CONTINUOUSWAVE

I'm concerned about the mounting height of our new 2020 Yamaha 90Hp on our old 1978 Montauk 17. The anti-ventilation plate appears to be about 2-3 inches ABOVE the deepest point of the transom. The Yamaha dealer pointed out that on most boats, the anti-ventilation plate should be roughly flush with the boats bottom (deepest point of the transom). The top mounting bolt is in the lowest of the four holes, so it could be lowered. However the dealer is concerned that the bottom bolt cannot be mounted any lower, due to the transom splashwell's limited height.

Has anyone with an older Montauk 17 experienced this concern and perhaps have a solution?

It appears the prior engine was mounted using a non-BIA hole standard. This is not a surprise. Many dealers installed motors in a unique way.

You may want to check the measurements of the hole positions. See below:





The tested and proven standard for mounting an outboard on a Boston Whaler boat pre-2002 is to start at 2 holes up when following the BIA standard pattern. Further refinements up or down are made based on prop selected, typical loading and typical sea/water state the boat will see.

Due to the position of the lower bolts, there is no other height that will work without filling in bolt holes with plugs (not dowels).

While the boat can operate in its current position of 3 holes up. It may ventilate on hard acceleration, moderate turns and coming off a 2-3' wave/swell.

Phil, Thank you for your response. I don't suppose that you would know if the "lower" bolt holes on a Yamaha F90 are elongated, allowing the lower bolt to "slide" as the engine bolts are re-positioned to the other "upper" bolt holes? If so, I don't see any reason that the Yamaha dealer's technician wouldn't be able to reposition to "2 holes up".

I think the spacing of the holes is wrong and this is the only height allowable.

More importantly-

Take a long flat edge and place it on the keel extending back so that you can measure the vertical distance from the top of the flat edge to the bottom of the anti-ventilation plate. What is this measurement? If it is between 1" and 2" the motor may be at the recommended height, regardless of the mounting hole placement.