22' Planing Catamaran Design

Hi Guys, I am new to forum and see their are some very helpful members with good knowledge on the Boat Design topic.

I am busy studying boat design through Westlawn Institute of Marine Technology and I am on the lesson of multihull design.

So I have started on a 1/10 scale model of a 22ft planing cat plug which I will later use to build a mold and later a boat out of. The plug is built from balsa as the core and will painted with topcoat faired and painted again until clean enough to produce a mold out of.

But I have a few questions:

1. What is the general rule with DWL to BWL ratios, my text from westlawn only refer to big boats and mainly displacement hulls.

2. Is there a relationship between tunnel width/beam at the DWL compared to the sponson/amas/demihull (whatever you guys refer to it as) beam.

3. My text refers to tunnel height above the dwl, any rules or ratios you guys use? The text refers to 10% of the waterline beam as a general tunnel height for boats under 30ft.

Any advice would be much appreciated, pictures to follow on the progress.

 

These answers in this paper: Nazarov A. Small Catamarans: Design Approaches and Case Studies//International Journal of Small Craft Technology, RINA, Vol.157, Part B1 2015.

 

Hi Alik, thanks for the reply, where can I find this?

 

Order from RINA www.rina.org.uk

 

So here is what the hulls look like so far, still have a lot of work to do.

 

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This another model I made just trying a supposed tank test for Cat hull charcteristics.

I had one that wanted to broach or "sponson walk", and this one that handled really well.

 

Thanks!

 

My first impression is that the hulls are too full, forward, by a margin, you will get a fair degree of "bog" running into the back of waves, down-sea. Which is annoying, and washes off too much speed. And produces a lot of spray.

 

Ok thanks for the input I will make the entry a bit finer.

The other thing I forgot to ask (I havent ordered that paper published by RINA which might answer this), but what are your guys thoughts on square, semi-sqaure or round tunnels? Which handles better? All the guys down in south africa swear by the round tunnel, but I have been on square tunneled cats that also really rode nicely as well... Magnum 32' and the Magnum36' by Two Oceans marine and the Cobra Cat525 just to name a few.

 

Last one.

 

I think for 6.2m this is OK. I owned a 6.2m cat which we designed, and tested it in variety of conditions. It should be quite full at the bow for rough water performance, especially on following wave.

 

South Africa was an early adopter of power cat trailer boats, as was Australia, and no-one here was more influential than this chap, whose designs more or less cemented the popularity of the type. You will find this interesting, I think.

 

It will still do the job, of course. and it is a little difficult to tell from the pictures, but that was the impression I got, fuller than it needs to be, it doesn't matter that much if the bows plunge a bit in a following sea, they still won't cause a broach.

 

Interestingly. old Bruce in that video, says the bows need to be full enough, well I can say for certain that Bruce's cats were nowhere near as full as the OP's. He got it pretty right, bog was negligible. I have seen a few alloy cats that were as full as the OP's, but the best alloy cats were less full.

 

Another thing in small cats I am not enamoured with, is high bows, which might create an impression of seaworthiness, but causes too much aerodynamic effect, and raises COG, it is better to cop a bit of green water over the bow, and deal with it on the foredeck. rather than try and stop the bows dipping in at all.