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  #1  
Old 06-08-2014
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Prop Dilemma

I have an Islander Freeport 38, also known as an Islander 38C. The original engine was a 42hp Pathfinder 50. Not sure what transmission it had or the gear ratio. The prop was a Sailor 2 blade 17 RH 8. I repowered it with Yanmar 4JH2-TE rated at 63hp at 3600rpm with a KBW 20 transmission.

The people who installed it also changed the prop to a three blade 17X14. The prop size (they said) required a 1.25" bore size while the shaft is a 1" which I had to keep because of the size of the shaft passage through the keel. Since the shaft had a smaller keyway then the prop they used a bore reducer and a stepped key to fit both the prop and shaft. After about 200 hours the key broke allowing the prop to spin on the shaft. I had it hauled and the mechanic fitted another key to it. This broke after 40 hours. My question is in two parts.

1. Does anyone know of a prop that is compatible with my engine and transmission that has a 1" bore that will fit my shaft, either 2 or 3 bladed?

2. Can my original 2 bladed prop be adapted to this engine and transmission by changing the pitch. I realize that it will be under proped but right now at 3200 rpm (80% of rated capacity) I max out my hull speed anyway. It seems that I can foresake high end speed and still be ok, but not sure how that will affect the wear on the engine.

Since my prop comes out of the keel there is very little restriction on diameter of the prop. Any response will be greatly appreciated.
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Old 06-08-2014
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Re: Prop Dilemma

A broken KEY is a symptom that the prop (if the shaft connection is TAPERED) was not PROPERLY honed to fit the shaft ... ... OR if a straight shaft connection - that the prop was installed with insufficient 'interference fit'

An 'interference fit' (press fit) means that the 'bore' of the prop should be 'smaller' than the OD of the shaft - by about 0.001" to 0,002" per inch of shaft diameter. This is standard industrial practice for all shafting involving couplings, gears, and props, etc.
What 'holds' a prop (gear, coupling, etc.) to a shaft is FRICTION between the mating surfaces. In 'good' / standard mechanical design practice a KEY is NEVER used to transmit rotation loads ... the key/keyway is there ONLY as a 'backup'.

If this is a straight shaft connection and your prop 'easily' can be slid off without using a 'gear puller', then there isnt sufficient 'interference fit' or 'press fit'. The usual remedy here is 'new shaft' machined to the proper dimensions; or alternatively, the bore of the prop should be 'welded' and a new proper sized bore machined into the prop. I would NOT agree to have the prop 'over-bored' and a cylindrical 'insert' be 'pressed' into the bore as a remedy, not without WRITTEN GUARANTEE that such a remedy would be effective to X amount of YEARS of service.

If this is a tapered shaft connection, then the surfaces should be 'honed' so that nearly 100% of the mating surfaces are in contact (no gaps of the surfaces) AND the shaft nut should be properly torqued to 'drive' the mating surfaces together to provide the proper FRICTION.
The remedy for a tapered shaft connection is to HONE the surfaces: apply 'bluing' to the mating surfaces, apply 'grinding compound' to the mating surfaces, and while the shaft is held firm the prop is rotated ... until ALL the bluing (and a wee bit of metal) is 'ground' off both mating surfaces.

Summary:
From your description:
If tapered fit, then HONE the mating surfaces
If 'straight fit', then new shaft or new prop.
The max. HP for a 1"ō shaft is about 30-35HP.
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Last edited by RichH; 06-08-2014 at 12:40 PM.
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Old 06-08-2014
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Re: Prop Dilemma

Agree with Rich (intended to write someting along those lines, but Rich beat me to it). Those that mounted your new prop made a really bad job, without basic mechanical understanding.

62 HP usually requires a shaft dia > 1", that is why it is somewhat difficult to find props for this engine with a 1" shaft fitting.
If you do want to find a new prop with these dimensions, then I propose you contact directly the major prop manufacturers (as Folding Propellers - Flexofold , Gori propeller, Kiwi Prop Home of the Kiwi feathering prop to name some few. I have actually contacted all these and got good and fast responses).

Do not use the old prop dimensioned for the 42 HP engine you had. It would be a waste.

/J
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Re: Prop Dilemma

I agree with what you are saying. The shaft is a tapered shaft and and they used a bore reducer to make the prop fit the shaft. It is a tapered nylon sleeve that fits over the shaft and makes the prop fit the shaft, however there is no way to lap it with this type of fitting. That is why I would like to go back to a prop with a 1" bore. The problem is that I was told that they could not get a prop with a 1" bore that had the diameter and pitch needed for this transmission and engine specifications. I'm not sure if that is true and I will be consulting with some prop experts next week. I was also told that this bore reducer and stepped key is quite common and rarely causes problems. Not sure if I believe that now.
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Re: Prop Dilemma

IMO - a (filled) Nylon sleeve will never be able to transmit that HP via a 'friction fit'. If a bore reducer MUST be used, then SS or bronze ... but it MUST be 'honed' for the 'fit' or it too will be vulnerable to 'slip'.

Simple and obvious solution - rebore/reglass/retube the keel, etc. and use the correct and applicable shafting.

Last edited by RichH; 06-08-2014 at 01:04 PM.
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Re: Prop Dilemma

I have never seen a plastic bore adapter land or sea that went above toy size that worked

Your original prop is the very best short term solution and as lon as the tack is watched will not cause harm



This is a 1" flex-o-fold hub with 12 x 7 blades (16 HP) so 63 would be be a freaking big jump

The blades are able to be swapped in minutes it just a question of what they think
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Last edited by tommays; 06-08-2014 at 01:29 PM.
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Re: Prop Dilemma

RichH, the shaft comes through the trailing edge of the keel and only extends out about 6", just enough to get the prop on and a donut style zink on it. I'm not sure how that is reinforced within the keel, whether it is just fiberglass or if there is some metal incasing the shaft passage and the cutlass bearing. I will take a look at it when I haul the boat in a couple of weeks. That is certainly an option although I'm not an accomplished enough fiberglass tech to attempt it on my own so I will have to factor the cost in of having someone do it for me. A new shaft is cheaper than a new prop so that will be a factor also.
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Re: Prop Dilemma

Your boat is a renown Bob Perry design.
Id suggest you contact Bob at Robert H Perry Yachts Designers Inc. Home Page and ask him the correct/best way to affect the shaft tube modification (for consulting fee of course).

Id be more concerned about finding the appropriate 'stern tube' (cutless) housing for an increased shaft diameter. Bob would definitely know if a 1-1/4 stern tube/cutless housing from a Tayana-37 or similar design, etc. would have the direct fit up dimensions to your Islander. With your present stern tube / cutless housing change-out to be able to use a 1.25" shaft ... Im thinking that this total 'tube' conversion can begin to become quite expensive.

If this is all too much trouble/expense, Id recommend to begin to seek out the current new prop manufacture's application engineering department to see if they wont sell you an exchange prop with either a custom made 1" tapered bore; ..... or at least, a NON-bored prop hub that can be taken to a 'precision' machine shop to have the proper correct dimensioned tapered bore made (and 'honed' against the current shaft to ensure concentricity and 'in round' of the assembly of the two.) That would eliminate the usage of a complicated weak and vulnerable 'adapter'. If the current new prop manufacturer would agree to a special order, Id also suggest to send them your shaft and let them do the 'concentricity' and honing assembly work of the prop TO the shaft, etc. This would be contingent upon the old shaft being in 'prime' physical shape; otherwise - "Hello Aquamet ........ " Marine Machining & Manufacturing - Home

All it usually takes is lots of boat-money !!!!!!!

Last edited by RichH; 06-08-2014 at 04:05 PM.
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Re: Prop Dilemma












I designed a set of tools that allows me to address it in various ways





The mixers i repair are to big to go in machine tools so you have to bring the tool to them
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Re: Prop Dilemma

I think a 17X10 pitch would be a good size for you. Shops can repitch to 9'' and maybe a bit more to 10". So anything over 9" should come with an improvement in key life, and get you to hull speed. Total cost $70!
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