hydroplanes

UNTIL NOW, the amateur boat builder whowas interested in a hydroplane had to be contentwith an ultra-light, temperamental racing hulldesigned to carry only one or two persons.

In spite of recent breakthroughs by boat manu-facturers in taming the race-bred, three-point hy-droplane for more general use, build-it-yourself

This hydroplaneseats four

By ARTHUR MIKESELL

Designed by Alan Scott, this modernthree-point runabout

can top 50 mph with a 65-hp motor

Sleek and sassy, this three-pointer combines the excitement of a racer with runabout versatility

1409

building a runabout, continued

1410

No building form is required, but temporary legs which support the frame should be secured to the floor

The bottom-side planking aft of sponsons must be slitto form inner chine. A lift rail covers this joint

plans for a true recreational three-pointer justwere not available.

So we decided to ask Alan Scott to design arecreational hydroplane. These were the designrequirements we gave him:• An easy-to-build three-point plywood run-about approximately 14 ft. long for use with amotor in the 45 to 65-hp. range.• A clean, modern topside design, featuring aroomy cockpit with seating for four.• The capacity to operate efficiently while tow-ing a water skier.

This was a pretty tall order; but when we re-ceived the plans and built the Hydro Dynamic, itturned out to be a rousing success on all of thespecifications we had requested.

If you've never driven a three-pointer, thenyou've missed one of the biggest thrills in boating.Let your imagination go, and slip into the driver'sseat of Hydro Dynamic for a test run. As youease the throttle lever forward, the boat comessmoothly up on a plane just like any normalplaning hull. But continue to advance the throttle

1412

and Hydro Dynamic suddenly climbs to herpoints with an effect like cutting in the after-burner on an F-104. The boat seems to be flyingover the water rather than skimming along thesurface, and you feel that almost imperceptible"walking" action, found only in three-pointers, asthe sponsons slide from wave to wave.

Of all the air-lift hulls (sea sled, catamaran,etc.), the hydroplane design is the most sensitiveto weight. Since this boat is designed for generalrecreational use rather than flat-out racing, thesponsons were made wide and flat, with a rather

deep entrance. This may not be the most efficientdesign for racing speeds but it brings the boat upon its points sooner and provides support for aheavier load, enabling you to enjoy the advan-tages of a three-point ride while towing a skier orcarrying extra passengers.

But a word about materials before we go on tothe construction of Hydro Dynamic. The hull ofthe pilot model was planked with AA-grade ex-terior fir plywood. Naturally, solid-core marineplywood can be substituted here, but it will raisethe cost of the boat substantially. Likewise, fir

1413

hydroplanes

For a flush joint between 3/8-in. aft bottom plankingand 1/4-in. forward bottom planking, glue a 1/8-in.-thick batten over the butt straps at this joint

After securing the aft edge of bottom bow planking,clamp fore end to bow beam and add other fastenings.The outside bow planking notches to center section

The sponson framing will require considerable fairing.To save time, you might cut this bevel oversize witha saw, then finish it with a rasp

A completed sponson includes two rails which slantinward, providing a slight "tunnel effect" to gainadditional lift. Round the ends as shown here

.r

building a runabout, continued

was used for all frames, battens and other struc-tural members, but mahogany has a betterstrength-to-weight ratio and would make the boatsomewhat lighter. However, mahogany is moreexpensive.

All joints are both glued and screwed, and allexterior fastenings should be countersunk slightlyand concealed with wood putty. While cadmium-plated steel screws are specified in the materialslist, you could just as well use 2 lbs. of 14-ga.1-in. serrated bronze nails (Stronghold, Anchor-fast or similar) in place of the 1-in. screws.

Fiberglassing the entire boat would have addeda small amount of extra weight, so only the seamsand rear sponson surfaces of the pilot model wereglassed. However, this is a matter of personaltaste, and you may well feel that the returns ineasy upkeep and appearance are worth the extracost and slight sacrifice in performance (less than1 mph).

Caulking compound should be used when in-stalling the lift rails and also at the bottom tran-som joint. If you plan to glass the seams or theentire boat, be sure that you don't use an oil-based compound as this type prevents properbonding of the resin.

1414

Bucket s e a t s a r e framed with light b a t t e n s ob t a inedby cut t ing 1 x 2 s t o c k in half on a c i rcular s a w

Two dress-up touches which aren't absolutelyrequired are the special seat cushions to fit thesquare bucket seats and the vinyl deck covering.The seats come in a variety of colors. As for thevinyl-coated deck fabric (Du Pont, Nautolex orsimilar), it won't scuff, crack or peel and is easyto keep clean. We used a teak pattern, but itcomes in other patterns and colors. The teakseemed most fitting but the choice is yours.

Before you begin building Hydro Dynamic,make sure that you understand every construc-tion step. Study the drawings—particularly theexploded view on p. 1410—until you know thewhole boat like the back of your hand. Then,after "building the boat in your head," begin theactual construction by cutting out and assemblingthe frames.

While most of the side and bottom membersof each frame are simply straight pieces of 1 x 4stock, Fig. 5, the transom, top beams and bowbeam must be cut to a curve. To avoid errors,make full-size paper patterns of each one, usingthe grid system to scale the drawings up to actualsize.

make a tracing toolYou can make a handy tracing tool for trans-

ferring these patterns to the wood by salvaging agear from a discarded alarm clock and mountingit on a nail between the legs of a wooden clothes-pin. Tape or tack the paper onto the wood, runthis tool around the edge of the pattern and you'llproduce a neatly dotted outline of the framemember on the stock.

Begin with frame 4, the transom. This is madeup of an inner frame faced with 1/2-in. plywoodand reinforced with a 2 x 4 cross beam. A 1 x 4running across the bottom on the outside providesadditional strength. Since the keelson, bottombattens, sheer battens, etc., will all be notched intothe inner framing and butted to the plywood,cut these notches before mounting the framemembers. Then fasten the framing to the plywoodwith glue and 1-1/4-in. No.-8 f.h. wood screwsspaced 2 in. apart. After assembling the transom,cut out the transom knee, Fig. 5, and mount it onthe transom. Note that it must be notched to fitover the bottom framing member.

Next, cut and assemble frame 3, using three1-1/2-in. N0.-8 f.h. wood screws at each cornerjoint. The dashboard mounts on supports runningaft from frame 2. While you may cut out thesepieces now, installation will be easier if you waituntil the hull is turned.

Frame 2, the step frame, is somewhat more

1415

Back-to-back seating takes advantage of every inchof space with rear legroom extending under the well

building a runabout, continued

complicated. The bottom crossmember must bemade double thickness since it also acts as fram-ing for the sponsons. The forward thickness fitsbetween the two side framing pieces and isnotched for the outer sponson framing member,"A" in Fig. 2. The aft thickness runs the fullwidth of the boat and isn't notched. Thus, thenotches in the forward thickness must be cut be-fore it is secured to the aft thickness. Use glueand 1-1/2-in. N0-8 f.h. wood screws to mount it.

You'll have to add a 1 x 2 cleat on the aft sur-face of the bottom crossmember of frame 2 toserve as a nailing strip for the 1/4-in.-plywoodbottom planking. This cleat must be notched toaccept the two outer bottom battens before se-curing it to frame 2. Be sure to make the innernotch large enough to accommodate not only thebatten but the nailing strip which runs along itsouter edge and butts to the aft surface of frame 2.After cutting notches, secure the cleats with glueand 1-1/4-in. N0.-8 f.h. wood screws.

Assemble frame 1 as shown, cut out the bowbeam, and you are ready to begin framing thehull. Although no complex building form is re-quired, you should have a stable, level nailingplatform on which to assemble the hull.

You will note from the photo on page 1411that the frames are supported on legs above thebuilding floor. Since we built Hydro Dynamic in ashop with a wood floor, it was no problem to toe-nail these legs directly to the floor. However, ifyou plan to build the boat in a concrete-flooredgarage, assemble a skeleton building platformfrom scrap lumber and attach the legs to this.Use C-clamps or screws to mount these tem-porary legs on the frames.

installing the keelsonOnce all four frames are supported at the

proper height and correct distance apart, Fig. 4,install the 1 x 2 keelson using glue and two 1-3/4-in.N0.-8 f.h. wood screws per joint. Next, mount thesheer battens on the frame, securing each jointwith glue and two 1-3/4-in. N0.-8 f.h. wood screws.Note that these must be attached to the outerends of the curved bow beam.

The chine battens go on next. They must becurved rather sharply from frame 2 forward tothe bow beam, so in order to make the bendingeasier we suggest that you slit this portion byrunning it through a circular saw. After slitting,fasten the chine batten to frames 2, 3 and 4 usingglue and one 1-3/4-in. N0.-8 f.h. wood screw perjoint. However, don't fasten it to frame 1 yet.Instead, bend this slit forward portion through

1416

Towing a skier, the Hydro Dynamic speeds over the water, staying well up on her points

the notch in frame 1 so that the end meets thejoint between the sheer batten and bow beam.

Trim the end of the slit batten to fit flushagainst the sheer batten, then fill the slit with glueand clamp the chine batten in place. When theglue has hardened, secure it to frame 1 and the

bow beam, then fasten it in place with glue andscrews. A reinforcing block is required at thisjoint, too, though it shouldn't be installed untilafter the center deck beam is in place.

Mount the bottom battens next, trimming theend of each to fit against the bow beam. Secure

1417

them with glue and one 1-3/4-in. N0.-8 f.h. woodscrew at each joint.

Before fairing the frame and beginning the jobof planking, you might mount the three foredeckbeams. While the operation isn't absolutely nec-essary at this point in the construction, thesebeams will make the hull slightly more rigid, thusproviding extra insurance against pulling it outof alignment when planking.

Since most of the hull surface is flat, the framewill require a minimum of fairing. Remove thetemporary legs which have supported the hullthus far and place the hull on the floor. Thenfair all framing surfaces until they will mateevenly and smoothly with the plywood planking.

use plywood panelBegin planking with the 4 x 8-ft. panel of 3/8-in.

plywood. Place this in position on the frame andattach it temporarily with four 1-in. N0.-8 f.h.screws. Then crawl underneath and mark thelocation of all framing members on the plywood.Also mark the location of the forward edge, soyou will know where to install the 1 x 2 buttstraps which reinforce that planking joint.

Next, remove the panel and predrill holesabout 2 in. apart for 1-in. N0.-8 f.h. wood screws,using a double row of screws along the transom.

After installing the butt straps between thebottom battens, coat mating surfaces with glue,apply calking compound to transom edge and se-cure the panel to the frame.

Once the 3/8-in. panel is installed, you canmount the nailing strips along each side of it.Made by cutting a 1 x 2 down the middle, thesestrips are attached to the two bottom battens andelevated slightly so as to bring the 1/4-in. plankingup flush with the 3/8-in. planking.

In order to achieve the bend necessary to formthe inner chines, you'll have to make a 54-in.saw cut in the two outside sections of aft plank-ing, Fig. 3. Secure these two sections as you didthe center panel, using a double row of screwsalong the 54-in. slit.

The forward bottom planking is also 1/4-in.plywood, so glue a 1/8 x 1-in. batten along the foreedge of the 3/8-in. planking to bring it up to theproper height. After securing the 4 x 6-ft. centersection of planking, make paper patterns of thetwo 4-ft. outer sections and mount these. Beforeyou start on the sponsons, install the side plank-ing, following the same procedure used in plank-ing the bottom.

Each sponson is framed with two longitudinalside pieces cut from 2 x 6 stock, Fig. 2. Aftermounting these framing members, add a taperedfiller block at the fore end to serve as a nailingstrip for the planking. Then cut out the 1/4-in.sponson planking, bevel the fore edge to fit flushagainst the bottom planking and secure with 1-in.N0.-8 f.h. wood screws and glue.

Before turning the hull over, install the eightlift rails. Those on the sponsons should slanttoward the center of the sponsons, while those onthe aft planking slant toward the center of theboat. Taper and round the fore ends of the tworails covering the 54-in. slits at the inner chines.Use calking compound and 1-in. N0.-8 f.h.screws to mount all rails. Finally, mask all fas-tenings, fiberglass the seams, and give the bottomat least three coats of paint.

Then turn the hull over, resting it on a paddedsurface so as not to mar the finish or the bottomof the hull.

Add the deck beams and battens, if youhaven't already done so, then mount the dash-board and add the perforated hardboard bottomto the glovebox. After fairing the deck framing,install the deck planking and the basic hull iscomplete.

cockpit is linedFor appearance, the cockpit of the pilot model

was lined with 1/8-in. perforated hardboard. Thisdoes add a certain amount of extra weight, andcan be omitted if desired. However, if you wishto do this, attach light l x l nailing strips to thebottom of the boat and secure the hardboard tothese and the deck battens.

Then conceal the upper edges under therounded coaming strips.

Next, make the four seats, Fig. 7, and mountthem on 1/4-in. plywood brackets attached to thebottom battens. The self-bailing well and fuellocker, Fig. 6, will complete the boat. If you wish,drill 1/2-in. holes at the corners of the well to allowwater to drain out. The fuel locker is a simple boxconstruction of perforated hardboard framedwith 1 x 2s. If you require more space, make thelocker longer so that it runs between the rearseats.

Finally, paint the interior of the cockpit, installthe vinyl-coated deck fabric and the boat iscomplete and ready for the water.

1418