When the right length motor is hung on the transom of a boat, the cavitation plate (the large plate that extends out over the propeller) should be even with the very bottom of the boat. At least if its too long you can raise the motor or "build up" your transom. Thats it.
I have put a long shaft outboard on a short shaft transom (inflateable) and it worked ok. Since you are using a relatively small outboard, I don't think you will encounter the dire consequences suggested. The long shaft will slow you down, and you are more vulnerable to hitting bottom.
Short vs.
Generally, short shaft outboards are 15 inches, long shaft outboards are 20 inches, and extra-long outboards are 25 inches. Since the length of the shaft of your outboard is associated with the size boat, smaller engines will generally have shorter shafts.One non-invasive way of raising the transom on an aluminum boat is to create a "lift" — a bracket that fits over the transom — to increase the motor's height. The completed bracket forms a channel, and the top of the channel holds the top of the outboard bracket a few inches above the transom of the boat.
This thing is solid! It is so easy to hook up with zero hydraulics and it travels up and down faster than the standard hydraulic
Jack plate.
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Short vs.
Generally, short shaft outboards are 15 inches, long shaft outboards are 20 inches, and extra-long outboards are 25 inches. Small portable outboards ranging from 2 to 20 horsepower are usually sold only in short or long shaft versions. High powered boats usually only come in long and extra-long variants.The transom is the flat vertical section at the rear of the boat. It is where the outboard is mounted, and some boat owners place the name of the boat. The height of the transom is calculated by measuring from the bottom of the hull to the top of the transom, using the center line.
If you set the motor back far enough without jacking it up, a short shaft will work on a 20" transom. Never argue with idiots as they will do nothing but bring you down to their level and then beat you with experience.
A boat rigger's rule of thumb is that the motor can be raised one inch for each eight to 10 inches of distance between the transom and prop. As the prop moves further aft, it's also likely to be in cleaner, “harder” water, and be more efficient.
Converting your long shaft outboard motor to a short drive shaft does, however, mean changing more than just the drive shaft. Before you start the conversion, you need to be sure your boat can handle a shorter shaft.
Standard shaft lengths are 15, 20 and 25" (38, 50 and 63 cm). Those shaft lengths are an industry standard but there are exceptions. For mid-range engines, the most common shaft length is 20" (508 mm).
your cavitation plate should be right at the water level. As a rule of thumb it should be 1 inch above the hull for every 12 inches back. (if you have a eurotransom, or bracket or something.) If your cavitation plate is under water you might get better performance if you raise the motor.
The rule of thumb is based on weight alone, and says you should have between 40 and 25 pounds of weight for each horsepower. For example, a 5,000-pound boat could have an engine with between 125 and 200 horsepower.
When they remounted the engine the mechanic said on most all pontoons the cavitation plate should be about 1 inch below the bottom of the tubes. Originally mine was mounted in the second hole from the top and he changed it to the top hole to lower the engine about 1 inch.
The shaft dimension depends on the size of the boat's engine. If your boat has a small engine then you'd need a short shaft. High powered boats would need shafts that are either long or extra long in length. Whereas a sailboat, for example, would need an extremely long shaft.
Standard shaft lengths are 15, 20 and 25" (38, 50 and 63 cm). Those shaft lengths are an industry standard but there are exceptions. For mid-range engines, the most common shaft length is 20" (508 mm).
Short vs.
Generally, short shaft outboards are 15 inches, long shaft outboards are 20 inches, and extra-long outboards are 25 inches. Since the length of the shaft of your outboard is associated with the size boat, smaller engines will generally have shorter shafts.In most cases, 8-20 hp motors will provide plenty of power to effectively propel your boat, and you'll find you get better fuel efficiency as an added bonus. If your boat's capacity plate lists a 40 hp maximum, a 20 hp motor should do you fine. You'll get good handling and plenty of thrust to move through choppy water.
12 Foot Jon Boat Speed
10-15 mph is a good expectation for this boat size. This boat length will usually allow for a 5hp engine, with some models permitting up to 10hp engines. The 5hp engine will put you in the 10mph range, while the 7.50-10hp engines will push you 15+ mph.Cavitation plates are mounted to the lower units of boats to help increase performance. They act as a planing surface to get your boat on top faster. They give you better bow response when trimming your motor. They keep water down at your prop and intake improving water pressure.
In architecture, a transom is a transverse horizontal structural beam or bar, or a crosspiece separating a door from a window above it. This contrasts with a mullion, a vertical structural member. Transom or transom window is also the customary U.S. word used for a transom light, the window over this crosspiece.
Adding a hydrofoil to an outboard does provide a performance boost much of the time. On a 16 footer with a mid-sized outboard and no tabs, for example, a hydrofoil will usually level out the ride by forcing the stern up and the bow down, and will end or greatly reduce porposing.
Tips for selecting the right size hp for portable outboard motor for your boat.
| Motor HP | Boat length (feet) | Boat length (metres) |
|---|
| 5hp – 15hp | 8' – 14' | up to 4.2m |
| 9hp – 20hp | 11' – 16' | up to 4.5m |
| 20hp – 40hp | 13' – 18' | up to 5.0m |
| 40hp – 75hp | 14' – 20' | up to 5.5m |
You will need at least a 48-inch ruler/measuring stick/tape measure in order to measure the length of your golf clubs. Place the measuring stick directly behind the golf club. The stick should make contact with the shaft of the club and be placed on the ground right at the heel of the golf club you are measuring.
Shaft length:
The shaft length announced by the outboard manufacturer is the vertical distance between the inside of the clamp and the cavitation plate. (We will not argue about names here but the proper name for the cavitation plate is ant ventilation plate). Shaft lengths come in multiples of 5" (127 mm).You can often tell that an outboard is too low if you are experiencing sluggish speeds, poor handling, excessive spray, porpoising, or even water pushing up into the cowling. That's a very brief, basic accounting of symptoms, but you'll usually find at least one of these symptoms if your motor is too low.
A boat rigger's rule of thumb is that the motor can be raised one inch for each eight to 10 inches of distance between the transom and prop.
Can I use a bigger motor on my boat than what it's rated for? It is not a violation of Coast Guard regulations to install or use an engine larger than specified on the capacity label, but there may be state regulations prohibiting it, and restrictions from your own insurance company regarding this.
If you are measuring for a lower unit replacement and your original lower unit is off of the engine, simply measure from the top of the waterpump to the top of your driveshaft. A shaft length that falls closest to 15 inches is short, closest to 20 inches is long, and so on.
Measuring The Transom Angle (Degrees):
Slide forward until the short side of the rule (16") touches the transom straight up. Measuure the distance from the keel point to the inside corner of carpenter's square ruler (see sketch). Each 1/4" is equal to 1° degree.The transom is the flat vertical section at the rear of the boat. It is where the outboard is mounted, and some boat owners place the name of the boat. The height of the transom is calculated by measuring from the bottom of the hull to the top of the transom, using the center line.
In order to get the correct shaft length needed for your boat, you first have to measure the distance from the mounting spot on the transom, down to the waterline. Then, take this measurement and add 20” to get the desired shaft length. If you are stuck between two sizes, err on the larger size.
