On a stock transom a 20" is perfect. A 25" will cause more drag and with the same hp and same engine you will not get top end max knots. Trust me. Lots of trials and errors. My dad is running twin 20" on an extension built too high for it. It cavitates and he cannot plane properly. With 25" he would have been fine but hard head my dad
.Mahalo. I'm sure we'll have our share of trial and error before this is done..lolOn a stock transom a 20" is perfect. A 25" will cause more drag and with the same hp and same engine you will not get top end max knots. Trust me. Lots of trials and errors. My dad is running twin 20" on an extension built too high for it. It cavitates and he cannot plane properly. With 25" he would have been fine but hard head my dad
. I used to fish with a friend of mine who had a Yamaha 90 20" and we flew to Miloli'i at 29 knots to help tow back a bigger boat. A few months later we installed a 90HP ETEC with 25" shaft which we thought would be overkill for the boat. Boat would not hit 25 knots. No matter what. Too much drag. He sold that motor in less than a few months. Bought another 90 short shaft but sold the boat shortly thereafter. For the Unicorn.... an 18' Alii kai. Not extended. But built by the Berdons stretched. That boat was wonderful until he sold it for a Force 18 and then my dream boat. A Bolos 20.
