Prop / Strut Clearance

[Photolomy]

We have an issue where there is only 1/8" clearance between the port prop and the strut, but almost 2" between the stbd prop and strut. The port shafts are connected with a 10 1/4" muff coupling, while the stbd shafts are connected with a 12 1/4" muff coupling. This would seem to explain the difference in clearances between stbd and port. However, Sams says that the correct size muff coupling is 10 1/4". My question is, what should the clearance be between the strut and prop? Surely it should be more than 1/8", correct? And what might another source of this lack of clearance be? It is a 1978 53' MY. In other words, if the 10 1/4" muff coupling is correct, why is the prop almost touching the strut?

 

[Pascal]

There is a formula for that spacing, i think its 50% of the shaft diameter but i couod be wrong. so for a 2" shaft it would be 1"

Somone must have replaced the shaft or coupler at some point...

 

[Photolomy]

Yeah, we are going to have to measure things closer to get to the bottom of this. Any ideas on how to eek out another 1/2" of clearance? I wonder if the transmission couplings are the culprit. In other words, they were too short and they addressed this on the stbd side with a longer muff coupling, but not on the port side (because they couldn't find a second longer muff coupling).

 

[Cricket]

Pascal, I always thought it was shaft diameter, not half of.
Maybe I'm thinking about distance between strut and prop???

But, Michigan Wheel ideally wants 20% of prop diameter to be the difference, 15% is the minimum.
"The clearance between the propeller blade tips and the hull bottom should be at least 15% of the propeller diameter and ideally, 20% or more. So for example, a 20” diameter propeller would have 3” minimum clearance and better still 4” or more. A common guideline for recommended propeller to strut or keel clearance is 20% of propeller diameter measured between the propeller blade edge and strut leg or keel. This is often measured at a point on the propeller blade edge about 70% of the distance from the shaft centerline to the blade tip."

https://www.miwheel.com/resources/faq/inboard/

 

[Pascal]

I think you re right, it is shaft diameter not half

 

[Boatsb]

Hatteras should have all the measurements for the shafts, couplers and what the original props were. I doubt it left the factory that way. It's possible someone did some work and changed out a shaft or coupler.

I believe 1/2 the shaft diameter is the maximum space between the prop and bearing. I've also seen spacers added at the transmission coupler that absorbed vibration or something like that. Not really the way I'd do it though.

 

[Photolomy]

Is the hole through the muff coupling all the way through? In other words, are the two shafts butted up against each other? This is what the couplings I have seen are like. In which case, whether the coupling is 10 1/4" or 12" wouldn't make a difference to the overall shaft length, which points to the shaft being too short.

 

[Photolomy]

Hatteras should have all the measurements for the shafts, couplers and what the original props were. I doubt it left the factory that way. It's possible someone did some work and changed out a shaft or coupler.

I believe 1/2 the shaft diameter is the maximum space between the prop and bearing. I've also seen spacers added at the transmission coupler that absorbed vibration or something like that. Not really the way I'd do it though.

Something like this? ...

https://propshopinc.com/Drivesaver-...sions-Flexible-Shaft-Coupling-5756A-p678.html

 

[Boatsb]

I dont know if theres one large enough though. I've not seen one on a hatt.

 

[bigbill]

max distance is shaft diameter, less is better so you dont get as much whip. i'm sure 1/8 is too close, being that small of a gap might slow the water flow down thru the bearing

 

[adammilne]

I really like the replies..

 

[Photolomy]

To follow up, we chose the DriveSaver option ...

https://www.gcsmarine.com/content/drivesavers/drivesavers

Did exactly what we needed. Pushed the entire shaft back 1" and now there is ample clearance between the muff coupling and forward strut, and the prop and rearward strut. The right size was available at the local distributor, and fairly easy to install (according to the yard). Don't know exactly why we were short on the port side, but is moot now. Alignment and bearings are all good.