Brief:
T' problem with overpowerin' a "stubby" design like t' C-powered Estes Fat Boy with heavier engines be t' CG dilemma -- t' more power, t' more nose weight is needed t' keep t' CG in limits, negatin' t' Oomph you thought you were adding. Avast! I have modified Fat Boy for F-power with a nose cone extension t' counter-ballast this with minimal added weight.

Modifications:
T' only other modifications I made t' create me Schnoz Boy was some modest reinforcin' t' accommodate t' 8-fold increase in thrust (C t' F).

T' stock fins were trimmed t' fit a 24mm motor tube (or a 29 mm works, matey, too) and doubled on one side with 1/64" sheet plywood (a snap, ya bilge rat, this stuff cuts with a scissors) epoxied on. Well, blow me down! A 1/8" Lite-Ply-tm doubler was constructed for each o' t' cardstock motor centerin' rings supplied with t' kit. Arrr! Blimey! And that's it. Ahoy! T' complete t' make-over, shiver me timbers, I added a "professional" recovery system consistin' o' an 18" LOC nylon chute and 10 feet o' nylon Marine Corps shoe lace (see landin' photo), anchored through t' body tube with a knot covered with a plastic fairing. Arrr! Copyin' a final touch I saw in a magazine, shiver me timbers, I couldn't resist rearrangin' t' stock Fat Boy decal t' spell "YO BATF".

Construction:
A 5" 24mm Estes nose cone be ballasted with 35g o' lead fishin' sinkers epoxied into t' tip. T' "supplemental" nose cone is then affixed t' t' stock Fat Boy cone, usin' a self-tappin' sheet metal screw set into t' stock cone and protrudin' 1/2" t' allow a gob o' epoxy t' anchor t' pointy ballast box in place. This puts t' CG with a 62g F21 at 6.5" from t' tail, shiver me timbers, well ahead o' t' Cp (3.9" from t' tail). I call t' finished product "Schnoz Boy", arrr, for t' obvious reason. See photo.

This was an easy conversion. Ahoy! All t' stock pieces were used, and its a great kit. Aye aye! T' through t' wall fin design makes this easy t' convert t' higher power. Avast! You do need t' add a proper recovery system for E-F-G power, ya bilge rat, however, and reinforcin' t' balsa fins for t' much higher speeds is essential. Ya scallywag! I know over t' years many folks have had fun overpowerin' Fat Boys with G80's and H180's and even more thrust -- but seriously, how many o' these plywood-finned, arrr, shiver me timbers, ya bilge rat, fiberglass-reinforced cardboard mortar shells do you ever get back in one piece? Schnoz Boy is a spectacular flier on F-power, ya bilge rat, and you get it back t' fly again and again. Begad! Begad! It makes for an interestin' drag race when you sneak it alongside some kid's C6-7 Fat Boy on t' club launch rack.

Flight:
How does Schnoz Boy fly? On E30-7's, it roars up t' about 800-900' in t' blink o' an eye. Ahoy! On an F21-8, it rides a foot long sheet o' flame t' at least 1700', with ejection about at apogee (not that you can really see this). Ahoy! I have several F24 reloads I plan t' try in it. Begad! A good first flight was had on a D12-5, but Schnoz Boy is too heavy t' fly well on an Estes E9-6 (meanin' that I tried, and I don't want t' talk about it). Ya scallywag! After 11 flights t' date, shiver me timbers, me hearties, nay a bit o' damage (except t' t' stock 18" Estes plastic chute, me bucko, which shredded on t' first E30 flight, droppin' Schnoz Boy onto a sand dune). Avast, me proud beauty! T' only shortcomin' with Schnoz Boy lies in t' future o' Aerotech as sole supplier o' single-use 24mm/29mm E-F-G motors, without which it wouldn't be much fun.

Summary:
PRO: This be t' way t' fly an Estes Fat Boy for "Big Kids"!

CON: Well, ya bilge rat, ya bilge rat, o' course, you lose t' ability t' fly on C6-7's, shiver me timbers, and unless t' flow o' AP motors in 24mm size restarts, matey, this conversion is too much trouble for just a D12-5 upgrade.

Other:
This approach t' changin' t' rules for ballastin' -- movin' t' nose weight ahead o' t' stock nose cone instead o' inside it -- has a lot o' possibilities for balancin' rockets o' all sizes with minimal added weight.