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Message: <blockquote data-quote="cerevisiaephilus" data-source="post: 357374" data-attributes="member: 13421">This has been bothering me for a while, bored at work now, so I thought I'd ask. The Yamaha performance bulletin for the 40/30 jet and the 60/40 jet are very similar. Same total boat weight, more or less, same top speed, same acceleration, if anything, the 40/30 does better. Does the 60/40 outperform the 40/30 only when you increase the total weight of the boat? Can anyone explain the physics behind this? I guess I'm confused by all the talk about different sized jet lower units affect performance. Seems like if they both have the same RPM and the same impeller size, how does horsepower matter? Exactly what is producing the resistance on the impeller? The physics of a propeller make a lot more sense to me. And I understand the chart on the Jet Outboards website, just not the actual mechanics of how a bigger power head on the same size pump can make a difference. https://www.yamahaoutboards.com/sites/default/files/bulletins/bulletin_4stroke_midthrustjetport_jet-drive_g3b_1656ccjdlx_f40jea_2011-06-13_alm.pdfhttps://www.yamahaoutboards.com/sites/default/files/bulletins/bulletin_4stroke_midthrustjetport_jet-drive_g3b_1656ccj_f60tjr_2010-05-18_alm.pdf</blockquote>