the resistance of tall ships

[liviper]

Actually,I want to talk about tallships like this ,lagre steel sailing ships , I don't kwon if I got the meaning of the word tall ship accurately.And i think they don't need a keel.

[liviper]

[quote=daiquiri;288754]I can confirm that the link above is broken. I also get the warning that the pdf file is damaged.



You are confusing the therm "keel" with the therm "fin keel".

Thank you for correcting me.

[liviper]

Originally Posted by ancient kayaker

Googling your question "predicting the speed of sailing yacht" found lots of references including this one: http://www.gidb.itu.edu.tr/staff/ins...ions/Cesme.PDF

The keel behavior is affected by the presence of hull surface at one end and the effect of heeling on the performance of the keel would be significant, however a Google search should reveal useful references for allowing for these factors.

Although it might be relatively simple to get a reasonable approximation for a "modern" (last 100 years) yacht design with its separated keel and hull, for a tall ship with a long keel blended into the hull shape perhaps the only way to get all the factors taken into account would be to make a model and perform tank testing.

Thaks for your particular expatiation,what I got is also about how to compute resistance separately for the hull.But what about the resistance of the hull,when it get a heel and leeway angle.Is there some information about this.
Do these resistance make any sense.

[ancient kayaker]

Originally Posted by liviper

... I can donw load the article http://www.oossanen.nl/download/pete...ing_yachts.pdf but I can't open it ...

I emailed Perry van Oosanen and he has replaced the damaged pdf file; it can now be opened. It seems rather large!

[ancient kayaker]

Liviper: the article in Perry van Ossanen’s site appears to address modern yacht design rather than the classic tall ship design that you are interested in, although I haven't had time to examine it in detail yet.

There is a sentence addressing the reduced drag due to the reduction of wetted surface that occurs when a modern sailing hull is heeled. This is known to happen on some beamy flat-bottomed sailing skiffs, but may not be so for a relatively narrow "tall-ship" hull even if it is more or less flat-bottomed with no external keel. In addition the extra drag due to turbulent flow transversely across the hull is likely to be a major factor in drag resulting from heeling on a reach (across the wind).

[liviper]

[quote=ancient kayaker;292720]Liviper: the article in Perry van Ossanen’s site appears to address modern yacht design rather than the classic tall ship design that you are interested in, although I haven't had time to examine it in detail yet.

Thanks kayaker,i received their email to.
But i am not read it yet.I kown it is difficult to compute the two resistance,and so far the simpleness and maybe the best way to get it is regression analysis from the experiment data,but it has somany limitations.I just try to collect information, and to find a feasible way to solve this problem,if it is possible.

[fralkolynn]

Originally Posted by liviper

Actually,I want to talk about tallships like this ,lagre steel sailing ships , I don't kwon if I got the meaning of the word tall ship accurately.And i think they don't need a keel.

I presume that by tall ships, you mean the multi-masted sqare-rigged beauties of previous centuries.

Their stability was due to lots of weight low down in the hull. If they had no cargo, they would ballast the hull with rocks or whatever heavy stuff was available.

Due to the nature of water, this stabilty is almost impossible to re-create in miniature. Where the full-size ship would have a draught of ten to twenty feet, a model only draws a few inches. A huge amount of lateral resistance - due to the compression of water at depth - is thus lost.

It seems that the only way to keep a model tall ship upright is by adding a detachable false keel to lower the centre of gravity, thus reducing the heeling motion.

[ancient kayaker]

It's true that the righting moment of a scale model varies as the 4th power of the scale factor whereas the heeling moment of the sails varies as the 3rd power. Because of this a scale model of a large sailing ship needs a subsantially lighter breeze and an extended, weighted fin keel to sail well, or a much reduced sail area. Large sailing ships like windjammers got their stability from their sheer size by virtue of the scaling factor above.

The hull shapes of the large steel windjammers are not so much different from a modern tanker or cargo vessel, so I imagine the resistance calculation of a computer hull design application would yield adequate results.

As to what can or cannot be ignored, because of their stability and the multi-masted configuration - resulting in a relatively low center of effort - a large sailing ship does not heel anything like as much as a small sailing boat. I suspect that for most practical heeling angles its effect on resistance can be ignored.

Whether the wave-making and form drag are negligible is beyond my expertise. I have found that, for the tiny canoes I design and build, skin resistance predominates until speed reaches a significant fraction of hull speed. If I scale such a hull the curves look much the same.

A large sailing ship such as the Preussen shown in a previous post, 400 ft, maximum speed 20 k, spends most of its time below 50% of hull speed. If the resistance formulae apply in that size range then the skin resistance at typical speeds under sail amount to well over 80% of total resistance. But this could be BS.