Prop Shaft

[Propless]

Bingo !!! There are a lot of people that have really good luck with Propshafts 7/8.....1"...... 1 1/8 !! Super Stock, K-Boat & GN Boats. There are always going to be failures but they can be minimized with care & fitement !! I only bring up 7/8 because that's a challenge in its self.

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.

[ogshotgun]

a pm someone just sent me lmao..." Why don't you just go prop less on these V-DRIVE boats would be much easier"

[ogshotgun]

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.

My opinion on the 7/8" shafts were on a factory boat with a few hundred horsepower, and it costs too much to change everything to a larger shaft so its what they have and they try to make it work, thats my opinion aim sure GN7 will tell the rhyme or reason...BOB

[Propless]

My opinion on the 7/8" shafts were on a factory boat with a few hundred horsepower, and it costs too much to change everything to a larger shaft so its what they have and they try to make it work, thats my opinion aim sure GN7 will tell the rhyme or reason...BOB

Maybe he'll teach us both after his nap. This was no factory low hp set up, it was from a real SS boat. It uses an 8 deg Casale with a 1 1/8" output shaft, an adaptive 6 bolt coupler (1 1/8" to 7/8"), drop through strut with a 7/8" bushing, and a 1" prop. Looks like it was a 1" shaft that was turned down to 7/8" right in front of the prop.

[Propless]

Less rotating weight, like anything in racing equals free hp.

Didn't see your reply when I posted. That makes sense, but it sure seems like a risky place to try and save weight. Especially when they had a huge 1 3/8" shaft between the V-drive and the engine.

[Propless]

Another thing the smaller the diameter the more torsional twist allowed before failure. Only problem here is, too large of diameter swing equal more chance of a shaft to become a pretzel with any amount of side load or thrust.

So your saying with two shafts of equal material, the smaller shaft with allow more torsional twist before failure ? That's interesting, I wouldn't have thought of that.

[gn7]

Some what rotational mass, but they could always skirt much of that with a rifled drilled 1 or 1 1/8 shaft and actually be stronger.
Its the drag/lift of the larger shaft, including the larger strut. Not all of the teams use them, but bet your ass the top ones are. Its not a matter of some low "stock" HP thing. Just the expense and hassle of finding and /or having made 7/8 RH props makes it an overly expansive proposition.

Then you look at the size of the collection of props in Rankins trailer and the reality of it slams home. Knowing that each one of them has an extremely short life span makes it even more daunting.

I don't know how much shorter life the shaft is over a 1" shaft. They see less than 700 HP. How much worse is that than some heavy ass GNs kicking out 1000+ in rough water. And there a couple running 1" shafts with reasonable success.

But the cost of relatively short lived 7/8th props just scrub some lift and drag seems a little over the top. But I have talked to guys that have switched from 1" to 1 1/8 shafts in a heavy v bottom, and they swear they can tell the difference. So in a little flattie, who knows the real effect.

[Propless]

Yes, if everything stays linear.
ANY side load will ruin that.

Gun drill that shaft and now it's stronger torsionally and lighter, freeing up more hp.

That's interesting, and you made me go take a better look at my parts. I hadn't noticed it before (never actually looked) but the motor to V-drive shaft (1 3/8") is hollowed out. Its got a 7/8" hole down the center. I guess they really wanted to get every last 10th of a hp they could out of this thing.

[gn7]

Yes, if everything stays linear.
ANY side load will ruin that.

And it is amazing, but the so called "whip" of a twisted shaft is pretty much mythical IMO.
Look at a tiny ass drill bit in a drill press. You can press to the point the bit bows, but it doesn't whip. The bow stays pretty much put. AS Westernaero says, so long as the shaft doesn't experience any side loading, the smaller shaft will absorb more angular twist under the same load without failure, but for a lower number of cycles.

[Propless]

Some what rotational mass, but they could always skirt much of that with a rifled drilled 1 or 1 1/8 shaft and actually be stronger.
Its the drag/lift of the larger shaft, including the larger strut. Not all of the teams use them, but bet your ass the top ones are. Its not a matter of some low "stock" HP thing. Just the expense and hassle of finding and /or having made 7/8 RH props makes it an overly expansive proposition.

Then you look at the size of the collection of props in Rankins trailer and the reality of it slams home. Knowing that each one of them has an extremely short life span makes it even more daunting.

I don't know how much shorter life the shaft is over a 1" shaft. They see less than 700 HP. How much worse is that than some heavy ass GNs kicking out 1000+ in rough water. And there a couple running 1" shafts with reasonable success.

But the cost of relatively short lived 7/8th props just scrub some lift and drag seems a little over the top. But I have talked to guys that have switched from 1" to 1 1/8 shafts in a heavy v bottom, and they swear they can tell the difference. So in a little flattie, who knows the real effect.

Bob, have you ever seen or heard of a shaft like mine, 7/8" shaft but uses a 1" prop ? Looks like a 1" shaft that's been turned down to 7/8" from the props taper forward.