Prop Shaft Shim and Alignment

[volzalum 280]

On my 2015 FS44, my boat came from the factory with a 15” prop. The boat performed awesomely with that prop until the end of the first season and I noticed significant Gelcoat burn. Centurion put a 3/16 Inch steel pate on the boat which only allows a 14” prop. The last 4 seasons have been very frustrating performance wise. I am about to make some adjustments to get back to a larger prop. I took measurements and have sufficient motor mount adjustments to ship the prop shaft strut to gain some back. By my calculations, I have enough clearance in the prop shaft log to change the prop shaft angle 1 degree after engine alignment.

On the FS44 there is a recess where the strut anchors. I am having a Stainless plate (~7-5/8 x 3-5/8) made to sit in the recess that has a 1 degree taper across the plate (bow to stern) which is a little over ⅛ inch on the long side. This will align the strut bearings with the new prop shaft angle.

Additionally, I am going to remove and reinstall the steel plate the way the factory should have done it - by recessing it into the fiberglass.

Between the shim and the recessed plate, I should gain nearly an inch of clearance on the prop. I’m hoping to get to a 16 inch prop size, but may have to settle for a 15.5 inch after everything is secured. I know that a 15 worked great, so anything larger will be even better. I will document this with some pictures and discussion once I get it complete (hopefully by mid-July). 

Edited May 25, 2020 by volzalum

[Kevin Baugh 78]

 recessing the metal plate seems like a feasible idea but I personally would not try changing the angle of the drive shaft

[volzalum 280]

1 hour ago, Kevin Baugh said:

 recessing the metal plate seems like a feasible idea but I personally would not try changing the angle of the drive shaft

I’m actually not changing the angle. I will realign the motor to keep the same angle. The tapered shim just fills in the gap between the strut and the hull to make sure the actual angle between the engine/v-drive and prop shaft stay the same. So essentially I will be lowering the aft part of the engine about ¼ inch, raising the front of engine and v-drive about ¼ inch, and then filling in the space created where the strut mates to the hull. So I’m rotating the triangle between them without changing any angles. The shaft log (Through hull) has plenty of clearance and the strut bearings will still be perfectly aligned as will the shaft at the v-drive. 

[volzalum 280]

And I do get that you are talking the angle of the prop shaft to the hull, which is currently 16 degrees, and with my shift will be 17 degrees which may raise the nose slightly under thrust. I can say that the larger prop gives more rise on the rear (I had to add 250 pounds to the nose when switched to the 14 inch prop just due to prop change) so I’m hoping that with a 15.5 or 16 inch prop the rear lift will counteract the change in thrust angle of 1 degree. 

[volzalum 280]

I ordered the Prop Shaft Strut Shim today. After recalculating everything, the original angle between the hull (where the strut mounts) and the prop shaft is 15 degrees. I am going to change that angle 1.5 degrees and take it to 16.5 degrees. By calculation, the thrust efficiency at 15 degrees has 3.4% loss, and at 16.5 degrees will have 5% loss, so still relatively minor.

For my case, the stainless steel plate will be 3.5 inches wide by 7.5 inches long (fill the bottom of the recess on the boat) and will taper from 0.36 inches at the front to 0.58 inches at the back along the 7.5 inch length. By calculation, it will gain me almost exactly 1 inch of clearance (with the recessed plate) so I am still hoping to adequately clear a 16 inch prop. The trailer has sufficient space to clear up to a 17 inch prop without striking the trailer brace.

[Troy 234]

Would changing the angle of the engine have any issue with oil in the engine? If you drop the Front of the engine (rear of the boat) and raise the rear of the engine to accommodate the different strut angle will that along with having the rear of the boat down so much surfing have a negative effect on the way the oil is in the pan? 

Just thinking out loud. 

[volzalum 280]

I am changing the angle way less than my ballast changes the angle every time I surf, so my guess is there will be zero impact on how the engine performs. The only detriment will be the increased loss in prop efficiency in the water.

[Jlagos 23]

sounds like quite an endeavor. Have you tried changing the propeller to one with a different pitch to try and offset the loss in size of propeller?

[volzalum 280]

I have. The boat ran great with the 15 inch prop but has been a dog since Centurion put the plate on which limited the clearance. The 14 inch prop regardless of pitch just does not have the same bite, so its puts the engine (XR550) outside of its torque band. I tried multiple props with Acme, and we could never get it closer than current one.

The actual shim is just 8 bots for the strut and then the four engine mounts to align. It actually doesn't look like it will be overly difficult. The hardest part will be reaching the nuts under the engine for the strut bolts. The routing to recess the plate will be slow just to make sure I do not get outside of my footprint for the plate. Since the engine alignment has not been checked since the 25 hour inspection, it probably needs an alignment anyway.

Edited May 28, 2020 by volzalum

[volzalum 280]

Here is the wedge shim I had made for my boat. It’s out of 304 Stainless Steel.

[Jlagos 23]

Interesting, good luck with the routing. 

[volzalum 280]

Luckily for the routing, the steel prop wash plate does not have the same curvature/recess as the fiberglass, so I will not have to remove much (just a little along the edges), and then put an epoxy barrier over the fiberglass and then the steel prop wash plate back where the edges are all flush with the fiberglass. Of course the shape of the recess in the prop wash plate is why the prop size was reduced so much when Centurion put it on. The plate in the picture above is the one for the prop strut to keep the bearings aligned to the through hull prop log.

[boardjnky4 8]

I'm really interested in how this turns out. Not a Centurion owner anymore, but sincerely interested for my current boat. I believe I can already squeeze a 15.5" on my boat, so I really only need to gain .25-.5" to get a 16" on.

Where did you order the plate? Somewhere local?

Once concern I have is within the shaft log. How much clearance is required around the shaft for driveline flex? Is there any flex?

Edited June 5, 2020 by boardjnky4

[volzalum 280]

I had the plate custom made at a machine shop. I gave them the dimensions and thicknesses I wanted and they machined it to the taper.

For mine, I will verify the shaft clearances through the log after getting the prop shaft aligned through the strut bearings. It looks like the only potential change (worst case) is I may have to remove ⅛ of fiberglass on the bottom of the hole at the exit of the shaft log. The log itself is very large and has plenty of clearance so its just to make sure I can at a minimum slide a zip tie around the shaft where it exits the hull/

[boardjnky4 8]

On 6/5/2020 at 0:05 PM, volzalum said:

I had the plate custom made at a machine shop. I gave them the dimensions and thicknesses I wanted and they machined it to the taper.

For mine, I will verify the shaft clearances through the log after getting the prop shaft aligned through the strut bearings. It looks like the only potential change (worst case) is I may have to remove ⅛ of fiberglass on the bottom of the hole at the exit of the shaft log. The log itself is very large and has plenty of clearance so its just to make sure I can at a minimum slide a zip tie around the shaft where it exits the hull/

Get a chance to install it yet?

[volzalum 280]

No. Its a project for tomorrow into Saturday (It has rained every evening this week and last weekend). I have everything apart, but still need to remove the exhaust line so I can reach the bolts under the engine.

[volzalum 280]

I’ve still got to button up everything and tighten the engine mounts, and get the strut bolts 4200’d and tightened, but here are a few pics of the change in clearance. On the supercharged engine, it is a pain to get to the six bolts. I’ll post some more pics later this week of the alignment and prop log clearances.

After talking to ACME, they are recommending a 15.5x15 prop even if I have clearance for a 16” one, so I am going to hold off on countersinking the steel plate to see how the 15.5 performs. I should have slightly more clearance for the 15.5 than I currently have on the 14  

 

[boardjnky4 8]

Nicely done!

[Hokiebob 0]

so what clearance did you have w/the 15" and what do you expect w/the mods and 15.5"?

[volzalum 280]

with the 14 inch prop (after the factory put the steel plate on, I had just over ½ inch clearance to the plate. Without routing the steel plate to be flush mounted, I expect right at ⅝ inch clearance on the 15.5 inch prop. I ordered the Acme 2847 today (based on Acme's recommendation). It is a 15.5x15x0.075 Cup prop. If I have any cavitation or slipping, I will countersink the plate this winter (which would get me to just over ¾ inch clearance.)