Posts: 3480
Location: Elk River, Minnesota | Hiya,
First...a question. How hot and humid was it when you ran the boat? If it was really hot and humid, the loss in RPM's would be expected. When the air is as juicy as it has been (at least here in MN) makes for a "wet" mixture of fuel and air which will not burn as efficiently as when one runs their motor in, say 70 degree temps and normal humidity levels.... My Yamaha 90 2 stroke is very finicky in this regard as well. Like you I can normally get to about 41 mph at 5400 rpms with my normal load, but on days like this past Tuesday, it would be hard pressed to get RPM's up to 5100.
The other issue that takes place with the warm temps as well is that with water, it expands, therefore your boat, although unable to tell by just looking at it, sits lower in the water, even while at speed which increases the wetted surface of the hull. More wetted surface means more friction, which robs speed and forces the motor to work harder to keep a specific speed.
When you combine those two elements, performance is robbed pretty dramatically.
Not sure if this is the case with your boat, but something nice to know nonetheless...
Steve |
