20" transom ? motor

I've got a question about motor and transom heights. How come if manuf. knows transom height they don't or didn't make motors so cavitation plate was even with the bottom of the boat? My Johnson is a long shaft supposedly for a 20" transom but cavitation plate is 2" below bottom. There's got to be a good explanation. Also the Mercury that I took off before this motor was the same way. Too low, or is it?Thanks!

I hear ya'. It seems the boat manufactures say a 15" transom, but they don't take into account the angle of the transom, resulting in a short shaft motor being slightly long.

First off..........transom angles run in the neighborhood of 12-15 deg. in general. For outboard powered boats.
Proper way to measure the transom is to take a straight edge and place it on the bottom of the boat extending out the back.
Measure straight up (for the non-rocket scientists) or perpendicular (for the rocket scientists) to the straight edge to the top of the transom. This is your true transom height.
Lots of curve balls as well in this. Boat manufacturers in general will err on the low side so that they do not get complaints in their customer service depts about cavitation during turns or while planing off. To keep the complaints to a minimum they cut the transoms down and that drops the engine down as well. End of the issue for the boat manufacturer. All boat manufacturers will have different heights for different boats........enjoy!
Engine manufacturers claim short shafts are 15" and long shafts are 20". This is just a rule of thumb measurement. Evinrude/Johnson will be different than Merc who will be different from Suzuki who will be different from Yamaha and so on. Not all engines are the same and some are not even close. Same thing with water pick ups on the gearcases. Not all MFRs place them in the same place.
Each time you want to properly set up a boat and post questions in here about proper set up take everything with a grain of salt if that person answering you has a different model boat or a different brand of engine.

What he said is 100%.

I will add that I know nothing of Merc, Zuke, OMC, BRP, or Nissan. Yamaha is all I know. Even some of the "short" shaft Yamahas are different lengths. The specifications are usually either in the rigging guides or they may be on their website. But a 25 HP outboard "short" shaft is longer than a 15 HP "short". Even though they're supposed to be 15". We're not talking inches, maybe 1/4" but theres a difference. Water pick ups on different motors even same manufacturer are sometimes in different places. Some of the Yamaha 9.9 pickups were just under the av plate. Some others were in the "normal" location. At least a few more have the normal pickup, with a separate hole on each side just above...adn if the hole is not in the water, it won't pump. Been there, done that. Seems like it was a 40 HP.