any prop calc boffins out there??

M

monkey_trousers

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bf494.co.uk
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
 
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.
 
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
 
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
 
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
 
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.
 
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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.

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that frightens me in itself! buying a new a big bonze prop! ouch!
 
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!
 
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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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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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! /forums/images/graemlins/tongue.gif
 
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.
 
[ 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.

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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!
 
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?
 
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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?

[/ QUOTE ]

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
 
Hum- If it won´t rev in neutral, then it isn´t 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 hasn´t 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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Hum- If it won´t rev in neutral, then it isn´t 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 hasn´t 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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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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