Why I'm Following Sven Yrvind

[sharpii2]

Speed is also and in my opinion mostly important for its influence on the necessary loading when considering an oceanic crossing (let'say a > 30 days sailing). For two mini boats having a certain % of average speed difference in coastal sailings, this % can increase a lot when considering an oceanic crossing. And moreover an extra loading, not always easy to put low in the boat, can degrade the ultimate stability. So speed has several consequences in cascade. In my opinion, there is minima to say it is a sailboat : to have a minimum 4 knots speed potential when sailing upwind (flat sea, with force ~ 3 to 5) , and to avoid too heavy DLR, i.e. remain < 350 (when D = full load displacement).


50% faster in average speed means a lot difference in length, 2,25 in average at same Froude, e.g. a L 5m >> L 11,25 m
>>> may I reformulate your dilemma with let'say ~ 22% faster equivalent to ~ 50% in length, e.g. a L 5m >> L 7,5 m
Well, I will choose the slower as long as the two above conditions (> 4 knots upwind, DLR < 350) can be fulfilled...

Thank you for those fine links.

I was looking at your answer to my hypothetical question. And I played around with the numbers you gave for the slower boat.

To get to windward at 4.0 kts (which I think you mean 4.0 kts made good to windward), you need to be able to actually sail almost half again as fast. The hypotenuse of a right angle, with equal length sides, is about 1.42 times those sides. And since "hull speed" is about 1.34 times the sqrt of the LWL, I calculate you will need LWL of 20.1 ft approximately. With a LWL that long, your D/L ratio of 350 or less will be easy to meet.

To fit within my arbitrary "mini" sum limit of 30 ft, it could have a 6 ft Beam and a 4 ft draft.

As for a boat going 50% faster needing a much different Froude number, that was part of my point. The faster boat will probably need a longer LWL. This is true. But it need not be over twice as long. The faster boat could be lighter, and be able to sail significantly faster than the typical S/L of 1.34 times the sqrt of the LWL. The faster boat could also have a significantly higher S/D than the slower one, and could reach its top speed during a greater proportion of the time.

To do this, assuming it's made of similar materials and techniques as the slower one, it will need to: be more lightly constructed, have a greater ballast moment, and a bigger sail plan. All of this does not necessarily make it less seaworthy. But it does make it more vulnerable to significant if not fatal damage, if it runs into bad weather.

Speed is good, but IMHO, it comes at a cost.

It's interesting to note that IIRC Sven has never had to be rescued while spending great amounts of time on the ocean in his creations.

 

[JPE]

...
It's interesting to note that IIRC Sven has never had to be rescued while spending great amounts of time on the ocean in his creations.

True, but three of his latest designs haven't met their trip goals, not even close. His designs are safe, I'll give him that, but they are not practical. Far from it.

The way I see it we can rate different journeys with "mission points". Lets say that an Ireland to New Zealand non-stop is 100 points. So to make the trip safely, that's the minimum you need. To meet the requirement of mission points, you are given a certain amout of "design points".

First of all, any boat has a maximum design point density. My thesis is, that you just can't pack that 100 points into a boat that is about 6x1.2x0.2 meters. No way. I'd say boats the size of Exlexes can carry a maximum of 60 points. Yrvind disagrees, or, at least did up untill his latest post.

Second, to reach the goal of mission points, you have to be careful with how you balance the design points in you design. Max out on safety, and you won't have enough left for, say, performance. There is a minimum acceptable threshold for any design area. This is what I also claim happened with Exlex and Exlex minor. Sven is very safety oriented. Too much so.

Now, I hear someone roaring in the background "you can't over emphasize safety". Yes you can: for example, we have cars. To make your holiday trip, or daily commute extra safe, you could upgrade the car to a tank. Super safe, yes. Also super impractical.

Your design points add up to your mission points. In reality this might be a more complex formula, but anyways. If you try to pack in more points than your design can carry, you actually end up subracting the extra points from your total, giving you even less mission points. Sound familiar?

 

[A II]

. . . . Sound familiar?

No, all in all it sounds like a bunch of nonsense with some jumbled facts in between.

 

[Dolfiman]

To get to windward at 4.0 kts (which I think you mean 4.0 kts made good to windward), you need to be able to actually sail almost half again as fast.

I would mean 4.0 knots speedo, so ~ 2,8 Knots made good, as a speed potential for wind 10 to 20 Knots. It is seen as the minimum safety performance to either be able to go off a wild coast or to join a shelter against the wind. On the other hand, far enough from a coast during an oceanic crossing, the opportunity of doing successive tacks upwind is rare, better to wait the wind to change of direction. 4 knots VMG is of course a better performance but a bit too much for the minimum sailboat herre we want.
With my VPP and several numerical boats with various DLR and canvas ratio, the results upwind by 10 to 20 Knots falls into a Froude range of 0,29 (at min by 10 Knots) to 0,35 (at max at 20 Knots or first reefing). So, with 4 knots and Froude 0,29 , one can estimate the minimum Lwl, it is 5,13 m. Then, when considering DLR < 350, one can estimate the full load displacement, it is D < 1695 kg. In résumé, my minimum sailboat for an oceanic crossing would be : Lwl > 5,13 m and D (full load for the crossing) < 1695 kg, my starting point to develop a project and check the other requirements.

 

[JPE]

No, all in all it sounds like a bunch of nonsense with some jumbled facts in between.

 

[MoeJoe]

He is on his way again, out on the sea. And he has made a long post, commenting our discussion here:
An abstinence syndrome https://www.yrvind.com/an-abstinence-syndrome/

 

[Will Gilmore]

Sven wrote, "Adding wave creating length to a boat reduces her resistans. It is a better alternative of creating speed.
It is better for the environment, it is also cheaper for the builder than adding sail area and ballast keel and winches and rigging, its also more gentle."
Adding length adds wetted surface, as well as displacement. Yes, a longer hull has a higher hull speed, but it also needs
more power to reach that hull speed. If Sven doesn't increase sail, but only adds length, it will slow him down even more.

And yes, I worry for him. Has he not read about the mischievous orca pod?

-Will (Dragonfly)

 

[Clarkey]

I hope he has left with the boat more appropriately loaded this time (but the cynical side of me imagines that he is clinging on to all the food and water like some kind of security blanket). The speed itself is not an issue - it is the inability to make to windward that makes Exlex more of a stunt than a practical means of transport. Likening it to a mountain bike is frankly a bit insulting to mountain bikes.

If he has ditched the excess weight then there is still an opportunity to test the mettle of this boat rather than dreaming up the next one.

 

[JPE]

 

[sharpii2]

True, but three of his latest designs haven't met their trip goals, not even close. His designs are safe, I'll give him that, but they are not practical. Far from it.

The way I see it we can rate different journeys with "mission points". Lets say that an Ireland to New Zealand non-stop is 100 points. So to make the trip safely, that's the minimum you need. To meet the requirement of mission points, you are given a certain amout of "design points".

First of all, any boat has a maximum design point density. My thesis is, that you just can't pack that 100 points into a boat that is about 6x1.2x0.2 meters. No way. I'd say boats the size of Exlexes can carry a maximum of 60 points. Yrvind disagrees, or, at least did up untill his latest post.

Second, to reach the goal of mission points, you have to be careful with how you balance the design points in you design. Max out on safety, and you won't have enough left for, say, performance. There is a minimum acceptable threshold for any design area. This is what I also claim happened with Exlex and Exlex minor. Sven is very safety oriented. Too much so.

Now, I hear someone roaring in the background "you can't over emphasize safety". Yes you can: for example, we have cars. To make your holiday trip, or daily commute extra safe, you could upgrade the car to a tank. Super safe, yes. Also super impractical.

Your design points add up to your mission points. In reality this might be a more complex formula, but anyways. If you try to pack in more points than your design can carry, you actually end up subracting the extra points from your total, giving you even less mission points. Sound familiar?

I agree with you.

The question is, is just where is the balance point here?

Supposing I was designing a sub-compact auto to go as far between gas stations as possible. Naturally, I would want it to have decent acceleration, and to have the ability to easily keep up with the minimum speeds on the freeway.

But, I suspect I would stop at that point when it comes to engine size. In every regard, this vehicle would be judged by most as a poor performer.

They probably wouldn't care that it can travel 500 miles between gas pumps. But I would care. And maybe a few thousand would-be car owners would care too.

Where I think Sven fails is getting the balance right. I think his loaded S/Ds are off by about half. I think his Ex Lex Minor would be improved greatly by doubling the sail area, or doubling the S/D by increasing the sail area and decreasing the loaded displacement.

Maybe 500 miles between gas stations will have to do, rather than 1000 miles between them.

My guess is that Ex Lex Minor is dragging her transom. I can see no other reason she can not sail to windward when there is plenty of wind.

A double-ender, if it has a relatively sharp stern end, would probably not have this problem, but might suffer from lack of initial stability sufficient to stand up to any kind of rig when so deeply loaded.

Indeed, when I was finally able to do stability calcs on my Lola 520 design, I was dismayed to discover it would not stand up to its more normal S/D of 15, and ended up with about 2/3rds the sail area I originally intended.

So, 500 miles it is between gas pumps--or about 3,000 nm between reprovisioning.

 

[Will Gilmore]

My guess is that Ex Lex Minor is dragging her transom. I can see no other reason she can not sail to windward when there is plenty of wind.

What about that rig says windward performance to you?

-Will (Dragonfly)

 

[JPE]

Well, anyone with any understanding of aerodynamics will state the setup is not optimal.

 

[sharpii2]

What about that rig says windward performance to you?

-Will (Dragonfly)

Granted. It would almost certainly sail upwind better with a higher AR rig. But even with this one, it should be able to make some windward progress. Even square-riggers can sail to windward. And even if they have just one square sail.

 

[skyl4rk]

If Exlex were my boat, I would add leeboards and double the sail area with an easy to reef rig, either junk or roller furling rig. If you look at the Portuguese television video, at about 3:00 you can see the boat rolling quite a bit. I would not want to have to sleep on the boat at that dock, I wonder what it is like in rough water. An outrigger might help reduce rolling.

 

[JPE]

Even square-riggers can sail to windward. And even if they have just one square sail.

Having two (or more) masts is actually not good for pointing ability af a sailboat. The sail(s) in front disrupt the airflow for the following sail(s).

In addition to the very low aspect ratio, the sail setup of Exlex and Exlex Minor is not able to form an optimal wing profile.

Put those together, add the overload and the transom it sunk, you get the performance Sven got.

During test sails Exlex Minor did display satisfactory ability to point. Nothing spectacular IIRC, but enough for safe sailing. I'm too lazy to check, but but again IIRC Sven was specifically not aiming for a boat that would be able to sail high into the wind. A slender hull would suggest that, and as such it is a weird design choice for a beam reach to running boat, but no, guess it's not a crime though.