Minimum Distance Between Cutlass Bearing and Shaft Zinc

[Han]

Hi All,

Another question for everyone.

When we installed our engine (Beta 28) the guy who sold us a prop said that we need a minimum of 1/4" clearance between the cutlass bearing and the shaft zinc - which is flush up against the forward edge of the propeller

We recently discussed with another boat owner and he figures this is much too small, as the engine might ride forward slightly on its mounts when underway, causing the zinc to rub up against the cutlass bearing, restricting water flow and causing all sorts of problems.

In the "acceptable clearances" section of this link it recommends at least 1/2 inch for a smaller engine than ours. Parts

Any experts out there who can set us straight here? I haven't been able to find much online at all. Our prop sits in an aperature with no strut at all. Total shaft length is very short, approximately 24 inches.

This picture isn't the best angle, but it gives the idea.



Thanks.

[delmarrey]

No! That zinc will block the flow of water thru the bearing eventually burning up your bearing.

You need a zinc nut which attaches behind the nut on your prop.

Do you know if your cutlass bearing is pressure fed or from inlet holes in the side of the keel wood? Pressure fed will have a hose attached behind the shaft packing and running to the raw water pump.

If so, a 1/2" gap maybe acceptable.

For strut hung cutlass the distance is 1-1/2 the shaft diameter and with no zinc between.

[Sandero]

I have a small space as well between the front of the hub of the prop and the cutlass bearing. I can fit a standard doughnut, but this prevents lubrication of the cutlass bearing. Also your prep will pull your shaft forward a bit when motoring forward.

What I do is have the yard machine away some of the doughnut to allow 1/4" in front of the doughnut zinc.

This lasts a year until I haul out and replace. I have had no problems with overheating / burning up the cutlass bearing.

If you have trouble with the prop nut solution, this will work.

Give it a try!

Here are pics w and w/o the zinc installed:

[David M]

At 90 bucks an hour, paying the yard to put the zinc on a lathe and shave it down might get expensive.

I would be more inclined to use the fairwater "cone" zinc that fits over the propeller nut. You want proper flow of water inside the cutlass bearing. Use a little blue Threadlock on the machine screw that secures the cone zinc onto the nut. The machine screw can come loose as the anode loses material from electrolysis.

[delmarrey]

Originally Posted by David M

At 90 bucks an hour, paying the yard to put the zinc on a lathe and shave it down might get expensive.

I would be more inclined to use the fairwater "cone" zinc that fits over the propeller nut. You want proper flow of water inside the cutlass bearing. Use a little blue Threadlock on the machine screw that secures the cone zinc onto the nut. The machine screw can come loose as the anode loses material from electrolysis.

A lot of the cone zinc's I've seen has a slot in it for a cotter key.

[senormechanico]

If you don't already have a shaft flexible coupling, measure to see if you have enough room to add one. It would push the shaft back by the thickness of the coupling.
That's what I did.

Steve B.

[stephan birke]

the general rule is that the clearence should be the double axle diameter. If this is not possible put a prop nut anod instead or as in most cases don´t use any since the axle and prop material is very close in the electrical chain. if you don´t put zinc the prop will give some copper away and there won´t be any shells growing.

[4HMainer]

As a bearing supplier and stern gear/gap advisor when fitting quicKutters with over 10 years experience and many 100's of vessels from 1" to over 8" shafts.

Maximum fore and aft shaft movement we have seen is 18.00mm (25.40mm=1")

That's 9.00mm fore and 9.00mm aft, we advise a min gap of 12.00mm on new cutter fittings as we instal a spool on the prop hub which reduces the existing gap. So the 6.35 (1/4") gap you have is tight it is likely th eanode will contact the bearing carrier and it could at that point in time restrict water exiting the bearing, measure the engine position/movement (strap a marker pen to the rear of the engine so that it presses on a pad of paper) if it only moves 3.00mm it is OK to have a 6.00mm gap.

We came to this figure as we are happy with 3.00mm for water to exit the bearing and allow for the 9.00mm of shaft movement towards the bearing. In some cases we have installed systems with 7.00mm gaps and have seen the spool touch the bearing carrier but it is only momentary as the drive is engaged, under steady motoring you don't see this much movement probably 4-5.00mm.

The gap is not really related to shaft size as we don't see much greater movement in bigger shafts and ideally you want to keep this gap as small as possible to reduced shaft flex aft of bearing.

[Blue Stocking]

--or you can fit a shaft brush inside, between the coupling and the stuffing box.
My boat has similar clearance problems to yours, and thats what I did.

PS love those gudgeons and pintles

[shipleft]

Originally Posted by stephan birke

the general rule is that the clearence should be the double axle diameter. If this is not possible put a prop nut anod instead or as in most cases don´t use any since the axle and prop material is very close in the electrical chain. if you don´t put zinc the prop will give some copper away and there won´t be any shells growing.

.......... Yah! And you won't have a propellor left to worry about cleaning off any barnicles ! And BTW, it is zinc the prop loses, not copper, nice red, soft copper is all that remains. S

[delmarrey]

Originally Posted by shipleft

.......... Yah! And you won't have a propellor left to worry about cleaning off any barnicles ! And BTW, it is zinc the prop loses, not copper, nice red, soft copper is all that remains. S

Actually, marine props are aluminum copper alloy with either nickel or manganese added for strength, with maybe some zinc thrown in to make it more flexible, depending on the manufacturer. Marine : Nickel Aluminum Bronze : Aluminum content and corrosion resistance

But the aluminum will still be the sacrificial metal to go first.

[4HMainer]

Originally Posted by stephan birke

the general rule is that the clearence should be the double axle diameter.

Not that I'm aware of? boat builders often aim for a gap from P bracket to prop of shaft to shaft and a half, to allow for some tolerance in engine positioning.

Rule is to get the gap as small as possible which for a soft mounted drive is 1/2" or 12.00mm, twice shaft diameter is way too much and not any rule of thumb I've come across. I have seen worse but the large gaps are usually associated with vibration and bearing wear issues. I specify clearances on new bearing installations.

[delmarrey]

1-1/2 x shaft dia is standard to allow for the passage of water thru the groves of the cutless bearing.

[OceanSeaSpray]

Originally Posted by senormechanico

If you don't already have a shaft flexible coupling, measure to see if you have enough room to add one. It would push the shaft back by the thickness of the coupling.
That's what I did.

Steve B.

Adding flexible couplings on floating shafts (i.e. unsupported at the shaft seal) can create far more problems than anything else in my experience. They cause long shafts to become unstable and wobble. Flexible couplings should be used when the shaft cannot "follow" the engine only. With short shafts, it can work either way.

When it comes to water "flow" to the cutless bearing, try keeping a cutless bearing dry underwater... it is not going to happen easily.
As many have said, there is a need for mechanical clearance under load with full propeller thrust, but (long) shafts also get slightly shorter in cold water (thermal contraction) and some gearboxes with a cone clutch can actually rely on some shaft displacement to engage, so it can be a good idea to check clearance with forward gear engaged. Some old Volvo reverse gears were like this.
I agree with 4HMainer's suggestions for minimum gap, but I have also seen many small engine installations with less gap and no issues. Due to my rope cutter configuration, I run a 4mm gap only with 40HP and a 19'' propeller without any issues. If you don't quite have enough at the worst-case point, you will see evidence of rubbing at the next haul-out. If it is severely too tight, you are going to hear it before anything else!!

Eric