IOM design about to come off the drawing board

Just thought I would share this picture. New design IOM currently undergoing CFD (computational fluid dynamics) analysis on a computer. The boat is an original design developped from a combination of learnings from other classes, input from a few IOM sailors and a rather in depth discussion of math and physics with Hal Potts. This screen capture is taken at a rather obscure angle such that people can get an idea of the shape, but not give too much away . . . yet.

The line that looks like a waterline at the bow, and follows through to the stern is NOT actually the waterline, it is the connecting line that controls the blend between the different sections used. Changing the position of this line can dramatically change the shape of the hull. The actual waterline is where one would expect it to be even though it is not currently shown.

I will be tweaking the lines just a little bit more, especially in the bow sections before making the plug. Always interested in input, especially from people like Roy who have an impressive amount of IOM experience!

As an aside. I found it interesting as I was playing with these lines, that the IOM rule does a VERY good job of keeping boats fairly similar based on a very simple rule. at 8.8 lbs of displacement, and limiting the maximum rocker to 60 mm, all the shapes that can be developped fit into a relatively narrow window. A very neat rule! Maybe I will switch classes afterall!


[edited immediately after initial post for typos and to clairify waterline]

Glad to have a potential convert. I’ve felt the class rules yielded a relatively competitive fleet. Do a search for the Bagheera (a new swedish IOM design) it looks vaguely similar.



what program are you using for your CFD calculations? I have always wanted to get my hands on a copy of SPLASH to test some design ideas of mine.


just for info


if it isn’t broken, don’t fix it!

The IOM Rule is deceptive in its simplicity. A lot of designers find that it’s not that hard to come up with a good boat for the class, but very difficult to take the next step and develop a consistent winner. If nothing else the rule rewards attention to detail and the ability to obtimize all of the elements of the hull/sails/addendage package.

The design pictured here looks to be in the ballpark of competitive boats, but it’s hard to tell without a few more views and accurate dimensions. Next problem is to figure out the shape and placement of appendages.

Access to CFD analysis for model boats has been hard to come by up to this point; would be very interested in the results and what boats (if any) were used for comparision purposes.

Thankyou all for the replies and pieces of insight.

Roy is right on all counts. In order to be competative in the shooting match in this class, all aspects of the boat have to be right on! I find it very interesting that some boats in the hands of the designer look very fast, but dont quite show the same potential in the hands of another builder/sailor! I very much take to heart your “deceptive in its simplicity” comment as this is some of the feed back I have gotten from many IOM skippers. Being VERY new to the class, this is something I will have to learn about first hand over the next couple years.

As for Computational Fluid Dynamics, several very good questions were raised, one of which is my largest stumbling block! I can analyze a hull shape in the water, at any degree of heel, or with the bow pressed etc etc, and I can develop numbers of all kinds to “rate” a performance, however, without a known boat to calibrate the numbers against, they are relatively meaningless. My options along these lines are to either

  1. compare them to some of my existing USOM designs that are on the water
  2. make a computer model of an exisiting IOM with a good track record and use that as a bench mark
  3. take a guess on the first design, build it, sail it, and then use it as a bench mark for iterative improvements.

Each of these aproaches can be good and bad. Benchmakring against a USOM I can start doing right away, but the boats are almost too different. . .apples and oranges. I REFUSE to model someone elses boat as I dont want to ever be accused of knocking of someone elses design. Just by putting one in the computer, I might be influenced by something I measure or see, and I dont want that to happen. Im not trying to reinvent the wheel or anything, but I dont want to plagerize. As for waiting for a year. . . I guess I will do that. Calibrate the model and iterate.

As for my source for CFD resources. . . I have to respectfully keep that under my hat. I apologize for this, but by disclosing my resource, I might loose access to it. I hope you all understand. There are however some relatively inexpensive CFD add on programs for Solid Works and a couple other solid modeling packages.

The one area of learning that I am trying to improve as we speak is the ability to model surface waves. I feel as this can have a substantial impact on hull design and is possibly neglected in some peoples CFD analysis. Currently, I have a way to get partial results on wave action that when looking at mock ups of known full size boats seem to be giving reasonable perdictions as to a boats efficency and behavior in waves (comparing say a farr 40 to a C&C 40 in 1 foot chop).

Once again, thank you very much for the posative input!

-Todd Brown