Show cart
Design & Illustration

Hull types in waves

Jeff Allen testar Frej i brytande sjö

Several times the last years I have had discussions with the people from the companies I am working with about different hull shapes and how they perform in waves. One common question is why I often suggest long waterlines and vertical stems on my designs when most sea kayaks have raked and upturned stems – sometimes delivered with the discreetly suggested notion that "that is what real sea kayaks look like".

Usually I try to explain that stems alone do not dictate the behavior of the kayak in different conditions – you have to look at the rocker, volume distribution, lateral balance, length/width ratio etc – and that it is hard for customers in certain segments of the market to accept that things have changed since British Eskimo epigones some fifty years ago established the concept "Sea Kayak".

Some years ago, when I built my first Black Pearl I thought that the Greenland style kayaks had an advantage of moving smoothly and silently in waves, and that the surfski style hull was faster but noisier and wetter – which seemed logical since the Greenland hunters were more concerned with being able to silently sneak up on their prey than with speed, and that surfskis, built for speed, could be forgiven a few bad manners on the wind.

But in the last years, I have again and again seen surfski style hulls going against short seas efficiently and with moderate pitching (very convincingly experienced with my Njord) – and accompanying kayaks with long overhangs hobby-horsing wildly and splashing down into the waves. 

When analyzing the mechanics of a wave passage it became quite logical.

Something like this:

In the image below I have a Valley Nordkapp representing the old style classic sea kayak, with looks and reputation promising excellent behavior at sea, and my Panthera representing a modern surfski inspired hybrid kayak. The dimensions are similar (Nordkapp 545x53 cm, Panthera 559x52), as are the volumes (Nordkapp 380 liter, Panthera 375), but most other data are fundamentally different. Panthera has a Cp of 0.563, Nordkapp lower (I have not found the exact number, but guessing from the volume curve it should be approx 0.52). The LWL is 467 cm for Nordkapp and 552 for Panthera.

The blue curve is the volume distribution below the waterline, Panthera with more volume out in the extreme ends and less in the middle, Nordkapp with slim ends and a fat belly (Kretschmer might have said pyknic had he been into kayaks ;-). The red lines show the efficient lever, the length of which determines the pitching amplitude (the length as determined as the point where the volume curve reached a predefined volume, the same for both hulls).

skrovformens betydelse

When a wave hits the stem it is the lever that triggers the pitching. A 50 cm wave lifts first the fore and then the aft point of the lever 50 cm. Every part of the kayak beyond the lever will pitch more – and more excessive the longer the overhangs. And more amplitude means more splashing and slowing down.

The animation below is from my page about hull shapes (in Swedish only) and is an attempt to show what happens (hover over the image to run the animation). The kayaks here are my Frej in the foreground and behind it an Anas Acuta.

Nordkapp (and to a certain extent also Anas Acuta) is an exceptional sea kayak, proven on spectacular expeditions across the North Sea and around Cape Horn and Australia – provided you don't push it. Panthera and Frej are also very good sea kayaks but they may get you there less tired, in style and comfort and in a shorter time.

Comments

Very interesting. Have you considered that the Greenland Eskimos generally went on short hunting trips, and usually in relatively good weather? I would love to see a similar comparison between the Aleutian baidarka hull shape and the surf-ski, since they have much more volume in the ends, as well as the bifid bow and unique stern

I agree. Greenland kayaks were used primarily for short hunting trips in waters close by. But there are stories about fighting sudden offshore wind for many hours or even days, so challenging conditions must be handled with kayak performance and solid seamanship.

I imagined that a baidarka would behave more like the modern surfski, being long and slim with high-volume ends. The sea otter hunt would require them to cover larger areas, and to get to the hunting grounds efficiently they used to connect the baidarkas in pairs and hoist a sail. But baidarkas are rare in Sweden and I have tried one only shortly and on flat water.

Their sea-keeping abilities, seem to depend on the hull configuration rather than the hull's flexibility that has been suggested.

Det ska bli intressant att uppleva denna skillnad. Jag sjösätter min Frej senare i veckan och har endast min Black Pearl som referens sedan tidigare. Mycket intressanta animationer!

Hoppas du hör av dig med dina upplevelser...

With cedar strips about to start covering the forms of my Frej, this article made me even more happy with my choice of your designs (as if that were even possible) as well as even more excited to get on the water with what I hope to be a true to form version of this boat! Great read, love your attention to scientific design.

Thanks Sean. Look forward to hearing if you agree with me when your Frej is launched.

Gäller dessa jämförelser även Pantera vs Black Pearl, Hunter, Nanoq?

Finns det inga fördelar med det mer Grönländska formatet?

Själv har jag alltid tänkt att dom fallande stävarna introducerar kajakens bärighet mera gradvis och därför rör sig mjukare och "stampar" mindre. Känns inte som en självklarhet att det måste gå stökigare bara för att stävarna rör sig mer upp/ner? Kan en spetsigare kajak penetrera brytande motsjö mer? Och kan det vara en fördel i vissa situationer? Kan en kajak med kortare vattenlinje vara mer lättsvängd vid samma totallängd? Kan olika totallängd, vattenlinjelängd, språng, volymfördelning bete sig olika beroende på vågornas höjd och längd?

Bara några tankar...

Jo, självklart har de grönländska skrovformerna fördelar. Resonemanget ovan gäller främst om man försöker hålla lite fart i krabb sjö. Annars går Black Pearl, Hunter och (framför allt) Nanoq väldigt mjukt och fint i sjön – en behagligt kontrollerad rörelse även om den har större amplitud än de mer surfskiliknande kajakerna. De här kajakerna går också tystare i sjön under förutsättning att man inte pressar upp farten så mycket att förskeppet dyker och skrätter.

Utfallet beror även på vågornas längd, höjd och form. Alla kajaker har sin akilleshäl i vissa vågförhållanden, och skrovform i förhållande till vågor är en komplex ekvation, med många motstridiga faktorer som skall balanseras. Problemet jag resonerar kring ovan gäller främst i vågor upp till ca 1,5 meters höjd – vid högre vågor blir våglängden så stor (> 11 m) att skillnaden gradvis minskar för ett 5 meters skrov.

Kortare vattenlinje ger förstås bättre manöverbarhet om allt annat är lika – men i fallet ovan får Panthera och Frej sin manöverbarhet genom krökt köllinje istället för genom kort vattenlinje och är därför lika manöverbara. Men jämfört med en gammal engelsk grönlandsepigon med tämligen rak köl är skillnaden stor.

Hi. thanks for the writeup. The idea of a wave passing the kayak making it rock and causing resistance may be correct, but with a kayakker in the kayak the real motion up and down will be dampend quite a bit, also the kayakker will often move back and forth going over a wave to counter that same movement. Can you recalculate the animation with a 75Kg kayakker and the weight of the kayak itself, to see how it may really behave ?

Hi Christian, the kayaks in the animation are loaded down to the design waterline and thus include the weight of a paddler in the cockpit. Since this weight is positioned very close to the center of motion, the dampening effect will be small. But if the paddler is alive and well and keeps rocking back and forth, the result could be just about anything – at best reducing the pitching considerably, as I have indicated here.

Post a comment