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Joe - I think your article is excellent, and a good reference article for most. There are two assumptions that I assume you are using, and which could be complicating factors if those assumptions are incorrect in a given situation.

1. The top bolts are drilled at the BIA standard and recommended 1-7/8" Centerline down from the top of the transom. Although this should be case for maximum transom stress safety, it is not always the case. I have seen top holes drilled higher, and even have that situation on one of my boats. Often an installer will do this because of a need to raise the bottom bolts slightly. So everyone should check that situation.

2. Not all engines are designed with the same middle section dimension, either 20" or 25". This can be difficult for a consumer to measure or determine. Engine manufacturers do not generally give this dimension. In other words, the difference between the vertical dimension from the top bolt hole to the anti-ventilation plate can vary, often considerably.

Regarding the E-tec 90 on a Montauk, where I keep reading the engine has to be mounted all the way up in the 4th bolt hole, I find it very hard to believe that the engine is running 2-1/4" high. This is the realm of high performance surface piercing racing and limited purpose speed, with special cooling intakes needed. Not even many props can truly handle this height. My best guess is that the Evinrudes have a longer than 20" mid section length, so that an engine up 2-1/4" may actually only be up 1-1/4", which is considered plenty high for most applications. Regarding the mid-range Mercuries, and Big foot gear cases, Mercury actually indicates that these are longer than normal, and need higher mounting. I can verify this from my own experiences. When I changed the power on my 18 Outrage from the older Mercury "towers", I had to raise the newer 90's up another 1" to acheive the same height, with same prop line. The 90's are at least 1" "taller" than the older engines, and also taller than my Merc V-6's.

On my Montauk, I have the jackplate mounted Merc "tower" running about 1-1/4" high, and even with a Laser II high performance prop, this is all the boat can handle for all around good holding and performance. There is no way an E-tec 90 could be running 2-1/4" high for all around use, without major slippage and ventilation. So I assume the E-tecs have a longer/taller mid section.

About the only way to completely predict mounting height would be to keep track of actual situations, as has been done with the E-tec 90 on Montauks.

I guess my point here is that if you mount an engine in the third set of holes, a good starting point for maximizing performance, don't assume you are up 1-1/2". It may be less if the engine has a longer than standard shaft length.