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The Ship

Snorri Under Sail:
How does she go?

Click on the images below to view enlargements

It's one thing to have an authentic reproduction of a Viking ship - all sturdy wood and iron


Photo credit: Andrew Marshall

Snorri during sea trials

 
rivets, long oars bristling from the sides - and another to have a reproduction that also works well. When Rob Stevens and friends were building Snorri, a 54-foot replica of a Viking knarr, the unspoken question - "will she sail?" - hung in the air like sawdust.

Taking into account some of the features of the original knarr's design - almost no keel, "old-fashioned" square sail, rudder on the side instead of at the stern - she seems to sail quite nicely, thank you.

Stiffer is better

 



Terry Moore,
Snorri's captain

Terry Moore, the ship's captain, says, "Overall, I was pleasantly surprised at the way the boat handled. It's very stiff and stable. When you step aboard, you don't feel the boat dip at all - it's like stepping onto a basketball court."

This stability comes from 13 tons of rock ballast and the boat's ample width. Stiffness is important because, Terry says, "You don't want it to heel much. You don't want to get water in this undecked, totally open craft. Down you'd go. Ten to twelve degrees is a lot of heel for this boat."

Using the wind

By modern standards, a square sail is not very efficient. Captain Terry reports that "she can point to within 50-55 degrees of the wind without luffing the sail. But because there's little keel and no centerboard, there's a lot of leeway, increasing this angle to a resultant 60-65 degrees. You compromise, and try to steer a course that will let you move forward somewhat efficiently.

"Off the wind, she sails very well. Her best point of sailing is a broad reach, with the wind on the quarter. She has made up to 9.5 knots in those conditions."

Handling the sail

How much trouble is it to tack and trim sail? Terry says, "Tacking is difficult. It can be done with five or six people, but seven or eight is better. You have to move the beiteass [tackpole], switch the sheets around. And the sail is backed for most of this process.

"On the other hand, jibing is ridiculously easy. You just bring the sail around so it stays perpendicular to the wind as the boat wears."

Easy steering

"The tiller is easy to handle," says Terry. "The boat is sensitive to fore-and-aft trim, so once the ballast is correctly placed, there is no weather helm - it won't automatically point up into the wind. That's a good thing for this rig - a square sail would just get backed."


 
Okay, but what about the rudder?


Any discussion of Snorri's sailing qualities inevitably turns to the rudder, and the mysteries surrounding its shape, size and functionality. The rudder also draws a lot of interest because it has taken the blame for last year's breakdown in the middle of Davis Strait. But it's not as simple as that.



Talk always turns to rudder performance

 




What really happened in '97

Contrary to popular belief, the rudder did not break. Rather, the rudder broke the boat. And the breakage did not necessarily occur because of the force exerted by turning the ship, although that force can be considerable.

Master boatbuilder Rob Stevens explains: "Very few people really understand what happened with our
 

Rob Stevens,
Snorri's master
boatbuilder

rudder. In fact, I don't think I had it figured out until several months later. The rudder itself didn't fail, by the way; it was the framing.

"The original drawing that I went by shows the tiller fairly far forward. I talked to a number of people and they felt that moving the rudder back couldn't hurt. Where I made my mistake was that although I made the frame at that point more rugged than it's shown on our plans, it wasn't anywhere near as rugged as it should have been.



Rob puts his finger on the framing problem

 



"It was only upon going back to Europe and looking at models of other Viking ships that I realized the framing [where the rudder is attached] is different from what we had - very rugged. This part of the boat didn't exist in the original wreck [Skuldelev Wreck #1]. And so when these plans were drawn up, they had no way of knowing how it should have been framed back there. It turns out that it was very rugged."

Rob explains how the rudder failed: "I think that either all the pressure on the rudder throughout the sailing, or the grounding [the rudder got a good whack when the ship grounded on a submerged rock], or both of them, started pulling the frame forward. The rope that holds the rudder on comes through the hull, wraps around the frame and that's what holds the rudder tight. The rope should have come through right at the frame, but it came through about six inches forward, and so there was a forward pull on it.



A steel-and-polyester line fastens the rudder to the side of the ship

 

"Over time, it just sucked the treenails right through the planking and the boat started to leak. We put the sea anchor out and nailed shingles and tin cans and what have you over the hull and it seemed to be fine. And we moved where the rope went. About six hours later the rope broke, and that's when the rudder floated straight out and folded the sheer plank over.

"The rope, called Combi-line, is actually half steel and half polyester. It comes through the hull, comes through the rudder, wraps around a block of wood that acts like a big knot and then comes back on itself.

"Then you can pull in against this knot into the hull. Our line had a crimp fitting, a metal sleeve just crimped on it, and the weakest point of that fitting is right next to the sleeve.

"This crimp fitting was right at the pivot point of the rudder. And if you want to break a piece of metal, you just sit there and flex it back and forth. In six hours it broke. We were using the same size and type of line that the replica ships in Europe are using, but they have an eye splice in theirs. That way, the strain is spread throughout the whole thing, and I think that's why they haven't had problems with their lines."

Captain Terry Moore puts more emphasis on wave action as the cause of the failure. In the hours just before the failure, Snorri was sailing in a "beam sea," with wind and waves coming at right angles to the boat. These waves caused Snorri to roll sideways, back and forth. The force of the water against the rudder blade pushed it away from the hull, like a huge lever prying against its attachment point. The rudder pulled the side of the ship outward at that point, unfastening and moving some of the interior framing. And the voyage was over.

Speculative rudder questions

Which edge is the front?

No one has ever found a rudder still attached to a Viking ship. So there is debate over the most fundamental question - which edge of the rudder is the leading edge? What's the front, what's the back?


Prevailing
belief -
thin edge
forward,
tail aft

Tail forward
provides a
better
hydrodynamic
shape
 


A hydrodynamically correct
rudder has the fat part forward

The more popular opinion is that the little tail is on the trailing edge of the rudder. But if this is true, the rudder's shape in cross-section is backward according to modern thinking on rudder shape.
 

 


Typically, modern rudders are shaped like an airplane wing, and for the same reason - to provide "lift." This lifting force helps to steer the boat in the same way an airplane wing's lift helps the plane get off the ground.

No matter which edge of the Viking rudder points forward, some lift will be generated by the shape. But putting the thicker end forward improves the rudder's efficiency considerably.

Tail or nose?

What is that little tab for, and where does it go? Artifacts seem to indicate that the pivot point of Viking rudders was right down the center of the rudder. If that's the case, then a symmetrical rudder, with both edges having identical profiles, would have a tendency to turn sideways in the water. Forces on both sides of the pivot point would be equal. In this scenario, the tail, a "trim tab," was added to provide the force that would keep that edge trailing.

Another tab-aft theory concerns the pressure of the water against the rudder. When turning the ship, water pressure will be greater on one side of the rudder than the other. The water on the high-pressure side has a natural tendency to slip around the edge of the rudder to the other side. Where it succeeds, it creates a vortex, or "backwash," that reduces the lifting effect on the other side of the rudder. The tab may have been provided to attract the high-pressure water and give it a better path to slip off the rudder. The backwash would then occur just behind the tab, and would interfere less with the rudder's lift.

Still others say that the tab was on the front of the rudder, not the aft. If this were so, the rudder would then have a cross-sectional shape that corresponds to current thinking.

What function might the tab have in this situation? One explanation is that with the boat under motion, there would be considerable force from the water pushing the bottom of the rudder backward. Counteracting this force would quickly become tiresome for the person at the tiller. So a line would be tied from the rudder tab to someplace farther forward on the ship. This would keep the rudder in a vertical position and reduce strain for the helmsman.

A unique theory - the leather sheath

Engineers and designers at Paine Yacht Design in Camden, Maine, have been involved with the puzzle of Snorri's rudder. Chuck Paine was driving to work one day when he asked himself, "What if the Viking rudders were made of two materials, but only one - the wood - survived?"

Chuck Paine's leather sheath would improve the rudder's effectiveness

 
Chuck envisioned a leather sheath that would be fitted to the rudder below the waterline. This sheath would have three effects:

1. It would increase the surface area - and therefore the effectiveness - of the rudder blade.
2. It would modify the reverse-wedge shape of the rudder into something closer to the modern foil shape, improving lift.
3. Because leather is flexible, it could bend to relieve excessive force, reducing strain on the rudder attachment.

Did the Vikings use leather sheaths? There is no evidence that says yes or no - but it's an interesting idea.

Rudder evolution

Rob Stevens sheds some light on rudder size and shape: "When the first rudder we had did not work well, just a regular old board worked better in the water. And then we had a naval architect design a rudder that was in keeping with modern theory, but completely backwards [in cross-section] from a Viking rudder. And that worked. So we thought okay, a board works, and a modern rudder works, and our [smaller] Viking rudder doesn't. It's sort of like any big object will work.

"A Viking rudder was not as big and it was very finely tuned, but the weakest point on a Viking rudder is the rope holding it on. My feeling is that that was the limit of their technology. They were as good as they could be using their attachment method. That was it, they had reached a dead end.

"Earlier, the Mediterranean boats had rudders weighing ten times as much as a Viking rudder - as much as ten tons. But they had a different attachment method. The final evolution of the Mediterranean rudder was the Viking rudder - the end result of at least 3,000 years of evolution of a side rudder.

"The further aft you put the rudder, the better it works. In Northern Europe, when the stern rudder was invented, it coexisted with the side rudder for 300 years. You'll see paintings with boats that have both a stern rudder and a side rudder. But I think one of the advantages of the stern rudder is it doesn't have to be very efficient, it doesn't have to be very well-designed, whereas the side rudder seems to have to be extremely well-designed to work."

"Tuning" a Viking rudder

One of the things I've been finding out is that there are patterns. The 30 Viking rudders in existence have some ratios that are somewhat the same, as far as length and width and what have you. That's something John Gardner discovered while we were making the two rudders and laying them out - that when you connected the lines of the forward edge and the after edge of the rudder, they hit at the very top of the center line of the rudder head.

In other words, if you know the rudder is going to be 12 feet long, and say you know it's going to be 24 inches wide, you just sit there and make a triangle and there's your rudder.


Snorri's "improved" rudder (left) has pivot point forward of center. The original Viking-style rudder pivoted at the centerline.


"And then the ruddersmith would know that you have to add this little heel to make it unbalanced, so the after edge of the rudder will trail in the water. And the lower three feet of the forward edge was cut away a little. You follow these rules and you put it on, and then you fine-tune it.

"I think cutting away the lower edge was the fine-tuning. I was talking to a guy in Norway who's made a bunch of Viking rudders and he says it takes him 10 to 15 tries of fine-tuning a rudder, shaving away a little more and a little more to make it work the way he wants it. And the Viking rudders are probably only half the surface area that a naval architect would say a rudder should be in relation to the underwater surface area of a boat. So I mean, it must have been really finely tuned."

On the other hand...

"And then there's always the possibility that Viking rudders don't work. Because everybody I talked to over in Europe said that basically you sail with ballast and sail trim, and that the rudder is just to make corrections to your course. So maybe a Viking rudder will never work as well as we expect rudders to work nowadays.

Changes for '98

What does Rob have planned for this year? "What I'm going to do when I go back is reframe the rudder area the way the Vikings would have framed it. I might move the rudder farther aft. When I was in Denmark a month ago talking to the people in Roskilde, they thought the rudder was still too far forward, even though it's aft of where it's shown on the plans. And I'm going to get a piece of line with an eye splice in it rather than a crimp fitting.

"We also sent two new rudders over there [to Nuuk, Greenland], Viking-style rudders, and we're going to experiment with them to see if we can make them work. But if they don't work well enough for Terry to feel safe going on this trip, then we'll switch back to the rudder we used last year. With the new framing I'm planning, I don't think it's too big for the boat."