Resistance prediction methods/series for catamarans

Hi there,

I am looking for resistance prediction methods/series for catamarans around 10 m lenght, 6 ton displacement and 10 knots of speed (Froude number around 0.52). Any recommendations?

Thanks

 

You can try HERE as this excellent research may give you a good first indication.

 

We use following methods:
- Molland series - see link from AdHoc, for round bilge shapes
- Muller-Graf series - for semi-planing catamarans with chine
- Savitsky planing method with hulls interaction factor - for planing catamarans
- For narrow and light hulls, there is 'slender hull' method, this is originally a graph published by Doubrovsky in his book. However use with care, as comparing with tank tests seems the method underpredicts the resistance.

 

Which is also available HERE.

But with these parameters, the M-G series is generally outside your scope. But may offer you some guidance in addition to the Molland paper.

 

10 knots on a 10m powercat is only just over the hump speed. So it would be best to either aim for displacement speeds of under 8 knots or go for mid teens as a minimum speed.

see here for an example of one of my home built designs in that size range



Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Thanks a lot everyone, will have a fun weekend studying.

Yes the froude number is 0.52. Maybe at around 0.4 (~8kt) would make more sense.

 

I did a similar investigation for a sailing catamaran, see the document attached to the quote#51 and the comments around.
Calculating friction resistance, drag coefficient? https://www.boatdesign.net/threads/calculating-friction-resistance-drag-coefficient.62553/page-4

 

Hi all. There is some time we arfe working with this series and we get som values of the Residuary resitance - weight ratio negatives for centains Volumetric Froudes. Does anybody knows if perhaps, there is some printing error in the coefficients values. Any help, most welcomed. Regards

 

Hi.

I have found this calculator gives not unreasonable results for the power and cat specs I am interested in, but my interest is small motors on sailing cats.

Free to use Propeller Sizing Calculators https://www.vicprop.com/free-propeller-sizing-calculators

It predicts about 5 knots from 4hp on a 32' cat weighing 5300 lbs. Interestingly it predicts only 5,5 for a 6hp. When I input beam I double the hull beam to account for both.

I would be very interested if anyone knows of a better calculator for speed predictions for displacement situations.

 

I've used them in the past and not come across this...

Can you post your numbers/data?

 

That is a really fine power cat! Congrats Richard.

 

Can you test mine, SA-VPP Power Catamaran 1.1, based on Molland and al. results for Froude 0,4 to 1,0, and a connection with Holltrop for lower Froude < 0,4. It is a spreasheet application working with Open Office or Libre Office.

 

Here in my Excel spreadsheet, you can see there are values for some Froude that they are negatives....????

 

Thanks Dolfiman, I will check out with my data. We worked with Molland with good results for slow and medium speed. We are working with some high speed model with chine for easy and amateur construction...

 

Ok, fine, just to mention a typo mistake in my User Guide, it is :
Cms : midship section coefficient (section / Tc Bw) , and not of course .... (section / Lw Bw).
(This coeff Cms is used in the Holtrop / coeff c5 formula)