Battery placement

[vegatern]

Does anyone have a comment or an idea about moving the batteries
out from under the cabin sole? I am reluctant to move them back into
the cockpit lockers because the boat already has an 18 gal. fuel tank
in the port locker and 17 gal. water tank in the starboard locker.
Adding additional weight there seems unwise. I am thinking of moving
them to the storage space directly behind the forward water tank, at
the aft end of the v-berth, but wonder if the distance from the
engine, an MD7A, is too great. Any comment would be welcome.
Craig Hirt
Tern #1519

 

[pjacobs55]

Craig:
I'm not sure about the weight distribution issue (sounds like a good
place as all your other weight is aft) but you'll need to run heavier
gauge cables from the battery back to the engine and fuse panel. I
think the biggest load is your starter motor, so if you can size the
cables to satisfy that you should be ok with the rest of the loads.

Peter.

 

[alanderjakob]

Of course there is allways a comment... First things first. Why do you
want to find another place for your batteries? In my opinion the place
is good, except for one reason: flooding. If the water level inside
the boat is rising you will loose power early wich obviously isn't
what you want. Been there, done that.

It is wise not to load too much gear and accessories in the cockpit
lockers since the boat will be very stern heavy, as you already have
figured out. Keeping the boat's longitudinal trim fairly constant is
important to achieve maximum sailing speed. On the other hand it is
important to keep the mass moment of inertia (not sure about the
translation) low. This is done by reducing weight far from the centre
of gravity. With a large mass moment of inertia the boat will try to
move trough the seas instead of over them and the ride will be
unpleasant. In very choppy seas the boat will lose a lot of speed due
to this and it might be impossible to make speed ahead if you are
close hauled and trying to beat through heave seas.

The batteries are originally placed almost in the centre of gravity
and if you move them to the suggested place behind the watertank, the
distance from the cg equals placing the batteries at the aft end of
the cockpit. Neither is great but there are not that many places to
chose from.

Anyway, it won't be much of a problem dimensioning the cables to the
batteries. I am not certain about the capacity of your alternator but
i suppose it is 35A. To get a good battery charging you would like to
se 13.8 V at the batteries at a charge current of abot 10 A per
battery. Since your alternator probably cannot sense the voltage over
the battery, you have to minimize the transfer losses. You will need
approx 5 m cable. If you chose a cable with cross sectional area 25
mm^2, your voltage drop at 10 A will be 0.07 V wich is good. If you
use the same size, 25 mm^2, for the starter motor your voltage drop
will be approx 0.35 V while cranking the engine.

Please note that the above given figures are theoretical and the
installation has to be made carefully to achieve these values. There
must be no weak spots. Do also note that the cables can be smaller
than stated above, but it will take longer time to fill up the batteries.

/Jakob

 

[pjacobs55]

I've always wondered if the area around the rudder post would work.Nicholas Walsh
Nicholas H. Walsh P.A.
111 Commercial Street
Portland Maine 04101
Tel. 207/772-2191
fax 207/774-3940

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[vegatern]

Thanks for the great help. It doesn't seem that it would take
much water below to cover the batteries, and adding the loss of power,
lights, GPS, etc. to whatever problem that would cause the cabin to
flood, well, just seems to be adding problems to problems. Your right,
I am reluctant to move more weight out of the area of center of
gravity. The original fuel tank has been removed already and replaced
with one higher in the cockpit locker, to combat a problem of starving
the engine. I am considering putting it back and using it as strictly
a storage tank, and using a manuel transfer pump to move fuel to the
cockpit tank when that one is low, or, perhaps having a new bilge tank
made for water. This would get some weight back low in the boat.
Another idea was to leave the original batteries where they are but
insall a third, seperate battery in a more water resistant location.
I wouldn't know how to wire the third one in though, I asume it would
have it's own circut and switched seperatly. Not sure what I will do,
but I think I'd rather go sailing and think on it. Thanks for the
suggustions

 

[alanderjakob]

You are wellcome! Of course you should install a third battery. This
will greatly increase your power availability since filling up the
last 20% of the battery takes a lot more time than filling it 60-80%.
This will give you much more stored energy per time charging. Wiring
is real easy. You conect the negative terminal to the other batteries
negative terminals. Then you run a new wire from the positve terminal
to a new main switch. Then you connect the the service battery
switches in parallel.

I think the most elegant solution is to place the batteries in water
tight boxes in the original place. Just remember that you need a small
ventilation hose from each battery. I am thinking of making such an
arrangement on my own boat. It might be difficult to find such boxes
though, I'm thinking of making them myself in GRP.

I know the starvation problem too. We installed an electric membrane
pump in series with the mechanical feedpump to solve this problem. It
works great!