Savitsky Power Prediction

Will,

Excelent question, what you are after here is "Pe", Propulsive Efficiency or Propulsive Coefficient.

This really rates it's own thread but I'll give you a couple of short notes.

Reference L&E Principals 2nd Ed. pp191-193, 199-204
Professional Boatbuilder #59, Alan Gilbert, Speed Predictions, pp60-62
Skene's Elements, 8th Ed, pp86

L&R say use the propeller efficiency figure from the prop charts, but then later they mention fudging that for a water jet which is their example boat. (from 65% down to 50%) The highest Pe I have found is with surface drives at high speed, (40+ knts) up to almost 70%. The lowest I have found is waterjets at low speed, about 42%. The usual figure used to be 50%, so double your ehp for required. Modern twin inclined shafts with 4 bladed wheels are about 60%, outdrives 60-65%. Kinney mentions 10% for folding 2 blade props!

The only way to predict this accurately is to measure boats and compare them with your predictions.

All the best, Tad

 

Thanks Tad - and I think you're right... it probably does deserve its own thread....
You're response leads me to another - very broad-reaching, rather off-topic, and definitely worth its own thread - question.... Just how did you drag up those references so quickly? I've been battling with a decent way to archive all the info I gather for ages - time spent studying means that I've not had the time to come up with a decent system yet.....

 

Another point... what we really need is a prediction spreadsheet that includes input for the drive system being used - apart from the Arneson calculator, I don't think I've come across one that allows for the use of anything but traditional shafts & props....

 

Will,

Here's an urban myth attributed to Einstein. During an interview someone asked Albert his phone number, he replied "I don't know it, but I know where to look it up!".

A long time ago, around 30 years, I started writing all the useful tidbits in a little notebook. These are small, 3" by 5", the first one was ring bound and filled long ago, some of it's penciled entries are unreadable now, and I've moved on to several more. But I still have and use the first one, minus a few pages that have fallen out of the front, just wear and tear. The newer ones are still expanding. Most people I've worked with have copied everything out of these books into their own notebooks.

These booklets start off with the really useful stuff, address and phone number of the current place of business! Then we have decimal equivalents for feet and inches, Planimeter scale correction factors(not used in a while), then a number of pages of conversion factors, then reams of form factors and ratios, SAE bell housing sizes, block coefficients, steel plate weight, chain weights, engine cooling areas, weight estimations coefficients, things like capsize screening formulas, sheeting angles and loads, mast rake for various boats, rig dimensions, construction scantlings, d/l ratios real and guesstimated, various dimensions used by certain builders. Lots of good stuff.

This simple reference is supplemented with files in the filing cabinet. My main reference is called "Design", below this are " Preliminary Design, Sailboat Design, High-Speed Powerboats, Lobsterboats, Sportfishermen, Long-Range Powerboats, etc" These are supplemented with files like "Rudders, Keels, Steering, Engines (a bunch of these), Specifications, Appliances, Gas Turbines, Noise Reduction, etc." In addition I have files full of drawings and brochures of various boats, mostly by size or type, and equipment and hardware files. That plus a file for each job I've done.

Beyond that there are about 400 dedicated books and something upwards of 6000 periodicals in here. I now lose things more often than I find them!

Tad

On your second note, BHP or installed power would just be another column in the spreadsheet with a Pe input up top. Go to it!

 

Atahawaii,
I have heard about this excel solver before but could not access it. I do a lot of excell programming and this will certainly help. How do i access it?

Thanks in advance

 

Rx - you pays your money, they send you the stuff......

 

Those who want something a little more physics-based than Savitsky's method can consult Prof. Ernie Tuck's latest contribution to "Slender Planing Theory" at:

http://internal.maths.adelaide.edu.au/people/etuck/index.html

See the paper towards the bottom of the page.

I look forward to seeing a spreadsheet version

Incidentally, many people here seem to be "stretching" Savitsky's formula in a variety of directions. Has anyone done any experiments on these variants to see whether they are even in the ballpark?

Leo.

 

The Savitsky calculations at www.hawaii-marine.com/templates were checked with great scrutiny for validity with Savitsky's sample calculations provided in his original paper. The calculations provided in the template are slightly more accurate than Savitsky's because parabolic instead of linear interpolation was implimented for determining the equilibrium conditions. At the time Savitsky wrote his paper powerful spreadsheet programs were not available. If he wrote it at a later date he might have also incorporated a parobolic interpolation method. I have not checked the other variants available, so I cannot speak for them.

 

If I recall correctly Solver is in an older version of Excel. But I may be mistaken. There are new tools like "Goal Seek" and "Scenario" in the tools menu of the newer Excel versions. The problem with these alternatives is that you need to directly interact with them to get the answer each time you do the calculations. I was looking for an automatic method that did not involve this personal interaction with these functions for each set of calculations.

The main problem with automating the Savitsky Method is that two formulas need to be iteratively worked out backwards. One good method, for solving this numerical analysis problem, is the Newton Raphson Method. But this method requires programing that uses iterative do loops. So as far as I can tell a macro should be able to do this. But a recent problem I have experienced with macros is that Microsoft is now requiring security certificates for them in the newer versions of Excel. I wanted to completely avoid this hassle and the extra expense, so a propriatary was method was developed in the Savitsky Spreadsheet located at www.hawaii-marine.com/templates. This proprietary method does not involve programming with do loops or use macros or involve time consuming interactions with functions like "Goal Seek" and "Scenario." The goal of automatically generating results is met in the product at this website.

 

Sorry, I wasn't clear enough about what I meant. I am more concerned whether Savitsky's formula captures the physics of the problem accurately (if at all), especially when it is applied to slightly curved shapes and other variations.

Cheers,
Leo.

 

Atahawaii,
Thanks.What they have mentioned is a solver that will create a formula based on a graph. manually, i can do this but not all the time. or the more complex formulas. I understand that it is the reverse of a graph. the graph goes in first, then the formula is created.

Rx

 

Savitsky´s power prediction

A couple of years ago i asked for a sample of this method and a copy of Savitsky´s paper. I understand you have information about this matter. Would you be so kind as to share it with me.
I have this mail address: humbertor92@hotmail.com
and hrra2004@yahoo.com.mx
thanks in advance

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Does Savitsky model slamming/wave loads? I see nothing in the model that would suggest it does (eg. sig. wave height). If not, does anyone have anything that does model this phenomena?

thanks

 

I have programmed Savitsky in program SeaPower. It comprises 28 resistance methods + propellers.

you can download demo program www.sea-power.net

rgds

Davor

 

i need an excel sheet of savitsy's power prediction can any body please post it and if possible the papers also.