Power CAT - How much wide?

Hi,

Sailcats are usually wide. Power Cats however are mainly narrow cats. I believe a narrow cat has a better maneuverability at high speeds. Also longitudinal stability is better. Narrow boats also may be lighter for better speed performance.

If the beam between hull centers is named BCB then the Length/beam ratio of the catamaran is called LBRC, being LBRC=LH/BCB.
In cases where LBRC is = 2.2, the longitudinal and transversal stability will come very near to the same value (Terho Halme).

In case of coastal cruising in calm waters:
The question is if we do not care about short turning radius and we do have a light boat, can we use an LBRC: 2.2 or lower for a power catamaran with planing hulls? What about an LBRC of the order of 1.8-1.9? Is it dangerous? What would be a lowest reasonable value?
What are your comments?

Thank you.
Regards
APP

 

It you are considering calm water only and cruising at displacement speeds there is not much limitation except the added weight of the tunnel structure when going wider. Most likely you will be more limited by things like finding a suitable berth for the boat in a Marina than anything else.

You should still have a good turning radius. With wide hulls your propse are very wide apart and using one prop in reverse will turn the boat on a dime with that kind of hull spacing.

If you inted operating in waves with some speed then the situation is pretty different. Such wide hulls will adversely affect the roll motions and corkscrewing motion in quatering seas.

 

Beam of power cat is a function, first of all, of design speed. One needs wider cat at hump range; in planing range more narrow tunnel is optimal.

Then, add transportability limitations, parking, arrangement, etc.

The graphs shows relation between horizontal clearance c/L and volumetric Froude number, where c is width of tunnel.

Source:
Nazarov A. Power catamarans: design for performance// 2nd Chesapeake Power Boat Symposium, 2010.

 

Power Boat - Wide Cat

Thanks to all for your replies.
Alik: I think "Nazarov's optimal" is more to avoid roll motions than in higher resistance and loss of speed. So in calm waters the curve might be different, or not?
Regards
APP

 

No, this is exactly for resistance

 

Hi APP,
It may be better to rephrase your question to be more specific to your needs and aspirations, - - or are you simply posing an academic question? - It is not really clear what you seek...

 

optimal tunnel width also depends on the shape of the demi-hulls to some extent. Attached picture is from Yermolayev et al. (1997) Hydrodynamic Features of High Speed Catamarans,’ Hovercraft and Hydrofoil, Vol. 16.

Figure only considers tunnel with as a function of demi-hull beam (not LWL), which is not entirely correct but the general trends can be considered correct.

Nazarov's optimal goes to a maximum separation of C=0.75 which means L/B<1.5 I dont think I have ever seen such an extrme L/B. Normally sailing cats are L/B >1.8.

 

This graph from Yermolayev does not makes much sense, because optimal tunnel beam/clearance depends on length not beam of demihulls. This is quite simple: wave pattern is related to length first of all, so length defines interaction in tunnel and thus optimal size of tunnel. There is some relation of c to demihull section shape - I agree, but discrepancy is very high.

As to c/L ratio referenced in my graph, it is based on existing designs plus calculations.

 

What I meant with Yermolayev's graph is that that the Froude number ranges to requirements for wider tunnels based on demi-hull shape provide the right trends. Length is more improtant - agreed! Best is to consider length, demi-hull beam and demi-hull shape when choosing an appropriate tunnel width.

Typo in my reference above. Yermolayev's papers was published in 1977 (not 1997).

 

Changing BCB with Michlet software

I tried, using the same Cat, to change BCB and with Michlet software to calculate total resistance and compare output (up to 30 knots). No special results.

Perhaps, someone with a professional software could come out with some reasonable figures changing the distance between center hulls, keeping all other data fixed including displacement.

Regards
APP

 

Send a PM to Leo Lazerous, He is an expert...

 

You mean L. Lazauskas? Here is attached his related article
Regards
APP

 

Yep, - that is the person... (Leo, pardon my misspelling)...