monkey_trousers
regular
Reged: 02/01/2008
Posts: 103
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Quote:
Your symptoms are classic of being over propped. When you haul, compare what you have with what you have calculated you need. Then go along to a prop manufacturer such as Clements (there are many more) and ask for their recommendation and quote.
that frightens me in itself! buying a new a big bonze prop! ouch!
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Converting a Converted MFV Fifie.
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Tranona
regular
Reged: 10/11/2007
Posts: 1549
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Thats why you have a boat! If the job was not done properly when the new engine was put in, go back to whoever did it and give them hell!
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chrishelen
regular
Reged: 07/01/2007
Posts: 1328
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Have you ruled out the usual things for slowing a boat down, you have allready said the engine is in good nick so have you got a dirty bottom?is there a grassy growth on your waterline,barnacle growths? all will become clear at haul out,hopefully something simple.
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monkey_trousers
regular
Reged: 02/01/2008
Posts: 103
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Quote:
Have you ruled out the usual things for slowing a boat down, you have allready said the engine is in good nick so have you got a dirty bottom?is there a grassy growth on your waterline,barnacle growths? all will become clear at haul out,hopefully something simple.
pretty much, don'y think she's a dirty girl at all. was antifouled just over a year ago and was sat in freshwater in the caley all the time till we moved her
been some really good responses though, (as ever - thanks boys and girls), and reckon, it might be a combination of gearbox ratio and the old prop. new box and prop might do the trick...
but I can here that old kelvin clickety clicking in the back of my head though. who needs reliability and a good supply of spares when you can have an old boat that goes clickety click anyway! 
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Converting a Converted MFV Fifie.
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srp
regular
Reged: 10/05/2006
Posts: 1327
Loc: IoW
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Using Propcalc.xls (you should find it with Google) I get 18 x 9, or 17 x 12 as an alternative, to give you 8 knots That's by making a few assumptions along the way. I think I've read that 1" of diameter is equivalent to about 3" of pitch, which would explain the difference in the 2 sizes suggested. And it's not that different to the figures calculated by Peterduck earlier on in this thread, so it might serve as a starting point for you. It would be handy to know what size prop is actually fitted at the moment, but it sounds as though it will be a fairly large diameter one, which a high revving engine will struggle with.
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www.corribee.org
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monkey_trousers
regular
Reged: 02/01/2008
Posts: 103
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Quote:
Using Propcalc.xls (you should find it with Google) I get 18 x 9, or 17 x 12 as an alternative, to give you 8 knots That's by making a few assumptions along the way. I think I've read that 1" of diameter is equivalent to about 3" of pitch, which would explain the difference in the 2 sizes suggested. And it's not that different to the figures calculated by Peterduck earlier on in this thread, so it might serve as a starting point for you. It would be handy to know what size prop is actually fitted at the moment, but it sounds as though it will be a fairly large diameter one, which a high revving engine will struggle with.
need to get the underwater mystery resolved as soon as I think. only thing I've got to go on is an old pic I found, but I don't know if this was before or after the new engine went in, or even if this prop is still on it. this was when she was on the slip at macduff 6 years ago
http://bf494.co.uk/media/users/mark/66622.jpg
been trying to guess how big it is based on the standard scots workmans head unit of measurement!
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Converting a Converted MFV Fifie.
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DownWest
regular
Reged: 25/12/2007
Posts: 226
Loc: S.W. France
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All diesels have governers, forget about taking it off, essential part (and integral with) the injection pump. As said, if it revs in neutral and is in good nick as you say. then fine.
Andrew
Overpropping usually means lots of black smoke. Any sign?
Edited by DownWest (05/06/2008 07:47)
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monkey_trousers
regular
Reged: 02/01/2008
Posts: 103
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Quote:
All diesels have governers, forget about taking it off, essential part (and integral with) the injection pump. As said, if it revs in neutral and is in good nick as you say. then fine.
Andrew
Overpropping usually means lots of black smoke. Any sign?
hardly any black smoke at all when pressed hard.
all in all I think we have a combination of gearbox ratio being way way wrong, and having found some pics of her last time she was hauled out, the prop looks very small, so might have been put together to match the shaft speed from the box
in neutral she won't rev over about 1550, although the engine is rated for max HP at 2750
so the plan is new box, and new prop (double ouch!) with a 2.5 or 3 to 1 box we can get up to 24 28" prop at a shaft speed which is much more suited to the boat's design and then maybe get something like the hull speed
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Converting a Converted MFV Fifie.
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DownWest
regular
Reged: 25/12/2007
Posts: 226
Loc: S.W. France
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Hum- If it wonīt rev in neutral, then it isnīt going to rev in gear. I think you need to talk to the instalers. Just because the factory spec says 120hp/2750 rpm doesnt mean the throttle limit stop hasnīt been moved. The rpm is directly controled by this setting. Silly question: have you checked that the throttle lever has full travel? So it hits the stop.(it is on the injection pump)The lack of smoke might indicate this
Andrew
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monkey_trousers
regular
Reged: 02/01/2008
Posts: 103
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Quote:
Hum- If it wonīt rev in neutral, then it isnīt going to rev in gear. I think you need to talk to the instalers. Just because the factory spec says 120hp/2750 rpm doesnt mean the throttle limit stop hasnīt been moved. The rpm is directly controled by this setting. Silly question: have you checked that the throttle lever has full travel? So it hits the stop.(it is on the injection pump)The lack of smoke might indicate this
Andrew
did that by throwing the advance on the pump forward till it hit the stop from the engine room, with my other half in the wheelhouse watch the rev counter, so I really do think we are in 'governor' territory
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Converting a Converted MFV Fifie.
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