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Old 27-07-2015, 11:30   #1
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What is the correct distance between cutlass bearing and prop hub?

This is my current configuration (about 3 inches on a 1 inch prop shaft)

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Old 27-07-2015, 12:03   #2
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Re: What is the correct distance between cutlass bearing and prop hub?

You can probably get away with it but will feel more vibration. That seems an excessive amount of shaft showing.
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Old 27-07-2015, 12:05   #3
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Re: What is the correct distance between cutlass bearing and prop hub?

The dimension to which you refer is called "prop overhang". A general rule of thumb, as put forth by the shaft manufacturers, is that the overhang should not exceed one (1) shaft diameter.

As mentioned, this is a rule of thumb, and the severity of consequences for exceeding it depend on things like the diameter and mass of the prop, H.P. of the engine, gear reduction in the transmission, etc. The problems that can occur include vibration from shaft whip, cutlass wear, shaft breakage, high steering load on the tiller or wheel, and similar goodies.

Probably wouldnt hurt to shorten yours up a bit if possible......

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Old 27-07-2015, 12:06   #4
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Re: What is the correct distance between cutlass bearing and prop hub?

It does look to close to the rudder. There is a formula for the distance from the deadwood. Something about 2/3 of the cord min? Maybe.
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Old 27-07-2015, 12:09   #5
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Re: What is the correct distance between cutlass bearing and prop hub?

While I don't profess to be an expert, I am surprised that there is no egg shaped zinc on the shaft. Also there needs to be space between the cutlass bearing and the zinc to get good water cooling to the cutlass bearing. All in all, I don't think 3 inches is excessive.

I would check the shaft flange at the transmission side to make sure that the shaft is fully inserted and locked.

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Old 27-07-2015, 12:35   #6
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Re: What is the correct distance between cutlass bearing and prop hub?

My first thought was: "Where is the shaft zinc?"

And I would add a line cutter to the shaft too ("spurs").
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Old 27-07-2015, 13:11   #7
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Re: What is the correct distance between cutlass bearing and prop hub?

Thanks for all the responses.

With regard to the zinc, the photo was taken right after doing some work on the prop - don't worry - a zinc was put on afterwards

I have had a lot of transmission problems, and have fixed some possible contributing causes, but this is one that has not been addressed.

So I guess am going to shave this shaft down to leave just enough room for a 1' donut zinc.
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Old 27-07-2015, 14:07   #8
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Re: What is the correct distance between cutlass bearing and prop hub?

Sgt, note the prop nuts on the pic Steady posted.

I suppose there is some debate to be expected, but some from folks say the half nut goes on first and the big nut is actually the lock nut. I'm one of those folks.
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Old 27-07-2015, 14:09   #9
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Re: What is the correct distance between cutlass bearing and prop hub?

Correct, 1/2-nut 1st.
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Old 27-07-2015, 14:13   #10
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Re: What is the correct distance between cutlass bearing and prop hub?

Thanks for that.

I'll be pulling the shaft this winter to get it shortened, and when I put it back on I'll be sure to get my nuts in order.
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Old 27-07-2015, 14:13   #11
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Re: What is the correct distance between cutlass bearing and prop hub?

Quote:
Originally Posted by sgtPluck View Post
This is my current configuration (about 3 inches on a 1 inch prop shaft)

Attachment 106156
Here's the engineering decision process I'd apply.

For vibration reduce the overhang to be as short as possible.

For optimizing flow to the prop i'd keep the prop 40- 50%, of the distance between the keel and the rudder, as measured from the back of the keel.

Finally You might like to shorten the prop shaft so you can still fit a clamped anode.

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Old 27-07-2015, 14:38   #12
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Re: What is the correct distance between cutlass bearing and prop hub?

Just be aware that shortening the shaft may result in prop cavitation if too close to the hull and in turbulent water flow.
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Old 27-07-2015, 15:09   #13
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Re: What is the correct distance between cutlass bearing and prop hub?

Good points about distance from hull, thanks.

This was a re-power, so I'm going to check the original installations and try to get that distance as close to as originally intended by the boat builder (Cape Dory) as possible.
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Old 27-07-2015, 18:03   #14
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Re: What is the correct distance between cutlass bearing and prop hub?

Regarding the exposed shaft length- if it ain't broke, don't fix it. Especially considering the the minimum gap between the cutless and prop (or anode or whatever is next in the drivetrain) should be 1-1.5 shaft diameters.
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