Propellors

[blackrose]

I like being able to get the boat where I want it with very little revving up and minimal bother

Not at all concerned about racing anyone Or time trials (which I would win anyway ) but I really do like a bit of low end torque for maneuvering. It makes a big difference in my experience.

 

Edit to do something

I agree, and a correctly pitched prop can only help in manoeuvrability at slow speeds. My engine has plenty of torque at low revs and I can spin it around almost within its own length at just over idling. I don't think the manoeuvrability of a relatively big boat could be much better. I've been on boats with much "boatier" hull shapes that can't do this (no BT used).

 

The chimney isn't at the front - it's about 1m forward of the centre of the boat

 

 

Hi all boat back in water engine now pulls to 3000 rpm si right on the button for max power cruises at 1600 and still stops ok big plus is it will now go slow so expensive zinga doesnt get damaged perfik

 

Peter

Glad to hear it worked. I may even consider reducing the pitch of my prop further in light of your results.

Edited July 16, 2015 by blackrose

[Tony Dunkley]

I agree, and a correctly pitched prop can only help in manoeuvrability at slow speeds. My engine has plenty of torque at low revs and I can spin it around almost within its own length at just over idling. I don't think the manoeuvrability of a relatively big boat could be much better. I've been on boats with much "boatier" hull shapes that can't do this (no BT used).

 

 

A high rate of turn capability, which, without getting too technical, is effectively what you were demonstrating in your video clip is mainly a function of the area of the immersed longitudinal section of the hull and the rudder area. A flat bottomed wide beamed boat has a smaller immersed longitudinal section area than a narrower beam vessel of the same displacement, and will turn tighter with a rudder of the same blade area as the narrower beam (or 'much boatier' shaped) vessel.

Your boats' excellent turning ability has nothing whatever to do with either propeller pitch or the engine torque curve, but everything to do with it's length, beam and flat bottom making it more directionally unstable than a vessel with a better hull form. The relatively small propeller hiding behind a low aspect ratio balanced rudder will also contribute towards a small turning circle.

Edited July 17, 2015 by Tony Dunkley

[mark99]

 

 

Your boats' excellent turning ability has nothing whatever to do with either propeller pitch or the engine torque curve, but everything to do with it's length, beam and flat bottom making it more directionally unstable than a vessel with a better hull form.

 

Slightly related.

 

There was a study of some fish species (Perch in fact) who are inherently unstable in water. It turns out nature had provided them with this benefit as they were predators and a tiny fin movement could manouver them very quickly rendering prey fish unable to out manouver them.

Edited July 17, 2015 by mark99

[blackrose]

Your boats' excellent turning ability has nothing whatever to do with either propeller pitch or the engine torque curve, but everything to do with it's length, beam and flat bottom making it more directionally unstable than a vessel with a better hull form. The relatively small propeller hiding behind a low aspect ratio balanced rudder will also contribute towards a small turning circle.

I was simply using turning as an example, but the fact is that low speed manouverability on my boat improved after repitching my prop. I think the OP said the same thing in a private message to me as well. Because we are now able to go slower, the prop repitch has allowed better control and maneuverability at low revs.

Edited July 17, 2015 by blackrose

[peterboat]

I was simply using turning as an example, but the fact is that low speed manouverability on my boat improved after repitching my prop. I think the OP said the same thing in a private message to me as well. Because we are now able to go slower, the prop repitch has allowed better control and maneuverability at low revs.

Exactly right Mike far easier to moor up because I have less push at low rpm a joy in fact. My boat is the same as yours spins in its own length useful at times

 

Peter

[magnetman]

Good news Peterboat and thanks for telling us the outcome

[peterboat]

The boat does 10 kph at 3000 rpm which for me is about the same as before so all seems to be ok there as well

 

Peter

[blackrose]

Following Peter's re-pitch, I was wondering if I should reduce the pitch of my prop further. It was originally 19" x 13" and Noris' reduced it to 18.5" to 11.5". The half inch off the diameter was just to take the blade tips away from the skeg and counter where they might cause cavitation, but both Crowthers and Noris agreed I shouldn't really be reducing the diameter.

 

Following the re-pitch my engine only gained about 250rpm (300rpm engine now revs to 2200rpm in gear in deep water), but according to the calculations it should have been closer to a gain of 400rpm (based on 1" of pitch reduction gaining 250rpm and 1" of diameter reduction adding about 100rpm).

 

Anyway for me 1" of pitch reduction = 166rpm (ignoring the reduction in diameter), so I calculate if Noris reduced the pitch by another half inch the engine would still only rev to 2283rpm and 1" reduction would be 2366rpm. I know the correlation is probably not linear, but I guess that calculation is close enough.

 

It hardly seems worth the bother of another re-pitch for such a small gain, especially when Noris warned about the prop losing bite if it's too flat.

Edited July 29, 2015 by blackrose

[Tony Dunkley]

Following Peter's re-pitch, I was wondering if I should reduce the pitch of my prop further. It was originally 19" x 13" and Noris' reduced it to 18.5" to 11.5". The half inch off the diameter was just to take the blade tips away from the skeg and counter where they might cause cavitation, but both Crowthers and Noris agreed I shouldn't really be reducing the diameter.

 

Following the re-pitch my engine only gained about 250rpm (300rpm engine now revs to 2200rpm in gear in deep water), but according to the calculations it should have been closer to a gain of 400rpm (based on 1" of pitch reduction gaining 250rpm and 1" of diameter reduction adding about 100rpm).

 

Anyway for me 1" of pitch reduction = 166rpm (ignoring the reduction in diameter), so I calculate if Noris reduced the pitch by another half inch the engine would still only rev to 2283rpm and 1" reduction would be 2366rpm. I know the correlation is probably not linear, but I guess that calculation is close enough.

 

It hardly seems worth the bother of another re-pitch for such a small gain, especially when Noris warned about the prop losing bite if it's too flat.

 

Is that a typo ? , , , , , It sounds as if you're saying that your engine should rev to 3000 rpm to produce it's rated horsepower, but it's only managing 2200 rpm.

[magnetman]

Isuzu 55 is 55hp@3,000rpm apparently.

 

So it does seem to be well overpropped if it only goes to 2200rpm.

 

I suppose if a smaller prop was put on and engine allowed to rev higher it would behave too much like an egg whisk. Perhaps a 3:1 gearbox would be worth considering although the setup is apparently perfectly good as it is so best not to tinker !

 

Blackrose - what happens when you get to the 2200rpm? Is there any more travel on the Morse control? Black smoke? Overheating?

 

Its an interesting boating subject.

[Tony Dunkley]

Isuzu 55 is 55hp@3,000rpm apparently.

 

So it does seem to be well overpropped if it only goes to 2200rpm.

 

I suppose if a smaller prop was put on and engine allowed to rev higher it would behave too much like an egg whisk. Perhaps a 3:1 gearbox would be worth considering although the setup is apparently perfectly good as it is so best not to tinker !

 

Blackrose - what happens when you get to the 2200rpm? Is there any more travel on the Morse control? Black smoke? Overheating?

 

Its an interesting boating subject.

 

It sounds as if it's gearbox reduction ratio or the engine power output that's the problem. The propeller is already too 'small' for good efficiency on a boat of that hull form and displacement. If the engine is performing close to or on it's rated power curve, then the incorrect reduction is what's limiting the engine to around 75% of it's rated maximum output, or around 40 bhp instead of 55 bhp. Take into account the low efficiency of such a small high revving propeller on a slow heavy displacement hull, probably in the region of 45% at best, and there is well under 20 bhp left to actually push the boat along.

The comparison with an egg whisk is very apt, high revving propellers on slow heavy boats just can't move enough water to be efficient.

Edited July 30, 2015 by Tony Dunkley

[magnetman]

It sounds like a larger diameter prop is not possible due to the distance between the bottom of the counter and the top of the skeg being too small. I wonder if a 4 blade prop would make any difference* ? I personally would prefer a larger power unit (70hp) but I think most of those boats have 55s in them and seem to be okay, depending on what OK means.

 

*(4 blade prop and 3:1 reduction box I meant)

That would be a lower cost option than repowering with a bigger engine and might make a big difference.

Edited July 30, 2015 by magnetman

[Tony Dunkley]

It sounds like a larger diameter prop is not possible due to the distance between the bottom of the counter and the top of the skeg being too small. I wonder if a 4 blade prop would make any difference* ? I personally would prefer a larger power unit (70hp) but I think most of those boats have 55s in them and seem to be okay, depending on what OK means.

 

*(4 blade prop and 3:1 reduction box I meant)

That would be a lower cost option than repowering with a bigger engine and might make a big difference.

 

You've got right to the heart of the matter there.

So many poorly designed hulls with unsuitable and inefficient engine and sterngear installations have been produced and sold in recent years that the so-called boatbuilders who churn them out have actually succeded in educating most of their victims (or customers as they prefer to call them) into believing that the poor performance of the vast majority of newly built canal boats is acceptable and as good as can be expected.

Edited July 30, 2015 by Tony Dunkley

[Neil2]

These issues have been subject to discussions before on CWDF but this has been one of the best and most constructive I reckon.

 

I don't have Tony's vast experience but I would agree with the last comment. The problem is that to the vast majority of folk who buy narrowboats the priority is internal living space. A narrrowboat is by necessity a compromise but it seems to me the balance is way out at the moment and builders get away with it because of the general ignorance of the boat buying public. And, of course, most narrowboats hardly go anywhere from one year to the next anyway.

 

My own personal experience is I have an engine that is designed to operate at or near maximum revs but will not because the prop is "too big". Except it's not. The prop is the right size for the boat and though I could get the engine to rev out by fitting a much smaller screw, I know the correct, albeit much more expensive, solution is a higher ratio gearbox which is eventually what I intend to fit.

[blackrose]

 

Is that a typo ? , , , , , It sounds as if you're saying that your engine should rev to 3000 rpm to produce it's rated horsepower, but it's only managing 2200 rpm.

No it's not a typo. Before I had the prop repitched it only revved to 1950rpm. Reducing the pitch by 1.5" released 250rpm and a lot more power, but perhaps I should have reduced it a bit more. You have to remember that the Isuzu 55 produces maximum torque at 1800rpm so although more power is available at higher revs, I think it's probably a case of diminishing returns above that point.

[nicknorman]

No it's not a typo. Before I had the prop repitched it only revved to 1950rpm. Reducing the pitch by 1.5" released 250rpm and a lot more power, but perhaps I should have reduced it a bit more. You have to remember that the Isuzu 55 produces maximum torque at 1800rpm so although more power is available at higher revs, I think it's probably a case of diminishing returns above that point.

It was a typo, you put 300 instead of 3000.

[blackrose]

 

You've got right to the heart of the matter there.

So many poorly designed hulls with unsuitable and inefficient engine and sterngear installations have been produced and sold in recent years that the so-called boatbuilders who churn them out have actually succeded in educating most of their victims (or customers as they prefer to call them) into believing that the poor performance of the vast majority of newly built canal boats is acceptable and as good as can be expected.

I think perhaps you need to read back through the thread. The pilot who took my boat across the Bristol channel thought the performance was very good. This was after the prop repitch. Following Peter's results I was just wondering if by repitching a little further I could have got even more power out of it?

It was a typo, you put 300 instead of 3000.

Ah, sorry!

[bargemast]

Liverpool boats didn't do too well on Blackrose concerning the engine, gearbox and prop they put in the boat without adapting them to each other and studying the boats needs.

 

Therefore a while ago there were serious overheating problems, mainly because the engine was asked to work too hard fighting against a prop that was much to heavy to reach the revs that were given by the engines manufacturer.

 

The cooling systems has been made bigger to handle the overheating problem, but there is still the problem with the 800 revs missing, so the engine can never get to it's designed maximum of 3000n and is still struckeling and working too hard.

 

It would be good if you could get (lend, borrow if possible) another prop that will allow your engine to get to 3000n or at least much closer to, to see the difference it will make.

 

Don't expect to gain a lot of extra speed though as the hull insn't made for speed, but if you're lucky you may find that you'll use a bit less of fuel as the engine is working closer to what it should have done since day one.

 

Peter.

[blackrose]

Blackrose - what happens when you get to the 2200rpm? Is there any more travel on the Morse control? Black smoke? Overheating?

 

No black smoke or overheating. There's a bit more travel on the Morse control but no more revs at 2200rpm. It revs to 3000rpm out of gear so I know it's not Morse adjustment.

 

Liverpool boats didn't do too well on Blackrose concerning the engine, gearbox and prop they put in the boat without adapting them to each other and studying the boats needs.

 

Therefore a while ago there were serious overheating problems, mainly because the engine was asked to work too hard fighting against a prop that was much to heavy to reach the revs that were given by the engines manufacturer.

 

The cooling systems has been made bigger to handle the overheating problem, but there is still the problem with the 800 revs missing, so the engine can never get to it's designed maximum of 3000n and is still struckeling and working too hard.

 

It would be good if you could get (lend, borrow if possible) another prop that will allow your engine to get to 3000n or at least much closer to, to see the difference it will make.

 

Don't expect to gain a lot of extra speed though as the hull insn't made for speed, but if you're lucky you may find that you'll use a bit less of fuel as the engine is working closer to what it should have done since day one.

 

Peter.

Mostly correct apart from the previous overheating issues. This was caused solely by not having adequate skin tank cooling. Not as you said, because the engine was working too hard to swing the prop, although I admit that probably didn't help. However, since I added the extra skin tank it's never overheated and that was done before the prop was repitched, so the skin tank size was the problem.

 

It's interesting to hear people's different take on this. The pilot (who actually steered the boat for the best part of 3 hours at max revs or thereabouts), seemed to think the engine, gearbox and prop were well matched.

Edited July 30, 2015 by blackrose

[Tony Brooks]

My take on it is that the best full throttle fuel economy occurs at roughly half the maximum revs. The maximum torque point is somewhat lower than the absolute maximum revs. My boat is propped to reach maximum torque speed and I do not worry about not having the extra speed to get maximum power. I am not even sure a prop 100% matched to the boat would achieve that as the torque is falling off when you get towards maximum power. In my view to get to maximum power would tend to imply a slightly under optimum sized prop.

 

The whole thing is something of a black art so if I can get acceptable fuel consumption, acceptable speed, acceptable acceleration (ahead and astern) I am happy and would not worry over a few hundred revs.

[bargemast]

No black smoke or overheating. There's a bit more travel on the Morse control but no more revs at 2200rpm. It revs to 3000rpm out of gear so I know it's not Morse adjustment.

 

 

Mostly correct apart from the previous overheating issues. This was caused solely by not having adequate skin tank cooling. Not as you said, because the engine was working too hard to swing the prop, although I admit that probably didn't help. However, since I added the extra skin tank it's never overheated and that was done before the prop was repitched, so the skin tank size was the problem.

 

It's interesting to hear people's different take on this. The pilot (who actually steered the boat for the best part of 3 hours at max revs or thereabouts), seemed to think the engine, gearbox and prop were well matched.

 

The overheating was caused as you say because of the skin tank not being adequete, that doesn't sound good to start with for a company had build quite a few boat before yours, and if people were prepared to pay a supplement with even more powerful engine, so even less adequate, not very serious if you ask me.

 

I'm glad the pilot was happy with the well matched engine, gearbox and prop, he was probably (I hope for him) a much better pilot than an engineer, if an engine can't reach it's maximum revs, it can't be a good match.

 

Peter.

[bargemast]

My take on it is that the best full throttle fuel economy occurs at roughly half the maximum revs. The maximum torque point is somewhat lower than the absolute maximum revs. My boat is propped to reach maximum torque speed and I do not worry about not having the extra speed to get maximum power. I am not even sure a prop 100% matched to the boat would achieve that as the torque is falling off when you get towards maximum power. In my view to get to maximum power would tend to imply a slightly under optimum sized prop.

 

The whole thing is something of a black art so if I can get acceptable fuel consumption, acceptable speed, acceptable acceleration (ahead and astern) I am happy and would not worry over a few hundred revs.

 

A few hundred revs wouldn't worry me either, but in this case 800n difference, it's quite a bit more than only a few hundred revs.

 

If I count idle at about 800n and maximum at 3000n there are 2200n that the engine can work with, so the missing 800n is a bit more than 35% less, that can't be the most efficient way of using the engines designed performances.

 

Peter.

[nicknorman]

In my view to get to maximum power would tend to imply a slightly under optimum sized prop.

 

I don't think you can generalise as there are so many variables. Our Beta 43 has max torque at 1800 and max power at 2800. With our prop it will rev to about 2750. It already goes quite fast at tickover (2mph or so). If the prop was such that it only revved to max torque rpm I think it would be far too coarse, and of course you would be limiting the max power to around 33hp. One also has to bear in mind that it has a 3.5kw travel power, a 175A alternator and as well as a smaller engine alternator, so there has to be enough spare torque lower down the rev range to service these. It is not unknown for us to put the tumble drier on (2kw from the travel power) whilst charging the batteries at say 120A from the domestic alternator, and this has a very noticable effect on torque margin such that the rate of engine acceleration coming off idle is noticeably much more sluggish.

[nebulae]

Interesting that 3,000rpm is needed to achieve full power. I have an Isuzu 42 in a 50ft narrow boat. The engine is rated at 42hp by Isuzu. However H.P.I.(the engine marinisers in this country,now not trading) restricted the rpm to 2,000.Obviously,if the engine was fitted to a gen set,for instance,it would be set to run at its rated maximum of 3,000rpm.On a boat,I think the noise at that speed would be unacceptable with this unit. Consequently,the engine,which is rated at 42hp at 3,000rpm,is probably only putting out about 25hp.at the gearbox output.2/1 box. Allowing for 45% efficiency,I have 11hp to push the boat along. Food for thought. I have a chart,prepared by the Manchester Ship Canal Engineers Dept which shows our boat has a theoretical top displacement speed with 42hp of 6.5 knots.In practice,flat out in deep water,the max.speed is 5.9knots.(engine wound out to 2,100rpm).The boat handles O.K. at canal speeds.Will not pull more than 2,100rpm with this prop.

[Tony Dunkley]

It's interesting to hear people's different take on this. The pilot (who actually steered the boat for the best part of 3 hours at max revs or thereabouts), seemed to think the engine, gearbox and prop were well matched.

 

You must consider the pilot's thoughts in the context and circumstances in which he expressed them.

Firstly, he would almost certainly have had only the performance of similar relatively modern canalboats with which to compare the performance of your boat, and there is no doubt that modern boats don't make efficient and effective use of the engine power they have in comparison with the commercial carrying boats of days gone by. Size for size, a fair comparison would be a Leeds and Liverpool Shortboat. Fully loaded a typical power/displacement ratio of around 0.4 bhp/ton, and light (empty) around 1.0 bhp/ton as against a modern widebeam canalboat, with a minimum of around 1.6 bhp/tonne, and usually much more. Overall performance and speed capability were very similar to that achieved by modern widebeams, and by any standards, that is something that todays so-called boatbuilders should be thoroughly ashamed of.

Secondly, you were paying him to get you and your boat to wherever you were going, and not to provide an assessment of the engine and sterngear installation. As a paying client, unless your boat was a floating deathtrap he would probably be hoping that you would engage him again should the need arise, so it's extremely unlikely that he would be inclined to be critical, or make adverse comments, about your pride and joy. Personally, I would say what I thought, but I've always had a talent for offending people by volunteering unsolicited opinions and saying things they may not want to hear.

Edited July 30, 2015 by Tony Dunkley