Moderated Discussion Areas
  ContinuousWave: Whaler Performance
  Prop Calculator

Post New Topic  Post Reply
search | FAQ | profile | register | author help

Author Topic:   Prop Calculator
jimh posted 06-08-2002 11:50 PM ET (US)   Profile for jimh   Send Email to jimh  
This message thread is for comments about the prop calculator. See
peteinsf posted 06-09-2002 01:30 AM ET (US)     Profile for peteinsf    
Thanks Jim, very cool...


bigz posted 06-09-2002 04:08 PM ET (US)     Profile for bigz

Look familiar!

Jim we need a site search 'engine' more than a copied so called prop. calculator!!!!!!!!!


dgp posted 06-09-2002 05:32 PM ET (US)     Profile for dgp  Send Email to dgp     
Jim, based upon actual GPS numbers your new propcalculator is fast by 2 MPH. Also, changing the prop slip factor does not affect the resulting MPH, at least on my application.
jimh posted 06-10-2002 12:10 AM ET (US)     Profile for jimh  Send Email to jimh     
Sorry Tom, but the Prop Calculator is implemeted using Mod-Perl. The RBBI calculator uses Java Script. It is not a copy.

Mine just happens to resemble the RBBI model in is fundamental approach, but that's because it's a pretty fundamental approach and it was interesting that it could calculate any of the five values.

Mine works better, in my opinion, since it does not constantly trap you into an annoying error message if you have five inputs. On the other hand, the RBBI works without server involvement.

My son and I worked it out between periods of the Red Wings game yesterday to relieve the tension of the triple overtime.

As for problems with the SLIP factor not working, I don't see that here. Try slip = 0% versus slip =99% and you should see changes.

george nagy posted 06-10-2002 02:13 PM ET (US)     Profile for george nagy  Send Email to george nagy     
Does anyone off-hand know the gear ratio of a 1997 150hp v6 gearcase for an omc?
Jerry Townsend posted 06-10-2002 02:26 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
George - my 1996 catalog shows 1.85 (reciprocal of 0.54 --- 14:26). --- Jerry/Idaho
Taylor posted 06-10-2002 04:01 PM ET (US)     Profile for Taylor  Send Email to Taylor     
I think dgp's problem may this: Once I do the a caclulation, I then want to vary one parameter and see another change. The first time through I did not realize it could calculate any value, I was focusing on finding my pitch, so I changed pitch, hit calculate and was surprised that MPH did not change. Duh... but it stumped me for a minute. Maybe a check box for which is the calculated value?
george nagy posted 06-10-2002 05:34 PM ET (US)     Profile for george nagy  Send Email to george nagy     
What I don't fully understand is the calculated top speed. There would seem to be a few more variables to make this determination such as hp and boat size.
Jerry Townsend posted 06-10-2002 06:52 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
George - strictly speaking you are correct. However, there is a trade-off, in that there are at least two approaches to making the speed calculation; evaluating the performance of the boat/engine system or just looking at just the 'screwing' action of the prop - as done in the prop calculation. Both approaches have their limitations.

The 'prop propulsion' calculation is totally based on the prop 'bite' as it 'screws' it's way through the water. One unknown input parameter in the calculation is the slippage which the calculation is totally based on and which will vary widely. As such, the calculation might get you in the ball-park - but again, many factors (condition of the prop, cup/no-cup, number of blades, pitch, fresh/salt water, elevation, wind, speed, will affect the slippage.

BUT - any calculation will be faced with similar problems - that is, who knows what the wetted surface area of a given hull is? or the surface tension coefficient, or the drag coefficient of the hull? Evaluating these parameters are more difficult that taking a 'wag' at the slippage. --- Jerry/Idaho

jimh posted 06-10-2002 07:41 PM ET (US)     Profile for jimh  Send Email to jimh     
The calculator is not a storehouse of knowledge. It depends on the submission of data to just use some simple alegebra to compute the answer. So if, for example, you think you might like to see how fast a boat goes with a certain prop pitch, it will tell you. But there is no guarantee that your engine will be able to turn the prop at that speed.

For example, I have a 17-inch prop on my boat and I can turn it to 5400 RPM. If I raise the pitch to 21-inches the calculator will compute a much higher speed, but I know that there is no way my engine could ever turn a 21-inch prop to 5400 RPM.

The higher the pitch of the prop, the more horsepower it takes to turn it, and thus props are usually sized according to the engine horsepower. Just to suggest a few common props, you might see this:

50-HP = 15 in. pitch
70-HP = 17 in. pitch
100-HP = 19 in. pitch

Those might apply to a lightweight boat. If the boat is heavier, the pitch will have to be lowered.

By the time you get to 200-HP you can be turning props with a pitch of 23-inches or more.

jimh posted 06-11-2002 10:02 AM ET (US)     Profile for jimh  Send Email to jimh     
Based on some situations described above, I made the error messages appear in red. It sounds like people were getting error messages but not noticing them.
Taylor posted 06-11-2002 12:39 PM ET (US)     Profile for Taylor  Send Email to Taylor     
Hard to miss *that* error message :) If it was there before I might have missed it because it was in the text area, rather than inside the calculator box. I'm veering into WebUI esoteria here, but I've noticed that users are more likley to notice error messages near the point of interaction. But I agree that its better not to trap you in a error messsage dialogue such as 'you blew it, ok?'.
tlynch posted 06-11-2002 01:51 PM ET (US)     Profile for tlynch  Send Email to tlynch     
One suggestion that could make this tool slightly better. If it kept track of which parameter it calculated last, and if all 5 parameters are filled out it could calculate that parameter again and update the new value.

For instance:

1) Insert all parameters except Speed.

2) Hit calculate, it calculates Speed.

3) Change pitch.

4) Hit Calculate, it does nothing. It would be nice if it calculated speed again.

Todd Lynch

george nagy posted 06-11-2002 03:11 PM ET (US)     Profile for george nagy  Send Email to george nagy     
Do I double the mph if I have twin engines?
george nagy posted 06-12-2002 11:14 AM ET (US)     Profile for george nagy  Send Email to george nagy     
I am serious about calculating twin engine speed. I'm considering buying a boat with a semidisplacement hull and am trying to figure out which engine(s) to power with. The boat is 25'X8.5' and I desire 20knots plus cruise speed. I'm told a single 150 hpdi yamaha will cruise at 18 knots so perhaps twin 80-100 yamaha 4 strokes will get me closer to the low 20's. sorry if my question seemed "smart-allecky".
jimh posted 06-12-2002 11:44 AM ET (US)     Profile for jimh  Send Email to jimh     
Regarding twin engines, no you don't double the speed.

Here is a real-world example. On my boat I have twin engines with 17-inch pitch props. When the engines turn 4000 RPM, the boat goes 25 MPH.

If I had a single engine with the same gear ratio, turning a 17-inch prop at 4000 RPM would produce the same speed.

It is likely that the prop diameter would be bigger on the single, and the horsepower would have to be double the twin's horspower, but the boat would go the same speed (or close to it).

Typically, speed varies with the square-root of the horsepower. If you want double the speed, get four times the horsepower.

Richard Quinlivan posted 06-12-2002 10:34 PM ET (US)     Profile for Richard Quinlivan  Send Email to Richard Quinlivan     
Does the calculator automatically add one inch to the entered pitch to account for the greater effective pitch of a cupped prop?
jimh posted 06-13-2002 12:05 AM ET (US)     Profile for jimh  Send Email to jimh     
The calculator does not apply any science or knowledge to your input. If you tell it the pitch is 17 (inches), it takes you at your word.

By the way, there is no way for the calculator to differentiate between cupped or not-cupped props (at the moment) so there is no way for you to communicate to the calculator that you are using a cupped prop.

Many propellers have progressive pitch and the number that is designated as the "pitch" is not reflective of a single, constant pitch across the blade.

jimh posted 06-13-2002 12:08 AM ET (US)     Profile for jimh  Send Email to jimh     
By the way, I am enjoying all the feedback on the Prop Calculator, please keep it coming. Because of the highly modular design of the code that generates it (the code is all object-oriented programming) it is quite straightforward to add new features. My son, the real programming power behind this, and I have been talking about some improvements to be made. We'll add them as time and coding permits.
tabasco posted 06-13-2002 07:40 AM ET (US)     Profile for tabasco  Send Email to tabasco     
Thanks Jim ......couldn't get it to work. However, when I put in O % slip it works. Thanks ....this is fun.
jimh posted 06-17-2002 11:25 PM ET (US)     Profile for jimh  Send Email to jimh     
We had to take some time off while the Red Wings won the Stanley Cup, but after the Championship celebrations died down a bit, we got back on the case and implemented several of the suggestions regarding the user-interface of the Prop Calculator.

If you give it five parameters, the calculator now tries to guess which parameter you meant to have recalculated. To let you see which parameter was actually calculated the cells containing that value are shaded in a darker color (if you have style sheets enabled in your browser).

Also implemented was a choice to change the units of the BOAT SPEED parameter, now available in either MPH, KN (nautical miles per hour), or KPH (kilometers per hour).

The cool part of the calculator is the modular nature of the code.

The HTML is contained in a template.

The appearance of the HTML in your browser is controlled mainly by style sheets, like most of everything else on this website except the forum areas.

The calculations are contained in a separate program module (or "class").

The connection between the user and the sever is accomplished with a CGI Application module ("class").

Don't think I understand all of this quite yet either, as my son the computer programmer knitted most of it together. I hacked out some of the HTML and Style Sheets.

Let me know how you like the new variant of the Prop Calculator.


Jerry Townsend posted 06-18-2002 01:16 AM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Jimh - I have not tried the calculations - but one technique that I have used for years - program the logic so that with a given parameter selected/specified, a nulled (zero) input means calculate that parameter while a non-zero input means that you are inputting that value for that given parameter. That takes the guess-work out of it. ------ Jerry/Idaho

Post New Topic  Post Reply
Hop to:

Contact Us | RETURN to ContinuousWave Top Page

Powered by: Ultimate Bulletin Board, Freeware Version 2000
Purchase our Licensed Version- which adds many more features!
© Infopop Corporation (formerly Madrona Park, Inc.), 1998 - 2000.