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Sizing your Propeller

“Hey, I understand that you will answer a boat question in exchange for a drink,” I heard from the other end of the Fo’c’sle bar.


“I’ve been known to do that – what’s on your mind?”


“The propellers on my 44’ sport-fisher are getting a little old and I think it’s me to replace them before fishing season starts. Now, I’m not sure what size they are, but I have seen some great deals online. Frankly, I feel like hauling the boat out to have them sized and fitted is a rip-off. What’s the big deal about the size, as long as it fits? The listings also have different pitches and number of blades. Can’t I just grab a good deal on a set of props online and have my diver change them out?”


“Yep, you’re right - good luck with your bargain propellers and thanks for the drink.”


“Well, wait a minute Krabby – you don’t think my plan to save some money on my new propellers will work?”


“Doesn’t sound like you want to know the real answer, so cheers!”


“Ok, I get it – give it to me straight.” said the sport-fisherman.


“Well, it’s not that easy,” I said. “Ok. Here’s another drink”....


Fitting the right propeller for your boat is not that difficult, but you need to go through the process if you want positive results. A correct sized propeller can make your boat run smoother, quieter, and more efficient. In a boat like yours, the fuel savings could pay for the new wheels you’ll need in a couple of seasons. If you understood a little about how the propeller works, you would be more likely to appreciate the importance of going through the process of getting it right.


The theory of how a propeller works is real simple, but fine tuning for the precise diameter, pitch, number of blades, and type of propeller is an exercise best left to the experienced professionals that do it every day. Usually, the prop shop will include the cost of engineering as part of the sale of the new propeller. If you want to par cipate, and perhaps save a few bucks, you need to give the propeller shop some informa on to use in their calcula ons. Some of the informa on they will need are the boat hull type (planing, semi-displacement, full displacement), length overall, length at the waterline, beam, draft, and weight.


Next, they will need some engine info: horsepower and RPM, reduction gear ratio, propeller clearance, and shaft size. Lastly, they will need some sea trial data. Make a spreadsheet with RPM and speed with about ten inputs spaced through the RPM range of your engine. This will help calculate the performance with your existing props, so they can make a more accurate adjustment to the new prop size and pitch. When you haul the boat to remove the old props, the prop shop will take them to ensure the numbers stamped on the hub are accurate.


Here’s what the numbers mean: the first number is diameter - that one’s easy. The next number is pitch, this is where you have to pay attention. Pitch is the theoretical distance (in inches) that the propeller would travel in one revolution if it were to turn through solid material, like a block of wood. Imagine a screw going into wood. That’s why we sometimes call a propeller a screw - the original idea came from Archimedes’ screw.


Now you know that each revolution of the prop will theoretically move you forward the distance equal to the pitch of the propeller, but that is not how fast your boat will go. This is because the propeller is actually turning through water, not a block of wood. Since water is a liquid, not a solid, the water allows the prop to slip a little. You have probably no ced when you look over the back of your boat that the water flowing past the prop and rudder moves a little faster than the boat is moving, this is the slipstream. The percentage of slip is a helpful number in calculating the right diameter and pitch of your propeller.


Now that’s the basic theory, but the next step gets tricky. We need to calculate the pitch to determine the speed and the diameter for the correct load to accommodate the horsepower of the engines. Then optimize the ratio between the pitch and diameter for maximum efficiency. Yes, there are online resources that make the basic calculations, however, the last bit of tweaking is more art than science.


The spreadsheet you made during the sea trial will help compare the theoretical calculations with real- me data. The comparison and a bit of experience will get you the best results. Even if you were to only gain a little in the way of efficiency, the fuel savings alone will be worth it.


When you get your new props, they need to be fitted to the shafts. This is where perfect is close enough. This must be done with the boat on the hard to get it right. The slightest imperfection of fit will cause vibration that can ruin cutlass bearings and keep you from enjoying the benefits of the boat’s new props.


Still want to save a few buck and get the bargain props? Guess you owe me another drink.

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