Outboard Motor Size
A larger boat doesn’t necessarily mean a larger motor. Although there are different ratings for different classes of boats, a small power plant can be more effective than a larger one. Conversely, an outboard motor can easily overpower a small boat and create unsafe conditions at high speeds. Guidelines and requirements differ between motorboats and sailboats. And while there is some overlap, these considerations apply directly to sailboats.
Engine power has to do with how much water a boat displaces. For sailboats, smaller is better. If you’re a bit of a math geek, the exact formula is 4 horsepower for every 2200lb of weight. Coupled with a propeller size, which can be determined using a propeller calculator, you’ll get a rough estimate to use as a guideline to start shopping. This is a good first step, since size is essentially a fixed variable. Though it’s worth noting for those who are buying a sailboat directly from the manufacturer, that actual weight will increase after delivery – once all other rigging and outfitting has been completed.
Physical size of your outboard motor is an important consideration and is directly related to the design of your sailboat. An outboard motor is made up of three parts from top to bottom:
- The Powerhead – Houses the engine. The bulbous part of the motor.
- The Midsection – Houses the exhaust system. Varies in length and design.
- The Lower Unit – Propellers attach to the gearbox. Submerged when operational.
Shaft length is an important design element and should be considered when purchasing a motor. A shaft that is too short will obviously prevent the propeller from being submerged in water, while a shaft that is too long will extend the propellers too far. Not only will it decrease the efficiency of your engine, it will create unnecessary drag. Know your transom length when looking at different models.
When an outboard motor is not being used, it should be stowed in its upright position. Some of the larger motors have an automated switch that will raise it out of the water, but some must be secured manually. Make sure everyone who sails with you is capable of lifting and securing the motor out of the water in case of an emergency.
Outboard Motor Power
Any kind of motor installed on a sailboat (inboard or outboard) should be viewed as a tool to help with maneuvering. Although there are some very skilled sailors out there who can sail into their slip without the aid of a motor, many harbors have restrictions that either don’t allow for the use of full sails, or there simply isn’t enough room to maneuver. A motor with both forward and reverse gears helps tremendously with docking.
While there is no exact correlation between boat length and total weight, the following is a rough guideline:
- 1-4 HP for boats up to 20’ (approximately 1000-2000lbs)
- 4-18 HP for boats between 20-30’ (approximately 2,000-10,000lbs)
- 18-34 HP for boats between 30-40’ (10,000lbs or more)
There are some things to consider when deciding how much horsepower you need or want. Location and the type of conditions you expect you’ll be sailing in is one of the biggest factors. Heavy seas and high winds typically associated with open ocean sailing will put more strain on your engine, and in some cases overpower it, whereas an engine that is heavier than needed will add unnecessary weight when racing. If you plan on motoring for long distances, consider purchasing an engine that will stand up to a lot of use.
Less HP is required for:
- Boat Design – Single hull boats made out of fiberglass require less power.
- 2-Stroke Engines – This is due to an overall lighter weight engine and higher torque.
- Diesel Engines – Diesel delivers more torque because the rate of compression is greater.
- Bigger Propellers – More surface area means more water displacement.
- Location – Motoring on lakes and rivers requires less power than open ocean.
- Distance – A smaller engine is suitable for shorter distances.
More HP is required for:
- Boat Design – Catamarans and heavier boats (regardless of size) require more power.
- 4-Stroke Engine – Engine weight and an extra step of compression yields less power.
- Gas Engines – The rate of compression for gas engines is much lower than diesel.
- Smaller Propeller – A smaller propeller displaces less water.
- Location – Open ocean, with tides and currents, will strain a smaller engine.
- Distance – Cover more distance when wind conditions are poor requires a larger engine.
Outboard Motor Cost
There is no way to quantify how much you will pay for any given motor. But there are several costs associated with owning an outboard motor that are definitely worth considering when making your purchase.
Certainly, a lager, more-powerful engine is going to be costlier than a smaller engine with lower horsepower. But as mentioned earlier, size is not necessarily a guarantee of performance and efficiency. At the same time, there’s only so much you can get out of an engine before you exceed its capability. Larger engines tend to help with resale value should you choose to sell your boat at some point, but a boat outfitted with right motor to begin with will always perform better than a motor that’s large just for the sake of it.
Factor in maintenance costs and fuel when looking at models. You want to run your engine at around 90% of its max RPMs to balance proper fuel usage and with wear and tear. Making a few calls to marine mechanics to inquire about an engine you’re interested in will give you a lot of information a sales person won’t be able to give you. The good news about outboard motors is that most of them are portable, which means you won’t have the added cost of either paying a mechanic to come to you, or having to get your boat to the yard, which usually requires help from a very good friend willing to spend all day driving and sailing back and forth.
Owning a boat requires constant care and maintenance, so a little knowledge goes a long way. While an outboard motor is not required for sailing, it’s a convenient addition that can greatly increase your enjoyment out on the water. Being patient and spending time researching engines will not only help you make the correct purchase but will help you take advantage of a great deal when it presents itself. Whether you sail the Caribbean, or race off the coast of California in a catamaran, there is an outboard motor that’s just right for you.