Propshaft R&R

[tuner]

My 98 Mackinaw has a slight vibration just before getting on a plane, I can see the shaft log run-out at this point then it disappears by 20mph. A new prop, new cutlass bearing and coupling alignment to within 0.003" did not resolve the problem. I suspected the shaft may have some run-out but put it off due to the horror stories of coupler removal. Last week, I built a heavy duty puller based on designs for sale, it is 1/2" plate with a 3/4" NF bolt and nut. The coupler pattern for my Liberty 5000 transmission was easy to find and simplified the project. Fortunately for me, with the help of a 1/2" impact gun, the coupler budged and I had the shaft out in about an hour.

I took the shaft and coupling to Ryan at Wilmington Iron Works for inspection and repair. There was only a small amount of run-out in the shaft to repair then reface the coupling. The shaft and coupling looked great and included new keys, set screws, and paint on the coupler.

I tested the boat with a different prop from Damon ( 17x14 3 blade) vs the previous prop (15x15.75 4 blade). The results were good at low speed and the boat practically jumped out of the water, but the prop also produced a new vibration or harmonic at higher speeds that was troubling. I have since reinstalled the 4 blade and will retest when weather and work permits to verify if the issue was due to the prop.

Rough looking, but it got the job done! I'll make it look nice when I have time



Shiny like new!



The big wheel, I had high hopes for!

[My My Hey Hey]

Nice job on the tool. Except for the new vibration did you like Damon's prop better? Does it vibrate on his boat at the same rpm's?

[Quik Fix]

Does the prop sit that close to the strut?

[tuner]

Damon said he really liked how the prop performed on his boat, the only downside was that the max rpm was too low.

The prop made a sound and vibration that was unlike the 4 blade, I could see the vibration in the water in the bilge. I need to test the 4 blade and be sure that everything else is ok. I wonder if it could be from pressure waves from the prop striking the hull, the prop is fairly close to the hull. It is a shame because the boat pulled like a tractor, I bet the prop would be great is snotty conditions.

[tuner]

Does the prop sit that close to the strut?

The prop is closer to the strut than the 4 blade, I'd say just over an inch of exposed shaft. I cut the shoe back because the blade just touched it. The prop looks much closer to the lower portion of the strut than it actually is.

[gary s]

Brand new never used prop or has it been worked on? Did you paint the coupler before you put it on or after?

[Ship]

The distance between the hub of the prop and strut should be 1-1/2 times the diameter of the shaft or so my memory recalls. It looks closer in the picture. That would possibly be your vibration. Prop wash against the strut.

[tuner]

Brand new never used prop or has it been worked on? Did you paint the coupler before you put it on or after?

Please re-read the thread.

[gary s]

I did read your post if you don't want help fine.....

[tuner]

The distance between the hub of the prop and strut should be 1-1/2 times the diameter of the shaft or so my memory recalls. It looks closer in the picture. That would possibly be your vibration. Prop wash against the strut.

The maximum recommended distance from the coupler is 1.5x the shaft diameter. The closer the prop is to the bearing the better it is supported, if you get too close, water may not flow as well through the cutlass bearing. The only area that may be too close is the hull. There are numerous engineering papers that recommend 15-20% of the prop diameter for clearance to the hull, but many small inboard boats, like ski boats, routinely run much less.

[tuner]

I did read your post if you don't want help fine.....

I'm not trying to offend you Gary.

From post #4: "Damon said he really liked how the prop performed on his boat, the only downside was that the max rpm was too low"

From post #1 "The shaft and coupling looked great and included new keys, set screws, and paint on the coupler." Do you really think I would paint a critical surface like the flange?

[Ship]

The maximum recommended distance from the coupler is 1.5x the shaft diameter. The closer the prop is to the bearing the better it is supported, if you get too close, water may not flow as well through the cutlass bearing. The only area that may be too close is the hull. There are numerous engineering papers that recommend 15-20% of the prop diameter for clearance to the hull, but many small inboard boats, like ski boats, routinely run much less.

The water has to flow past the hub of the prop as well. the strut acts as a pillow block and the shaft diameter allows spacing to accomdate it.

[tuner]

The water has to flow past the hub of the prop as well. the strut acts as a pillow block and the shaft diameter allows spacing to accomdate it.

The hub on the 17" prop is longer than the 15 and larger in diameter than the strut where the bearing is (my puller bolts barely fit over the hub) . The longer hub reduces the clearance to the bearing.

If the prop was 1/2" farther aft, it would only increase the hull clearance, water flow would be the same at the hub. Damon mentioned that he had some clearance from the shoe and about 1.5" from the hull, and no problems.

I hope to retest this weekend.

[Damon]

Hi Steve,

ACME used it as a test prop, so they gave me a good deal on it (it was in like knew condition from the factory). The prop ran smooth for me when I had the 351W HO (it was my favorite of the 4 or 5 props I tested back then). It was just was about 250-300 RPMs shy of my targeted WOT. If it were me, I would take the old 4-blade (or the 17" 3-blade) to a prop shop to have it checked out just to rule it out as the the cause. The prop hub seems really close to your strut. IIRC, the tip of the blade had more clearance from the hull, and was out further from the bottom of the keel on my Mackinaw.

Go ahead and keep the prop. It would have just sat in my garage indefinitely anyways. I hope you can make it work out somehow.

[tuner]

Hi Damon,

I ran my 4 blade (15x15.75) on Sunday and the driveline was smooth, in fact, the best it has ever been! I am happy that the shaft repair and coupling facing made such a difference, I was starting to wonder if my 4 blade had any issues. Wilmington Iron works did not think the small repairs would help my problem. It may be that I am more sensitive (OC?) about vibration and noise than most.

The picture is misleading. The 3 blade hub was 1" from the bearing, the 4 blade hub is 1.25" from the bearing. I could barely get my palm between the blade and the hull.

I will take the big 3 blade to Wilmington propeller and have it checked. The prop didn't shake the boat like it was bent, or when you get a plastic bag stuck, it was a higher frequency vibration that felt like pulses from each blade rather than each revolution. Otherwise, I can see why you were impressed with the prop.

The differential carrier in my old Willys jeep broke (the bearing sheared off), so I have a new iron in the fire and the Mack will sit as is for a bit

It is very kind of you to let me keep the prop, I hope I can make it work.

Steve