Engine Buying Tips: How to Select the Right Horsepower for an Outboard Boat - 11/06/2018

The engine is the most important and most costly component in your boat. To a large extent, as does the engine, so does your boating experience. Making a mistake on your propulsion package -- either under powering or over powering your boat -- can be costly. Here we present Installment #7 from Martha Comfort’s Boat-Buying Handbook.

Engine Buying Tips
Do you want to go 75 mph or 7 knots? The speed of your boat is one of its most important attributes.
Make a mistake and it will cost you dearly.

Speed and the Boat’s Hull

The speed you need will be an important factor in engine selection. If you wish to travel at Mach1, you will find the purchase price of your engine and boat will be high. The hull which houses those expensive turbocharged or supercharged hotrods has to be incredibly strong because the faster your boat hits the water, the harder it gets. Just think of your car running over a series of speed bumps placed every 30' across the road, then imagine what would happen to your car if you traveled over a mile or two of those bumps at 70 mph!

The more high-technology materials, such as Kevlar and carbon fiber, that are used in the hull, and the more stringers and bulkheads that need to be put into the boat, to say nothing of a thicker, cored laminate, the more the hull will cost. If you ever wondered why high-performance boats are so expensive with so little inside them, now you know.

With these things in mind, let us now look at eight major types of small boats and see what engines might be best for your application—

1. RIBs and Inflatables

Inflatables and RIBs are not as easy to power as one might think. The days of the little kickers are over except for the compact inflatables that one can roll up and store in a boat’s lazarette or trunk of a car. Usually the weight of the engine and how much the owner can lug will be the determining factor on max horsepower, usually 9.9 to 25-hp for small inflatables.

RIBs take more horsepower because they are far heavier than inflatable boats. All boats are weight sensitive, and RIBs are no exception. For that reason, owners should get as light an engine as possible that has a lot of low-end torque. That is why 2-stroke outboards are often favored by many RIB owners.

Engine Buying
RIBs take more horsepower and need strong low-end torque.

Many RIBs actually draw more water than a similar-sized all-glass or aluminum boat because so much of the beam is the inflatable tube itself, which is not submerged as much as is the fiberglass hull it is attached to. This means more horsepower is needed until they are on plane, and then they fly due to reduced wetted surface.

The key with RIBs is to get the boat on plane without having to put everyone in the bow to trim the boat while it is building up speed. Depending on the size of the RIB, we would recommend from 40-hp to 50-hp as a minimum on the small end, say 12’ to 14’. Here, we recommend 2-stroke engines. The really big RIBs can have two or three big outboards on their transom and they need them in order to go fast and carry a load.

2. Aluminum Outboard-Powered Boats

These boats can be powered by anything from 50-hp to 300-hp, depending on the size of the boat and the mission. Typically, these boats are used for fishing, so the size of the engine depends on the load to be carried and how fast the owner wants to go and how far. We recommend that boat buyers calculate the anticipated total weight of the boat with passengers, fuel and gear, then decide what best cruise speed they want to attain if they plan on fairly long runs.

Engine Buying
Alumacraft Edge 185 Sport.

When equipped with 250-hp or 300-hp outboards, these types of aluminum fishing boats can typically have a WOT of 55 mph to 60 mph, or a bit more with two people aboard. But you don’t have to put engines that large and expensive on a 16’ to 24’ aluminum fishing boat. Boats in this size range with 115-hp to 150-hp engines typically have their best planing fuel economy in the 23-mph to 28-mph range with these size engines, and have a WOT in the high 30s or mid-40 mph range.

Before moving up into the large outboard engines, make sure that you really need that much horsepower. Some 90-hp engines in 18’ aluminum boats with two people aboard will perform almost as well at cruise and have a WOT in the low 40s.

16' aluminum boats are popular and a 60-hp engine will typically propel them from 25 mph to 30 mph top speed, depending on load.

[Check out BoatTEST.com’s “Aluminum Fishboat” type to get a good idea of what you might need…]

3. Bass Boats

These very specialized boats are designed for one purpose only -- bass fishing on relatively small and calm bodies of freshwater. For many anglers, their bass boat is a thing of rare beauty and they treat their boats much like collectors of antique cars pamper their classic (and expensive) automobiles. While virtually all builders of bass boats design them for tournament fishing, most people who buy them actually do not enter tournaments and instead use them for low-key recreation fishing enjoyment.

Having identified at least three categories of bass boat owner (tournament anglers, casual fishermen, and bass-boat aficionados), it is not surprising that the power needed for all three purposes would be quite different. Boat-builders and engine makers spend millions of dollars each year promoting bass boats powered by very large outboard engines.

Engine Buying
Ranger Z520C.

Anglers engaged in tournament fishing are going to want the biggest engine on the transom that the boat is rated for, in most cases, and that means from 225-hp to 300-hp. But more casual anglers, particularly ones fishing on smallish lakes, have no need for such prodigious amounts of horsepower.

For example, a 2,800-lb. fiberglass bass boat powered by a 150-hp outboard will have a WOT speed in the 50-mph range, and will have its most economical cruise in the mid-30s with two people aboard. Who needs to go faster than that? If your lake is small, perhaps an even lower horsepower engine would serve the purpose. In that case, your major consideration would be finding a like-minded angler come re-sale time.

Traditionally, bass boats have been the domain of 2-stroke outboard engines. They are light-weight and have a lot of low-end torque which makes for fast hole shots.

[Check out BoatTEST.com’s performance tests of bass boats to get a better picture of what you need…]

4. Bay Boats and Skiffs

Like bass boats, bay boats are designed for a very specific application – saltwater fishing in skinny, protected conditions. Bay boats go where the water is flat, so like the bass boat, they don’t need much freeboard. And, like the bass boat, their bottoms are relatively flat, like that of a skiff, and if powered by large outboards they can go lickity-split.

Builders and engine makers often display these boats -- which are typically 16’ to 24’ long -- with 250-hp to 300-hp engines. You must ask yourself, how fast and how far do you need to go on an average fishing trip? The answer may be 50 miles or more, in which case you may well want the extra juice so that you don’t spend all of your time traveling to and from the fishing spot.

Engine Buying

On the other hand, if you will be fishing just a few miles from home or the launch ramp, why do you need all of that horsepower? Perhaps 100-hp or 150-hp is all you need. Again, the best cruise speeds and WOT speeds of the boat should be your guide, given the body of water you’ll be ranging over.

Unlike bass boat owners, bay boat owners do not care so much about hole shot speed so they are open to both 2-stroke and 4-stroke engines, so long as they are competitive in fuel economy. But remember, 4-stroke engines have weak low-rpm torque. They will not be as quick to plane as will be a 2-stroke engine.

5. Sportboats, Runabouts, and Deckboats

Something on the order of 90% of all sportboats are powered by sterndrive engines. However, the last five years or so there has been a resurgence of interest in outboard power for sportboats as more and more builders have made these models available. This development is giving consumers a good option that they largely did not have 10 years ago.

Engine Buying

When selecting power for a sportboat perhaps the most important thing to avoid is under-powering the boat. Some boat builders and dealers typically will install 3.0L 135-hp engines in the smallest sportboats to keep the total price of the boat as low as possible. They call them “entry level” boats. This package might be sufficient for a youngster just starting out or for a newbie couple, but for slalom skiing and hauling a boat full of guests it is simply not enough.

A single 115 to 150-hp engine can produce reasonable performance for an entry-level fiberglass sportboat from 17'-19', depending on the load. With it, a power boat in this range can have a top speed from 40 to 45 mph, with cruising speeds in the mid 20s.

Engine Buying
Four Winns HD 270 OB.

Consumer Caveat: Be careful not to overpower a small boat of any type. Always follow the USCG max hp rating that appears on the plate on every boat. Over-powering a boat can make them hard to control and dangerous.

Larger fiberglass boats will need larger engines and today we are seeing 22' to 25' sportboats powered by 250-hp to 300-hp engines.

For Towing Sports: Boat owners wanting to tow waterskiers and wake boarders should consider engines with strong torque in the lower RPM ranges, because it takes lots of power to get participants up on their boards. And even more power is required when there is a boat load of guests.

6. Center Console, Walkaround, and OB Small Express

Anglers going long distances will want to carefully balance fuel consumption, best cruise speed, and range. After a long day of fishing it is no fun coming back at night to an island, or even a poorly marked inlet at low speed because of low fuel. It is always wise in a small boat to get back before sundown, and that means you have to know how fast you can go with the fuel you have left onboard, and what time to bring the lines in.

Center console boats from 20'-25' can usually perform well with a single outboard from 200 to 300-hp, depending on load and top speed expectations. Over 25'-26', twin engines are typically recommended.

Consumer Caveat: Buyers should remember that most boat tests are done with just two people aboard. All small boats are weight-sensitive, so owners planning on loading a boat with guests should plan on robust power options.

Big Single or Smaller Twins? All outboard engines are much more reliable than they were 15 years ago. As a result, many offshore anglers are opting for the advantages of a larger single rather than smaller twins, equaling the same horsepower. Twins will weigh more and burn more fuel at a given RPM and speed -- thus reducing range. On the other hand, some anglers feel more comfortable offshore with redundant power. Take your pick.

Engine Buying
Scout 350 LXF.

Somewhere around 25’ most boats going offshore will be powered by twin outboards, typically from 200-hp to 250-hp. Around 33' to 35’ offshore anglers in center consoles will hang three engines on the transom with anything from 225-hp to 350-hp each. At about 40’ some center consoles and express fishboats start hanging four engines. Whether or not all of this expensive iron on the transom is necessary is a matter for the individual buyer to decide depending on his or her mission.

Make sure you know what octane fuel your large outboard is recommended to run on. Some require high test fuel.

[For a good idea of the speeds and fuel consumption you can expect for center console look at BoatTEST.com’s tests…]

8. Pontoon Boats

For the last decade pontoon boats have been the most popular type of boat on the market. The invention of the tri-toon and new center console designs have made pontoon boats faster so that they can now be used for skiing and towing, in addition to cruising and entertaining.

Engine Buying

Most pontoon boats have twin toons and these are displacement boats; they can only be pushed so fast and adding big engines will not make them go faster and is counter productive. Depending on how large and heavy the twin toon is, most can be pushed close to their maximum speed with a single 90-hp engine, and many people are quite content with a 50-hp engine driving their toons.

Tri-toons are another story. Because of their added buoyancy, these boats can get up and plane and typically have WOT speeds in the 40-mph range. Some new tri-toons can go 55 mph to 60 mph, when powered with twin 300-hp engines. There is even one large pontoon boat on the market with triple 300-hp engines. They can be powered with 150-hp to 300-hp single engines.

[Look at BoatTEST.com’s tests of pontoon boats to get an idea of how much horsepower you will need…]

Towing Considerations: Owners wishing to tow water skiers or wakeboarders must buy a tri-toon. Because all tri-toons are hard to get up to what we might consider "planing speeds," an engine with strong low-end torque is advisable.


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