This is more on one side than the other. It looks a little worse in the pics below than it does in person due to shading and transom angle, but it is 2 to 3/32nds... I have been keeping an eye on it, and it didn't exist when I redid the boat 6 years ago.
My first thougt was 'gee whiz my 250 lb engine is yanking the transom off'.. However, the more I look at it, I think it might be something else. Number one, I want to make sure it doesn't get any worse, and number 2, you can see that the varnish and the wood underneath is splitting where a screw goes through the knee and the transom.
Looking all around the outside edges of the transom, the edges are all dead on.. there is no deflection anywhere that suggests the transom is moving away from the plank ends. Looking at the inside of the transom, there is at most just a cracked seal of varnish between the cross support and the transom, and there is an aluminum bar support at the center of the transom that is rock solid. Although I cannot lay a straightedge across the outside back of the transom because of the motor brackets being in the way, it doesn't look like there is any bow out there.
I have trailered the boat perhaps 5000 mi the past few years, I am wondering if the separation is due to the sides of the boat wowing out a little... Maybe rocking back and forth on the trailer is allowing the sides to move out just enough to open up the inside end of that right angle knee.
I also realized that I stopped using a strap several years ago that wrapped over the sides and down to the trailer. Given that there is no cross supports across the gunwale until the dash, maybe the sides have drifted out a little. I would think that it wouldn't take much deflection to open up the short span of the knee.
Since I have never replaced the brass carraige bolts through the transom, I plan to do that first. That won't help this situation , but Andreas and others have found their 60 yr old bolts to be in pretty rough shape.
Then I think I will try to lift the motor skeg to take the weight off the transom, and shoot one or two recessed SB screws wherever I can from the inside, through the knee and into the transom. Although I hate to put a screw where there wasn't one originally, I would rather not drill out the plugs and redo the old screws and have to refinish the transom... but I might.
So back to my original question... have any of you seen this sort of separation in your transom knees... If you have a well, I doubt you would ever have this problem.
John.