How to measure shaft on outboard motors the simple way
If you're standing up in your front yard staring at your boat or moving through used entries online, you've possibly realized that understanding how to measure shaft on outboard motors will be way more important compared with how it looks at first glance. It's one of these things that seems such as it should become standard, but obtain it wrong by even five inches, and you're taking a look at a boat that won't plane, a motor that overheats, or a prop that kicks upward more spray compared to a broken open fire hydrant.
The truth is, outboard manufacturers have mainly standardized these lengths, but boats haven't always followed suit. Whether you're repowering an old hull or even buying your very first kicker motor for a dinghy, you will need to be accurate. You don't need to be the particular person at the boat ramp along with a motor that will sits too deep in the water—or worse, one that's sucking air mainly because it's too short.
Why the exact measurement in fact matters
Before you grab your tape measure, it assists to understand what's on the line. If the particular shaft is too longer, you're creating unwanted drag. The engine sits too strong, which forces the engine to function harder, burns more fuel, and may in fact make the boat feel sluggish or even "heavy" at the stern. It can also mess along with your trim plus tilt angles, making the boat deal with like a shopping cart with a poor wheel.
On the flip side, if the shaft is actually short, you've got an actual problem. The propeller needs to become fully submerged within "clean" water—water that will hasn't been bubbled up by hull. If the motor is too higher, the prop will cavitate, which fundamentally means it's re-writing in air plus bubbles instead of pushing water. Not only does this particular kill your velocity, but it can also lead to the engine overheating because the intake of water (which cools the particular engine) might not really stay consistently marine.
Finding the right points to measure
Whenever you're figuring out how to measure shaft on outboard engines, a person aren't actually calculating the entire length associated with the motor through the top of the particular cowing to the particular bottom of the skeg. That's a common mistake. The "shaft length" will be actually a particular distance between 2 essential points on the motor.
First, look from the mounting group. This is the particular part of the particular motor that hooks over the top of the particular boat's transom. Your starting point for your measurement is the inside of the installing bracket hook . Think of it as the place where the motor actually "sits" on the particular wood or fiberglass of the boat.
The 2nd point will be the anti-ventilation plate . Most people call this the cavitation plate, even though "anti-ventilation" is formally the proper term. It's that flat, horizontal metal fin located just above the propeller.
To get your own measurement, run your tape measure through the top of the mounting bracket (where it would relax on the transom) straight down to the particular top of the anti-ventilation plate. That range is your shaft length.
Standard industry sizes you'll run into
Once you have your quantity, you'll probably notice it's not a random figure such as 18. 3 inches. The marine sector generally sticks to some standard categories. In case your measurement is definitely close to 1 of these, that's your official shaft size:
- Short Shaft: Usually around 15 inches. They are common on small aluminum fishing boats, inflatable tenders, plus small sailboats.
- Long Shaft: Usually around 20 inches. This is actually the "standard" regarding many mid-sized runabouts, center consoles, and larger pontoons.
- Extra-Long Shaft: Generally around 25 ins. You'll see these on deep-V hulls or boats with very high transoms designed for just offshore use.
- Ultra-Long Shaft: These are 30 inches plus are typically arranged for massive just offshore boats with several engines.
Don't panic if your measurement is away from by a half-inch or even so. A "20-inch" shaft might actually measure 21 inches based on the brand name (Mercury, Yamaha, plus Honda sometimes have slight variations), but it still falls into the "Long Shaft" category.
Don't forget to measure the motorboat too
Knowing the motor's shaft length is only half the battle. You also have to know how to measure shaft on outboard needs for your specific boat. In other words, you require to measure your transom.
To do that, discover the center point of your transom towards the top. Run the particular tape measure straight down to the particular very bottom of the hull (the "V" or the lowest point of the particular boat). This is your transom height.
Ideally, your motor's shaft length should match your own transom height as closely as you can. If you have the 20-inch transom, you need a 20-inch (long shaft) engine. If the transom is 15 ins, you will need a short shaft. In case you try to put a 20-inch motor on a 15-inch transom, that propeller is going to be 5 inches deeper when compared to the way it needs to be, which is usually a recipe for hitting rocks and wasting gas.
What if your own boat has a bracket or a jack plate?
If your ship has a jack plate (a metal bracket that enables you raise or lower the engine manually or hydraulically), the rules alter just a little bit. Jack china allow you to use a motor that might not become a "perfect" suit because you can adjust the elevation. However, even with a car lift plate, a person still want to be inside the ballpark.
For motorboats with an offset bracket (where the particular motor sits the foot or 2 behind the real hull), the drinking water actually rises a bit as it exits the back of the vessel. In these instances, performance boaters often mount the electric motor slightly higher than they will would on a standard transom. But intended for 90% of us, sticking to the "plate-level-with-hull-bottom" rule is definitely the safest bet.
Common errors to avoid
One thing I see constantly is individuals measuring the electric motor while it's tilted up. You can't get an accurate read that way. Make sure the motor is trimmed most the way down so it's top to bottom. If the electric motor are at an angle, your tape measure goes to give you a diagonal reading, which will certainly be longer than the actual up and down shaft length.
Another tip: when you're measuring a good engine that's currently mounted on the boat, it's much easier with two people. Have one main person keep the end associated with the tape towards the top of the transom and the other read the particular number at the cavitation plate. In the event that you're flying solo, a small spring clamp can hold the tape measure in place at the particular top so it doesn't slide away while you're inclined on the back of the boat.
Final thoughts on the perfect suit
It might feel like a lot of work for the single number, yet getting this ideal saves you so much headache down the road. A motorboat that's properly propped and has the correct shaft length will certainly handle better, proceed faster, and keep your engine healthful for a long time.
When you are stuck between sizes—say your transom is 17 inches—you're in a bit of a "no man's property. " In this specific case, most boaters opt for the longer 20-inch shaft and use the small shim or a mini-jack dish to raise the particular motor up these few extra ins. It's always simpler to make the long motor sit down higher than it is to make a short motor achieve deeper.
Take your time, measure twice (or 3 times just to become sure), and you'll be ready to hit the water without worrying regarding whether your brace is doing the job. After most, the whole stage of being away there is to relax, not to stress over precisely why your boat will be dragging its ft.