Well said, Monsieur!
Far too many modellers are fooled by those seductive capacity ratings, to the extent that some believe they can even run two brushless motors from one SLA battery and obtain an hour's sailing from one charge. If I've seen one Huntsman or Perkasa lumbering round a pond like a narrowboat, weighed down by a huge brick of a battery, then I've seen a hundred. It makes me weep. It takes a great percentage of the motor's power just to get the battery moving, never mind getting the model up on the plane.
Anything faster than a tame tug is indeed
NOT suitable for SLA batteries.
I run my springer (with a big nice #174 45mm prop from Raboesch) with a BLDC motor off a 6V SLA """7.2 Ah""" battery. When cruising around, total amp draw is around 2.6A which would give me almost 3 hours of constant run time. Effective run time (because you're not always cruising all over the place) is 4 hours +.
But when I start pushing my 25kg-shoe-box-shaped barge or towing a heavy ship around, current draw goes through the roof: 11A at full throttle. Datasheet says run time would be around 25min with a constant 11A load. Actual run time is closer to one hour. I was actually surprised the first time I took my barge to the pond. I had somewhat forgotten that SLA batteries were so "weak".
Unless they are of the special high-current, deep-discharge "leisure" type
Even the "deep discharge" - at least those that are affordable - perform only marginally better. I'm tempted to look at LiFePo4 batteries, for that matter. They seem to offer a good balance between "usability" and discharge/charge rates. And they are getting cheaper.
I have two 12V / ""12 Ah" batteries in my Odin tug. Good enough as the overall power draw isn't that bad and I needed around 13kg of ballast anyway, but this is as "ideal" as it gets.
On smaller boats, SLA batteries might not be that good a choice.