Triton Shaft Tube Alignment

[jhenson]

I am working on the Yanmar 2GM installation today. The engine jig is ready to be placed into position, so I can make the bed molds. I ran a mason?s string from dead center of the cutlass bearing through the center of the cabin. Tube centers were determined by turning a couple of plywood discs on the lathe. The disc at the aft cutlass bearing end of the shaft tube has a hole bored through the center about the diameter of the mason?s string. The other disc on the forward end of the shaft tube has a 3/8? hole to reduce the aperture of the tube so I can better determine the forward center of the shaft.

It became very clear to me today that the shaft tube does not run parallel with centerline of the boat. If I instead run the string through the center of the shaft, the line runs to starboard by an estimated couple of degrees (as viewed facing forward). Let me also say the cutlass bearing ?appears? to be centered in the shaft tube. I remember the shaft having very little play in the cutlass bearing when I removed the A4, so I think it to be relatively new.

I have retained and marked the location of the A4 bed. When I place it back into position, a bird?s eye view leads me to believe that it also runs along the centerline of the shaft tube and not the centerline of the boat.

The string (adjusted to the shaft tube) lies roughly 4 inches from center at the forward cabin bulkhead which is roughly 13 feet from back of the stern tube. I calculate that angle to be about 1.47 degrees. Maybe that is insignificant, but it looks somewhat troubling when you view it down the cabin.

Is this significant enough to warrant the installation of a new stern tube? I really don?t want to add something to the list, if I can help it. If it isn?t that significant, I?m guessing that the engine alignment would need to be with that of the shaft tube and not the centerline of the boat, so as to minimize the amount of cutlass bearings ahead. I know some aircraft engines have a slight angle from center to help oppose the rotational torque of the propeller, but, unfortunately, I think this would be in the wrong direction for the Yanmar.

Joe

[Peter]

Is this to compensate for "prop walk"?

[Tim]

I'm not at all surprised to hear that your shaft tube is not precisely on the boat's centerline. Remember that Tritons were built quite roughly and by the seats of the builders' pants, particularly in the earlier years of production. Nothing in Tritons is on center, perfectly plumb or level, or even symmetrical from side to side, so, like the bulkheads and other interior items, I am sure the shaft tube was installed by eye and using simple measurements rather than with any sort of the more sophisticated means that quality builders use today. At least it sounds like your A4 beds were installed in proper alignment with the shaft tube, off center as it may be.

It's not supposed to be off center, but the amount you're talking about is no problem as long as you align your engine to the actual shaft position--and not to the centerline of the boat. 1.4? is a pretty small offset, though ideally everything would be perfectly aligned and measured and "just so". I doubt you'd notice any effect on the propulsion of your boat if you left it alone.

It doesn't take much to introduce a slight error, which is why adjustable engine mounts and taking the time to align the engine are so important in any boat. I'd be interested to see just how closely any shaft tube is installed in most production boats, at least in the older ones built before today's higher-tech and more controlled building practices.

Here's a picture of the alignment string that I ran on Glissando when I built the engine foundations. It looks roughly on center, or at least on the center of the opening. I never tried to determine if it was spot on or not.



If you feel like adding another project to your list, it's really not very difficult to replace the shaft tube. I did this when I built the Daysailor. Note that while I made all efforts to keep the new tube properly aligned with the centerline of the boat (successfully, as far as I know), I actually waited to permanently install the stern tube until the engine was installed on the new beds, so that I could adhere the tube into exact alignment with where the engine ended up. Here's how I did it on that boat.

Link: Replacing a Triton stern tube

[jhenson]

Thanks Tim,

How much air space would you say I need between the exhaust riser and the bridge deck structure. My exhaust riser was installed sideways, and I noticed yours is aligned fore and aft. Is there any problem leaving it the way it is? It looks like the exhaust hose would wrap around the transmission to port and make the bend to the Vetus muffler.

Joe

[Tim]

Well, the riser is where the raw water is injected, so it doesn't get as hot as a dry stack would--not that it doesn't get warm, though. If you were worried about clearance you could always wrap the riser in some of the fiberglass dry-wrap insulation.

My riser might be a couple inches from the forward edge of the cockpit, and about 1" from that red battery cable boot.

My exhaust riser was installed sideways, and I noticed yours is aligned fore and aft.

When you say "sideways", do you mean it's just twisted 90? so that the riser is aligned athwartships, or do you mean that it's actually lying on its side, rather than vertically?

Having it aligned athwartships, or at any other angle besides in line with the longitudinal centerline, is not a problem as long as you have the clearance needed for the riser and hose, etc.

[jhenson]

Tim,

It is athwartship. There is also a siphon break between it and the heat exchanger that yours does not have.

Joe