Catspaw Dinghy
The picture below is a 'Plank on Frame' version of the Catspaw Dinghy as designed by Joel White’s interpretation of Nat Herreshoff’s classic Columbia Dinghy.
This boat is a real classic design. I guess you could even say that 'They do not build them like this anymore'.
The rowing version weighs about 150 pounds while the sailing version weighs about 200 pounds considering the addition of the Centerboard case, Centerboard, Mast boom, Sails, and Rudder.
Plans are still available for this 'Plank on Frame' constructed Catspaw Dinghy.
The Aluminum
Catspaw Dinghy
This aluminum version of the Catspaw Dinghy is an interpretation of Joel White’s interpretation of Nat Herreshoff’s classic Columbia Dinghy.
I chose this version of the Catspaw-Dinghy for redesign in aluminum construction because it cover all the aspects of 'Bezier Design and Construction'. It is ideal for a novice Builder's to get their 'Feet wet' before taking on a larger Bezier Design. At the end of this build you will have something to be proud of along with homing your sheet-metal working skills before taking on a larger 'Bezier Design and Constructed Hull'.
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Conversely, this would be a good design for an Established Boatbuilding company who would like to possibly expand their product line. Invest a little money and get the feel for building True Round hulls. You can even Look at building the aluminum Catspaw Dinghy as a fun project!
About the Redesign:
Designing a true round small boat such as the Catspaw Dinghy for aluminum construction required a lot of 'Thinking outside of the Box'.
Creating a computer of the originial 'Plank on Frame' hull form was fairly straight forward. I did, however add a little more tumblehone, just my preference, at the stern. I also corrected what look to me as a bit of negative surface curvature near the bow. Besides that the hull is a good interpertation of the originial design.
Now that I have a Computer model that looks the same with the static hydrostatics as the orginial design a structure for aluminum need to be calculated resulting in the same Static Hydrostatics.
I am expecting that the base rowing aluminum version of the Catspaw-Dinghy to come in at 175 pounds while the Sailing version at about 225 pound. A bit more than the 'Plank on Frame' version. A good indication that the aluminum redesign wouldl have the same rowing and sailing feel as the orginial.
Accepting that all the above Design work is true. the boat must still be buildable using todays design and construction.
As a Reader, you must have a lot of question about the Design and Construction methods.
Let me ask the first question:
Since this boat is constructed of aluminum, will it not sink to the bottom like a rock if it capsizes and is swamped leaving the crew to the 'Grace of God'.
Floatation:
Being constructed of aluminum the boat has no possibility of floating in a swamped condition, therefore the aluminum version of the Catspaw Dinghy will require flotation, which is neatly placed under the rowing stations
According to my calculations the minimum floatation required, in cubic feet, to support a swamped Catspaw Dinghy and 300 pounds of crew is 2.95 cubic feet.
The floatation designed into the hull under the seats is 4.140 cubic feet and notably 1.190 cubic feet above the minimum floatation required by Law.
The image below shows those void spaces.
The following are other Design and Construction in no particular order. If you do not see your concern here, Text me at 617-653-1063 or email thesecondwind@outlook.com
Framing system:
The aluminum version of the Catspaw Dinghy will have no Transverse frames - Only Longitudinal frames opposite the 'Plank on Frame' version which has no Longitudinal Frames.
The below rendering shows five (5) longitudinal frames, (magenta), between the sheer longitudinal and Center Vertical Keel - CVK.
The longitudinal are temporally supported by nine (9) 1/2" plywood, (white), temporary Hull Formers until the shell plating . In essence the longitudinal frames support the shell plating and the shell plating supports the longitudinal frames.
This is the exact opposite of the 'Plank on Frame' version where the transverse frames support the hull planking where the hull planking supports the transverse frames.
Hull Former
The below image represents a typical Hull Former cut from 1/2" dimensionally stable plywood.
The former is show in section view attached to the Strongback's vertical framing structure.
The slot indicate the location of the Longitudinal frames. Full-size patterns, in PDF file format, are standard in the Catspaw Dinghy's buildingto the Plans
The below image is a detail view of the Longitudinal slots.
- Note that the depth of longitudinal slot is no deeper that the depth of the longitudinal itself -1.250" depth x 0.250" in thickness.
- Because the longitudinal frames the only point of reference you will have to align the shell plating, they must be held securely held in place.
- To secure the longitudinal to the former a small angle clip (1.250" x 1.250" x 0.125") is screw to the former in line with the slot .
- As per 'Bezier Design and Construction' the longitudinal's will be marked where it crosses every Former. After you are satified with the fit a every crossing between the Formers and longitudinal clamp the angle clip to the longitunidinal, then tack weld the clip where shown in the above drawing. Than remove the clamp.
StrongBack
The Strongback shown below in plan and profile views is constructed from 3.500" x 1.500 and 5.500" x1.500" common lumber. Other materials can be used if you like as long as you keep the distance's between Formers as shown.
Remember that the accuracy of the Build all starts here! Everything that follows depends on the accuracy of the Strongback.
Shell Plating:
Finial Building Plans for the Catspaw-Dinghy are still on the Drafting Board
(Coming Soon)
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Updated - June 22 2025
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