J Boat Comparisions

My next project will be a 1:16 scale J Boat. Been a typical engineer we research the facts & figures before making informed decisions.
After reading a lot of books and web pages I am confused by the figures in the following table.
I thought that the power of a sailing craft can be formulated from SA/displ (a rough guide) I was amused to find the later built boats where down on this on this figure. Does this mean that they where slower???

Download Attachment: [ Comparision Table.doc](http://www.rcsailing.net/forum1/data/JohnB/20047192386_Comparision Table.doc)

So if I wanted a race winner I would build the Enterprise or Shamrock V.

What are your thoughts???

Tried to insert the table into the post, but it came out all mixed up.


While I am no expert on the J Class, there are a few things to keep in mind. The J class (like the 12 meter and IACC classes) are built to a rule. Under the rule, if you add sail area or decrease displacement (in order to increase the SA/D ratio) you need to change something else (probably waterline length). It is a tradeoff. Since these are heavy displacement boats, they will be limited in speed by their waterline length. These boats have long overhangs which increase the waterline length when heeled and pressed. These overhangs as well as other design facets of the boat evolved over the life of the class. So more than likely, the lower SA/Disp ratios used in later years were found to be faster because the waterline length could be increased with those boats and still meet the rule.

Endeavor II was considered by most experts to have been the fastest J. I’m not sure if when you scale the boat down, the same holds true. I know the effects of wetted surface area do not scale perfectly, so it may be the case that other J designs are faster models.

  • Will

Will Gorgen

According to Taglang/Chevalier’s book, Whirlwind comes out the best on VPP calculation. That being said, remember that an AMYA J Class boat has a modified rig compared to the full-size boat (single jib, lower mast). Also, the Whirlwind hulls that started the class are 1/20 rather than 1/16 (if I remember correctly) so you take an LWL penalty if you use one of them. Frankly, I think there are so many confounding factors that I would just pick a sentimental favorite and concentrate on tuning :slight_smile:



check your SAD formula (e.g. at www.sailingusa.info/cal__sad_ratio.htm). The displacement needs to be squared and then cube root taken to take the units to be the same as the sail area. The SADs were in the twenties, rather than fourties (it seems to me…)

Not sure that it changes the relationship of old Js vs new ones, but at least they seem closer…

Two cents worth-
I always thought that it was the sail area displacement ratio, for a given wind and sea condition. There’s art and there’s science


Good site, I will rework the figures.

FWIW I am tooling 4 different hulls at the moment for future supply to the J Class fleet, and I am in constant contact with the Class Commissioner John Hanks as we go through the process. The boats are a strict scale 1/16th and you are allowed to construct and use any sailplan that the original boat carried as well as the use of centerboards remotely controlled.

The Whirlwinds were “grandfathered” into the class since they were incorrectly manufactured, but were the original boats used.

We are prepping Endeavor II, Shamrock V, Ranger, and Rainbow for molds, with consideration to possibly adding Enterprise.
Rainbow has changed tracks to allow for an R/C operable centerboard since it appears to have the best use of the device of all the boats.

I am in the process of testing these 4 boats against each other since I was curious if they were going to show advantages over one another that would show one hull being significantly better and they are actually quite even. John has shown me several shortcuts that have all proved to be good in determining performance not the least of which is that the boats are usually overpowered and sail better with reduced sail area. As of this writing, Shamrock seems to have the edge, but further testing will undoubtably change that.

Stay right on that 3/4" to 1 foot scale so you will be class legal. We are quadruple checking all measurements because of the immense amount of work that is going into the plugs.

Good luck with it, it is a lot of work!

Here are the revised figures using the SAD formula (e.g. at www.sailingusa.info/cal__sad_ratio.htm).

Download Attachment: [ Revised SA-D ComparisionTable.txt](http://www.rcsailing.net/forum1/data/JohnB/20047205234_Revised SA-D ComparisionTable.txt)

What does it show?

That the Enterprise & Shamrock 5 are the higher performers, I think!!!


You must have a major workshop/boatyard!!!

Can you email at designtech@xtra.co.nz as I would like to discuss hull layouts extra.

Er, remember, if you are working off the published numbers, those are almost always the rated sail area, which is an artificial number based on mainsail luff and foot, and size of the fore triangle. Actual areas, particularly when running under the big “quad” jibs, were substantially larger.



Were’nt the “quad jibs” a measuring loop hole, as the jibs where measured by the hoist lenght and the perpendicular distance???

The sail area for each hull is specific and class regulated. You have to use the class #'s. The list is on the class rules, if you need it I can get it for you.

Unless you are in <7 kts you can’t be fast with that much sail area with the hulls as they are. (even with the 2" addition to the keel)

I am working with the genoas now, and getting better speed, the trick with be how quick in stays she has to be to compete. (ie… make up for time lost tacking)