To be clear, horses can’t pull boats. (Okay, unless the boat is a barge towed along a canal.)
So why do we use horsepower to measure boat engine strength? Furthermore, why do we use horsepower to measure any engine’s capacity? What is horsepower, anyway?
To put it simply, horsepower is… confusing. It equates to the amount of work (e.g. energy) required to lift approximately 160 pounds three feet in the air in one second (which is apparently the capability of the average 18th Century draft horse). At least, that’s the arcane measurement the steam engine pioneer James Watt conjured up. Crystal clear, right?
Suffice to say, the horse pulling/lifting weight whatnot comes out to approximately 740 watts of power. Horsepower, then, is a unit of power measurement that equates to the ability to create usable force. In this case, it’s the force to drive your boat over the surface of the water.
Now, how much horsepower does your boat need? Well, this depends on the boat and on how you use it.
Horsepower to Boat Weight: A Matter of Ratios
The bigger and heavier your boat, the more horsepower you need to keep it moving. So first let’s discuss boat weight, then we’ll explain power. Horsepower.
Deck Boat Weights
Deck boats are rather light vessels compared to cabin cruisers and oil tankers, but many deck boat captains have the need for speed, so don’t let their lighter loads make you think a smaller motor is sufficient.
An average 18-foot deck boat will weigh between 3,000 and 4,000 pounds.
Many a 26-footer will have a wet weight of nearly 6,000 pounds.
Pontoon Boat Weights
The average pontoon boat weighs 2,200 pounds, but as these user-friendly vessels come in lots of different sizes, let’s look at two more examples here.
A 16-foot Avalon Eagle pontoon boat weighs just 1,350 pounds.
A Suntracker Regency measuring 27 feet in length weighs 3,300 pounds. That’s dry weight—no fuel or oil or water or other fluids. And no gear or crew weight, either.
Cabin Cruiser Weight
Now we get into a category where weight varies widely, and based not only on boat length but also on size. Add more levels, you get more weight, see?
A smaller 25-foot cabin cruiser will easily weigh in at 6,000 pounds.
A 30-footer will often ease the scales over 8,000 pounds.
And at around 40 feet, you’ll be dealing with a boat weighing between 10,000 and 15,000 pounds or more.
So How Much Horsepower Does a Boat Need?
At safe cruising speeds, we can use a user-friendly plug-and-play formula to calculate a boat weight to horsepower ratio. Mind you, this is for reference, not the be-all, end-all guide. But here it is…
For every forty pounds of boat weight, you need one horsepower to motor along over smooth waters at 20 miles per hour.
So for a 4,000-pound boat, that’s 100 horsepower for smooth, steady cruising. For an 8,000 pound boat, that’s 200 horsepower. And so on.
But you don’t wanna be stuck topping out at 20 MPH, right? As many pontoon boats can comfortably motor at more than 30 MPH, with deck boats and cabin cruisers often going even faster, let’s now calculate the HP needed for that speed.
A boat weighing 4,000 pounds with the desired cruise speed of 30 MPH requires at least 160 horsepower in the engine (or engines). For an 8,000 boat, you will need more than 320 HP to move along at 30 MPH. And for a 12,000-pound boat (at wet weight, FYI), you’ll need a minimum of 480 horses under the hull. Or back in the outboards.
How Much Horsepower Does a Sailboat’s Motor Need?
Yes, most sailboats have motors. While it’s controversial among sailing communities, sailboats should have motors both for safety (should sails or rigging become compromised) and simply to charge batteries. An old rule of thumb says that for every 2,200 pounds of weight (AKA every Metric ton), you need four horsepower.
So, for your 4,500-pound sailboat, you should have an eight or nine HP power motor. A Swan 48, on the other hand, weighs in around 36,000 pounds and will need at least 65 proverbial horses in the motor (if said motor is to adroitly move the boat under slack winds with furled sail).
Using Beyond Cruising
You know how much HP your boat needs to propel itself, but what about pulling waterskiers with the need for speed? Or what if you plan to tow a large inflatable “island” around the lake or even a jet ski? Take into account the usual activities and extra gear when considering your overall HP needs. When it comes to some boating activities, a higher horsepower equals higher thrills.
On the other hand, the trolling fisherman will be just fine with the base weight-to-horsepower ratio.
Choosing the Best Motor for Your Boat
If you already own or are buying a boat with an inboard motor, changing out the engine is, of course, a serious undertaking. It’s a project best left to the professionals. (Or left alone altogether based on cost!) But if you’re wondering how to choose the best outboard motor for your boat’s horsepower needs, then here are some questions to ask yourself:
Two- or Four-Stroke?
I’ll be frank. The two-stroke motor vs. four-stroke motor debate is coming closer to being settled these days. Today, two-stroke motors waste less fuel than older versions and four-stroke engines aren’t much heavier than their two-stroke associates. So don’t get too hung up there.
How Many Motors?
Are two motors better than one? Or three better than two?
A horsepower is a horsepower, so one 300 HP motor is just as good as two 150 HP motors or three 100 HP outboards. In fact, multiple motors can reduce speed and power by adding more weight. If you’re worried about an engine failure stranding you at sea, though, two motors are always welcomed.
The Octane Issue
Unless you have a high-performance speedboat, you really don’t need a high octane motor that burns higher priced fuel.
3 Outboard Motor Brands to Consider
Take time to shop around on your own. Buying a new motor is a serious investment. But if you end up purchasing one of these brands, you’ll be doing your boat right.
The Mercury 115 is a 360-pound motor that, of course, packs 115 HP. This is a great motor for a smaller deck boat or pontoon boat. In addition, the motor can be paired up for faster cruising or deep sea fishing vessels.
This powerful engine is rated at 150 horsepower, which is impressive for a motor that weighs less than 420 pounds. The E-TEC 150 safely operates at up to 6,000 RPM.
Okay, the Torqeedo Deep Blue motor is crazy expensive, but it’s also crazy awesome! Why? Because this all-electric outboard can compete with its gasoline-swilling brethren. You can get Deep Blues that equate to 40 or 80 HP gas engines. You’ll just have to lay out more than twenty grand, is all.
Making the Most of the Boat Horsepower You Have
If you already have a great boat and a great motor (or two or three), there are some relatively easy additions that’ll squeeze a bit more speed and performance out of your hardware.
The first is to have the hull blueprinted. Boat blueprinting involves fixing every imperfection, removing objects that cause drag, and generally streamlining all parts of the boat that touch the water.
Second, simply upgrade your propeller. Stainless steel propellers (check price on Amazon) cost about five times as much as aluminum props, but last longer and deliver more speed and power without flexing under heavy strain.
Off You Go!
So now you know why we use horsepower as our unit of measure for power output. You also know approximately how much HP you need for various types of boats.
But, do you know the maximum horsepower output of a Nimitz-class aircraft carrier?
Why that would be 260,000 HP.