maximum hull speed for cat

[Nauti Cat]

Is the formula for max. hull speed the same for a cat as a mono hull.
If different --What is the formula?

[David M]

Many catamarans are not limited to their hull speed. I did this with a Hobie 18 frequently. An 18 foot boat has a hull speed of 5.9 knots. I frequenty hit 20 knots and above and was not planing. You need to have a high sail area to displacement ratio, a high righting moment and narrow hulls. Catamarans do have a hull speed but it is not their limiting factor. Same with powerboats, once they reach hull speed, if they have any more power then they start to plane. Some monohulls can plane as well.

The most common formula is the square root of the length in feet times 1.34...the speed units are in knots. You will see constants that varie somewhat from 1.34.

[44'cruisingcat]

There was research done in the UK that indicated that when waterline length to beam ratios exceed around 8:1, the limitations caused by wavemaking drag become much less important. Most cat hulls are better than 8:1, and I'd say all tri's would be.

For boats with L:B ratios of less than 8:1 to exceed hull speed they need to plane, which requires enormous amounts of power. Boats with higher rarios can exceed theoretical hull speed by large amounts without planing, and don't require anywhere near as much power.

There's a video here of a 50 foot power cat at 30 knots in full displacement mode. It is powered by a pair of 375hp diesels - very modest power for a boat of that size and performance: Allura Marine | Welcome

[Nauti Cat]

Guess I need to be a little more specific: 46' power cat w 17.5 beam
37 tons. Max. hull speed by standard formula for mono hull would be 8.6kts
So, would this be the most efficient max speed? It does have twin 370hp that have top end of 22kts., but I'm trying to figure the most effecient speed for long voyage.

[44'cruisingcat]

Really you'd need to establish that by testing. I think Bob Oram said that in the tests they did, the Allura boat was at it's most fuel efficient at 15 knots - 37 litres per hour total, = 2.5 l/nm, and 3.0 l/nm at 20 knots.The very best economy might lie somewhere in between.

At 7 knots it used 24 l/h = 3.4 l/nm, so economy was better at 20 knots.

That boat is 16 tonnes and 55 feet.

It would probably depend on a lot of things - the engine characteristics, the props etc etc.

[MidLandOne]

Originally Posted by Nauti Cat

Guess I need to be a little more specific: 46' power cat w 17.5 beam
37 tons. Max. hull speed by standard formula for mono hull would be 8.6kts
So, would this be the most efficient max speed? It does have twin 370hp that have top end of 22kts., but I'm trying to figure the most effecient speed for long voyage.

For the power cats I am used to dealing with the following generally applies. But I make the point that these are very well designed efficient foil assisted vessels so will not necessarily apply to others. They are also bigger than yours.

- Most efficient fuel usage per mile run is when the boat is on the plane, not in displacement mode.

- Once fully over the hump and properly planing the fuel use is constant per mile run ie going faster uses no more fuel per mile run, and that has been so with vessels capable of 40 knots.

But vessels vary, and with cats especially with their displacement, and unless you can get reliable information from the boat's designer you need to do sea trials to measure the fuel consumption.

[nautical62]

To add a bit to an above post, the formula I've most often seen is Maximum hull speed (displacement) equals The square root of the water line length times that vessel's hulls K value. The K value for many monohull cruising boats this may be about 1.34, but it will be very different for most catamarans - mostly higher K values which means a faster hull. My understanding is most cruising catamarans don't plane, they just have very high k values. However, one must also note that with less buoyancy, for a given length, adding weight to a catamaran will greatly increase the wetted surface as well as other characteristics which will slow down potential speed more than a monohull of comparable length.

Charles Kanter goes into how hull shape and other factors contribute to the K value as well as moving characteristics in his book: "Cruising in Catamarans"

As one approaches the theoretical maximum displacement speed, resistance increases more and more, so you get less speed gain for a given increase in power. At slower speeds, wind and current will play a larger role. I think it's hard to talk in terms of maximum efficiency without knowing more. Your most fuel efficient speed to cross an ocean in a power cat is probably to get into an ocean current and prevailing wind, throw out a sea anchor and turn off the engine. The most efficient speed for you will include how important your time is to you. What I think you really need to do is get out in your boat and test the fuel efficiency at different speeds and decide what trade-off you are happy with.

I know in a previous outboard powered pocket cruiser I had, I could go about 7 knots at full throttle with a 10 HP outboard, but there was a lot of pressure on the rudder and the boat did not behave well. At less than 3/4 throttle, I could still do 6 knots comfortably with a notable savings in fuel. That was my typical cruising speed under power.