Matching the right horsepower to your boat is crucial to ensuring a safe and balanced boating package.
Matching the right horsepower to your boat is crucial to ensuring a safe and balanced boating package. In most cases there is a choice of engine size for a particular hull with a minimum and a maximum rating within the safety guidelines. These recommendations should be strictly observed.
There are very real dangers in choosing the wrong horsepower engine. Excess power can mean excess weight: hulls have been known to break up, or refuse to turn one way. At full power they have been known to flip over backwards or ride on their chime and capsize.
Hulls of similar length, but of different design and purpose, may well be fitted with different sized engines. The power recommendation for one 5.5m boat may be between 90 and 200 hp; on another craft between 75 and 115. One may be designed for speed, the other for cruising.
If you follow the table below it will act as a general guide. For each engine it has the maximum boat length recommended for each hull style. If you are still confused with matching information please contact your local dealer with details of your current boat and they will assist.
Choosing the right shaft length
Selecting the correct shaft length is fundamental to the correct matching of boat and engine. Outboard engines come in a range of different shaft lengths to assist the boater in selecting the correct engine.
Should the wrong shaft length be chosen the performance of the boat will be affected. For example, if a long shaft is fitted to a low transom the propeller will be too deep in the water. The engine will probably operate but its efficiency would suffer and excessive fuel would be used trying to drag the lower unit through the water.
If a short shaft engine were fitted to a transom requiring a long shaft the propeller would be too high, and cut through air and water, possibly causing damage to the engine. Cavitation may occur when making turns.
To determine outboard shaft length requirement, subtract your boat transom height and the motor transom height. The difference should be approximately 0-2 inches which is the installation height range.
Always check your boat to match the correct length. Yacht auxiliary motors are often long shaft. Most inflatables, sailing dinghies, fishing and rowing boats tend to be short shaft. Many yachts require an Extra Long Shaft.
As a general rule the outboard should be installed so the anti-cavitation is parallel to the bottom of the boat.
Choosing the right starting system
The start system is the method with which you start the engine and there are two different methods, recoil and electric.
Recoil start is a manual process, this works by the user pulling a starter cord that is attached to the engine that leads to a start in the engine cycles. The advantage of a recoil start is that it reduces the weight of components, is lower in acquisition cost and removes the need to carry a battery on board for engine starting. Recoil start is normally found on smaller horsepower engines where less effort is required.
Honda 4-Strokes are very easy to start this way, our new BF8 and 10 have been uniquely designed to start first time with only a one-handed pull on the cord, taking the effort out of this starting system.
As it sounds, the engine is connected to a battery and starts on the turn of a key on a control box- just as you would have on a car. This is a standard feature on all our engines greater than 30 horsepower. Electric start is also an option on the rest of the range as low as 8 horsepower. Hondas smaller horsepower models feature tiller handled electric start that works from a button on the tiller arm rather than through a remote control box.
Installation of the engine
The setup of an outboard on your boat is crucial to the performance and handling of your craft. An improperly installed outboard motor can result in the motor being dropped into the water, the boat being unable to cruise straight ahead, limited engine speed and higher fuel consumption.
Always match the engine to the recommendations as established by the boat manufacturer. Every new boat will have a manufacturers power recommendation: the maximum rated power is usually stamped on the manufacturers plate on the hull. Do not exceed this stated maximum horsepower.
Install the outboard at the stern on the centre line of the transom. Ensure that the boat is well balanced.
As a general rule the outboard should be installed so the anti-cavitation plate is parallel to the bottom of the boat. The water level must be at least 100mm (3.9 inches) above the anti-cavitation plate with the motor not running, otherwise the water pump may not receive sufficient cooling water, and the engine will overheat. If the height is too low, the idle port could go underwater, causing engine damage or hard starting.
While operating the boat, check the tightness of the clamp screws occasionally. Re-check after first 15 minutes running during each use. Tie a strong rope through the hole on the stern bracket and secure the other end of the rope to the boat. This will prevent accidental loss of your engine.
Attach the stern bracket to the transom and tighten the clamp screws.
If the outboard requires bolts, apply a silicon sealant (three bond 1216 or equivalent) to the outboard motor mounting holes.
Set the outboard on the boat and secure with the bolts, washers and lock nuts.
The propeller delivers the thrust to push the hull of the boat through the water. The size, shape and number of blades on the propeller will have to be selected to suit the size of engine and the shape and weight of the hull if the boat is to reach the design speed.
Choosing the right propeller will give:
A comfortable ride
Economic fuel consumption
Optimum engine rpm
If the wrong propeller is fitted you get:
A bumpy ride
Higher fuel consumption
Poor engine rpm
Generally, on smaller horsepower outboards the standard propeller that is supplied with the engine is suitable for most applications.
On larger horsepower engines the propeller will need to be matched directly with your boat and application. Hondas largest engines are supplied with a choice of aluminium propellers.
In case of unforeseen circumstances it makes sound marine sense to carry a spare propeller at all times.
Points to take into consideration when choosing alternative propellers include:
Type of boating you do (fishing, water-skiing, cruising, inland navigation )
The size and weight of your boat
The engine horsepower
The performance you expect to achieve
Pitch and Diameter
Getting the correct pitch and diameter of your prop will depend on the boat and engine combination.
Imagine a corkscrew turning and driving itself into the cork of a bottle. Each full rotation will drive it down the distance that it takes the tip of a section of the blade to complete a rotation of 360 degrees. Take any propeller and stand it on a flat surface. Walk it, by hand through 360 degrees following the path of one blade tip and measure the distance it walks along the flat surface. That is the pitch. This information will be stamped on the propeller along with the diameter.
In theory, one revolution of the propeller will move the boat through the water the distance of the pitch. However, in reality the blade is moving through water which acts as a lubricant and allows the blade to slip.
The diameter of the propeller chosen for an outboard engine will fall within a fairly small band. Depending on the size of the engine the propeller drive shaft will be a specified distance from the anti-cavitation plate. So a propeller can be selected with a slightly different diameter for the same amount of pitch, depending on application.
A slightly larger diameter will increase the thrust from a propeller of similar pitch. The blades will be larger and therefore have more contact with the water.
Both diameter and pitch must be taken into account when selecting a propeller.
Setting up the propeller
Put the boat in the water with its normal load and, assuming the engine is run in, take it up to maximum speed. The tachometer reading should be at or near to the maximum recommended rpm. If the engine is revving above this level a propeller with an increased pitch or diameter is needed.
As a general rule, a difference of 1 inch of pitch can alter the engine speed by as much as 300 rpm. A propeller with a larger pitch will reduce the top rpm, whereas the lower a pitch will increase the rpm.
Continuous excessive revs can cause serious damage to an engine. Serious damage can also be caused when a propeller of too large a pitch is used, especially when the engine is under load.
Why choose a Honda engine?
Honda have been building 4-stroke outboard engines since 1964, longer than any other manufacturer. In fact, we’ve only ever built 4-stroke marine engines. Why? Because we are committed to developing high performance engines with the lowest possible exhaust emission levels. Because of their fuel economy and reliability. And, perhaps even more importantly, because they are clean and quiet.
Honda. Designed to be in tune with their surroundings.
Honda engines look as good as they feel. Their metallic silver finish was specially chosen by our designers to compliment the widest range of boat colours, shapes and sizes. They reflect the colours of the boat, the water and the quality of Honda’s engineering. Designed to be in tune with nature, many engine parts are recyclable and at launch the BF 90 won three industry design awards.
Honda. The world’s largest range of 4-stroke outboard engines.
Honda are the only manufacturer to produce such a wide variety of 4-stroke outboard engines. From the compact and reliable 2 horse-power engine to our latest and most powerful 225hp models, we have an engine for every kind of operation in virtually every kind of boat. Whatever power you need, you’ll find it in a Honda.