What is Prop Rattle?

Prop rattle is the noise from a prop rattling.  Okay, Mr. Obvious, what does that mean and is it harmful to my boat?  First, no, it is not harmful.  But it can be loud and annoying.  It happens at idle speed when the boat is in gear.  It sounds like something is VERY wrong with the motor.  Which, by the way, is a great source of material to freak out first-timers on your boat.  Especially your daughter's boyfriends.  "Hear that?  Uh-oh, sounds like something is wrong with the motor.  We might not make it back to the dock.  Anyone know how to use an oar?  Anyone bring an oar???"  The blank stares you get are just priceless...

Prop rattle only happens at idle while the motor is in gear.  Only stainless steel props will rattle.  Aluminum props won't.  This is because the stainless props are very hard and brittle, while the aluminum is soft.  This is one of the disadvantages of stainless steel.  But, it can be fixed easy enough. 

Why does the hardness of the prop matter?  Well, a motor gets it's power from the explosions that occur inside each cylinder.  These explosions are not continuous, but rather sequential, meaning one cylinder fires, then the next, then the next, and so on.  This happens so fast that a motor normally seems smooth and continuous.  But it's really not.  If you were to slow it WAY down, you would be able to hear each individual bang of the cylinders as they fire.  It would sound like a hammer hitting a nail over and over.  At idle speed, this can be slow enough for a hard stainless prop to "feel" each "bang, bang, bang" of the cylinders firing off.  This is what causes it to make a "rattling" sound.  As soon as you speed the motor up a little, it goes away because the inertia of the prop catches up and it runs smoothly and no longer reacts to each "bang" of the cylinders.

How do you fix this?  There are special hub kits like the Mercury Flo Torq IV hub shown above.  These hubs are slightly "soft" and absorb the shock between the prop and the shaft at low idle speeds.  This shock absorption will prevent the prop from rattling.  

Another option to fix this is to cake the marine grease between the black delrin sleeve and the inside of the prop.  This has worked fairly well for me.  It quiets the rattle by ~90%.  Since the amount of grease needed to do this costs about 10 cents (vs $100 for a new hub) it is worth a try.  Just cake it around the outside of the black delrin sleeve shown in the photo below, and coat the inside surface of the prop where the delrin sleeve will touch the prop.  The sleeve goes between the shaft and the prop, so caking it with grease absorbs most of the shock.  You can still hear the rattle, but it is MUCH muted, and not nearly as annoying.  For 10 cents worth of grease, it works for me.

 

By the way, Mercury's Flow Torq hub kits are "breakaway" hubs.  I think Yamaha makes a similar hub called SDS (if I have that right).  The purpose is if you hit something hard with your prop, the hub will break before your lower unit gears break.  The hub sleeve that breaks is a $50 part.  Your lower unit is a $1500 part.  Which would you rather have break?  Get a breakaway hub for your boat.