Propeller | Shaft | Strut & Bearing | Stern Tube & Stuffing Box | Coupler & Drive Savers | Alignment
or "You Should Have Kept up With Your Zincs"
How to kill a prop:
- Hit stuff
- Poor bonding or letting zincs go too long
- Don't tighten the prop nut enough
Your prop is the first item on our list. The first thing to do, before you haul out your boat, is to check the pitch. This sounds more technical than it is; it’s really very simple. Your prop will have two numbers stamped on it, for example 13x15. This means you have 13 pitch and 15" diameter. Take your boat out of the marina, and run engine to MAX rpms in forward. Really rev the engine up, as high as you can make it go, and watch the tachometer. Better yet, watch where you’re driving, and have an assistant watch the tach. Compare the maximum RPMs to the number listed in your owner’s manual for the engine. Standard for a Yanmar 3GMF is about 3400 RPMs, if I were to see 3800 on the tach, I would know the prop is not a steep enough pitch. This means if I had a 13x15 prop, I would have the shop switch me from a 13 pitch to a 14, making it a 14x15 prop. If I were to see 2400, I would know that the pitch needs to decrease (from 13 to 12). Generally, call the prop shop if there's a question, they usually have a data sheet you can fill out and they can re-pitch or replace your prop if necessary, or you can just live with decreased engine power as so many boats do.
Once you haul out, check for corrosion – clean the prop thoroughly with a scotch-brite pad and look for lacy or crumbly edges. You can also check after you remove the prop if the boat is still in the water. Tap (gently) with a small hammer and see if it “rings” like a bell or “thuds”, or somewhere in between. Is it bright pink? If so replace your prop and try to keep up with your zincs in the future, so you don’t have to do this again. Do not leave a corroded prop on; they have a tendency to do things like break off blades when you need them the most.
Next remove your prop – if the boat is in the water use a prop puller and a hammer – be careful with the hammer not to bend or smash the soft bronze. Also helpful is scuba gear or the ability to hold your breath for a long time. If you are out of the water (much better) apply a puller to get steady pressure (leave a nut barely threaded onto the end of the shaft so it doesn’t fly off when it pops) and heat the prop with a torch. This hurts the prop much less than beating on it. Keep heating it until it pops free (this may take a while). Let prop cool before touching it. Never point the torch at the cutlass bearing unless you want to melt it and buy a new one.
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| For sale, 1/3 off | Bad cracking caused by mis-alignment | Severe electrolysis |
While the prop is off, you can take it to a shop if necessary for inspection or repair, or if it's fine just put it back on after you inspect the rest of your drivetrain. When you put it back on, a few tricks: lap the shaft before you put it back on, and consider greasing/polishing the propeller.
Lapping is the process of finely machining the surface of the prop and shaft so they mate securely, and it's easier than it sounds:
- put a tiny dab of lapping compound on the shaft
- slide the prop on without the key in place
- spin the prop back and forth on the shaft by hand, smearing the lapping compound around and hopefully making a very slight grinding sound
- remove the prop and wipe off all the lapping compound
Polishing the prop gives it a better bit in the water, and decreases friction to provide more efficiency
Greasing the prop keeps growth off, sometimes for years. Hard grease or a grease pencil is the best, as softer grease will rub off quickly.
What the yard has that you don’t have –
Acetylene torch to heat the prop quickly and pop it off without hammering -
- You can use a regular butane torch, lots more elbow grease.
- You shouldn’t because you can bend or dent the prop and melt the cutlass bearing if you point it the wrong way.
Prop pullers for every occasion -
- You can use a standard gear puller, find one that fits and hang on to it for next time
- You shouldn't because the wrong puller can damage the soft bronze of the prop or crush the threaded end of the shaft.
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| Prop pullers for every occasion |
Next we will inspect the shaft.
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