When mud blows out and won't refill to give the prop bite you are not going to go anywhere because of prop cavitation. I have noticed that this can sometimes be remedied with the Prodrive motor with full power reverse. On the Prodrive, when you go in reverse you are actually ratcheting the drive portion of the motor 180 degrees. Your prop is now under the boat, which becomes a huge cavitation plate to keep the mud from blowing out. Doesn't always work, but when it does you are glad you have full power reverse. You can see videos of this on the Prodrive Outboards website.