I hate to say this but those are some wishful numbers out of a stock 260 hp engine. The stock 260 engines are only 2 bolt blocks with all cast internals. I don't think they would last long at that RPM range plus you are way out of the operating range of that engine. Now if you heavily modified a pair of 350's then sure, but we are talking about stock engines that are 25 years old. Personally I'd stop trying to get it to 62 with a prop swap, most likely its not going to happen. 58 mph is respectable for an older 242 with the original engines.

You also need to remember prop slip. A typical 242 single will have slip numbers around 12-14% at WTO a twin may be in the 7-11% range. A pair of stock 350's spinning at 4600 with a 21 mirage would give you a top speed of 57 MPH with a prop slip number of 7%. Which is the best you can possibly hope for. I think 10% is a more realistic number. Now different props are going to change this number as you have seen. Either way to get to the magic 65 I think you are going to have to rebuild your engines for more output. Out of curiosity, how many hours are on them? Here is a great tool for props and speed. http://www.mercuryracing.com/propell...calculator.php

The engines have to be fresh for sure...or they're not delivering 260 HP.

I think 63-65 on gps should be feasible with twin 350 mags that are running strong in the best of conditions. 242 with a stock 502mag (390hp) will run 62 with a 22-bravo and will run 63 with a Hydromotive Q-IV on gps. A strong, true 535hp motor(think 509) has gotten a 242 to 70 that I know of. Granted there is more weight with additional drive and another motor but you are talking small blocks and alphas on the back so the weight isn't exactly doubling what is in the big-block and bravo boats. Also the angle of attack and COG on the twin boats are a little different and will get the boat riding on more the back end and running little less "wet" than the singles. I am interested in knowing if the same hook on the 242 signles exist in the 242's with twins (speaking only from 87-91) and how that might play into things

Another thing to remember is that a 242 with a 330/454 uses a 21 pitch mirage as a stock prop. If your only able to spin a 21 with twins my guess is that your engines are really tired. You should be able to spin at least a 23 maybe 25's. 25's would put you at 65 mph on the prop slip calculator. What are you guys with stock 350 twins using for props to hit 65 MPH? Have you done a compression test to see what compression is on your engines?

what should a single 350 use for props?

Couple different factors...Alpha or Bravo? Gear Ratio? Think you should start around a 19-21-23 Mirage or 22-24 Bravo without knowing too much but others can better assist

Unless it was rerigged it would be alpha with 1.5 gear ratio.

Yup alpha 85'.

A 17 or 19 pitch Mirage would work well.

It'll take 23 Mirage's to get the 242 to run 65. It's the same prop's they were also putting on the 272's with the 350/alpha combo. If you can't spin the 23's, that's why you're not getting the top end.