View Full Version : Seakindliness

Foster Price
09-12-2000, 04:52 AM
Hello Guys

I've just been reading a series of letters between John Morwood and Peter Tangvald, published in "Solo Cruising" an Amatuer Yacht Research Society publication of October 1965. J Morwood was editor at the time.

They discussed many aspects of the concepts involved and made several propositions, some of which were:

beam/lenght ratio based on the formula Beam^0.5=LWL^0.333 - This sems to be a good guide to me.

Short ends - ie no long overhangs, probably with a transom stern.

MOderate draft with a low aspect keel.

Pitching motion able to be adjusted through use of ballast/weight

Veed sections

They didn't agree on light versus heavy displacement - lets continue this one and see what we come up with.

Regards - Foster

PS I'll scan and post the articles if anyones interested - let me know.

[This message has been edited by Foster Price (edited 09-12-2000).]

Ian McColgin
09-12-2000, 08:14 AM
By that theory Granna (an LFH MarcoPolo 53'lwl 10'b 5.5'd) is about 4' too narrow and my old Goblin (Alden 43' schooner 33'lwl, 12'b, 4.5'd) was a couple feet too plump. Both schooners are quite seakindly in very different ways.

I find Marjahj (sp??) "Seaworthiness: The Forgotten Factor" (or words to that effect) to give a really good look at how many factors interact.

09-12-2000, 01:49 PM
Is seakindliness definable? Or is it like Best=What I Like? Paper the page with numbers. Bury me with statistics. Does it all come down to what feels good? My notion of comfort is probably different from my adrenalin-junky son's.
Seaworthy is perhaps a different matter, ultimately reducing to arrivals equaling departures.

Ian McColgin
09-12-2000, 02:52 PM
I believe that seaworthiness is definable and objectivly verifyable but also that it comes in a number of different forms. I also think that it works differently for different general sizes of vessel.

For example: One of the great early MORC boats was Ray Carlson's 22' Cutlas, reverse shere and transom, hard bilges and fin keel - rad for early 60's. Like any small boat, she was pretty uncomfortable in a wild storm but she's super seaworthy whether bashing into it, runing off at speed, or lieing ahull.

But you gotta be nuts to lie ahull in something as heavey as Granna since the impact of a boarding wave goes up exponentially with both tonnage and surface area and the boat's ability just slide off under the blow diminishes by similar exponential factors. In all of this, the hull strength becomes the critical breaking point. In the Cutless or any other wee couple tonner, the ratio to hull strength towers over any possible level of impact while Granna's far stronger hull does not have such an advantageous ratio. This strength to weight bit is like a guy I used to climb with. I could press 250# with reps and half squat 500# - far more than Super Squirt could do - but he could do about 50 chin-ups. Strength to weight.

Granna has superior seaworthiness because she can heave to quite high into the wind and the present a small target or she can run off with very small quarter waves and thus small risk of being pooped.

My opinion to the extent that rules have any bearing is:
IOR Rulebeaters cannot be as seaworthy as real boats. You gotta keep the keel and especially the rudder in the water all the time as a basic element of seaworthiness.

The smaller the boat, the greater the relative value of 'high' displacement since it eases an otherwise choppy motion.

The smaller the boat, the less impact design has on seaworthiness so long as the hatches are strong and the design is not IOR. It can be fat or thin, infact as the boat gets smaller it can get porportionaly tubbier without under seaworthiness problems. There are seaworthy little 22'lwl x 8'+b things but a 50' waterline with an 18' beam might charge like a luge but would also be stabile upsidedown and would also impose incredibly loads on the rig and hull appendages.

That's why those round the world racers end up capsized with no keel.

It's not the one thing, it's the harmoney.

09-12-2000, 04:22 PM
Seaworthy -- the boat will survive (details of conditions to be survived, crew, ... are important).

Seakindly -- you will survive inside her, and be willing to do so again.

Ian McColgin
09-12-2000, 04:46 PM
I think they are the same because if she's not seakindly and bangs around hard she'll damage her gear and become unseaworthy.

So,now that I think carefully about your distinction, maybe 'seakindly' applies more to the design and 'seaworthy' to the empiracle condition of the boat.

All I know is that some 60' sled with a name like 'Il Pipistrello fuori di Inferno' is unlikely to be seakindly ever and will not remain seaworthy for even 25,000 miles - about as good as a Yugo.

09-14-2000, 11:34 PM
"When an owner is unable to claim speed, handiness or beauty for his ship he calls her a magnificent sea-boat. Seaworthiness depends on the crew, gear, strength of hull and only slightly on its shape. A fast ship may appear a bad sea-baot when she is crashing into the waves and sending aft sheets of spray. Let her reduce sail and she may be as dry and comfortable as her less able sister."

RD Graham and JEH Tew, "A Manual for Small yachts", 1938.

Ed Harrow
09-15-2000, 11:58 AM
I've never tossed my cookies, but have been under the influence of diesel once (enough). This summer I was on a modern, trick, fast, 38' boat in Buzzards Bay with a SW chop. This boat had incredibly harsh, choppy, action. I assume this is the consequence of 7+ feet of keel that's only a few feet long, fore and aft. All in all an unpleasant day, tho I was still able to appreciate lunch! http://media4.hypernet.com/~dick/ubb/smile.gif

09-18-2000, 01:48 AM
Let me try and offer a serious answer to the issue of seakindliness. Peter Tangvald, of course, had a Harrison Butler designed boat of great loveliness and orthodoxy. The tome I quoted from above suggests that any boat larger than 5 tons (Thames measurement, so say 25ft LOA or so) should have some of her ballast inside to ease her motion. That is very old-fashioned thinking, and Harrison Butler himself would not have approved of it.

I think that compared to the yachtsmen of a century ago we have come to prize windward performance far more highly and are willing to make great sacrifices in comfort to get it. That is really the point that I was trying to make, earlier.

I do agree with Ian's point - a seakindly boat imposes much lower stresses on her structure in a seaway and once we get above the size of very small boats, this becomes very important.

Foster Price
09-18-2000, 06:05 AM
This is interesting because none of you have mentioned formulas such as waterplane loading etc that some designers/commentators resort to, and in the end neither (so it would appear) did Tangvald, Even though he must have been used to thinking in engineering terms.

Has anyone seen a published copy of the lines of Tanvald's boat (I only have the sketches in his autobiography)

Regards - Foster

Ian McColgin
09-18-2000, 09:02 AM
Formulae are useful in engineering and partial testing for a total boat, but do not create the boat, any more than various harmonics formulae can create good music, especially in all it's diversity, though some of the rules may well help a composer solve a knot in a new fugue or blues riff.

That said: and staying within at least a visual type, anyone seen a good side by side analysis of (or better yet, sailing) between a suitable Archer/Atkins type ketch as something a little finer looking like LFH's Diddiaki. I'd love to try a few hours of say Force 5 or so in each, with good shelter just an easy reach away . . .

Then we can go the other way. I felt the old Cal 40 to be both sea worthy and sea kindly for her intended use, but afer a night thrashing around in one in a good wind (40kt) and a juvenile sea (5' or so but only about 60' crest to crest so really noxious and steep) I'm not so sure that type is just the thing for, say, the Irish Sea.

One last thing - speed (within the limits of a generally wholesome hull) is clearly a very good thing. The dead round shape, like that misbegotten Czarist battleship experiment, is not an option, even though nothing more seaworthy than a bouy.

'Course, you can get the the near cousin of a bouy with modern trimaran design. The idea (and the interior is designed to facilitate this) is that should the boat flip, you flood the bow so she floats perpendicular. Wait out the storm in relative safety and when it's calmer start pumping the bow out with some inflation on the mast head so when the verticle does go to horizontal, it's the right way up.

S. Russell
09-19-2000, 12:44 PM
Ted Brewer developed a motion comfort ratio formula which I believe you can find in Danny Greene's "Cruising Sailboat Kenetics."
I've often wondered about this relationship of performace to seakindliness and how one would measure seakindliness. When describing the Joseph Conrad Villiers said,"She was extraodinarily handy but a bit small in a seaway." I suspect most of us small boat sailors would consider the Joseph Conrad a large boat with very easy motion so it might all be relative, I'm afraid.

09-22-2000, 06:52 PM
Yes Ted Brewer also wrote several handy light displacement paperbacks in which he developed his "comfort ratio" idea -- "Explaining Sailboat Design" is one of them I think. Our library has 3 of them. Don't remember the numbers but it worked when I was looking for a boat.

09-23-2000, 02:30 PM
The original thread here was entitled 'seakindliness,' if I'm not mistaken. Certainly this encompasses seaworthiness, but to my pointed head it is aimed directly at 'comfort.'

Dave Gerr, Ted Brewer and Weston Farmer all give the same formula in their writings, that a waterplane area in square feet divided into the displacement in pounds should equal 64, the weight of seawater. A higher number or lower number will produce a livelier motion or a slow sick-inducing heave.

Weston Farmer, at least, gives examples of well-known vessels with such a "comfort factor" number in his book From My Old Boat Shop.

If I have the formula backwards, forgive me; I'm no math type.

Ross M
03-10-2005, 08:22 AM
It's amazing what you can find down here. I was looking around last weekend and found this one, which struck me as a good candidate for the Designs/Plans index I one day hope to complete.


03-10-2005, 08:29 AM
My experience in big boats is limited, in kayaks the boats I've liked dissapear in some conditions, it's just water and motion, others remind you there's something in the water that needs to get moved or is being moved.

Ross M
03-10-2005, 09:29 AM

I once read that a really good bike is one that disappears beneath you. It becomes physically transparent, in effect.

Is this what you mean?


[ 03-10-2005, 10:35 AM: Message edited by: Ross M ]