HERE WE HAVE Sure Mike II, ideally calculated to take the largest stock outboard motor, the Mercury Mark 55. This boat is 21 feet long with an 8-foot beam. She probably comes closer to the size of small cruiser desired by most men than any other boat of this type we have run in the Boatbuilding Annuals.
She is carefully sized, having been swimming around on my sketch pad for a couple of years. As to her running qualities, little more can be asked from a boat this size.
I've approached the problem of construction from all angles and have decided on a seam-batten job, with planking specified in the usual manner. If it is desired, the topside strakes may be cut from 3/8-inch 5-ply Super Harbord marine grade plywood. I should plank her bottom in regular seam-batten fashion, using mahogany or white cedar planking of the thickness specified.
I decided on this type of construction on the basis of (1) my own experience handling large plywood panels in a home shop, and (2) after much talk with men who have tried to plank a hull this size in "one hunk." The condensed gist of all thought runs like this:
The average man isn't equipped to loft a generated surface hull this size. He hasn't the tools or know-how to get a fair job. A hull of smaller pieces —that is, straked planking—will build easier in this size boat for the average amateur. It will take longer, but the result will be stronger, fairer and somewhat heavier. This latter feature is an advantage in water that dances a bit.
Of course, any boatbuilder who is mechanic enough can build this boat, planking her of plywood throughout. It is well here to mention to the novice that when a large panel of plywood, such as a topside, is both swept and twisted, the cross sections that result are not straight-lined sections.
They have curve, arc, belly to them. Hence the framing of an accurate
frame for a boat this size becomes a job for & skilled loftsman.
The straight-framed job or a boat with a majority of straight frames, can be easily planked seam-batten style in strakes. Be warned that no design with a preponderance of straight frames can be planked with plywood. What you'll get is a series of pillows between frames.
Sure Mike II can be inboard powered. The Gray Model 620 would make an ideal installation. The Universal 60 hp Unimite would be another good choice of engine. A Chris Craft B, 60 hp, 133 cubic-inch motor would also turn in an extremely good performance. Red Wing also has a 60 hp 133 cubic-inch model that has well-engineered features.
The insert elevation section shows enough of the inboard motor version to enable you to install the job. The skeg and rudder on the lines drawing give you your dimensions for this option. The Gray 620 will want a 12-inch diameter by 10-inch pitch three-blade wheel. The 133 cubic-inch engines, commonly built around the Hercules block, will swing an 11-inch diameter by 9-inch pitch three-blade wheel. The Universal Unimite will want a 13-inch diameter by 8-inch pitch three-blade wheel. Differences in torque characteristics and piston displacement are the reasons.
Speeds of around 20-22 miles an hour can be reached if your boat is in dry condition with a good uncluttered bottom. Under the same conditions about the same speed can be had with the Mercury Mark 55—perhaps a shade better as the weight/hp ratio favors this big kicker.
Though Sure Mike II is 21 feet overall, the added length is but a bit more than previous designs published in the Boatbuilding Annual. She is just three feet longer than
Simplex, published in the 1954 Annual, and but four feet longer than Sundance, which appeared in our 1953 Annual. Both these latter boats were out boarders, and had sufficient accommodation to allow them to be classified as "cruisers" under the modern definition of the term. The slight added length and beam of Sure Mike II, however, makes her much larger than the boats her forerunners' were. This is because lengths vary per the square; bulk varies per the cube.
Hence, Sure Mike II, while no leviathan, is very much more of a ship than the outboard cruisers of the finger-bowl-washroom and candle-
Weight and length do much the same thing for a boat that weight and wheel base do for a car. Where comfort and adequacy are musts there is no substitute for either.
I believe that by extending your purse and muscle a little to get this boat instead of something less roomy, you'll cure at the start the usual disease that hits home builders—"biggeritis."
Old-timers in the boating game can tell you that you will become infected with "biggeritis" during the first month you're afloat in your first boat. She'll loom like a circus tent as you first begin to plank her, this first boat of yours, and she'll shrink to a pumpkin seed as you begin to use her. Why this is true is beyond me—I've been a midwife to boating appetites for only 40 years and have never learned the cause. I only know that it is true.
But I doubt that in settling on Sure Mike II you'll suffer much from this
The information published here is sufficient to enable anybody to build her. Her lines must be laid down full-sized on a smooth floor. And an ideal floor can be made from three panels of 1/4 inch by 4 foot by 8 foot common grade plywood.
Old-timers at the boatbuilding trade do not need to be told that this full-sized lay-down and fairing up is a must. Some beginning boatbuilders, however, think a boat should be built like a piece of machinery or a chair, from a multitude of small drawings of detailed parts. A boat cannot be built this way. She must be watertight, so you lay her down to get the hull shape, the keel shape and the shape of the knees and their bevels; also at the same time fairing up obvious errors in scaling the original small (3/4 inch to the foot) drawings.
This gives you the shape of the hull to the outside of the skin. This outside shape is the important thing.
All pieces inside the vessel are then fitted to the frames and bulkheads, using dimensions from centerline out as given for the accommodation plan, and heights from the waterline for inboard fixture and joinery heights. All boats are built this way if a fair and easy building job is to be had.
The frames are to be diminished by the thickness of the planking, and are to have extensions which will meet your building floor.
These are erected upside down over the centerline and station line grid marked out on your building floor. The frames in this condition must be created plumb, and horned square with good bracing.
I should treat the transom cheeks as a frame in itself, making due allowance for the fact that the projected view on the body plan of the lines is not the true shape of the frame: it must be "expanded." That means to take your half breadths for this frame along the rake of the transom face.
This means that the transom cheeks form a separate frame, raked. The 3/4-inch 7-ply transom can then be scribed for shape, sawn and fastened in place, using a good grade of casein or semiwaterproof gunk.
The transom on Sure Mike II is especially framed to take the largest of outboard motors. A 16-1/2-inch transom height will work for a Mercury Mark 55; it would be well to procure your motor beforehand, or measure the one you will use. It is so easy to dope out the transom opening and framing you want as you are at this stage, rather than doing it the hard way (by "unmanufacturing") later.
Let me insert a word here about the transom hood Idea. It seems to be wanted by many. It does dampen noise, but that is all. All enclosures around outboards I have ever had anything to do with always failed to keep the motor running cool, despite water circulation, and the motor always ran rich because there was no way to get stale smoke away from the carburetor. Cover the topside and the forward face, but don't try to fully enclose the motor—it is against all principles of air circulation. Air, to a degree, always follows a broad transom. And don't, for heaven's sake, put the motor on a bracket outboard. She'll load up in a following sea, also the strain on the transom is no good.
When the frames are up, plan out the spiling of the planking, and run in the battens, seeing that they land well in the gains through the frames, and at stem and transom cheek. Battens are best streamed in after the frames are erected, cutting the gains at that time. Fairer planking lines always result.
When the battens are installed. I'd commence planking the topside. Start at the chine, work toward the sheer. Three strakes of plywood are called for in this topside planking. The 3/8 inch thickness will be quite light, not too expensive. The 1/2-inch 5-ply stuff will be best, but it costs more than the 3/8 inch. I'd choose 1/2 inch if Sure Mike were my boat.
The drawings show the fastening size and frequency for this portion.
Next apply the garboard plank, and work out toward the chine. Use strong 1/2-inch planking here, so that when sanded you'll finish 1/2 inch. This is none too thick in a boat this size. The thickness has proved serviceable) but you can't go ramming logs with it. Plain, good boatbuilding wood is chosen for the bottom, because it will swell, get tight, and can be easily repaired. It has been my experience that 3/8-inch plywood has been about the top limit of thickness amateurs can bend without a full crew, much experience and a goodly number of expensive clamps. This thickness is not enough for the bottom of a boat this size. Hence the solid planking.
Soon your hull will have taken shape and you are ready to sand and paint her. A good rugged disc sander in the hands of a skilled man who won't make gouges is the best tool; a reciprocating sander which takes longer and wears out more sandpaper is the treatment for the outboard face of the skin. Several coats of grain-filling priming paint, like Firzite, will be needed, and the priming coat of paint is then installed.
The cabin coaming, the deck line and the framing of the cockpit sole are just plain straightforward carpentry and there is no need to burn up paper or midnight oil on that subject here.
Work from the waterline for your vertical heights. Work from the centerline out for your berth faces.
Individual desires always creep into one's execution of the interior of any cruising boat. The sky is the limit as far as trickiness is concerned, but don't forget that this boat, like all other careful designs, is designed to a weight. Performance is a matter of horsepower per pound, and behavior is a matter of loading the shape to the correct angle of attack. If you change weights, try to put in a cast iron stove, install the old family refrigerator and build a dog house for Rover, she then becomes your design and headache—not mine.
One 25 hp Johnson or Evinrude would give a good 14-15 miles in this boat. Two 15 hp motors would provide a safety factor) furnish 30 hp for about 16 mph.
Do not eliminate the skeg shown if you build her for an outboard. This item is there for a purpose: to make the boat sweet in a following sea and to take the yaw out of her when you have to check down to meet wild weather. Skegless outboards work on picnic days only.