I have no comment apart from this reaction to the statement ;

"So from your observation the ... wave piercing bows do not reduce pitching?
(I thought that was what all the hype/design was about?)"


The word hype is not entirely unjustitified as it looks like it is more and more used as a marketing tool HOWEVER there are some noticable benefits from a trully wave piercer hull design. Let me explain what the theory is thus far.

Take a look at the attached picture and preferably open it in a new window so you can swap easily from text to graph and back.

The name wave piercer is a bad name in personal opinion. All fine entry bow sections are wave piercing and nearly all catamarans have fine bow entries. Therefor a Nacra 6.0 bow is just as much a wave piercer as that of the flyer A-cat.

However the name was made famous by the Flyer A-cat design which has a special volume distribution. In fact it re-arranged its distribution of displacement which AMONG OTHER THINGS make it EVEN less sensitive to passing wavelets than normal (wave peircing) bow sections.

Let look at the graph :

The red line models the bow sections of V-shaped hulls like Prindle 16, H16, Dart 18's.

The blue line depicts the "wave piercer" bow sections with INVERTED volume distribution when compare to the V-shaped bows. This bow section is found on Flyer A-cats, Capricorn F18, Hobie FOX, Hobie FX-one.

The green line belongs to U-shaped bow sections as is found on the older nacra's and most of the F18's.

The I-20 and I-18 are somewhat of a mix between mode red and green.


The graph features two axis, The verical one reflects displacement = restoring force. The horizontal one reflects the depth to which the bow is submerged.

On the left and underside of the graph you see the partially subnerged bows; All are depicted creating equal restoring force.

On the right and up you see the fully submerged bows.

In the graph you also a green dotted line. This depicts a bow section which has the same shape as the U-bows BUT where the builder has just trimmed the deck away to give the appearence of a trully wavepeircer hull shape but didn't rearrange the volume distribution. Lets say that these designers are only after the marketing aspect of the wavepierer hype but haven't been succesful in implementing it in the right way.

What happens ?

First of all, All sections give the same restoring forces when fully submerged (apart from the trimmed U-bow that is)

Now the V shaped bow clear has te worst wave and dive resistance. It also has the worst wetted surface area. It needs to be deeply submerged in order to provide enough force to keep the boat level under the forces of the sails. This relative deep submergence result in relatively high wetted area.

With each gust it need to submerged noticably deeper to create enough restoring force to counterbalance the gust or wahtever is causing the dive. This is the result of the upward curving line. Most of its volume is place high in hull and it take alot of depth to make use of it displacement.

Look at the grey dotted line to see how far the other hull shapes need to be pressed down to achieve the same restoring force. Noticably less.

Wavelets ? Because the biggest portion of the volume is placed high in the hulls, each passing wave will hit this part and then free this part time and time again. This results in a strong aggrevated oscillation to waves. This portion which is only wetted by wave makes up a big portion of the total volume. Its reaction reflects this.

To the first improvement the U shaped bows.

Here the wetted surface is imporved as well as the dive resistance and wavelet reaction.

All these things are clear from the graph. For a given restoring force the bow needs to be depressed less => dives less deep. Its ration of volume to circumference is better than that of teh V-bows and therefor it will have a better wetted surface ratio. Its lineairisation around its hull = level depth is less inclined that that of the V-bow and therefor its increase / decreases in forces produced by bow displacements are less in magnitude when a wave passes and wets the reserve volume in the bow. As a result this bow reacts less severly to passing wavelets. Of course big wave (swell) are so large that they will completel submerge the bow and make the induced forces equal again. So it can't ingnore the big wave just the smaller wavelets.

Now to to the wavepeircer concept.

Rearrange the volume so it rest low in the hull and remove the same amout from high up.

This will make the underwater body resemble a circle even more and reduce the wetted surface area in relation to the displaced volume.

It is also very clear that this setup will lift the hull up in the water and give more wave clearence to the beams. A side effect.

Again for a given restoring force the bow needs to be depressed even less than the U-shaped bow which in turn is depressed less than the V-shaped bow.


The dive resistance has a perculiar character.

Dive resistance is not entirely governed by force alone. Because diving is a motion and not a stationairy state the concept of energy and work govern dive resistance and not retoring force.

Do this mind experiment. Take a bottle and fully submerge it under water with its neck point up. Than do the same with the neck down. The force required to keep it fully under is the same but the path down was easier (lighter) when the neck was pointing down. This illustates that is requires less work / energy to surpress the bottle upside down then when it is right side up. If you compare this to the Wave piercer bow you'll see that teh rig have to pump in more energy into depressing the wave piercer bow fully than it does in depressing the other two shapes. Ergo its dive restance has been increased by rearrganing the volume.

The amount of work or energy needed is equal to the area enclosed between the individual graph and the horizontal axis. Clearly the U-bow is superior to the V-shaped bow and the trully wave piercer bow is superior in this respect to the U- shaped bow. EVEN though all have the SAME maximum restoring force when fully submerged !

Now what happens when a builder take a U-shaped hull and just trimms the deck to resemble the wave peircer bow section by imitating the sloping deck of it. Green line with the dotted ending.

For the first part it acts no differently than a (old) fully U-shaped bow of normal height. As long as the wavelets don't come higher than its new deck level the reaction to wavelets will be identical too. However when the wavelet (chop) comes higher than the deck than it does react less severely than the older bow section of greater height. This advantage it does have.

What are the downsides of this trimmed deck approach. First of all the area under the graph has been reduced ergo the amount of work required to full submerge the bow is less than before. Therefor its dive resistance has been reduced. It is now somewhere between an older U-shaped bow of high stem and the V-shaped bow.

Also this approach doesn't reduce the wetted surface when sailin level with the water line. But to top it off it's maximum restoring force is reduced too. Meaning that the threshold for overpowered sailing has been reduced as well. In short the higher stem U hull (parent) can be pushed harder before the bows disappears under the water level. We all know that a pitchpole on a reach is not far away when the water hits the deck. Such a 15 %- 20% reduction in maximum restoring force will be noticed on reaches. Please notes that this threshold is only valid with respect to a semi stationairy situation. A gust is a dynamic phenomenon and there the maximum restoring force plays a secondairy role with respect to energy thresholds. Later more on this.

So a trimmed back bow without a rearranged volume distribution will only have a limited positive effect (Less reaction to large wavelets) against the noticable cost of less dive resistance, no different reaction to smaller wavelets, lower maximum retoring force = less ability to be driven hard on reaches / downwind or less margin, whatever you want.


So a true "wavepeircing" bow is more complex than just trimming a deck or reducing volume in the bow. It must go together with a more complex volume distribution to work properly.


Now a smart guy will notice that the Wave piercer hull in my example has a higher slope at its operational depth than the others and therefor is will react more strongly to passing wavelets. That is true in my example. BUT there is a trade off.

With very small wavelets this is the case. BUT these also have very short wave lenghs; often much smaller than the hull length. So when a small wavelet lifts the bow than its little brother will lift the middle section and a third brother will lift the stern as well and thus cancelling eachother out.

The problem of oscillation starts with bigger wavelets which have minimum wavelength of about a hull length (often called chop). These wavelets are significantly taller than the smaller wavelets.

Not lets go back to the graph.

Look at the green line. A chop wave of only halve the bowheight causes the restoring force (Now aggitating force) to grow to its maximum.

Now look at the blue line. A chop of 3/4 the height of the bow caused the restoring force (Now aggitating force) to grow to its maximum.

We know the maximum forces are the same therefor the wavepeircer bow requires a LARGER wavecrest to pass to be put in the same magnitude of oscillation. That is in this example.

Now a good designer will see the increase in the dive resistance and may decide to trade part of that away for further reduced chop oscilation by reducing the total volume of the bow section and take the hit in lowered maximum restoring force as well.

He can use this trade-off to his advantage while the trimmed deck designer can NOT. This is because the trimmed deck designer has actually reduced dive resistance by his trimmed deck instead of increased it. In short the trimmed deck designer has made no gains which he can than trade off for more benefits.

I hope most of you understand this explanation. I tried to keep is as simple as possible. There are more secondairy and high order effects but these are relatively small than the main issues discussed.



Now why do I write this down ?

To show that it is important the seperate the hype from the real benefit of a hull shape with rearranged volume distribution as seen in the Flyer A-cat design and which is often refered to as a wave-piercer hull.

Simply mimicking the sloping of the deck is not enough. The hulls need to be completely redesigned. And not only the bow section as we did in this example. and of course one mustn't go overboard with it.

We should also take care not to let the hype give the true wave piercer hulls a bad name. It is very healthy that the hype is collapsing but we should not throw away the real advances that were made with the flyer design.


Wouter