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Author Topic:   Dauntless 180 w/ 135hp Optimax RPM Question
bravocharlie posted 07-16-2005 08:08 AM ET (US)   Profile for bravocharlie   Send Email to bravocharlie  
I have a 2004 Dauntless 180 with a Mercury 135hp Optimax. The outboard is fitted with a stainless steel 17 inch Mirage three blade propeller.

My question is:

With the motor trimmed all the way in (0.0 trim)and going to 3/4 full power to get up on plane, the RPMs are at 4000 +/-. As the boat levels off on plane, the RPMs appear to drop to 3500+/- without any throttle adjustment and the motor appears to (sounds like) it shifts gears (down one to a lower gear).

Is this normal? And, if so, what is exactly happening?

Whalerider posted 07-16-2005 12:22 PM ET (US)     Profile for Whalerider  Send Email to Whalerider     
It's normal. The prop is slipping (through the water) at low speeds and under load. As you gain speed the prop hooks up (with the water) and away you go. If the prop has vents, its working exactly as it is supposed too and will be more noticeable.
bkloss posted 07-16-2005 07:28 PM ET (US)     Profile for bkloss  Send Email to bkloss     
I have the exact same setup as you and mine does just the opposite. If I do a whole shot and say rev to 4000 rpm, once she gets up on plane and the boat levels out - there is less resistance (boat and motor) causing an increase in rpm at the same throttle setting. I then throttle back to decrease rpms and then trim out relative to the seas.

I have a dolfin, maybe that creates a difference or maybe that my motor is raised 2 holes - but I never have any slippage with the S/S 17P.

bsmotril posted 07-17-2005 12:31 AM ET (US)     Profile for bsmotril  Send Email to bsmotril     
The prop is ventilating until it hooks up with solid water. This is good to a certain extent in that it lets the motor rev quickly into its' peak powerband. If your mirage prop has the vent holes, that is why it is happening, and those holes are there by design to allow the motor to rev faster out of the hole. If your hub does not have the vent holes, it is likley that the motor is trimmed to high for a clean hole shot. Try it again with the motor trimmed fully in, and then when the boat comes on plane, trim it out for best efficiency. BillS

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