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Engine Alignment

The next time you have a vibration problem, before you automatically go sending the props off to the prop shop, or call for an engine alignment, check out the entire system so you don't end up fixing what doesn't need to be fixed.


Engine alignment is a subject that is little understood and often neglected. Most boat owners aren't even aware that engine alignment is one of those things that requires occasional maintenance, and is one of the last things to be considered when some kind of drive system problem develops. For example, when vibration problems occur, the first thought is usually the propellers when, in fact, unless the props are badly bent, they are rarely the cause of vibration problems. More often than not, the source of the problem lies elsewhere. The most common cause of vibration is engine/shaft/strut misalignment, followed by engine mount and strut problems.

The subject of alignment is really not very complicated, but it does involve several other factors which most people are not aware of. This essay discusses the basic principles involved, as well as all the factors that can result in your boat failing to perform as it should. Anyone can understand it, and by taking the time to study the issue, you will not only know what to look for, but can save yourself a lot of money by heading off problems before they develop. Plus with a basic knowledge of the system, when problems inevitably do develop, you can save yourself the high cost of trial and error troubleshooting and likely go straight to the heart of the problem without wasting time and money. Here's a short list of the problems that can be caused by engine/shaft alignment faults:

  • Rapid cutlass bearing wear.

  • Misaligned strut galls shaft, requiring shaft replacement.

  • Causes stuffing boxes to wear out and leak, not infrequently sinking the boat.

  • Bent or broken shafts

  • Drive system vibration that can damage transmissions, engine mounts and the boat hull itself.

  • Vibration causes  damage to other systems.

  • Transmission failure caused by increased stress on the rear output shaft bearings and gears.

  • Loosening of struts, causing leaking and possible sinking.

  • Oscillating propeller shaft causing stuffing box clamps to loosen and work free, flooding or sinking the boat.

  • Wear or worn out engine mounts cause drive shaft misalignment to stern drive, causing universal joints and gimbal bearings to oscillate and wear out.


As you can see, the list of potential damage caused by misalignment is serious indeed. The good news is that drive systems tolerate a lot of abuse and are very forgiving. The bad news is that a lack of general understanding of these systems often translates into more abuse than the systems can withstand.

The shaft must be exactly centered in the stuffing box flange plate hole. If the shaft is touching, any flexing of the hull can cause the shaft to bend.

This shaft is badly misaligned in the bearing.

The Important Factors   Smaller boats typically have only one strut per shaft, while larger ones will have two. Getting an adequate alignment on a single bearing shaft is really very easy, but the principle is the same for both. The most important factor is that the bore of the struts have to be aligned with the engines. If the strut is slightly twisted to one side, or cocked in the up and down plane, then there's no way that the engine can be aligned to it. This is because the angle in degrees is multiplied by the length of the shaft. Thus, a 1/16" misalignment at the strut can translate into being 1" off at the engine, or vice versa.  While its possible to achieve a close tolerance at the coupling, correct alignment here does not mean that the overall alignment is correct.  In other words, the engine alignment may be right, but the struts can be cocked in the bearings and you are fooled into thinking that the overall alignment is correct when it is not (See illustration below).


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.

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.

Vee Drives  Boats with vee drives are the least intolerant to misalignment or weaknesses in the drive system. This is because the shafts are very short and reverse direction. Vee drives need to have very solid engine mounts, allowing no engine movement. Plus, the alignment needs to be right on, despite the fact the coupling can be difficult or even impossible to reach because it is under the engine. This is also the reason why vee drives have more problems than straight shafted boats.

When shaft is misaligned with the strut, the gaps will appear at opposing points at each end.

Testing for Bearing Wear This is very easy. Simply lift up on the end of the shaft as hard as you can. Use a long board as a lever if necessary. If the shaft moves up and down, its worn and is ready for replacement. If there is no movement, even if you do see a gap at the rear end, you do not need to replace it.

Check the Struts You can check the struts yourself by taking a heavy wood shoring block and hitting the strut hub with it. Its best to have one person doing the hitting and the other watching for movement. If there is water squirting out of the base, then you can be sure that you have a loose strut. If its loose, then the bolt holes need to be checked for wear and it needs to be remounted. Also note whether the hull bottom is deflecting when you hit the strut. If it is, you have a problem.


David Pascoe,

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