Too long a shaft

[Cadence]

Originally Posted by jkishel

Power is equal to torque times rotational speed. The shaft must be strong enough (diameter and material) to deliver the torque required to turn the propeller at the desired speed. Because the propeller is pitched, it converts the applied torque to propeller thrust, moving the boat forward or aft. So you could say that the torque is “absorbed” by the water behind or forward of the propeller.

If your happy with that.

[aqfishing]

Originally Posted by jkishel

No confusion here - the OP asked how to determine that the shaft was adequate to drive the propeller. The answer is to first calculate how much power is needed to turn the propeller, given its geometry (diameter and pitch) at a specified rotational speed. The shaft needs to be sized to deliver this power. Of course the engine must be capable of delivering this power at the specified rotational speed. Hull speed has nothing to do with it, this power is required to turn the propeller at the specified rotational speed even with the boat tied to the dock.

You cannot determine shaft diameter from prop diameter. You must take engine kw into account.
In all the vessel builds (4) I have been involved in the equation starts with what performance etc you want, then you spec the engine and gearbox, from that you spec the shaft and propellor using the engine specs. Not ever have I ever heard of someone taking a prop working back from there.

One can therefore assume the OP’s engine and prop are matched properly, although I suppose one should check.

When we have had to asses slightly worn or pitted shafts the surveyor would look at the amount of “meat” left in the shaft. We would then have a naval architect calculate the min shaft thickness required (the marine surveyor would also do his calc) along with a safety margin. Not once did the either one ask me for my prop size, but they always asked for engine and gearbox specs.

After all, the engine determines how much torque will be loaded on the shaft.

[Jim Cate]

After all, the engine determines how much torque will be loaded on the shaft.

IMO, the engine only determines how much torque COULD be loaded on the shaft (that is, how much the engine can ultimately supply), but the prop and the mass and drag of the hull actually determine how much is applied to the shaft. Consider if the "prop" was a flat disk... in such a case it would spin freely (more or less) and little torque would be applied to the shaft, even though the engine could provide much more.

It is a complex relationship between engine, hull and prop, enough so that the prop wallahs don't always get it right the first time, and why having a prop with variable pitch is kinda useful.

JIm

[Kastaway]

If the shaft were to be designed to withstand the propeller coming to an immediate stop whilst the engine revolutions coincided with the maximum delivered engine torque, then maximum engine torque developed and gearbox ratio would need to be considered.
If it was thought that this was an unlikely consequence and that the prop would more likely be stalled by having water thrown at it, then the expected torque loads that the shaft would likely experience would diminish somewhat. Consider the prop, most of the loads would go away without it. (…….oh no - I think the next post will be……. “consider the engine Kastaway, most of the loads will go away without it”.) I think if you compared the required torque to shear the bolts on the output flange of your gearbox to that of the shaft you won’t be concerned about the shaft being able to transmit all the torque that your propeller calls for.

[Searles]

Max over hang 1.5 dia of shaft min blade tip clearance 10 pc prop dia max shaft speed .1200 rpm in a displacement hull ,simple rules change them at your peril .⛵️⚓️

[Searles]

If the shaft speed is too high ,cavitation of the prop may become a problem with increased vibration ,plus the pitting of the blades ,this mainly applies to displacement hulls ,adjust the gear ratio to max the shaft speed and the life of the skeg bearing ,it all works together

[Nicholson58]

The long space between the prop and shaft increases the moment arm so that lateral forces are turned into greater bending moments on the overhung shaft. Lateral loads come from: chance struck objects, uneven fouling build up, interference of the water flow by the hull, mechanical unbalance of the prop parts, misalignment throughout the assembly. In addition, all rotating shafts can go wonky near their critical speed. The less stiff the shaft, the lower the critical speed(s).

You are correct to worry and to take action. Your long overhang will permit cyclic side loading (bending) that will cause vibration and possible fatigue damage to the shaft.

Let’s just assume the engine, transmission, prop and shaft diameter were reasonably engineered to begin with. Your only problem is with the geometry change from the strut modification.

The long overhang also increases greatly the bending loads for a struck object. The impact to one blade colliding with debris or being forced to stop instantly has a much greater chance of bending the shaft and strut.

Before you shorten, will you want to leave space for a shaft cutter?

You might move the zinc ahead of the cutlass in your plan.

Are you happy with the coupling? Want to add an Aquadrive CV joint?

Shorten to minimize the distance from the cutlass to the prop but leave enough space for water flow through the cutlass. BTW I dive with a SS bicycle spoke for cleaning out the cutlass grooves before leaving the harbor.

Ours pictured is 24” prop with 50mm shaft, variable pitch. The gap is about 1/4 inch. No room for a cutter.

[kaisardog]

OP, you need a copy of dave gerr's propeller handbook. and your gearbox specs. and michigan wheel's propeller calculator.

[ChrisPuget]

Whatever you decide, you may want to run it by Tacoma Propeller. They have been very helpful at times. There must be a good prop shop in PA also.

https://www.tacomapropeller.com/shop/

[merrydolphin]

Folks, he’s not asking about shaft dia or prop dia or pitch or anything but how far from the strut should the prop be? And I think that several people have answered that both directly and in reference.

[Icarus]

If you are worried cut the shaft and leave enough room for the anode.

[jkishel]

Originally Posted by merrydolphin

Folks, he’s not asking about shaft dia or prop dia or pitch or anything but how far from the strut should the prop be? And I think that several people have answered that both directly and in reference.

The PO also asked “how can I verify the shaft is adequate for the prop?”

[col50]

Originally Posted by jkishel

The PO also asked “how can I verify the shaft is adequate for the prop?”

Exactly!