How much shaft can be out of P-bracket

slawosz

slawosz

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Hi,
I have 3/4 inch shaft. Now distance from propeller and P-bracket is around 12 mm, but as I am ordering a new shaft I would like to install a narrow anode and rope cutter on propeller side of shaft. I think I will be fine, but in case unknown unknown I don't want to make a mistake and have too much shaft out - in case of some unballanced vibrations. New distance will be 50 mm from propeller to P-bracket and propeller will be longer as I am 90% going for folding one.

PS. I might overthink it but its better to be safe than sorry.
 
Not sure why you would put the anode after the p bracket, the rope cutter details should tell you the recommended clearances, you need enough to allow for water flow to cutless bearing when driving forward as shaft will push towards engine. We set ours at about 12mm
 
slawosz said:
Hi,
I have 3/4 inch shaft. Now distance from propeller and P-bracket is around 12 mm, but as I am ordering a new shaft I would like to install a narrow anode and rope cutter on propeller side of shaft. I think I will be fine, but in case unknown unknown I don't want to make a mistake and have too much shaft out - in case of some unballanced vibrations. New distance will be 50 mm from propeller to P-bracket and propeller will be longer as I am 90% going for folding one.

PS. I might overthink it but its better to be safe than sorry.
Click to expand...
The anode goes forward of the P bracket NOT aft (leaving 10mm or so clear shaft). The 12mm between the prop and the P bracket is correct. If you are fitting a Stripper then you need 36mm between the P bracket and the prop ropestripper.com/strippers.php which is quite acceptable. If you are fitting one of the less effective disc citters then the same principles apply - 12mm plus the thickness of the cutter.
 
I agree about the anode being more conventionally placed before the p-bracket. Apart from space issues, there's the hope that it might save your shaft/prop from either striking the rudder or dropping into the sea (and the subsequent rapid flooding of the boat) should the shaft coupling fail.

My understanding is that propeller overhang should be max 1.5 times the shaft diameter, i.e. max 29 mm, in your case. As far as I'm aware this is measured from the aft of the bracket to the front of the prop hub. Given this guideline, it looks like your 50mm is out of spec, and the current 12mm is well within that. Can you move the anode around, and what clearances does the
cutter manufacturer recommend?

I might be in the market for a new shaft myself so will be watching this thread with interest.
 
slawosz said:
Isn't it better to have anode as close to the propeller as possible for best effectivenes?
Click to expand...
The recommended position in front of the P bracket is fine. It is directly bonded to the shaft and in line of sight to the prop. I would expect minimal wear on it anyway , particularly if you paint the propeller..

Just for infotmation props are commonly protected by hull anodes several feet away from the prop. The key is electrical binding between the 2 which is often problematic with shaft drives.
 
sailoppopotamus said:
I agree about the anode being more conventionally placed before the p-bracket. Apart from space issues, there's the hope that it might save your shaft/prop from either striking the rudder or dropping into the sea (and the subsequent rapid flooding of the boat) should the shaft coupling fail.

My understanding is that propeller overhang should be max 1.5 times the shaft diameter, i.e. max 29 mm, in your case. As far as I'm aware this is measured from the aft of the bracket to the front of the prop hub. Given this guideline, it looks like your 50mm is out of spec, and the current 12mm is well within that. Can you move the anode around, and what clearances does the
cutter manufacturer recommend?

I might be in the market for a new shaft myself so will be watching this thread with interest.
Click to expand...
The 1.5*diameter is just a rule of thumb. The 38mm when fitting a Stripper is fine with a 3/4" shaft. Has been done may times over the lat 30 years or so without problems.

The norm without a cutter is 10-15mm which allows water to flow in and out of the cutless and crucially to allow fore and aft movement of the shaft when going in and out of gear. Not so critical on little engines, but with bigger engines and soft mounts this can be as much as +/- 6mm. Of you look at the design of the stripper you will see that the fixed cutter is located in a V shaped block to allow for this movement.

This is an AM10 fitted to my boat with a 30mm shaft and overall shaft overhang aft of the cutless housing of 40mm. We had the shaft made after the engine was installed so that it was exactly the correct length.

IMG_20220916_123848.jpg
 
sailoppopotamus said:
My understanding is that propeller overhang should be max 1.5 times the shaft diameter, i.e. max 29 mm, in your case. As far as I'm aware this is measured from the aft of the bracket to the front of the prop hub.
Click to expand...
For unknown reasons my shaft protrusion after the aft p-bracket face is more like 2-2.5 shaft dia, propeller is Max Prop. 6k engine hours and everything going fine, no vibrations nor undue stress to the rubber thingy inside the bracket (too late to look for the correct term), which by precaution I changed after 5K hours. I put an olive anode on the shaft forward of the P-bracket, a collar anode between the p-bracket and propeller (where there is also a sort of rope cutter), the propeller is perfect and both anodes get corroded, I see no reason to change anything. FWIW, as it is customary to say :)
 
To the OP: I would put more faith in Tranona's actual experience in doing this as opposed to my vague understanding of how these things are meant to be engineered.

We do not know what cutter OP wants to fit. So perhaps some information on that might be usesful. At any rate, I think I would err towards having a shaft that's too long, rather than one that's too short. The former seems much cheaper to rectify.
 
The disc cutters are much thinner than a Stripper and the fitting instructions will specify the gap required.
 
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