24v motor on 12v outboard

[GW267000]

Thinking about installing 24volt power assist (already have from dads old boat) to twin 200 OX66 but would like to use only three total batt. Any ideas on how to wire this. Would only run power assist while running and not while *****ing for around 6 hours??

[99yam40]

what is the model of the 24 volt power assist?
Some of them are auto select for 12 or 24

[GW267000]

Dont have model # with me but it a 24volt only as in 2008 that is what it was bought for. Wish it was the auto sence.