Transom Height and cavation plate location

DaveP

DaveP

Active member
To all you boat and motor experts. I just built a boat posted in " Started Another Duck Boat Build". The plans were for a twenty inch transom, long shaft motor. My understanding on outboard motors come in 15" short shaft, 20" long shaft and 25" extra long shaft. I got a good deal and purchased a '99 yamaha 25 hp 2 stroke with electric start; but it was an extra long shaft. So I rasied the transom 5" to exactly 25". I placed the motor on the boat for the first time yesterday. The prop extends below the keel, but the cavation plate is about 2" above the bottom of the transom, placing it well in the water. With the cavation plate above the bottom of the transom will this effect the motors performance?
 
The cavitation plat should be 1 to 2 inches below the hull. With how you have is now when you are on plane the cavitation plate will be 2 inches out of the water and the motor will cavitate and overrev, especially when you turn. On the boats we build we try to get the cavitaion plate 1 to 2 inches below the hull and that seems to work out great for us. If the motor sits too high we just cut the transom down. I had to do this on my last boat, a devlin bluebill, and i eliminated the cavitation problem.
 
Carl, That is what I thought. I already cut the transom down to 22'' inches. Which should put the cavitation plate about 1" below the boat. On another forum someone told me Yamaha extra long shafts under 40hp are actually 23". Which in my case seems to be true. Had I place the motor on the boat before finishing it, I would have seen this, but I did not want to get any oil or grease on the transom before finishing it.
 
You also have to remember that the bottom of the transom is not necessarily the bottom of the boat. You may have to give it a test run and see what happens..then adjust. Good luck.
 
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