[Dale Tanski]

So....
For every gallon of water you have in your tank, it will require (absorb) 8.34 BTU's to raise that gallon 1 degree F. The big difficulties are the variables such as sea temp and refrigeration run time.  As the water in the tank rises, the lower the temperature differential between the expelled water out of the reefer unit gets and the tank temperature. This drops the efficiency rate of the reefer which means it will need to run longer. 
Also, the warmer the water in the tank the greater the risk of algae growth issues.  I'm sure the heat exchanger designed by the refrigerator manufacture is capable of handling the corrosive requirements of sea water. KISS.

Dale

[S/V AMITY]

   I agree. The wild cards are the heat transfer value of the hull skin, immersed area of the skin in the tank, seawater temperature and the cubic heat-sink volume.  The only non-variable is the conductive value of fiberglass.  I have no clue as what that value is but it must be quite low.  Since we operate in coastal New England, peak seawater temperature is in the mid to upper 60's; much lower in the spring & fall.

   Since the sole non-variable in this mix is the r--value of the hull skin,  the only solution I see is to hook it up and see what happens.  Concerning algae and other undesirables thriving in the tank, that problem is covered with a small amount of bleach we add to the tanks anyway at avery refill.

   What may throw a monkey wrench into all this is the amps the cooling water circulator pump will draw.  That might be a killer...

[P69]

The other variable with this concept is that the settee water tanks are likely not integral with the hull.  On my 367, the settee water tanks are actual tanks that were put in and glassed to the hull. If it's the same on the 365s, there is likely to an air space between the hull and the part of tank that presses against the hull. That'll slow the heat transfer quite a bit.

[S/V AMITY]

  P69... Good point.  Will check to see if that's the case on Amity.

[S/V AMITY]

  This from Sea Frost this morning:
           
  Jim,

   The average heat per hour is about 100 BTU's.  Any tank will maintain sea water temp with that small amount of heat added.   The pump is not protected from running dry but in my experience loosing prime for a few minutes has not killed it.  I think some water stays in the pump to lubricate the impeller shaft.

[Dale Tanski]

Tanks on my boat were seperate as well.  There were places where there was 1/2" of space between the hull and the tank.
Dale