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 Boat Design Forums Assignment help please!

#1
01-08-2013, 06:37 AM
 Anjana Join Date: Jan 2013 Rep: 10 Posts: 4 Location: -

I'm not sure if this is the right place to post my thread and im sorry if its not!
I am studying for a Master in Yacht Design (i'm an Interior Architect) and we have some assignments to calculate various parameters - resistance, power, cavitation, etc using Max surf, Navcad, etc.
I'm really new at this and have been struggling to figure out the best way to solve all the calculations and completing my assignments!
Our professors (mainly Naval engineers) haven't exactly given us a lot of mathematical information and i'm really confused.
If anyone has time to help, i would REALLY appreciate it!
Thank you!
#2
01-08-2013, 09:59 AM
 Crowsnest Junior Member Join Date: Jun 2012 Rep: 18 Posts: 44 Location: Madrid
Hi Arjana:
Would like to know what mathematical backgound have you been given. Im not familiar with the teaching systems at your country.
There are many kinds of software that could help you, but two points must be taken into account.
Accuracy is better or worse depending on which software, which methods are choosen for each part of the calculation.
With no background knowledge, software only is not useful and could lead to very big errors. This is not a simple matter of clicking a key.
Hence, a first thing to be known, is what kind of ship and which sailing conditions are those calculations related to.
If you only need only aproximative calculations, the web and this forum are full with very useful procedures.

Regards
__________________
Multiplicanda, non est ponenda sine necesitate.
#3
01-08-2013, 10:19 AM
 Anjana Join Date: Jan 2013 Rep: 10 Posts: 4 Location: -
Since most of the students are designers they've just given us basic knowledge to enable us to understand how different factors affect ships.
Maxsurf and Navcad are enough to complete the assignments.
These are the requirements :
a) For the given parameters rescale the attached hull form
b) Calculate the resistance for given speed
c) Choose propeller by using Wageningen series (3,4, and 5 bladed props, twin propeller)
d) Calculate power (PE,PD,PB)
e) Check cavitation on the propeller
f) Compare power requirement with similar boats
g) Report all calculations

Assumptions
• Shaft angle is assumed to be 6 degrees
• Rudder span is assumed to be ½ draught in span, rudder aspect ratio of 2
• Antirolling fins 4 in total, ¼ draught in span, aspect ratio of 2
• Bilge keels are 20 % of the length long, 20 cm wide
• Shaft brackets are T/10 long, 15 cm in chord length
• Max propeller diameter is T/2
• Engine power and rpm can be selected from attached engine (choose an initial engine power twice the effective power)
• Reduction ratios should be chosen in ratios of 3,3.5,4,4.5,5,5.5,6.0,6.5,7.0

Our teacher has given us an Npl round bilge hull to base our calculations on. We have to resize the hull based on some given dimensions (which i did).
Next he's given some assumptions for appendages - i've managed to figure out that we need to calculate the area of each appendage to put the values into navcad to calculate total resistance!
I guess i need to stop panicking and go step by step!
I'll try to look around the forum for some answers, thank you so much!
#4
01-08-2013, 02:57 PM
 DavidJ Senior Member Join Date: Jun 2004 Rep: 441 Posts: 219 Location: Canada
This is actually an easier assignment than you think. Just go from a) to g) in order. Navcad has places to input most, if not all, of that information.

All you have to do is scale the hull in Maxsurft. Make sure your reference points and units are correct. Then copy the new vessel parameters from Maxsurf into Navcad.

Fill in the rest of the given information into Navcad.

Look up other boats that seem similar and compare their installed power to what you came up with.
#5
01-09-2013, 04:28 AM
 Anjana Join Date: Jan 2013 Rep: 10 Posts: 4 Location: -
Hi David,
Thank you for your reply. You're right the assignment isn't as hard as i initially thought it was. I think i panicked a bit
I need to calculate the areas of each of the appendages so i can put them in Navcad to find resistance.
Hopefully i'll be able to finish without getting stuck again. Some of the parameters in Navcad are new and confusing to me.
#6
01-09-2013, 06:56 AM
 Anjana Join Date: Jan 2013 Rep: 10 Posts: 4 Location: -
You said to follow steps a-g in order.
Step b says calculate resistance - don't i first have to calculate all appendage areas, then put in data from step c: propeller information? Navcad asks for propeller details as well to calculate resistance. Or do i skip that and calculate resistance without propeller data?
Thank you again
#7
01-13-2013, 05:59 PM
 Tim B Senior Member Join Date: Jan 2003 Rep: 841 Posts: 1,438 Location: Southern England
Nothing too hideous in that assignment. I'd suggest that you work through one case by hand, so you get an idea of what the software is doing behind the scenes.

Tim B.
__________________
Open Source Marine Charting - openpilot.sourceforge.net
Open Source Vessel Dynamics opendynamics.engineering.selfip.org

#8
01-14-2013, 12:40 PM
 DavidJ Senior Member Join Date: Jun 2004 Rep: 441 Posts: 219 Location: Canada
Sorry I haven't been near a computer for a few days. Boat design is a cyclical process. There is no 100% right answer here. Likewise, you will not likely get the "best" answer the first time through. Navcad does not need appendage information or a propeller in order to calculate resistance. It is assumed that you will get a better answer when you put in more information.

The basic answer is that you need a preliminary resistance/PE (Effective Power) in order to calculate a propeller. You will then choose a 3-bladed propeller and check for cavitation. If it is in the cavitation range then move up to a 4-blade. Then you will need to re-run your powering estimate because a 4-blade is less efficient. If you get to f) and you find that your power numbers are completely out of whack you may have to go back to b) and try again.

One thing I will say about that is that you will probably find the installed HP of the similar vessels is much higher than what you've calculated. Part of the reason will be loses due to efficiency. Around 40% of your power can be lost to do loses in gearing, the propeller, bearings, inside the engine itself, etc. This is why your prof said to choose an initial engine power double your PE. However, in the yacht world boats usually have more power than required for the designed displacement speeds. I don't really know why this is. It's probably a question for someone involved in marketing.

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