By Fleet 1:

One of the more common questions for our sailboats is “How big (or small) of an outboard do I need?”

There are three factors to consider with the San Juan 21, Horsepower, Weight, and Shaft Length.

A 4 HP motor is plenty of power to get our boats to hull speed in all but the most extreme conditions. The boat can only move so fast through the water, that’s hull speed and it’s around 5.77 knots. A bigger motor won’t move the boat much faster than that. Facing high winds can slow the boat down and here a larger motor will get the boat back up to hull speed.

Larger motors will allow you to get to hull speed without going full throttle so they can be quieter and have less vibration.

Electric outboard motors are enticing with the idea of no smoke, fuel, or noise. However, they don’t save any weight as the motor and battery weigh as much as an outboard. It does allow you to put the weight forward so there is an advantage there but the output of these motors isn’t all that spectacular. Best Guesses are that these will push our boats to about 2 to 3 knots. Ok if all you do is just want something to get away from the dock.

Another factor is the length of the shaft. A standard shaft or short shaft motors are usually around 15”. Long shaft motors are around 20” or more. A short shaft motor can have problems with the prop lifting out of the water in rolling waves and/or if a crewperson goes too far forward, especially with the stock motor mount. This wouldn’t be much of a problem except that water cooled motors can damage their water pumps quickly if they don’t have water running through them. Mounting an adjustable motor mount that can lower the motor further than the stock mount will help. 
The Weight factor is the most important to racers. Our boats are sensitive to weight on the stern so racers want the lightest motor they can get.

Except for the Honda, Yamaha, all the 2.5 and 3.5 hp 2 stroke outboards are made by Tohatsu. The Nissan, Merc, and Evinrude are all the Tohatsu motor. The difference between the 2.5 and the 3.5 is only the carburetor so it’s worth it weight wise to get the 3.5. These motors weigh about 28 to 30 lbs. and will push a San Juan 21 to about 5.5 knots.
The current Tohatsu based outboards are all 4 Stroke now. The 3.5hp 4 Stroke weighs 41lbs according to Tohatsu, the 2.5 is likely similar. The Nissan, Mercury, and Suzuki 4 Stroke 2.5/3.5 hp engines are all rebadged Tohatsu's.
Honda has a long shaft, air cooled 2 hp 4 stroke motor that is light weight at 28 lbs. and works well for Racers. It pushes a Juan to about 4.3 knots.