I do that, I weigh stuff, just never seems to be good enough. I did a danish breakfast roll batch the other day that worked, if I tried again odd are it would not.
Im going to try this tomorrow if I have the time...
From your posts I'm assuming this is not your first boat.
A 40hp on a 13 foot whaler should get you mid 20 mph i would guess.
If you are only getting 8 mph something is major wrong.
I can tell roughly my rpms based on sound
A tach will tell you what exactly your rpms are but your ears...
One of my early careers had a boss tell me 'it takes pennies to make dollars', same with wasting anything. I'd say I'm better than the average American by far but still have a long way to go, including ensuring I smell two stroke smoke in the morning....
You should ask about seafoam next....
My thought would be to use as instructed unless you know you will use just for trolling. If planning on running on plane stick with oe instructions.
This is the one I'm eyeballing
https://www.oreillyauto.com/detail/c/extreme/super-start-extreme-standard-flooded-side-post-battery-group-size-78-800-cca-115-minute-rc/ssbp/78ext
Ok looks like any side post unit fixes my problem. I (will) have lifepo up front next year for my trolling motor but also want the rear as a backup to act as a kicker if both main motor is out and lifepo gets drained. That is why I was thinking deep cycle. If I go non deep cycle what could...