Catamaran Hull Speed

[CapnHand]

The hull speed formula for displacement monohulls is 1.34 x the square root of the waterline length. When planing, a monohull will exceed it's hull speed.

Obviously this formula doesn't apply to catamarans. Even when not planing, they will travel much faster than what the hull speed formula predicts. Is there a formula or general guideline for catamaran hull speed related to waterline length and / or hull length:beam ratio?

[Giulietta]

Here, read this...

[Jeff_H]

Few Catamarans are actually planning vessels. Most are semi-displacement boats (like most modern IMS/IRC derived monohulls). Both forms of semi-displacement boats achieve speeds that are higher than normal hullspeed (i.e. 1.34 x the square root of the waterline length) by minimizing wave production. In the case of multihulls, this is done through a very narrow waterline beam to length ratio, and minimal interference between the waves produced by the other hull(s). Semi-displacement mono-hulls cheat a bit by using very fine entries to minimize the size of the bow wave that they produce. In both cases, clean hull forms and minimal drag is critical to overall performance, but properly designed, semi-displacement boats can achieve sustained speeds that can literally exceed twice their theoretical hull speed without planning.

Of course as Alex points out, and as the article that he linked to explains, few cruising cats achieve passage speeds that exceed or even match those of modern performance mono-hull cruising boats.

Respectfully,
Jeff

[sailingdog]

Most modern catamarans have a LWL to Beam ratio of about 8:1. Most modern trimarans are a bit higher than that.

My 28' trimaran has been up to 15 knots without planing or surfing.

[CapnHand]

Thanks guys.

[Rockter]

Damn multis.

They leave me standing.

[sailingdog]

Yup... is true... leave boats a lot bigger than mine in my wake.

Originally Posted by Rockter

Damn multis.

They leave me standing.

[BigCat69]

This article posted by Kantor explains how to calculate hull speed for catamarans, based on the width of their hulls (in proportion to the length.)

Southwinds - January 1999

To get the adjusted hull speed for the catamaran design on my website at http://www.dunnanddunnrealtors.com/Catamaran .html .
you need to know that the waterline length of one hull divided by the beam of one hull is 12. That gives you a 'k' factor of 3.3 - the k factor replaces the usual 1.35 that you multiply times the square root of the waterline length to get the hull speed.

The square root of 64 is 8, and 8 times 3.3 equals 26.4 knots. Obviously, if the boat is heavily loaded it's going to take a bunch of wind to hit hull speed, and you have to be careful not to turn it over or pitchpole it.

My old voyaging monohull, the Batwing, rarely made its theoretical hull speed of 7 knots. We had a few 165 mile days, but when you do that in the trades you are usually getting a half knot lift from current. It was really a comfortable boat to voyage in, though, and we very very rarely turned on the engine. I mean we'd go weeks without turning on the engine, and usually sailed in and out of anchor. We cooked, lit, and when cold, heated with kerosene, ate canned and dried food, and so on. This was back in the 70s. Our power usage probably averaged less than 10 watts a day. I laugh when I think of all the electrical stuff voyagers find necessary today.

This was before the GPS system, too. As the saying goes, we were never lost, but occasionally we were confused for a few days-until the sun and horizon came back.

[sctpc]

Under body shapes: keels and centerboards, Beam & hull-beam ratios, Beam overall – overall wide beam versus standard beam, Displacement but I can`t find any mention of the optimal hight of the bridge deck cabins, Giuliettas post mentions under deck clearance and it`s extremely important but if you build the bridge on top off the bridge deck would that make it extremely unstable as the center of gravity would be to high making the cat useless in any swell.

[sailingdog]

The problem with catamaran design is that to make the bridgedeck have standing headroom you either must make the cabin excessively tall or give up the clearance under the bridge deck. That is why so many of today's catamarans have such poor upwind performance. They've got too much windage to sail well to windward.

Originally Posted by sctpc

Under body shapes: keels and centerboards, Beam & hull-beam ratios, Beam overall – overall wide beam versus standard beam, Displacement but I can`t find any mention of the optimal hight of the bridge deck cabins, Giuliettas post mentions under deck clearance and it`s extremely important but if you build the bridge on top off the bridge deck would that make it extremely unstable as the center of gravity would be to high making the cat useless in any swell.