RE: Speed Boat Project
Looking at the specs for AA disposable batteries, it looks like Duracell Ultra cells are your best bet.
Duracell Ultra (MX1500) has a quoted internal resistance of 81 mOhm.
Standard Druracells (PC1500 and MN1500) internal resistance is 120 mOhm.
Energizer cells are 146mOhm.
If you draw 5 amp from your 5 cell pack the voltage available to your motor will be as follows:
Duracell Ultra 5.475v (27.375w)
Duracell 4.5v (22.5w)
Energiser 3.85v (19.25w)
If the manufacturers specs can be believed the Duracell Ultras should give almost 50% more power than the Energisers.
I suggest that you try one pack of cells with the 6v Speed 400 or two packs with the 4.8v version.