Resistance Model

I am running a resistance model on a very high L/B catamaran and am running into an issue where NavCad doesn't have any predictive models that are particularly relevant. Does anyone know of a model that is designed for something in the 18-20 L/B range, either software or a published paper on the topic?

 

Is this part of your course work, or is it for a boat that you would like to build yourself?
If the latter, would it be feasible to build a model, and test that?
It need not be any fancy test rig - many years ago we tested a model using my boss's 30' motorsailer. We used a wind surfer mast as an outrigger to tow the model in calm water off the side of the motorsailer (rather than in the wake). With strong fishing line attached to a spring balance (as used by fishermen for weighing their fish) via a pulley on the end of the mast to measure the resistance at different speeds.
And it worked very well.

 

I assume L/B or 18-20 is for the demi-hulls, not the overall L/B. Michlet by Leo Lazauskas could be useful but it may no longer be available. Michlet https://www.boatdesign.net/threads/michlet.30472/

In addition to the resistance of the individual hulls interference effects between the hulls may be significant. I don't know what if anything Michlet did to account for interference effects.

What Froude number range and displacement/length ratios? For lower Froude numbers and displacement/length ratio with L/B or 18-20 skin drag will be dominant and wave making drag will be insignificant.

 

David,

The design speed is supposed to be 6kn, which results in a demi-hull Rbare of 80.1N (Holtrop) and a Fn of .378.

Correct the L/B is for each demi-hull not the overall design, and I have no good way yet to model the interference resistance. So I am really facing two issues, a) the L/B is currently at 21.2 which is pretty far outside the reasonable range for everything and b) the interference issues as mentioned. But with this L/B I suspect, but don't know that the interference is going to be very, very low just because the wave generation should be pretty minor. This is predicated on the idea that with this extreme L/B and the shallow draft (5.3") the generated waves are going to be pretty trivial. But I would really like some way to evaluate this other than 'I kind of think this is true,' which strikes me as a particularly poor amount of data.

I have also considered reducing the length just to pull in the L/B a bit and make it more reasonable to model, as well as reducing surface area a bit. But then I end up chasing my tail with a deeper draft and not much less wetted surface so the effects appear to be somewhat minimal, but since I don't have a resistance model I trust its kind of shooting blind. And since I am trying to keep to less than 6" of draft it doesn't take shortening it much before I hit problems there as well...

 

Its for coursework so a model is pretty unlikely. I did something similar to you for another project with a loadcell and my powercat, but for this I don't see it happening. But who knows, one of the advantages of being a student is I can get time in our tow tank... So if i find the time I may 3D print a model and chuck it into the water just to see what happens. It would be a pretty fun senior design project if nothing else.

 

I found some old copies of Michlet to play with, but I couldn't find any data on the circumstances it works well with. So at this point I am planning to download OpenFOAM and a wrapper and see if I can get anything useful out of a cursory CFD analysis (lol ya right).

 

There might be some useful stuff in this paper?
A Practical Design Approach including Resistance Predictions for Medium-speed Catamarans https://www.academia.edu/24912665/A_Practical_Design_Approach_including_Resistance_Predictions_for_Medium_speed_Catamarans

Or this by Tony Molland?
https://eprints.soton.ac.uk/46409/1/127ShipScience_Report.pdf

Tony Molland and Mustafa Insel also did a lot of work together on resistance of power catamarans - here is one of their papers.
https://www.researchgate.net/public...ponents_of_high_speed_displacement_catamarans

If you google them a lot more info should come up.

Excellent! 3-D printing sounds like the way to go nowadays - I built our model from balsa - and unfortunately on the last run it decided to nose dive and went under (we were experimenting with LCG......) and we never saw it again (unfortunately the fishing link chose then to break, otherwise we should have been able to fish it out).

 

I proposed a power catamaran VPP here, in case this can match with your project :
SA-VPP power catamaran https://www.boatdesign.net/threads/sa-vpp-power-catamaran.65725/

 

Series 64 hull.

 

Thanks for the suggestions!

I have been trying to convince my school (UNO) to hire a grad student to explore using an infinite Z axis 3d printer to print large scale test models. I have the schools backing, but sadly they can't find anyone to take on the project and I don't have time. But I suspect a large format printer should be able to print out a sizable, testable hull form fast and cheaply enough to make running new Series runs reasonably easy.

 

Perfect, thank you very much.