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Easy way to find the C.G on boat

Old 09-24-2012, 08:45 PM
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The water does drain when on a lift and then stops. My guess has been that the configuration holds the water along its length until tilted. Now that ain't so good to have 6 gallons of salt water splash the bottom of you engine once you go out.
I have been on the Manufacturer's forum and find that others complain that they can't pump the bilge down.
Probabkly going to install a second manual Mini pump at the transom.
Thanks for the thoughts.
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Old 09-25-2012, 11:51 AM
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Still,
We have dual bilge pumps on one float. The rear pump is not in the bilge but has a 3/4" hose run to the front of the transom drain plugs. This gets all the water out out of the bilge by activating the pumps after each run....except for a few spoonfuls.
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Old 09-25-2012, 12:48 PM
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OK, I've asked this question before in another post. Is there an ideal center of gravity in a non step V hull performance boat? If the boat doesn't fly level then obviously weight could be shifted fore or aft to adjust but that could be done by trial and error without ever know the CG of the boat (knowing the CG and calculating it after several adjustments may help dial it in quicker but without a known target it is still trial and error). I have a 28' boat with twin engines, bravo drive and extension boxes. It is very stern heavy. If there was a target for the ideal CG I could adjust some items to try and reach that condition and then sea trial it to see how it works on my specific hull. Anyone?
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Old 09-25-2012, 03:56 PM
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With a 30' Scarab and a similar set up, we ended up lengthening the inter strakes to be about 6" behind the CG (depends to speed).
This was with method that was suggested by Mercury at Lake X. My suggestion is to test first, build some temporary strake extensions out of oak (screw & glue). When you are happy (boat is stable with the right attitude), you can get a FG shop to build permanent strake extensions.
This was the single most important thing we did to this boat Scarab over the last 25 years. In 1987 the boat ran mid-60's, we ended up at 100+ and it was rock stable. The length of the strake extension will depend on the speed...trial and error on flat water was the test method to finalize the optimum strake length.
If you are looking for a quantum leap in speed on an older boat, work on the bottom and drive height.
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Old 09-28-2012, 09:52 AM
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I took a quick measurement on the water line and transom depth. I then graphed out a large triangle with the water line horizontal (A), the transom line verticle (B). The next step was to divide the triangle vertically in steps from the transom. This way I could slide the length of the horizontal (B) to form an overlaying triangle. I calculated the total area of the first triangle and then divided it in half. That way I could find the area of the "overlaying triangle" to equal the 1/2 area of the "Total triangle". Area=Displacement. So 1/2 should be CG ???
First rough test: CG 42.5% / 57.5% of length . Hummm close to 50/50

Next idea is to attach a 4' level near midship while afloat and shim it to level, from outside the boat. Then have some one get on the boat and take a step at a time forward until the level is back to balance.
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Old 09-29-2012, 11:05 AM
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Went back to my fuzzy math and found that I mixed up "equal areas" with the length measurement. CG was not 42/58. Sorry ! Equal areas (displacement) fell roughly 24" from transom using this therom.
I tried the "Level float" test yesterday. Interesting. I expected my canal to be calm as it is a secondary feeder and just as I set up, naturally a boat passed. Ever try to hold the boat steady and look at a level ? Things settled down and I attached the level.I then shimmed the level and had my wife (not 200#) step from the stern and go forward one step at a time until the level came back to balance. I marked the spot with tape. Had her go forward and back to justify. This gave a CG closer to 70" from stern. More reasonable.
Scratch the equal area therory for now. Remember fuel level and shifting can affect the second test.
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