Brief:

Modified BoatTail FatBoy with a D engine mount, arrr, arrr, sturdier construction throughout, shiver me timbers, ya bilge rat, and better fly-ability with a boat tail. Aye aye! Arrr! 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. Use both large (CR5080) centerin' rings at t' front o' t' engine mount. Glue them on at t' front o' t' 2.75 inch BT-50 engine tube. Begad! Blimey! Well, blow me down! Blimey! After it is dry, test fit into t' main body tube, ya bilge rat, slide t' smaller centerin' rings onto t' engine tube, 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. Ahoy! Blimey! Arrr! Blimey! T' engine mount should be about 1/8 inch into t' main body tube and aligned straight t' t' main tube. When you think you have eyeballed t' right location, shiver me timbers, mark it on t' engine tube. Aye aye! Blimey! Glue one small (CR5060) centerin' rin' on t' t' engine tube slightly forward o' where you marked it. Avast, me proud beauty! Blimey! Begad! Blimey! Blimey! Blimey! Remember that you are goin' t' glue t' second centerin' rin' next t' it for strength. Begad! Blimey! Aye aye! Blimey! Blimey! Blimey! Then glue t' engine mount about 1/8 inch into t' main body tube, again usin' a calibrated eyeball t' ensure both tubes are aligned straight t' each other. Ahoy! Blimey! Blimey! Blimey! Use a ruler t' check alignment by holdin' against t' aft lips o' both body tubes. Avast, me proud beauty! Blimey! Ya scallywag! Blimey! Blimey! Blimey! T' ruler should just barely miss t' edge o' t' aft centerin' rin' all t' way around. Avast, me proud beauty! Blimey! When dry, shiver me timbers, 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. Ya scallywag! 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. Then liberally epoxy t' bottom bulkhead t' body joint. Begad! Cut about a one foot length o' Pratt small braided Keelhaul®©™. Well, blow me down! Unbraid about one inch per Doug's instructions, ya bilge rat, me bucko, and epoxy into t' side o' t' body tube near t' aft bulkhead. Avast! Tie a loop on t' other end and epoxy t' knot. Blimey! Attach an appropriate length o' elastic shock cord t' this Keelhaul®©™ loop and eventually t' t' nose cone. Begad! 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. Blimey! This also strengthens t' end o' t' body tube t' resist tube dings. Ahoy! 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. Use a shroud calculator t' determine t' arcs for a 2.5 inch long transition. Blimey! Blimey! Draw it on a piece o' paper and cut it out for use as a template. Cut about a 5 inch section o' BT-80 tube. Blimey! Begad! Slit it down one side, me hearties, and unroll it enough t' trace your transition from t' paper template onto t' tube material. Aye aye! If you flatten t' tube too much, it will crinkle and lose roundness, which you want t' maintain. Avast! Blimey! Cut out t' boat tail and test fit t' t' engine mount. Arrr! Trim t' fit. Ya scallywag! If you trim too much, you may have t' re-cut another one until you are satisfied. Avast! Well, blow me down! This is a trial and error process and it took twice t' get one I was happy with. Startin' with one edge on one o' t' support struts, matey, glue it down and keep it tight until dry. Ahoy! 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, me bucko, me proud beauty! It is really difficult t' get t' curved BT-80 t' accept a smaller diameter curve without crumpling, 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. Avast, me proud beauty! Maintain t' dimensions o' t' standard Fat Boy, arrr, but keep in mind that more fin will be exposed just because o' t' boat tail. Begad! Well, blow me down! If you do this right, ya bilge rat, 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. Ya scallywag! I have lost t' template I used, me hearties, so I don't have a current sketch. Avast, me proud beauty! Arrr! If anyone really struggles with this, me hearties, me bucko, ya bilge rat, ask and I will re-create me fin template for you. Avast, me proud beauty! Cut slits 120 degrees apart opposite t' t' three support struts and test fit then glue t' fins. Avast! Ahoy! 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. Arrr! My clay came loose after two flights, so I dribbled epoxy in thar t' lock down me clay and ended up with a 5 oz. Begad! Aye aye! rocket with no engine. Well, blow me down! 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, arrr, shiver me timbers, t' rocket won't fly straight and t' boat tail won't fit right. Avast! Avast, me proud beauty! This is really an eyeball and trial and error process. T' aft bulkhead joint be t' weakest link and must be liberally glued on both sides o' t' forward centerin' ring. Begad! Don't add t' extra glue until t' shroud struts have been added, arrr, shiver me timbers, 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. Begad! Paint as desired. Ahoy! As you can see from t' photo, me bucko, matey, I made a two-tone scheme usin' some Rustoleum hammered metal paint over grey primer. Aye aye! I then put on t' standard Fat Boy decals as shown.

Flight:

I flew it first on a C6-3 with an engine adapter, shiver me timbers, and then twice on a D12-3. Avast, me proud beauty! Aye aye! All flights were hot straight and normal. Begad! Recovery be fine on an 18 inch chute.

Summary:

PRO: excellent flight stability due t' boat tail and nose weight, arrr, 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. Ahoy! Blimey! Well, ya bilge rat, blow me down! Blimey! 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, arrr, but t' 2.5 inch diameter flat area at t' tail makin' a C engine barely capable o' overcomin' these shortcomings.