70hp Merc on 12' boat.. help

Scisor34

Cadet
Joined
May 6, 2010
Messages
20
This boat is 11'4" long, and I have a 1979 Merc 70 mounted on a 10" setback jackplate with a 10 3/8 X 14pt 3 blade prop. I'm unsure of the prop manufacturer. I'm in the "dial-in" process, The jackplate it came with had no adjustments so I had to redrill the holes, and think I'm about correct on the motor height. With the motor trimmed completely down, it didn't throw it up on plane like it should, and running WOT 5100rpm (still trimmed all the way down)I was hardly able to trim motor up at all without it wanting to jump to 6000rpm. There was plently of motor left in the water, and I wasn't able to get any bow lift so my top speed was only 39mph. I know it's impossible to know what prop will perform best without testing it, but anyone have any advice? or ideas? If I didn't give enough info, please let me know. Thanks
It's a 2-seater and probably weighs around 600#, but that's a total guess.
 

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steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: 70hp Merc on 12' boat.. help

Was it venting when rpm jumped or did it accelerate?
Did you ever try to pick up the boat?I would guess 300-400 lbs.
unless its a real brute.
On that little boat I would think at least a 17" pitch.
When running at speed the antivent plate should be visable.
 
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