Are we making any real design improvements??

.....

 

Now, Wardi, if you are going to include appendages like foils under "design improvements", then I guess the use of carbon spars and better materials is fair game, right? ...

Steve

 

Yes absolutely, I certainly have not ruled out any of these at all.

What I am saying though, is that it is the lowering of the Centre of Gravity of the rig and reduction in pitching moment which gives the efficiency improvement, not the weight saving.

 

Yah, but.... A hull is no more "efficient" with a lower CG and pitching moment than the same hull with higher values - it just is able to sail faster.

Sound familiar?

 

.....

 

Here is the problem I see with using the perameters you list. Something like a Melges 24 will look to be very inefficient because of its light weight and large sail area. To make a boat look efficient, using the perameters in the way you are evaluating them, all you need is very little sail area, a lot of ballast and a narrow beam. The sail area need to be minimized since it is treated in a linear nature. Displacement is compenasted for so that there is no advantage to making a boat light so there is no reason not to have a deep heavy keel. Now that you have much more stability than needed you can narrow the beam to reduce wetted surface area and wave drag. Now this configuration will likely be reasonably fast up wind but it will be a stone on any other point of sail. Take a look at the boats that are very efficient in your data and see if they follow these trends. I bet most of the efficient boats tend to be heavy and have little sail area and the least efficient tend to be light with lots of sail area.

 

Wardi,
I'm getting confuddled here (not a new thing, to be sure ) You have since the beginning of this thread been talking about hte efficiency of the hull, and yet you are willing to consider that foils are an allowable way to improve this. Why not just stick a honking big engine on the back? It would have as much bearing on hull efficiency.

Steve "not QUITE joking...."

 

.....

 

Sorry for the confusion, this was just an example of the fact that not everything in yacht design costs money. It is a reference to another thred here on foiler design...nothing to do with this thread on yacht design.

 

I'm sorry to sound like a broken record, but I still feel that what could be interesting with any comparision like this is WHAT and WHY certain design differences causes a specific result. And to be able to get that info without having to resort to CFD.

The comparision is indeed interesting, but I do wonder about it's usefulness...
What, if anything, can actually be conluded from it, I wonder?
---That we all like different kinds of boats perhaps?---

 

.....

 

First of all, I still say that there is a problem with your analysis if you think that a the speed of a 1960. 1970's or 1980's boat would appear to be faster if compared to a more modern design filtered either by real sailing displacement or by actual sail area. To understand the reasons that modern boats are faster would require a more sophisticated analysis than you can do off of the limited data that is readily published since the speed gains have come from a few big advances plus a whole host of pretty subtle changes.

When I read a statement like that I really have to question how much you have really looked at the current performance yacht design process, at least as it is practiced by the better design offices. Heeled resistance has been an important area of design for at least a decade now. With computer modeling and better and fairly accurate VPP programs, one of the big performance gains is in the area of heeled performance. (Pre-internal ballast) IMS type forms from the better design offices tend to be remarkably balanced even at pretty high angles of heel and rudder angles tend to be smaller and with lighter loadings. The levels of sail trim required to keep them at speed and balanced is quite small compared to the magnatude of trimming that older designs required to stay at speed. Modern designs seem to hold their speed longer in a changing condition and gain back speed losses much quicker as well.

Beyond that, when you say things like "as it is easier to stack crew on the gunwhale.........than address this difficult issue.", all I can say is, 'you obviously have a very short memory'. I have been racing sailboats for roughly 40 years now. Over this period, crews have gotten smaller and lighter. From the beginning of my sailing carreer these crews have sat out on the rail (or hiked tooth and toenail on smaller keel boats). These days a 40 footer is sailed with a crew of the size and weight that we might have used on a 33 to 35 footer in the 1970's and 1980's. Except at the most extreme grand prix levels, crew weight is not the determinant that it once was.

When you look at the best of the (pre-internal ballast) IMS type forms, these are boats that benefit from all kinds of subtle messaging of their hull forms, buoyancy and weight distribution, foils, rig proportions, hardware and deck layouts, sail designs etc. While some of these may be slightly adjusted to accommodate percieved loopholes in the rule, at least until recently, these improvements have produced steadily faster boats that are much easier to handle no matter how they are measured.

Respectfully,
Jeff

 

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Wardi,

I don't want to criticize your work because it is very extensive. However, in your last post, you said that no one yet thought the work was unsound.

I think that using Taylor's series as a reference for yachts is very misleading. First, have you seen what a Taylor series ship looks like? Compared to a yacht-type hull, this is like towing a brick through the water. I can tow a series of different disp/length Canines through the water (Chihuahua all the way to St. Bernard), but scientifically, I shouldn't be allowed to extrapolate this data to sailboats.

The big problem relates to wave-making resistance. First, a ship will have a radically different wave-making pattern than a yacht. Secondly, especially at high froude-numbers, a highly-efficient modern sailboat will rise-up on a plane.

A Taylor series ship could NEVER rise on a plane. In the same manner, a Melges 24 normally sails on a plane. This is due to difference in trimming moments, heave, LCB, and LCF, which are mostly combined in the distribution in the sectional area curve.

A ship has one function of wave-making resistance versus Froude number. A full keel, older, yacht will have something that looks a lot different. A modern dinghy-type yacht will be different than the older yacht.

This is why your prediction thinks that older style yachts fair better than newer yachts. Most simply explained, it can't predict the performance enhancements of things such as planing.

What I would say is: if you want a baseline comparison for a sailboat, the first order of business is to pick a model that looks something like a sailboat.

I'm sorry if I may have criticized your work too much. It still is a good starting points, but I don't think you can use Taylor's series to compare different types of sailboats.

-Jon

 

Anyone tried http://www.sailrater.com/? I would assume the formula is similar to Americap, which is probably based on both the IMS VPP and PHRF results.

(See also http://www.boatdesign.net/forums/showthread.php?t=3741)