While every rower is different, year after year we see people making the same mistakes, so I try to tackle the most common ones, their causes and how to fix them. As I've said before, I imagine my audience as primarily scullers but hopefully sweep rowers will find some good advice here too.
Number one by far is digging deep. We often see oars with 1/2 or more of the shaft buried in the water. The obvious cause is that the hands are too high, but usually this is caused by the body going up. It goes up at the catch and then the hands go up along with the body. Once the body begins to go up, it tends to keep going, causing the blade to go deeper as the drive progresses.
The key is to catch with the forearms and not the body – to keep the body still. Then when you start the drive, move the body horizontally.
Use your glutes and core to push the body to bow rather than trying to drive the shoulders to bow. To help feel this, try starting at the catch with the blade already in the water. It's much easier to drive horizontally when you don't have to think about the catch and the little bit of up that is needed to drop the oar in the water. You should also try rowing with as little pressure as possible - the oars will float at the right depth if you just let them.
Many people also have difficulty staying clean at the finish, whether that be washing out or getting stuck.
Washing out is caused by pulling the hands too low at the finish and literally finishing the drive with the blades no longer in the water. As long as the arms are pulling the oar handle towards the body, you want the blade to be partially in the water. The top half can be out so as not to get stuck, but the bottom half should still be in.
Then, just before you feather, release the pressure and start the feather, allowing the bottom edge of the blade to feather out of the water.
If you're getting stuck, make sure you are allowing part of the blade to come out square as you approach the end of the drive. If you feather as you continue to draw the oar handle towards you, you may cause the blade to feather under the water and get caught. Make the feather the beginning of the recovery, not the end of the drive.
To help feel and see this, try sitting at the finish with the blade squared and partly buried. (You can also do this with just one blade so you can watch just one at a time) As you feather the oar, think about pushing both down and away as you feather. I imagine my hands going down a little slide. Remember down and away. Top half of the blade comes out square and then the bottom half of the blade feathers over the water as it moves to bow.
If you're only getting stuck on one side, and it's probably the port side, you may be letting one hand finish too low relative to the other. This will sound counterintuitive but you'll need to bring that hand in a little higher to keep the boat from dipping down and causing the oar to get stuck. Also, make sure you have your weight centered in the boat. Often we lean towards the dock as we get in the boat and you want to center yourself as you start rowing.
Novice scullers in particular have trouble going straight. Putting in miles in the boat and becoming a more adept sculler with better catch and finish timing will probably fix that, but if you find yourself always going in one direction, you're most likely pulling longer on one side than the other. We're big advocates of nesting the hands for the crossover on both the drive and the recovery.
To do this, the left hand needs to be slightly above and to the stern of the right hand - think knuckles of the right hand brushing against the heel or palm of the left hand. Once you nest though, you need to keep the hands always in this relative position. If you over reach with the right hand at the catch (easy to do as most of us are right dominant) you will find yourself going to starboard. If you pull through too far with the left hand at the finish, you'll find yourself going to port. Check to make sure you always have the left hand slightly ahead and this way you will pull the same length arc on either side allowing the boat to go straight.