The Wrong Prop can Wreck Your Motor!

You need to have the correct prop to prevent unnecessary wear and tear on your motor.  More specifically, you want a prop that keep your motor from “lugging”.

Fortunately there is a simple test you can do to see if your props is sized correctly.You will need to perform this test with your boat because even if two boats have the exact same boat/hull/motor combination, they could require different props.  This is because the prop size also depends on how much weight you carry, such as passeng

ers, coolers, anchors, fuel, etc.  

How you normally use your boat factors into the equation.  For example, do you mostly go fishing by yourself (which would be a lightly loaded boat)?  Or do you usually fill your boat to capacity with people (which would be a heavily loaded boat)?  The prop you need for each of these two situations could be different, even though you have the same boat/motor!  But, you of course don’t want to change props every time you use your boat, so what you want to do is split the difference.  You want to lean towards how you normally use the boat, and select the best prop for that situation.  For example, if testing (I'll describe testing in a minute) shows that you need a 13 inch prop when your boat is heavily loaded, but a 15 inch when lightly loaded, and you run your boat in each of these situations about 50% of the time, then you might want to buy a 14 inch prop to split the difference.  If you normally run a heavily loaded boat, and only go out occasionally by yourself, then you might want the 13 inch prop since that is the best choice for your situation most of the time.  If you fish alone normally, and occasionaly take out a bunch of people, then you may want the 15 inch prop.  The point here is you want to get the prop that is optimized for how you normally use your boat.

This is why it is often said “selecting the right prop is an art, not a science”.  As I mentioned, there is a simple test you can do to make sure you DON’T have a prop that will damage your motor.

Here’s what you need to do.  You need to take your boat out on the lake, loaded like you normally would use it.  Warm up the motor and then do a full speed run trying to hit your max speed.  Note the max RPMs at WOT (Wide Open Throttle).  Now go back and check what the motor manufacturer recommends for the RPM range for your motor at WOT.  Every motor, regardless of how it is used, will have a range specified.  This range can be found in your manual, or online, or it might even be stamped on the motor.  You want a prop that will allow your motor to run within this range at WOT.


 
If your test shows you are running too low on RPMs, then you are lugging your motor and you NEED to get a new prop.  If you are running too high on RPMs, you might be bumping against the rev limiter on the motor and leaving some extra speed and power “on the table” that you could be making better use of with the right prop.  NOTE: If you sometimes have a heavy boat (lots of people), and sometimes have a light boat (just you), you should run the test under both conditions and compare with the manufacturer's range.

So what exactly is lugging an engine?  If you’ve ever driven a stick shift car in 4^th gear when you should have been in 2^nd gear, you were lugging the motor.  You were putting too much stress on the motor for the RPMs you were running at.  You probably felt the car shake.  The motor might have sounded funny too.  Maybe it stalled on you.  All of those are bad for a car motor.  It causes it to wear out quicker.  The problem with a boat motor is it is much harder (perhaps impossible) to know you are lugging it because it’s much harder to hear or feel it on a boat.  If you do this occasionally on a car, its no big deal.  Just try not to do it next time.  But since a boat doesn't have a transmission and is always locked into one gear ratio, if you have the wrong prop, you are lugging the motor every time you use it, and you may not even realize it.  THAT IS BAD FOR YOUR MOTOR!

So how would the number of passengers on a boat affect the RPM test?  Actually, it’s not just the passengers, it’s really about the total weight.  There’s no quicker way to load down a boat than to fill it with people, but you also want to consider coolers, live wells, anchors, and anything else.  The more weight on the boat, the slower the RPMs will be at WOT compared to the same boat lightly loaded with just you on it.  A heavy boat could require that you drop down to a lower pitched prop. 

For example, lets say when you have 12 people on your boat, your RPMs at WOT is 6000 RPMs.  When it’s just you on the boat, you hit 6400 RPMs at WOT.  If the manufacturer says the motor should run between 6000-6400 RPMs at WOT, then your prop is perfectly “dialed in” and you can rest easy that you are not causing unnecessary wear and tear on your motor. 

So, what should you do if you performed the tests with a heavy and lightly loaded boat, and you find you are outside the RPM range?  You will probably need to change props because the idea here is to select a prop that allows your actual RPM range to match what the manufacturer recommends.

If your RPMs are too low, then you have too high of a pitch, and you will need get a prop with a lower pitch.  Conversely, if your RPMs are too high, then you need more pitch.  The rule of thumb is that 1 inch of prop change will change your RPMs by roughly 200 RPMs.  That's up or down.  Use this rule to help guess how much pitch change you will need. For example, if you are running a 15 inch prop and running 400 RPMs too low, you would next want to try a 13 inch pitch, which should add roughly 400 RPMs (or 200 RPMs per inch of pitch). 

What if you want your boat to go as fast as possible?  What if you want the quickest hole shot (acceleration)?   What if you want the best fuel efficiency when cruising?  These are all valid priorities and there are different props to choose from that will optimize each of these.  That’s a whole different topic, but I will say this:  Regardless of whether you are looking for top speed, or acceleration, or fuel efficiency, you ALWAYS want to have your motor running in the RPM range the manufacturer recommends, at WOT.  If you aren’t, you have the wrong prop and are causing unnecessary damage to your motor.

One last comment.  What if you are running both under and over the manufacturer's range?  This is probably a rare scenario, but let’s say your recommended motor range is 6000-6400, and you are running 6500 RPMs (over revving) with a light boat and 5800 (under range) with a heavy boat.  Both are outside the range.  What should you do?  I would lower the pitch an inch so the bottom number comes up into the recommended range (in other words, make the 5800 come up to 6000).  The downside is this will cause your upper number to go even higher.  But remember, the goal here is to not lug your motor.  You want to make sure you are never "below" the low number of the range.  If for some strange reason that causes you to go over the top of the range with a light boat, then I would just make a mental note and watch your RPMs whenever you go WOT when you are out alone, and back off on the throttle a little when you need to so you don’t go past the upper RPM limit.  The rev limiter on the motor will remind you if you forget!  This might be a pain, but in my opinion, this is the best trade off to make sure you are not lugging the motor.