I was thinking of changing my tips and getting rid of the external rubber flapper. Is there a rule of thumb to how high the tips must be from the water line to not use external flaps? Is there a rule of thumb on the risers being vertically higher than the tips? I see boats with no flaps and the tips are practicaly under water. I just don't want to do something stupid and end up hydro-locking my motor.
BTW the motor is MPI with a stock cam.

Just make sure you get a set of tips with good internal flappers and you will be fine. Corsa's are good and have a lifetime guarantee. They make a little racket at idle though.

Shah,

There IS an actual figure (measurement) that Merc uses to determine that, regardless of what type of tips you utilise, you are in the safety zone as they consider it.
The reason I know this is because I asked them to investigate a 'ghosting' salt pattern I had in our exh manifolds some time back and they immediately asked for the distance of the tips from the waterline.With 3" extensions on the manifolds and a stock engine we were OK.Sorry I can't remember what the figure was but now you know that Merc does have a number in mind !

Be careful with removing the external flaps. Like Griff said, get some tips with decent internal flaps and you should be OK. In my setup, the tips are actually below the waterline, but I have about a 12" rise to get to the manifold. I use external flaps and had installed internals but my exhaust puked them out the back. Just to be safe I "burp" the throttle when coming off plane to keep the pressure on the right side of the flap.