Dalslandia Posted October 19, 2014 Report Share Posted October 19, 2014 (edited) I bought a copy of the Yachting Monthly tests, this was mainly looking at folding props butthey did include an axiom and a standard prop. The Axiom did well astern bollard pull 181kg against 173kg standard stopping in 7.7 against 9.3 seconds propwalk was good too at 9.3% against 13.9% However ahead max speed was 7.1 knots against 7.5 on the standard bollard pull 233kg against 264kg standard I have asked Axiom about this and they say that the competition was overpropped and if they had overpropped they would have done better ahead. So inconclusive here. Most of the information out there is either very technical or way to simplistic, I have just about got my head around propeller diameter and pitch sizing but propeller area calculations elude me. However I wonder if on our boats often the problem is not propeller area. As I understand it the area of the blades does not matter a lot until you get to the point where you are putting more power in than a given area can cope with and the prop cavitates. This would explain why 2 props of nominally same diameter and pitch perform differently if one has a lower area and fails to take the power? The prop suppliers I have asked to quote me have been unforthcoming on this point I suspect most have a limited range of prop sizes and sell what they have rather than what you need, Crowther have a good reputation but the term "high efficiency" does not seem very specific. This info say it all, we did know the Axiom was about as good both ways, the standard prop isn't far away in reverse. 7.1 to 7.5 knots sound like it's not a big difference, and it isn't in speed/time, but in thrust power and fuel saving, if the conventional was over propped, it probably didn't allowed the engine to develop full power either. and top speed and bollard pull would suffer. to increase speed from 7.1 to 7.5 takes 18% more thrust power (thrust HP) and if the tests was made on same boat/engine the difference is in propeller efficiency, it also means the standard prop will save up to 18% in fuel at the same speed. Edited October 19, 2014 by Dalslandia Link to comment Share on other sites More sharing options...
Dalslandia Posted October 19, 2014 Report Share Posted October 19, 2014 (edited) ISTR that you fitted an Axiom, Gibbo. Or am I misremembering? A better prop can't throw less water unless it throws it faster. The forward force comes from the rate of change of momentum of the water being thrown back by the propeller. N As you say, the thrust comes from displacing a mass of water, larger volume at a higher rate make more thrust, a propeller is just a tool to make that happen and can only make it more or less efficient, and they do. so no prop wash no thrust. A more efficient prop have less prop wash in relation to forward speed, it is the mass thrown back that make the thrust, so to make it efficient it should take as a big area and accelerating it as little as possible, but there also is a optimal diameter for the power and speed and rpm, and a practical diameter limited by strength, space and hull drumming/noise. with a fixed diameter, more "prop wash" make more thrust, more effect, but less efficiency (power in/power out) The pitch should be so that every blade radius work at its best L/D (lift / drag) but cavitating and acceleration/stopping demands decrease in the alpha (angle) and increase in blade area, (lower lift coefficient demand a larger area for same effect) so it takes a twist in the blade to make the whole propeller work at its optimum The Axiom label there prop with diameter and blade angle, so there (example) 18X20 is 18" diameter and blade have 20 degree angle, so not pitch in inch, boat props use the angle at 70% radii for nominal pitch, so that 18X20 is 14.4" In the airplane world, I often see the static thrust (bollard thrust) is talked about and tested, measured, we can design a propeller for high static thrust, bigger diameter, blade area, less pitch, but at cruise it make a poor propeller, airplanes that have (often) a high speed range and top speed, compered to boats, the speed very much change the optimal diameter on airplanes, faster plane make the diameter smaller. an airplane tied to a birch don't fly very well, and the efficiency is zero, so even on boats, if we prop the boat for high bollard thrust the forward speed and efficiency will suffer. but ok for a tug boat. Edited October 19, 2014 by Dalslandia 1 Link to comment Share on other sites More sharing options...
Neil Smith Posted October 19, 2014 Report Share Posted October 19, 2014 I paid £550 for my axiom 18" prop and was quoted £475 by crowther so not much difference really, just hope it lives up to all there claims. Neil Link to comment Share on other sites More sharing options...
carlt Posted October 19, 2014 Report Share Posted October 19, 2014 (edited) I paid £550 for my axiom 18" prop and was quoted £475 by crowther so not much difference really, just hope it lives up to all there claims. Neil I would have expected a new, unproven product to be significantly cheaper than a prop manufactured by a long established company acknowledged to be amongst the best in the business. Edited October 19, 2014 by carlt Link to comment Share on other sites More sharing options...
Dalslandia Posted October 19, 2014 Report Share Posted October 19, 2014 I paid £550 for my axiom 18" prop and was quoted £475 by crowther so not much difference really, just hope it lives up to all there claims. Neil What blade angle did you order? Link to comment Share on other sites More sharing options...
Neil Smith Posted October 19, 2014 Report Share Posted October 19, 2014 I think they said it was 25% but would have to check with them. Neil Link to comment Share on other sites More sharing options...
Miffy Posted October 29, 2014 Report Share Posted October 29, 2014 I think the blog by Ray Oakley about his experience with fitting an Axiom prop to his narrowboat "Stronghold" makes informative reading: http://nbstronghold.blogspot.co.uk/2012/05/axiom-propeller-saga.html Ispoke with Ray recently and he is still convinced and happy with the results. Link to comment Share on other sites More sharing options...
nicknorman Posted October 29, 2014 Report Share Posted October 29, 2014 The trouble with those sort of reports is they aren't very scientific. He went from a chipped and bent prop of unknown origin and quality, to an axiom carefully "tuned" for his boat. Perhaps no surprise that there was an improvement but whether putting a quality "normal" propellor on would have had the same effect remains unanswered. Link to comment Share on other sites More sharing options...
MtB Posted October 29, 2014 Report Share Posted October 29, 2014 I find it hard to believe he is getting 4mph on the cut at 1400 rpm with no breaking wash, yet only 5pmh in deep water on the Thames at 'peak revs' (whatever that means for a BMC, I'd imagine about 2,000rpm). Axioms are the same as Ecofans. The don't stand up to technical analysis, yet their users seem completely convinced of their effectiveness. MtB Link to comment Share on other sites More sharing options...
Timleech Posted October 29, 2014 Report Share Posted October 29, 2014 I find it hard to believe he is getting 4mph on the cut at 1400 rpm with no breaking wash, yet only 5pmh in deep water on the Thames at 'peak revs' (whatever that means for a BMC, I'd imagine about 2,000rpm). MtB ...also he seems to be implying that his increased fuel consumption was due to a leak into the sump. How many litres had he 'lost' that way??? I don't dismiss all the claims for the Axiom, by the way. Just some of them Tim Link to comment Share on other sites More sharing options...
MtB Posted October 29, 2014 Report Share Posted October 29, 2014 ...also he seems to be implying that his increased fuel consumption was due to a leak into the sump. How many litres had he 'lost' that way??? I don't dismiss all the claims for the Axiom, by the way. Just some of them Tim Yes I noticed that too. I don't think this bloke is very analytical or has much truck with piffling detail... I'm more inclined to think he made a mistake in his arithmetic calculating his fuel consumption. Like you I suspect, I'm certain 0.6 litres of fuel an hour being dumped into the sump of his BMC would make the oil level rise one helluvalot more than the 1" he reports! MtB Link to comment Share on other sites More sharing options...
RLWP Posted October 29, 2014 Report Share Posted October 29, 2014 You would need to know the surface area of the bilge. Ours is about (from memory) 0.5m wide and 1.5m long, making 0.75 sq M. I make it that 25mm of diesel in there is around 19 litres Richard Link to comment Share on other sites More sharing options...
MtB Posted October 29, 2014 Report Share Posted October 29, 2014 You would need to know the surface area of the bilge. Ours is about (from memory) 0.5m wide and 1.5m long, making 0.75 sq M. I make it that 25mm of diesel in there is around 19 litres Richard Ah did he say bilge, I missed that bit. (I must be turning into cotswoldman!! ) MtB Link to comment Share on other sites More sharing options...
nicknorman Posted October 29, 2014 Report Share Posted October 29, 2014 Ah did he say bilge, I missed that bit. (I must be turning into cotswoldman!! ) MtB No, he said into the sump / engine oil. Link to comment Share on other sites More sharing options...
RLWP Posted October 29, 2014 Report Share Posted October 29, 2014 No, he said into the sump / engine oil. Ahhh, much less than 19 litres then (I didn't read the post properly either). Perhaps a bit over 1 litre as the marks on a dipstick are usually around 2 pints apart Richard Link to comment Share on other sites More sharing options...
Dalslandia Posted October 29, 2014 Report Share Posted October 29, 2014 No it is the sump, using the dipstick.. was the extra fuel consumption due to higher speed? 1,6/1 liter means 60% more power going 17% faster like going from 4 to 4,7 how far is it to Oxford? I guess it depends on where you start :-) but you guys know better then me the geograpy Link to comment Share on other sites More sharing options...
Dalslandia Posted November 6, 2014 Report Share Posted November 6, 2014 I think they said it was 25% but would have to check with them. Neil Have you got your propeller? what size did you get? Link to comment Share on other sites More sharing options...
David Mack Posted November 6, 2014 Report Share Posted November 6, 2014 Ahhh, much less than 19 litres then (I didn't read the post properly either). Perhaps a bit over 1 litre as the marks on a dipstick are usually around 2 pints apart Richard So you are not "my friend Richard the diesel engineer" then? Link to comment Share on other sites More sharing options...
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