# Delftship Design Question on Design Drag vs Hull Ratio

Discussion in 'Boat Design' started by Fanie, May 17, 2009.

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### FanieFanie

Hi guys,

I have drawed the same hull in delfship but variations in the shape.

The hull aspect ratio is said to determine how fast it is. In other words if the hull aspect is 15:1 it is supposed to be a fast hull.

In Delftship however the hull with the 15:1 ratio shows huge resistance, 2200N at 8 kn.

The same hull drawed for least resistance only shows an 8.9:1 ratio, but resistance is only 375N at 8 knots and 500N at 13kn.

Displacement for both is about 1500kg. Draft for both is around 350mm. Both are 10m LOA.

Which should I work on ? Maybe I'm misunderstanding this.

Design drag or hull ratio

2. ### Guest625101138Previous Member

Something does not gell here Fanie.

You have two boats that are 10m long. They both have the same displacement. They both have the same draft. However one has a beam of 10/15m and the other 10/8.9m. How can that be?? One will have almost twice the beam of the other.

Anyhow I would be careful of results that Delftship produces for drag.

Something to aim for - the lowest drag 1500kg displacement, 10m long hull for 8kts will have a calm water drag of 326N. It will have 790mm beam and 330mm draft.

Rick W

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Sure there is no doubling up...like one being a mono and the other a catamaran, or some other "click this" in the program which is messing up the calc's?
Do you have any other design/hulls to compare this result against, this way you can ascertain which is correct for your current design, if either!

It could be a simple case of 'GIGO'

what do the frictional results look like, the two should be the same, or very very close. Check them against standard ITTC to see if the calculation is correct.

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### FanieFanie

The 15:1 hull is almost streight and flat like a motorised hull would be.
The 8.9:1 hull is more rounded like a sailboat should be.

On the 8.9:1 hull, what I did was to evaluate the position of each point and adjust it to where the drag is the least. If I attempt to improve the aspect ratio the drag goes up.

Try it yourself, you'll see.

5. ### Guest625101138Previous Member

But I already know what the lowest drag hull looks like. Why would I want to try to do any better when I know I cannot!

Rick W

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Fanie

what are the calculated frictional components for each?

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### FanieFanie

Youre lucky then mine doesn't give the the the option of aspect ratio and drag.

I'll fiddle with it some more, see if I can get it figgured.

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### daiquiriEngineering and Design

Try exporting model to Freeship+ and see if the problem persists. As far as I could see, Delftship free (if you use that one) has only 2 methods for the evaluation of resistance (Holtrop and Delft series) while Freeship+ has a broader choice of methods. Some of them are more suitable for slender hulls (kayak-type), like KAPER by John Winters.

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### FanieFanie

Thanks Daiquiri,

I'll have a look tonight. It bothred me that the 15:1 hull is going to be so slow

10. ### Guest625101138Previous Member

Fanie
The attached is the Delftship data for the optimum hull.

The Delft Series is nonsense. The KAPER is almost spot on.

As I stated earlier, you have to use the drag information from Delftship with caution. How is it possible that the drag could be a factor of 2 from one set of data to the other. In this case the KAPER series is much closer.

Rick W

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### fcfcSenior Member

Scope of delft series is Lwl / Bwl from 2.73 to 5. At 8.9 or 15 , you are way outside of this statistical analysis.

There are also limits on L/D , LCB, LCF, Cp, Cm, loading factor.

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