Need new prop what make and size ?

[Ex Brummie]

3 hours ago, RufusR said:

Never been on a river yet unfortunately awaiting my retirement to be able to go out more that 2 weeks to get to and cruise on river though I guess I could make the nene if I wanted . It seems to rev pretty freely though when pushed to emergency stop etc 

When using 'emergency stop' you will only be 1:1 from the gearbox. The engine will find at 2(.09):1 it is harder to turn the prop. Think of the gears on a bike and the effort needed to pedal.

[alan_fincher]

1 hour ago, Ex Brummie said:

When using 'emergency stop' you will only be 1:1 from the gearbox.

Can you explain this please?

Are you saying the reduction ratio is different in forwards and reverse?

As the manual states the gearbox can be run in either direction, to suit LH or RH props, that surely can't be the case, can it?

[Ex Brummie]

My PRM 150 is 2:1 forwards, but 1:1 in reverse.

[mad dog n' Englishman]

Best to gather as much info whilst out of water re existing prop and clearance, ring Crowthers, answer their few simple questions and let them give the best advice. Be ready for a small shock, My 22x22 prop is going to be 6 weeks in the making and £900 to pay. Sure it will be worth it tho.

Garry.

[alan_fincher]

17 hours ago, Ex Brummie said:

My PRM 150 is 2:1 forwards, but 1:1 in reverse.

But that simply doesn't square with the statement that you can use an idenical box whether you have a LH or RH prop?

Anyway, why would they do this, it makes no sense to me at all?

Is yours some kind of "special"perhaps?

[cuthound]

3 minutes ago, alan_fincher said:

But that simply doesn't square with the statement that you can use an idenical box whether you have a LH or RH prop?

Anyway, why would they do this, it makes no sense to me at all?

Is yours some kind of "special"perhaps?

Perhaps he is referring to the "mechanical lock" device, to be used as an emergency fix in the case of hydraulic failure, mentioned in the last paragraph below?

It doesn't alter the ratio of the gearbox though.

Extract from PRM150 website.

"PRM150

Full hydraulic operation, lightweight, compact & rugged
 

The PRM150 marine gearbox is purpose built for use with both pleasure craft and commercial boats; its twin countershaft design provides separate oil-operated multi-disc clutches (which need no adjustment) for ahead or astern drive allowing full rated power to be transmitted continuously in either direction.

 

The reduction ratios offered (1.53:1. 2.09:1 and 2.82:1) are all available for left-hand or right-hand propeller rotation in "ahead", making PRM150 particularly well suited to twin engine installations.

The heavy duty aluminium alloy gearcase is internally ribbed for rigidity and strength and consists of two separate halves to facilitate servicing.

The hydraulic operating system functions on normal multigrade engine oil, avoiding the need to use automatic transmission fluid, and ensures rapid response to movements of the operating lever for good boat handling. The operating lever has a positive neutral detent and is suitable for use with proprietary single lever remote control operating systems.

Robust and reliable, the hydraulic system is nevertheless provided with a mechanical lock up device for added security, so that in the unlikely event of a hydraulic failure the boat can be brought safely back to port. Access to this device is via the detachable manifold on the rear of the gearcase which also houses the hydraulic control valve."

[alan_fincher]

17 hours ago, mad dog n' Englishman said:

Best to gather as much info whilst out of water re existing prop and clearance, ring Crowthers, answer their few simple questions and let them give the best advice. Be ready for a small shock, My 22x22 prop is going to be 6 weeks in the making and £900 to pay. Sure it will be worth it tho.

Garry.

When it was suggested we might need a new prop on Flamingo, Crowther's quoted a large 4 figure sum, and at least a 6 week lead time.

We were put on to FAL Propeller Services in Scotland, and a prop the same in both diameter and pitch was about 2/3 the price, and available almost immediately.  Quoted as "heavy duty", I have no idea though  how it might have compared with a Crowther one.

In practice it didn't come to that.  Crowther had said our existing prop could not be re-pitched, after T Norris had failed an attempt at it, but FAL Propeller Services said it definitely could.  Within a few days I had the prop back, refurbished and changed from about 15" pitch to 21" pitch.  Cost was, (from memory) about 1/5 of what Crowther's wanted for a new one.

I can't speak too highly of FAL Propeller Services, and would urge anybody who is told by Crowthers or Norris that something can't be done. to check with them as well.  I suspect someone geared up to dealing with the much bigger props on some fishing vessels has the ability to take on tasks that those used to dealing with narrow boat props can't always do.  Even if you need a new one, you may well save a lot of money.

 

Edited September 6, 2017 by alan_fincher

[Tony Brooks]

12 minutes ago, alan_fincher said:

But that simply doesn't square with the statement that you can use an idenical box whether you have a LH or RH prop?

Anyway, why would they do this, it makes no sense to me at all?

Is yours some kind of "special"perhaps?

I think there will be a SLIGHT ratio difference between ahead and astern because one will need an idler gear fitted between the two main gears so slight is the word. maybe 2:1 and 1.95:1

Edited September 6, 2017 by Tony Brooks

[Ex Brummie]

1 hour ago, Tony Brooks said:

I think there will be a SLIGHT ratio difference between ahead and astern because one will need an idler gear fitted between the two main gears so slight is the word. maybe 2:1 and 1.95:1

There is certainly a greater difference than .05 : 1 when I select reverse. I note that the Newage literature quotes the ratios in the "ahead" mode, and also warns about using reverse mode 'in emergency only' as it applies heavier stresses on the box. It also quotes that when using the emergency fixed drive, that it is only available in the 'ahead' mode, so presumably, although using the same box, it must be set up for different rotations.

[blackrose]

On 05/09/2017 at 20:29, Ex Brummie said:

My PRM 150 is 2:1 forwards, but 1:1 in reverse.

Really? I didn't realise that!

[blackrose]

Sorry to hijack the thread but when I had my prop re-pitched at Norris, the 1.5" reduction in pitch and 1" reduction in diameter only achieved an extra 225 rpm. The result in terms of improving the boat's performance was excellent, I've got a lot more power now, but the increase in revs was less than expected and the 3000 rpm engine still only revs to a maximum of 2225 in gear in deep open water. The original 3 blade R/H prop specs were 19" x 13" (revving to 1950 rpm) and the prop is now 18" x 11.5", but I probably should have reduced the pitch to 11" or even 10.5".

Norris told me that they could reduce the pitch further if required, but warned me about making it "too flat" as it would lack "bite". My (linear) calculations suggest that another 1" of pitch reduction (to 18" x 10.5") would only give me another 183 rpm taking the maximum engine revs to about 2400 rpm.

PropCalc is saying I should go down to 10" pitch but I would rather stay slightly over-propped rather than under. So my question: is it worth reducing the pitch by another inch to gain an extra 183 rpm?

Data Input

Waterline length in feet:	55 feet
Beam at the waterline in feet:	12 feet
Hull draft in feet (excluding keel):	2 feet
Vessel weight in pounds:	64000 lbs
Engine Horsepower:	55 HP
Number of engines:	1
Total Engine Horsepower:	55 HP
Engine R.P.M. (max):	3000 RPM
Gear Ratio:	2.09:1
Shaft R.P.M. (max):	1435 RPM
Number of shaft bearings (per shaft):	1
Desired speed in Knots:	6 knots

Horsepower Calculations

This will calculate the maximum horsepower and torque available at the prop(s).
Total available horsepower at the engine(s):	55 HP
Total available torque ft/lbs at the engine(s):	96 ft/lbs
Horsepower loss of 3% per gearbox:	- 1.6 HP
Horsepower loss of 1.5% per shaft bearing:	- 0.8 HP
Total horsepower available at the propeller(s):	52.5 HP
Total torque ft/lbs available at the propeller(s):	192 ft/lbs

Speed & Power Calculations

Basic displacement speed and horsepower required
Displacement hull speed (1.34 X sqrt of waterline length):	9.94 Knots
Minimum horsepower required at propeller(s) for Hull speed:	139.6 HP
Calculations based on desired speed and available HP
HP required at propeller(s) for desired 6 knots speed:	28 HP
Estimated maximum speed with existing 55 horsepower: This is the speed we will use for the propeller size.	7.37 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	18.8	X	9.9
3 Blade	17.9	X	9.8
4 Blade	16.9	X	9.6
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.

Edited September 8, 2017 by blackrose

[nb Innisfree]

My experience for OP's info:

Innisfree is a standard 60' cruiser stern narrowboat which permitted a comfortable max diam prop of 17". Original engine was 2.5 litres producing 33 bhp @2000rpm. Crowther prop for this engine was 17 x 14 x 75% blade area, this proved to be spot on, forward and reverse, very impressive in fact.

When the engine was replaced with a Lister Petter LPWS, 1.85 litres producing 40 bhp @3000rpm I opted for Crowther reducing pitch and blade area to 17 x 12 x approx 60-70% BAR. Keith at Crowthers predicted it would be a bit over propped which proved correct, however it cavitated badly in reverse. At the earliest opportunity when dry docked for reblacking I replaced it with a standard prop 17 x 12 x approx 55% BAR, this worked out well but still a bit over propped achieving approx 2700rpm with large alternator at max output (approx 8-10 bhp) and approx 2900rpm with alt off load.

Ideally I reckon 17 x 11.5 X standard 55% BAR would be as good as it can get taking into consideration the large alternator. 

Edited September 8, 2017 by nb Innisfree

[Dalslandia]

What is an Horse Power? ( ok we all know it is an horse lifting 75 kg one meter in one second, or 33000 lb one feet in one minute ( 550 lb one feet in one sec.)) but how many of those does the Engine make, in real Life installed in the boat, if the stated Power is in SAE BHP it might be so that the installed Power is 80-90% of the stated.

ad a big alternator on a small Engine and the "losses" is even bigger, as Innisfree say.

 

this have to be in the calculations or in the mind whan calculating the propeller. just because the propeller don't like the sales talk from the Engine Company.

 

Edited September 8, 2017 by Dalslandia

[RufusR]

Sorry for late reply . Boat yard told me 18 x12 will be over propped particularly as we are running a large domestic alternator he recommended 16 x14 . Will keep taking advice got time before we dock the boat again

[alan_fincher]

15 minutes ago, RufusR said:

Sorry for late reply . Boat yard told me 18 x12 will be over propped particularly as we are running a large domestic alternator he recommended 16 x14 . Will keep taking advice got time before we dock the boat again

As common advice is that 1" of diameter is equivalent to somewhere like 1.5" of pitch, an 18" x 12" will be broadly equivalent to a "16" x 15".  That's not radically different from 16" x 14", and I think as a paper exercise it would be hard to say that the former is "over propped" whereas the latter is correct.

[MtB]

On 17/09/2017 at 15:09, RufusR said:

Sorry for late reply . Boat yard told me 18 x12 will be over propped particularly as we are running a large domestic alternator he recommended 16 x14 . Will keep taking advice got time before we dock the boat again

Like Alan, I was thinking the two sizes are near as dammit the same.

Does this yard have a 16x14 prop in stock they need to get rid of, just possibly?

Cynical? Moi?