Alkaline dry cells typically have much greater charge density than lead-acid gel cells, so for an equal mass they will last noticeably longer. The trouble is that they also have much greater internal resistance, so you lose a fair bit of that power as heat build-up in the cells. The lower the output current, relative to the capacity of the cells, the less this power loss will be.

Alternativley, this means that you need a higher nominal voltage to the battery when running high-current devices like bilge pumps.

I have found that I can get 15 to 20 minutes of pumping with the Rule 500 and ten AA alkalines (nominal 15 Volts). The extra cells are necessary to overcome the effects of internal resistance and supply an actual 12 volts to the pump.
Nine D cells (nominal 13.5 volts) would give you over 3 hours for about the same battery weight as one hour of pumping from a gel cell. The extra (ninth) cell is necessary to overcome internal resistance and supply the pump with 12 volts.

Since you don't mention the specs of your 12 volt dry cell, then we can't estimate how much power is lost as internal resistance, but from your estimate of 5 US gallons in 40 seconds, it would seem that the pump output is down by only ten percent or so. This is an acceptable loss. Go for it!

: This weekend I bought a Rule 500 electric bilge pump, and a 12 V dry cell
: battery (instead of a lead-acid gel battery). The combination pumped out a
: 5 gallon bucket in about 40 seconds, so it seems to work fine. Advantages
: of the 12 V dry cell are that it cost only $11.00, it is lighter than the
: gel cell, and it doesn't need to be trickle-charged.

: I just don't know how long it will keep pumping, relative to the heavier
: battery (I suspect it doesn't have as much "power"). Any
: electrical-engineer types have a feeling for how these two power sources
: compare?

: Thanks!