Re: WOT on 4.3 merc
If you've had the motors tuned and they didn't put them - properly - into base timing mode, you'll see a noticeable drop in performance due to retarded ignition timing. Couple that with falling out of the peak HP band for that motor and...slow.
Obviously, the data doesn't add up.
I will say, with modern progressive pitched props we've run into some darn low slip #'s for the "Quoted" pitch.
Those Fas-Trac hulls are great and efficient, but more difficult to set up. The smaller ones put so much air under the boat, many "Good" props ventilate when any kind of trim is applied.
Diameter and blade area changes can significantly affect RPM's. The rule of thumb is 1" decrease in diameter will increase RPM's by 400 as well as the more commonly quoted 150/200rpm change per inch of prop pitch. Your mileage may vary.
You mentioned a 14" diameter prop. It is not uncommon to see a 400rpm difference between same pitch Lazer II(~14"dia.-higher rpm) and your Mirage(~15"dia.lower rpm) with same Mercury quoted pitch.
Since this is a new to you boat, did you sea trial the vessel?
If you've had the motors tuned and they didn't put them - properly - into base timing mode, you'll see a noticeable drop in performance due to retarded ignition timing. Couple that with falling out of the peak HP band for that motor and...slow.
Obviously, the data doesn't add up.
I will say, with modern progressive pitched props we've run into some darn low slip #'s for the "Quoted" pitch.
Those Fas-Trac hulls are great and efficient, but more difficult to set up. The smaller ones put so much air under the boat, many "Good" props ventilate when any kind of trim is applied.
Diameter and blade area changes can significantly affect RPM's. The rule of thumb is 1" decrease in diameter will increase RPM's by 400 as well as the more commonly quoted 150/200rpm change per inch of prop pitch. Your mileage may vary.
You mentioned a 14" diameter prop. It is not uncommon to see a 400rpm difference between same pitch Lazer II(~14"dia.-higher rpm) and your Mirage(~15"dia.lower rpm) with same Mercury quoted pitch.
Since this is a new to you boat, did you sea trial the vessel?
