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billh

Prop size for 2DM?

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Can anyone give  me a definitive  correct diameter and pitch for the (left -hand)  prop? Details: National 2DM,1000rpm max  , Brunton's box, 2:1  reduction, 70ft narrowboat. Thanks in advance for any info.

 

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I have transcripts of the Cardex system for the 'British Waterways' South Eastern Division fleet and the details are given as:

 

National 2DM 18½ @ 1000, Brunton 2:1 reverse/reduction gearbox, 3 blade left hand propeller 24’’ diameter x 17 pitch

 

:captain:

 

 

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Going on memory, Denmark, although only 40’, had a PRM150 with a 22x20 prop. Don’t know if that’s any use to you, but it’s a start!!

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8 hours ago, billh said:

Can anyone give  me a definitive  correct diameter and pitch for the (left -hand)  prop? Details: National 2DM,1000rpm max  , Brunton's box, 2:1  reduction, 70ft narrowboat. Thanks in advance for any info.

 

Is this a modern fabricated hull or a historic? How much space is there between the top of the skeg and the uxter plate on this hull?

 

If enough space to accommodate a 24" blade then I'd say 24x17LH as suggested by Pete's BW records sounds just right. 

 

 

Edited by Mike the Boilerman
speeling

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Persia had a Crowther 24x20 SC on 55 ft which I challenged when we bought her and was assured it was right she draws 30” stationary

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6 hours ago, Mike the Boilerman said:

 

Is this a modern fabricated hull or a historic? How much space is there between the top of the skeg and the uxter plate on this hull?

 

If enough space to accommodate a 24" blade then I'd say 24x17LH as suggested by Pete's BW records sounds just right. 

 

 

It's an historic. The uxter "plate " is wood and there's 30" to the skeg. Now I'm really puzzled, I was expecting the recommended prop pitch to be more than the existing 21" not less! As it is , although the prop can be described as adequate, the engine never seems loaded up, even at full gas,I would have expected some black smoke  at that point. So the 24"diameter sounds right, but reducing pitch to  17" sounds like a bad move? There's no sucking air in once she's on the move and the  clutch is not slipping. Perhaps we need to check the prop is actually  the claimed 24x21?

Thanks  to all for information so far.

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18 minutes ago, billh said:

It's an historic. The uxter "plate " is wood and there's 30" to the skeg. Now I'm really puzzled, I was expecting the recommended prop pitch to be more than the existing 21" not less! As it is , although the prop can be described as adequate, the engine never seems loaded up, even at full gas,I would have expected some black smoke  at that point. So the 24"diameter sounds right, but reducing pitch to  17" sounds like a bad move? There's no sucking air in once she's on the move and the  clutch is not slipping. Perhaps we need to check the prop is actually  the claimed 24x21?

Thanks  to all for information so far.

Presumably the boat in question is DAPHNE, which is fitted with an exG.U.C.C.Co. Ltd. National 2DM :captain:

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1 hour ago, billh said:

It's an historic. The uxter "plate " is wood and there's 30" to the skeg. Now I'm really puzzled, I was expecting the recommended prop pitch to be more than the existing 21" not less! As it is , although the prop can be described as adequate, the engine never seems loaded up, even at full gas,I would have expected some black smoke  at that point. So the 24"diameter sounds right, but reducing pitch to  17" sounds like a bad move?

Although I'm sure I have seen Pete quote it before, I'm staggered that it is suggested that the boats were originally (or at some stage) fitted with 24 x 17 for a National (or presumably an RN)

 

That sounds far, far too small to me for a 2:1 box.

 

Flamingo came with a Brunton's propeller that was 25 1/4" diameter that was estimated to be about 17" pitch, (so larger overall than the suggestion here), and was woefully under-propped - that on a Lister HA2 with a 3:1 box.

 

As the Lister runs at 1,800 max, and the National at only 1000, I'd expect the National to possibly need the larger prop of the two - not smaller.

Flamingo's prop was re-pitched to 25 1/4" x 21", and could still go larger - it is certainly not overloading the engine.  So I wouldn't want anything smaller on your set up.

 

Putting the numbers for a 70 foot boat on 3 foot draught into the Vicprop calculator, and using the numbers for your engine and gearbox gives me 27 x 25, (yep that sounds massive, but that's what it says).  Why on earth you would want as little as 24 x 17 I can't imagine - it makes no sense to me, and  I can't see that stopping at all in an emergency.

Edited by alan_fincher

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5 minutes ago, canalboat said:

Unfortunately its a bit late for you to tell H&W that they were wrong.

I don't need to - I have tried operating a similarly sized boat with a larger prop, (and a 20% faster shaft speed) and a similar horsepower engine, and it was a nightmare.

A 24 x 17 prop will not stop a 70 foot working narrow boat in a hurry with a National 2 pot on 2:1 - I'm absolutely convinced.

I am in complete agreement with Bill that I would expect the recommendation to have been to increase the pitch on a current 24 x 21, (and ideally increase the diameter a fair bit too)

Why in the name of heaven would you willingly reduce pitch if it is not a great performer now, and the engine is demonstrably not over-loaded.

Maybe H&W (who had no experience in designing such boats, nor probably of the "modern" engines being installed) came up with that number, but can it be established that the boats ran for years with propellers of that size - I simply struggle to believe it would not quickly have been found to be totally undersized.

 

 

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24 x 17 sounds about right looking at the ex national blade that was fitted to Banstead when we got her, the GU boats were all towing a butty which does affect the size of the blade slightly, they would also have not followed the modern practice of overpropping to make a nice sound for hours of running at low speeds. it would be interesting to see how the original blade works with a loaded pair, if you can find enough water!

 

Tom

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1 hour ago, billh said:

It's an historic. The uxter "plate " is wood and there's 30" to the skeg. Now I'm really puzzled, I was expecting the recommended prop pitch to be more than the existing 21" not less!

 

What diameter is the original prop? 

 

This is just as important as (and probably more than) the pitch. 

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2 minutes ago, Mike the Boilerman said:

 

What diameter is the original prop? 

 

This is just as important as (and probably more than) the pitch. 

 

24", according to this......

 

1 hour ago, billh said:

Perhaps we need to check the prop is actually  the claimed 24x21?

 

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I wonder if the prop was specified slightly undersize to allow the engine to work harder? Many say a diesel engine is kept in better shape if under load most of the time. 

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5 minutes ago, BWM said:

I wonder if the prop was specified slightly undersize to allow the engine to work harder?

 

That would make an engine work less hard, not more.

 

A better reason for under-sizing a prop is to keep the boat down to a sensible speed at tickover.  

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but that would not have been anything to worry about for the GUCCCo as london would not have been full of fat boats tied up for miles with people living on them as an alternative to housing, and any boats would have been tied up properly.

I believe the main reason for the slightly smaller blade is that they were always towing.

Tom

 

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53 minutes ago, Mike the Boilerman said:

 

So what power is your Lister at 1,800rpm, compared to the National at 1000rpm?

Lister circa 22 HP, National (it says) 18.5 HP

 

Not a huge difference.

When I had to work out what to do with "Flamingo" the Vicprop calculator came surprisingly close to the recommendations of others, and the prop I now have is more or less what it said.

 

In the scenario described here it comes up with 27" x 25" (rounded down to neraest whole number).

That's so far different from 24" x 17" as to be ridiculous, (in my view!)

 

 

.

 

 

Data Input

Waterline length in feet: 70 feet
Beam at the waterline in feet: 7 feet
Hull draft in feet (excluding keel): 3 feet
Vessel weight in pounds: 45000 lbs
Engine Horsepower: 18.5 HP
Number of engines: 1
Total Engine Horsepower: 18.5 HP
 
Engine R.P.M. (max): 1000 RPM
Gear Ratio: 2:1
Shaft R.P.M. (max): 500 RPM
 
Number of shaft bearings (per shaft): 2
Desired speed in Knots: 5 knots

Horsepower Calculations

This will calculate the maximum horsepower and torque available at the prop(s).
 
Total available horsepower at the engine(s): 18.5 HP
Total available torque ft/lbs at the engine(s): 97 ft/lbs
Horsepower loss of 3% per gearbox: - 0.6 HP
Horsepower loss of 1.5% per shaft bearing: - 0.6 HP
 
Total horsepower available at the propeller(s): 17.4 HP
Total torque ft/lbs available at the propeller(s): 183 ft/lbs

Speed & Power Calculations

Basic displacement speed and horsepower required
Displacement hull speed (1.34 X sqrt of waterline length): 11.21 Knots
Minimum horsepower required at propeller(s) for Hull speed: 98.2 HP
 
Calculations based on desired speed and available HP
HP required at propeller(s) for desired 5 knots speed: 8 HP
Estimated maximum speed with existing 18.5 horsepower:
This is the speed we will use for the propeller size.
6.47 Knots
 
At this point it is important to note that all of the calculations above are based on full RPM and HP. Most engines are rated to run at a percentage of thier full RPM. This is what will determine your maximum cruising speed. The propeller sizing calculations below are based on 90% of full RPM. This gives the engine some reserve power to allow for variable loading in the vessel.
 

Propeller Size

Number of blades Diameter (inches)   Pitch (inches)
2 Blade 28.4 X 25.4
3 Blade 27.1 X 25.2
4 Blade 25.5 X 24.7
 
The propeller sizes shown above do not contain calculations for cavitation or blade loading.
If you find that the recommended propeller is too large to fit your vessel, you can try increasing the shaft speed. Failing this, you can reduce the diameter and increase the pitch at the expense of your propeller efficiency. The rule of thumb is 1 inch of diameter is equal to 1 1/2 to 2 inches of pitch.
44 minutes ago, BWM said:

I wonder if the prop was specified slightly undersize to allow the engine to work harder? Many say a diesel engine is kept in better shape if under load most of the time. 

That's the opposite of what will happen.  Over-proppong puts the engine under additional load. not under-propping.

Edited by alan_fincher

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1 hour ago, alan_fincher said:

Lister circa 22 HP, National (it says) 18.5 HP

 

Not a huge difference.

When I had to work out what to do with "Flamingo" the Vicprop calculator came surprisingly close to the recommendations of others, and the prop I now have is more or less what it said.

 

In the scenario described here it comes up with 27" x 25" (rounded down to neraest whole number).

That's so far different from 24" x 17" as to be ridiculous, (in my view!)

 

 

.

 

 

Data Input

Waterline length in feet: 70 feet
Beam at the waterline in feet: 7 feet
Hull draft in feet (excluding keel): 3 feet
Vessel weight in pounds: 45000 lbs
Engine Horsepower: 18.5 HP
Number of engines: 1
Total Engine Horsepower: 18.5 HP
 
Engine R.P.M. (max): 1000 RPM
Gear Ratio: 2:1
Shaft R.P.M. (max): 500 RPM
 
Number of shaft bearings (per shaft): 2
Desired speed in Knots: 5 knots

Horsepower Calculations

This will calculate the maximum horsepower and torque available at the prop(s).
 
Total available horsepower at the engine(s): 18.5 HP
Total available torque ft/lbs at the engine(s): 97 ft/lbs
Horsepower loss of 3% per gearbox: - 0.6 HP
Horsepower loss of 1.5% per shaft bearing: - 0.6 HP
 
Total horsepower available at the propeller(s): 17.4 HP
Total torque ft/lbs available at the propeller(s): 183 ft/lbs

Speed & Power Calculations

Basic displacement speed and horsepower required
Displacement hull speed (1.34 X sqrt of waterline length): 11.21 Knots
Minimum horsepower required at propeller(s) for Hull speed: 98.2 HP
 
Calculations based on desired speed and available HP
HP required at propeller(s) for desired 5 knots speed: 8 HP
Estimated maximum speed with existing 18.5 horsepower:
This is the speed we will use for the propeller size.
6.47 Knots
 
At this point it is important to note that all of the calculations above are based on full RPM and HP. Most engines are rated to run at a percentage of thier full RPM. This is what will determine your maximum cruising speed. The propeller sizing calculations below are based on 90% of full RPM. This gives the engine some reserve power to allow for variable loading in the vessel.
 

Propeller Size

Number of blades Diameter (inches)   Pitch (inches)
2 Blade 28.4 X 25.4
3 Blade 27.1 X 25.2
4 Blade 25.5 X 24.7
 
The propeller sizes shown above do not contain calculations for cavitation or blade loading.
If you find that the recommended propeller is too large to fit your vessel, you can try increasing the shaft speed. Failing this, you can reduce the diameter and increase the pitch at the expense of your propeller efficiency. The rule of thumb is 1 inch of diameter is equal to 1 1/2 to 2 inches of pitch.

That's the opposite of what will happen.  Over-proppong puts the engine under additional load. not under-propping.

 

2 hours ago, Mike the Boilerman said:

 

That would make an engine work less hard, not more.

 

A better reason for under-sizing a prop is to keep the boat down to a sensible speed at tickover.  

Only in the sense that it would bog down before reaching the top end of the rev range, not a useful situation when loaded and towing. In the same way that powerful, torquey engines in trucks and four wheel drive vehicles are geared down. 

Edited by BWM

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Now then

Battersea is running a National with Bruntons 2/1 box and a 27" x 17"

Loaded and towing a 16" pitch was recommended to me, my experience is that with deep water we could pull more pitch however this one is enough at times when running shallow bits of the cut along with modern alternators.

I would recommend as big a prop (Diameter) as is safe to fit as you get more grip and stopping power, be very cautious of trading smaller diameter for more pitch cos you will get more prop walk. you are welcome to come and try ours or get together to compare, feel free to get in touch.

Bob Tidy

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Thank you all for your useful and diverse replies. I now have two more questions:

Does anyone still run a GU motor ,National with the original 24x17, what is the performance like ?

Why is a lesser pitch desirable for towing? I have some experience towing with another motor (Kelvin engine,no reduction ,221/2x ?) and that is just as capable as when not towing. The Kelvin has a much better "grip" of the water than the National and stopping in  reverse is dynamic ( and quick)!

We have done the Vic Prop calculation and that comes close to Alan F's , 27x25. This also corresponds  also  with our local sterngear professional's (Crowthers) calc. The 27" might be a bit too big, reduce to 26'' might be ok.

There's no panic about this  potentially expensive modification , as I  said, the performance as is, is adequate.

I did wonder if there had been a clerical error at H&W , later perpetuated by British Waterways over the size of the prop, but that seems unlikely.

Thanks again

Bill

 

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On ‎05‎/‎07‎/‎2018 at 23:24, pete harrison said:

I have transcripts of the Cardex system for the 'British Waterways' South Eastern Division fleet and the details are given as:

 

National 2DM 18½ @ 1000, Brunton 2:1 reverse/reduction gearbox, 3 blade left hand propeller 24’’ diameter x 17 pitch

 

:captain:

 

 

I'm not disputing that's what may have been fitted, but there were many boaters such as my old mate Bob who were forever putting their boat on the cill and giving the blade a bit more lift - he would have done it to mine given half a chance!

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On 08/07/2018 at 22:46, Bob Tidy said:

Now then

Battersea is running a National with Bruntons 2/1 box and a 27" x 17"

Loaded and towing a 16" pitch was recommended to me, my experience is that with deep water we could pull more pitch however this one is enough at times when running shallow bits of the cut along with modern alternators.

I would recommend as big a prop (Diameter) as is safe to fit as you get more grip and stopping power, be very cautious of trading smaller diameter for more pitch cos you will get more prop walk. you are welcome to come and try ours or get together to compare, feel free to get in touch.

Bob Tidy

same on towy is a 27inch prop

when I spoke to a few blade places the newerboats they were putting 24inch props on. ours just fits under the uxter plate by an inch at each point might have a pic somewhere

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On 09/07/2018 at 11:20, billh said:

Thank you all for your useful and diverse replies. I now have two more questions:

Does anyone still run a GU motor ,National with the original 24x17, what is the performance like ?

Why is a lesser pitch desirable for towing? I have some experience towing with another motor (Kelvin engine,no reduction ,221/2x ?) and that is just as capable as when not towing. The Kelvin has a much better "grip" of the water than the National and stopping in  reverse is dynamic ( and quick)!

We have done the Vic Prop calculation and that comes close to Alan F's , 27x25. This also corresponds  also  with our local sterngear professional's (Crowthers) calc. The 27" might be a bit too big, reduce to 26'' might be ok.

There's no panic about this  potentially expensive modification , as I  said, the performance as is, is adequate.

I did wonder if there had been a clerical error at H&W , later perpetuated by British Waterways over the size of the prop, but that seems unlikely.

Thanks again

Bill

 

towy stops dead i can go full speed and throw reverse and will stop very quick. my Tickover in forwards is a tad quick but i love it. your happy to test it any time although it will drain the cut quite happly

Edited by billybobbooth

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