# Resistance calculation

DELFTship forum Hydrostatics and stability Resistance calculation

• Author
Posts
• #34887
andrea
Participant

I’ve successfully designed a seaplane hull, and I am now trying to analyse the results from the Resistance Tool.

However, the program doesn’t display any results relative to speeds higher than 5 knots. What can I do?

Thank you very much

• #34892
otaku
Participant

Have you verified that you set the start and end speeds and the speed steps? Or, is there something about your hull that causes the resistance to suddenly drop off to a vertical-like line and reports resistance as zero?

• #34948
andrea
Participant

I think that the settings for the plot are correct, I’ve been working for two weeks on the hull but I still haven’t figured out how to calculate the resistance.

I attach the model I’ve created

Thank you

• #34949
Marven
Keymaster

None of the integrated resistance calculations is applicable to this kind of hull shape. You’ll at least need a planing hull resistance calculation, but even better would be a true CFD calculation.

Most of the statistical methods integrated in delftship will stop when the hull speed is obtained, that’s why the resistance at 20 knots is not calculated.

• #35218
lebar
Participant

Hi marven,
what do you mean by “hull speed is obtained”; i have the same problem and the calculation stops at 6kts for “my three hull” ship
eric

• #35219
lebar
Participant

In fact I would wish to know whether at what speed my hull (with 0.3m draught) will plane or not; Do you think this is measurable with delftship
Then I could optimze the profil of the wet surface to minimize the water resistance

• #35220
Marven
Keymaster

6 knots probably is the maximum (hull) speed for your vessel, corresponding with Froude number 0.40. Planing starts approximately at froude range 0.60-0.70.

• #35221
lebar
Participant

Fn=V/(G*L)0.5
L is 7.5 m
V is to determine
G is 9.1
How is defined Fn when you don’t know V ?

In the case of the trimaran is the formula applicable?
The botton hull shape of the main hull is broadly similar to my rib’s hull (zodiac PRO 5.5m) but more narrow so it “looks” longer in term of “finesse” (? in english). So according to what I understand this hull should be faster than my RIB (30 kts with a 6OHP when planning)…6 kts is quite few
good night

• #35222
lebar
Participant

Another way to ask the question is: Why the Fn associated to my 7.5 m hull is 0.4? taking into account that I hope to reach some speed over 50kts …

• #35290
Brecht Staelens
Participant

is it posible to calculate the resistance while driving backwards?

• #35291
Marven
Keymaster

Well, you can always try to turn your boat 180 degrees around. The algorithms used however are based upon a specific type of ship. You cannot used the Delft series, which is based upon sailing yachts, to predict resistance for planing motr boats for example. And of course the methods are also based on ships sailing with the pointy end in front;)

In short, it’s possible to fool the software by rotating your vessel, but the results will be useless.

• #35402
Aaron
Participant

I need an answer as soon as possible if you can. I have a sea plane hull similar to the original topic poster, however when I run the resistance calculation I obtain a graph that loops around and is apparently not a function. Is this also due to the way delftship calculates resistance. Why does it work for boats but not a sea plane? If you can elaborate on WHY, I would be greatly thankful.
Aaron

• #35405
otaku
Participant

Well, you can always try to turn your boat 180 degrees around. The algorithms used however are based upon a specific type of ship. You cannot used the Delft series, which is based upon sailing yachts, to predict resistance for planing motr boats for example. And of course the methods are also based on ships sailing with the pointy end in front;)

In short, it’s possible to fool the software by rotating your vessel, but the results will be useless.

Hmmm… That raises a question.

I had not until recently and then not again until reading this realized that calculations are (in my mind, crippled) in that someone, somewhere, deciced that Naval Architecture shall measure from the right, pointy end on the right, and that “tradition” flowed henceforth through time.

Given the computer age, I personally, strongly feel that modeling software in any industry should ask the user the USER’s prefered model orgin and then calculate from there. One Nav Arch I asked a couple of years ago had no specific on why it is that way, but posited that rudders were drawn first and therefore the architect or drafter started from the left and rolled the drawing scrolls up from the left.

Regretfully (wryly/snidely), I learned to draw ships (space ships) from the left to right, bow to the left, sine it seemed logical to them that since we typically use rulers and measuring aids numbered lowest at left, the art would follow function. Some real-world naval architects follow suit, but “tradition” has crept its head into the computer world and I suppose there is “resistance” to most firms providing calcs that work with the USER orientation, not implacable tradition.

So, now, I that while I could (and did) rotate my models 180, because my paper drawings are that way and because transferring information would be less insane, all the resistance calcs are worthless (not just because my hull is far, far larger than yachts, and later it finally was strongly impressed upon me that Delft series results are not applicable to frigate-type hulls), or rather, meaningless. (This makes me wonder and tempted to rotate the model again, bow to right, and running calcs on both. I thought I had checked just before committing myself to doing so, back in 08, but now I cannot recall, hehehhe…)

Thing is is that I don’t recall the manual stating the bow must be on the right. The calcs didn’t display a preamble of such a requirement, either. However, I am not a Naval Architect, and naturally would not expect to be constrained by programming or tradition. I generally thought it was something traditional, not that it was convenient for titleblocks to be read easier.

• #35444
Martijn Braam
Participant

Regarding your seaplane calculations: If you can get your hands on a copy of Fluid-Dynamic Drag of Dr. Ing. S.F. Hoerner 1965 Chapter 11 deals with the resistance of water-borne craft and the Theory of planing.

It will give you some directions on lift drag and effect of the shape of a planing hull.

Succes, Martijn

• #35445
lebar
Participant

Hi, martijn
May i get Ã  copy too? I word also on a planniing Hull AndrÃ© this software does not intÃ©grate That…
Many thanks,
Eric

• #35446
Martijn Braam
Participant

I can scan the chapter 11 it in to a .pdf file for you and send it to you next week when I’m back at work.

Happy holidays, Martijn

• #35447
lebar
Participant

Nice, thank you.
The only Book i found is related to air plane Only And chap 11 is about longitudinal stability
Cheers
Ã‰ric

• #35448
Aaron
Participant

If i may get a copy of that PDF as well, I would greatly appreciate it. If you need my email or whatever, just let me know.

• #35462
Martijn Braam
Participant

I’ve attached the pdf file. Good Luck Eric Have a nice flight

FluiddynamicdragHoerner.pdf

• #35464
lebar
Participant

Hi Martijn,
Thank you very much. I start to read from now
Cheers
eric

• #35465
Martijn Braam
Participant

Dear AAA3, Airbus?

I’ve send the .pfd to Eric in reply #2289. You can find the 5 Mb there.

Greatings Martijn

• #35632
John R. Coil
Participant

I’ve got a question about the Delft Series for resistance calculation: is there a limited range for prismatic coefficient that is valid for Delft Series calculations?

Marven stated earlier that it is for “sailboats” but my sailboats just seem to break the thing. Even without keels added to my model (with it managed to produce a literal figure-8 up and down the graph) it shows (for my last model) resistance actually decreasing between 8 and 9.25-9.5 knots. Sometimes these plotted shapes can be rather funky, taking a negative slope after the first peak has been reached. This seems usual for the somewhat high Cp hulls I’ve been producing (I seek to err on the side of having a Cp suited for hull speed based of statements I’ve read here and there … and a Cp of 0.6-0.62 should result in a SLR close to 1.3).

Fn is only around 0.4-0.45 for all the hulls I’ve contrived so I just don’t see the program ‘thinking’ that the thing is planing either.

• #35640
Marven
Keymaster

Can you perhaps post an image of the resistance curve here?

• #35643
John R. Coil
Participant

Here’s the resistance sans keels (KAPER seems to work):

Here’s the figure-8 (a curve fitting artifact to be sure, but a similar rise and fall in resistance) with keels (both don’t like it):

The hull in question is nearly identical (especially underwater) to the one I put up in show me your ship.

• #36304
Peter Edmonds
Participant

A somewhat belated contribution from a brand new user.

For planing hulls I have been using the Taylor Model Basin Series 62 monohedron – 12.5 deg deadrise, which I have in part set up as an easy to use Excel tool This handles the lookup from Excel tables for drag and trim, the interpolations for length/beam and displacement/length, and presents graphs of drag against speed. An alternative is the TMB Series 65, 25 deg deadrise, which I have yet to implement into Excel.

They are both published, but are a bit labour-intensive to do the lookups off the curves and then the interpolations; hence the part implementation into Excel to support my own practice.

The seaplane float case is interesting. I haven’t looked at the model, but if it has the conventional step I would consider the bottom forward of the step only for the planing surface. This will be how it is working in areas of interest – say above 10 – 15 knots.

I don’t think the standard TMB approach of 4 positions of LCG is going to work well. I would look to determining trim angles over speeds for all the positions, then match the trim angle to the anticipated aircraft pitch angle prior to take-off (around level flight angle?), then use the drag corresponding to your assessed trim angle.

Displacement will be an issue, as aerodynamic lift (from wings) will be quite significant as lift-off speed is approached. I would suggest taking displacement as full, 70% and 40% of full. You would then have to start guessing from known seaplane take-off experience as to weight on floats as against speed. However, I expect you could use assessed “weight on wheels” against speed for similar aircraft on runway take-off. I expect experienced pilots would have a feel for this.

In light of this, I would be interested as to how you solved your drag analysis.