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Author Topic:   Propellers With Hub Kits

Smithsm posted 05-17-2009 03:12 PM ET (US)
I have a FICHT 225 which I believe is a 15-spline shaft. I see some propellers have a hub kit. Is this to protect the prop in the case of a strike or is it to allow the same prop to fit different shafts with different splines?
jimh posted 05-17-2009 03:34 PM ET (US)
Both.
TransAm posted 05-17-2009 03:37 PM ET (US)
Both. Most hub kits will allow the drive sleeve to fail before major damage is done to the lower unit/gears. Changing propellers between different branded and splined motors becomes much easier and less expensive by simply changing hubs. In addition, some hub kits like Mercury's Flow-Torq III & IV also address prop rattle and shift clunk.
jimh posted 05-17-2009 04:22 PM ET (US)
Almost all outboard motor propellers have hubs which are designed to provide a breakaway in the event of a propeller strike. Just a few outboard propellers have solid connections between the propeller and the propeller shaft, and of these most are racing propellers, not everyday propellers.
The initial material used to provide the breakaway was rubber. The hub was fitted to a rubber collar that was then fitted to a shaft coupling. Rubber is an excellent material because it provides some elasticity.
From an old song by Martin Mull:
"I used to be plastic,
and so inelastic.
Now I'm flexible."
The rubber hub helped cushion the load of the propeller coming onto the gear case when shifting. Without the cushion you get a CLUNK when shifting. The plastic couplings do not provide this cushion.
Mercury developed a rather fancy coupling in which flexible rods connect two parts of the plastic coupling, one coupled to the shaft and one coupled to the propeller. This plastic hubs with elastic couplers improve the elasticity and cushioning of the hub, making it more like a rubber hub. They are expensive, however, typically $50 more than a standard plastic hub.
The plastic hubs have the advantage of being replaceable in the field. The rubber hubs generally are not. You typically need some type of press or arbor to compress and insert the rubber into the hub.
Using plastic hubs which can be fitted to different brands is a great advantage for the propeller manufacturer. It makes his product more useful for more customers. It is also useful for the boat owner, at least the owner who likes to change engine brands a lot.
Smithsm posted 05-18-2009 03:24 PM ET (US)
JimH
thanks
It sounds like I would be better off buying a hub kit and seperate prop rather than a used OMC prop without a kit.
am I reading this correctly ?
I have an option of buying a stainless prop locally from the dealer that is OMC and will require no hub or getting a used prop and new hub.
I never used a hub before but it sounds like there are advantages.
jimh posted 05-18-2009 08:18 PM ET (US)
The only real operational advantage for a boat owner in using a propeller with a plastic hub coupling is the notion that in the event of a failure of the hub it could be replaced in the field. Of course, this assumes you are carrying a spare hub plastic insert with you at all times, that the failure of the hub will be in a way that you can replace the failed component, and that the time and place of the failure will be convenient for you to perform the replacement.
I have found that propellers with rubber hubs have smoother and quieter shifting. I have never had the occasion to need to replace a hub in the field. If the hub fails due to a propeller strike, there is a very good possibility the propeller will have damage to the blades, so whether or not the hub is replaceable becomes a moot point; you can't use that propeller anyway.
The most advantageous situation for a propeller with a plastic replacement hub is when the hub fails due to too much torque on the propeller shaft. This generally only happens on very high horsepower, high torque engines. You will not be twisting the hub on a propeller with just normal engines and normal use. If you have a very powerful engine, and you run the boat in unusual circumstances, such as running at high speeds in large waves where the propeller comes out of the water and re-enters the water at high speed, you might twist the hub. But in normal operation, most boaters never spin or twist a rubber propeller hub. It is not that common.
The plastic hub inserts are also prone to melting. If they melt in place you may not be able to easily replace them in the field. A melted plastic hub may need to be chiseled out of the propeller.
I carry a spare propeller with me. If I had a propeller strike or if I spun the hub on the main propeller, I would just change to the spare.
Many propellers are made only with a design that requires a plastic hub, so you may not have a choice if you want to use a particular propeller and that is the only way it is made.
I currently own four propellers. Three have rubber hubs and one has a plastic hub.
The other advantage to a boat owner with a plastic hub propeller that is available for many brands is at the time of sale. If you want to sell a propeller, and it can be fitted to almost any motor, it gives you more potential customers to sell it to. In this way, you have the same advantage the manufacturer had when he sold the propeller.
Smithsm posted 05-18-2009 08:47 PM ET (US)
JimH
thanks much again
Skip Smith

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Author	Topic: Propellers With Hub Kits
Smithsm	posted 05-17-2009 03:12 PM ET (US) I have a FICHT 225 which I believe is a 15-spline shaft. I see some propellers have a hub kit. Is this to protect the prop in the case of a strike or is it to allow the same prop to fit different shafts with different splines?
jimh	posted 05-17-2009 03:34 PM ET (US) Both.
TransAm	posted 05-17-2009 03:37 PM ET (US) Both. Most hub kits will allow the drive sleeve to fail before major damage is done to the lower unit/gears. Changing propellers between different branded and splined motors becomes much easier and less expensive by simply changing hubs. In addition, some hub kits like Mercury's Flow-Torq III & IV also address prop rattle and shift clunk.
jimh	posted 05-17-2009 04:22 PM ET (US) Almost all outboard motor propellers have hubs which are designed to provide a breakaway in the event of a propeller strike. Just a few outboard propellers have solid connections between the propeller and the propeller shaft, and of these most are racing propellers, not everyday propellers. The initial material used to provide the breakaway was rubber. The hub was fitted to a rubber collar that was then fitted to a shaft coupling. Rubber is an excellent material because it provides some elasticity. From an old song by Martin Mull: "I used to be plastic, and so inelastic. Now I'm flexible." The rubber hub helped cushion the load of the propeller coming onto the gear case when shifting. Without the cushion you get a CLUNK when shifting. The plastic couplings do not provide this cushion. Mercury developed a rather fancy coupling in which flexible rods connect two parts of the plastic coupling, one coupled to the shaft and one coupled to the propeller. This plastic hubs with elastic couplers improve the elasticity and cushioning of the hub, making it more like a rubber hub. They are expensive, however, typically $50 more than a standard plastic hub. The plastic hubs have the advantage of being replaceable in the field. The rubber hubs generally are not. You typically need some type of press or arbor to compress and insert the rubber into the hub. Using plastic hubs which can be fitted to different brands is a great advantage for the propeller manufacturer. It makes his product more useful for more customers. It is also useful for the boat owner, at least the owner who likes to change engine brands a lot.
Smithsm	posted 05-18-2009 03:24 PM ET (US) JimH thanks It sounds like I would be better off buying a hub kit and seperate prop rather than a used OMC prop without a kit. am I reading this correctly ? I have an option of buying a stainless prop locally from the dealer that is OMC and will require no hub or getting a used prop and new hub. I never used a hub before but it sounds like there are advantages.
jimh	posted 05-18-2009 08:18 PM ET (US) The only real operational advantage for a boat owner in using a propeller with a plastic hub coupling is the notion that in the event of a failure of the hub it could be replaced in the field. Of course, this assumes you are carrying a spare hub plastic insert with you at all times, that the failure of the hub will be in a way that you can replace the failed component, and that the time and place of the failure will be convenient for you to perform the replacement. I have found that propellers with rubber hubs have smoother and quieter shifting. I have never had the occasion to need to replace a hub in the field. If the hub fails due to a propeller strike, there is a very good possibility the propeller will have damage to the blades, so whether or not the hub is replaceable becomes a moot point; you can't use that propeller anyway. The most advantageous situation for a propeller with a plastic replacement hub is when the hub fails due to too much torque on the propeller shaft. This generally only happens on very high horsepower, high torque engines. You will not be twisting the hub on a propeller with just normal engines and normal use. If you have a very powerful engine, and you run the boat in unusual circumstances, such as running at high speeds in large waves where the propeller comes out of the water and re-enters the water at high speed, you might twist the hub. But in normal operation, most boaters never spin or twist a rubber propeller hub. It is not that common. The plastic hub inserts are also prone to melting. If they melt in place you may not be able to easily replace them in the field. A melted plastic hub may need to be chiseled out of the propeller. I carry a spare propeller with me. If I had a propeller strike or if I spun the hub on the main propeller, I would just change to the spare. Many propellers are made only with a design that requires a plastic hub, so you may not have a choice if you want to use a particular propeller and that is the only way it is made. I currently own four propellers. Three have rubber hubs and one has a plastic hub. The other advantage to a boat owner with a plastic hub propeller that is available for many brands is at the time of sale. If you want to sell a propeller, and it can be fitted to almost any motor, it gives you more potential customers to sell it to. In this way, you have the same advantage the manufacturer had when he sold the propeller.
Smithsm	posted 05-18-2009 08:47 PM ET (US) JimH thanks much again Skip Smith