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21 Foot Formula Prop selection help needed
Hello
I just replaced a 260 HP motor with a 411 HP motor - small block bravo drive. the boat is a 21 foot formula F206 LS - v hull sport boat. previously with 260 HP it ran 60 mph at about 5500 rpms with a 17 inch pitch Mercury Revolution 4 prop with 150 HP more I have no idea what i might need for a prop. i'm interested in top speed the new motor can run at 6000 rpms. it's a blue print marine engine. I ordered a 21 inch pitch and they are on back order. do you think that's too much for my application? Should I go with 19 or 20 inch pitch? Thank Cliff in Maine |
What gear ratio does your drive have. |
Originally Posted by obrien
(Post 4652764)
What gear ratio does your drive have. |
Something isn’t right with your numbers. With 0% slip you would be running 59 mph Figuring roughly 12% slip you would be just shy of 52 mph. Either your tach, pitch, or speed is incorrect. How are you getting your speed? |
That speed sounds way off but just off formula hills and that power I bet you end up with a 21 or 23 mirage or mirage plus. |
If you keep in the Rev 4 line up a 21" will be too much. I would not go bigger than a 19" Rev 4 IF using that prop line. Your fastest top end will be with either a Laser II or Mirage in 19-21" pitch range. Bumping performance up with a small block does not equate to throwing big pitch props at them. The power is made in the upper reaches of the rpm band but there isn't the torque to swing a large blade, large pitch prop. So, nearly the same pitch, maybe a 'little' more but you will just spin it faster.
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No way a 21' Formula runs 60mph with 260 hp. Break out a GPS and prepare to be disappointed.
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