Ignorance Exposed

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PeteThePen1
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Ignorance Exposed

Post by PeteThePen1 » Wed May 26, 2021 9:53 am

Hi Steamboating Colleagues

I am having a great time with my new copy of Gerr's Propeller Handbook but have now hit a snag. I wish to transpose one of his formulae but my algebraic skills are on the wrong side of rusty. I have made several attempts, then put numbers in and got obvious rubbish out. Please could anybody who remembers how help out on this one?

Gerr Formula 2-1 Page 10 - Re-expressed in terms of SHP

SL = 10.665 / cube root(LB/SHP)

Regards

Pete
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Re: Ignorance Exposed

Post by cyberbadger » Wed May 26, 2021 1:04 pm

Pete,

I came up with:

SHP=(LB*SL^3)/1213.0608

-CB
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Re: Ignorance Exposed

Post by fredrosse » Wed May 26, 2021 1:50 pm

Dimensional Analysis is a fundamental element of ALL Science and Engineering, and applying it to your algebraic manipulations may solve your problem.

The fundamental algebra property is that one can perform the same operation on each side of the equation, and the equation remains valid. Multiply, divide, add, subtract, raise to any power each side of the equation, and it remains valid. This is all you need to know about algebra for the simple stuff comprising 99% of 19th Century Engineering.

The second, and equally important aspect of this problem is dimensional consistency. All formulas/equations in ALL of Science and Engineering must be dimensionally consistent. That is, the units on each side of the equation must match. Often Newton's discovery, relating Force, Mass, and Acceleration for Newtonian motion is also involved, (F = M x A), or other equalities found by various historical figures, such as (550 feet-Poundforce / Second = 1 Horsepower) from Mr James Watt, or V = I x R for simple electrics, etc.

Throughout ALL Science and Engineering, one can multiply or divide ANY term in ANY equation by the non dimensional number one, and not change the equation nor its validity. Note that the units involved must also be included in the equation.

For example: we know 12 inch = 1 feet

Divide both sides of this equation by the same value, and the equation remains valid;

12inch/1feet = 1feet/1feet

Notice that the right hand term here cancels feet/feet, thus no units on the right, and 1/1 = 1, so this results in the right hand side of the equation equal to a Non-Dimensional 1 = 1 (ND) The "ND" term just reminding the analyst that this is indeed a Non-Dimensional Number. Thus:

12inch/1foot = 1 (ND)

So we can use this Non-dimensional value of "one", and multiply, divide, square, etc. any term in any valid equation by this number and not change its value. A most rudimentary example here"

My boat is 19 feet long, how many inches long??, OK to multiply by 1 (ND) ;

19 feet boat length x 12 inch/feet = 19 x 12 boat length inch = 226 inch boat length

Gerr's Propeller Handbook, my copy I cannot find just now. When I find it I will try to answer your question, but until then, have fun.
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Re: Ignorance Exposed

Post by PeteThePen1 » Wed May 26, 2021 3:04 pm

Thanks CyberBadger & Fred

I am grateful for you assistance and suggestions. Since posting the request I found a tutorial on transposing equations and found that it was nicely pitched. Of course, having gone through it I did the first two exercises in the list at the end and got them right. Confident of my new prowess I had a go at Q 10. Oop! Nowhere near so I think a little more revision is required!

I will also try out some numbers in your suggested answer CyberBadger and see if the answer looks sensible.

Best wishes

Pete
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Re: Ignorance Exposed

Post by barts » Wed May 26, 2021 3:30 pm

The values from this equation are typically about 20% high - they predict more power is needed than is actually required - according to my brother, who is a naval architect. This is of course a safe and appropriate approach to powering a vessel; there's allowance for hull forms, etc, and it provide a margin for errors/wind/waves, etc.

My brother's web site is https://www.woodenboatdesigns.com/home . There are some pictures of really nice boats built from his plans that are worth seeing.

- Bart
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Re: Ignorance Exposed

Post by DetroiTug » Wed May 26, 2021 4:18 pm

Opinion: Since a steam engine's torque is constant, the biggest bite the prop can practically take is better. On the tug, it originally had a 20 x 20 three blade propeller. Went up to a 21 x 23 four blade propeller. Results: The engine RPM dropped considerably, has better bollard pull, uses less steam, gained speed. The paddlewheeler at very low RPM does not need considerably higher pressure to make hullspeed.

This result also causes confusion about the PLAN formula.

Ron
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Re: Ignorance Exposed

Post by barts » Wed May 26, 2021 6:21 pm

DetroiTug wrote:
Wed May 26, 2021 4:18 pm
Opinion: Since a steam engine's torque is constant, the biggest bite the prop can practically take is better. On the tug, it originally had a 20 x 20 three blade propeller. Went up to a 21 x 23 four blade propeller. Results: The engine RPM dropped considerably, has better bollard pull, uses less steam, gained speed. The paddlewheeler at very low RPM does not need considerably higher pressure to make hullspeed.

This result also causes confusion about the PLAN formula.

Ron
In general, steam engines are more efficient when loaded heavily. Fitting a bigger propeller reduces RPM, and the engine is now working harder even though the cut-off is the same and the throttle is wide open in both cases. Steam consumption is of course reduced, and boiler pressure may well climb if the boiler was too small to pop the safety at full speed before, further increasing efficiency. For some boats, a step up drive is necessary if insufficient room is available for a larger propeller. A larger prop is preferred, however - no gear train losses or noise, and better 'connection' with the water.

- Bart
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Re: Ignorance Exposed

Post by Lopez Mike » Thu May 27, 2021 6:06 am

Ron,

It can't be quite true that the torque is constant. If that were the case then we could get any power we wished by just turning the engine at a higher r.p.m. There are unavoidable flow losses in the ports as r.p.m. increases for instance.

It IS true that the torque stays quite constant down to zero speed. And with the usual sized boilers we have the sustainable engine power is pretty much set by the boiler output anyway.

With I.C. engines the peak power occurs at a certain average intake port velocity that is strongly correlated with how direct and low loss the port design is. For instance the range is all the way from 250 f.p.s. for L head (flat head) engines all the way to well above 300 f.p.s. for modern high performance engines with very direct ports.

I would be most interested if any research has survived as to the effect on steam engine peak power r.p.m. v.s. port velocities. A quick look at the port designs of steam locomotives over the last years of design and production shows that the designers were very aware of the issue.

Mike
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Re: Ignorance Exposed

Post by barts » Thu May 27, 2021 6:23 am

Lopez Mike wrote:
Thu May 27, 2021 6:06 am
Ron,

It can't be quite true that the torque is constant. If that were the case then we could get any power we wished by just turning the engine at a higher r.p.m. There are unavoidable flow losses in the ports as r.p.m. increases for instance.

It IS true that the torque stays quite constant down to zero speed. And with the usual sized boilers we have the sustainable engine power is pretty much set by the boiler output anyway.

With I.C. engines the peak power occurs at a certain average intake port velocity that is strongly correlated with how direct and low loss the port design is. For instance the range is all the way from 250 f.p.s. for L head (flat head) engines all the way to well above 300 f.p.s. for modern high performance engines with very direct ports.

I would be most interested if any research has survived as to the effect on steam engine peak power r.p.m. v.s. port velocities. A quick look at the port designs of steam locomotives over the last years of design and production shows that the designers were very aware of the issue.

Mike

Torque of steam engines drops with RPM, of course, due to resistance in the valves, ports, etc.

My steam engine design books suggest 4000-6000 fpm average exhaust velocity, 6000 to 8000 inlet port velocity - much lower than gas engines. Of course, the ports are more convoluted on a conventional steam engine.

- Bart
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Re: Ignorance Exposed

Post by DetroiTug » Thu May 27, 2021 11:37 pm

I was just making the point that loading the engine down rather than let it freewheel and throw steam away appears to be better. We get into this discussion occasionally when it's mentioned that steam cars don't need a transmission. They do. The piston cylinder arrangement remain constant in regard to torque, mechanical resistance as well as port flow resistance is the cause of diminishing torque at high RPM. White and Brooks (and I think Doble) ran a two speed transmission for this reason.

Steam engines can be compared to stepper motors, both have all their torque at very low RPM. Both lose torque with increasing RPM, the stepper motor, it's resistance in the windings.

Ron
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