Dinghies aren't race boats, and they don't exactly make great ski boats. They're designed for safe transportation.
So when choosing an outboard motor for your dinghy, more horsepower isn't always the answer.
Instead, stability and efficiency should be at the top of your list of things to look for in an outboard motor.
What makes an outboard motor more stable and efficient depends a lot on the specifications of your dinghy.
Determine Your Horsepower Needs
Let's start by addressing what everyone's concerned about - horsepower. Bigger is not always better. Be sure to check your dinghy's capacity plate before choosing an outboard. That being said, if it's within the limits stated on the capacity plate, you're fine. With aluminum john boats and v-bottom fishing boats, you may want to size up to a 15 or 20 horsepower motor.
Let's assume your dinghy has a maximum horsepower rating of 20 hp. This is the most horsepower you should ever find on a dinghy (ideally), but the fact that your dinghy is rated for up to 20 hp doesn't mean you should equip it with the maximum power it's capable of handling.
To find an adequate horsepower for your dinghy, determine the mid-range. The mid-range horsepower for your watercraft is half way between 2.5 hp and the maximum horsepower listed on your dinghy's capacity plate. So if your dinghy is rated for a maximum of 20 hp, your mid-range horsepower would be 11.25 horsepower. However, you're not likely going to find an 11.25 hp outboard motor, so just round up or down to the nearest size. A 10 hp or a 15 hp outboard will better suit you.
Determine Your Dinghy's Weight Limit
Your dinghy can only hold so much weight, and that weight includes you and your gear. To determine how heavy of a motor you can equip your dinghy with, you'll need to look again at its capacity plate.
Your dinghy's capacity plate will tell you what its maximum weight capacity is, but it'll tell you in a couple of different ways. First, in larger letters and numbers, it will list what the maximum passenger limit is for your watercraft. It'll say something along the lines of "4 PERSONS OR 500 LBS.," but the 500 LBS is not the total weight limit for your watercraft.
Below that, you'll see something more comprehensive, along the lines of "650 LBS. PERSONS, MOTORS, GEAR." This will be the total weight your dinghy can handle. Subtract from that your weight, the maximum weight you'd ever carry in passengers, the maximum weight you'd ever carry in cargo, and the remaining weight will be the most your outboard motor can weigh.
Determine Your Transom Height
You also want to make sure the prop will reach below the boat. At the back of your boat, measure the hull from top-to-bottom in the center. This will be your transom height. Your outboard motor will be mounted at the top of the transom, and the shaft of your motor will extend to the bottom of your transom.
The prop should extend just below the bottom center of your hull, allowing it to pull water from beneath the boat and push it behind you. Outboard motors come with different shaft lengths, and you'll want to make sure that the one you choose has a shaft equal to your transom.
Think Practically About How You'll Use It
This one's a little less technical, but it's equally just as important. Will you be using your dinghy every day? If so, you'll probably leave the outboard mounted to your dinghy and it won't be an issue. If you'll be using it weekly or monthly, you're going to need to remove the outboard to store it between uses. A lighter motor will be much easier to lift on and off of the transom. After all, you don't want to drop it in the lake. That would be one expensive anchor!
Will you be using your outboard for transportation across a large windy lake? If so, a bit more horsepower might help keep you on track so you don't drift too much in the wind. You'll get from point A to point B much more quickly and efficiently. However, if you're primarily using it to move you around a calm, quiet lake, less horsepower may suit you fine to more calmly and quietly move across the lake.