15 Hp 4 Stroke Long Shaft 'Speed' - Mercury vs Yamaha

[cabovic216]

I recently bought a 2005 15 Hp Yamaha 4 Stroke with Electric Start and Power Tilt to replace my 2001 Mercury 15 Hp 4 Stroke Manual.
To my amazement, I ran the Yamaha yesterday (same boat - Lund 14') and the Mercury seemed to go a lot faster.... the Yamaha's prop is 9 1/4" x 11, however, I don't know the prop size that was in the Mercury, although it was the stock prop.
I'd expected the Yamaha go at least as fast as the Merc., I'm not trying to set any speed record here, but the Merc.would go about 15 mph (boat loaded with 2 people), and the Yamaha will barely do 10 mph.
Any idea if this is driven by different props or is the Merc. is just faster ? I trimed the Yamaha engine and 10 mph was the most I could get (without trimming, it would only do 8 mph). The Yamaha only has about 25 hrs on it, while the Merc. had about 150 hrs of operation. I realize the Yamaha is a bit heavier with the starter and alternator.

The Yamaha seemed to run quieter, but still at about the same speed while trolling.

Thanks for any comments on this subject in advance.

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Re: 15 Hp 4 Stroke Long Shaft 'Speed' - Mercury vs Yamaha

Was the mercury a long shaft too? that 9 14 X 11 prop has quite a bit of pitch for what you are pushing.
the stock prop on the mercury I think is 9 1/2 x 9 1/2. Pushing an old 14' Starcraft I get 16 to 17 mph (gps)
with 2 fatskis and gear. 21.6 mph with one fatski. If your prop is too deep it will kill top end. Good luck!