The 6-8 amp draw is only when the pump
is actually running -- if you are taking enough water onboard that would lead to draining your battery bank, you have more pressing issues than a new bilge pump
. With a modest 450 a/h bank (4 x Trojan T-105, eg) you would have to have the pump
running for over 25 hrs to draw you down under the recommended limit of 50% discharge.
You can get creative with the plumbing, installing a Y-fitting so that both the electric pump
and the manual pump use the same through-hull -- but I probably wouldn't do it myself. If you do elect to do it, make sure the Y is close to the t/h on the downhill side of your hose runs. Don't forget to install an anti-siphon loop in the hose from the electric pump.