Propeller

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jonlaw

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Im in the process of buying a used 36 foot yacht. The survey has highlighted the facts that the two bladed propeller is rather large at 42cm and the 8cm of 25mm shaft between propeller and cutlass bearing is large and may give rise to vibration.
I'm tempted to have the shaft shortened and a properly sized propeller fitted for peace of mind. Am I wasting my money ?
Any thoughts ?

Regards

John
 
Is the recomendation to change the propeller based on just the diameter on a 1" shaft? or is it because it is overgeared? Remember that the current may prop matches the gearbox ratio and if you change the prop then a courser pitch will be required.

Do you not require the 80mm gap to fit a stripper?

Iain
 
The survey states

" It is recommended that consideration is given to shortening the propeller shaft in order to reduce the length of shaft between the P Bracket and propeller to an absolute minimum. If excessive vibration is experienced from the stern gear, then consideration may be given to fitting a propeller shaft and bearings of a larger diameter, more suited to the sixe of the propeller"

To be honest, Im buying from quite a distance and only viewed the boat twice, I cant remember if a stripper is fitted.
When first viewing, the propeller did strike me as very large.
But to replace the shaft, bearings, coupling, seal etc seems excessive, why not reduce the propeller size
I have this idea that maybe the previous owner had a nice shiney folding prop, and wanted to keep it, so he fitted something that 'will do' to sell.

John
 
A 16 1/2" propeller sounds a lot for a 1" shaft but it depends on the gearing and on the pitch of the prop. What engine is in the yacht?. Is it a slow running one or is it a higher speed modern one? Have you been out in it under power?. If so did it seem ok? or have you just looked at it out of the water?

Your surveyer may have found something but perhaps not.

How old is the yacht? The original shaft on many 33/34ft yachts was certainly 1".

Iain
 
My Autoprop is a 16 inch on a 1 inch shaft. I could have had a 17 inch on the same shaft, so I don't see yours as being excessively large.

The 8 cm is more of a problem. Normal advice is to have this clearance as small as possible to give support to propeller lateral loads. However, it has presumably been done for a reason, which I suggest may be that the propeller will foul or come very close to the hull if the shaft is shortened. If this is not the case then I would shorten the shaft as this should be relatively cheap and easy to do. Otherwise you may need to bite the bullet, shorten the shaft and fit a smaller prop. Your performance under power will suffer as a result, diameter has more effect than pitch.
 
I would say try it first and see how it behaves under power. If overpropped, you can get it re-pitched - I had this done on a 15" x 9 prop to reduce it to 8" pitch - it made a lot of difference. The firm that did it said they could go another ½" to 1" less pitch if necessary.

dickh
I'd rather be sailing... :-) /forums/images/icons/smile.gif
 
Get it properly sized by a pro or the engine manufacturer. I just re-engined with a used (only 65hrs) Volvo and they were very helpful. Boat now goes like a train.!
 
To be honest I would say that things are about right. Depends on the HP of the motor. If it is around 18hp then do not worry. An 8 cm gap is little more than necessary but not likley to cause vibration. More likely to get bad vibes from a worn cutless. As someone else said you are going to need at least 5-6 to fit a stripper. If you go into http://www.nannidiesel.com/ and then calculations, you can get a good rough idea of both prop size, shaft size (minimum) and even max hull speed. You even get a max length of unsuported shaft but that is between two bearings. Even still I would not worry.
 
You do not say what the engine/gearbox is, and therefore how many shaft horse power you have available and a what shaft speed. If you are going to fit a new prop then I would suggest you fit a three bladed for greater efficiency and if you value your sailing a folding or feathering proeller is advised if you can afford it. I fitted a 3 bladed Flex-o-fold to my 37 footer and found that the sailing performance went up by about 3/4 of a knot. The boat has a 53hp yannmar with a max shaft speed of about 1300 rpm. The actual requirement to drive the boat at max hull speed is about 28hp, but the extra power kicks in when going to windward is a heavy sea. The prop fitted is 17 x 13 but from the sounds of your boat your engine is less powerful so you might not need such a big three bladed prop.

Yanmar were free with their advice on what to fir and their advice turned out to be better than the propeller people.

Hope this helps

Chris Stannard
 
Careful: conversions

I think somebody may be confusing centimetres with millimetres. A stripper needs about a quarter of an inch, 6 millimetres. The figure given for the bearing to prop measurement was 8 centimetres, that is more than three inches in old money. I would agree that we don't need to panic over 8 mm, but 8 cm is a different thing.
 
I fitted an Autoprop 18", and had to have the shaft lengthened to allow the blades to rotate. So if you are considering changing the prop, and might have a 'clever' one, you might need a longer shaft.
Also a stripper will need space.
 
I fitted a Stripper about 2 years ago and from memory you want about 30 to 35mm gap between the prop boss and the P bracket.
I think the Stripper website has this information - www.ropestripper.com

dickh
I'd rather be sailing... :-) /forums/images/icons/smile.gif
 
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