Discussion in 'Ocean Yacht' started by Trinimax, Jul 30, 2018.
Fatigue related to their own duty cycle.
Taper fit, keyway fit, material selection, shaft diameter, corrosion, bumping into soft bottom or hard objects, lobster/crab pots, ropes, etc.all can be contributors.
You can work backwards from engine hours now to estimate the shaft life for your application, but you will be missing information like how many times the props/shafts bumped into something.
Shaft speed is another major contributor to lifespan. A lot of the older boats had a 3:1 or 4:1 reduction versus 2:1, so the shafts were spinning a lot slower...….then again I've seen boats with the same shafts go decades and decades with no issues......one big MY I know of, the owner used to damage a prop what seemed like every other time he ran the boat.....and still has original shafts 30 years later...…...LOLOLOL...….
My last boat, 1962 42' Matthews Conv. Sedan had the original bronze shafts. Powered by G.M. Alpine green 6V53s and not a problem with them. Now I have an '81 Ocean with original shafts and no problems. The modern sport fish has so much power that I also think aggressive use of the throttle whether it buy docking, "blasting off" to plane, or during fishing in the heat of the moment, over time contributed to the shaft failure.
All boats need time to react steadily, not in a split second by means of aggressive throttle .
Agreed with the posts, there are so many factors and variables that its hard to pin point one possible source of failure, heck our previous boat was a 1989 38 ocean with 375 hp 3208 cats, that had 1 1/2" shafts with a 1.5:1 gear ratio. We owned and ran that boat from 2007- 2015 with no shaft issues with the original shafts. Now with this 2001 boat we have this issue, with supposedly more robust 2" shafts with a 2:1 ratio.
One thing is for certain, I will be having the shafts crack tested every haul out, which I aim to be every 2-3 years.
I am kicking myself a bit, I am a class surveyor and the classification rules state that crack testing should be performed on both ends of the propeller shaft at a minimum of every 5 years. So it looks like I should make sure to apply some of the class rules to my own vessel.
thanks again for all the help and I will let you know how it all goes.
While that might be the rule on class vessels, in reality I have never seen anyone have their shafts tested, nor have I even seen yards try to upsell it on yachts under 100', unless there has been some propeller damage.
Well I finally got my new shaft and prop 2 weeks ago and I hauled out last week Monday to make the swap. I pulled the port prop and did a dye check on the key way and all looked well, so fingers crossed. Overall the new shaft, coupling and reconditioned prop came to about $4000. with another $500. for the shipping to Trinidad. The prop shop did a great job of matching the new prop with the old one, as my speed went up a little bit and my rpm was in the same range as before. I would like to thank everyone here for their advice. One this is for sure that its good to have the boat back running after 6 weeks of down time
Some pics from the job
Good news to get her back and running with a performance gain.
Hard to tell from the picture angles , the design target for the distance between the Forward Face of the Prop Hub and the Aft Face of the Strut Barrel is usually kept to 1.0 x Shaft Diameter or less. I typically try to aim for 0.5 x Shaft Diameter. Just something to keep in mind for the future.
You are correct on that, the new shaft was shorter than the old one, which we thought was an issue at first, but it appears that it is the other way around. we measure and it is just about 1 " from the hub to the aft face of the strut, which is just about right for the 2" diameter shaft. next haul out, I might look at pulling out and shortening the port shaft to reduce the "whip"
Wow Trinimax, I missed your post when this happened. Glad you've gotten it sorted. Let's hope this is one problem I don't have!