An IACC-Inspired Student Boat

I don’t know whether to call this a very simple model yacht or a very complicated toy boat :slight_smile: but in any case here’s the first cut at a student free-sailing model that is intended to be relatively inexpensive and quick to build. Most student programs in the past have emphasized building over sailing, whereas I wanted to do just the opposite: get the boats on the water as quickly as possible and spend the bulk of the program teaching the physics of sailing and the art of tuning and trimming. Free sailing boats are best for this because there’s really nothing to it except tuning and trimming. The boat sails to windward on balance alone and to leeward using a simple sheet-to-tiller gear called a “Marblehead Gear” in the US and a “Clyde Gear” in the UK. Spinnakers will be used to provide the “thrills and spills” factor.

I chose to mimic the general appearance of an IACC boat for a modern look. This choice, and the constraints that the boat was to be constructed by first-time builders posed many problems whose solution may be of interest. By component they were:

  1. Hull. The hull form had to be balanced so the boat would track in uneven winds (see the Hull Design thread for more than you ever wanted to know about hull balance :-)) The hull form had to be forgiving in terms of weight and location of CG so that the students were not faced with a set of constraints beyond their ability to meet. The weight tolerance was solved by placing the bilge curve above the LWL, so that buoyancy increases sharply as the hull is pushed down into the water. This also helps prevent submarining on the run, and gives reserve for RC gear if so desired. The CG tolerance is provided by the overhangs. The hull is 30 inches long. It was designed using Freeship (now Delftship) with the balancing done by hand using the Turner metacentric moments method. The plug was CNC milled by Phil’s Foils and the hulls made by Nigel Heron, who has the mold if anyone else would like one.

  2. Ballast. This was the toughest nut to crack, as there is simply no way a program such as the one I envision could expose young people to lead. The prototype boat uses steel shot “potted” in resin and molded in an RTV rubber mold. This has a density about 5/6 that of solid lead. I also intend now to experiment with bulbs made from model rocket nose cones loaded with lead shot, as a chap in the “other” forum described. The boat in the pictures carries 30 oz of ballast on a 44 oz displacement boat, and appears adequately stiff. She’s a quick little thing, getting up to hull speed in a real hurry.

  3. Rig. With all due respect to the folks at Thunder Tiger, I think an IACC model with a mast crane and a backstay just plain looks wrong. The function of a backstay is threefold: a) put a controlled bend in the mast to set the sail; b) take the force of the sail on the run; c) tighten the jib stay so the luff of the jib doesn’t sag. In this boat a) and b) are taken care of by sweeping the lower spreader back 15 deg. When the diamond stays are tensioned they curve the mast, and having them behind the mast gives plenty of fore and aft stiffness. The jib stay problem remained unsolved until I saw Graham Pugwash’s footy rig with the rigid stay and had one of those forehead-slapping moments :slight_smile: [ aside: Rigid jib stays were used by UK Marblehead class designers in the 1960’s, and on L. Francis Herreshoff’s famous R class boat “Live Yankee,” which was so fast, and so expensive, it killed the class.] The sails are shaped with battens of spring stainless wire. The square top is held up by a triangular batten tied to the mast at its top and bottom. Rig tension is by DuBro tensioners and nylon thumbscrews, to eliminate the cost and effort associated with turnbuckles.

The pictures show the first prototype which was slapped together at a dead run to check out basic shapes and locations. Next iteration will involve a jib radial to keep the jib foot parallel to the deck. The aspect ratio of the main will be increased by increasing the luff 2" and cutting an inch or so off the chord at the head. The rudder will be made deeper, and a proper fin installed. I may also do a RC version.



hey earl
that is a great idea. the students can learn a bit. AND have their own yacth? is there any room in the future that a student can put a radio in? it does not have to be in any class. but they could enjoy sailing with the rest of us
well done old chap

I didn’t know that you were going to make the hulls available to people through me. The mold was a flash, and intended a quick run. I’ll have to make a proper production mold for it.
Any suggestions for a retail price?

Hi Earl

Rigid jib stays still all the rage for UK vane sailing in the “A” class:

Best of luck with the project!

Second sail in light air with the new jib. The jib radial is a DuBro 2137 ball link drilled out to take a carbon rod; the 2-56 transverse screw clamps it to the desired vertical angle. Works very well.

The Mark II rig. Mainsail head attachment changed to get rid of the excessive twist, higher aspect ratio, a little more sail area. Picture taken in 6-8 kts. Lack of a backstay doesn’t seem to be a bother.



Keep the pictures coming Earl. The more I see the boat, the more attractive it becomes. I’m the stereotype of a 10-thumbed builder but its making me think of trying once again to make a decent hull:blush: