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Does my prop look fine in this?


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2 hours ago, IanD said:

If you look at the recommendations for marine propellers you'll find the following (propeller diameter=D):

 

http://www.ricepropulsion.com/TNLS/Clearances.htm

 

Minimum space to deadwood (end of swim) = 0.27D -- about 4" for a 16" prop. A bigger gap than this is even better to get good water flow into the prop

Minimum space to rudder = 0.1D = 1.6" -- but should be at least 2" to reduce risk of jamming with debris. Too big a gap can reduce rudder effectiveness

 

These recommendations are for slow boats like trawlers not power boats, so narrowboats should be similar.

 

If the shaft bearing plus gap to propeller puts the prop closer to the hull than the 0.27D recommendation then you get more prop noise and lower thrust. Note that this assumes the end of the swim is pointed not blunt, if it's blunt then the gap needs to be bigger, add approximately the width of the blunt end to the minimum gap (e.g. 1" width of blunt end, minimum gap = 5" for a 16" prop) -- if you want to be exact, extend the sloping sides of the hull backwards until they meet, and take the 0.27D from here.

 

Looking at the OPs photo, the gap to the (very blunt-ended!) hull is much smaller than recommended, and the gap to the prop (can't see this) is much bigger -- just like a lot of narrowboats because it's easier, you don't need a protruding stern tube to support the shaft.

 

Probably yet another reason they don't slip through the water as well as they could and use more fuel, just like all the other terrible design practices that some boatbuilders use to churn out hulls which swim like a brick...

 

I think there's something in this, you see so many narrowboats with the prop slammed right up against the swim but if you observe "proper" boats there is always a fair distance.

 

Peter Nichols sadly no longer builds narrowboats but I'm sure his boats have a distinctive stern tube arrangement that sets the prop well clear of the swim, and Nichols boats do have a reputation for cutting through the water.  I seem to remember looking at an old Water Travel boat that also had a fair amount of stern tube extending the prop well clear of the swim. 

 

But how many narrowboat buyers today could care less about what is going on below the waterline. 

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13 minutes ago, Neil2 said:

 

I think there's something in this, you see so many narrowboats with the prop slammed right up against the swim but if you observe "proper" boats there is always a fair distance.

 

Peter Nichols sadly no longer builds narrowboats but I'm sure his boats have a distinctive stern tube arrangement that sets the prop well clear of the swim, and Nichols boats do have a reputation for cutting through the water.  I seem to remember looking at an old Water Travel boat that also had a fair amount of stern tube extending the prop well clear of the swim. 

 

But how many narrowboat buyers today could care less about what is going on below the waterline. 

Me. Extended stern tube to get the prop away from the swim. 9' swim at the stern, 7' at the bow.

 

Modified 25% aspect ratio Schilling rudder with end-plates, designed to work up to +/-75 degrees deflection, at which point there's no forward thrust at all, it just pushes the stern sideways like a thruster.

 

Will probably cost a grand or so to get fabricated though, which is probably why most people wouldn't bother...

GB2136374A.pdf

Edited by IanD
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22 hours ago, Neil2 said:

I'm curious - is there anyone who is 100% happy with their propeller?   Don't we all cruise around thinking, maybe a bit bigger, maybe a bit smaller...  

 

I remember meeting a guy who had just had his BMC 1.5 reconditioned, and he was moaning because it was smoking when it didn't before.  Then it turns out he had also changed the prop from a 17 to a 19 because "I wanted it to sound like a trad".  He just assumed the "new" engine would have more power.   

Yes, I am.  Crowther high performance prop, it has a wider blade area. could not be better in my opinion. Sad that they have quit.

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23 hours ago, Neil2 said:

I'm curious - is there anyone who is 100% happy with their propeller? 


I went from 50% to 90% happy this weekend.
 

I switched a pair of 18x15 3 blade props for 17x11.5, skewed 4 blades, while changing the pulley ratio on my electric motors to spin at max 1100 rather than 800rpm. 
 

The reduction in vibration is more than I dreamed possible - a combination of less energy per blade passing the hull, rudder etc, and a bit of extra clearance. 
 

Next project, rudders! 
 

 

 

E320487E-5CF0-415F-AE73-CC80E557043E.jpeg

Edited by Thames Bhaji
Edited to make the prop specs the right way round!
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FWIW Innisfree requires 17" dia. Best dia is fixed by clearance available, if pitch then is OK for engine power and prop rpm a wider blade is the way to go, standard blade area (BAR) is 55%.

More power requires more area, not more pitch, more area is obtained by bigger dia and/or wider blade if dia is fixed. Rarely is BAR considered important, most rely instead on pitch. 

With first engine - 2.4 litres 33bhp @ 2k revs 100 ft pounds @ 1650 rpm - Keith of Crowther props specified 17 x 14 x approx 75%  (BAR wasn't stamped on prop} he was spot on, and he didn't need to consult a formula he told me straight away! It pulled max revs of 1,950 - 2:1 gearbox.

Later LPW4S 1.750cc 40 bhp @3k revs 100 foot pounds @ 2k IIRC? with same box had 17 x 12 x 55% BAR and was slightly overpropped, 17 x 11, or maybe 11.5 would be ideal. 

 

 

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11 minutes ago, nb Innisfree said:

FWIW Innisfree requires 17" dia. Best dia is fixed by clearance available, if pitch then is OK for engine power and prop rpm a wider blade is the way to go, standard blade area (BAR) is 55%.

More power requires more area, not more pitch, more area is obtained by bigger dia and/or wider blade if dia is fixed. Rarely is BAR considered important, most rely instead on pitch. 

With first engine - 2.4 litres 33bhp @ 2k revs 100 ft pounds @ 1650 rpm - Keith of Crowther props specified 17 x 14 x approx 75%  (BAR wasn't stamped on prop} he was spot on, and he didn't need to consult a formula he told me straight away! It pulled max revs of 1,950 - 2:1 gearbox.

Later LPW4S 1.750cc 40 bhp @3k revs 100 foot pounds @ 2k IIRC? with same box had 17 x 12 x 55% BAR and was slightly overpropped, 17 x 11, or maybe 11.5 would be ideal. 

 

 

Instead of a bigger BAR 3-blade you can also go to a 4-blade (usually about 75%-80% BAR as standard), which can also reduce prop noise.

 

Nothing wrong with bigger pitch, but if you go too far the efficiency drops -- normally want to keep diameter/pitch between 0.6 and 0.8 for slow boats like narrowboats, anything above 1.0 should be avoided unless there's no option (e.g. slow-speed direct drive -- and even then more blades (e.g. 5) and/or higher BAR (e.g. 100%) is usually better. It's what ships do when they can't get big enough diameter props in, especially heavily-loaded single-screw ones like container ships, some of their props are truly works of art...

 

https://www.wartsila.com/marine/build/propulsors-and-gears/propellers/wartsila-fixed-pitch-propellers

Edited by IanD
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I'm afraid all this teccy talk goes over my head, but my set up is a 62ft Colecraft shell with a BMC 1.8 engine (38bhp). It had a 16/12 prop, and revs of 1800rpm were needed to maintain 3mph even though the engine didn’t sound as if it was working any harder than other boats I’d driven (hire and shared) which used far less revs for the same speeds.

 

When out of the water, several boatyards had said the 16 inch prop looked a bit small, like an egg whisk they joked. Yet Colecraft confirmed it was the original prop they fitted in 1996, and that it was suitable for the boat.

 

But the engine was sometimes overheating on rivers so in 2016 I changed to an 18/12 prop with the belief that it would help push the boat along better and require less revs, especially in deeper waters. It made no difference to the overheating (eventually solved by increasing the skin tank size). But 5 years later and I still have that larger prop and it is definitely better in rivers and deep water, but on shallow canals I have to be careful about creating wash.

 

I used to worry about the ‘higher than normal’ revs but I don’t anymore. Having had 40+ years boating experience the engine sounds fine to me. It uses hardly any oil, and it isn’t smoky. I fear I’m tempting fate here, but having done nearly 13,000 hours including a lot of hard work in tidal waters such as the Trent, Ribble/Douglas, Thames Tideway, and the Mersey, it’s showing no signs of having suffered as a result of having 5 years with the larger than recommended prop, or due to the high revs that it’s always had.

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4 hours ago, IanD said:

Modified 25% aspect ratio Schilling rudder with end-plates, designed to work up to +/-75 degrees deflection, at which point there's no forward thrust at all, it just pushes the stern sideways like a thruster.


I’m considering something similar and am curious - if you have a 25% aspect ratio, does that mean it’s length is 4 times it’s height? I assume not, but either way would be interested in a photo if you have one? 

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28 minutes ago, Grassman said:

 

 

 

 

 

 

I'm afraid all this teccy talk goes over my head, but my set up is a 62ft Colecraft shell with a BMC 1.8 engine (38bhp). It had a 16/12 prop, and revs of 1800rpm were needed to maintain 3mph even though the engine didn’t sound as if it was working any harder than other boats I’d driven (hire and shared) which used far less revs for the same speeds.

 

When out of the water, several boatyards had said the 16 inch prop looked a bit small, like an egg whisk they joked. Yet Colecraft confirmed it was the original prop they fitted in 1996, and that it was suitable for the boat.

 

But the engine was sometimes overheating on rivers so in 2016 I changed to an 18/12 prop with the belief that it would help push the boat along better and require less revs, especially in deeper waters. It made no difference to the overheating (eventually solved by increasing the skin tank size). But 5 years later and I still have that larger prop and it is definitely better in rivers and deep water, but on shallow canals I have to be careful about creating wash.

 

I used to worry about the ‘higher than normal’ revs but I don’t anymore. Having had 40+ years boating experience the engine sounds fine to me. It uses hardly any oil, and it isn’t smoky. I fear I’m tempting fate here, but having done nearly 13,000 hours including a lot of hard work in tidal waters such as the Trent, Ribble/Douglas, Thames Tideway, and the Mersey, it’s showing no signs of having suffered as a result of having 5 years with the larger than recommended prop, or due to the high revs that it’s always had.

 

I think Calcutt used to fit a 19" prop to their larger boats, but with a 3:1 ratio.  I had a 47 foot boat with the BMC 1.8 that had 17x12 which was ok but I would get black smoke if really hammering it on a river, so I can see why Colecraft would fit a smaller prop on a bigger boat, I did feel mine was overpropped once I got it into deep water.  The BMC is a high revving engine that does need to be worked quite hard and I suspect a lot were fitted with insufficient cooling.    

Edited by Neil2
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54 minutes ago, Thames Bhaji said:


I’m considering something similar and am curious - if you have a 25% aspect ratio, does that mean it’s length is 4 times it’s height? I assume not, but either way would be interested in a photo if you have one? 

My boat isn't due to be built until next year, so no photo. 25% aspect ratio means that if the rudder is 400mm long front to back (chord), maximum thickness will be 100mm (25% of chord), pivot is 100mm from the front -- you can see the shape in the patent I attached. Endplates are added at the top and bottom to stop the water flowing off the ends and increase lift -- this drawing shows a profile for a similar (not identical) MacLear Thistle rudder, the rudder shape is in blue and the endplate is orange, numbers are percentage of chord. Height between endplates should be a little bigger than propeller diameter, probably 450mm in my case.

 

Looks very strange compared to a typical narrowboat rudder (very thick, wide endplates) but tests showed that it could make a ship pivot almost on the spot with no forward movement at all; the one in the patent is optimised for low-speed manoeuvrability and working up to very large angles e.g. 75 degrees, flat-plate rudders stall at less than 45 degrees.

rudder_profile.png

Edited by IanD
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