Displacement Weight

New to this board and need some help.I am looking at two sailboats one being a 37' LOA with 11' 7" Beam and 21,000 lbs displacement and the next sailboat is a 40' LOA with 12' 2" beam and 16,890 lbs displacement. In some cases size does matter but I do not know how to compare the displacement weights.Would the 40' not do as well in rougher water, being the lighter??
Any words of wisdom would be appreciated.
Thanks,

 

A better indication of how well a yacht will do in heavy weather is the D/L (displacement length ratio) the higher the number the more comfortable the motion will be in a confused sea. There are a number of other things you can look at as well. Several attempts to make a "comfort" formula have been tried over the years and some work better then others.

The two boats you describe should look very different in hull form. The hull shape at a glance will give you a good idea of how she'll behave. Narrow entry angles, flat stern sections, low free board means a light air boat, usually with a lofty rig. Not a comfortable boat when the conditions go south. Bluff bows, fuller sections, heavier construction, beam drawn foreword and aft means a cruiser where windward performance isn't as important as making a good cup of coffee while eleven days from Lisbon.

 

Thanks for the input, PAR.Makes since.The 37' is an Endeavour and the 40' is an Irwin.
Thanks, again
Steve

 

"Displacement" doesn't actually mean a lot when comparing the work of different designers. Some use empty weight, others use empty weight plus a sandwich and a pair of shorts. Some use half load, others use fully loaded condition.

Strangely enough there are conventions for that sort of thing among NA's, but a lot of designers seem to consider themselves above all that, and consequently they tell people what they want to hear. It is the age of marketing, after all.

Ideally 'displacement' should be clarified. What you really want is empty weight (lightship) and fully loaded displacement. Then you have something to compare.

 

I agree, displacement on its own is not terribly helpful in determining whether a boat will "do well in rough water." As a starting point, Nigel Calder has a helpful article in the April issue of Sail Magazine - "Crunching the Numbers". He covers ballast ratio, displacement to length ratio, sail area to displacement ratio, length to beam ratio etc. in assessing a boat's characteristics/performance for cruising.
In addition, of course, as Par suggests, one must consider the hull shape. Robert Perry in 'Sailor's Secrets', International Marine,1999, for example, asserts that in his experience, rocker (the fore and aft curvature of the underbody) is more important than whether there are 'U' or 'V' sections in order to avoid pounding in heavy seas.
You must also consider keel shape, hull scantlings and construction details, the rig design and specifications, rudder design and construction, deck layout, interior layout etc., etc.
Finally, since all boat designs are compromises, there are no ultimate cruising boat designs. The issue is really, what compromises best suit your budget and intended purpose for the boat. For example, is a shoal draft critical for the areas you intend to sail? Are you planning ocean crossings in this boat, or coastal cruising with an occasional short open passage -e.g., the Caribbean. Will you be sailing in a cold or warm climate? Will you be sailing single or short-handed, or with plenty of crew? How important to you is windward performance? The list goes on and on.
I seem to recall that Robert Perry used to provide a service where for a reasonable fee, he would assess a boat's design for your purposes. I'm not sure if it was for any design, or merely his own. In any event, I'm sure that there are a number of naval architects that would undertake the exercize on your behalf. Considering the costs involved in the purchase of a boat, it would be cheap insurance towards getting a boat that you will be happy with.
Brad kelneck

 

Nothing "on it's own" Brad, will be helpful in determining how well a craft will do in a rough go of it. That said, a boat with a 500 D/L will be much more comfortable then a yacht of 200 in a confused conditions.

The subject would easily cover the contents of a book or two, and has. The original question was about two very different yachts. The displacements show just how different they are. The 37'er is near half as much larger then the 40'er with 10% less LOD. Clearly the shape of these two boats can give an experienced eye a clue as to how they be in a sloshing.

Contrary to what some would have you think, most reasonable designers, list loaded displacement (full tanks, crew, gear & lockers full with stores) manufactures of production boats list shipping weight for advertising and transportation issues.

Cruising yachts are the hardest design to fulfill, as it requires the design to meet the largest set of criteria asked by the client. The needs of one rather experienced couple for a cruising yacht, will be much different then another equally experienced cruising couple also looking for a similar boat.

Ultimately, the selection boils down to how much experience do you have? When looking for a yacht in this size range, you should be moving up or down from a previous boat where the qualities weren't quite what you wanted, but you have a real good idea what they are.

Personally, if it was me, I'd go for the 40'er, because it would go well in light air compared to the 37'er which may require much more air just to get way on. If I was heading, as most folks do, down the coast a bit then back, this would be a better boat for near shore conditions. If you plan to make major crossings or passages then the 37'er, being much thicker, may be the better pick as it has some mass and therefore some higher level of comfort in the trade winds.

 

My point, Par, was precisely that- no single factor, including displacement or displacement to length ratio will be 'terribly helpful' in answering the question. That was why I suggested the article by Nigel Calder as a good starting point. That's why I suggested that he may wish to consult with a naval architect.

If we are talking about the Endeavour 37 and the Irwin 40 of about 1982 vintage, the following additional dimensions should apply (from the 1982 'Sailboat & Equipment Directory') :Endeavour waterline 30', Irwin 31' 1"; Endeavour maximum beam 11'7, Irwin 12'2", Endeavour displacement 20,000 lbs., Irwin 16,890 lbs; Endeavour ballast 8000 lbs, Irwin 7000 lbs; draft Endeavour 4'6", Irwin 4'3".

In addition, the Endeavour narrowed considerably toward the stern while the Irwin carried more beam, much further aft. The Endeavour had considerably more rocker than the Irwin and wider side decks. Both boats, I believe, had long fin keels and spade rudders (the line drawings don't show their underbodies, so I'm going from memory and comparison to other boats from their respective manufacturers at the time). Which boat would be better in heavy seas (or 'rougher water', as was the original question)? Ignoring construction details, the rig, weather/lee helm, interior lay-out ,etc., it should be the Endeavour and not merely because of the increased displacement. The narrower beam to waterline length,narrower ends, increased rocker, increased draft to waterline length and yes, increased displacement should all make the boat more comfortable in heavy seas, especially to windward.

We should also bear in mind that the displacement figures are for an unladen boat. The weight of crew, safety equipment, water, diesel, food and clothing will add considerably to the actual displacement. What is significant is that the additional weight should be the same for the same couple taking the same trip on either boat. Adding the same weight to two boats will typically have a greater negative effect on the performance of the lighter displacement boat - in all winds and sea states. Advantage again, Endeavour.

Finally, we have to be careful in assuming that the Irwin will be a better light air boat. I don't have the relative sail areas for the two boats (using sloop rigs in both and a 100% foretriangle). We also don't know the WS (wetted surface area) figure for either boat, although we can safely assume that it is considerably less on the Endeavour based upon their respective beams, waterlines and hull shapes. The point is, of course, that SA/WS is a far more accurate predicter of light air performance and, as Nigel Calder points out in the article I referred to, "surprisingly, heavier boats with relatively low SA/D ratios sometimes have relatively high SA/WS ratios and, as a result, can have surprisingly good performance in light air." Further, the heavier displacement boat with a relatively low WS has an even greater advantage sailing in light air with a left over chop - once they get going they have momentum to keep plowing through waves. Finally, sometimes the better light air boat is so because it is over-canvassed for all but the lightest conditions. Having to reef down in anything over 10 knots may not be the best situation for a relatively novice sailor.

Where we are in complete agreement, Par, is with respect to the relevance of the type of sailing Sea Wings ultimately wants (and can realistically expect) to do. It may be that neither of these boats will suit his wants or needs.

Brad Kelneck

 

As stated, there have been a few attempts at a comfort formula, none really work well across the board, but a guide. I meant no offense Brad, just that this is such a difficult topic to address, without a much better understanding of what to look at, when looking at and into boats.

I have to admit I was guessing at what these two boats may be, I'm acquainted with both designs of various vintages. Both are fine little yachts and my interest in the 40'er but a wish to skid down some rollers running an inlet. Just the boat for coastal work, the majority of cruisers are (yes, another assumption) not working the trades for weeks on end.

The numbers can be quite deceiving, as many a design has proven. The best and surest way to judge a design is to sail her in a combination of conditions (not always possible) and get a feel for how she does, talk to owners and skippers of the same design. They'll all be biased in some way, but you'll get a feel. Other then that, lots of study and sea time . . .

Good Luck,

 

Thanks Par. No offence taken by me and I'm sure none by Seawings. Your suggestion that he speak to owners of existing boats and go out for a test sail is absolutley right on. Lets face it, the numbers can lie - and at best, they tell only a part of the story. Hopefully we've given Seawings some things to ponder. And good luck to him.

Brad Kelneck

 

I recently read an assertion that comfort at sea was best predicted by comparing displacement with the area of the waterline plane. Essentially, you were comparing bouyancy with weight to get an idea of heave. For a given weight, less area in the waterline plane produces a more comfortable (slower) heave. Can't remember if it was in Dave Gerr's book or another. A recent design of mine was predicted to be quite comfortable, because despite its light displacement, its low waterline plane area made its heave slow.

 

Neither boat has a particularly comfortable motion. My mother owned an Endeavour 37 and it had one of the most uncomfortable motions of any boat that I have ever sailed in my life and neither are particularly good heavy weather boats.

Jeff

 

See: http://www.endeavourowners.com/boats/e37/e37sloop.html

Which boat do you like better? You can't make all your decisions on the numbers.

I don't really know a lot about the Endeavour Yachts, but as i recall they started by buying an Irwin design and building the boat with better quality and upscale features. Irwin was always a mass market and charter boat company, as far as i know.

 

I don't especially like either boat. You are partially correct that Endeavour got started by buying a set of obsolete Irwin molds for a 32 footer. The Endeavour was very inferior in quality. The Endeavour version eliminated the centerboard and went from a high density ballast to lower density ballast. I once was in a marina were there was an Endeavour 32 next to the original Irwin 32. We spent one evening comparing the two. The Irwin was generally an upgrade in almost everything that we could see.

Jeff