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, arrr, negatin' t' Oomph you thought you were adding. Blimey! 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, arrr, matey, too) and doubled on one side with 1/64" sheet plywood (a snap, this stuff cuts with a scissors) epoxied on. Ahoy! A 1/8" Lite-Ply-tm doubler be constructed for each o' t' cardstock motor centerin' rings supplied with t' kit. Blimey! Arrr! And that's it. Arrr! T' complete t' make-over, 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), shiver me timbers, anchored through t' body tube with a knot covered with a plastic fairing. Copyin' a final touch I saw in a magazine, 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. Arrr! T' "supplemental" nose cone is then affixed t' t' stock Fat Boy cone, shiver me timbers, 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, well ahead o' t' Cp (3.9" from t' tail). Aye aye! Avast! I call t' finished product "Schnoz Boy", for t' obvious reason. Avast, me proud beauty! See photo.

This be an easy conversion. Ya scallywag! Begad! All t' stock pieces were used, me hearties, and its a great kit. Ahoy! T' through t' wall fin design makes this easy t' convert t' higher power. Avast, me proud beauty! Ya scallywag! You do need t' add a proper recovery system for E-F-G power, arrr, me hearties, shiver me timbers, however, and reinforcin' t' balsa fins for t' much higher speeds is essential. 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, me hearties, matey, how many o' these plywood-finned, fiberglass-reinforced cardboard mortar shells do you ever get back in one piece? Schnoz Boy is a spectacular flier on F-power, me bucko, and you get it back t' fly again and again. Well, blow me down! 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, me bucko, it roars up t' about 800-900' in t' blink o' an eye. Aye aye! On an F21-8, arrr, it rides a foot long sheet o' flame t' at least 1700', ya bilge rat, with ejection about at apogee (not that you can really see this). Avast, me proud beauty! I have several F24 reloads I plan t' try in it. Ahoy! Avast! A good first flight was had on a D12-5, arrr, 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). Avast, me proud beauty! Ahoy! After 11 flights t' date, arrr, nay a bit o' damage (except t' t' stock 18" Estes plastic chute, arrr, shiver me timbers, arrr, which shredded on t' first E30 flight, matey, droppin' Schnoz Boy onto a sand dune). Well, blow me down! T' only shortcomin' with Schnoz Boy lies in t' future o' Aerotech as sole supplier o' single-use 24mm/29mm E-F-G motors, ya bilge rat, 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, o' course, you lose t' ability t' fly on C6-7's, and unless t' flow o' AP motors in 24mm size restarts, 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.