Limits to the beam of a catamaran ratio ?

I'm roughing out a 11m lightweight power catamaran floor plan and was thinking a 6m beam.

Is there a rule of thumb for this ?
Benifits / disadvantages to wider boats other than marina fees.

 

Hi. There are few "golden rules" but it depends on what type of power catamaran you are after. If you want light and fast, using planning hulls, then you can have relatively wide shallow V shaped hulls about a quarter of the overall beam. The only downside is fuel costs. If you want fuel economy and cruising speeds about 10 knots (peaks of maybe 20) then you go for thin hulls (length to beam from 12 to 1 upwards) and separate them as far apart as your build specs will allow accepting added overall beam often equals added weight. For reference Roger Hill type power cats versus Malcom Tennent canoe stern power type cats.

So what I am really saying is what is your overall requirements. How much accommodation do you need. Is it an offshore boat or a coastal cat. Do you want comfort and moderate speed or just high speed. What fuel range do you want at what speed. Is the build going to be high tech light weight, reasonable tech or EG strip plank cedar and plywood. Do you want inboards or outboards etc.

One of the better plan guides is provide by Richard Woods Skoota cats which has a 36 footer. Web site at: Sailing Catamarans - Click here for Power Catamarans (8 designs) https://www.sailingcatamarans.com/index.php/designs-2/6-powercats

Also if "Fallguy" is viewing the thread he can provide some valuable insites into the design and build of a Woods Skoota power cat. Hope you find what you want.

 

For hydrodynamics/resistance - as wide as possible
For structure - less beam as possible
For weight - less beam as possible
For resistance - as long and slender as possible, i.e less weight, per unit length
For stability - as wide as possible
For seakeeping - less beam as possible
For cost - take your pick of the above.

Plus what Old multi noted above.

 

Thanks.

11m X 6m would be within an acceptable range ?

I was aiming for a relatively lightweight aluminium boat, with a foam core/glass fitout, hoping for more complex structural design in exchange for the thicker materials. I understand the wider the beam the added weight compounds. The objective is to have an efficient boat with a pair of 70ish HP outboards with a 15kn cruise and a top speed of around 20kn @ better than 1.5lt per nm, maybe 1.2lt, am I kidding myself.

On a sailing cat they can run one engine cruising to save fuel, would this also be possible on such a power boat.

It would spend most of its useful life as a weekender (1 to 7 days) and be a coastal cruiser and occasional longer runs to some remote areas.

 

Try this paper.

https://www.researchgate.net/public...-displacement_ratio_and_breadth-draught_ratio

 

Given you dimensions, and requirements (SOR?) of an 'efficient boat', you will be starting with a length displacement ratio minimum of 6.5-7.0.
That means your total full load all up displacement on 11m length, would be around 4.00 tonne.

So, can you get your boat to have a max displacement of around 4.0 tonne...if you can, then you are on the right track.

 

I don't know, I was hoping for a target light displacement of 3t. I'd be disappointed if it exceeded it by much. Loaded for an extended cruise it will easily exceed the 4t at the time of departure but I accept that is not the norm.

 

Thank you. I'll have read tonight.

 

If you max displacement is around 3 tonne, post build, then your L/D ratio is 7.6..which is good. Anything above 7.5 is good.
Let's see how close you can get to that displacement...as it is easier said than done!

 

Gonna be hard to come in a 6000 pounds in 11x6 with Yammie 70s.

It isn't practical.

If you go too narrow hulls; you end up needing to make hull corrections.

More important than beam is lcg & lcb matching.

More important is bdeck clearance,

More important is a hull that meets the laminate specification.

Stay away from the danger of pretending you can get an 11m cat in at 3t.

Shoot for 11-12 to 1 for l/b per hull. My boat was 32' by just under 3' wlb or about 11 l/b. Now it is longer after some corrections due to lcg/lcb issues.

Anyhow, best to plan for 5 ton. See how Ad Hoc responds to my remark. He is on the right track discussing these challenges and he is the expert; not me.

 

Our cat is 12 m & does all that you are asking for. 2 x 60 hp Yamahas, just touches 19 knots, cruises at 13- 14 knots. 1.25 per nm per motor. weighs 3 ton.
Built out of Duracore, trips up & down the coast, Ballina to Hinchinbrook. A one off Orams design. Paul.

 

Tell us more and the displacements in all states.

 

Similar to Triroo, I designed and built a 10.5m power cat with 2 x 90 hp Suzukis, maxing out at just over 25 knots, cruising around 13 knots at 1 litre per mile (2 x 0.5 lpm), weighed 3 tonne at launch and
designed displacement of 4.5 tonnes. Many Bass Strait trips and delivered Melbourne to Brisbane and sold (eventually). New owner early 2023 and now considering returning boat to Melbourne!
Just goes to show that it is possible to achieve OP's objectives.

 

On outboard fuel consumption.
Old rule of thumb that is generally close. Carburettor & EFI two strokes - divide HP by 10 for full throttle GPH (in US gallons - 3.8 litres), so a 90 can suck 9GPH (~35l/h ) with its neck wrung - pull the stick back a third and halve fuel consumption. For 4 stroke divide HP by 20 and the reduce throttle by a third rule is applicable. If you happen to have a direct injected 2 stroke that runs divide HP by 15. Near enough for rough planning but it is easy to check consumption against engine hours to obtain an accurate picture of your rig in different conditions. Very useful for an estimate of how much a prospective purchase will drink, but don't bank on it.