Viking tumblehome sterns

I have always wondered which is better for taking waves over the stern:

• the surfboat stern, or the
• Scandinavian tumblehome stern?

The Viking longships of fame had surfboat sterns, but I just learned that the cargo ships of the time, or Knarrs, had the tumblehome stern long ago. My thinking is that the Knarrs were heavier and that has something to do with it.

Knarr:

 

There is nothing good about taking waves over the stern, but I suspect you want to know which stern is best for taking waves under it, to prevent being pooped. All boat design is about compromise. For example, when running down waves, an overly buoyant stern can cause the bow to dig in and cause the vessel to broach to.

The stern of the Knarr is part of the shape of a broad beamed clinker built vessel, dedicated to carrying cargo. The longships also were propelled by oars as well as sail and were long and slim, so as glide through the water. Lovely model isn't it?

http://en.wikipedia.org/wiki/Medieval_ships

 

Maybe the tumblehome was to increase the strenght of the entire stern area rather than a just a function of wave taking design. The idea being similar to bending a 90 deg. lip or a rolled lip on a bucket to prevent it from deforming when under load. In the case of the bucket internal loading outward . In the case of the freight carrying Knarr to absorb the shock of a following sea trying to instantly move tons of cargo. Just an idea and knowing those master builders it would have been a design feature they would have built in. Yes beautiful models.--Geo

A yacht is not defined by the vessel but by the care and love of her owner

 

VN.

The creation of the stern of the Knarr and that tumblehome, is a function of its construction. No stern post; whereas the longship stern reaches to Valhalla.......

http://home.online.no/~joeolavl/viking/osebergskipet.htm

Jag måste sova nu.

P

 

Ok man of a thousand interpertations is my theory a plausable one.--Geo.

 

The most likely reason is.....................aesthetics. Viking ships included features that were not solely functional. My bet is that the builders decided that short of chunky stern shape suited a knarr. By the way, it's the same as the bow of course. This might indicate that suitability as a stern was not the only consideration.

 

Decorative they were but that feature was definately a secondary thought when it came to seaworthyness. The stern on these freight carriers was more rounded than the bow, which had a very fine hollow entry. Max. beam was carried further aft to increase volume. They didn't have proper fasteners to create a transom as we know it today so like all Scandinavian vessels of their time and many today they build in the double ender design. By broadining the beam and locked into a stern without a transom there was only one choice. Bend the planks in as tight a curve as possible. This then presents a much flatter wall of impact to a breaking following sea and in their earlier experiments with this new design it might have resulted in a type of implosion damage. IE too much flexing of the entire stern area. To offset this they would build in tumblehome which creates a quazi sphere shape to the oncoming forces of the sea. As you know the sphere shape is the strongest of all possible geometric patterns. The more i think about it the more logical it sounds.--Geo.

A yacht is not defined by the vessel but by the care and love of her owner.

 

I think this is a bit like marine historians of 3011 arguing about the functional relevance of the clipper bow on Herreshoff's Araminta. I mean he was a master desiger so it must have had some functional relevance if he included that feature, right?

Your "flatter wall of impact" doesn't work because what you are actually talking about is a stern where the planks are heavily curved.

 

The wall of impact i am referring to is this blunt curved stern verses the traditional stern of a typical Viking ship which in essence is a copy of it's fine entry bow. When one is compared to the other the impact energy would have been much greater. -Geo.

A yacht is not defined by the vessel but by the care and love of her owner

 

Do me an engineering study that proves the stern would have had insufficient strength without the tumblehome.

 

There is a clue in the photograph of the model Knarr. The boat is coming alongside a wharf of sorts and the inward slope of the stern being clear of projections, would prevent snagging lines at the staithe. Examining the model below the waterline, there is a keel that rises towards the stern and it seems to fade into the construction aft. There might be a small stern post or it could be camera flash. :idea: Knarrs also had a deeper draught than longships. However the model could be wrong?

http://www.tripadvisor.com/LocationPhotos-g499196-L_Anse_aux_Meadows_Newfoundland_and_Labrador.html

http://home.online.no/~joeolavl/viking/baatbyggerier.htm

http://vikingove.mysteria.cz/vikingska-lod-9.html

http://www.johnburdenart.com/illustrations 02/illustrations 02.html

http://home.online.no/~joeolavl/viking/norse-shipbuilding.htm

At this point, it might be wiser to change focus and admire other examples of the boat builders' art.

http://forum.woodenboat.com/showthread.php?127208-dutch-punter

 

After much painstaking and scholarly research, I have become convinced that the shipwright chose this three-dimensionally curved stern shape because it reminded him of the ample buttocks of his beloved Gunnhild (a formidable woman renowned for her sturdiness and carrying capacity).

 

As well as a willingness to go all the way (to distant shores and return, you naughty boy!).

 

The use of tumblehome has historically been for "working" the boat. In most cases it permits a crew member to stand with their belly at the rail, without having weird foot positions to make them unbalanced as they work (tending nets, hauling pots, fending off, etc.). With the sole a little wider then the rail, a man can stand normally and put his back into the work, even leveraging against it. If the sole was narrower then the rail, the crew would have to lean over to get at the rail, placing them in a precarious position.

In terms of Viking craft, some art work was used on these craft, very little of it was for "looks", with most having religious or wives tail mythology attached to them. Warrior shields had two natural purposes, protection and identification, figureheads where purely mythology, with possible owner/builder and/or king over tones as well.

Tumblehome aft, has always been a solution to loosing crew over the side when working near or at the rail, at least until yachts began twisting working, commercial and naval craft into desirable "toys", in which case then "looks" came into play. Previous to this, it was difficult to justify a design feature for looks alone.

Much later, when ships became burdened with deck cargo and huge armaments, the use of tumblehome was incorporated full length along a hull, to help center these large masses of bronze and iron. Again a purposeful use of tumblehome, rather then aesthetics.

Aesthetics for the sake of display didn't really come to be, until national navies started working the world's oceans in an effort to spread their influence. In this, garnished up war ships would speak volumes of the mother country's wealth and status in the world. If one country's diplomat sailed into the harbor of another, with a trade agreement to sign, the gaudiness of the ship's accommodations and trim where as important as their attire and manners at the dinner table.

 

Tumblehome doesn't help to center the weight of cannons. Assuming equal numbers each side, the total weight is centered with or without tumblehome. What tumblehome can do is lower the CoG by reducing the upper deck wieghts, and help keep gunports clear of the water through larger angles of heel.