ketch lead

[Hannu Rantala]

Hello!

Do somebody know how much is the recommended lead for fin keel masthead ketch?

[gonzo]

It depends on sail area proportions, shape of keel and shape of hull among other factors.

[Hannu Rantala]

That's true, but maybe it is possible to give a general value, for example Larsson says that long keel ketch should have a lead of 11-15% of Lwl.

And I'm speaking theoretical lead measured of theoretical CLR and CE in upright conditions.

[mmd]

Skene's "Elements of Yacht Design" suggests about 20% using CLP without rudder and 1/2 mizzen area.

[gonzo]

The limitations of Skene's, and I'm looking at it now, is mainly that it refers to a particular hull shape. For a beamy and shallow or narrow and deep hull, the numbers don't work right. Also, his hulls have even waterline and buttock lines. In a modern type hull, with straighter lines and sharp turns, the centers of effort change a lot with heeling.

[mmd]

Not to be too fussy, but I have a certain level of disrespect for "rules of thumb" regardless of the pedigree. But your point is well taken, Gonzo.

[gonzo]

I think "rules of thumb" are fine as long as you are aware of the limitations. For example, scantling rules, when applied to the intended application, produce adequate designs.

[Hannu Rantala]

Is there any procedure to find out suitable lead for certain yacht. I have preliminary design of such ketch. Which measurements should be known before deciding the amount of lead?

[Andy]

Due to the flexibility of configuration of the ketch or yawl rig, that the amount of lead can be varied and will vary much more than for a single mast. You can sail with any combination of all the sails on board, and for each combination the lead and balance will be different (sometimes extremely so, eg no mizzen but full main and foresail). Many canoe yawls were designed to take advantage of this - the mizzen was just a 'steadying' sail to help balance the boat rather than a driving sail. The manoueverability of these boats in confined spaces is due largely to this. Perhaps the best solution is to work out the overall centre of effort and lead for a number of sail combinations, and bias the final lead towards the most used configuration. I would also be tempted to go with Larssons figures as Skene was desiging for long keel boats which are much less sensitive to variations in lead than short chord fin keels.

My two cents!

Andy

[Hannu Rantala]

So, the good old way of spying is the best...
By the way, ketch rig has mizzen in the bow side of rudderpost, and yawl aft...or is there something wrong? What do you mean saying flexibility of configuration?

Thanks for the comments!

[gonzo]

A yawl has the mizzen mast aft of the rudder post or the waterline. Also, yawls generally have a smaller mizzen sail. Long keel boats are remarkably forgiving to changes in CE.

[Andy]

Hi hannu

Flexibility of configuration means that there are many more sail and reef combinations available to a two masted boat. Also, as everyone knows, you can tune a rig fore and aft with rake and prebend to alter the centre of effort - with two masts you again have many more tuning possibilities...

Andy

[Stephen Ditmore]

The library at Mystic Seaport is where the designs of Sparkman & Stephens and many others are archived. The Seaport Museum also owns an S&S yawl and has undertaken to gather considerable sailing data on it. The people who've been doing this work should be able to answer your question in an informed way. http://www.mysticseaport.org/

I'd also encourage you to arrange a sail on an existing ketch for which you can obtain a sailplan and underwater profile. Do the mast rakes and other details match the drawing? Calculate the Center of Effort the Larsson way.... where the 25% keel chord intersects the waterline 45% of draft below the DWL. Best if the bottom of the boat is clean and the sails are decent & recent. When you're on board don't worry about the effort needed to steer - that's a different issue. You need a way to measure the rudder angle in degrees fron CL. In order to get the boat to sail straight with the tiller (or quadrant) 1.5 - 2.0 degrees to weather do you have to reef the mizzen? How much?

One other thing I'm advocating: Instread of quantifying lead against waterline length, make it a percentage of the CE height above DWL. The reason why is long & involved. Trust me, this is what everyone should be doing. Perhaps I should call this something other than lead; how about the RPP (Rig Position Proportion)?

[Hannu Rantala]

Thank you for the tips about mysticseaport! But do they have fin keel
ketches also?

Measuring CLR is guite clear...I have used Larsson's way.

What do you think is advantage of measuring lead of CE
height above waterline? Is this new method? As I know, present method is
only a percentage, which tells us better the amount of lead relative
to yachts size.

Hannu

[SailDesign]

Hannu,
The reason for using some fraction of the height of the CE is the same as the reason for lead, that the CE is offset from the CLR as the boat heels - the more the heel, the more the offset, so the "lead" would need to be larger with increasing mast height.
Steve