Power in reverse

[PaulG]

2 hours ago, Debs said:

We have not long bought a 45' narrowboat but we are finding that it doesn't have much power in reverse. This means that it's really hard to stop as well even when going at very low speeds. When we are at standstill, it will move backwards but very slowly. To resolve the issue, would we be looking at getting a bigger propeller or an Axiom one? The one we have at the moment is a 3 bladed, bronze right hand rotation on a PRM 80 2:1 gearbox. We don't know what diameter it is though.

I've got a 55 footer with a 27hp.

That's rubbish in reverse as well...

Don't waste your money on an Axiom prop.

Snake oil, IMO.

[Neil2]

An Isuzu 25 on a 2:1 ratio is not going to work on a 45' narrowboat unless she is very fine underwater. 

If you fit the "correct" prop, it will be too small to stop the boat effectively, if you fit a much bigger prop the engine will struggle to turn it.  

I bet it has something like a 15" screw.  You could try something a bit bigger, most NB's are a bit overpropped anyway, an inch on the dia could make a lot of difference, assuming the boat's design underwater will allow it. 

 

[Iain_S]

5 hours ago, mrsmelly said:

(snip)

Ask yourself why only " axiom " props are axiom prop shaped?I

Although the "Axiom prop shape" has changed a bit.

I personally think that the shape parallels the development of the propellor since 1836, and is now at 1914, or thereabouts .... 

[Iain_S]

Looking at the spec. for the PRM 80 gearbox, it looks as though both the 2:1 and 2.474 : 1 have 2.474 :1 as the reverse ratio, so an engine with the 2:1 box will have to turn about a fifth higher in reverse to get the same propellor speed.

As others have said, the boat's underwater shape will also have a big effect.

(One of our local canal societies has an outboard powered workboat that can be a handfull to stop: if too much power is put on in reverse, the wash just bounces off the transom, and cancels the effect of the reverse thrust of the prop.) 

[Murflynn]

5 hours ago, mross said:

My 60' NB has an 18hp engine and does fine.  But it is a trad engine, so quite torquey.

good point.

most folk have no idea what the nominal BHP figure means in practice; referring to the torque is much more meaningful in the circumstances under discussion, not forgetting to multiply it by the gearbox ratio.  In a good set-up the torque figure relates to the prop blade area, which is critical to stopping quickly.

[rusty69]

6 minutes ago, Murflynn said:

good point.

most folk have no idea what the nominal BHP figure means in practice; referring to the torque is much more meaningful in the circumstances under discussion, not forgetting to multiply it by the gearbox ratio.  In a good set-up the torque figure relates to the prop blade area, which is critical to stopping quickly.

Quite tourqey isn't really a meaningful figure though 

[mross]

10 minutes ago, rusty69 said:

Quite tourqey isn't really a meaningful figure though 

Nor a meaningful spelling!

[Athy]

Something one can resort to, perhaps.

[Neil2]

8 minutes ago, Athy said:

Something one can resort to, perhaps.

Do you set crossword clues when you're not posting on CWDF?

[rusty69]

9 minutes ago, mross said:

Nor a meaningful spelling!

Indeed. Still probably more important to have a meaningful meaning than a correct spelling imo. 

[cereal tiller]

1 hour ago, Athy said:

Something one can resort to, perhaps.

Torquay?

[MtB]

On 18/07/2017 at 15:02, mross said:

My 60' NB has an 18hp engine and does fine.  But it is a trad engine, so quite torquey.

 

I think you are inappropriately conflating blade diameter with engine torque

Yes your boat probably stops well because it has a large diameter blade, but this is possible  because of the hull design, not because of the engine. 

It was possible to specify the large blade because your boat has more blade clearance than the average narrowboat fitted with a modern low torque buzzy engine. The same type of hull design tends to be fitted with a Torquay engine, but I hold that you could fit a low torque high speed engine into your boat along with a suitably high reduction gear (say 4:1 or 5:1) and it would stop equally effectively with the same blade.

[Dharl]

Another vote for getting the gearbox checked out first.     If all ok going ahead but issues in astern that sounds like the problem.   

[Athy]

10 hours ago, Neil2 said:

Do you set crossword clues when you're not posting on CWDF?

Tee-hee.

No, but the habits gained through ten years of regularly doing (and sometimes finishing) the Torygraph crossword die hard!

7 hours ago, Mike the Boilerman said:

 

The same type of hull design tends to be fitted with a Torquay engine,

...or perhaps a Dursley one.

[rusty69]

8 hours ago, cereal tiller said:

Torquay?

Watery Fowls? 

[Neil2]

7 hours ago, Mike the Boilerman said:

 

I think you are inappropriately conflating blade diameter with engine torque

Yes your boat probably stops well because it has a large diameter blade, but this is possible  because of the hull design, not because of the engine. 

It was possible to specify the large blade because your boat has more blade clearance than the average narrowboat fitted with a modern low torque buzzy engine. The same type of hull design tends to be fitted with a Torquay engine, but I hold that you could fit a low torque high speed engine into your boat along with a suitably high reduction gear (say 4:1 or 5:1) and it would stop equally effectively with the same blade.

Looking at some of the "marine" engine manufacturers data it seems like the 2:1 reduction ratio is almost universal for some reason, what I mean is if you look at the engine range they almost always recommend the same gearbox ratio regardless of output.  I think this leads to a tail wagging dog situation where for the lower powered engines the engine/gearbox set up dictates the fitting of a prop that is probably too small.  It's been pointed out many time on this forum that especially with narrowboats you need to start with the ideal prop size and work back from there. As you say, heavy deep draughted boats usually end up with a high torque low revving engine but it doesn't have to be that way. Eg I know two Hudson owners one has a Gardner the other a Kubota. 

The design of modern narrowboats exacerbates the situation with short swims designed to maximise cabin space and engine bays just big enough for a 2/3cylinder motor. 

 

 

[alan_fincher]

15 hours ago, Neil2 said:

An Isuzu 25 on a 2:1 ratio is not going to work on a 45' narrowboat unless she is very fine underwater. 

If you fit the "correct" prop, it will be too small to stop the boat effectively, if you fit a much bigger prop the engine will struggle to turn it.  

I bet it has something like a 15" screw.  You could try something a bit bigger, most NB's are a bit overpropped anyway, an inch on the dia could make a lot of difference, assuming the boat's design underwater will allow it. 

 

You seem to have overlooked the fact that OP has confirmed that it seems fine when in ahead gear, so clearly an Isuzu 25 on a 45 foot narrow boat is fine the vast majority oif the time she is using it.

It is not a big engine, but in my view should be adequate, and, unless the hull is very poorly shaped, it should be perfectly possible with the right prop and set up to get adequate reverse and stopping power.

 

[Rickent]

2 hours ago, rusty69 said:

Watery Fowls? 

The best one was flowery twats, no idea how they got away with that one.

[Rickent]

[MtB]

On 19/07/2017 at 08:36, alan_fincher said:

You seem to have overlooked the fact that OP has confirmed that it seems fine when in ahead gear, so clearly an Isuzu 25 on a 45 foot narrow boat is fine the vast majority oif the time she is using it.

It is not a big engine, but in my view should be adequate, and, unless the hull is very poorly shaped, it should be perfectly possible with the right prop and set up to get adequate reverse and stopping power.

 

 

I'm not understanding the significance of that when the discussion is revolving around the factors affected blade performance is astern. Blades are not symmetrical in their performance in ahead and astern (Axioms aside), they are optimised for improved 'grip' of the water in ahead at the expense of grip in astern.

This problem seems to be far more pronounced with small diameter blades than large. Did you not have exactly this problem with flamingo when you first acquired the boat? No 'brakes' due to a small blade? OR did you fix that by re-pitching the existing blade rather than by fitting a bigger diameter blade? I don't remember the outcome.

Once the OP gets to measure the diameter of the blade on their boat, more light will be shed on the problem.

 

[alan_fincher]

4 hours ago, Mike the Boilerman said:

 

I'm not understanding the significance of that when the discussion is revolving around the factors affected blade performance is astern. Blades are not symmetrical in their performance in ahead and astern (Axioms aside), they are optimised for improved 'grip' of the water in ahead at the expense of grip in astern.

 

I was querying only the assertion that " An Isuzu 25 on a 2:1 ratio is not going to work on a 45' narrowboat unless she is very fine underwater. "

It doesn't say " An Isuzu 25 on a 2:1 ratio is not going to work in reverse on a 45' narrowboat unless she is very fine underwater. "

I agree that a whole host of things come into play, including the shape and blade area of the prop, as well as whether is is roughly symmetrically shaped or distinctly asymmetric.

Flamingo's performance  was very poor both in ahead and in reverse, primarily because the prop proved to have far less pitch than was ideal.  It has been re-pitched from somewhere around 15" to 21", and the improvements are fairly spectacular.  That said, stopping is still not that brisk, but we are talking a heavily ballasted 72 foot boat of maybe 25 tons, not about a modern 45 footer, much shallower in the water.

I think a 25HP modern engine should be able to power a modern 45 foot boat adequately, but prop changes may indeed be required.  I wouldn't touch an Axiom with a barge pole though!  Our BMC in a 50 foot boat was only circa 30HP, and that boat flew along, and stopped very well.  Most of the latest modern narrow boats  simply have larger engines than there is any reasonable need for IMO.

Edited July 19, 2017 by alan_fincher

[Athy]

Whilst we're on horsepower, can anyone put me right on this point? Old engines' outputs are, as far as I know, measured in "horsepower", e.g. I think a JP2 produces about 25 h.p. and a 2LW about 28. Even the still sort-of-in-production DM2 has a similar rating. But these are obviously not the same sort of horsepower as, for example, the 25 quoted for the 1100 c.c. Isuzu. Is the latter a measure of brake horsepower, as it is for modern cars? If so, when did the transition take place, i.e. people started rating boat engines in b.h.p.? I wondered this when Alan mentioned his 30 h.p. B.M.C. engine, which, if I'm correct, is a power unit whose origins go back to the 1950s.

[mross]

Some horsepowers are calculated, but most are measured on a brake dynamometer.  The old Citroen 2CV was 2 french horsepower but I think this was based on cylinder bore.  Citroen went on to produce long-stroke engines as these were more 'tax efficient'.

[MtB]

33 minutes ago, Athy said:

Whilst we're on horsepower, can anyone put me right on this point? Old engines' outputs are, as far as I know, measured in "horsepower", e.g. I think a JP2 produces about 25 h.p. and a 2LW about 28. Even the still sort-of-in-production DM2 has a similar rating. But these are obviously not the same sort of horsepower as, for example, the 25 quoted for the 1100 c.c. Isuzu. Is the latter a measure of brake horsepower, as it is for modern cars? If so, when did the transition take place, i.e. people started rating boat engines in b.h.p.? I wondered this when Alan mentioned his 30 h.p. B.M.C. engine, which, if I'm correct, is a power unit whose origins go back to the 1950s.

 

HP and BHP are the same, sort of. BHP is a sub-sype of HP. As mross says, BHP is a historic term telling you the type of device used to measure the actual output of the engine. In this case a brake. 

A brake is a wheel of known thermal mass with a band around it to create some resistance to it turning. The engine turns the brake and the brake band tightness is adjusted so the engine speed is constant. Then this is run for a measured amount of time, and the temperature of the brake wheel is measured at the beginning and end of the test. The HP of the engine can then be calculated from change in temperature together with the mass of the wheel and the length of the test. The output calculated in this way is BHP.

Another way to calculate the HP of the engine is to measure the consumption rate of the fuel over the period of the test, and use the specific energy of the fuel for the calculation. This actually tells you the HP input of the engine rather than the output, so frictional losses inside the engine are not taken into account. Hence the BHP method is regarded as more useful amongst petrol heads, as their prime concern is absolute power output, rather than niceties such as efficiency.

Nowadays engine power output is stated in kW, or some unit called a "ps", which I think is the same as a BHP.

Edited July 19, 2017 by Mike the Boilerman

[Athy]

27 minutes ago, mross said:

Some horsepowers are calculated, but most are measured on a brake dynamometer.  The old Citroen 2CV was 2 french horsepower but I think this was based on cylinder bore.  Citroen went on to produce long-stroke engines as these were more 'tax efficient'.

Thanks. So would those old Nationals, Listers, Gardners etc. have had their power measured by this dynamometer?

The French horsepower were fiscal, as you suggest, like the old R.A.C. Horsepower: something like 1 cv per 250 cc (the 2CVs made in the '50s were 425 cc, later ones had a "big" 602cc engine and in fact were taxed as 3cv even though they were still marketed under the original name because everyone knew what a "Deux Chevaux" looked like.) Actually, some of them may have been badged as "2CV6", just to confuse the enemy.

6 minutes ago, Mike the Boilerman said:

 

HP and BHP are the same, sort of. BHP is a sub-sype of HP. As mross says, BHP is a historic term telling you the type of device used to measure the actual output of the engine. In this case a brake. 

A brake is a wheel of known thermal mass with a band around it to create some resistance to it turning. The engine turns the brake for a measured amount of time, and the temperature of the brake wheel is measured at the beginning and end of the test. The HP of the engine can then be calculated from change in temperature together with the mass of the wheel and the length of the test. The output calculated in this way is BHP.

Another way to calculate the HP of the engine is to measure the consumption rate of the fuel over the period of the test, and use the specific energy of the fuel for the calculation. This actually tells you the HP input of the engine rather than the output, so frictional losses inside the engine are not taken into account. Hence the BHP method is regarded as more useful amongst petrol heads, as their prime concern is absolute power output, rather than niceties such as efficiency.

Cor, thanks. I never knew that temperature came into the calculation.

But that still doesn't explain (to me at any rate) why, for example, my 2LW is rated at 28 (b.) h.p. when it is (to me at any rate) obviously far more powerful than a modern buzzy 28 engine. Perhaps another factor, such as torque, comes into the equation?