On shallow water with bottom full of rocks and stones, both the rudder and the propeller can easily break.
What is worse is that if the rudder is stuck and you keep trying to turn, the small servo can fail. If your rudder has the potential to hit the bottom, consider to cut it shorter a little bit. If you are using a plastic propeller, consider to use a metal one instead.
This video shows a high speed brushless boat, also with a shorter rudder (about 2 inches):
A metal rudder can be cut short a little bit. Sometimes several mm difference is all that is needed for smooth sailing.
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