(A Sudoku adventure in RC boat building)
by: Wm. Brown, Yeoman
Pima Micro Yacht Club,
The challenge comes from several directions. The finished product, however, is an extremely weight conscious and very finely tuned international racing machine. Being among the lightest of the competitive RC sailboats, the demand for
effective consolidation and distribution of weight, and, effective use of sails is critical. If these considerations aren’t enough, the kit presents itself with its own set of challenges.
Having built three Solings (10 Lbs) and two Victorias (5 lbs), I personally found the Micro Magic (2 lbs) the biggest and most rewarding challenge. Yes, completing my boat, was fun.
The kit, first developed and manufactured by Graupner in Europe, comes
with a number of notable challenges. The hull and deck are fabricated from a fairly “soft” and flexible ABS material. The two components are brittle to impact and subject to distortions from inappropriate glues. More on that later. The challenge of effectively bringing together the hull and deck is formidable, and, is not appropriately addressed in the kit instructions. At the Micro Magic National Competition in Phoenix last Fall, I observed that deck / hull union was apparently a source of frustration for more than one competitor. The kit itself comes with very limited step by step construction dialog. Full size schematics, however, are good and complete. A word of caution regarding the alignment dimples on the deck and hull, … ignore them. My rudder was 12+ degrees off vertical, and required my cutting an unscheduled hole in the aft deck. The keel deck hole was too small and was 1/8 in. off center line. The stern deck / hull alignment was off 3/16 inch. Other smaller kit “discoveries” included: the main and jib booms being a little short for effective sail outhauls, and, the hatch closure being poorly designed, misaligned, and not user
friendly. The most important issue not addressed in the kit instructions deals with effective placement of batteries. Being the heaviest single item in the boat, battery location deserved a more thorough discussion.
A critical eye and common sense caught a majority of the kit’s shortcomings. On the positive side, the kit provides an array of opportunities for creative and rewarding problem solving, ….I found it the fun part of building a Micro Magic.
Get ready. I hate bowsies! In this discussion you will note that I favor the use of a proven alternative. For the purpose of securing cordage, I have opted for the use of 3/16 in. nylon machine screws. Where used, a slightly enlarged 3/16 hole is drilled into the appropriate location and a nylon nut is epoxied to the underside. Using a small screw driver, cordage is adjusted and secured under the head of the machine screw. After
five boats and no failures, I enjoy this simple arrangement!
Let’s Build the Boat
1. Selection of glue
Prior to starting construction, I took five pieces of scrap ABS
from the kit and tested the adhesive and thermoplastic qualities of three glues and four epoxies. Glues included Super Thin CA, Thin CA, Gap Filling Medium CA, and Shoe Goo. All appeared safe to use, but, Medium CA, with prep sanding, proved to have the best tensile adhesive quality. Shoe Goo didn’t make even a decent filler. Medium CA worked very well on both ABS and Carbon Fiber (CF). Two part epoxies included Devcon Plastic Welder, Ace Plastic repair, Loctite Extra Time Epoxy(60 Minute), and Loctite Marine Epoxy (two hour). With prep sanding, CA and Loctite epoxy worked very well on the deck / hull union. Medium CA was flowed between the deck flange and hull and Marine epoxy used for gap filling. All the above, especially the Loctite Marine epoxy, sanded and finished very well. A review of the literature reveled that some glues have proven to be too “Hot” for ABS. Caution should be taken.
2. Deck/Hull Union
When bonding the hull to the deck, the challenge is to effectively push the hull out and against the 5 mm deck lip, ie: flange. The non rigid hull does not uniformly align against the deck flange. My solution was to create a 5 mm deep X 3 mm wide trough on the bottom (inside) surface of the deck. This trough uniformly forced the hull wall against the overhanging deck flange. Using of lots of close pins, the fence was uniformly pre-positioned and glued 3 mm inside the deck flange. Medium CA was used for bonding. (See Photograph) With the fence in place, the hull wall was prep sanded, placed in the trough, and secured with compression rubber bands and tape.(See Photograph). Medium CA was flowed into the trough to complete the union. With the edge of a finger and Marine Epoxy for filler, a uniform, strong, and esthetically smooth bead of epoxy was laid down between the hull and flange.
3. Jib boom weight Management
Rather than simply attaching the jib boom weight to the leading end of the jib boom, I elected to: (1) cut the head off a small 3/32 in. threaded brass bolt, (2) and insert the bolt into the forward end of a non kit 4mm DIA
CF boom, and (3) apply medium CA to secure the bolt in place. I then tapped the kit brass jib boom weight to match the bolt thread. This created a fore and aft adjustable jib boom weight. (See Photo.) The weight was secured in place by a like threaded nylon nut.
4. Forestay to the jib boom Attachment
The forestay, coming down from its pivot point on the upper mast, is attached to the jib boom using a small 1/8 in. dia. split ring. The ring was placed on the aft end of the threaded screw described above (#3). This
arrangement is very simple, very accessible, and very removable.
5. Boom to deck Attachment
My goal here may appear to be making the attachment as complicated as possible. But, not so. The attachment actually is a response to several
Goal #1: Get the boom as close to the deck as possible;
Goal #2: Make the boom height off the deck adjustable;
Goal #3: Reduce tension, friction, and torque in the short cordage between the jib boom and its deck attachment.
Solution: (1) Make the cordage longer (from 1/4 in. to 1 1/4 in.), (2) secure the cordage to a point below the deck, (3) for adjustment, return the cordage to a point above deck. Using 3/16 in. dia. styrene tubing, continue the angle of the jib boom stay through the deck, around a 3 in. radius, and returned the loop to exit the deck 3 in. behind the entry point. In this water tight tube, the tension, friction, and torque of the stay is distributed over both a longer distance and a longer piece of cordage. After exiting the
hull, the stay (now a halliard) is taken to an adjustable machine screw located at the base of the mast (See opening preface). Note: The length of the jib boom halliard is now adjustable, and, the stress on the cordage is greatly reduced. This arrangement provides quick adjustment of both boom height off the deck, and, in conjunction with the back stay, the fore and aft tilt of the main spar (mast). Using a small screw driver, both adjustments can quickly be made when there is a change in wind speeds.
6. Back Stay/Transom Attachment
I opted for: (1) an 8 in. piece of cordage anchored on the transom, (2) extending this cordage up to a 1/8 “ split ring on the backstay, and (3) returning the cordage down to the transom. The return cordage is secured on the transom under the head of a nylon machine screw (see Preface). Sounds complicated, but, it makes for a very simple adjustable backstay. With the twist of a screw driver, you are ready for adjusting and securing. No bowsies.
7. Topping lift and forestay Adjustment
Note: The way I elected to configure these two items, would traditionally make them halliards; i.e.; Both pieces of cordage attach to a moveable item, follow a course up the mast, and then return to the deck. The common mast top return for both halliards is located approximately 27 in. up from the bottom of the mast. Returning to the deck, these halliards are secured under two very adjustable nylon machine screws (see Preface and Photo). To achieve a horizontal line to the screw heads, a 1/4 in. high X 3/4 in long deck fence is CA’d to the deck near the nut heads. The cordage (halliards) is passed through two 1/16 in. holes in the fence to the screw head. Again, a small screw driver affords the opportunity to adjust and secure these two halliards. No bowsies.
8. Battery Weight Management
Note: This is a biggie! The battery in the MM is the heaviest single component in this two pound boat. In the five pound Victoria or the 10 pound Soling, the placement of the same weight is important, but, not as critical. The dilemma: How can you maximize the fore and aft tuning of the heaviest object in the boat? Ideally you would like to be able to move the battery to any location between the rudder post and the keel trunk. If this could be accomplished AND position the two servos in line with the
battery, you could achieve ideal beam to beam and fore and aft weight symmetry. The heaviest items in the boat would be as low as possible in the hull, aligned amidship, and be adjustable. A Solution: (1) Make a tunnel under which the battery can slide from the rudder post to the keel
trunk, (2) mount the servos in line on top of the battery tunnel. The lateral weight symmetry for both components would be ideal.
Fabrication: The tunnel is fabricated from three pieces of 1/16 in. Lexan. Lexan is a premium, poly-carbonate and can be purchased at Lowes in 12 in. squares. The tunnel has a 5 X 2 in. top and two lateral walls 4 X 1.5 in. The walls are angled out from the top at 30 degree angles. The three components are bonded with Med CA. Two identical Hittec HS-5085mg
servos are mounted end to end on the top of the tunnel. The battery, resting on the floor of the hull, fits comfortably under the servos. Lateral shifting of the battery is controlled by two 8 X 3/16 in. fore and aft, styrene “fences”. These battery guides are CA’d parallel and 3.75 in. apart
inside the hull wall. The width of the “trough” is dependent upon the width of the battery plus about 1/4 in. on each side. In this trough, the battery pack should move freely from the rudder post to the keel trunk. The battery / servo tunnel may be secured to the hull floor by whatever
means you wish. I enjoyed the use of small conveniently located nylon machine screws. A nice feature of the tunnel concept is that it is totally removable from the boat, …servos, servo arms, and battery. Tuning battery fore and aft: Note: don’t get lost here, – this really is simple. First determine the direction and posture you would like your battery pack to rest, – while under the tunnel. The direction and path of the battery leads going to the receiver must be given consideration. Once the position of the battery in the trough is determined, CA a piece of string to the aft (rear) end of your battery pack. Then CA a piece of string to the forward end of
your battery pack. Next, fabricate “returns” for these strings from 12 in. squares of scrap ABS. Drill a 1/16 in. hole in each return. CA one return near the rudder post and the other to the side of the keel trunk. Thread the aft battery string through the rudder post return and the forward
string through the keel trunk return. With the battery tunnel secure in the hull and servos installed with arms attached and sheeted, bring the two battery tail strings up and over the tunnel and tie them together. You now have a loop that: pull one string and the battery will glide back, pull on the other and the battery pack will glide forward. With a nylon machine screw
anchored on the top of the tunnel, you can secure or release the ends of the loop and position your battery in an infinite number of fore or aft positions. The servos remain anchored and in line on top the tunnel. The servo arms move free and un encumbered.
9. Aligning the rudder post
Caution: Do not use the dimples on the deck to align your rudder.
The goal here is simple: Make the rudder and keel perpendicular to the deck. Accomplishing this is not challenging. Start by moderately oversizing the rudder post hole located on the
cockpit floor. With the brass rudder post through the hole in the hull and extending up and
through the oversized hole on the cockpit floor, slide the rudder into place. Put a block under
the rudder to prevent it from falling out. This support will be used to tune the alignment of the
rudder fore and aft, and, beam to beam.
Next, place two identical small square pieces of wood on the deck edge (the gunnels) on each
side of the servo access port (hole). Bridge, beam to beam, the two pieces of wood with an 8
or 10 inch bubble level. With the boat hull in it’s cradle and rudder independently supported,
view the level from behind (astern) and level the deck. Next, drop an overhead plumb line and
bisect the boat transom. With the deck bubble level leveled, the line will be perpendicular to the
deck. The plumb line provides a visual reference for aligning the rudder and rudder post –
beam to beam. To align your rudder post vertically (relative to the cockpit floor), place your
level fore and aft on the cockpit floor and raise or lower the hull bow to achieve bubble level.
The rudder and rudder post are now vertical, – beam to beam and fore and aft. Using the
cockpit floor, It is now time to slip the snug fitting ABS collar over the brass rudder post tube,
and, with CA, “lock in” your alignments. When dry, remove the rudder from the rudder post.
With a mirror and a piece of wire, apply marine epoxy around the two in-hull rudder post exits.
Reinforcing these two stress points will protect the hull from the brittle nature of ABS.
With the rudder post now solidly secure, use a fine metal file or drummer grinding disc, carefully
hone the lower brass tube rudder housing flush to the hull. Leaving 1/16 in. extending from the
cockpit floor, perform the same procedure on the upper end of the brass tube rudder housing. It
is now time to trim the upper margin of the rudder. This margin should duplicate the contour of
the hull. Installed, the movement of the rudder from side to side should clear the hull by the
thickness of a piece of paper. The cockpit rudder servo arm should move smoothly from side to
side and be parallel to the cockpit floor.
- As noted, the ABS hull of the MM is brittle and vulnerable in impact situations. My hull came in a new box with a 8 mm crack on the bow, – at about the water line. A packet of accessories was in the immediate area. I resolved the problem with a 1in. X 2 1/2 in. piece of single ply, medium fine, fiberglass. The patch was laid vertically inside the bow and bonded with slow setting, two part, marine epoxy from Ace Hardware. A very small external cosmetic application of the epoxy resolved the problem. Surfaces were prepped by a light sanding. This relatively simple procedure was also performed at the junction of the keel box and the hull. To avoid future impact problems, I suggest following this procedure when constructing any Micro Magic.
As noted, I personally enjoy using small nylon slotted machine screws and epoxied nylon nuts to secure, (1) the main and jib sheet length adjustments in the cockpit, (2) the back stay adjustment on the transom, (3) the jib toping lift and forestay halyard adjustments, and, (4) the jib boom deck adjustment.
When binding the deck to the hull, there is a transition issue where the 90 degree deck flange stops and the slope of the aft deck to the transom starts. If nothing is done to reinforce this deck / hull transition, you end up with an unreinforced 90 degree hull to the deck glue job. Not good !! I opted to continue my styrene “fence” (see Deck / Hull Union above)
through this transition, and around the inside margin of the transom. Taping the deck and hull together and then flowing medium CA between the fence and hull provides a much more stable union. Cosmetic filling was accomplished with slow setting, two part, marine epoxy and fine
My main spar (mast) is unstayed. If, however, you wish to incorporate standing stays, I suggest not using the items provided in the kit. Glue failures are common. I and others suggest the following: 1. Make a 1/2 inch round disc from scrap kit ABS, 2. Drill a 1/16 in. hole in the
disc, 3. Stick the head of a small cotter pin through the hole in the disc and flair the legs of the pin out and flat, 4. Epoxy the flared legs to the bottom side of the disc, 5. Drill a 1/16 in. hole through the appropriate location on the deck, 6. From the bottom side of the deck, stick the
head of the cotter pin through the deck, then, 7. With the head of the cotter pin protruding through the deck, CA the top of the disc to the bottom side of the deck. On the deck top, use marine epoxy to cosmetically finish the area around the protruding cotter pin head. If this arrangement fails, you have demolished the deck of your boat and the boat will soon be resting at the bottom of the pond.
Reducing drag. Drag and friction are speed killers.. Sailboats are unique, – they interact with two media, air and water. Separately or jointly, both have the potential of producing significant friction or drag. Air we manage and control with sails, hulls we manage by fairing the trailing edges of the rudder and keel. The trailing edge of each of these components should be
sharp, able to cut paper, ….or at least cheese. Another form of drag is surface tension and / or turbulence. Researchers have demonstrated
that very finely sanded (400 grit or more) and unpainted surfaces have less friction than shiny painted surfaces. Most of RC boaters do not paint below the waterline. They keep their keels, rudders, and hulls micro sanded and very clean.
Painting suggestions: As noted above, painting below the water line is not recommended. Micro sanding and painting above the water line, however, is both fun and provides visual identification of your boat. Prep sanding with 300 – 400 grit paper and using Krylon Fusion (for plastic) has been successful for most in our sail club. Care should be taken
to maintain the recommended 12 -14 inch distance from the hull, and allow an average of 1 hour drying time between thin applications. Thoroughly mix the contents of the spray can. If your finish “corn flakes” on you, let it dry, sand it smooth, and, start again.
Mast Options: I elected to use an unstayed, 6.0 mm OD / 4.8 mm ID, 28 in. long, carbon fiber mast. I fabricated an aluminum mast crane. To prevent the CF from vertically splitting, I found it necessary to tightly wrap the mast head with fine cordage and glaze it over
with epoxy. The forestay and toping lift halliard return is located 26.75 in. from the bottom of the mast. With two holes in it, the small piece of 1/4 in. thick Lexan is contoured to the mast and secured with medium CA. The bottom of the mast is plugged and glazed with epoxy.
Boom options: I elected to use 4.0 mm DIA hollow CF tubing. A small cotter pin inserted and CA’d in the back of both booms provides a good outhaul attachment.
Sail options: A review of the literature strongly suggests purchasing custom sails.
Wet Tuning. When completed, Micro Magics’ small size makes it ideal for putting it in water and noting it’s resting “posture”. Using a plum line and viewing the mast from behind, you can accurately determine if the completed model sits flat on the water. A mast that is not vertical will respond differently on port and starboard tacks. Viewed from the side, the plum will tell you the amount of fore and aft rake present in the mast. As noted earlier, correlating mast rake with battery location in a two pound boat is critical.
I hope this information provides some insight into the fun you can have creatively solving problems. On the pond, this jewel of a boat will let you celebrate both your imagination and your ingenuity. It has for me. bb
Share & Related