design for light airs

Not only is she a joy to sail, she's easy on the eyes. Nice.

 

I would like you to clarify your criteria to compare things, as I do not follow you. Which are the parameters you fix and which don't?
Also I would like to know the anser to my second question in the same post, if you're so kind.

 

Question:

Should I worry about the raised the center of gravity because the cabin has been converted from wood to steel?? And, I'm not sure whether the change in spars from wood to alloy compounds the issue, or alleviates it. Thoughts? TIA.

I'm trying to get the plate thickness, but I doubt that information is available. The owner who made the cabin and spar changes has passed away.

BTY, I REALLY APPRECIATE the help I've been getting in this thread. I'm new here, but am finding this place to be a great help.

p.s. I found out this boat has always been a sloop. Chainplates for a yawl were added just for grins. I think the stay sail arrangement was also added later. In other words, she was and is a sloop, with a forestay added when the new mast and boom went on.

 

What is her SA/D? TIA.

 

Is it a big cabin, or just the original design made in steel? Anyway it is a heavy boat. I don't think it will make a big difference, unless it is a monstrous thing.

I have run your data through a small sailboat analysis program and I will post the results.

The sail area you have given me is HUDGE. Are you sure? You say that the working sail area is 519 sq ft and to that I should add the staysail. In a cutter rig you go to windward with the three sails, so you should add that one, but I believe you are mistaken in the sail areas.

This boat, with 519 sq ft, was a fast boat in its time and even today it is not a slow boat, but with the staysail, it would be a fast boat even for today’s standards. I mean a fast boat limited by its short LWL. If that sail area is correct, this is a boat that with 10 K winds will do probably 5 to 5.5 knots to 45º of true wind. But with 15k winds will not make a lot more, probably more half a knot or so.

See if those numbers are correct and then give me the data of your previous boat. Then you will be able to compare both boats. It is not perfect, but it will help.

 

Vega:

The cabin looks quite natural. Just found out that the plate thickness is .096", which is about 2.5 mm.

If you are having a hard time with the numbers, I think I know why. Another steel boat I reviewed had a more modern design by Koopman, again built in the Netherlands. It's keel was longer and had less depth. Displacement was 24,000 lbs for a 34' boat. There are many 39' steel Folkes in the area. I forget the displacement of these boats, but I think it's similar.

6 tons for seems light for steel. If you increase displacement, the sail area looks more inline. I don't think this is a 6 ton boat. That's almost in the range of plastic boats for this size.

My previous boat:
32'10" LOA
27' 3" LWL
11' beam
6' draft
4800 lbs ballast
11,000 lbs displacement
678 sail area per one website I found, but I do remember that the SA/D was 16.6.

Fin keel; skeg-hung rudder.

* * *

The warts on this Buchanan boat are starting to come out. Fuel tankage is 140 gallons (wow) but water is only 13 gallons. Prior owner meant to put in a watermaker. (Not my style.)

Worse: headroom, V-berth and setee length are too short for my heigth. I think the ergonomics kill this boat for me, unless I rip out and the redo the interior. Sigh. Maybe I should look at the Koopman again. Of course it's 2.5 times the price.

 

The other thing I forgot to add, is that the current owner (he's owned this boat twice) says that when the new spars went in, they were the same length as the original spars, but rigging and spar section was increased to increase rig security.

I wonder if he's wrong; that perhaps the mast height was increased.

Still, I think the better explanation is that we are working with a displacement that is too light for reality.

 

Just to compare old and new boats:
Fisrt, let's compare boats with similar overall length and recognized as reasonably good performers in their time.
(Note: Stability data estimated, not from designers/manufactures)

LUDERS 50
LH = 14,93 m
Lwl = 9,15 m
Bmax = 2,75 m
Draught = 2,13 m
Disp = 10000 kg
Ballast = 5450 kg
Sail area = 87 m2
Power = 27 HP

Displacement /Length Ratio D/L = 364,1
Sail Area /Disp. Ratio SA/D = 19,05
Power / Disp. Ratio 6*HP/D = 7,34
Hull speed HSPD = 7,34 Kn
Potential Maximum Speed PMS = 8,25 Kn
Velocity Ratio VR = 1,12
Comfort Safety Factor CSF = 1,29
Motion Comfort Ratio MCR = 50,61
Screening Stability Value SSV = 19,6
Angle of Vanishing Stability AVS = 151,67 º
Roll Period T = 5,62 Sec
Roll Acceleration Acc = 0,02 G's
Stability Index SI = 2,04

NEW BENETEAU 50
Lh = 15,09 m
Lwl = 13,3 m
Bmax = 4,49 m
Draught = 2,05 m
Disp = 15520 kg (1586 kg load)
Ballast = 2900 kg
Sail area = 105 m2
Power = 100 HP

Displacement /Length Ratio D/L = 184
Sail Area /Disp. Ratio SA/D = 17,15
Power / Disp. Ratio 6*HP/D = 17,52
Hull speed HSPD = 8,85 Kn
Potential Maximum Speed PMS = 9,49 Kn
Velocity Ratio VR = 1,07
Comfort Safety Factor CSF = 1,82
Motion Comfort Ratio MCR = 32,14
Screening Stability Value SSV = 180,62
Angle of Vanishing Stability AVS = 112,34 º
Roll Period T = 3,28 Sec
Roll Acceleration Acc = 0,12 G's
Stability Index SI = 0,73

Comparative: LUDERS 50 - BENETEAU 50.
Lwl ------------ 9,15 ------ 13,3
Displ. --------- 10000 ---- 15520
Sail Area ------- 87 ------- 105
SA/D --------- 19,05 ----- 17,15

Luder 50 probably is a better performancer in light airs than the Beneteau 50, basically due to its bigger SA/D ratio, not its shorter Lwl. Probably Benetau 50 has also bigger wetted surface, but I do not have this data for either boats.

Although Luder's HSPD is 7,34 kn against Benmeteau's 8,85 kn, the Luger will develope a higher one when underway because of the longer overhangs, probably coming close to Beneteau's.

Next thing, I'll try to find two boats with the same Lwl and afterwards two with the same Displacement, and make comparisons. But not today.

If somebody wants to compare another pair of boats, please tell me where I can find the basic data, and I'll gladly do it.
Cheers.

 

Vega-
Thank you for the above. A very nice concise description of the boilerplate argument of light vs heavy displacement sailboats. I would suggest however that it treats the SA/D ratio as the single defining parameter in defining speed through the water at velocities less than hull speed. The SA/D of any sailboat represents how much force is available to act upon the resistance of the wetted area of that design. To my mind however it fails to fully explore the relative force required to overcome this resistance when the SAME wetted area is expressed over the highly variable hulls forms under discussion. To wit: is the wide shallow hull as easy to drive as one which is narrow and deep? How does the refinement of a good entry and lines in the hull carried aft effect resistance? Degree of longitudinal hull symmetry? Slack bilges vs full?
Can a long deep CCA racer be directly compared to say a Westsail 32 of the same displacement? If we decide that they carry the same sail (though the Westsail has greater form stability), will they both achieve the same velocity at 7 knots of wind and therefore be equally efficient?

I would argue that as hull design varies from purely ballast to purely form dependent stability, that the drive from the force acting to overcome the resistance of the wetted area varies in a manner which is not simply that of f=ma. I understand that for this discussion that the boats have been simply compared by same length over the range of heavy to light. All of the above must be applied to vessels of the same displacement and differing hull forms.
Furthermore that the design may violate same simple parameters which are required of sailboat in use... Perfectly stable upside down anyone?

To bring this back to the thread:
"Do some of the older designs offer advantages for cruising boats in light airs"
I would suggest that we explore the relative efficiencies of hull forms and the overlain degree to which these forms are in keeping with seaworthiness as it relates to this question of light air performance.

I would present my Rhodes as an example of a "traditional" from which is in fact good in light air by effectively translating the available power of the sails into speed. She is 41' and 20,000 lbs yet ghosts along well in light going. Need I say it.... Love is blind

 

Guillermo:

Any chance the 10,000 displacement for the Luders is a typographical error?


Thanks for that very apt comparison. The difference in motion and stability statistics is amazing.

 

Hiracer:

Yes, you got that weight wrong, but also the ballast.

The most known boat of Alan Buchanan was the fiberglass Halcyon. The prototype was the first Ellen Mac Arthur’s boat, the one that made her fall in love with sailing. She cruised extensively around the British Isles with that boat. In her book 'Taking on the World' Ellen describes this boat ( Cabaret) as having 'a big heart'.
http://www.boatsandoutboards.co.uk/view/F40251/


The only steel boat from Buchanan that I could find was a 31ft with an 8.9ft beam and a displacement of 9T.

http://nl.apolloduck.com/display.phtml?aid=33873

This nice 37ft wooden one (with a LWL of 26ft) weighs 8T.

http://yachtworld.com/core/listing/...rency=USD&access=Public&listing_id=29424&url=


So I would say that your boat would probably displace between 10 and 11T.

But what you really should do is email Mr. Alan Buchanan (yes, he is still alive and active) and ask him . See the email here:

http://www.yachtworld.com/core/byp/...y=295&subcategory=150&startPagination=0&10000


I will consider 11 T, estimating a ballast of 5 T and 10 T with a ballast of 4.5 T.

The graphs seem alright now, especially the one considering 10 T. It shows what is expected: not much speed, but a lot of motion comfort and seaworthiness.

I will also post graphs of your previous boat (Wauquiez 32ft?) and of a modern light big production 36 ft cruiser, a Bavaria 36. Finally I will post a comparison between the three boats (considering the Buchanan D = 10 T)

I guess that you are going to understand better what I have been saying

 

Vega:

I tried to email Mr. Buchanan but the website taking the email (his email address not being provided) would not take my email. I will have to write him a letter.

As to displacement, I have suspected from day one that the displacement was not 6 tons, despite the owner claiming to have the design plans showing 6 tons. And the vessel is documented at 10 tons net, 12 tons gross.

But I must confess I've never seen a steel hull sailboat under 40 feet with a ballast-displacement ratio approaching 40%. They are almost always around 30% and sometimes even lower. The reason, of course, if the mind-blowing length-to-displacement ratios. Designers tend to short-change the ballast with small steel hulls to reduce sail area and total displacement.

Thus, if a I may without being rude, I'm not confident about your ballast figures. I suspect that actual ballast is quite a bit lower. If you check other similiarly sized steel sailboats, you will see a pattern of light ballast relative to total displacement. Assuming displacement is about 20,000 lbs., I would hazard a guess that ballast is in the neighborhood of 6,500 lbs, at best.

One of the objections I've read about steel-hull sailboats under 40 feet is that they may not, in fact, be good ocean-crossing vessel because they habitually do not have enough ballast. Their range of positive stability is not up to the task, sort of the dirty little secret of small steel sailboats.

In any case, I agree that this Buchanan vessel will probably not perform well in light airs because the SA/D is probably not up to the task--narrow hull form notwithstanding. This boat would be interesting to me only on the condition it's displacement is closer to 6 tons than 10 tons, given the amount of sail area.

Still, the point that bntii makes about hull form being underweighted in any discussion about light airs is valid, in my opinion. For example, none of the dimensional or numerical analysis that your have provided makes any allowances for hull form. This is not to disparage. It just an observation of a perceived blind spot that I am beginning to think may be industry wide.

What condemns these older designs is the low SA/D ratio. Yes, lack of form stability is the issue, but surely better compomises are available than the current obsessions with ultra wide boats we see so much of today.

Your comparisons are illuminating on how much modern design has sacrificed in the single-minded pursuit of speed and interior room. I'll never look at our PHRF numbers the same.

 

Bntii,

First of all let me say that I love sailboats and particularly beautiful sail boats and for me, the most beautiful are the classical and the traditional ones.

Each kind of boat has its qualities and the good old ones are a lot more comfortable in a seaway and as you said have a lot better inverted stability.

But regarding speed, especially light winds and flat seas, light boats are a lot faster. They can also plan (with enough wind) and are even faster downwind. As I have said, the only situation when a deep keel heavy cruising sailboat can be faster than a light beamy boat is against to windward in a lot of wind and troubled seas.

That’s why the boats have handicaps and heavy traditional ones have a lot of “compensated time”. Racing in real time, they don’t have the slightest chance.

Is this important? Not for me. Sailboats are slow and what counts is the pleasure they give, the way they sail. That’s why some of the fastest ocean racing sailors have traditional classic sailboats as their personal boats.

If I could I would not resist switch any boat I could have for a 12 William Fyfe yacht, never minding the practicability, speed or even maintenance costs.

As you have said, love, beauty and pleasure is everything (or almost), in what concerns sailing.

But not in what regards speed in light winds. Take a look at the speed of this graph regarding the speed of a light modern 40ft cruiser with 8K winds and compare it with the speed you know your 41ft boat makes with the same wind and don’t let the love blind you

See what I mean? And this is a mass produced cruiser boat and this graph doesn’t come from an advertisement but from a test sail made by one of the best European sailing magazines.

But your boat is a lot more beautiful

About hull speed and light winds, as you can see the 40ft is approaching hull speed with light winds.
With less wind than this a cruiser will be motorsailing.

A boat with a longer waterline can reach 3/4 of hull speed quite effortlessly, but after that a lot more power will be required. For a 40ft boat that speed is 6.4 and for a 50ft it will be 7.1. Both boats will be able to reach this speed (3/4 of hull speed) with 10k of wind.

So, even with light winds (10k for a cruiser) a 50ft can reach 7.1K but a 40 ft will only reach 6.4 . That’s why I have said that for a cruiser hull speed and LWL are always important.

This was what I have tried to say in the post 27 when I explained that I was talking about cruising boats, not racing boats.

Racing sailing boats are the only ones for whom any wind counts, even those winds that would make any cruiser go motoring.

 

You probably are not familiar with yacht design in the sixties and before. Since the 70’s, boats have begun to get beamier and beamier, relying more and more in form stability and this has permitted boats to have a lot less ballast, for the same initial stability.

The effectiveness and importance of hull form in the boat initial stability is so big that in the 2004 Vendée Globe, Mike Golding, having lost the keel and ballast, was able to go on sailing for more 50 miles, and that has not prevented him from being 3rd overall (the boat’s Architect told him that he could still sail if he didn’t go over 30º heel, using also some water ballast).

Some photos of the boat sailing with keel, without keel and the missing keel

If you look at older boat plans, you are going to find that I am not far away. Look for instance at this Koopmans Sr. boat plan. It is from the late 70’s (I think), not as narrow as the boat we are talking about and the proportion is already 40%.

 

Well, I've worked with available numbers, but it seems to be correct to me. Luders designed several racing (and cruising) boats, with SA/D ratios in the 19+ zone and very long overhangs. So numbers I worked with seem to make sense.
Interesting to realize that the LUDERS 50 (A cruising yawl) having a D/L of 364.1 (so a heavy weighter by today standards), has a SA/D ratio of 19.05 so a big sail area for the displacement. BENETEAU 50, being heavier (15.5T against 10T) has a D/L of 184 and a SA/D of 17.15
The LUGER 50 probably outperforms the BENETEAU 50 in all rhumbs in light winds (No spinnakers, of course). In somewhat higer winds probably beats it also when going to winward.
If wind and waves increase, other factors come into consideration, as total mass, longitudinal inertia, exposed hull lateral area, forward volumes and the like. I have not enough info to risk an opinion on this.
For sure the LUGER 50 has a clear disavantage when in opened rhumbs and stronger winds, when the flatter aft sections of the BENETEAU 50 create dynamic lift.