are you using a high amp speed controller ?
Whether you go brushless or brushed, a springer isn't really a high amp application.
Here's the hardware I used in my springer.Yep, that's a 625W motor (under much higher voltage than the 6V it gets in my springer though) and a 60Amp car ESC
Actual amp draw averages ~1.5A with the "basic" 40mm 4-blade prop, cruising around half throttle. I get around 4 hours of run time from a 6V/7.2Ah lead battery. Both the motor and ESC are total overkill but that means I don't have to worry about cooling them. And you'd have trouble finding a brushless ESC below 40 amps (unless you go for tiny ESCs for tiny cars ... but they end up being more expensive).
Even towing the Spirit of Archimedes (ie: almost constant full throttle) I still got an hour and a half off the 7.2Ah battery. That's with the bigger 45mm/wide blade prop.
what have you used to make your towing post and how is it mounted ?
I used the towing posts (samson posts ?) that came with my kit. They're made of metal.
Posts go through the deck. I epoxyed them in place (I made some "formwork" with masking tape on the underside and poured epoxy).
You'll notice the hull has three transverse beams and piece of plywood near the bow. Front post lines up with that plywood "plank". Rear post ends up right next to second beam from stern. Once deck was epoxyed to hull, I added small plywood strips under the deck around the posts and put a more epoxy bonding the extra plywood strips, towing posts and either the beam or the bow "plank" together. Had to work with the tug upside down. Not really easy (had to use a mirror to look inside).
Deck and hull were sealed with G4 marine primer which really stiffens the plywood. All joints where reinforced with epoxy fillets (ie: heat up the hull with hair dryer, pour some 30 min epoxy which gets very fluid due to the heat, tilt hull around and let epoxy cure with hull at the right angle; repeat). Took some time but the hull is really stiff now.
Was worth the effort 'cause both posts are really strong. I can pull my springer out of the water using the posts.