Center of balance

Designs for inboard or outboard power

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FFRCOBRA
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Center of balance

Post by FFRCOBRA » Tue Aug 25, 2015 8:39 pm

I have a question about motor placement and center of balance.

I'm building the belle isle and putting the motor in. The plans show the center of balance between frames three and four here:
Image

With the motor hanging on a load leveler, the center of balance of the motor and trans would be the chain on the orange hook (from the hoist mounted above). You can see the center of balance marked on the boat between frames three and four. Do I simply line these two up and put the motor there? This position puts the motor further back about 4 or 5 inches than what the plans show. Image

Am I thinking correctly?

Thanks
Craig

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sscobra
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Re: Center of balance

Post by sscobra » Tue Aug 25, 2015 10:03 pm

Craig, the center of balance on the plans is for the whole boat including the motor, not just the hull by itself. I would mount the motor as shown on the plans. There is some variance in weight between the different engine and transmission combinations, but I am sure the designer placed a typical engine as shown on the plans to achieve the center of balance. Once you get it in the water, if you don't like the way it sits (floats), you can add ballast in the bow or transom area to level it. Skip
Built the Glen-L Monaco, 2016.

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billy c
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Re: Center of balance

Post by billy c » Wed Aug 26, 2015 4:40 am

try to get it installed as drawn, but if anything, mount the motor slightly back of the drawn location. make sure your running gear has proper angle and engine height fit under the deck as it is close. i think your PCM sits taller than my Merc
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sands35
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Re: Center of balance

Post by sands35 » Sun Aug 30, 2015 1:35 pm

If you need to adjust the CG of the boat, you have a big battery you can move around as well.

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Bill Edmundson
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Re: Center of balance

Post by Bill Edmundson » Sun Aug 30, 2015 5:45 pm

As Billy C said, if anything slightly back!

Bill
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sands35
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Re: Center of balance

Post by sands35 » Mon Aug 31, 2015 7:16 am

Slightly back is simpler to trim out. Simple to move weight forward or add trim tabs. But if the CG is too far forward, it can't be trimmed out with trim tabs.

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steveh41
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Re: Center of balance

Post by steveh41 » Mon Aug 31, 2015 11:32 am

Agree with all of the above. Would double check to see if any mods needed to frame 3 to clear the trans. prior to installation.
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Bill Edmundson
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Re: Center of balance

Post by Bill Edmundson » Mon Aug 31, 2015 11:41 am

Frame 3 mods. are often needed. Be sure that you can get to the coupling and that you have room to get the packing nut open to replace packing.
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Ozzieboat
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Re: Center of balance

Post by Ozzieboat » Mon Aug 31, 2015 11:34 pm

FFRCOBRA
There is some good reading about "Centre of Buoyancy" in News Letter No. 100.
ozzieboat

FFRCOBRA
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Re: Center of balance

Post by FFRCOBRA » Tue Sep 01, 2015 2:53 pm

Thanks everyone for the input. In my original post I said that lining up the two lines put the motor about 4-5 inches further back than the plans show.
Most of the responses say to mount it slightly back from drawn location or error to the 'slightly back' direction.

Will the 4 or 5 inches I mentioned equal the 'slightly back' position that others have mentioned?

One more question- if slightly back is more like 2-3 inches and I'm talking 4-5 inches....am I over thinking this? Ha!

Craig

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Aussie
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Re: Center of balance

Post by Aussie » Tue Sep 01, 2015 11:18 pm

Hi Craig,

Yeah mate, you are overthinking it... sometimes a good thing but can drive you nuts also...
At 23 ft long those few extra inches wont be that important... if you are between frame 3 and 4 give or take some you should be fine although back is better....
Some pics of where my engine sits in the Belle...

Jamie
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