Catamaran resistance

Hello everyone.
I have workboat catamaran L = 19 м, B = 12 м. Distance between hulls - 4 m. Displacement 315 t. Hard chain hull.
Cfd in Open Foam show us very high towing resistance for 10 knots. Actually almost double from expected. What’s may be wrong? Any ideas from forum experts? Can it be a bad interference of waves between hulls?

 

Draught over 2.5m .How it looks like at the bow ?

 

The displacement is unrealistic; forgotten a decimalpoint?

 

Draft is 2.6m

 

You have a slenderness ratio of 3,5 per hull, using the 19 m length as waterline length. This is unrealistic, and resulting in extremely high resistance. Something is wrong with your design or the values you have given.

 

So just on those metrics, a box of 19 x 4 x 2.6. That equals 198 x 2 = 395m3 or roughly 405 tonne.

This means your bow will need some 'shape so say take off 15%-20% and the stern is most likely prismatic so 0% , so best case 324 tonne.
Whilst the numbers "suggest" this is possible, it makes little sense, as it is ostensibly a box with a pointy bow.

Your LD ratio is just 2.76, or 2.8.
That is woefully low...

Agreed, something is not right!

 

Hull is similar to this one AQS Marine - AQS https://aqs.no/vessels/aqs-marine/
It is a workboat.

 

That link does not provide any info on its displacement or loading capacity. Scaling your displacement value to that size would still be unrealistic; a decent barge shape would be more efficient.

 

As you can see there is almost no freedom for any kind of optimization. My question is why our CFD show almost double resistance in comparison with the prototype? What’s may be wrong? AQS Marine - AQS https://aqs.no/vessels/aqs-marine/

 

Make vertical surfaces between hulls straight and parallel from the bow to a stern.

 

We test 3 different versions of bow profile, but resistance becomes even higher. Looks like there is some bad interactions between huls.

 

Is the bridgedeck really fully wetted on its downside? The pictures suggest this ...

Also have a look to the flow around the transoms. I would expect a hollow behind them but the pictures suggest they are fully wetted - giving a large low pressure area ... may be you need to set a stream separation condition there, or a much finer mesh on this area ...

Im not a expert, just a idea.

 

Yes that is a classical mistake. In this condition, you have a huge transverse wave rolling between the hulls. I suspect that the CFD algorithms have difficulties with this wave condition. I've seen examples of this in reality, where designers/builders have lost control over weights during the design/build stages. We even see the shadow of the situation close to hump speeds with planing catamarans. And the phenomenon occurs also with asymmetric cats, although to a lesser extent.

 

Honestly,
If you are still wondering why the resistance is so high after seeing the pictures, it would be time to get some help.
If this is a real commercial project, the budget allows it.
If it‘s a learning fun play around project then just experiment and see what you learn the hard way. Also reading some books might be useful. Or just go play with real boats.
Any optimist sailor at the age of 6 knows that if your are pushing a square bow under water is slow.
Playing around with boats is the most fun thing anyone can do in his life.
Have fun