On paper how much faster is a 15 HP compared to a 9.9

[909]

Just picked one up today

Doesn't feel much different.

1976 Evinrude 9.9

1976 Evinrude 15

How can I make the 15 faster ?

 

[909]

 

[kbait]

My ‘85 15hp pushes my 14’ deep/wide alumacraft 23 mph
Tested a ‘79 9.9hp on same boat - 18 mph.
Both motors were tuned and running normally with standard 9x10 props. I’m sure a 9x8 prop on the 9.9 would have helped on my fairly heavy boat.

 

[ryan 98]

4 mph might not be too far off. Speed and power are not exactly linear. I think the rule is close to double the power for 30% more speed. There's also the question of what prop pitch there is between the two motors. The 15 should have more get up and go in it. I think in those years both motors were even the same block. Just different carbs and maybe reeds. But I'm probably wrong on that. If it's a similar prop on both, and they both rev at similar rpms you speed is going to be pretty close. But it should have more in it with the boat loaded up

 

[F_R]

The 9.9 and 15 both came from the factory with the same prop. But the 15 has more horsepower, and runs at higher RPM, so it can swing the prop at higher RPM, which translates to higher speed on the same boat.

Having said that, the actual top speed can vary considerably according to boat and load. If the 9.9 struggles to get up on plane but the 15 planes it with ease, it will be quite faster. At best I wouldn't expect more than 3 or 4 MPH. Your experience may vary.

 

[ryan 98]

All add that my tinner does about 30 with a 9.9 and just shy of 40 with my 18. So that follows the 30% rule close enough. Even tho I basically only drive it around just on plane, both motors would perform the same there but I still use the 18. Once you load it up heavy with gear then you really see the difference in power and for the most part I only use the tinner for back country camping where it's loaded up with gear. If your just after speed and have an empty boat try for more prop pitch. Should pick up some speed. With those speed numbers I'd guess your boats pretty light to start and the 15 can probably spin a bit more prop on it. Also rase the motor up on the transom so the cavitation plate is level with the bottom of the boat, you can go higher but make sure it's able to pump water. Should pick up some speed there. But it all comes back to the fact it's a tinner not a race boat and an extra few mph isint really that important.

 

[oldboat1]

Change out the impeller before more trials, if you haven't already done that.

Find a light 12' Starcraft, and operate from the center seat using a tiller extension (articulated extension for full control).

 

[ryan 98]

Change out the impeller before more trials, if you haven't already done that.

Find a light 12' Starcraft, and operate from the center seat using a tiller extension (articulated extension for full control).

60s narrow barrel back style tinners go like hell too. And kinda stable. I think the wind just isint as big of a factor from the low profile. All attach a pick of mine that has seen well in to the 40s... theoretically... I can't exactly advocate for fast tinners since their the one boat that has scared me more than an 90mph 15 foot hydrostream. When the boat only weighs 100 pounds wind becomes a big factor. But if one were to theoretically go for speed, a hull design like this works way better than my super light 90's crest liner or whatever brand high side tinner. Crappy photo I know, but it's the only one I have of it. It's a 61 built somewhere around here in Canada. But a bunch of brands made similar things and their cheap to find now.

 

[909]

Thank you for your replies.

I will measure the RPMs of both motors and report back. I feel the 9.9 revs higher by sound alone.

If the 9.9 revs higher than the 15 what would be the cause of that ?

What causes an engine to not rev as high as it should ?

The 9.9 hp has CDI , while the 15 hp has points / condensers.

I put in a new OEM impeller kit.

 

[ryan 98]

Thank you for your replies.

I will measure the RPMs of both motors and report back. I feel the 9.9 revs higher by sound alone.

If the 9.9 revs higher than the 15 what would be the cause of that ?

What causes an engine to not rev as high as it should ?

The 9.9 hp has CDI , while the 15 hp has points / condensers.

I put in a new OEM impeller kit.

Start out at the props on them. Are they identical? Either of them bent or damaged? There's no way the 9.9 could be reving higher than the 15 at a slower top speed if they have the same prop. Unless it's set up different on the transom and there's more prop slip with the 9.9.

 

[909]

15's numbers are getting better

Props are the same. Everything is identical.

Only diff is the 9.9 is CDI (stronger spark) and the 15 is points and condensers

 

[909]

Great deal came along, but I'm curious...

Is a newer engine able to outperform the older one ?

 

[jbcurt00]

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