Hullspeed Question

[buet2012]

Hallo,
I am a new member of this forum. I am a student of NA program. I'm learning Hullspeed. I am trying to calculate engine power of my model. So i need resistance of my ship. So when i tried I faced a problem in Hullspeed during resistance analysis about analysis method. It is not a problem, Its a Confusion. That is -

What is the relation or difference between Holtrop Resistance, Slender
body resistance and fung resistace in Hullspeed?

For 10 knots speed my models Resistance is about 50 kN (Holtrop), 170 kN (slender) and 28 kN (fung). why this variation for same model? Which is the final resistance of my model for 10 knots speed? I’m very confused. please Help!!!!

[DavidJ]

You need to be very careful about what you are doing when you use these kind of dimensional analysis speed predictions. You must be absolutely sure that your test vessel's parameters fit perfectly within the framework that the chosen method requires. They only give reliable results if you are using them on a vessel that fits within the tested parameters. I recommend reading this excellent article from Hydrocomp's website on the use of this type of powering estimate:
http://www.hydrocompinc.com/knowledg...mmandments.pdf

I have not used hullspeed in a long time. I do not remember how much info it gives you on the different methods. I seem to recall that it showed the parameters in a side bar. Particulars of your vessel that fell outside of the necessary values were highlighted in red or something like that. If a method has too many red values or if your numbers are WAY outside of the appropriate range then you really should not be using that method. How many is too many? How much is too much? Only experience can answer that.

If possible you should try to locate the original journal articles where the methods are described. That will give you the best idea as to how they work and whether it is appropriate for your vessel. It's also a good learning experience to try punching one of these into a spreadsheet just so you know what is going on inside that magic box.

Good luck

[buet2012]

Originally Posted by DavidJ

I recommend reading this excellent article from Hydrocomp's website on the use of this type of powering estimate:
http://www.hydrocompinc.com/knowledg...mmandments.pdf

I read your article but i don't understand what is the relation this article with Hullspeed. My problem is difference between Holtrop, Fung and slender body method in Hullspeed. Thank you for your comments_t][t

[groper]

They use different algorithms which make better predictions for different hull types... so you need to use the method that suits the hull type your modelling...


Slender body method should only be used when the hull your modelling has a length to beam ratio of 6:1 or greater - like multihulls or warships etc...

The following is taken Directly from the manual;

Methods For Displacement Ships

Holtrop
This algorithm is designed for predicting the resistance of tankers, general cargo ships, fishing vessels, tugs, container ships and frigates.

Compton
This algorithm is designed for resistance prediction of typical coastal patrol, training or recreational powerboat type hull forms with transom sterns operating in the displacement and semi-planing regimes.

Fung
This algorithm is applicable for resistance prediction of displacement ships with transom stern hull forms (generally used for larger vessels than Compton). The regression is based on data from tests on 739 models at the David Taylor model basin and consists over 10 000 data points, Fung and Leibman (1995).

[buet2012]

Originally Posted by groper

They use different algorithms which make better predictions for different hull types... so you need to use the method that suits the hull type your modelling...

My ship is a Fishing vesel.
L=40m
B=9m
D=5m
d=3.8m
Cb=0.71
Which method I've to use for calculating resistance

[zeroname]

You can use holtrop method if ur ship do not have any hard chine. if its planning type then use Savitsky method.

[groper]

We have no idea what your hull is, despite your measurements...

But at a guess, based on your hull dimentions, you have a trawler type hull? If so, van Oortmerssen algorithm is probably best suited to a trawler type hull...

[buet2012]

Originally Posted by zeroname

You can use holtrop method if ur ship do not have any hard chine. if its planning type then use Savitsky method.

My ship is bilge type with bulbous bow . If i use holtrop method result is 50kn resistance for 10 knot speed. Power comes 360 hp. Is it correct or practical?

[NickT]

Hi, I'm also a student and am using hullspeed for my final thesis. This is how I see it:

For all methods exept for slender body method (SBM), hullspeed assesses if your design falls within the criteria. Just check if it does for the different methods in the "data" window. Use the one that fits your design best.

The reason your SBM prediction is so high is probably beacause you have not taken into account the virtual appendage. You can check if hullspeed forms the mesh correctly by turning it on in the Display tab.
Even if it does, I would not use this method however, because your design is not slender. L/B should be 5 or greater, yours is 4.4. All this is covered in the hullspeed manual which you can find on the formsys website.

[buet2012]

Originally Posted by NickT

I would not use this method however, because your design is not slender.

But another methods result is practical??????

Holtrop- 50 kN
Fung- 28 kN
what you say about that?

[groper]

I would say, WHY ASK STUPID QUESTIONS?

GO AND READ AND LEARN - you obviously have NO IDEA what you are doing.

[buet2012]

Originally Posted by groper

I would say, WHY ASK STUPID QUESTIONS?

GO AND READ AND LEARN - you obviously have NO IDEA what you are doing.

I think you don't understand what i'm trying to say. I know that my knowledge is weak about ships power and resistance. For this reason i post this topic for getting some help. If you don't know about this please don't comment but change your language.

[NickT]

Originally Posted by buet2012

But another methods result is practical??????

Holtrop- 50 kN
Fung- 28 kN
what you say about that?

Please rephrase your question, I don't understand what you mean.
I kind of agree with groper, although I would not say it like that. There are a lot of books about ship resistance and propulsion. If you look on sites like scribd.com you can even download them. There are also a few webinars on the formsys site about hullspeed which I suggest you take a look at.

[zeroname]

Originally Posted by buet2012

My ship is bilge type with bulbous bow . If i use holtrop method result is 50kn resistance for 10 knot speed. Power comes 360 hp. Is it correct or practical?

can be correct .. check with some reference vessel. power will be increase more if u decrease the efficiency in hullspeed.

[idefix26]

I think the Holtrop Method may be correct for your ship. However, hullspeed power data can be captious. Because, the efficiency data is important to calculate required engine power. The efficiency value is general propulsion efficiency. You need propulsion data to calculate that.

In brief, in hullspeed, resistance data is correct and power data is variable. It is important.