Can't really speak to the conversion, since everywhere you look the conversions vary wildly. There are posters here that know much more about this comparison, but I like to keep things much simpler.
BUT, one thing ,is that an internal combustion engine's power(HP) is rated at a speed at which I never use my motor
. In fact, the place at which this horsepower is rated might be who knows where on the motor
I would have to say a 9HP internal combustion engine
delivers much less horsepower at a slower speed . . . such as the general speed of a sailboat, and in my case quite a bit below "hull-speed", a speed at which it begins taking much more power to maintain or increase. There has to be a sorta sweet-spot where electric and IC engines equal out when an IC engine
is running at a somewhat slow cruising speed.
An electric motor
at slow speed will have much more torque than an exploding-type engine at a similar slow speed. That amount of torque at a slower speed, if properly used could be used much better than the torque of an exploding engine at the same speed.
Best thing to do, is to just get the motor and see for yourself. If it's a MinnKota motor, look up Kipawa propellers. Evidently these props increase the thrust(pushing power) of those electric trolling motors.
I'm waiting for someone to report how much these propellers help more underpowered 12 and 24 volt trolling motors on a 25+ foot long sailboat.
For me, I'd rather have more battery
and less gas on board. Of course, to me, a sailboat moving at 2 knots in dead air is okay.