monkey_trousers
regular
Reged: 02/01/2008
Posts: 74
|
|
might not be the right place to ask this, but feel a bit more at home in here!
Help! can't work this one out at all at all
43 foot 6 boat. think I'm wrongly propped. 120hp ford lehmann, 1.5:1 box, max revs we can get is 1600, engine is rated max revs 2750 (cruising 1800)
at max revs we can get, slack water, no tide/current to push, we get about 6knots
previously she had a Kelvin k3 66hp at 300revs, dunno about box though, think the kelvin boxes were 1:1??
I have a suspicion that when she was re-engined they retained the old shaft and prop, been trying to do the sums to see if it is a logical conclusion that the big old kelvin prop for those revs would hold her back that much?? does that sound logical?
other alternative is I suppose there's a governor on the engine (how do they work then and where are they and how do you remove them!)
hull speed should be 9-10knots or so (I think!)
any ideas?? what should she be swinging with the ford at 2750 revs
she's coming out the water shortly so I'll be able to see what she currently swinging
--------------------
Restoring an old MFV Motor Fifie
http://bf494.co.uk
|
markdj
regular
Reged: 31/07/2002
Posts: 672
Loc: Strangford Lough, Northern Ire...
|
|
If you have the weight of the boat then give Andy a call at BT Marine Propellers.
I think there are also prop calculation software out on the net but not sure where.
|
Peterduck
regular
Reged: 10/04/2002
Posts: 1010
Loc: Melbourne, Australia
|
|
Just looking up the graphs and nomograms in 'Skene's Elements of Yacht Design' I get a prop of 18" diameter x 12.5" pitch [by the short method]. If the current prop is close to that it can be modified. There are workshops which will do this. The pitch can be [carefully] changed,and it is quite remarkable how much difference a little bit off the tips of the blades will make to the performance of a prop.
|
chrishelen
regular
Reged: 07/01/2007
Posts: 1194
|
|
Hi monkey trousers, if you "google" prop pitch there are some good sites that do the calcs for you.
|
Tranona
regular
Reged: 10/11/2007
Posts: 1377
|
|
Just put Propeller calculations into Google and you will get lots of sites you can use yourself. You have all the necessary information. Do the sums to get a ball park size and then go to any propeller manufacturer and ask them for a quote - they will advise the recommended size and you will be able to check against your calculation.
Not reaching full revs suggests current prop is too big, probably in diameter if it is the original.
Hope this helps
|
chrishelen
regular
Reged: 07/01/2007
Posts: 1194
|
|
These prop pitch calc sites assume the engine is not clapped out.
|
Peeriemaa
regular
Reged: 16/11/2005
Posts: 56
|
|
You have increased the revs at the shaft by a factor of about three, so the propeller requires more torque than your engine can supply. The relationship is the slower the revs, the bigger the diameter and higher the efficiency of the prop. You will need to reduce the diameter of the prop, so do not be surprised if the prop design software you use drives you in that direction.
Nick
|
gdallas
regular
Reged: 27/04/2002
Posts: 98
Loc: Hertfordshire
|
|
I have a copy of Dave Ger's 'Propellor Handbook' that I'd pass on for a small RNLI donation. PM if interested
|
monkey_trousers
regular
Reged: 02/01/2008
Posts: 74
|
|
all up she's about 25 tons, and the engine has 380 hours on it, and starts an runs like new
I used http://www.castlemarine.co.uk/pitch.htm and bashed some numbers in and got an estimated hull speed of 8.5knots and a diameter/pitch figure, but I'm not convinced of my own abilities with the science of it all
fundamentally though, I feel there's something along the way that's not right, maybe its the old kelvin prop still, which is so big its holding her revs back, or the engine is governed to keep it back for some other reason?
need to get her hauled at as soon as we can to see what she's swinging down there and then do some more head scratching (an no doubt cheque book stretching) and get some one who knows what they are looking for to see if she does have a governor on the lump. As well as everything else she burnt a hell of a lot more diesel than I'd expected at 1500 revs on the trip back.
one mad alternative that presented itself today was the possibility of pulling the new fordy/borg warner box out and sticking a big old kelvin lump/box back in! Maybe flogging the fordy and box would pay for the kelvin to go back in!
logically that probably is an insane thing to do, but you know what, I can here that clickety clackety sound already!
historically, from the family that owned her way back when she was kelvin powered, they would get around 9knots from her (10 with a good shove from the tide) So the old girls has it in her somewhere along the lines
--------------------
Restoring an old MFV Motor Fifie
http://bf494.co.uk
|
Tranona
regular
Reged: 10/11/2007
Posts: 1377
|
|
It is unlikely to be the engine. If it goes up to full revs in neutral, that is until the governor limits it then there is nothing in theory to prevent it doing that under load.
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.
|
Previous
Index
Next
Threaded
Edit
Reply
Quote
