[cbrendanjarvisi]

Dial Indicating Shafts Recently a number of articles have appeared in boating publications about dial indicating shafts while in the boat. I'll explain why this should never be done. As noted above, boat hulls can and do change shape over time. Further, the shape of the hull changes when hauled versus afloat. The changing shape of the hull is likely to cause the shaft to bend. Now, you may look at a 1.5" stainless shaft and think that it's cannot be easily bent. In fact, if you suspend it on its ends, even gravity will bend it considerably. Gravity will bend a 15' shaft a lot. more

[richardmacdonald68]

In fact, the most commonly used engine mounts for gas and small diesels is a mount that does a very poor job of maintaining alignment. Shown in the photo above, this mount not only permits the engine to rock forward and back, but up and down and laterally also. It literally allows the engine to move like a hobby horse.

[daemox]

When there is significant misalignment involved, the struts and strut bolts should be checked. To do this, I usually pick a heavy piece of shoring found in most boat yards and give the strut several good whacks. If the strut deflects or shudders, its not stable. If water squirts out from the base, its loose. Also note whether the whole bottom of the hull is defecting when you hit the strut. If so, there's no point in realigning anything unless you first do something to make the strut base stable. If the struts are fluttering because the bottom is weak, the entire system is unstable and must be corrected. more

[ralphmcclure]

The strut in photo #3 carries the same size shaft and prop as in #2, both on a 40' boat. But the later is double the thickness in both width and cross-sectional area, with a substantiallly wider base. There is no doubt that this strut will hold the shaft steady. Also notice that the length of the strut bore is double the length of that in photo #2. [links]

[Dark_Ghost]

Finally, when you rotate the shaft by pulling on the propeller, you're moving the shaft against the rubber bearing, further throwing your dial indicator readings off. As any trained machinist will tell you, there's only one way to dial a shaft and that is on a calibrated roller bed. One simply cannot dial a rotating object without a reference base and expect accurate results. And a shaft in a boat doesn't provide such a reference. [links]

[kathysafari]

But that's not all. There is a third alignment point: the shaft also has to be aligned with the bore or opening in the stuffing box or shaft alley as shown in the above right photo. This is the opening where the shaft exits the hull. Typically there is about 1/4' total clearance here (1/8" all around). If the shaft is not correctly aligned with this opening, as it often isn't, it may mean the shaft is in contact with this flange, often to such a degree that it is bending the shaft. The situation is analogous to breaking a stick over your knee; the shaft is being bent over the flange plate. Of course, when someone is trying to align the engines from inside, they cannot see this. This is why attempts to align the engines can end up causing more problems than it solves.

[Margaret Stukel]

The Nature of Inboard Drive Systems It is a common belief that engines and shaft couplings have to be aligned to tolerances of a few thousandths. There is some misunderstanding about this. Yes, the shaft coupling to transmission coupling needs to fit within several thousandths, but what were talking here is the coupling fit, not the shaft alignment. This is an important distinction that is often confused. I'll explain why.

[sethsethseth]

If the mounts are the vertical stud type set in rubber in an aluminum base, these are the kind that are prone to rocking back and forth, particularly with heavy diesel. Check the stud to make sure that its centered in the base with the vessel at rest. If its leaning in any direction, the mount is stressed and the system is out of alignment (see photos above ). more

[jimmy]

Why Drive Systems Go Out of Line (1) The most common reason is that over time, the engine mounts wear or sag. Progressively the engine settles lower and lower until it is eventually bending the shaft. (2) Hull changes shape. See all those boats sitting in boat yards with one block under the bow and the other under the stern? Supported at only the ends, would you suppose that the hull is sagging just a bit? Sure it is, and that's throwing the system out of alignment. (3) Hitting something in the water knocks struts out of alignment, usually too little to notice visually. (4) Engine mounts are too weak and permit too much engine movement. (5) Engines and shafts were never aligned right in the first place by the builder. This is more common than you might think.

[Guillermo]

A laser can be used to “sight through” a shaft. Here the laser has been positioned in the aft-most shaft bearing, and aimed at the transmission output coupling. When it’s centered, the shaft droop will have been eliminated. The pilot bushing is damaged, dents can be seen between the 12:00 and 2:00 o’clock position.

[mstate2]

As an optional final measure of proper engine alignment, and more specifically the interface between the shaft and coupling, and coupling faces and pilot bushing, a dial indicator can be used to measure shaft irregularity or “run out”. Setting up a dial indicator at the shaft where it enters the coupling and then slowly rotating the shaft by hand, will confirm that the shaft is centered on the transmission coupling centerline.

[chrisadamshtm]

When I’ve discussed this issue with some mechanics they’ve expressed skepticism, ‘How could a two inch diameter steel shaft sag?’ they asked. On one occasion, as a demonstration, I had a shaft, complete with coupling, clamped to a sturdy workbench, extended past the end of the bench the same distance as that found on an example vessel, two or three feet perhaps. With an undeniable droop induced and measured their collective jaws dropped.

[autoinsurance1]

Why do we still need the magnetic compass when we have GPS? Many people do not appreciate that the GPS, when it shows 123 degrees magnetic is showing COG, due to the set of the current and leeway carrying the boat sideways. The actual heading may be completely different.

[Luther]

Careful adjustment will not remove all the errors if the compass has not been aligned with the fore and aft line of the boat. With the compass amidships it is easy to line up the lubbers line with the bow of the boat, but if the compass is offset you need to measure the distance of the compass from the fore and aft line. more

[xcikni]

Because magnetic influences affect the compass needle differently on different headings the deviations will not be the same on every heading. And this explains why I cannot adjust a compass on the bench in my workshop. [links]

[LZ]

Since the magnetic compass was first used by the Chinese for navigation sometime around 1044, it has become an essential instrument on every boat, yacht & ship.

[Insti]

The inbuilt compass is housed in the dome underneath:

[MH80]

You may have seen soft iron spheres or washers mounted either side of the compass. These are used to correct for errors caused by induced magnetism. Without the soft iron it is still possible to make a partial corection but there may be significant errors on NE/SE/SW/NW. [links]