RichR
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Registered: 17th Oct 01
Location: Waterhouses, Staffordshire
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This is a bit of a random technical question and isn't related to cars at all and I'm guessing noone will be able to help but stranger things have happened.........
I have:
- Resistance Curves
- Speed/Powering Curves
- Engine Powering Curves
The customer has requested acceleration in terms of time for 0-10, 0-20 and 0-30.
I can extrapolate to calculate the Speed against horsepower, giving me the overall speed for set horsepower; as such, I'll know the exact horsepower required to give 10, 20 and 30mph; from this, I'll know the exact engine speeds in RPM required to achieve 10,20 and 30mph and also the resistance at each speed but that's where I draw a blank - I need to work it out in terms of seconds - Any ideas?
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Steve
Premium Member
Registered: 30th Mar 02
Location: Worcestershire Drives: Defender
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would the calcuations used to determine 0-60 based on BHP and weight for the performance calc i designed for vagweb help?
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RichR
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Registered: 17th Oct 01
Location: Waterhouses, Staffordshire
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possibly, I'll give anything a try - its a stupid request to be honest; I can tell them 0-30 will be in the region of 12-15seconds so why they need to know the other two figures I'll never know - practically why would they need it!
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John
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Registered: 30th Jun 03
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Can you not just tell them 4-5 and 8-10.
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ed
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Registered: 10th Sep 03
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Just tell them 0-10 will be 5 seconds, 0-20 will be 9 seconds and 0-30 will be 14 seconds. 
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RichR
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Registered: 17th Oct 01
Location: Waterhouses, Staffordshire
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resistance curve wouldn't show that though - acceleration in theory would be quicker 0-20 than 0-10; 0-30 ie the initial acceleration is slower, then 10-20 is pretty quick but 20-30 is a little bit slower in terms of time. Its not linear acceleration
This particular customer is a complete arse, they'll tie us down to a contract with that in it if we give them figures and refuse the boat if we're over in reality
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John
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Registered: 30th Jun 03
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Well why not say 7 and 12 then.
Acceleration is a simple bit of calculus iirc, I couldn't even do it years ago though so I've got no chance now.
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ed
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Registered: 10th Sep 03
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Do you do FEA on your designs? I'm sure ANSYS would tell you if you're stuck - would probably take about a week of processing though 
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RichR
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Registered: 17th Oct 01
Location: Waterhouses, Staffordshire
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therin lies the problem - have to go back today - John, from my experience, we're likely to be 0=10, 6/7 seconds - 0-20 = 10/11seconds - 0-30 = 15/16seconds
BUT here lies another problem, different propellers give different figures; i.e. a fast accelerating propeller will give a slow top end and vice versa and then we can only record using stopwatch and GPS and whilst GPS is accurate, it isn't instant - fine for top end but not for acceleration - I hate this job - I've had a week off, I wanted something easy for my first day back!!!
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ed
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Registered: 10th Sep 03
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I'm thinking back to all the stuff I learned for my pilots licence. These equations aren't all that simple actually, there are a lot of variables! Prop, two types of drag, power, torque, the amount of the hull in the water...
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RichR
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Registered: 17th Oct 01
Location: Waterhouses, Staffordshire
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precisely; longer lengths but smaller waterplane area coefficients actuall give greater speed - position of centre of gravity, angle of attack, props - too many variables to give an accurate estimate without testing IMO
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alan-g-w
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Registered: 9th Nov 07
Location: Glasgow
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Is the boat you're doing it for a one off?
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adiohead
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Registered: 28th Sep 01
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Just make it up
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Steve
Premium Member
Registered: 30th Mar 02
Location: Worcestershire Drives: Defender
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cant you just take it out and time it
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John
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Registered: 30th Jun 03
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Build a remote controlled exact scale model.
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RichR
Premium Member
Registered: 17th Oct 01
Location: Waterhouses, Staffordshire
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quote:
Originally posted by Steve
cant you just take it out and time it
not built yet - pre build contract speeds normally only relate to the maximum achievable speed which by comparison is easy to work out - but to work out the time taken is a whole other level
All we normally record is a 0-30 time on the actual boat trials at the end fo the build, this given, I've produced a scatter graph of 0-30 tiems for various baots; added a linear trendline and am blagging it with the fact that the acceleration curve is actually logarithmic, so, in theory, using points taken off the linear trendline should all fall on the underside of the actual; i.e. if it says we'll do 0-10 in 7/8 seconds, we're safe because in theory we'll be looking at 5/6seconds etc
[Edited on 29-11-2010 by LiVe LeE]
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RichR
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Registered: 17th Oct 01
Location: Waterhouses, Staffordshire
User status: Offline
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quote:
Originally posted by alan-g-w
Is the boat you're doing it for a one off?
Yes, pretty much every boat we make is 
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