Prop Shaft

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Originally Posted by Propless

Sense you brought it up, and sense this thread seems to back on track. Whats the thinking behind the 7/8" shaft ? Is it that its lighter (easier to spin up) or that its smaller and has less drag ? I only ask because the hardware (hardware from a SS boat) that came with my project included a 7/8" shaft. I wasn't planning to use it, just curious about how a race boat can get away with such a small shaft, and what advantage it would have. I cant imaging them running a smaller weaker shaft if there wasn't some kind of advantage to it.

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simply put a 7/8 shaft will pull 4-5 mph on a SS than a 1"

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Originally Posted by white tortilla

simply put a 7/8 shaft will pull 4-5 mph on a SS than a 1"

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WOW that's a big difference ! Sometimes its hard to remember just how much every little thing can add up to.

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Originally Posted by Propless

This probably the stuff from that boat. Story is these parts came from the SS 44 boat that flipped at Long Beach in 07.

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ill check into that lol

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Originally Posted by white tortilla

simply put a 7/8 shaft will pull 4-5 mph on a SS than a 1"

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so does a 16' boat

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Originally Posted by ogshotgun

so does a 16' boat

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Maybe, but keeping it set and hammered becomes more of a challenge.

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Originally Posted by ogshotgun

so does a 16' boat

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I've seen pictures of a 16' Biesemeyer flat mold. I'd like to play with one of those using a hopped up sbc.

Something to consider when dealing with centerless ground stock is you can't determine the roundness with a set of mics. The process of centerless grinding can produce a shaft out of round way beyond the size tolerance and it will mic good. Every subsequent machining process will be affected by that . If the shaft mics in size and wont fit your coupler this would be an indication. You either have to deal with people and material you really trust or invest in tools to verify it yourself.

Separately from the above , centerless grinding is a primarily a high production process , setup and the guides for doing a step grinding from 1" to 7/8 would be a relatively pricey thing and finding a shop to do one-off stuff on a c-less would be difficult.

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Originally Posted by sangerdan

Separately from the above , centerless grinding is a primarily a high production process , setup and the guides for doing a step grinding from 1" to 7/8 would be a relatively pricey thing and finding a shop to do one-off stuff on a c-less would be difficult.

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The 2 shafts I have seen that have reduced diameter, both ends were still 1", and only the area was reduced, with a very generous fillet. So what goes on inbetween is of little concern provided it is reasonably centered between the 2 original areas. I also wouldn't want to bet they were ground, that was an assumption I my part. They could possibly have been reduced on a lath and "polished" Not sure the accuracy of reduction is all the critical. Even if it had to be done by flipping the shaft in the lath due to length, I don't see what it would matter.

The diameter accuracy of the prop shafts is evidently a lot less critical than I would have thought just a few years ago. Because all the shafts I have ever seen were nothing more than mill roll. I have a 17-4 shaft that actually has a very slight depression groove the length of the shaft almost like you would see in welded tubing, and was in service as a multi championship ski boat in the 60s. The groove is kind of handy because you see any twist that might occur in the shaft just by looking at it. Its still pretty much dead straight.

The poster mentions having to enlarge his coupler to 1" so I guess I assumed only the prop end was at 1" ,hence what I said about the expense. Not looking to endure the wrath of anyone but I doubt any prop shaft is just mill finish without being either turned or preferably ground. Part of the finish machining process is to get rid of the surface imperfections and such from the mill.