On older and smaller sailboats, a popular motor choice is the gasoline outboard. These motors offer the ease of replacement of any outboard, but suffer from the fire risk of carrying gasoline onboard. A common argument for these motors arrangements is that you will already be carrying gasoline for your dinghy outboard, now you can power both with the same tank!
Gasoline is a very dangerous substance on a boat. The liquid is highly flammable and the fumes are explosive! A small problem can easily become a life threatening explosion that will engulf the boat in flames.
Gasoline inboards complicate this problem further by bringing the entire unit inside an enclosed area where fumes can collect and await detonation.
Modern boat builders have recognized the risks involved with gasoline, and have since moved over to diesel motors. Diesel fuel is flammable, but not explosive and there is no fear of the fumes blowing your boat up.
While much less dangerous, there is still a safer option yet: electric. Electric motors require no fossil fuels at all, and therefore pose no risk of fires or explosions from fuel leaks. Instead, electric motors are powered by massive battery banks that feed them the needed amps to keep the propeller spinning.
As with everything, there are pros and cons of each.
Gasoline outboards are very inexpensive and if they die, they are the easiest to replace. Their fuel is a hazard, but it is usually kept in a red plastic tank in the cockpit. If the tank starts to deteriorate, it can be easily replaced. All of the fuel and fumes is kept out of the cabin, so there is much less risk of an accidental explosion.
Gasoline inboards are inexpensive to buy, but costly to replace and repair. They also bring gasoline and all of its hazards inside the boat where deadly explosions could occur. Gasoline engines are also not that efficient and will struggle to push a sailboat at hull speed for days on end without any rest.
Diesel inboards are work horses! They will keep banging along as long as they have oil and fuel. They are expensive to purchase, expensive to replace, and expensive to maintain, but they will power you along without complaints or rest! This is one of the key selling points of a diesel inboard in a sailboat. If you cross an ocean and have no wind the entire time, a diesel motor will be able to power you along the entire way without complaining. While they are an invaluable tool to those who have them, they are also a smelly companion in your cabin. That classic "boat smell" is a combination of the toilet hoses and diesel motor. It isn't always very strong, but it is always present.
Electric motors are the last of these options. They will never explode because they have no fuel, but they will also never push you across an ocean without complaints. As they power you along, they will draw on your batteries. More range means more batteries which means more weight, which means more power to push you along. It is an endless cycle that will never get you very far. Electric motors should be seen as "short term" power options. Say you need to motor into a marina berth, or out of the way of a cargo ship, this is where the electric motor will shine. It will power you along on short distances without problem, but it will run out of electricity long before you make it across the ocean.
Electric motors are best for those who plan to sail. In the situation where there is no wind while crossing an ocean, you would be forced to sit there as you wait for wind to come (which it will eventually do). Cost is another variable. All electric motors achieve the same end result, and the different brands available offer a huge spread in prices. We went with one of the cheaper options by Electric Yachts and it has been a blessing! It always runs when we need it and it didn't break the bank to purchase or install. There are other brands that are many times more expensive, but they too achieve the same goal.
The choice about which motor is a personal one. If you can't really afford anything, a cheap used outboard will be your friend. If you are a power boat disguised as a sailboat, a diesel will be your best friend. And if you want to sail, an electric motor will be your best friend.