Laser alignment of boat propshaft bearings ? 3

[Paulista]

I have just redone the planking on the bottomsides of my wooden power boat.

I now need to align a pair of prop-shaft struts, a longer one at the stern and a shorter one just aft of the point where the prop-shaft enters the engine room. The distance bewteen these struts is typically about 5-6 feet.

These props struts contain the "cutlass bearings" that support the prop shaft and the bearing centres need to be aligned within 1/16 " (1.6 mm).

I was thinking of machining a pair of circular aluminium "caps" that would fit (concentrically) over the ends of the cylindrical cutlass bearings.

On the centreline of one of these caps, a pen-like point laser source would be fitted. Similarly, a translucent screen with cross-hairs on the other cap.

The pen-like point laser source on the assembly of the longer prop-strut would be the transmitter. The cross-hair fitted translucent screen on the shorter strut, the receiver. In effect, you would "align as you see".

This is not unlike some of the current automobile steering alignment systems on the market.

Is this feasible ? What would be the desirable characteristics for the pen-like point laser source in view of the required alignment to within 1/16" (1.6 mm) ? Is such a point laser source available for a reasonable amount ?

Thanks for any suggestions.

 

[GregLocock]

yes, that is a good way of doing it. If you can find a laser with a good cylindrical body you are most of the way towards a robust solution. Install the laser into an adapter sleeve into the bearing housing. Now spin the sleeve and laser assembly, observe the diameter of the circle that the spot makes on the target. Then bring the target half way back towards the laser and repeat. If the circle is the same size the laser is eccentrically mounted, if it is half the size then it is skewed axially in the sleeve.

When you've got that sorted out, you just set it up from each end.




Cheers

Greg Locock

 

[patprimmer]

I have disagree a little with your methodology.

In my experience the bottom of a boat moves around a bit when the boat is on rails on dry land, floating statically in the water, and moving at speed.

Wooden boats are further compromised by swelling of the timbers as they absorb water.

To have the best compromise in shaft alignment under operating conditions you have to eliminate, reduce or allow for the variables. The amounts of movement I have seen, make your order of accuracy seem meaningless.

For example. I changed ring gears on a speedboat. The original ring gear had just over 1" clearance with the log seal fitting in the bottom of the boat. The new ring gear had about 3/4"clearance. The new ring gear wore a 1/8" grove in the bottom of the boat when the boat was run at speed in rough water. This was a fibreglass boat, so it is not a direct comparison, but there must be some relevance.

I generally try to estimate the load distribution, and shape the trailer so that the load on the boat approximates the load when running. I do this by pure intuition and guess work, then look at wear patterns in the bushes, and correct a little next time I replace them.

A lot will depend on where the engine bearers run and were the cross members are relative to skeg mounting points.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
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[TheBlacksmith]

Having been the maintenance officer for a fleet of wooden hulled Navy training craft, I absolutely agree that the final alignment must be done waterborne. We had one boat that was carefully aligned out of the water that burned up the cutlass bearing where the shaft exited the boat. When we disconnected the shaft from the transmission, the flanges jumped about 2 inches. You usually end up using feeler gauges on the flange faces or dial indictators on the flange rims.

Blacksmith

 

[patprimmer]

I use a 0.002"feeler gauge on the flange. And I turn the flange face in a lathe with the flange bolted to the shaft in exactly the same manner as it will be in the boat.

I always ensure the flange is a tight fit on the shaft and I machine flats on the shaft instead of holes for the grub screws. I then use flat bottom, not pointy, grub screws. This greatly increases the bearing area for the screws and avoids them cutting into the shaft.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Paulista]

Many thanks to you all. Let me tell you WHY I conjured up this rather unusual proposal for aligning a PROP SHAFT WITH ITSELF. FORGET THE GEAR BOX FLANGE (AND ENGINE) FOR A MOMENT.

The reason is that I never managed to align the port PROP SHAFT WITH ITSELF ! I would push the prop shaft through the stern strut cutlass bearing, with the latter´s through-bolts tightened up against the shaft log in the bilge. The shaft would then be pushed through the cutlass bearing of the shorter strut up front.

This shorter strut would be held in place by a jack (ground-up) and then the strut base-to-hull area shimmed until the shaft when rotated by hand, felt it was being freely suspended, between the two cutlass bearings. Once this was attained, the through-bolts of this shorter strut would now be tightened up against the shaft log - BINGO ! Result of exercise ? Shaft does not rotate freely anymore - cutlass bearings of stern strut and forward strut have just been misaligned!

So what if I actually had the stern and the forward strut cutlass bearings aligned perfectly with each other, prior to introducing the shaft into their midst ? ? Wouldn´t that be a nice - and less frustrating way to do it ??

Perhaps I was too finnicky about the cutlass bearing centres being aligned to 1/16", but what I was actually aimimg for was 1/8" (3.2 mm). My intended laser point light-source would need to be accurate to at least 1/16" at 5-6 ft (transmitter-receiver distance) to allow me the 1/8" overall result.

Yes, I agree that boat hulls move when in the water and out ! But if I am unable to align the port shaft with itself in the dry........I could actually be a lottery winner and this shaft would miraculously end up perfectly aligned with itself in the water !

In the past,have aligned the port and starboard prop shafts with themselves with the boat sitting on it´s cradle, and the cradle on a bow-stern incline in the boat hangar. I would never attempt this with the cradle on a port-starboard incline, with the hull skewing to one side and neithert have the resources to do so with the hull in the water (frogmen and the rest of it)!

Ah,yes, before I forget, I also align the shaft flanges with the shafts on a rotating-lathe, tighten up the respective bolts and then don´t touch this assembly again.

Once I am able to ALIGN THE PROP SHAFTS WITH THEMSELVES IN THE DRY, the following step is to have the shaft-flanges, aligned up with the flanges of their respective gear-boxes. THE LATTER PROCEDURE HAS TO BE CARRIED OUT WITH THE HULL IN THE WATER - PERIOD !

For some God forsaken reason, the port prop shaft gives me this trouble whereas the starboard one is well behaved in this respect.