# Centre of Lateral Resistance, Centre of Effort and lead

Discussion in 'Sailboats' started by cookie munster, Jun 26, 2017.

1. ### PrinciplesPrevious Member

Tinkerbell anchored in mid Atlantic

How can we stop and hove-to with a light surfboard ?

Taking the rudder out of the water

without the rudder the boat can hove-to with a sea anchor

Robert Manry, Frank Dye ...
The minimalist tribe taught us this way

Taking the rudder out of the water

The monster IMOCA thought it was a fishing boat of the 19th century

with a surf board you can not stop,
if you want stop you must anchor
with the rudder out of the water

Last edited: Jul 18, 2017
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### gonzoSenior Member

You will have to qualify your statement that traditional boats can't surf in heavy weather. Many fishing boats have been used in the surf for centuries.

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### DCockeySenior Member

What difference does the direction boat faces in the drawing make?

4. ### PrinciplesPrevious Member

In a boat the Hydrodynamic Center (CLR or Center of Pressure, cp) is ahead of the Center of Gravity (cg)

Naval architects draw bows to the right
Aeronautical engineers draw noses to the left

Nose, left--- CG ---- CLR --- Tail
Stern ------- CG ---- CLR --- Bow, right

In a traditional sailboat the Center of Pressure -the Hydrodynamic Center of Lateral Resistance (CLR)- is around 30%-40% Lwl from Bow and the center of gravity (cg) is around 49%-50%-53%

In a boat the rudder and the helmsman has too much work moving aft the water pressure to position the CLR aft the CG

This is a classic solution to move aft the water pressure, to move aft the CLR
and
This is a modern solution to move aft the water pressure, to move aft the CLR

Last edited: Jul 18, 2017
5. ### PrinciplesPrevious Member

body (Bow) + wing (Keel lateral force) + fin (Rudder lateral force) =
a Force aft of the center of gravity or just in the center of gravity

the problem is that a heavy sailboat have more nose that a SR-71 blackbird

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### PARYacht Designer/Builder

I'm not sure why this particular poster is attempting to make argument where none currently exists, but we've seen this type of personally before. I'm sure he'll find the same as previous members that have gone this route. It's one thing to make exception, but grandiose statements about we all being wrong, for aesthetic or other contrary reasons, seems at least self defeating. Maybe it's just Windship 225 trying to reappear . . .

7. ### PrinciplesPrevious Member

This a classic Solution, a very good solution,
I don't see was is wrong with this Solution

Ok, wrong forum to speak of sailboats

(PAR, Besides spitting your contempt you should be able to say something interesting)
(You drown in your own bitterness)

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### PARYacht Designer/Builder

Yep, this guy is going to last a long time on this forum . . .

Showing century old designs as "solutions" to his perceived "problems" again seems self defeating or absorbed, I can't tell which. Stormy Weather can hold it's own against a modern cruiser, but not against a modern racer/cruiser and certainly not against any modern racer, which is of course is what she was in her day. The top image, is just a slack bilged pig of a boat, with lots of everything wrong with it, in modern standards, though she is drawn facing in the correct direction.

So, tell us "Principles" what is your experience? I know mine (I've been helping here for over 15 years), so do many here, but who are you? Have you any designs in production? Have you ever actually designed and had a vessel built? Have you built any boats? Maybe you can justify your grandiose statements and conjecture with something other than "cut and paste" from WiKiPedia.

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### DCockeySenior Member

I still do not understand how the direction a boat is drawn makes any difference to its performance. I have seen boats drawn both ways. I also have seen boats with station location measured from the transom or sternpost with positive going forward, and with station location measured from the bow or forward end of the nominal waterline with positive going aft. Again no effect on how the boat performs. I am assuming the designer or other person doing calculations understands what is forward and what is aft, and acts appropriately. There is no reason the CG has to be to the left of any hydrodynamic centers in a drawing.

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### PARYacht Designer/Builder

Starboard profiles are traditional and given this posters need for antique ways of doing things, I'd think he'd accept this as the norm. In reality, it doesn't matter. From a presentation point of view, the port profile is more appealing to a left brained person (the vast majority of us).

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### gonzoSenior Member

The problem with that "solution" in the Channel cutter design you posted, is that they have a huge amount of weather helm. The mainsheet was attached to the aft end of the rudder head to compensate for the steering forces. They are not well balanced boats, but a design that stemmed from UK tonnage rules.

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### PARYacht Designer/Builder

Not only are they not well balanced, though the LWL's look fairly balanced, the rig placement doesn't take advantage of this, like it does on Stormy Weather, these puppies sail like crap, they're slow, they don't handle worth a darn, draw way too much for their length, etc., etc., etc. A classic example of an antique design that obviously hasn't benefited from modern thinking.

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### sharpii2Senior Member

The lines drawing on the top looks like that of a more traditional hull type, but one designed for racing. The keel drag is so extreme that it is almost delta shaped. I don't believe any working boat would have a keel such as this.

This keel, as drawn, was probably drawn this way for two purposes:

1.) to get a higher aspect ratio, since the aspect ration is determined by the square of the span (depth of the keel in this case) divided by its area, and
2.) to move the Effective Center of Lateral Area (ECLA) as far aft as possible.

My guess is the second goal was probably the more predominate one. In that case, if the bow wave theory is correct, the strategy probably didn't as well as hoped. Moving the ECLA that far aft would only give the bow wave that much more leverage to force the boat to turn up wind.

Just because some of something is good does not necessarily mean that a lot of it is better.

As far as the channel cutter type goes, I have heard that it makes a good cruising boat.

No it's not fast. And no, it doesn't plane.

But it can carry a lot of gear and sail at a reasonably fast cruising speed.

From what I have read, I have not heard of heavy weather helm issues. And that's with versions rigged with the more traditional gaff rig and versions rigged as masthead cutters.

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### PARYacht Designer/Builder

I'd offer the keel was drawn that way and referred to as "drag", because it was dragged up onto marine railways or onto carts on skids, to get dragged ashore in high tidal range areas (like the UK) and this keel drag permitted the hull to be floated onto and off the cradle relatively level.

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### sharpii2Senior Member

This is an interesting theory, and I can't say you're wrong. But, leafing through my Chapelle book, every long keel I see has drag. But the drag is very subtle, usually 1:8, rise over run, or less. And often it's a lot less.

My suspicion is that the drag served a different purpose. Perhaps it was to allow a deeper rudder.

Another possible theory is that the drag caused the bottom edge of the keel to move through the water at a slight angle. This would cause some positive upward pressure which could discourage water on the down wind side of the keel from taking a short cut under the keel to the windward side. This would presumably make the keel somewhat more efficient.

My guess is that the 18t and 19th century designers of these keels knew little or nothing about fluid dynamics. Aerodynamic theory was barely understood until the Wright brothers. But they were keen on what worked and what worked better.

The term "drag'' probably referred to the aft corner of the keel touching bottom first, then dragging along.

The only reason I can see for using a long keel on a modern boat would be to limit the sailing draft as much as possible, with windward performance being a secondary consideration.

In this case, a keel about half as long as the boat is probably close to the best compromise. As the keel gets longer and shallower, it's area must be increased to make up for its diminishing efficiency. Adding drag might help, but that increases the sailing draft. With a very long keel, where the drag would be the most useful (if my theory is correct), even a modest drag of 1:16 would increase the draft to such an extent that a shorter, higher aspect ratio one could be used instead and get the same sailing draft.

Attached is a sketch of a model I would like to build and test someday. It would have detachable fin keels of various proportions. Her it is shown with a keel half its hull length, with 1:16 drag, and a detached rudder.

It would be an interesting experiment to make another keel with the same length and area without the drag and see if it performs the same. If my theory is correct, the one with the drag will perform noticeably better than the one without.

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