I have an older Yamaha F100 on my 20MA (same as a F90, with higher RPM limit). I also have a 30" shaft tohatsu 6hp SailPro kicker. Both 4 stroke. Weights are 367 and 58 lb, respectively. I wouldn't put more on a stock 20" transom. But I love the security. I have a 20 gallon tank for the F100 which gets 5? MPG and 6 gallon portable tank for the kicker. I like the combination. Batteries and tank are under my console. I am way more comfortable off shore with a kicker. It will charge a battery, too.
A second 6gallon tank is a nice get you home margin on a 5MPG main engine. And a 5 gallon Jerry can is 25 more miles. Ballasted wherever you like.
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