Sailplan vs Center of Effort

[Oscarvan]

In the pursuit of educating myself on boat design I have been diligently chewing my way through the book pile, and applying the theory to my design.

The sailplan I envision is a gaff/cutter rig with topsail, think Channel Cutter. I have calculated sail area and located the center of effort, both for the full rig, and for main/stays'l alone.

I have located an approximate Center of Lateral Resistance.

The tops'l CE is almost directly over the CLR, so whether it's up or not doesn't make much difference. Main and stays'l alone put CE about 13% of lwl behind CLR. Mr Skene approves.

Here's my problem though. The jib is large, overlapping large, in fact it is almost as large as the main and tops'l combined. I want a LOT of canvas in light air. I want to be able to approach hull speed in 10-12 knots of wind. The big jib is intended for winds up to 10-12 knots. But it brings the CE forward. Waaay forward. Forward of the CLR, by, coincidentally, approximately the same 13% of lwl.

It is almost unavoidable to have this happen. If you put a big sail that far forward things are going to shift a lot. Now, close hauled this would give me a serious lee helm. Probably would not be a problem off the wind.

Is this a problem? I'm sure I could learn to live with it. I have little experience with this sailplan. Is lee helm unavoidable under certain conditions on a channel cutter? Moving the rig aft enough to trim it out would give me too much weather helm in a blow with main and jib alone...the mast needs to stay where it is....

Comments, suggestions, whiplashings? Thanks...Oscar.

[Ed Burnett]

You might want to re-read that paragraph in Skenes - 13% is nice, but not aft!

As for the huge shift in sail plan CA with changing jib area, welcome to what is pretty much problem no.1 in the design of this sort of boat. Checking your references will more or less turn it on it's head for you though.

[Tom Hunter]

I will start by saying that I am poorly educated on this subject. Maybe I will learn something from this thread.

My impression of channel cutters is that they have large gaff mains, and multiple, smaller triangles forward.

You seem to be advocating a small gaff main, and a single, large triangle forward. When you calculate this out, the math is (correctly) showing lee helm.

Why not just go with the big powerful mainsail to drive the boat. I would think this will be a lot safer than what you are proposing, and likely easier to sail as well. What performance advantage are you hoping to get in exchange for lee helm? Is it worth the danger that comes with lee helm?

Is it possible for you to post some pics or drawings of you design proposal? That will get you no end of commentary, some of it helpful.:)

[Lewisboats]

Lead is the distance of the CE of the sails AHEAD of the CLR. The percentage of WL normally is determined by rules of thumb related to the shape of the hull, the shape of the keel, the aspect ratio of the sails and the stability of the hull (does it heel a lot or is it stiff). Too much lead will increase weather helm...not eough will run to a lee helm. You should have a 3-5 deg weather helm with the rig you will use most and put up with more when necessary.

[JimD]

Too much lead will increase weather helm...not eough will run to a lee helm.

You sure about that? ;)

[Lewisboats]

I knew I was gonna mess that up :o. For the longest time I was under the same impression as Oscarvan about the positions of CE and CLR... Seems I can't manage to remember the relationship to balance properly either...Sorry.

[Oscarvan]

OK, I sit corrected. All of my paper guru's, dead and alive put the CE in front of CLR.....this is counter intuitive....looking from above you'd think the bow would be pushed away from the wind.....can someone 'splain this???

That may solve the problem though.....have to rework the numbers.

Tom, the rig is a large gaff main, tops'l, small stays'l and an assortment of jibs ahead of that on a long sprit, the biggest one being HUGE. Again I want to be able to put a massive amount of sail up in 10 knots of wind......

A picture......hmmmmm have to think about that.....something in me wants to surprise you guys when I start building.

[P.I. Stazzer-Newt]

Hmmm, that's counter intuitive. Figure CE behind CLR gives you weather helm......Back to the book.
...

Think of a tiller-steered boat where the CoE is aft of the CLR.

This has the boat tending to turn up into the wind - to counter this the helmsman pulls the tiller up towards the weather side of the boat - hence "weather helm".

[Thorne]

That's how I remember it also -- to sail straight on a reach, having to push the tiller toward wind = weather helm, tiller away from wind = lee helm.

[Oscarvan]

We all agree that weather helm = pushing the tiller to windward, as the bow wants to go to windward...

What I'm confused about is why a CE forward of CLR would cause that....again, looking from above you'd think the opposite.

[Hwyl]

You might want to re-read that paragraph in Skenes - 13% is nice, but not aft!

As for the huge shift in sail plan CA with changing jib area, welcome to what is pretty much problem no.1 in the design of this sort of boat. Checking your references will more or less turn it on it's head for you though.

OscarVan, you may not be aware that Ed Burnett is; well, Ed Burnett.
This thread should never have progressed past post #2.

Go back and look up lead. Ignore the rest of us.

I will say that for drifting sails CE does not matter too much, but 10 knots is not drifting

[Thorne]

Too many to's and froe's here for me to keep track of.

As far as I know (from pratctical experience, not theory), adding canvas forward increases lee helm, or balances weather helm depending on how you look at it. Anyway, canvas forward of the CLR pushes the bow downwind.

Canvas aft of the CLR pushes the stern downwind.

Balance the two forces and the boat is balanced -- of course you want the CoE to be a bit forward of the CLR so you get the desired small amount of weather helm.

I learned this from two boats. One was a friend's boat with savage lee helm, any strong gust put the stern into the wind. We removed the jib and it handled much better.

The other was my boat with savage weather helm -- any strong gust forced the bow into the wind. I added a bowsprit and jib and the boat now sails much better.

[P.I. Stazzer-Newt]

We all agree that weather helm = pushing the tiller to windward, as the bow wants to go to windward...

What I'm confused about is why a CE forward of CLR would cause that....again, looking from above you'd think the opposite.

It (CE forward of CLR) doesn't cause weather helm, its the (partial) cure.

A common mistake is to assume that having the CoE in line with the CLR created a perfectly balanced boat - this is very wrong - the lead (thats as in "Horse to water") suggestions of 5% to 15% are an indication of how far forward the rig must move for reasonable balance.

The weasel word here is "reasonable" - most people have a strong aversion to Lee helm - its scary - as a result most designers aim to retain some weather helm under almost all conditions.


B.T.W. Hwyl's right - Ed is one of the best known and respected Gaff designers in Britain.

[Oscarvan]

It (CE forward of CLR) doesn't cause weather helm, its the (partial) cure. A common mistake is to assume that having the CoE in line with the CLR created a perfectly balanced boat - this is very wrong - the lead (thats as in "Horse to water") suggestions of 5% to 15% are an indication of how far forward the rig must move for reasonable balance.

AH! That makes sense....the boat has weather helm all by itself......let me guess....fluid dynamics? Oh boy.....

Agreed with the effects of moving canvas around.....

I feel better now. Learning is taking place.....thank you all, and nice meeting you Ed.

[P.I. Stazzer-Newt]

If you want to get a feel for how the variables interact - go sail a laser - people who are good at this can sail round all sorts of complicated courses - without the rudder.

[Ed Burnett]

There are a couple of important things to realise here:

1. The centre of area of the sail plan (often referred to as the centre of effort) bears very little relation to the true centre of effort.

2. The centre of area of the underbody (often referred to as the centre of lateral resistance) bears very little relation to the true centre of lateral resistance.

The way these forces are generated is pretty complex, and the whole subject of helm balance is very complex. There is an awful lot going on here, to the point that the problem defies true or accurate analysis outside a towing tank, wind tunnel or at a pinch exceptionally rigorous computerised analysis (computational fluid dynamics or CFD for short).

So, what those of us in the real world do is use an age old method of comparing one boat with another, by analysing simple things that we can work out in five minutes. Over the course of time, it has been found that if you compare the fore and aft postions of the centres of area, wind in a few little fudge factors and bear in mind a couple of other things, you can get an idea of how a boat will balance. That's it. Very little theory, practically no science, and absolutely nothing to do with trying to predict where these forces actually act. It's just a way of solving a problem we don't have a good answer for.

Like many areas of design, when we don't have the time or the inclination to work out what is really going on (or when we are wise enough to realise that we will probably get it wrong if we do), we fall back on a pool of empirical data and experience. Start from a known position, make a small and measured change, and see what happens. So it is with helm balance and specifically the "geometric lead" method of assessing it.

So, unless you are looking for a PhD project, forget trying to explain why boats don't balance perfectly with a lead of 0%, just take a good look at some existing boats and use the lead method to compare your boat with them. Check they were well balanced though - you can't necessarily assume this and you don't want to go copying a dog.

[Oscarvan]

Point well taken Ed. I'm starting to understand the process better.

I'm also realizing I could be in grave danger.... Never was one to follow a recipe in the kitchen.... :D

Back to the books. Thanks all.

[Dick Wynne]

Ed: My recent build to an 1899 design carries a bit more weather helm than I would like - a N/A I discussed this with recently suggested this was common with modern builds to old designs, and that the only factor he could think of was different properties of cotton vs synthetic sail materials. Assuming, that is, the boat would have been well-balanced in her day. I can't afford to put this to the test unfortunately. Maybe someone has already experimented with both materials on the same boat. What do you think?

[John B]

Dick, one thing I have learnt from extensive reading over many years on this very subject and particularly related to turn of the century gaff designs is that there was a belief amongst designers ( some? all ?)that a boat needed significant weather helm in order to perform on the wind.
This is often referred to as an aside in the text, as if its so commonplace you wouldn't bother talking about it. The last place I recall reading it was an article in a relatively early woodenboat mag about the NY30 boats by Herreshoff. I don't recall the author but it was perhaps Bolger ? IIRC it was repeating a comment made by LFH about his father's designs." men were men back then " type thing etc

I'm not saying that your comments about superior modern cloth is untrue at all, in fact I agree,just that its another element in the mix.

edit.. as I look up from my desk to my 'gaff wall' I see in amongst the David Payne Rawhiti , Ed Burnett Rainbow, Logan Iorangi / Rawene/ My Waione and Steve h 's Ngatira, a NY 30 printed off a page that has " 35/woodenboat 23 on it "( LOL)
So the article may be in wb 23, or wb 35.


another question for Ed.
When I designed my rig I left the topsail out of my figuring. I did do a measurement to see what effect it might have but after it appeared fairly neutral I just left it out of additional plans/ experiments.
Is that normal or would you typically figure it in ?

[Dick Wynne]

. . . there was a belief amongst designers ( some? all ?)that a boat needed significant weather helm in order to perform on the wind . . .

Ah! Thanks John, very interesting.

[Hwyl]

I've read that too, but relatively recently (at least contemporary to my own life, which some may not claim as relatively recently) It was that the rudder and the keel combined to make an airfoil to drive the boat to windward, so the helm always had to be up to make this airfoil.

[Todd Bradshaw]

There are differences between modern fabrics and old natural fiber types, as well as the techniques used to shape them into sails, but as far as I know, the desired resulting shape is pretty much the same. Natural fiber may be more adjustable since the stability of synthetics tends more toward "what you build is what you get" and natural cloth can usually be stretched and tweaked more, but I'm not aware of any instance where you would make big sailplan changes based on the cloth selected. The goal is generally to design the sail and it's 3-D shape and do your best to build and maintain that - whether it's cotton, Dacron, Kevlar or polytarp.

Also, don't be too sold on the idea that the best headsail for light air is a huge one, because it often may not be. Sail area is great, but it takes a certain amount of weight to generate sail area and you may not have enough wind pressure to fly such a big hunk of fabric (and it does need to fly, even though it may be hanked to a stay). It's almost like the guy you see out on the water trying to sail in five knots who has a 170 Genoa up, but has absolutely huge sheets hanging off of it and weighting it down. The lightest jibs, made for the lightest winds, are usually drifters (made from spinnaker fabric and generally good up to 6-8 knots). Most are in the smallish range (maybe 130 or so) and they work better and fly better than larger sizes and the boat goes faster because you can keep them working and the airflow attached.

[John B]

There's a very well known if not famous NZ racing sailor , a friend of mine who I have enormous respect for. He says that 5 degrees of weather helm is optimal and thats good enough for me.

What I'm referring to Gareth, is that the perception 'back then' was probably significantly more than that. From my own experience when sailing deliberately out of trim ( say a staysail and main set because the wind has come up all mean and I need sail off) a boat with lots of weather helm does actually go very well to windward. Exhausting, but the boat is defaulting to a close winded sailing position. So I can see how in a different age , with slacker rigs and softer sails that this might be a ' design parameter'.

Anyway, I tried to dial that out on Waione to some extent with a shorter boom ,long bowsprit , bigger headsails and a very plumb rig and it worked pretty well. Easy for me , I had the boat, so I had the clr and the ce in a 'realtime' situaton.:rolleyes:

[Jase]

just been through this getting my new sail plan designed.... old COE was right on the mast with the Genoa up, used to truck along in a perfect line but i used to wonder where the boat would end up if i fell off.... new coe is back a bit so i get some weather helm... and consequently I hope goe to windward better (ignore hull as it was scaled from a photo and isn't correct for rudder placement etc.)http://www.classicyacht.org.nz/files/ForumJP/coe.jpg

[John B]

I'm going to talk to the handicapper about you buddy.:eek:

[Jase]

tee hee :D B's will rock this yr :D may have to get a guest handicapper for us and i'll do the rest :D

[John B]

Hmmph, I'll do it. Just give me a numerical version of 'dead last', that'll do jest fine..

[Woxbox]

Another way to look at this issue is to consider solely what's going on under water.
With weather helm, the keel is pointed to a slightly higher angle than the direction of the boat. The foil shape of the keel then creates lift to windward. The rudder is working at an even higher angle of attack to keep the boat on course, which produces more lift to windward.
With lee helm, everything is reversed. The keel is neutral, or worse yet, has an angle of attack to leeward, and the rudder, in attempting to get the nose up to windward, is also working to push the boat the opposite way. This is disastrous for windward work.
So you absolutely do need weather helm to go to windward. But, if there's too much, you have to throw the rudder so far over that it stalls out and creates turbulence, which is to say, lots of drag.
But dialing in the right amount of weather helm is a trickky thing, since it's a moving target. It depends not only on the sail you have set, but the trim of that sail, the force of the wind and the speed of the boat, angle of heel and who knows what else.

[JimD]

A simple google of something like 'causes of weather helm' will provide all sorts of useful, well written info for anyone who wants the long version.

[Ed Burnett]

Ok, this subject is opening up nicely. All relevent stuff which just underlines how many things contribute and why it is a hard nut to crack from a purely theoretical standpoint.

Firstly, Oscarvan, you are only in danger if you lose sight of where you are relative to other boats. At all costs, keep track of the various proportions, ratios and coefficients (nice big spreadsheet) which will tell you where your boat is "different" to others. Different isn't necessarily bad, but make sure you know what the consequences are likely to be and whether or not this is actually what you want. It is all to easy to veer off the road when developing a design - take a look around every now and again.

As for materials, I do think it makes a difference. Traditional rigging materials were all pretty stretchy relative to what we can use today, and I suspect this made everything rather more forgiving. Certianly we now see increased loads in rigs because individual pieces do not stretch and "share" the load around. It follows perhaps that all the cause and effect links are sharper making the boat as a whole more sensitive in various respects, including balance.

What I do know is that I am winding in far more lead in the design of rigs today than you would expect to see in old boats. The same goes for re-rig jobs. The logical next question is what these old boats were like to steer, and I rather think the answer is "heavy"!

It is probably the case that people were more accepting of this as has been suggested above. It is tempting to concoct a reason for something after the event - one hears all sorts of stories as to why heavy weather helm was a desirable attribute, such as "so the boat comes back to get you when you fall over the side" and so on. In truth however, just as many boat owners would do today, I rather think that this was just trying to shed a favourable light on the issue.

Then again, in a "softer" boat, perhaps without a prop apperture in the rudder (which makes living with imbalance harder work), it may have been that the problem was easier to live with.

It is the case that you want a bit of weather helm. In a fin and spade boat one would aim for a few degrees and optimise the foils with this in mind. In this case however you would have a partially balanced rudder so the actual helm load would be less. With long keels the rudder works rather differently, but still a bit of weather helm makes the boat easier to feel on the wind.