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Installing a Kill Switch for Emergency Engine Shutdown

DUCKWORKS, an online magazine for amateur boat builders, has an excellent, well-illustrated article about installing an engine kill switch on older outboards.  The article, by Max Wawrzniak, is available online at http://www.duckworksmagazine.com/05/columns/max/3/free.cfm.  The article also appears in Max’s book CHEAP OUTDOORS, The Beginners Guide to Making an Old Motor Run Forever, available from Breakaway Books.

A set of instructions for installing your own kill switch directly on OMC engines is available at the link below to view and/or print.

Kill Switch Installation

Racing with a 2- or 3-Blade Propeller

Several members have purchased 6- and 8-hp engines with stock 3-blade propellers rather than the standard 2-blade propeller. Inevitably, questions were raised about which propeller is faster and should both 2- and 3-blade propellers be allowed racing?

This past April the Technical Review Committee tested both propellers on two different boats, each with a 1973 vintage 6-hp Evinrude Fisherman engine. The manufacturer spec for these engines is 6-hp at 4500 rpm with a full throttle operating range of 4000 to 5000 rpm. Both engines topped out at 5200 rpm with the 2-blade propeller. The 3-blade propeller dropped the rpm by 800 to 4400 rpm but did not increase the top speed by more than 2%. Because of the higher rpm the 2-blade was faster out of the hole and had a broader mid-range power band but a 2% lower top speed. This does not seem to be an issue for course racing.

Down river racing is probably more influenced by driver weight where ballast is not carried. The 3-blade propeller may have slightly less tendency to capitate in the corners. Data aside, when we drag raced 3 boats side-by-side, #3, #9 and #10 seemed to be dead even (#3 with a 3-blade prop, #9 and #10 with 2-blades). Since the motors are all used, reconditioned or rebuilt, there will always be some variables in the motors. The end result is that either 2- or 3-blade propellers may be used depending on driver preference.

Boat #         2-Blade Propeller              3-Blade Propeller

3                17.7 MPH @5200 RPM       18.1 MPH @4400 RPM

10              17.7 MPH @5200 RPM       18.1 MPH @4400 RPM

Frame Alignment

The Cocktail Class Runabout is designed to have a straight run fore and aft from frame 4 to the transom. Any fore and aft curvature in this section the hull will result in pounding and hobby horsing at high speeds and reduce overall top speed. It is necessary to check this alignment at the chines and keel cut out prior to installation of the keel and sides. This photo and the drawing below illustrate the process of checking the alignment at frames 4, 5 and the transom

Note also in DWG 14 in the Building Manual that the keel is made from 1″ x 2″ clear fir or equivalent but the keel cutouts are only 7/8″ deep. This is to allow enough material for shaping the keel as it transitions from a flat bottom at the transom to a very shallow V at frame 5 and a sharp V at frame 1 and the stem block. When the keel is installed, the bottom of the keel should be approximately 1/8″ higher than the bottom edge of the transom. The excess material is then planed to shape; flat at the transom and into a V to conform to the shape at each frame. DO NOT pull the keel down level with the transom as this will create a fore and aft curve. Rather, plane it to shape so it is 7/8″ thick and flat at the transom and approximately 7/8″ at each frame with the appropriate V shape.

Frame Alignment Drawing

Stem Block - Keel Joints

Making the stem block/keel joint is one of the few difficult evolutions in constructing the Cocktail Class Runabout. The initial series of boats used a beveled butt joint held in place with a Spanish windlass until the epoxy cured. Cutting the front end of the keel to the proper bevel and length was problematic.  The use of a jig and frame kit eliminates the need of the Spanish windlass but still requires a beveled butt joint.

An alternate approach is to rabbit the stem block to accept the keel and then trim the keel flush with the front of the stem block. The depth of the rabbit in the stem block should be determined after the keel has been cut into three 1/4″ strips from frame 2 to the stem block. (Cutting the keel into three strips allows it to bend to the curve of the hull without breaking.) The keel is then epoxied in place and temporarily secured with a screw. When the epoxy has cured remove the screw, trim the keel flush with the iron of the stem block and seal the end grain of the keel with epoxy. These photos illustrate the various techniques for making the stem block/keel joint.

Making and Installing a Steering System

An inexpensive but reliable steering system may be made for the Cocktail Class Runabout using components purchased from your local hardware store. A detailed drawing and parts list for the steering hub is included in the Building Manual in DWG #25. The 10″ Go Kart steering wheel may be purchased from a local supplier or ordered online for approximately $20. The cheek block pulleys for the steering line are Harken Micro Cheek Blocks and are available from most marine supply stores such as West Marine. The steering line is 4mm and should have very little stretch. For tension control on the steering line use springs or shock cord as shown in the photos below.

Note: It is very important that there be sufficient tension in the steering system to allow positive and responsive control of the engine. A slack steering system will allow the engine to wobble left and right at high speed which may result in loss of steering control and the boat flipping over!

Approved Keel Modification

In 2015 the CCWBRA approved a keel modification which decreases the ventilation of the lower unit particularly when cornering.  The modification involves removing a portion of the keel at the end and sharpening the keel trailing edge.  While the modification is not required, all boat are encouraged to have this modification applied to race because not only does it improve your boats performance but it allows more predictable behavior when cornering and therefore promotes safety on the race course.

The keel modification consists of the removal of the last 12” and the end is to be tapered in plan view only.  Shown is a method of making the taper by first cutting a straight cut diagonally in plan view.  Then a smooth radius is added to either side.  This is shown only to suggest a method of construction.  See images below.

keel mod 1

 

keel mod 2