# Resistance prediction method

Discussion in 'Boat Design' started by Lukas, Feb 23, 2010.

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### LukasJunior Member

Dear all,

This is my first post and I hope that it is in the right place.

With a good composites experience and after lots of readings on boat design, I have been asked to design & build a 1.3m catamaran that is designed to carry GPS, depth sensor, sonar, etc. for an underwater research company.

It has to be remote controlled.

I have finished the drawings but I look for a resistance prediction method for this boat.

It is a round bilge catamaran, which particulars are:

LOA: 1300mm
LWL: 1253mm
Distance between hull axis: 1000mm
BOA: 220mm
BWL: 187mm
Depth: 250mm
Draft: 105mm
Displacement: 27kg
Prismatic coefficient: 0.775
Block coefficient: 0.537
1/2 angle of entry: 15.4°
Wetted surface area: 0.329m²

I guess that with such a distance between the two hulls, it can be treated as the sum of the resistance of two monohulls, can't it?

I still have to define the speed (I guess 0 up to 4 or 5 knots seems like a reasonable range), but to do so, I need to know if you could help me by indicating a good resistance prediction method for such a small boat.

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### LukasJunior Member

Any thoughts anyone?

I am sure there should be methods that could work. The model is small but rations should be quiet normal?

I have used ITTC for the skin friction resistance but struggle to find a relevant method for the residuary one.

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4. ### Guest625101138Previous Member

Lukas
If you post an iges 3D file of the hull I will do an estimate for you.

The hulls are very wide for their length. If you are serious about getting to 5kts with economic powering you might want to increase the length. Is there any reason why you are limititing to 1.3m?

Rick W

Last edited: Mar 2, 2010
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### LukasJunior Member

Thanks again Rick for the help.

I am really amazed by the different resistance compoents proportion.

Of course the boat is small (see particluars above) so the maximum theoretical hull speed is achived very quickly (3knots - Fn: 0.44).

Viscous resistance climbs regularly as expected but it remains a large proportion of the overall resistance curve (which was only expected at low speeds).

Do you have any idea why Rw remains so low? Is this an input error? A typical curve for a catamaran at high Fn's?

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### daiquiriEngineering and Design

Hello,
Is it possible to see some hull lines of your boat, if it is not a secret?
I'm asking it because the prismatic coefficient of your demi-hull (Cp=0.78) is rather high, in the range of planing hulls. Though you say it is a round-bilge, it still indicates that your boat might generate a sensible amount of dynamic lift at high speeds. How much - it will depend on the overall hull shape.

While Michlet does appear appropriate for the analysis of slender hulls like yours (but let's see the lines first...) at displacement speeds, it will not estimate the dynamic lift (and it's influence on drag) forces acting on the hull at high, above-displacement, speeds. So the previous graph should be taken with much caution at speeds above 4-5 kts (2.0 - 2.5 m/s).

Cheers

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### LukasJunior Member

This is not a secret at all! Here is it. Thank you for your time

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8. ### Guest625101138Previous Member

Hull speed has nothing to do with what speed a boat can achieve. It is simply the phase velocity of a wave train having wavelength equal to the LWL of the hull.

You might consider the drag low but it is up around 10% of the total weight at 2.5m/s. This is quite high for a cat - granted it is only a little hull so not unexpected.

The measured drag data I have for catamarans have more slender hulls than this. With the slender hulls I make the wave drag does not exceed more than 10% of the total drag throughout the speed range that goes to Froude of about 1.

You may find the drag is lower than predicted because I expect there will be some dynamic lift if you push the speed 5kts or more although it is not a good hull to generate lift.

It is a very heavy little boat. I build 6m long boats that weigh less than this.

Rick W

9. ### Guest625101138Previous Member

By the way you are going to need to have reasonably big props relative to the hulls to get acceptable prop efficiency.

Rick W

10. ### Guest625101138Previous Member

Another point to consider is that I can get my 6m long hull displacing 95kg to 6kts with the same power it will take your cat to get to that speed. So I do not consider the drag to be low for a 27kg boat.

Rick W

11. ### Guest625101138Previous Member

Attached is the same output screen showing data for my V11J hull. Power to drive this has been carefully tested so I have a lot a confidence in the Michlet data.

It is a much more slender hull than you are considering but I use it to show how little the wave drag can be.

If you are near water where sculls are used have a look at how small the waves are from these hulls.

I have also attached a chart with drag components of Michlet for a small monohull that is finer than you have. It is 1.8m LWL displacing 24kg. There is actual measured data on the graph as well. You will see Michlet is close to measured.

Rick W

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### LukasJunior Member

Who. Thanks Rick for all this information. Very interesting to get a comparison.

As I explained you by e-mail, both length and displacement are fixed... This is why I can't get L/B greater.

Thanks again.

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### marshmatSenior Member

Lukas,

The drag curves you found- wave resistance essentially flattening out above some threshold, and viscous drag becoming dominant- are about what one would expect for a slender hull being pushed to very high Froude numbers. At this weight and length, I would not expect top speed to be a priority, and a cruise of two or three knots would seem about right.

Michlet has been demonstrated to be fairly accurate for hulls within its realm of applicability (high L/B, smooth and fair, no protrusions or curvatures that would tend to induce flow separation). As with any computer model, though, its results are not gospel and are subject to human error at both the input and output stages. You have to give it sensible data, and be aware of the assumptions and limitations in Michell's theory, in order to produce meaningful results.

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### LukasJunior Member

Thanks again to all of you. Brillant forum with great understanding for amateur's questions...

I am aware of all the limitations and have tried to get as much reading & teaching by any ways than possible. This explains why I asked you this question as not sure about the results.

The reason why I was sceptic is because I have always heard / read / etc. that resistance was mainly due to friction at low speed and that this trend reversed as speed (and thus Fn) increases and that from there the major component of Rt becomes the wave resistance.

Remaking a Michlet analysis for lower speed and proves this and it makes sense now.

I was not aware that friction resistance gets again the major contribution to Rt for high Fn.

Interesting compromise then to find a hull that has a Low WSA for low speeds, high lift capability for transition Fn and planing speeds (often given by the mean of beam, chine, etc. all of these increasing WSA) and then a planing hull with a limited WSA for higher planing speeds...

Can't it be simple for once! Kidding. Thanks again for all this information

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### daiquiriEngineering and Design

Hello Rick,
The NPL4a graph (the second one in your last post) was already discussed by Leo in this post: http://www.boatdesign.net/forums/design-software/michlet-30472-3.html#post333153

Lukas, another source of error when modeling this particular boat (with respect to Rick's point-stern hulls) will be the wet transom - which is rather extreme in your hull. There is still no universally accepted mathematical model for wet transoms, so take that fact into account.

Let's make it clear - I am not telling you not to use Michlet for this task. Quite the contrary - use it, because it is a handy tool which can save you some time with your project (I wouldn't be saying it so easily if this was a full-scale boat for full-scale money, though). But, while using it, be well-aware of all the limitations it has, just like any other tool.

P.S.
Sorry guys, I see now that it has already been said in the last 2 posts while I was typing. you guys are fast. Good luck with your project, Lukas.

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