How much faster will 7 feet longer be?

[yellowwducky]

Hi all,

Just running through looking at ideas on catamarans. Its interesting to see the huge price increases that come from 7 feet in length; say 40 - 47 or 42 - 49. Its generally on the order of 50% more money.

So, xyz manufacturer, generally speaking, how much faster is the larger boat when making these kinds of jumps in size? I gather 40-42 foot cats can make 10-15 knots pretty routinely. So would 47-49 be pulling 20?

Regards

[Bene505]

I'm not sure it's just speed. For instance, take a 42 foot boat and one that is 7 feet longer. That's 1/6th longer, 1/6th wider and 1/6th higher.

7/6 x 7/6 x 7/6 = 1.59. So that's 59% more volume.

To an engineer or math major, 59% more boat for 50% more money is a deal.

IIRC, to an economist, there's something about marginal benefit that keeps most people away.

[yellowwducky]

Oh I know there is other elements than speed - but I know that longer boats are faster, I am just not sure if there is a 'rough' rule of thumb as to how much.

[Jeff_H]

In the world of Monohulls, with all things being roughly proportional except that one boat has a longer waterline length, the boat with the longer waterline (as a percentage of its speed) will be faster approximately by the square root of the longer waterline length divided by the square root of the smaller waterline. Of course that is only theory, in reality, far more important than theoretical hull speeds is the percentage of time that a boat sails at or near its theoretical hull speed. Here there are huge variations in the percentage of time that boats are near theoretical hull speed which is the reason that modern boats generally turn in dramatically faster passage times than more traditional boats with similar hulls speeds.

All bets are off when you talk about a mathematical relationship of length to speed in multihulls. While there is some relationship, most multihulls behave as semi-displacement hulls (rather than planing or displacement) and so their speed has very little direct relationship to thier length. Just to be clear, while it is true that specialized very high performance cats can hit enormous speeds, the normal 40-42 foot cats very rarely make 10-15 knots and most cruising cats never make the upper end of those kinds of speeds.

The reality is that in windspeeds under 20 knots or so, a modern monohull will actually be faster than most cruising cats on almost all points of sail.

Respectfully,
Jeff

[Valiente]

Unless you have substantial resources, a 49 foot cat is going to need a special dock (end of a finger) or mooring. It's a vast boat due to the width, in most cases.

Take this model: LAGOON 50' JACK SPARROW New Catamaran For Sale

That couldn't get through the sea wall of some places I know of.

[sailingdog]

It really depends a lot on the design of the two boats. You can design a slow 48' catamaran or a fast one. For instance, compare a FP Salina 48 to a Gunboat 48. There's little difference in LOA, but the Gunboat will probably smoke the FP..

I know that I regularly sail at 9-12 knots in my 28' trimaran, and I'm pretty sure that Chuckles frequently sees 10-14 knots in his 34' catamaran...

Boats grow in three dimensions, as pointed out above. A 49' catamaran isn't 17% bigger than 42' catamaran... but closer to 50-60% bigger.

Originally Posted by yellowwducky

Hi all,

Just running through looking at ideas on catamarans. Its interesting to see the huge price increases that come from 7 feet in length; say 40 - 47 or 42 - 49. Its generally on the order of 50% more money.

So, xyz manufacturer, generally speaking, how much faster is the larger boat when making these kinds of jumps in size? I gather 40-42 foot cats can make 10-15 knots pretty routinely. So would 47-49 be pulling 20?

Regards

[Bene505]

Originally Posted by Jeff_H

...The reality is that in windspeeds under 20 knots or so, a modern monohull will actually be faster than most cruising cats on almost all points of sail.

Respectfully,
Jeff

Can this be true? I had no idea that was the case.

[yellowwducky]

Yes, I am curious about that one as well. Of the cats I have been pondering in the 42-49 foot length speeds mentioned are in the 10-20 knot range. I was on a 41 foot monohull this past weekend and they are thrilled with 8 knots. I was under the impression that a 49 foot cat would smoke a comperalby designed for speed 42 footer.

[Jeff_H]

I probably should have qualified my statement that a modern monohull will generally be faster than a cruising cat in wind speeds under 20 knots by saying instead that a modern performance oriented monohull will generally be faster than a cruising cat in wind speeds under 20 knots .

I see a lot of 42 foot and under catamarrans here on the Chesapeake. My 38 foot boat which is not as fast as most performance oriented newer designs of a similar length has a very easy time running down and passing cruising cats of this size and type pretty much in all conditions up to 20 or so knots of winds. As windspeeds begin to approach 20 knots, cats get harder to catch on a reach but are still easy to catch on a beat or approaching dead downwind. I have not seen wind conditions where a Gemini 105 is faster than a modern 38-40 foot monohull, but then again I don't recall seeing Gemini 105's out sailing in heavier conditions where I have been able to gauge their speeds.

But even looking a passage times for the long cruising rallies, the cruising cats just do not seem to have that much faster passage times especially when cruising cats are compared to the better performing similar cost monohulls.

Jeff

[JohnRPollard]

Jeff's comments above are consistent with my understanding.

Speaking in lay terms (the only ones at my command), the issue seems to be that cruising catamarans displace much less and have less reserve buoyancy than a typical monohull. So loading them up with the same amount of cruising gear adds proportionally more weight, and tends to immerse their hulls more and bog them down.

Keeping them light seems to be key to their overall performance.

Whereas a medium displacement cruising design monohull of the same length can manage the same payload with less effect on performance.

That said, there are light displacement mono-hull designs that labor when you load them up, too. Check the "pounds per inch immersion" figure when considering some of the modern performance designs. Not all of them make good cruising boats. But the latest generation of performance cruisers seem to be striking a better compromise in this respect than some of the designs from a decade+ ago.

There are some catamarans that I'd consider, and which I'd expect to come closer to delivering on the promised speeds. Sailingdog mentioned Gunboats. The other designs I like are the Chris White Designs Atlantic Series. The Atlantic 48' would be high on my list.

Chris White Designs