If you are new to using outboard motors, you may have difficulties with the right height to mount your motor.
Many of us wonder what is the proper height to mount an outboard motor. In this article, we will help with learning more about how to achieve this.
Table of Contents
What is the proper height to mount an outboard motor?
The mounting height of an outboard motor is a large determinant of the following:
- The handling characteristics of the boat
- The safety of the boat
- The speed of the boat
Hence, when choosing the height to mount the outboard motor, you must put into consideration factors such as:
- The style of the boat
- The length of the outboard motor’s shaft
- The size of the boat
How do you set the height of an outboard motor on a boat?
Basically, the mounting height for outboard motors is the same as the length between the top of the propeller and the base of the motor. This distance is also referred to as the Shaft length.
The height of the outboard is determined by some factors including:
The placement of the exhaust port
In the outboard engine, the placement of the exhaust port on the lower unit should be above the water line by exactly one inch when the boat is idling.
Once this is achieved, the engine can idle without restricting the flow of the exhaust.
For every 10 miles per hour of speed attained by the boat, the outboard can be lifted one inch above its standard height. Note however, that the motor should not be lifted to a height of more than five inches from the standard position.
Lifting the motor has effects on the boat. It can
- Decrease the lift of the boat
- Increase the amount of time needed for the boat to reach the plane
- Increase the speed of boat
- Decrease the effort which is required to turn the boat
More so, severe issues gotten from lifting the motor may involve overheating of the motor. Once the boat is raised to a height which takes the water inlets for the cooking system away from the water, it simply means that there will be overheating of the system.
Boats that have extended transoms, or brackets which extend the mounting bracket for the outboard motor at the back of the boat, require a higher height for the outboard.
Basically, these brackets add one inch to the general height of the outboard mount.
What height should an outboard motor be set at?
For many outboard engines with less than 125 horsepower, the standard length is 20 inches. Even V6 bass boat engines also have a shaft length of 20 inches.
However, outboards, which are specifically designed or built for use in saltwater, has a longer shaft length.
The 10-horsepower Johnson outboard motor
This outboard motor was created between the years of 1949 to 1963 with the model name, Seahorse. This two cycle motor requires the use of both gasoline and oil combined together.
In the year 1957, alongside the original model, another model was produced by Johnson and this model included a longer shaft length.
While the original model had a serial number beginning with QD, this new model has a serial number beginning with QDL. The “L” indicates length.
It can be thus concluded that the only difference between these two models is the length of the shaft.
Features of the 10-horsepower Johnson outboard motor
The engine of the 10 horsepower Johnson outboard motor is a two cylinder engine that has a 2⅜ inch by 1⅞ inch bore as well as a stroke and a 16.6-cubic-inch displacement.
The gap on the spark plug gap of the 10-horsepower Johnson Seahorse motor is 0.03 inch.
It operates at about 4,000 to 5,000 rpm and is certified at 10 horsepower at 4,500 rpm. Based on the size and the weight of your boat, this 10-horsepower Johnson Seahorse can move slow enough for trolling all the way up to 20 mph.
Thanks to the vari-volume water pump, the engine can be cooled off thereby preventing overheating.
For this motor, the gear ratio is between 13 to 23 with neutral, one reverse and one forward gear. A twist-grip or optional remote controls helps to control the gears.
These gears are responsible for turning the three blade propeller which has a diameter of 8¼ inch and a pitch of an 8½.
Weights and Measurements
- Weight of motor (without the fuel tank): 67 Ibs
- Weight of empty fuel tank: 12 Ibs
- Height of stern (QD model): 15 inches
- Height of stern (QDL model): 20 inches
- Height of cover above stern: 22½ inches
Note that, for the motor to be tipped forward easily, the front of the motor should have about 21 inches of clearance inside the boat.
The Effects of Standard, Lower and Higher Mounting on a boat
On most of the engines in a boat, the anti-ventilation plate is usually aligned with the bottom of the vessel. This can be measured with the bottom of the vessel being parallel to the shaft of the propeller.
The mounting holes and brackets accompany the outboard and are aligned vertically to enable the boater to make adjustments where necessary.
Note, however, that there are exceptions to this rule. In the case where the mounting is not going as planned, consulting a marine engine dealer about the design of the boat is highly recommended.
Mounting the motor lower than the standard length can have some negative effects such as:
- It reduces the speed in faster boats
- It can cut into the efficiency of the fuel and affect your travelling capabilities in the boat trailer
- Mounting the motor lower than recommended can cause future boat repairs
- It can create excessive spray
- It can increase the case drag
- It can also reduce the clearance underwater
To know if your outboard motor has been mounted in the appropriate height, below is a simple test to follow:
- Start with the engine in idle
- Trim the motor out by one half to full trim
- Briskly accelerate but do this safely
If the propeller ventilates, then the motor is mounted appropriately but if it does not, then you know that the motor has been mounted too low.
A lower mount can be ideal in some cases as some fishing boats can work perfectly with their motor being underwater. Regardless, this should not be done for beginning boaters
For professionals, decisions on higher mounting can be taken. But it is important to keep in mind that the engine can overheat if there is not enough water reaching the engine to ensure that it is cool.
Also, the boater must consider the design of the boat transom support and whether or not it can carry the weight of a higher mount.
How low should an outboard sit in the water?
The motor can be raised one inch for each eight to 10 inches of distance between the transom and prop.
How deep should an outboard prop be in the water?
You should set the cavitation plate about 2 inches below water if you want to have the best for your outboard pop.
How do I know if my outboard is too low?
The outboard will experience poor handling, porpoising, sluggish speeds, or excessive spray when it is too low.
How do I know if my outboard is mounted too high?
Some of the signs will include low water pressure, prop letting go in turns, or lack of bow lift .
Benefits of higher mounts include:
- Additional horsepower
- Increased RPM
- Increased speed
- Improved experience while driving a faster boat
While these benefits can prompt the boater to raise the Mount higher, it is essential to be aware of the disadvantages of doing this. The disadvantage of a higher mount is that it can disrupt the steering capabilities of the boater.
In conclusion, it is recommended to follow the required industry standard for beginners.