I'm not sure which area you are asking about for how thick is the balsa core. Since you are talking about a manual pump, are you thinking of mounting it through the bulkhead that is the aft cabin "wall" or...?
The after cabin bulkhead, on my boat, is fiberglass skinned and balsa cored. Without measuring it, I'd say the core itself is about 1/4" to 3/8". There are areas where it ends, and there are flatter areas of just fiberglass.
In case you mean the hull: On my boat, the cabin "sole" is the hull, and the core is somewhere in the neighborhood of 1" thick. There is a depression near the aft cabin bulkhead, under the galley, that was meant for a bilge pump, and in that area it was laid up without core to make a small sump. At some point going "backwards" in years, I don't think that sump was built in though, and there was often a pump in the cockpit just aft of that bulkhead, to starboard, with a metal(?) shield over it. Of course an Angler will probably be somewhat different, with the shorter cabin.
Not sure if I helped, but maybe? Perhaps you could explain in more detail how you would like to mount the pump.
Sunbeam
