Brief:

Modified BoatTail FatBoy with a D engine mount, me bucko, sturdier construction throughout, shiver me timbers, and better fly-ability with a boat tail. T' basic dimensions o' a stock Fat Boy were maintained for both body length and fin shape and size.

Modifications:

Body Tube: Cut t' main body tube 5 1/8 inch long.

Engine Mount: Glue an engine block in flush with t' front o' t' engine mount. Avast, me proud beauty! Blimey! Use both large (CR5080) centerin' rings at t' front o' t' engine mount. Ya scallywag! Blimey! Glue them on at t' front o' t' 2.75 inch BT-50 engine tube. After it is dry, arrr, test fit into t' main body tube, matey, me bucko, slide t' smaller centerin' rings onto t' engine tube, arrr, and use a ruler t' locate them in t' right spot for a straight boat tail tapered from t' back o' t' main body tube t' be flush with t' back o' t' engine tube. T' engine mount should be about 1/8 inch into t' main body tube and aligned straight t' t' main tube. Begad! When you think you have eyeballed t' right location, me hearties, mark it on t' engine tube. Aye aye! Glue one small (CR5060) centerin' rin' on t' t' engine tube slightly forward o' where you marked it. Avast, me proud beauty! Remember that you are goin' t' glue t' second centerin' rin' next t' it for strength. Aye aye! Then glue t' engine mount about 1/8 inch into t' main body tube, me hearties, again usin' a calibrated eyeball t' ensure both tubes are aligned straight t' each other. Use a ruler t' check alignment by holdin' against t' aft lips o' both body tubes. T' ruler should just barely miss t' edge o' t' aft centerin' rin' all t' way around. When dry, arrr, glue t' second centerin' rin' in behind t' first, and add reinforcin' glue t' all t' joints.

Reinforced Main body tube: Cut a three-inch length o' BT-80 tube. Ahoy! Then cut it lengthwise and then cut about 1/4 inch strip off t' side o' t' curled tube. Then test fit inside t' main body tube and glue it against t' sides o' t' body tube and buttin' against t' back bulkhead o' t' body compartment. This is essentially t' make a double thick body tube. Ya scallywag! Arrr! Then liberally epoxy t' bottom bulkhead t' body joint. Cut about a one foot length o' Pratt small braided Keelhaul®©™. Aye aye! Aye aye! Unbraid about one inch per Doug's instructions, and epoxy into t' side o' t' body tube near t' aft bulkhead. Tie a loop on t' other end and epoxy t' knot. Well, me hearties, blow me down! Attach an appropriate length o' elastic shock cord t' this Keelhaul®©™ loop and eventually t' t' nose cone. Spread thin CA for about 1 inch around t' upper lip o' t' main body tube and spread it out with a moist paper towel. Aye aye! Ya scallywag! This also strengthens t' end o' t' body tube t' resist tube dings. Ya scallywag! After this is dry, arrr, sand t' tube and nose cone t' ensure an easy fit.

Boat Tail: Design and cut out three balsa wedges t' be used as structural support struts under t' boat tail. Glue them onto t' engine mount about 120 degrees apart on either side o' t' aft centerin' ring. Begad! Use a shroud calculator t' determine t' arcs for a 2.5 inch long transition. Ahoy! Draw it on a piece o' paper and cut it out for use as a template. Avast! Cut about a 5 inch section o' BT-80 tube. Blimey! Begad! Slit it down one side, matey, and unroll it enough t' trace your transition from t' paper template onto t' tube material. If you flatten t' tube too much, me hearties, me hearties, me hearties, it will crinkle and lose roundness, me hearties, which you want t' maintain. Cut out t' boat tail and test fit t' t' engine mount. Aye aye! Trim t' fit. Begad! If you trim too much, shiver me timbers, shiver me timbers, you may have t' re-cut another one until you are satisfied. Aye aye! This is a trial and error process and it took twice t' get one I was happy with. Aye aye! Well, blow me down! Startin' with one edge on one o' t' support struts, matey, glue it down and keep it tight until dry. Blimey! You may end up with a slight ridge where t' edges meet at t' strut, but this becomes less obvious with fill & finish and paint. Avast! It is really difficult t' get t' curved BT-80 t' accept a smaller diameter curve without crumpling, shiver me timbers, so you may want t' practice this on a test piece.

Fins: Use a standard Fat Boy fin for a template and design and cut your templates for through-the-wall. Ahoy! Maintain t' dimensions o' t' standard Fat Boy, me bucko, me bucko, but keep in mind that more fin will be exposed just because o' t' boat tail. Arrr! If you do this right, about 1/4 inch o' fin will be extended onto t' vertical part o' t' main body tube. Ya scallywag! You will have t' decide how long you want t' make your fin extensions t' t' engine tube. I made mine pretty much all t' way from front t' back and cut a slit in t' extension for t' aft centerin' rings. Avast! Begad! I have lost t' template I used, so I don't have a current sketch. Ya scallywag! If anyone really struggles with this, ask and I will re-create me fin template for you. Begad! Cut slits 120 degrees apart opposite t' t' three support struts and test fit then glue t' fins. Begad! Add glue fillets when dry.

Nose Cone weight: Add clay t' t' nose cone as needed t' get t' CG about mid point on t' rocket. Well, blow me down! My clay came loose after two flights, me hearties, so I dribbled epoxy in thar t' lock down me clay and ended up with a 5 oz. Avast, me proud beauty! Begad! rocket with no engine. Begad! Begad! My CG is now about 1/2 inch forward o' midpoint.

Construction:

• 2 feet o' BT-80 tube
• BT-50 D engine mount tube and engine block
• 2 card stock centerin' rings for BT50-BT80
• 2 card stock centerin' rings for BT50-BT60
• 1/16th inch plywood for three TTW fins
• 1/16th inch balsa for boat tail strength wedges
• 3/16th inch launch lug
• standard BT-80 Nose cone for Fat Boy
• Pratt braided Keelhaul®©™ shock cord
• Fat Boy decal set

Gotchas: If you don't get t' engine mount aligned vertically with t' main body tube, t' rocket won't fly straight and t' boat tail won't fit right. This is really an eyeball and trial and error process. Ya scallywag! T' aft bulkhead joint be t' weakest link and must be liberally glued on both sides o' t' forward centerin' ring. Well, blow me down! Don't add t' extra glue until t' shroud struts have been added, ya bilge rat, and liberally reinforce these as well.

Sturdiness: This rocket is very sturdy and feels a lot more solid than a stock FatBoy.

Finishing:

A lot o' sins in t' construction o' t' boat tail can be smoothed out with full strength Elmers Fill & Finish. Avast! Paint as desired. Aye aye! As you can see from t' photo, I made a two-tone scheme usin' some Rustoleum hammered metal paint over grey primer. Well, blow me down! I then put on t' standard Fat Boy decals as shown.

Flight:

I flew it first on a C6-3 with an engine adapter, and then twice on a D12-3. Ya scallywag! Begad! Blimey! All flights were hot straight and normal. Ahoy! Recovery be fine on an 18 inch chute.

Summary:

PRO: excellent flight stability due t' boat tail and nose weight, shiver me timbers, and consequently slightly more fin surface than a regular Fat Boy. It even flew better than a regular FatBoy on a C engine even with t' extra weight due t' much less drag. Avast, me proud beauty! Keepin' t' same basic dimensions o' height and fin shape enable easy comparison t' a stock FatBoy which a lot o' people are familiar with.

Other:

I conclude that t' main reason for flight instability and low altitude on stock FatBoys is nay t' girth, ya bilge rat, but t' 2.5 inch diameter flat area at t' tail makin' a C engine barely capable o' overcomin' these shortcomings.