George. I have recently bought yet another boat, last one I promise !!. It's a Savage Scorpion 2002 model 485 SF alloy and there are a few things I want to add to it to make it more enjoyable.
One of those things is a decent bow mount electric motor with spot lock and cruise control and all the bells and whistles, so I will never have to use an anchor again.
I am almost certain I will be buying the Motorguide XI 5 55lb 54" shaft, however I have been told that the 54" shaft will cavitate on any sort of chop and that I need a 60" shaft at least. These are out of stock in the 12 volt system, SO I am thinking maybe I have to go to the 24 volt unit 80 lb 60" OR 72" shaft, more money more room it's going to take up on the bow and harder to fit.
OK to get to the point. I have a 12 volt 185 amp hour deep cycle battery already for the 12 volt unit. IF I go to 24 volt do I have to get another 185 amp hour battery ? OR can I mix match with a 125 amp hour battery.
IF I don't use the same amp hour batteries , will it in any way interfere with the performance of the motor ? .
Cheers.

Frank I would stick with the 12Volt system I will explain why!!! first of all Your starting system is 12Volt If you fitted a Solenoid to your starting Battery and had a wire to each battery It would charge your AGM Battery while ever your engine is running, (if you are not sure about this I can always give you a Diagram)

Secondly if you have 2 different size batteries on 24volt your amperage becomes 50% of the combined amperage eg; 185Ah + 125Ah =310Ah at 12v
but only155Ah at 24volt

Thirdly there is a weight and storage factor to take into consideration Plus the fact that you cannot charge the 24volt system whilst on the water

I had a FOURTH But forgot what I was going to say

If it was Me I would stick to the 12volt system and if the weather was choppy enough to make the electric Cavitate You would probably NOT be using the electric anyway
If the 54'' puts the top of the prop at least 2 inches under the water I can't see a problem

Frank I also have an AGM battery which I run all of my electrics off because it does not take power from my starting Battery.

Thanks for that George, your explanation is exactly what I was worrying about. You have put my mind at rest. And after all IF the chop is that bad that the motor will come out of the water I probably wouldn't want to be out in it anyway. Think I will stick to the 12 volt but might hang out till they get the 60 " shaft model in stock.