Viking tumblehome sterns

Stability problems related to the influence of hull forms on it can be the same for both ancient and modern ships. One of such problems is pure-loss of stability (the phenomenon of decreased stability in waves)

As shown in figure (Belenky and Bassler, 2010), for exactly the same underbody and KG value, the angle of maximum GZ of the flared topside configuration is much larger than that of the tumblehome topside configuration. The difference in the value of the maximum of the GZ curve is even more dramatic. Stability failure near a wave crest is a phenomenon occurring in a very small encounter frequency, while the wave crest is slowly moving along the hull (worst in following seas as encountering time is bigger). As a result, heeling may occur almost statically.

For not decked boats as viking ones, a tumblehomed hull can dramatically increase the risk of flooding because of such pure-loss of stability phenomena. That's one of the reasons why they did not have such forms.

 

Agree about the Roskilde model.. Never been in that museum so don't know if it really is like it looks, or is it only a something with angle and lights of the picture. But... traditional scandinavian hull forms are, and have been very symmetric..

 

A bit offtopic but here comes the problem what I had with Dellenbaugh angle way back.. just didn't find a way to put it to words. But with flared topsides a boat with narrower BWL and therefore worse Dellenbaugh angle can still have better stability than expected..

 

Thanks for those papers, Guillermo. Regarding the reasons for tumblehome of old warships, the first paper says this:

This agrees with the Wikipedia page, and backs up my claim that the primary reason for tumblehome on old warships was to lower the height of the CoG by reducing upper deck weights.

As regards "centering" the weight of cannon on the upper decks, that would have no effect on the CoG if the deck weights were not taken into account (ie: cannon were moved laterally wthout changing hull shape) and therefore no effect on righting moment. However, what it would do is shorten the gyradius for the cannon, resulting in a slightly shorter roll period for the ship. So, PAR's argument would seem to be that the reduction in roll period was what was being sought.

Note that if tumblehome is employed you will get both effects at once: lower CoG (meaning increased righting moment and range of stability) and also shorter gyradius for the weaponry and for parts of the upper deck structure (meaning shorter roll period).

 

Yes, Dellenbaugh angle has rather to do with stability at small angles, while flared topsides and high freeboard have to do with stability at big ones. So a relative narrow Bwl but a high freeboard and flared topsides can provide safe overall stability characteristics. As a matter of fact passenger ships have small initial GM so they are tender (to provide a comfortable ride), as well as warships (to provide an stable "firing platform"), but a high stability at big angles.

Cheers.

 

Yeap...!

 

You sure about that? Knarrs were ballasted open boats (open in the sense that what decking they did have was not watertight). Better to not have a wave go over you. Take a close look at the picture in your OP. The stern shape doesn't even appear to have any significant tumblehome. It is flared at least to deck level. The tumblehome is very slight and only towards the tips of the stem and stern. Here's a clearer picture:



I really would not think this was designed to submerge in following seas.

 

You must forgive my engineering limitations and if i am reading those graphs correctly,(post #46) I'm still having problems getting my head around the idea that the straigh sided or flared hull has better stability than the tumblehome counterpart.Looking at the underwater cross sectional profiles, the tumblehome model has more beam and as such should have more initial (form) stability. I can understand beyond a certain heel that the tumblehome loses emerged volume (displacement) and suffers a higher loss in stability but for a fixed vessel length and this was their problem building in wood (Hogging) I think they simply increased the beam and made use of tumblehome to strengthen the hull form longitudionally by introducing curvature and as a by product result it gave them more inital form stability by more water level beam and the upper level loads more transversely centered. I can't speculate which design feature they were working toward at the time, a design that would strengthen the hull longitudionally, a design that would increase inital stability thus maybe in the limited engineering logic of their day, If she's more stable to start with and if we hold her within certain limits she'll remain that way. Regardless, if you introduce one of those design features the other is a by product. Possibly tumblehome was a feature to strenghten the hull longitudionally but as soon as the first vessel hit the water for general new vessel trials the crew were so impressed on how stable she was that, that feature not the design reason became the song of the day. Within certain limitations the tumblehome vessel is more stable and i'm sure they were well aware of that and strived to keep it so. Look how long it took for the cods head and mackerals tail to run out of favour in terms of speed not to mention a couple of more boat design "reverses" that are still in vogue.---Geo.

P.S. I realize this is off the opening query but tumblehome in general seems to have captivated the thread.

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

 

This whole centering thing is getting out of hand. This is not correct:

As long as the weights are evenly distributed each side, moving weights towards the centreline does not increase stability. Basic geometry tells you this because moving weights laterally, as long as it is done evenly on both sides, does not change the height or lateral position of the CoG. This means it has no effect on the metacentre and therefore no effect on righting moment.

 

Geo,
Posted graph's hulls are EXACTLY the same under water.
Tumblehome has little to do with initial stability. If you see the GZ curves, the initial GM (related with the slope of the curve at zero degrees) is the same or almost.
The adopting of a tumblehome had nothing to do with the willing to increase longitudinal strength. As a matter of fact it could even diminish it, by diminishing the deck's transverse section area. It has rather to do with what NoEyeDeer has said about keeping high decks weight under control.

Cheers.

 

Yeap!

Geo: I'm afraid you are misunderstanding (and mixing) what "forms stability" and "weights stability" are.

Cheers.

 

Well there you go i learn by a negative process, ahhh but i learn --- Tnx. Geo.

 

Just did a bit more checking around. The only more or less complete example of an original knarr is Skuldelev 1. All the replicas have been based on this ship. As you can see from the plans, the ship actually has a slight cod's head/mackerel tail form, with a bow that is fuller than the stern.



ETA: Plan is taken from here: http://vikingeskibsmuseet.dk/index.php?id=2018&L=1&tx_ttboats[pointer]=2&tx_ttboats[tt_boats]=5&tx_ttboats[backPid]=1846&cHash=9d0b89465e5e499ffa7d1b7b89c1dc17

 

Now i'm confused, the thread opened with a model on what was referred to as a Freighter style Viking ship whose design i thought we were referring to. My posts on Viking design was in comparison to that model. Looking at that model the max beam looks like is is carried further aft ending up with what looks like a stern that is more blunt in design than it's bow. I certainly agree the traditional Viking ship as we know them is fairley symetrical in design maybe with some small alterations depending on the area or builder much the same say as modern day Cape style fishing craft. So it would be more productive to compare apples with apples or to simply get back on thread by asking is that model an actual Viking ship model based on accurate info.---Geo.

 

Well that model is supposed to be of a knarr, and the only actual knarr we have for a reference point is Skuldelev 1. So, either that model was made to the Skuldelev 1 lines or the model's lines were invented by whoever made it. I don't know which is the case.

ETA: Looking at the image, my guess would be that it is modelled on Skuldelev 1 and the apparent broader stern may be an optical illusion caused by the angle of the shot. This is only my guess though.