Gearbox ratio and propeller pitch

[Graham.m]

 

Basically that.

 

Daniel

 

 

 

 

Some laboratory testing would be fantastic if you could get the time on the equipment, and enough time to work out and then validate some reasonable assumptions/simplifications/scaling. It would perhaps even work out cheaper, depending on what level of equipment you deemed suitable, and if you could get a good price on drydock/slip/cranage.

 

You could (would) model the hull as part of the test, assuming that was the aim, as well as modeling the floor of the canal, and or water surface. You do the whole lot full scale in a large hydrodynamic testing pool if someone would lend you one.....

 

Bollard pull would be very crude, and highly unlikely to be meaning full below a slow cruising speed of say 2mph. However a lot of boats spend a lot of time a 2mph, including most of the time you are maneuvering, as which point there would be no 'white water' anywhere to be seen. Certainly I agree that if you where aiming to test performance representative of a 4-5mph hull speed using a static test would be meaningless.

 

 

Daniel

 

As you say if one could get access to a full size tank etc. The costs would be astronomical v the financial return and I cannot see a way of justifying them on the basis of a return. Even the basic of lifting the boat out and transporting to and from the tank would be say £200 each for lifting in and out of the canal, hire of a crane at the tank x 2 say a £1000, trucking there and back say £3000. Just that bit could roll up £5000 without a test starting. If I was heavily into prop design I might think about it, I think first I might think about a redesign of the hull first, but there is another can of worms

 

 

No my money has to earn or give me pleasure so it can stay earning where it is and I will have a couple of days of play just seeing what the new prop does compared to the old

[Dalslandia]

As it is easier to cut wide belts from others leather, :-) you could test 10 slightly altered propellers 5 Axion and 5 normal propellers, with different diameter and pitch. ...

 

To compere different propellers without to much equipment, the full throttle test is best, need some repeteble test runs on each propeller, Speed will tell the thrust HP, rpm will tell how suiteble the prop is for the hull/engine combo, the difference in speed and rpm can be used to recalculate the power engine turn out, and then the difference in propeller efficiency. It is also possible to recalculate the cruise power from info from full throttle test.

 

Pollard pull test is useful for tugs and trawlers. even if speed range isn't so large on a NB or other canal boats, it effect the propeller, static thrust can be tested is the easiest to test and also the most useless, especially on airplanes, the question is what the propeller do and perform at normal cruise speed and or top speed. static it can partially stall the blades.

 

I checked the propking spread, I was not sure of your boats deplacement, but it gave a 18"x12" or 13", but that soft calculate with a BAR of 30%

 

the Swedish marin test center thumb of rule for a 50% BAR gave a 20"X10"

 

As you understand your hull/keel design isn't optimal for that engine power/rpm, if you hade 24" from skeg to uxter plate it would be better.

Edited December 20, 2015 by Dalslandia

[Graham.m]

Dalslandia, on 20 Dec 2015 - 09:34 AM, said:

As it is easier to cut wide belts from others leather, :-) you could test 10 slightly altered propellers 5 Axion and 5 normal propellers, with different diameter and pitch. ...

 

To compere different propellers without to much equipment, the full throttle test is best, need some repeteble test runs on each propeller, Speed will tell the thrust HP, rpm will tell how suiteble the prop is for the hull/engine combo, the difference in speed and rpm can be used to recalculate the power engine turn out, and then the difference in propeller efficiency. It is also possible to recalculate the cruise power from info from full throttle test.

 

Pollard pull test is useful for tugs and trawlers. even if speed range isn't so large on a NB or other canal boats, it effect the propeller, static thrust can be tested is the easiest to test and also the most useless, especially on airplanes, the question is what the propeller do and perform at normal cruise speed and or top speed. static it can partially stall the blades.

 

I checked the propking spread, I was not sure of your boats deplacement, but it gave a 18"x12" or 13", but that soft calculate with a BAR of 30%

 

the Swedish marin test center thumb of rule for a 50% BAR gave a 20"X10"

 

As you understand your hull/keel design isn't optimal for that engine power/rpm, if you hade 24" from skeg to uxter plate it would be better.

 

Lol 5 Axioms £3000 plus, 5 others tailor made say another £3000, plus changeover costs £2000 lol Call it £10,000, do that three times and I could buy Clarence again.

 

Trouble is the full throttle test does not work on the canals. Clarence has a draft of 26 inches, the average depth of a canal, water I mean not slurry probably not much over 39 inches, or it could be because of the profile, open Clarence up and she (old prop) would go to 4 mph and stick with lots of wash and froth. You could then drop the RPM 300/400 RPM wash drops and froth goes and still 4 MPH. In open water she would go faster. So it appears that what I call canal effect holds her back. So the full throttle test can only be done on open water. Mind you I have yet to try full throttle with the new prop on a canal.

 

Prop wise Clarence is 48 feet at waterline, 7-foot beam and draft 26 inches, displacement estimated at 15 Tonnes. The clearance between Uxter and skeg is 21 inches approx.

 

I think someone believed the Axiom 18 22 degrees was the equivalent of an 18 x16; the old prop was 20 x 16 according to the markings. Thanks for the calculations. Those props your calculations came up with suggest with the Axiom at 18 x 16 she is still over prop’ed.

 

I agree about static tests they tell you nothing about how the drive train will perform when moving. Thinking further even when I am towing there is only a few seconds before the boat is moving through the water gradually gaining speed. I can see no use for the test reference a narrowboat the more I think about it. Clarence is capable of both pulling and pushing with gentle force if you know what I mean. Not long ago the marina guys were having major problems with a boat; wind was beam on away from them and blowing the boat away. I was just leaving my berth noticed them struggling and took Clarence and tug pushed the boat back. Their faces looked horrified, but the guy on the boat told me after he did not even feel Clarence’s first touch no any great acceleration as she gentle pushed her broadside back onto the berth. Oh obvious question answer, U/S engine.

 

Some years ago I seem to remember there was a fashion for blowing air along the hull from a compressor within the boat, a hose across inside the boat to lots of nozzles forward on the boat and pumping air under the boat to lubricate it. Some said it was marvellous. Vague wondering what the effect it might have on a canal boat at 4 mph.