Here is a popular article commonly used to explain COG. This copy looks to
be posted by Hot Rods on JRCBD and he is thanking Dom (AC Lazar) who I
assume either wrote it all or at least most of it. Either way, here you go:

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This is an old artical done by one of the best riggers in the hobby. And it's
still THE source for rigging info.

With THANKS to Dom ( AC Lazer )

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I know I posted this once before but since the Hack Job is was all lost. These
pictures show how the CG of every boat is a little different. The 3 Blast Cats
below all have the same motor, same hardware and 2 have the same Aluminum Tuned
Pipes and one has a Metal Tuned Pipe. All 3 Boats have the CG within 1/8" of
each other. The pipe is placed in the same spot on the 2 end boats and they are
all sitting in the same place on the Work Bench. The Blue Boat is the only one
that has an extra layer of cloth, wish my purple one did

Find your CG the Proper Way and balance the Hull itself. Forget about the Spark
Plug to transom measurement :\

...Mark out and mount your Hardware to the transom, using a Sharp Pencil works
best. Take your time using a Square, a good straight edge and measure twice and
drill once. Once all your hardware is mounted place “everything” into the Hull
temporarily. This includes an empty Gas Tank, Engine and Mounts, Tuned Pipe and
Mount, Radio Box and Mounts, Servos, Battery, Drive Cable and anything else your
will have your boat rigged with when its completed. You may have to tape some of
the items in place into the Hull to hold them until you mount them permanently.
Place a Magic Marker mark at your desired CG on the Keel and place a Brass Tube
or Wood Dowel under your boat at your mark. Balance the Hull and all its
components on its Keel on the tube by moving the motor and other internal
components at your mark until the Hull will teeter totter or balance on your
Brass Tube at your mark. The higher the powered motor the higher (more forward)
your CG (Center of Gravity) should be set. Once the Hull is balanced mark the
location of all the items and start building and mount them permanently. 30-32%
balance point of the total length of the hull itself is a good number depending on
how powerful your engine is

Thanks and Good Luck



MORE Info:



The higher the power that you rig a Hull with, "usually" means a higher % of a
CG number. The CG number is the % of the total length of the Hull that the Hull
itself will balance out at with EVERYTHING in it, measuring from the Transom
forward on it's Keel. Meaning, once it's all built and you put a tube under the
Hull, it will teeter totter on a tube at that number.

Examples:
45" hull with 28% = 12.60"
45" hull with 29% = 13.05"
45" hull with 30% = 13.50"
45" hull with 31% = 13.95"
45" hull with 32% = 14.40"

Examples: When I built my AC Razor 45 Super Stocker. It's rigged with a STOCK
25.4 cc Sikk Motor and a Go Nads Canister Muffler. My balance point is around
12 3/4". Meaning the Hull itself will balance on a Tube with everything rigged
in it at 12 3/4". The total length of the Hull is 45", and 28.5% of that is
12.83". Less wetted surface means more speed and that's what is needed in the
SS Class. It runs 52 mph all day long now and never reaches the 60 mph speeds
that a Modifed Boat will run with a Modified Engine and a Tuned Pipe.

I just built another AC Razor 45. It's the Custom Metalflake Purple one. It's
rigged with a strong M&D Production MODIFIED 260 Zenoah, and a Jeroen Tuned
Pipe. I have the CG on that boat set at 14 1/4". It means that the total length
of that Hull is the same 45", but the Hull balances on a tube at 14 1/4". It
translates to, 31.5% of 45" is 14.18". It's because of the Higher powered
engine and Tuned Pipe, so the CG needs to be higher.

OK, now, I'm going to confuse you even more:

Here is another AC Razor 45 that I built. It has a 10.0+ hp SG Racing 35cc
engine from Italy in it and Tuned Pipe. I rigged that boat to set a SAW Record
with, that's it. Again, less wetted surface means more speed. I didn't built
it to race, but just to set a SAW Record with. I built that one at 29%, or a
CG or 13.05". I wasn't even sure what it would do at speeds and was planning
on having to add weight to the bow to hold it down on the water. It weighed in
at almost 20 pounds and stayed on the water just fine with that CG. I also used
different Trim Tabs to hold it down on the water once it reached 74.0+ mph
speeds during my testing here before the SAW Trials. It did set an XLS36 Record
for SAW, but the Hull was destroyed when we tried to set an Oval Record with it.
I've built another one, just like it, last year, but didn't even test it yet.
I built it at 28%, looking for even more SAW speed.

YOU MUST FORGET about that spark plug to Transom measurement, it actually means
NOTHING. Just pretend it don't even exsist.

WOW, this turned into a friggen Novel

Good Luck and hope this helps

Attached Thumbnails For some reason, the pictures don't come over. But the whole
artical with pictures is on the first page of FAQ forum.

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Not certain but I think the pics referred to are in the archive at JRCBD. If
anyone knows a URL for the FAQ Forum where the pics are please post it. TIA.