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What year is that 40hp Mercury?

Most newer motors have 4 or 5 bolt holes at the top and slots at the bottom (except Mercury where some have bolt holes also at the bottom instead of slots).
See this photo of a 40hp Yamaha.
http://www.whalercentral.com/images/p..._64/13.jpg

Are you sure 40hp is going to be enough for you?
http://www.whalercentral.com/articles...ticle_id=5

I have seen a pair of 40's on the back but never just one 40hp.
The boat is rated for 100hp max and 35hp min
http://www.whalercentral.com/userphot...lbum_id=14