Thread: Beth Sailing Canoe

Howdy Engblom,

For pure aerodynamic reasons it is better to stick all the area into one sail when possible.

The trend can be observed on boats where actual sail area is measured and any sail distribution can be adopted. No free area cheating like overlaps or unmeasured roaches. No Spinnakers.

In every case the jib become quite small relative to the mainsail or disappears completely unless the class rules stipulate it.

One class very familiar to me is the NS14. 100 square feet of actual sail, 14ft long with a beam of six feet and the mainsail can't be more than 18ft above deck. This is a very modest rig with limited span and low area. It is a restricted class .. within that framework and a minimum hull weight of 140lbs and a couple of measurements to stop waterlines getting too small and a crew of two adultish types .. it is a free for all.

All NS14s have about 27ft in the jib and 73 in the mainsail or similar to that ratio.

A single mainsail rig is not outlawed, and nobody has really exploited it effectively, but a lot of us wonder what would happen.

The main reason it hasn't been exploited is that the boat is so highly developed over decades and it would be hard to make the single sail rig competitive within a single season. It is just a too high risk strategy. But if a boat this modest is perhaps on the cusp of going to a single mainsail I suspect it is more open to many classes if their rules allowed it.

You arguments are good as counter arguments And they are the ones that would be proved right if a cat rig NS14 could not be made competitive after considerable trying

MIK

I am in no way an expert, and I have never even had the pleasure to sail in a real ketch rigged boat. The reason I am talking here about this topic (even though I do not qualify as novice even) is that because my claims, regardless if they are right or false, will get comments from experts and both me and other watchers might learn something.

When it comes to NS14, I think it is a boat where one sail makes sense. There is an upper limit on the sail area and with two persons aboard, you apparently are able to handle the heeling force.

When I took up the discussion, I was more talking about boats where you have no upper limit for the sail area. Even though the sails in a ketch would be less optimal aerodynamically than one single sail, I think from logical reasoning that a ketch can be faster (except for upwind, where it still does well) than a single sail rig because of the less heeling force provided there is enough of sail area. This topic I studied some time ago, and apparently the sailing community is divided and there are several persons supporting both views.

800px-Palanca-ejemplo.jpg

I think this image illustrates well my reasoning. The 100kg ball is balancing with a 5kg ball. if the 100kg ball is 10 cm away from the balancing point, the 5kg ball needs to be 2m away. It is not much heeling force you can handle if it is at the very end of the mast. Now in a rig, you have balls everywhere along the mast with decreasing weight as we get closer to the top.

(I am feeling a bit guilty for continuing this discussion in a thread about Beth Sailing Canoe, as it is not completely on topic, but it is so interesting to learn more about this )

It is a good area for discussion.

There are a few classes that have restricted sail areas and no limitation on span - they can have the tallest rig possible.

Some examples are the A,B and C class cats. The rig heights have been creeping up and up.



The 18ft skiffs however stopped their development now because of rising expense. They have unlimited sail area and unlimited span

But the 12ft skiffs keep developing. But also note that they can deal with heeling moment by swapping to smaller and smaller rigs. The 18 footers used to have four rigs and choose which one to sail with on a particular day .. so estimating what would happen to the weather was part of the racing kit of the skiff sailors.



The thing to note as you say is that the A-class has a very small sail area similar to the restrictions of the NS14. NS14 only 100 sq ft and A-Class - 18ft long by 8ft wide, no minimum weight, one crew on trapeze is only 150 square ft of sail.

So a lower rig doesn't develop much heeling force for the power of the multi platform so the rig goes up. Limiting factor has been the weight of the rig. With lighter alloy masts appearing about 20 years ago the rigs went up from 26 to 28 to 32ft. I think now with carbon masts they have gained some extra length.

The NZ Leander R-Class is a restricted area two trapeze dinghy 12ft 9" from memory - with a limited main and jib area but not very limited spinnaker.



You can see the difference in the 12ft skiff rig and the R-class rig despite similar power. In NZ the 12ft skiff is called the Q class.

I think with the dinghies with big rigs that the height is really set by the kite - you need a long enough luff to sail fast downwind (tacking downwind) without the kite becoming too low aspect - you run out of boat to sheet it to and luff length is much more efficient than area - just like our discussions about centreboard area.

So the kite pushes the rig up and the heeling forces upwind keep it down. And the jib becomes a low cost (in terms of heeling moment) way of adding a bit extra sail. In strong winds the skiffs that have too bigger sails for the say wind on the downhaul/cunningham in the main to bend the mast and blade out the top part of the sail. Compression loads force the mast to bend even more allowing heavy twist in the upper part of the mainsail. So you really are sailing on the jib and the bottom half of the main.

But the interesting thing with all the above pics is the squarish top main - first developed in the R-class about 40 years ago - mostly as a way to get depowering but also it worked to optimise the sail area under their rule - because the roach of the mainsail is free area.

But now with boats that can push the area up a square top is nice to get the area high but cut off that small triangular area at the top which provided the most heel for the least benefit. And also in a sense gets rid of the bit of mast in the old rigs that was holding up a ridiculously small bit of sail - lots of weight and infrastructure to not add much.

MIK

Boats with limited and fully measured sail area but unlimited choices about what area to put in which sails are Bembridge Redwings



And NS14s



A boat that looks kindof similar in the big main and small jib idea is the F40 multihull class.



But in its case the jib is a specified minimum size - I'm sure it would be immediately absorbed into the mainsail if that restriction was not there.

I do have a hunch that if you have unlimited sail area that if you limit the span it is better to make the rig out of several higher aspect sails ... So the taller the rig the more sense it makes to make it one single sail. Here you go from 1880s



But to go to BETH - I remember working up to the sail area question. And had chosen the area that was in some books about the American sailing canoes the rig was based on. They quoted 67 square feet. I thought the hull, being a box will have more stablity and also maybe we have learned a thing or two about setting up rigs as well as sailcloth improving so much. So I fiddled and once it got up to Moth type areas I felt satisfied. Much lower sailplan though.

Then I found out about the sail area options for the American Canoes - up to maybe 140 sq ft in big ketch and yawl rigs. Masts right out in the ends of the boat.

MIK

Originally Posted by Boatmik

It is a good area for discussion.
But the interesting thing with all the above pics is the squarish top main - first developed in the R-class about 40 years ago - mostly as a way to get depowering but also it worked to optimise the sail area under their rule - because the roach of the mainsail is free area.

But now with boats that can push the area up a square top is nice to get the area high but cut off that small triangular area at the top which provided the most heel for the least benefit. And also in a sense gets rid of the bit of mast in the old rigs that was holding up a ridiculously small bit of sail - lots of weight and infrastructure to not add much.
MIK

Take a look at this picture:
AERO-HYDRO PHOTO2-27A.jpg

In the picture smoke is let into the air passing through the sail, to show the flow of the air. The top of the sail actually became completely useless according to the flow. It will only produce a slowing down drag. Having the top cut off, you getter better use of the sail. Also, when looking at surfing boards, they have the tip away. If you look at any normal air-plane, they do not have a triangle shaped wing.

Anyone interested in this and the shape of the sail could take a look at State of the Art
The author of that site claims the triangle shape came from that it was the best option if you had a stayed rig and you need to fit the sail inside of the wires.

Originally Posted by engblom

Take a look at this picture:

Originally Posted by engblom

AERO-HYDRO PHOTO2-27A.jpg

In the picture smoke is let into the air passing through the sail, to show the flow of the air. The top of the sail actually became completely useless according to the flow. It will only produce a slowing down drag. Having the top cut off, you getter better use of the sail. Also, when looking at surfing boards, they have the tip away. If you look at any normal air-plane, they do not have a triangle shaped wing.

Howdy,

If that's his explanation... cutting off the end ...it is wrong. He really needs to bone up a bit. Or maybe he's having a joke before going on with the proper explanation.

You don't get rid of the end effects,You create a new end.

The sail, when it is creating lift separates high pressure slow moving air on the windward side from low pressure faster moving on the leeward side.

At the top of the sail the barrier ends a.d the high pressure air can whip around to the low pressure area. This does two things.

The difference between the pressures is lost over some area of the top (and bottom) area of the sail. This is a reduction in the power generated by the sail on some not small area at the top of the sail.

The second is that the movement of air from windward to leeward imparts a rotation in the air which "continues moving until a force acts upon it". So you have a spinning vortex starting at the top of the sail and trailing off downwind where it eventually dissipates. That's shown in your photo.

This vortex represents a large part of the induced drag and is unavoidable. But it can be reduced.

One way is keeping the same area but making the wing or sail longer. A-class cats or high performance Sailplanes. The limit for an aircraft is structural ... if the wing is too long it will break off or become cumbersome landing or at airports. Usually aircraft have the longest wing they can afford for their function and squeeze the necessary area into that length.

Making the tip narrower...putting less area in the affected zone. Mathematically the best way to distribute the lift is to have it distributed elliptically. Some make the mistake that the foil planform (actual shape) should be elliptical like a spitfire wing, but it is not true. But it is not too difficult to get the required shape using a battened sail The required pressure distribution can be achieved with a bottom section that is parallel then a top section that is tapered with one straight edge and the other edge with a kink. This is the way most aircraft go. it is freeing up a bit now because of CAD - more complex shapes are now calculable without the cost and accuracy being lost.

A couple of decades ago the sailboards think they found another solution. Because so much of this stuff is never actually tested and people are applying reasons they "think" apply. On the internet this means that "reasons" get repeated again and again until it seems they are true, but at best they are educated guess work - whereas at worst they are completely wrong - like the idea that hung around for most of last century about the "slot effect" air between jib and main is "accelerated" - it is completely and totally wrong. So when people give a reason ... you have to take it with a really big grain of salt.

Back to the sailboards. They started playing with much bigger sails than previously and had big head areas, however the head was very flat indeed. Then they used a large amount of downhaul to heavily bend the mast and the top of the sail became quite floppy - so it tended to twist a lot right at the top and trail downwind. This reduces the pressure difference between the windward and leeward sides of the sail. So it is another way of getting an elliptical pressure distribution without an elliptical shaped sail. Less loading on the tip of the sail and less vortex. Aircraft and hanggliders had been doing the same thing, more rigidly in the aircraft's case - usually for handling reasons where you end with more controllable characteristics as you reach the limits of control with a similar approach. At that time they had no restriction on area and the only limit was what a person could hold up. So getting rid of the loading at the top of the sail has some advantages there too and you can always load it up in stronger winds. Then they mucked everything up for the classes that allowed the development by saying that racing would be off if the wind was less than 15 knots ... so they didn't need a good low wind rig any more.

The third way is using an endplate or winglet. An endplate is a simple barrier at the tip of the wing. A winglet you see on most passenger aircraft now - it is an extension of the wing that swings up into the vertical plane with very small tip area. Or sometimes you see a much smaller winglet that extends a bit above the wing and a bit less below ... that is a bit of a combo of winglet and endplate.

So in general you can see that there is a real advantage in reducing the area of the tip, provided it doesn't reduce the area.

So why have sails developed the square top. It developed first in the NZ Leander R-Class.

Their reasons were that the span of the sail ... mast height was not limited (from memory). Sail area was limited to 110 square feet but the roach of the mainsail (outside the basic triangle described by the boom and mast) was not measured. So this was a way of getting lots of extra area, reducing the heeling moment of a small triangle the top of a more conventional sail (which has the same weight of spar and rigging as the more effective and wider areas below and they found it was very good for gust response so they could carry taller sails.

So let's look at the evidence from everything that has wings. There is an advantage to reducing the area at the tip. That's the rule.

However let's list the things that are particularly relevant to sailboats.

1. There is a limit of stability that is much lower than the strength requirement of the rig. A boat will fall over before the mast breaks - unlike an aircraft. Square topped sails allow the same sail to be lower for less heeling so allow area to be increased for some classes
2. The conventional sail with a triangular tip requires quite a lot of mast and staying relative to the small amount of work being done - this is the same with aircraft ... as pretty as a Spitfire's wing is ... almost nothing followed it.
3. It allows the sail to twist off more readily in gusts as per the original sailboard developments and the R-class.
4. Wind gradient - where sailboats are totally different from aircraft. Because of the drag against the surface of the earth the wind is slower close to the earth and gets stronger higher. So there is an advantage in a higher sail ... or more area up higher. My hunch is this one is the cruncher that makes the square top valuable. More sail up in slightly stronger winds. Don't underestimate this. Lift is proportional to the square of the velocity. So if you get another 7% of wind velocity just below the top of the sail compared to ground level then the force the sail can develop (all things being equal) is 1.07 squared is 1.145 or a 14% increase in power per unit area toward the top.

I would be expecting on the powering up side the wind gradient is the clincher for sailboats, whereas the square tip on many aircraft is primarily a manufacturing limitation as well as defined by the aircraft purpose.

On the depowering side the square top is very amenable to getting rid of the lift as the wind gets up and increased the dynamics gust response of the sail There would be zero difference if you radiussed that corner a bit to make it curved but it is easier square and to hold it out with a single batten.

I haven't seen any high level articles based on CFD (computational fluid dynamics - computer simulations of flow) that report on the real reasons ... as far as I can see it is all guesswork, some by people who know a thing or two and others who haven't much of a clue - I saw one guy saying his 12ft skiff sail that had a higher back corner at the top than the front corner was better for "endplating" and that was the reason for its superiority.

I would say that sailboat mainsails are so multifactorial just where the draft is fore and aft, how much draft, the amount of luff round, the stiffness of the mast at every height, restraining rigging at each height, individual batten stiffness, roach size, vang, outhaul, cunningham/downhaul tension in different winds, obstruction of sail by the backstay, how the headsail completely changes the flow around the main, wind shear, the actual wind gradient for the sailing site, wave loadings on the rigging, mast bend sideways, mast bend fore and aft, rig tension, cut of the jib, twist of the jib, sheeting of the jib, forestay tension and its influence on the jib shape, boat pitching, water roughness, wind roughness, interference of other boats, interference of landforms on general air characteristics on in an endless list.

It's just rubbish to say the performance edge is because of "this". Much like world peace - it's so multifactorial that people tend to have pet theories about the one thing they can change that will fix it. I would always prefer to say .. there is always peace in some part of the world.

That's how we humans are

There is some fact in the performance improvement for square top sails and like many others I am trying to guess ... GUESS ... what it is. But a lot of the discussion can be eliminated as poorly informed like the above comment/joke. Look out for CFD testing for me!

Here we have an article about some really good sailors and designers chopping the small triangular area off the top of their jibs with the result the boat wins over the fleet. Square top jibs and other sail innovations

You can see what happens by the style of the article. The skippers are cautious but the writing says the two boats "cleared out on the fleet"

And so people are already changing over to the square top jibs and the sailmakers will be very happy and have some guesses about why they might work. Happily the sailors here are being cautious. Soon lots of boats will have square top jibs and we'll never know whether they actually work!

It is certainly not worth 3 minutes in a race as the article suggests. Particularly as they haven't optimised what's happening for the dynamic behaviour under loading - it's just a cut and try approach and must be a long long way from optimal.

Add to that that many cat classes and some dinghy classes have had a batten at the top of the jib extending it outside the straight line for years and years. It is going to be aerodynamically equivalent to a square top anyhow because the extra triangle of sail is so small in the scheme of things. I really doubt that sailors can "feel" the difference when a few square inches of cloth are removed.

Placebos can be very effective. As can groupthink. Always look for evidence somewhere - based on reasonable and directly proven cause and effect. Otherwise it is just the wind blowing in the trees.

MIK

For an example of real careful analysis of cause and effect have a look at Arvel Gentry's articles about how the slot effect never existed. These came out in the early 70s when I read them as a schoolkid and you can still find people who will fight to the death about the "accelerated flow through the slot" even though it was debunked by the Smith group at Boeing (which included Gentry). Read them from top to bottom and let it percolate - it took me some time to understand but it changed my understanding totally.
Magazine Articles on sail lift by Arvel Gentry

It was worth writing all of that

But the original article is from Eric Sponberg who knows a thing or three. You will see that we are saying similar things about the necessity of twist, but square tops are not the only sails that twist, but any sail that is non triangular. In Australia we have been well away from triangular sails in boats without backstays for many decades now. I would have to go back to the 1950s to show that sails with big roaches were already moving away from the triangle.

Here is Evelyn on the Swan River in the '57 championships .. the sail looks triangular despite the twist hiding most of the head. The skiffs of this era often had a little head batten that would kink the top of the sail out by a foot or so - much like a square top.



Or the VJ class for juniors (who are quick on their feet) designed in 1929.



I wouldn't claim at all that roaches outside the straight line from top of sail to end of boom was Australian. The Kiwis were maybe faster than us in adopting. The Germans were pretty well into that sort of thing. Probably the Swedes too.

MIK

I am very satisfied

Thank you for a lot of information. Personally I am happy I posted the thoughts I found on Spoonbergs site about the cut off sail as it provided a such lengthy answer here.

Mik, this photo takes me back to my youth, when in about 1956 I had clapped out VJ for a year or so and after revarnishing the hull kept it at Avalon Sailing Club for a year or so. It could not compete so I just sailed around Pittwater with friends. Loved the plank to hike out on. Lost daggerboard once during a capsize and forgot the drainage plug once. When she got sluggish, I just made it to Taylor's Point and went to a house and asked for a sherry cork which fit nicely. As Avalon was geographically impossible for a kid too young to drive I sold it.
Peter Lord

Originally Posted by boatmik

Or the VJ class for juniors (who are quick on their feet) designed in 1929.



I wouldn't claim at all that roaches outside the straight line from top of sail to end of boom was Australian. The Kiwis were maybe faster than us in adopting. The Germans were pretty well into that sort of thing. Probably the Swedes too.

MIK

The Swedes bent the top of the mast on their "skärgårdskryssare" (skerrycruisers) to get the same effect.

and the Germans bent their gunter yard and roached their sail in the 20s and 30s, and the sails were fully battened..

Originally Posted by engblom

I am very satisfied

Thank you for a lot of information. Personally I am happy I posted the thoughts I found on Spoonbergs site about the cut off sail as it provided a such lengthy answer here.

A pleasure sir!

MIK

Had some more wind - maybe 5-7mph, and got the Beth sailing quite nicely. Took my 5 year old daughter with me, she sat in the front of the cockpit and enjoyed her sail. I'm getting used to the push-pull tiller and like it so far. Here are a couple of (shaky) videos my son took of us last Saturday:

Beth Sailing Canoe 15jun13 - YouTube

Pleasant sail on the reservoir - YouTube

20130615_151332.jpg

20130615_151555_7.jpg

20130615_150939.jpg

And here are a couple more pics. Could someone with more experience than me (which would be any) with balance lugs please comment on the set of the sails, halyard position and give me some pointers on what should be improved with my sail hoisting technique please?
Cheers Rich

Nice photos and videos! Thank you for sharing them!

Hi Rich,

The photos look great ... I'm waiting for the videos to download.

The set of the sails looks nice meaning the tension is nice.

There are a couple of tweaks to the position of the sails.

Both sails need to have the boom moved back as the booms are meant to be higher at the back than the front. Partially for aesthetics, but mostly to give you more space in the cockpit. If this is done you will be able to move the mainsail down the mast a little. So to list that out for the mainsail ...
move the halyard point on the yard back 100mm
Move the mainsail down the mast by about 100mm
Move the tack (bottom front corner of the mainsail - for other readers back so the boom distinctly cocks upwards

With the mizzen - it needs the back of the boom up higher. But the overlap of the sail to the mast needs to be right for the boat to balance properly. I found the sail needed to be further back than you .. but you should allow your feel of the boat to be the guide - you do have to check from tack to tack. The mizzen should be just luffing in the throat area when sailing efficiently upwind in moderate winds when you are starting to hike out consistently.

Here's my original sailplan drawing. To my eyes now the mainsail looks a fraction too far forward. But you can see the boom angles. The main boom does come down lower to near horizontal with mainsheet tension upwind because of the springy mast. But it does mean every time you tack and ease the mainsheet 6 inches or a foot then the space will magically reappear for you to move across the boat.



MIK

Howdy,

Just looked at the videos - I've added the first one to my playlist for storerboats!

The best location for sitting is to use the centre butt strap in the bottom as a reference. Sitting with body weight over the butt strap should be about right. a bit forward in really light winds (when there is no small passenger to squish). Upwind you don't usually have to move back much from the standard position unless the bow is really digging into the waves.

Downwind in most conditions you don't need to move back much initially in light and moderate winds .. just progressively in stronger winds to keep the bottom corner of out of the water slightly most of the time. In really big breezes you do end up a bit further back.

All in All .... looking terrific!

MIK