Re: 40 hp03 yamaha on 5000# house boat(prop?)
5,000 lb non-planing boat with 40 hp? Yikes. <br /><br />You'll want the smallest pitch and biggest diameter prop. This will give you the best thrust and highest rpm (you'll need it). Yamaha makes a 12 1/4 diam X 9 pitch for the 40. Trade the stock one in for that.<br /><br />2 or 4-stroke?<br /><br />FYI the 2-stroke 40 runs out at a max 5500 rpm with a 1.85 gear ratio. The 4-stroke runs at 6000 rpm with a 2.0 ratio.<br /><br />This means (without doing the calculations for you) that both the 2-stroke and the 4-stroke 40 produce about the exact same torque (WOT at prop) of 70 ftlbs. The 4-stroke's prop turns at a slightly higher rpm, so it would be slightly faster prop for prop, but hardly noticable. Since the 4-stroke's engine turns not only faster, but more often per revolution of the prop, it would probably have a better power and torque curve throughout the power band, especially under load. Ironically Yamaha's 4-stroke 40 is nearly identical in weight to the 2-stroke, so no performance lost with additional weight.
