The K-5m is a miniature version of the Nick Potter and L. Francis Herreshoff designed full displacement double-enders of the 1920's and 1930's that were rated under the Universal and International rules. The idea behind this design is to have a boat that is small enough to single hand easily, but still has a big boat heavy displacement (lead mine) feel. The boats produced under the Universal and International rules were very similar in proportions as discussed further below in more detail. The K-5m rates just less than 4.6 meters under a proposed 4.6 meter class rating rule which has been added to the Naval Architect's Notebook page.

A web page with photographs of the Potter and LFH double enders is here. The weight estimate, scantling calculations, and stability report for this design can also be downloaded from the Naval Architect's Notebook page. Construction of hull number 1 has just started, details are on the Building the "Pintail" page.

The K-5m is similar in size to a fleet of Finnish 5 meter yachts, the last of which were built in the late 1940's, but heavier and with more sail area that is consistent with the big double enders designed by Potter and LFH.

Finnish 5 meter "Diana" #29 1948___________K-5m

LOA______________8.54 m_______________8.50 m
LWL______________5.83 m_______________5.81 m
Beam_____________1.82 m_______________1.86 m
Sail Area__________21.27 m^2____________25.40 m^2
Displacement______1.55 tonne____________2.18 tonne

The boats built to the Universal rule (R boats, M boats, J boats, etc) and the International rule (6 meters, 8 meters, 10 meters, 12 meters) were similar in proportions and hull form. Here's why ...

Minimum displacement without penalty:

Universal___________________International
(0.2*LWL+0.15)^3 m^3________(0.2*LWL+0.15)^3 m^3

The minimum displaced volume without penalty for both the Universal and International rules is the sum of 20% of the waterline length plus 150 mm cubed. There is no credit for greater displacement. The result is the above formula represented a target displacement to optimize the rating under both the Universal and International rules. Designers could reduce displacement without penalty by shortening the waterline length under both rules.

Maximum Draft:

Universal___________________International
0.16*LWL+0.53 m___________0.16*LWL+0.50 m

The maximum draft limit is virtually identical under both rules. There is no advantage, rating or otherwise, to reduce draft, so boats were routinely designed to the maximum draft limit under both the Universal and International rules.

Freeboard:

Universal___________________International
0.06*LWL+0.18 m___________0.08*Rating+0.25 m
minimum at 1/2 LWL_________maximum freeboard deduction

A six meter with a waterline length of 7.2 m might have a midship freeboard of around 660 mm. The minimum freeboard allowed under the Universal rule for the same boat is 612 mm. The "ideal" freeboard for a Universal boat is a bit less than for an International boat.

Beam:

Universal___________________International
No restrictions______________about 0.295*Rating+0.06 m
__________________________minimum allowed

Although there are no restrictions on beam in the Universal rule, narrow boats are encouraged to minimize the measured "quarter beam length" which is "measured in a line parallel with the middle fore and aft vertical plane, at a distance from it equal to one-quarter of the load waterline breadth and one-tenth of this breadth above the load waterline plane". The minimum beam allowed under the International rule is about as narrow as is practicable for form stability, so the end result is the maximum beams of Universal and International boats were similar, that is, -- narrow.

The K-5m has a deck stepped mast with swept back spreaders, no running backstays, and no jumper struts. Rather than the full keel typical of the Potter and LFH boats, the K-5m has a 1950's style fin keel spade rudder arrangement which is relatively simple to build and should work just as well to windward and steer better than the old boats.

There are a couple of minimally sized pipe cots below for a nap or overnights onboard.

Construction is all Douglas fir with Okoume marine plywood bulkheads and deck. The hull is strip planked. Frames, keelson and keel floors are laminated. The whole structure is built using the Wood Epoxy Saturation Technique (WEST). Building this particular design from anything but wood just wouldn't be right.