# Calculating friction resistance, drag coefficient?

Discussion in 'Hydrodynamics and Aerodynamics' started by dustman, Jul 11, 2019.

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### dustmanSenior Member

I am attempting to design a sailing catamaran and need to determine how much sail area I will need.

Originally I discovered the psychosnail website and was using their calculator based Gerr's displacement formulas but over time pretty much everyone has basically been telling me the numbers it is spitting out are ridiculously low. So now I'm left to try and figure it out using drag equation calculators, which brings a new problem. What drag coefficient do I use in these calculations? The hull design I am going with will be very slender and streamlined so I imagine it would qualify as a "streamlined body" which seems to be associated with a drag coefficient of 0.04.

Is .04 the drag coefficient I should be using? Also, am I right in assuming that long thin hulls with a very streamlined shape at normal cruising speeds will produce insignificant levels of form and wave drag?

People are also telling me to just use sail area/displacement calculations to choose sail area. This seems haphazard to me, since different hull forms have very different characteristics.

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### jehardimanSenior Member

First of all, let me let you down easy...a vessel has a coefficient of drag because it has drag....the coefficient of drag does not determine the drag. As TANSL's spreadsheet alludes to, there are many things that impede the progress (I won't say forward, because a sailing vessel does not move directly forward) of a vessel. Each of these needs to be addressed to determine how much power is extracted from, and transfer to, the fluids around the vessel. There are whole books devoted to this subject, and lots of very limited rambling opinions interspersed with vague mathematical calculations and graphs of sometimes dubious worth. What you need to do is satisfy yourself that you have adequately calculated the viscous friction drag as effected by the pressure drag and sum the two. In your situation, the length, breath, and spacing of the catamaran hulls will complicate the matter due to unavoidable near field effects. Have a look see at MICHLET (Michlet | Naval-Architecture.co.uk http://www.naval-architecture.co.uk/?q=node/47)

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### DejaySenior Newbie

Prelimina.com has a free calculator based on "modified michell's theorem" that spits out drag values for different speeds, but you need to upload your boat hull as a 3D model. So it should be similar to michlet. Probably the best option short of CFD. Other than that I found a couple of spreadsheets like hullcalc.xls that use simpler formulas. Then you know how much force you need from the wind to reach a certain speed (or propulsive power at a certain speed).
Dolfiman also posted a spreadsheet to generate catamaran hulls and calculate their resistance here. I think it calculates resistance.

But there are so many things to consider with a sailing catamaran. Length and weight is most important from what I've learned on this forum. Plus stability to carry sails.

Why not look for an existing design and modify it?

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### DolfimanSenior Member

Thanks Dejay to promote Gene-Hull , but actually in the Catamaran version I don't compute the resistance at such, just the overall wetted surface upright or with small heel angles. I did it only in the Canoe version of Gene-Hull (also slender hull), a tentative VPP sheet in connection with the output parameters of the hull generation sheet, but focused only for Froude 0,175 to 0,325. See at quote #58 of Canoe length, efficiency and speed https://www.boatdesign.net/threads/canoe-length-efficiency-and-speed.61301/page-4
Assumptions of this VPP computation are detailed in the User Guide, pages 24 to 27 .

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### DejaySenior Newbie

Ah I see VPP stands for velocity prediction program haha. I'll have to look into how you do that with a spreadsheet! Sometimes in the future

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### gonzoSenior Member

The drag coefficient of a body moving along its longitudinal axis is not representative of a hull moving through water, unless it is going straight down wind. When you start point further upwind, the boat does not travel along its axis. You need to calculate resistance considering the direction in which the hull is moving, which will be much higher when close hauled.

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### TANSLSenior Member

This yes, this is a discovery

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### dustmanSenior Member

I'm just looking for a baseline. I understand that there are a number of things that will slow you down to varying degrees, leeway, waves, stuff growing on the hull, etc.

"a vessel has a coefficient of drag because it has drag....the coefficient of drag does not determine the drag" Which is why I attempted to specify the type of hull I would be using, one with minimal wave and form frag, which would leave me with a minimal drag coefficient, but what number that would be, I don't know. Streamlined body .04? Some vessels seem to have drag coefficients below this. How can that be?

If it helps get me a straighter answer:

30' LWL
16' Overall beam
Approx 8" Beam at waterline for each hull
Deep V cross-section with very fine entry and exit, basically a giant Hobie 16 with less rocker and more reserve buoyancy forward
1000-1200lb displacement
+/- 120 sq ft of wetted surface area

Based on what I have learned so far I am considering 150 to 200 sq ft of sail. Most likely a schooner rig with Wharram wing sails. I'd like to achieve 8.5 mph in a force 3 on most points of sail. I'd have a small aerodynamic cabin amidship, sitting room only. Going for minimal windage.

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### jehardimanSenior Member

Dustman, as you can see from my quote, all vessels are compromises knowingly or unknowingly made by its designer. Generally speaking, when an author says things like "Cd of a streamlined body is 0.04", they are actually talking about a generality made by plotting lots of collected data on similar vessels. Some may be as low as 0.03, some as high as 0.06 but most fall along a line that the author decides is 0.04. No more and no less. What makes a vessel closer to the 0.03 line than the 0.06 line can only be determined by analysis and experimentation across the design space...i.e. the so called "standard series" experiments.
While there have been standard series test for power cats (https://www.researchgate.net/public...-displacement_ratio_and_breadth-draught_ratio), I know of no published sets for sailing cats a-la the "ANTELOPE" towing tank tests or some of the work done by MARIN. Some students/labs may have been working on such tests in the near past, but the sponsors may not be willing to share that data. Other designers may have done it all CFD, refining (nor not) each successive built. Additionally, often material and construction must be considered which does not allow for the "calculated optimum" to be realistically built. You are not going to find "one with minimal wave and form drag" for your unknown hull form just tossed off in a boat forum. Because no one really knows what that is. You need to do the research and the work.

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This summary may be of some help in understanding the basics...

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### DejaySenior Newbie

Oh - does that 0.04 drag coefficient for streamlined bodies come from aerodynamics like e.g. wing data? If so - I don't think that is really applicable since you create waves in the water that create additional drag.

Have you read "THE DESIGN RATIOS A Naval Architectâ€™s Dozen"? He talks about a rule of thumb formula on page 19.

I'd also make a list of similar sized boats and put their length, displacement and beam width and sail area and speeds in a spreadsheet.

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### dustmanSenior Member

I have done lots of research and work as well as reaching out to those of you who may have a better understanding. In all my efforts I come up with all kinds of different answers, different formulas, differing opinions. Also, lots of ego of people who want to show how smart they are instead of actually trying to help where I'm at. I guess my only recourse is to become a mathematician/physicist/experimentalist so I can bypass the human factor. I have neither the time, money or drive to do this so I am seeking more general criteria with which I can design an efficient, safe boat that will wind up relatively close to my design goals. I have a quite good grasp of the basic principles of boat design, I just need some reliable numbers to go by.

I mean, what do you think? With my given design variables is 200 sq ft of sail gonna do it? According to the SA/disp calculator I should be "racing" along. I also don't want to overdo it and waste money and materials and get dumped over by an unexpected gust. I guess at least stability is more straightforward to calculate.

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### DejaySenior Newbie

Boat Design seems like a discipline where the more you learn the less you know and the less questions you can answer.

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### jehardimanSenior Member

Just build it because there is no single absolute correct answer, never will be. That is why it is Naval ARTchitecture, no correct answers, only better compromises. Your structural analysis, building skill, and weight management as well as the sailmaker's skill and your sailing ability will have more effect on boat speed and safety than which method you chose to calculate drag....the vessel will go as fast as it goes as permitted by wind and seaway.
FWIW, if you are really worried about losing the vessel to weather, don't build it and don't sail. The only totally safe boat is the one never built and the only risk free sail is the one never taken.
I'm not patronizing you.
The whole problem of naval architecture is an N-dimensional matrix with knowns, known unknowns, and unknown unknowns. It will never be 1+2=3. IF a naval architect had the lines and sailplan, and IF the naval architect determined it was viable to even construct the vessel to the plan, and IF the naval architect had the time to devote to the rigorous computation necessary, and IF the naval architect had due incentive, then MAYBE they could give you a drag with a confidence of +/-10%, which is about the best you will ever achieve real build/real water. The actual drag is one of the known unknowns. I always overestimate drag and put 5-10% in my hip pocket ready to pull it out if needed. There are too many factors, post computation, that effect the actual number, and the actual safety of the vessel. I have always said that the most important quality of a Naval Architect is a strong reasoned belief in your opinion. Many see this as an ego issue, but it is truly not. When standing in front of the sponsors about to spend \$100M, you'd better believe that the 20 knot ship will go 20.1 knots and not only make 19.9 on trials.

Doug Halsey likes this.

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### dustmanSenior Member

I think trying to design a good sailboat is one of most challenging things I have ever done. You think you understand it then you get slapped upside the head with the next variable which seemingly contradicts(but not really) the last thing you learned. And who knew you need higher math to predict the outcome of your design. I doubt the Polynesians had these problems. Perhaps I should take another approach altogether.

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