Prop replacement problem, what would you do/advise?

[Quidi Vidi]

Further to an earlier thread that I posted, we managed to beach and remove our prop the other week using our home made prop puller. I had bought a cheap replacement prop at the Solent boat jumble last year in the hope that it might fit. Our shaft is 1” dia. and the replacement prop is also 1” dia. at its widest end. However once the old prop was off we discovered that the shaft had been turned down to 3/4” at the widest end of the taper leaving a shoulder to the rest of the shaft. This meant of course that our replacement wouldn’t fit so, we cleaned off and painted the old one and the refitted it. Now to put you further in the picture we are over propped and get black muck out of the exhaust at full throttle and only about half revs when in gear. The old prop is 12x8 and using online calcs we need a 10x7, this has also been confirmed by Silette who offered to make us one but at a cost which we can’t afford.
My questions are:
Is it common to turn the shaft down to leave a shoulder, IE. Is it worth looking out for a replacement at Beaulieu?
Can I get my replacement prop sleeved to fit the shaft?
Should I get a new shaft? I really don’t want to do this due to difficulty removing old one and the costs involved
Can I get the old 12x8 altered to nearer the 10x7 I need?

Please bear in mind we don’t want to spend a lot of money and we usually do all the maintenance ourselves but I am not an engineer so I could use any advice that I can get. To that end we are quite prepared to beach and clean the existing prop every month or so to get whatever power we can.

 

[davidej]

I can't answer all your questions but a new shaft is surprisingly cheap by boating standards.

The last one I had (for a significantly larger boat) was not much over £100

 

[Jcorstorphine]

you can get it repitched

Try T Norris as they may be able to reduce your pitch and even take an inch or so off the diameter.

T.Norris Limited
6 Wood Lane
Isleworth
Middlesex
TW7 5ER.

email: sales@tnorrismarine.co.uk

Tel: 020 8560 3453 Fax: 020 8569 7446

 

[Quidi Vidi]

I can't answer all your questions but a new shaft is surprisingly cheap by boating standards.

The last one I had (for a significantly larger boat) was not much over £100

Thanks for your prompt reply and this may prove to be the easiest/best option. I had thought about £100 for a new shaft however the cost of putting her into a yard and the hassle of removing the skeg and getting to the awkwardly accessed shaft union in behind the engine worries me. I had thought to maybe cut an access hatch in the cockpit sole to make life easier when dealing with the gearbox, stern gland and generally the back of the engine but this would of course add to the expense.
As i'm not an engineer and owning a boat is relatively new to me, although i'm a fast learner, I would like to know what all my options are before doing something as drastic as removing the skeg and cutting access holes in the boat.

 

[Quidi Vidi]

Try T Norris as they may be able to reduce your pitch and even take an inch or so off the diameter.

T.Norris Limited
6 Wood Lane
Isleworth
Middlesex
TW7 5ER.

email: sales@tnorrismarine.co.uk

Tel: 020 8560 3453 Fax: 020 8569 7446

Thanks, i would be more than happy if they could repitch it to 5" or 6", even better if the could reduce the dia by an inch and pitch to 7"ish. Anything would be better than what i have, the replacement i'd bought was an 11" by 7" so slightly large but would have made a huge difference. It only cost me £20 so maybe i was expecting too much for it to actually fit as well. Has anyone done anything similar to this suggestion? Any ideas of cost, anywhere nearer to Gosport that i could try?

 

[dunmor]

you can buy bore reducers to enable your new prop to fit see link below.
http://www.deepblueyachtsupply.com/products/nylon-bore-reducers

 

[Quidi Vidi]

you can buy bore reducers to enable your new prop to fit see link below.
http://www.deepblueyachtsupply.com/products/nylon-bore-reducers

Absolutely fantastic, exactly what i need, off to do a google search for UK suppliers now.
It seems too easy and cheap, any drawbacks? Anyone used anything similar?

 

[Tranona]

Don't think you will find anybody selling them here, so you will probably need to order from the US.

It is unusual to reduce a shaft size like that. I expect it was done by a previous owner who bought a cheap prop that did not fit! 3/4" is almost unknown now as most modern engines need 1"/25mm. Smaller sizes were common many years ago when 6 and 8hp engines were more common.

If the current prop is sound you should be able to get reduced pitch and even some diameter removed. There are propeller specialists locally (Southampton) who would look at it - can't remember the names offhand, but Google is your friend.

 

[vyv_cox]

I would view any shaft with a turned taper that left a shoulder with deep suspicion. This is the most common cause of shaft failure in rotating equipment, where the newly created smaller diameter leaves a sharp, un-radiused fillet that is a massive stress raiser. Fatigue and fracture is the inevitable consequence. The cyclic stress in a propeller is considerably greater than in the pumps and compressors I am most familiar with, life expectancy being reduced accordingly.

At the very least I would inspect the radius(?) with a magnifying glass, dye penetrant being far better. Much better to change the shaft.

 

[Norman_E]

Good advice from Vyv Cox as usual. I think your best and safest course is a new shaft, either 1 inch or 25mm to match the existing coupling and cutless bearing sizes. As Vyv says that sudden reduction in diameter, with a shoulder with little or no internal radius is a huge stress raiser. It is probaby not a question of whether the shaft will fail, and the prop fall off, but when. It will probably happen when you have to go quickly from forward to astern in order to avoid something.

As for cost you may find that the new shaft costs little more, and possibly less, than having your existing prop re-pitched, and cut down in diameter, and you can try it with your bargain buy, assuming that has a standard taper. You may still be over propped, but by nowhere near so much.

 

[colhel]

Following on from Brians (OP) PM. Yes I can make you a reducer, I'd have to make you a new key too. Or if you prefer turn machine your prop diameter.
I would take note of Vyvs advice though.
Something a bit baffling here though, with the shaft so difficult to remove, the previous owner must have really wanted that prop. Getting a a good taper machined and keyed isn't a quick job, plus re-assembling.
I'm asking, are you sure she's over propped? Machinists are a lazy bunch of b******* you'll ever wish to meet. Always trying to get out early.

 

[Quidi Vidi]

Thanks for the words of reason guys, sound advice but now puts me in a real quandry. I take it that turning a shaft down isn't normal or standard practice then?

 

[Quidi Vidi]

Following on from Brians (OP) PM. Yes I can make you a reducer, I'd have to make you a new key too. Or if you prefer turn machine your prop diameter.
I would take note of Vyvs advice though.
Something a bit baffling here though, with the shaft so difficult to remove, the previous owner must have really wanted that prop. Getting a a good taper machined and keyed isn't a quick job, plus re-assembling.
I'm asking, are you sure she's over propped? Machinists are a lazy bunch of b******* you'll ever wish to meet. Always trying to get out early.

Thanks Colin, i double checked the calcs and Silette came up with the same size too so your right, it is a mystery, I have no idea how long this prop has been fitted. I only need the slightest furring of weed on the prop to make the exhaust spew out black stuff and for the max revs to reduce drastically. In neutral everything is fine and when the prop is cleaned everything improves dramatically. I'm certain (almost ) that its the prop size thats at fault.

 

[Norman_E]

Thanks for the words of reason guys, sound advice but now puts me in a real quandry. I take it that turning a shaft down isn't normal or standard practice then?

Certainly not normal, and potentially dangerous. Without knowing both the engine and the gear ratio it is not possible to calculate whether a prop size is correct for any given boat, but if your engine really is a 6hp Petter a 12 inch diameter prop looks far too big, and inabilty to achieve full RPM, accompanied by smoke at full throttle appears to confirm it. Remember that the volume of water being pushed backwards, and hence the power needed to do it increases as the square of the propeller diameter.

I hope that you have checked that the engine is OK, with good compression and no other problems, but little Petters are fairly tough beasts and if it starts OK and revs freely in neutral it is probably alright. Poor compression generally makes for difficult starting.

 

[Quidi Vidi]

Certainly not normal, and potentially dangerous. Without knowing both the engine and the gear ratio it is not possible to calculate whether a prop size is correct for any given boat, but if your engine really is a 6hp Petter a 12 inch diameter prop looks far too big, and inabilty to achieve full RPM, accompanied by smoke at full throttle appears to confirm it. Remember that the volume of water being pushed backwards, and hence the power needed to do it increases as the square of the propeller diameter.

I hope that you have checked that the engine is OK, with good compression and no other problems, but little Petters are fairly tough beasts and if it starts OK and revs freely in neutral it is probably alright. Poor compression generally makes for difficult starting.

Engine starts fine unless very cold when a squirt of oil in a special plunger improves things, not sure what this does or how it works.
Engine is 6HP, gearbox 2:1 reduction, lwl 19ft, displacement 4000lbs but probably more with engine, stores and crew etc. perhaps someone could check my calcs for me?

 

[peteK]

Further to an earlier thread that I posted, we managed to beach and remove our prop the other week using our home made prop puller. I had bought a cheap replacement prop at the Solent boat jumble last year in the hope that it might fit. Our shaft is 1” dia. and the replacement prop is also 1” dia. at its widest end. However once the old prop was off we discovered that the shaft had been turned down to 3/4” at the widest end of the taper leaving a shoulder to the rest of the shaft. This meant of course that our replacement wouldn’t fit so, we cleaned off and painted the old one and the refitted it. Now to put you further in the picture we are over propped and get black muck out of the exhaust at full throttle and only about half revs when in gear. The old prop is 12x8 and using online calcs we need a 10x7, this has also been confirmed by Silette who offered to make us one but at a cost which we can’t afford.
My questions are:
Is it common to turn the shaft down to leave a shoulder, IE. Is it worth looking out for a replacement at Beaulieu?
Can I get my replacement prop sleeved to fit the shaft?
Should I get a new shaft? I really don’t want to do this due to difficulty removing old one and the costs involved
Can I get the old 12x8 altered to nearer the 10x7 I need?

Please bear in mind we don’t want to spend a lot of money and we usually do all the maintenance ourselves but I am not an engineer so I could use any advice that I can get. To that end we are quite prepared to beach and clean the existing prop every month or so to get whatever power we can.

I had a Petter mini six on a Leisure 23 with a 12x7 prop not a big difference from your set up and I thought the prop was well matched with only black smoke at near max revs.
Are you sure its not a problem with engine.Blocked exhaust elbow/restricted exhaust perhaps.

 

[Quidi Vidi]

I had a Petter mini six on a Leisure 23 with a 12x7 prop not a big difference from your set up and I thought the prop was well matched with only black smoke at near max revs.
Are you sure its not a problem with engine.Blocked exhaust elbow/restricted exhaust perhaps.

That's the problem I can't get anywhere near max revs under load, wouldn't a blocked exhaust be affected equally under load or out of gear.
I'm open to ideas of other possibilities for me to check before I start removing skegs etc

 

[peteK]

That's the problem I can't get anywhere near max revs under load, wouldn't a blocked exhaust be affected equally under load or out of gear.
I'm open to ideas of other possibilities for me to check before I start removing skegs etc

See other posting on MD 2020,for some reason they appear to rev ok in neutral but under load they will not rev up,Im my case with a Perkins Perama when I stripped the exhaust elbow there was only a hole about 8mm,for the exhaust gases to go through.

 

[ithet]

Have you used the Vicprop calculator?
http://www.vicprop.com/displacement_size.php

Using your figures gives a two blade prop size almost exactly what you have (12.2x7.6). The 10x7 figure comes out for a four blade prop.

 

[misterg]

...wouldn't a blocked exhaust be affected equally under load or out of gear.

No - the volume of combustion products is much greater under load.

Andy