Opinions on if I should repower 18

[mikeyk]

Originally Posted by DonV

"BTW, the F70 and F90 are exactly the same in terms of weight, dimensions, etc. Only difference, I believe, are the stickers and ECU...and the price"

Not true, the 70 is 1000cc and the 90 is 1800cc, the 90 is much heavier, 100# more. The 70 is only available in a 20" shaft and the 90 comes in 20 or 25". The 70 is basically a motorcycle engine with great HP/weight ratio. It was designed for the 16 to 18' technical skiff market.

https://yamahaoutboards.com/en-us/ho...15-50-hp/70-50

https://yamahaoutboards.com/en-us/ho...115-50-hp/f115

I stand humbly corrected. I was using the F75 as a comparison. It appears that the F70 and it's weight difference between the F75 is better suited for lighter boats where 70 hp is the upper end spec for mfgrs. power and engine weight, i.e., skiffs. Pushing a heavier boat with a 70 at the 5800 Rpm powerband range would probably require smaller diameter and/or lower pitch prop to utilize full horsepower. I noticed that Yamaha does not divulge torque specs. but using the formula
Torque = Hp x 5252 / Rpm (5252 is a constant)
the F70 yields 63.4 ft/lbs. @ 5800 rpm, the F75 71.6 ftllbs. @ 5500 rpm