Model Slipway Tsekoa
HI Derek
The speed 600 are a good motor for fast planing boats, but are rather heavy on the current draw and don't start well at low revs. ideally you need some low current high tork 500 size motors. Slipway used to sell a low current version for their Trent kits and Component Shop stock a 555.
Assuming your batteries are 4AHr I am not surprised that they don't last long. Your only viable option to improve the duration is to replace the 600s with a low drain motor. The added benefit will be easier control round a steering course.
Dave
Live long and prosper
Dave