I have had two planing cats designed by Ron Given. The PT and the Tiger Shark.
While sailing my 14sq I often arrive at a place where the boat simply does not want to go faster. The hull is trying to go bow down and displacing a lt of water. (on my 14 foot PT up wind this was never the case. it simply kept going faster until it ran out of puff. It was also much more settled.
The Tiger Shark gave me several rides on a tight reach that were stunning. Steady. planing motor boat like feeling. Took it up to full speed and then wanted more.

I don't know whether it is B.S. or not. I haven't had the chance, or testes to try it yet. But the guy I bought my FX-1 from said that you can trap off the back corner of the wing in 15kts with the chute up and the boat will plane. He said you can get about 4 feet of the leeward hull in the water and thats it.

I'm new to running a chute and today I learned three very valuable lessons about running a spinnaker singlehand while trapezing.
1: Don't drop the spin sheat.
2: Don't drop the tiller. Things turn very hairy very quickly.
3: Don't go too deep when dropping the spinnaker, or be freakish-ly fast about getting it down. Accidental jibe, followed very quickly by almost capsize when it rounds up on the other tack.

The "constant" in the equation for displacement hulls is dependent largely on the fineness of the hull in question. The oft-quoted 1.34 constant is pretty good for hulls 1/3 to 1/2 as wide as they are long (fineness of roughly 2:1) but terribly wrong for hulls 1/10, 1/12, or even narrower in beam compared to length. The formula is still good, it's just that the constant changes dramatically. It's something like 4 or 5 for most beach cats; this works well with empirical data of maximum speeds such as Carl posted.

sail fast, y'all!

PS: My abilities (and lack of them) limit my top speed, too.

The laser dinghy has roughly 1/3 hull ratio and it too is not held back to 5 knots as its top speed.

There is only 1 true constant in Froude's law and that is 1.34. All the other constants are just tricks to make the formula fit the real life data where indeed the formula is not applicable anyway.

Froude's law only decribes the speed at which a wave on the watersurface travels in relation to its wave length. It has nothing to do with "theoretical hullspeed" apart from the fact that HEAVY boats in comparison to their waterline length are unable to travel faster then the wave system around them. In this situation the wave length of the wave system is then the same as the waterline length of the hull. Basically the hull is trapped between two wave crests and has insufficient power to force itself out of this craddle.

LIGHTWEIGHT boats tend to have plenty of power in relation to their size and as such can just overpower this limitation and accellerate to higher speeds. This is what catamarans, surfboards, skiffs and dinghies do. This can also be done on a larger scale with for example navy Frigats deploying gasturbines as engines. Another way around this cradle is to go onto foils or start planing. Naturally a navy frigat neither planes nor foils, it is too large for that.

Wouter

The equations we use to discribe fluid flow are mostly approximations. The equation for maximum speed for a displacement hull is really just a fancy curve fit. It works for basic hull shapes and if the ratios of the length, width and depth are within limits.

If you make the hull narrower, you have to make it deeper to support the same weight. At some point the hull gets deep enough that maximum hull speed equation is not valid anymore.

I know this explaination is not very satisfying but the real explaination takes about 3 chapters in a book on Fluid Dynamics and they don't even talk about it directly.

Bethwaite's book is a good top level start but he he doesn't go into the details that would matter if you started building something really strange like a hull that was 4 inchs wide and 5 foot deep.

Finally I am just talking about hull speed. The dynamics of the hull are just as important if not more. For example, a boat might have great speed but it really hard to tack or it pitchpoles or it hobby horses and stops every time it hits a wave more than 6 inches high , etc, etc etc

Wouter,

This is a good explaination of what I've always agreed with...that standard cats & tri's don't really plane. They are displacement hulls that have sufficient power to weight ratios to overcome the bow wave limit.

What performance benefits would you get from a boat that had most of its bouyancy on some kind of bulb(s) that were completely below the water surface? The actual "hull" could site above the water surface supported by vertical fins. This would eliminate huge amounts of drag from the water/air interface...and it the reason why submarines go faster while submerged.

HSV 2

Would the bulbed hulls be expected to leave the water for some good old fashioned bulb flying?

GARY

So... should we define cat hulls in general as "wave piercing displacemente hulls"?

It is called a SWATH (Small Waterplane Area Twin Hull).

The problem is that they don't have as large a waterplane area as conventional vessels (refer the name) and therefore have less load carrying capacity. This basically means that they will sink, trim and heel more than a conventional vessel under the same loads.

Not a problem for vessels that have a very definite load and operating environment, infact they can perform their tasks very will if designed correctly. They are also fairly good seakeeping vessels if they have enough freeboard.

It is a problem however for sailing vessels that operate with large variations in trimming and heeling moments from the rigs.

The idea is correct though, you only have to worry about friction drag if completely submerged. SWATHS generally have lower wave drag than conventional vessels.

HSV 2 is not a true SWATH, but it has similar principles applied to its design.

I don't like the term "wave-piercing" in reference to sailing cats. They are just narrow enough to not be as influenced by waves as other boats.

HSV 2 is a wave piercer in the true sense of the word. There's a couple of meters of bow that are actually under the water and the top of the bow bulb is sharp. Having said that, the original wave piercing ferry that ran across Bass Strait was not known for the forgiving seakeeping in bad conditions and was often called the vomit comet.

A better, but not as sexy, term is a semi-displacement hull or high-speed displacement hull.

Certainly not planing, and not really wave-piercing, all boats go through waves, cats just tend to be influenced less by waves than regular boats because they are generally narrower.

It's just that nearly all cat hulls can "pierce" or "cut" through the most important wave: the front wave created at maximum displacement speed.

One question: is piercing the front wave easier or harder than piercing other waves?

The front wave moves with the boat, others move in other directions.
The front wave has a height related to the hull's displacement, others have any height.
The front wave travels at a well known speed, others travel at any speed.

The front wave is definitely special. But this does not answer the question, so lets think different:
If it was harder to pierce (or cut) through the front wave, most cats would be able to pierce waves. Since this is not the case, the front wave must be easier to cut or pierce.

Ok, I'm convinced: calling any cat a "wave piercer" is a BIG mistake. They are "front-wave cutters", "wave-cutters", "front-wave piercers", "high-speed displacement" or whatever better describes their particular method to cut through the front wave.

Now, cat hulls are closer to wave piercing hulls than planing hulls, that climb the front wave and ride it. And cutting through the front wave requires at least part of the features of a true wave piercer. So... Why aren't all cat hulls built as true wave piercers? Due to lower payload only? Wave piercing is inefficient vis-a-vis some other mode? What are the reasons?

Damnit, I've commented on three different posts about wave-piercing hulls today and can't remember where the relevant stuff was to transfer to this discussion.

Generally speaking, all waves are the same, energy moving through a medium.

The bow wave however is going to be stationary relative to a constantly moving hull, so the energy "in the wave" isn't working against the hull, but rather the hull is generating the wave which takes energy. This is where the design of the bow needs to try to minimise bow wave generation.

Wind and ocean waves are still energy moving through a medium, but relative to the hull, that energy is generally coming towards the hull faster when going to windward than when going downwind (if overtaking the waves). These are the waves you want to have minimal effect on your vessels speed and seakeeping. An investigation into the natural pitching frequency of a cat hull relative to wave incidence period (which changes with boat speed and heading) would be interesting.

I'm not completely up on wave mechanics though so that's the extent of my wave discussion.

Don't forget this is all relative to a fast moving hull. Slow speeds will have different things to worry about.

My opinion why cats aren't built as true wave piercers, or why I won't design one as such, is because wave-piercers stereotypically have very small waterplane areas towards the bow. This means that under the influence of a pitching moment (from the rig in the case of sailing boats) the hull has to submerge further than a conventional hull shape at the bow to oppose this pitching moment. Not a problem if you expect minor pitching moments, but rigs can generate huge pitching moments...who hasn't pitch-poled one of our cats? There are other forces acting in the case of a pitch-pole, but I don't wanna go there today.

However, there may be other benefits to current "wave-piercing" like bows for fast beach cats. Mainly reduced windage upwind and reduced radius of gyration (because weight is taken away from the extremities of the boat). Maybe we should call them "wind-piercer bows" instead?

For a engine powered craft with a screw this indeed a viable concept and several vessels have been made like this.

However for a sailboat you have to content with a widely varying loading pattern due to the tall rig and varying winds. In effect vessels of the first kind are very stationary in their bouyance distribution requirements while sailing vessels are not. The latter need reserve bouyancy to handle gusts and other pitching moments due to being top-heavy (mast + rig).

Typically a crew is too slow to compensate for these pitching variations by repositioning themselves.

Wouter

Actually one of the more important effects of what many call wave-piercer hull design is reduced pitching by increased damping.

Designs like the flyer, capricorn and Blade have redistributed the bouyance in the bows to the keel line. This means that in flat water the boat has to pitch less to bring this volume into play then a conventional hull would. The volume is closer to the watersurface and brought into effect sooner.

In wavy conditions the difference is negligiable as the both bow downs have the same amount of bouyancy designed into them (difference is only found in the distribution) and the oncoming wave easily submerges the whole bow anyway. Of course underwater any given volume of air has the same bouyancy force no matter how deep it is submerged. This means that both hull designs lift the bow by the same measure when striking an oncoming wave. But after the wave has passed the "wave-piercer" design stabilizes itself quicker and with less up and down movement. When riding down a wave the "wave-piercer" lets the bow dive less deep when hitting the trough of the wave, therefor the boat can be pushed harder.

That is the true working of a "wave-piercer" bow. The name "wave-piercer" is therefor totally misapplied.

The raked back shape to the stem is a directly result of the fact that bow volume reduces towards the decks in "wave-piercer" bows. And if you have to planes (sides) moving towards eachother when going up then their intersection is a straight line that falls back. This is directly the opposite of when two planes move away from eachother when going up. In effect the raked back bows are direct practical result of the decision to redistribute the volume and concentrate it low in the hulls. It had nothing to do with cutting down on winddage or whatever. At least it was like that on the Flyers, Capricorns and Blades. Since a few years we are seeing a new fashion develop, mostly in the A-cats, where the stems are actually cut away further. The jury is still out on whether this is effective as a design or only because of the specifics of the A-cat setup. (i.e. no spi, relative long hulls for a rather narrow platform width etc)

Wouter

Luiz,

A craft never overtakes or cuts/pierces through its own bow wave. The bow wave travels along side the craft at exactly the same speed. Same for planing craft, they too never overtake their bow waves.

What happens it something different.

With displacement hulls the stern wave detaches from the hull and moves back this increasing the wave length of the system and allow it to travel faster, remember Froude's law. This means their is a higher water level at the stem then there is at the stern. This creates a pressure difference along the hull which leads to a force pointing to the rear of the craft = drag. This drag needs to be overcome by increasing engine power or sail power. The faster the craft moves the further the stern wave falls behind the hulls, the larger the pressure difference, the larger this wave-making drag. On beach cats this drag component is relatively small compared to other drag components like wetted surface drag and induced drag from the daggerboards and the sails themselves. As such, encountering this situation on a beach cat is hardly noticeable. The large rigs, needed to overcome much larger drag components, have ample power to simply overpower this "theoretical hull speed" limit.

On planing hulls the situation is a little different. The size of the wave system (bow wave, stern wave) is determined by the displacement of the hull. In effect if the displacement of the hull is reduced then so too will be the wave system. This is was planing does, it lifts the hull higher in the water thus reducing the amount of displaced water. In turn the wave system is made significantly smaller and LESS excess power is needed to overpower the "theoretical max hull speed". Of course planing itself requires additional power as well, but it was found that this requires less additional power then overpowering the wave system when in full displacement.

Of course foiling in an extreme case of the planing situation. Partial foiling or bruce foiling is somewhere in between.

Wouter

I am very new at this cat sailing... I had my boat out last weekend.. there was some decent wind On the lake there was a E scow, sunfish and two other H16's I was sailing right past the E scow and the sunfish.. but no matter what I could do I could not pass the other hobies.. unless It was in a puff wiht my main sail trim all in. I feel that my speed limit comes from lack of sail-ability..

I feel my main loss in speed was fear.. and now knowing how to trim the jib. Fear meaning as the boat would start to lift a hull then the opposite hull would start to dive a little and seem to almost dive so I would release the mainsail... or the jib but I am going to blame my speed limiting to stupidity is there a "sailing cats for Idoits" book out there?

oh and I didn't really know how competitive sailing was untill I bought this boat.. I was just out sailing for fun when I found myself in hot lapping againts old pro's...

and are E-scows just slow in general?

Get this:

http://store.catsailor.com/tek9.asp?pg=products&specific=jqcmhpjpe

Whole chapter on the 16. This book will teach you more than you can learn in three years on your own. Whether you race or not.

Doug

Yeah, I second that. Rick also has a terrific video series as well. You can't go wrong with any of Rick's instructional media.

Here is the CD. I paid the full price for the tapes. CD won't stretch or wear out if not scratched. Get it

http://store.catsailor.com/tek9.asp?pg=products&specific=jnrorrrrg4

Doug