design for light airs

I understand Sharpii2 to be saying an OPEN 50 would beat a 12 METER in smooth seas, but that the 12 METER would win in waves. Light winds in both cases.

http://www.oceanracing.org/boat.html#specs
vs.
http://www.12metre.com/yachts_canada2.htm

Do I have that about right?

 

Bob, would you please clarify your formula for me? (Just adding all parenthesis and */ symbols where necessary)
I asume units are imperial. Thanks in advance.

 

No.

That's a pretty big over generalisation. The Open 50 would have such a vastly greater S/D that it would easily overwelm what ever momentum advantage the 12meter might have.

The 12 may out point the 50, but the 50 would out foot it by such a degree that, even with many more tacks, the 50 could easily be the victor, even dead up wind in a lumpy sea.

In those conditons, I'd rather be the 12, though.

I heard of one 'Open' skipper comparing going up wind in rough seas in his boat to "...driving a truck with square wheels..."

Bob

 

Yes, I agree 100%.

 

Sorry about that. Guillermo. I'll do my best.

The units do not have to be imperial. They can be. inches, feet, centimeters, meters, or even cubits. The only thing to remember is that the volume MUST be in cubic values of what ever unit you're measuring it by. If, for example, you are making a small model. And you prefer to measure the model in centimeters, then the volume MUST be measured in CUBIC CENTIMETERS.

To keep things simple. let me define what I mean by 'Beam'.
1.)take the waterline Beam and divide it by eight.
2.)Measure another Beam that distance up, as if you were immersing the hull that much deeper.
3.)Add the two values together, then divide by two.

This is your value for 'Beam'.

Now for the formula:

Heft Ratio = (20 vol.)/Length*('Beam')^2

Length, by the way, is the actual length of the hull. Waves can't tell 'sailing
length' from waterline length.

If you think my definiton of 'Beam' is odd, look at a banks dory some day, with its greatly flairing sides and its narrow bottom. It is a much better rowboat than a sharpie with vertical sides and the same beam.

I hope this clarifies things a bit.

Bob

 

Thanks, Bob.

Having worked a little bit on it, I find it maybe more practical (not more accurate, absolutely!) for my postings in these forums' purposes, to use Bwl instead of the (Bwl + Bwl1/8)/2 you propose, because I find it may also provide a good clue to the boat's behaviour. Not so good as taking in account the flare of the topsides, as your formula does, but easier to quickly have a first glance based on the more commonly published data. I'll try and see what we get.

Using this simplification, we may do another theoretical two-boats comparison (With more or less the same LOA):

LUDERS 36 ( Data from here)

Lh = 10,83 m
Lwl = 7,62 m
Bmax = 3,13 m
Bwl = 2,82 m (as 0.9*Bmax)
Draught = 1,6 m
HD = 0,5 m (guessed)
Disp = 6800 kg
Ballast = 2380 kg
Sail area = 62 m2
Power = 23 HP

Ballast/Disp Ratio Wb/Disp = 0,35
Displacement/Length Ratio D/L = 428,67
Sail Area/Disp. Ratio SA/D = 17,55
Power/ Disp. Ratio HP/D = 1,53 HP/tonne
Hull speed HSPD = 6,7 Kn
Potential Maximum Speed PMS = 7,41 Kn
Velocity Ratio VR = 1,11
Comfort Safety Factor CSF = 1,67
Motion Comfort Ratio MCR = 36,72
Screening Stability Value SSV = 47,97
Angle of Vanishing Stability AVS = 120,54 º

Roll Period T = 3,62 Sec
Roll Acceleration Acc = 0,06 G's
Stability Index SI = 1,16

Righting Arm 20º RA20 = 0,13 m
Righting Arm 30º RA30 = 0,18 m
Righting Arm 10º RA10 = 0,07 m
Initial Metacentric height GMo = 0,42 m

Heft Ratio (simplified) HF = 1,54 (Using just Bwl)
Roll Period/Beam Ratio T/Bcorr = 1,16 (Comfort: 1 – 1,1)


BAVARIA 36 (Data from here)

Lh = 9,91 m
Lwl = 9,4 m
Bmax = 3,61 m
Bwl = 3,25 m (as 0.9*Bmax)
Draught = 1,55 m
HD = 0,4 m (guessed)
Disp = 5485 kg
Ballast = 1059 kg
Sail area = 55,6 m2
Power = 29 HP

Ballast/Disp Ratio Wb/Disp = 0,19
Displacement/Length Ratio D/L = 184,19
Sail Area /Disp. Ratio SA/D = 18,17
Power/ Disp. Ratio HP/D = 2,4 HP/tonne
Hull speed HSPD = 7,44 Kn
Potential Maximum Speed PMS = 8,37 Kn
Velocity Ratio VR = 1,12
Comfort Safety Factor CSF = 2,07
Motion Comfort Ratio MCR = 22
Screening Stability Value SSV = 145,47
Angle of Vanishing Stability AVS = 112,95 º

Roll Period T = 2,18 Sec
Roll Acceleration Acc = 0,2 G's
Stability Index SI = 0,6

Righting Arm 20º RA20 = 0,49 m
Righting Arm 30º RA30 = 0,66 m
Righting Arm 10º RA10 = 0,27 m
Initial Metacentric height GMo = 1,55 m

Heft Ratio (simplified) HF = 1,02 (Using just Bwl)
Roll Period/Beam Ratio T/Bcorr = 0,61 (Comfort: 1 – 1,1)

Values of Roll Period/Beam ratio against boat behaviour:
(Just to compare against Heft Ratio)
Source: Dave Gerr

T/B
0.5 Uncomfortable quick roll
1.0 Comfortable. Low end of desired range
1.1 Comfortable. High end of desired range
>1.1 Uncomfortable slow roll. Possible unstability

Measures for I,J,P,E were taken from here
If better data are available, I'd appreciate the info.

 

Heft Ratio formula redux

Guillermo:

I forgot to mention that when I use this formula, I usually count the max Beam as 'Beam'. The Bwl and the Beam 1/8th Bwl above Bwl Beam rule was really meant for boats that have a very unusual amount of midsection flair. Such as dories.

I wonder if there is a large co relationship between my Heft Ratio and the 'comfort ratio'?

Bob

 

Thanks, Bob. I'll use Bmax, from now on.

Motion Comfort Ratio formula is (Imperial units):
MCR = DISP / (0.65*BEAM^4/3 *(0.7*Lwl + 0.3*Lh))

Heft Ratio is:
HF = 20*VOL/(Lh*B^2)

Now:
VOL = Disp/64 (Sea water, Imperial units)
(0.7Lwl+0.3Lh) is a length factor, not very different from Lh for boats with today's relatively shorts overhangs. So for this kind of boats we could asume it as Lh making little mistake.

We can then write equations as:
MCR = DISP / (0.65*Lh*B^4/3)
HF = (20/64)*DISP/(Lh*B^2)

Now, dividing:
MCR/HF = (3.2/0.65)*(B^2/B^4/3) = 4.92*B^2/3

So, if I didn't make a mistake, relation between your Heft Ratio and MCR is more or less:

MCR = 5*HF*B^2/3 (Boats with short overhangs)

So, there is not a linear relation among MCR and HF, but anyhow it depends basically on Beam, so I think your formula could be used also to assess Comfort. I like your HF more than MCR because it doesn't depend on units. That's very useful. I will find out how both parameters compare for several boats. Have you done this already?

Just to check it now:
For BAVARIA 36 we have:
5*HF*B^2/3 = 21,53 and MCR = 22, so quite close.

But for LUDERS 36 (Long overhangs):
5*HF*B^2/3 = 29,54 and MCR = 36,72, so not working so well, as expected.

Cheers.

 

Guillermo, I don’t know where you have got the sail area for the Luders from; certainly not from where you say you have taken the information because they don’t give any sail area.

That sail area for the Luders 36 doesn’t make any sense. You can see that the Bavaria 36 has an initial stability (Rightening moment at 10º,20º,30º) around three times bigger than the one for the Luders. That’s a huge difference and that means that the Bavaria is capable of carrying much more sail than the Luders.

About the Bavaria sail area those numbers are not correct.

That boat (in the test) is equipped with a furling mainsail (an option) and furling mainsails are smaller, but even so the correct area is 63 m2. The standard boat (without a furling mainsail) had 67m2.

The weight of the boat that you give for the Bavaria is a Half-load weight. The lightweight is 4,700kg.
(All data taken from Bavaria Catalogue)

We don’t know if the weight of the Luders is a lightweight or an half load displacement weight.

About the AVS, you say that the Bavaria 36 has an AVS of 113º when the real AVS is of 131º. That’s a huge difference. That is a very good AVS for this type of boat. The average in this type of boat is between 120 and 125º.

I have already said to you that the program that you use doesn’t seem to be very reliable, at least in what concerns the AVS. You have posted once an AVS of 359º, and for the “Far Harbor 39”( Container Yacht) that program has given you an AVS of 121 when Robert Perry has calculated an AVS of 147º for that boat.

I think that your program only calculates (with some imprecision) the hull AVS, not taking into account the role that the superstructure has in the AVS. You can see by the stability curve for the Bavaria 36, that the boat has a global AVS of 131º, but if you only consider the hull, it has only 118º.
That explains the big difference in the Harbor 39. That boat is a Pilot House, and the big superstructure has a big positive influence in getting a better AVS.

About that comparison, the numbers that matter, regarding very light wind sailing are the weight, that translates in a bigger wetted surface, the kind of keel ( a long keel, versus a fin keel) and the capacity to carry sail, that is given by the RM numbers for the initial stability (10º,20º,30º).

With RM numbers around 3 times bigger, much lighter and with a remarkably smaller wetted area, the Bavaria 36 would very easily outperform the Luders 36 in light winds.

And by the way, the Bavaria 36 also had an option for a lead keel with 1.95 draft. That’s the one I have in my boat.

 

Thank you for the numbers on the Crealock 34 and J-Boat 32.

OK, the Buchanan has slid by, and now the focus is on another steel sailboat. Owner claims that displacement is 24,000 pounds for 34' LOA. This on a 1985 design and build. I know of 39 footers with less displacement.

I can't find another similar steel boat with this displacement. I'm finding displacement for this size steel sailboat of this era to be in the 15,500 to 17,000 pound range. I think the owner is wrong. I am going to see if I can get the design number and contact the designer directly.

Anybody have an opinion of the typical displacement of a 34' steel sailboat form the mid-1980s, i.e., normal proportions (11' 3" beam; 29' LWL; 6'3" draft)?

What is it with sailboat displacement? Seems I'm getting questionable information consistently from different owners. Geeze. When I had boat, I knew everything about it.

 

Well, I did some searching and found a sister ship, one foot longer, centercockpit, BY THE SAME DESIGNER, and displacement is 18,742 pounds.

No way this boat is 24,000 pounds. I'm guessing it's 17,500.

 

If you want a relatively light metal boat, why not an aluminium boat?

Sometimes you can find some old ones that are not very expensive.

Take a look at this one, it has a displacement of 14,000 pounds, it costs US$39,500 and it seems OK. It will not win races, but I like it.
Probably it is not very expensive because it has only a 10hp engine and I think that you can bring this price down.

http://yachtworld.com/core/listing/...rrency=USD&access=Public&listing_id=2585&url=

 

http://www.yachtworld.com/core/listing/advancedSearch.jsp might be helpful to you in your research.

It's not inconceivable that 17,500 is lightship or design displacemant and 24,000 is full load displacement [all tanks full, provisions, crew & passengers on board, dinghy & equipment on deck, anchors & chain, winches & windlass, air condisioning, genset, electronics (nav/radar/stereo), a little extra ballast thrown in.... these things add up!]

Any way to determine if she floats on her designed waterline?

 

Heft ratio corelates to comfort ratio

Guillermo:

Just got done doing a little math experimen to see if the Motion Comfort ratio related at all to th "Heft Ratio" I came up with years ago.

Here is the experiment:

Take a single design and scale it up by 20% each time, four times and see if the two ratios have anything in common. I did this using a spread sheet that I wrote for this purpose. Below are four derivitives of the same boat, starting from smallest to largest.

Name----Lola 20-----Lola 24-----Lola 29-----Lola 35
Length---20ft--------24ft--------28.8ft------34.4ft
Lwl ft----18.5ft-------21ft-------25.1ft-------30ft
Beam-----5ft----------6ft---------7.2ft--------8.6ft
Disp lbs---2800 lbs----4880 lbs----8465 lbs---14426 lbs

MCR------30.7----------35.9-------41.3--------46.5
H.R.-------1.78----------1.77--------1.8---------1.8

It took me a while to notice the pattern. (I Am not all that bright, you see) But what I am seing is that this particular boat will consistently feel like being on a boat roughly half again its size, even as it's scaled up. At least in the smaller size ranges. I have noticed that the difference tapers off a bit as the boat gets larger. 46.5 ft is less than one and a half times 34.4 ft. Just as 30.7 ft is more than than one and a half times 20 ft.

It is interesting to note that, although the co relationship is not perfect, it is probably close enough to be useful.

Bob

PS- for anybody who is interested in my "Heft Ratio". It is explained in some of my earlier posts on this thread. My value for 'Beam' is usually taken as the max beam of the hull. The exception is for hulls that have straight(ish) flairing sides that are quite high. 'Length' is generally taken as the length of the hull except where the hull has high over hangs such as a clipper bow and high counter stern. In that case, The Lwl is much more relevent.

 

I would consider aluminum. But I don't find any in my area. That's a good price, less that what I'm considering now, but it's 3000 miles away as the crow flies.

I suspect that aluminum sailboats are in demand here, so they don't come on the market often, sale quick when they do, and at a premium price. Steel is the opposite. I know, I know. There are reasons for this disparity.