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Debs

Power in reverse

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IanD    64
13 minutes ago, Mike the Boilerman said:

 

Do you have a reference for that please?

I ask because Daslandia here (who designs propellers) said recently (IIRC) the power transmitted by a propeller, all other things remaining equal, rises with the fifth power of the shaft speed. Your statement suggests it rises only with the square of the shaft speed. 

Many thanks.

Oops, my error -- for a prop on a low-speed displacement hull input torque is (at least) square of rotation speed, so power is cube, so at half revs power is 1/8 (approximately -- different sources give different power laws between 3 and 4 for different conditions, I haven't seen any as high as 5).

This is why a "high-torque" engine is pointless, for any engine with a prop correctly sized for maximum power the prop torque curve rises far more rapidly than the engine torque curve falls, so all that "spare torque" at low revs is unusable because the prop input torque is much lower still.

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36 minutes ago, IanD said:

Oops, my error -- for a prop on a low-speed displacement hull input torque is (at least) square of rotation speed, so power is cube, so at half revs power is 1/8 (approximately -- different sources give different power laws between 3 and 4 for different conditions, I haven't seen any as high as 5).

This is why a "high-torque" engine is pointless, for any engine with a prop correctly sized for maximum power the prop torque curve rises far more rapidly than the engine torque curve falls, so all that "spare torque" at low revs is unusable because the prop input torque is much lower still.

 

Here we are. Post 33 by Dalslandia, in this thread:

I sugest a half inch smaller diameter to start with, Crow now get 3000 rpm and some black smoke, if we consider a 3600 rpm beeing standard on this Engine size, we would Think 3600/3000=1,2 or 3000/3600= 0,83 would be a good adjustment for the Power the propeller load the Engine. (as aiming Point)

prop diameter change Power with 5th Power (all the other remaining the same)

0,833^1/5= ,964

13"x,964=12,53"

with 12.5" D the Power needed at 3000 RPM will be some 82% of what ever it is now.

going to 12" and 67% is needed, I Think it is to Little.

An extention of the exhaust pipe down into the water with a row of small holes as a piccola on the way down and squize the end oval might do good on the Music, on the video the noise limiter let the mecanical noise be heared, some lead rubber and foam insulation will do good.

Edited March 31 by Dalslandia

 

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blackrose    607

I haven't read every post so someone may have already mentioned this, but if the prop is too close to the swims it won't help in reverse because it will just be pushing water into the back of the boat. It needs to be at least a few inches away.

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Tony Brooks    594
27 minutes ago, mross said:

But, oddly, most cars are advertised with engine power in PS, not kW.

That's because it is overseen by the marketing guys. It would give a larger HP than  BHP so make the car appear more powerful than it really is. In my view car buyers who take much notice of HP rather than a test drive are asking to be mislead but that is another thread. The truck data I have seen quote Kw.

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IanD    64
17 hours ago, Mike the Boilerman said:

 

Here we are. Post 33 by Dalslandia, in this thread:

I sugest a half inch smaller diameter to start with, Crow now get 3000 rpm and some black smoke, if we consider a 3600 rpm beeing standard on this Engine size, we would Think 3600/3000=1,2 or 3000/3600= 0,83 would be a good adjustment for the Power the propeller load the Engine. (as aiming Point)

prop diameter change Power with 5th Power (all the other remaining the same)

0,833^1/5= ,964

13"x,964=12,53"

with 12.5" D the Power needed at 3000 RPM will be some 82% of what ever it is now.

going to 12" and 67% is needed, I Think it is to Little.

An extention of the exhaust pipe down into the water with a row of small holes as a piccola on the way down and squize the end oval might do good on the Music, on the video the noise limiter let the mecanical noise be heared, some lead rubber and foam insulation will do good.

Edited March 31 by Dalslandia

 

Dalslandia's "5th power" is power as function of prop diameter, not prop rpm -- and I'm not convinced that is correct either, with a constant pitch power is proportional to swept area which is diameter squared...

Various references confirm 3 as the theoretical power vs. rpm in the ideal case, and 3.2-4 as the power in practice with slower hulls at the bottom end. One curve I found for real prop power scaled to the Beta 43 gave 43bhp at 2800rpm and 4bhp at 1400rpm which is a power of 3.4 -- in other words, less than 1/10th the power (1/5 the torque) at half maximum rpm, which shows even more why an engine with high torque at lower rpm is pointless...

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Murflynn    437

there are many commodities that are routinely measured, or sold, in 'imperial' units.

e.g. tyres (diameter but not cross section), pipelines (and most other tubulars) used in the oil and gas industry, beer glasses, narrowboats, bricks (9"x4.5"x3" but commonly converted to 225mm, etc. for compliance with SI standards) and other building modules based on the 9" brick (tiles, etc.), plywood (8'x4' but again converted to SI).

I completed my engineering degree 51 years ago when we were in transition from imperial to MKS (as SI was known in those days).  There was a campaign for cgs but as we now know, MKS won the day.

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Murflynn    437
5 hours ago, IanD said:

One curve I found for real prop power scaled to the Beta 43 gave 43bhp at 2800rpm and 4bhp at 1400rpm which is a power of 3.4 -- in other words, less than 1/10th the power (1/5 the torque) at half maximum rpm, which shows even more why an engine with high torque at lower rpm is pointless...

as I understand it, those figures are for a Beta 43 connected to a prop fitted to a boat where the prop will absorb 43bhp at 2800rpm.  Of course the same set-up is not efficient at 1400rpm.  Alternatively the Beta 43 could be geared and propped to produce (approx) 23bhp at 1400rpm.

 

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IanD    64
17 minutes ago, Murflynn said:

as I understand it, those figures are for a Beta 43 connected to a prop fitted to a boat where the prop will absorb 43bhp at 2800rpm.  Of course the same set-up is not efficient at 1400rpm.  Alternatively the Beta 43 could be geared and propped to produce (approx) 23bhp at 1400rpm.

 

Which means it would never output more than 23bhp, in which case why have a 43bhp engine?

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Debs    9

I would like to thank everyone for their answers on my topic. I have to admit that some went totally over my head but all are worth considering. At the moment we are cruising the Thames Ring and getting used to the handling of the boat. If you feel inclined, you can read about the journey in "Cruising Diaries" with the heading "Our New Adventure". Thank you all, once again, for sharing your knowledge with me. Xxx

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Jess--    62
12 hours ago, Debs said:

I would like to thank everyone for their answers on my topic. I have to admit that some went totally over my head but all are worth considering. At the moment we are cruising the Thames Ring and getting used to the handling of the boat. If you feel inclined, you can read about the journey in "Cruising Diaries" with the heading "Our New Adventure". Thank you all, once again, for sharing your knowledge with me. Xxx

I found that getting my boat onto the deeper waters of the thames meant forgetting almost everything I had learnt about how my boat handled on canals

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Debs    9

It's so hard to figure out! We may look at getting a bigger prop when we get home though!

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Debs    9

It's so hard to figure out! We may look at getting a bigger prop when we get home though!

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Murflynn    437

It's probably quite hard to figure out, but you may decide to get a bigger prop.  :unsure:

 

ain't it a pain when you submit a post and think it hasn't worked so you do it again .......... AND AGAIN ?

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