Loss of reverse thrust

[Bevan Hill]

Hi Andrew.
You may recall I have a 2009 R21 based in Mandurah Western Australia and you have helped with advice once before. My current problem has me stumped. I have lost reverse thrust. I still have normal forward power however on putting it into reverse and revving nothing happens. I have checked that the shaft behind the gear box is indeed turning in reverse and assume it is not a gear box problem. Is the shaft solid all the way to the prop? I can only assume there is some strange answer in the joining of the prop to the shaft. Do they connect by means of a spline or a keyway or something else. I can only guess that a keyway may have failed to some degree which allows grip in one direction but allows it to slip in the opposite rotation. I have encountered this before but put it down perhaps to accumulated weed on the rudder which did not impede forward purchase, but on applying reverse, maybe sucked a wad of weed over the blades rendering them ineffective. That may have accounted for the fact that the problem seemed to go away, however this time the problem is sustained, suggesting something more tangibly mechanical. I thought I would ask for your wisdom before I go to the effort of lifting the boat from the water. Previously, short of taking the prop clean off, poking around the propeller revealed nothing to me.
I'd be delighted if there is a simple explanation. Thanks, Bevan Hill

 

[Andrew Custis]

Hey there Bevan,

It really sounds like a broken keyway to me. The reason forward works is because you have a tapered shaft. As you engage forward, the prop thrusts forward on to the tapered portion of the shaft. When you reverse, the prop slips back off the taper and doesn't engage. I am not sure if a diver can do this under the water so you may need to have it pulled.

Please email me directly if you need more help.

Andrewcustis@rangertugs.com

Thank you very much and great to hear from you!

 

[abcandjrc]

My money's on Andrew on this. I have witnessed a large twin-screw cruiser have the problem when the yard forgot to install the key. The prop was tight enough to let them make many miles and maneuver in several locks on the Tenn-Tom Waterway. But when they tried to back into a slip in extreme winds, as a hurricane approached, the higher load pulled the prop off the taper. The twin-screw boat became an extremely imbalanced single-screw. That was fun! 😱

 

[LADYBUG TOO]

A diver can do it just fine but the ease of putting it on the trailer if you have one is worth it. With the boat out of the water there are several other routine maintenance things you can do. The key replacement is simple. If she is on the trailer, I would remove the prop and check it thoroughly for nicks and dents that can cause balance and wear problems. Also, we usually put a little marine grease on the shaft to make removal easier. If the prop to shaft fit seems a bit sloppy you can wipe the shaft and prop down, put a coating of valve grinding compound on the shaft and then spin the prop with light pressure to get a smooth fit. Remove the prop and check the bore for a smooth even coating of compound. Whent you see that it is smooth, clean the shaft and prop bore, add some grease, install the key, nut, cotter pin and replace the prop nut zinc if you have one. I have an aqualung rigged with a long hose(hookah) and clean the prop and bottom once a month. Warm climates usually mean higher marine growth rates so we slime up easily even with the best of anti-fouling bottom paints.

Pat and Carolyn, Ladybug, Too

 

[bbrh842]

The probablilty is 99.9999 percent that Andrew is absolutely correct. The initial cause of the malfunction is most likely the prop nut was not tightened sufficently when the propeller was oringinally installed.

 

[Bevan Hill]

Thanks Andrew and all.
It is very re-assuring that the problem is easily diagnosed and solved. I plan to pull the boat tomorrow having checked with the marina workshop people that they are are well stocked with various keyway sizes. I'll just take it steady for the short trip to the ramp and approach with timid care. It's sobering to realise how much we take reverse thrust for granted when approaching jettys etc.
Bevan

 

[Bevan Hill]

Hi Andrew,
Your diagnosis was right on the money. I lifted 'Queen Bess' out and trailed it home where in no time the prop was off and there was the sheared key. It took no time to procure a replacement and to reinstall everything. The boat will probably be back in the water tomorrow and snuggly back on our jetty at home.
Thanks again for the prompt and helful response. Bevan