Propellor and shaft size mismatch

[GILow]

Hi all,

Long story short, I’d like to install my existing Kiwiprop, made for a 1.5 inch propellor shaft on to a 1.25 inch propellor shaft.

The 1.25 inch shaft will be new, made from scratch.

Looking at ways of making it all work.

Kiwiprop themselves have recommended a tapered bore reducer bushing from Tacoma Propellor, but I’m thinking that there might be a tidier way, given I am having a new prop shaft machined from scratch.

One idea I’ve considered is to weld a short sleeve on the shaft and extend the tapered section that way.

Anything else anyone can recommend?

(And for the record, I am looking to see if the boat can accomodate a 1.5 inch shaft since I am replacing the cutless and shaft seal anyway, but I think it will get tricky to go up a size.)

Matt

[deblen]

Taper adapters are available from several sources.


https://cassellmarine.com.au/product...taper-adapters

[tkeithlu]

It is not likely a problem, but remember that shafts are sized to absorb the twisting torque of a particular engine/propeller combination. When you put a bigger propeller on a shaft, you are moving toward applying too much torque to it.

[fstbttms]

Sell your Kiwiprop and buy one with the correct bore.

[ItDepends]

Of course there is a LOT more to propeller sizing than just shaft size…

How is it possible that this 1.5 bore Kiwi prop you have kicking around just HAPPENS to be a perfect match to the new boat/engine? Really?

But… if that is true… the taper adapters mentioned by deblen above are exactly made for this issue. They work great. The reasons posted about why you should not use them are wrong, and just made up.

But… boat drivetrains assembled from random parts without careful design are rarely successful, and if they are it is by blind luck…

[David Mathis]

Isn't this a little like getting head surgery to make your hat fit? Just get the right prop. Hang your favorite Kiwi prop on a wall then reminisce about the great memories.

[tkeithlu]

QUOTE: "The reasons posted about why you should not use them are wrong, and just made up."

ItDepends, I'm the only poster on this thread who suggested any caution in adapting a propeller to a different sized propeller, I did not say that adaptors should not be used, and what I said was not just made up.

[Lepke]

Since you're making a shaft, why not use a 1.5"? I've seen shafts with uncommon machining or welding break.

[Cheechako]

I used a bushing on one boat, but it wasn't needing a full 1/4" reduction, i think it was 1/8" overall. never was a problem, but seemed a bit jury rigged.

Welding will cause faster corrosion (unless heat treated after) and may distort the shaft.

[GILow]

Originally Posted by Cheechako

I used a bushing on one boat, but it wasn't needing a full 1/4" reduction, i think it was 1/8" overall. never was a problem, but seemed a bit jury rigged.

Welding will cause faster corrosion (unless heat treated after) and may distort the shaft.

Thanks for that. I hear you on the jury rigged bit, that's how it feels to me too.

Good point about the welding and corrosion. Maybe a mechanical connection instead...

[Montanan]

Carefully reference to see if your shaft and keyway will be compatible.

SAE & ISO options

Snipet:

All SAE tapers reduce in diameter by 1/16" for every inch of taper - ISO by 1 mm every 10 mm.
All North American manufactured and most Australian shafts will follow the SAE standard exactly.
In NZ & Australia taper length may vary and must be specified with 0.625" or 0.750" BSW nut optional. You will need to confirm these dimensions on imported vessels or those which may have had the shaft replaced and which may not be standard. Do not assume the replacement matched the original.
NB: BSW is the same as UNC in all the above sizes
The square keyway of 0.313" for an 1 1/4 shaft corresponds to 5/16"
The above metric tapers and nuts cover all Beneteaur Bavaria and Jeanneau specifications. Metric threads are specified as Diameter x Pitch - both in mm.
NB: A new key Is only normally supplied with SAE 1.250" shafts The 8x7 mm key above Is mounted 7 mm deep and 8 mm wide
Blank bored bosses are available for local machining to fit non-standard tapers and nuts .... In these situations we forward a blank bored boss for half deposit with machining drawings for the user to arrange local machining with someone who can accurately measure the shaft taper, thread and keyway and carry out the necessary machining and fitting. The boss Is then returned to us for final assembly payment of the balance and despatch. We are not a jobbing shop and do not machine one-off bosses. In a remote situation unless you have access to accurate engineering quality measurements It Is virtually Impossible to produce an acceptable product.
While the above ISO tapers cater for modern production vessels - there are many older metric shafts with a very wide range of tapers that differ In terms of taper ratio, length, keyway and thread size.
Unfortunately there Is no such thing as a " metric standard " taper.
Thus It Is very Important to be sure of all the dimensions for a metric shaft outside the major manufacturers who adhere to the above.

[Cheechako]

Originally Posted by GILow

Thanks for that. I hear you on the jury rigged bit, that's how it feels to me too.

Good point about the welding and corrosion. Maybe a mechanical connection instead...

There's going to be a point of diminishing returns trying to do this I think. Is the hub for the 1.5" Kiwi prop much bigger than the 1.25" is a consideration. I assume the prop diameter will work with your boat.

If you are making a new shaft anyway, I'm thinking it's time to go to 1.5" shaft if the conversion isnt too difficult. But I wonder if your stern tube would work with 1.5"? Probably not. So that isnt easy either unless the boat was intended for 1.5" to start and a thick wall cutlass in in there...

[GILow]

Originally Posted by Cheechako

There's going to be a point of diminishing returns trying to do this I think. Is the hub for the 1.5" Kiwi prop much bigger than the 1.25" is a consideration. I assume the prop diameter will work with your boat.

If you are making a new shaft anyway, I'm thinking it's time to go to 1.5" shaft if the conversion isnt too difficult. But I wonder if your stern tube would work with 1.5"? Probably not. So that isnt easy either unless the boat was intended for 1.5" to start and a thick wall cutlass in in there...

I'm trying to get a handle on how hard increasing to 1.5 inch will be.

I'm pretty sure the kiwi hub sizes are practically the same, certainly not enough to be an issue in this case.

[Cheechako]

Originally Posted by GILow

I'm trying to get a handle on how hard increasing to 1.5 inch will be.

I'm pretty sure the kiwi hub sizes are practically the same, certainly not enough to be an issue in this case.

If you really want to bad enough...
-The shaft looks to be not too long, can a machinist turn down a 1.5" shaft to 1.25 except the aft end (which fits the prop) and make a new keyway?? Might be doable with the right machinist but how much $?
or
- There is a Flame Spray process which sprays liquid metal onto metal for repairs etc. Spray the tapered portion of the 1.25 shaft to be larger so it could be machined to match a 1.5"
It ain't usually cheap...

https://www.youtube.com/watch?v=14eAnEXI4WA

[GILow]

Originally Posted by Cheechako

If you really want to bad enough...

-The shaft looks to be not too long, can a machinist turn down a 1.5" shaft to 1.25 except the aft end (which fits the prop) and make a new keyway?? Might be doable with the right machinist but how much $?

or

- There is a Flame Spray process which sprays liquid metal onto metal for repairs etc. Spray the tapered portion of the 1.25 shaft to be larger so it could be machined to match a 1.5"

It ain't usually cheap...

I ran the idea of cutting down an oversized shaft past my guy. He frowned a lot, but he does that with most things.

There's around AU$5000 plus a crap load of work plus expensive time on the slipway riding on this one. So if turning down the shaft incurs a bit of labour I won't mind. I said I'd fish around for some more ideas and then discuss it all further next week.

I'll look into that spraying thing.