View Full Version : Center of gravity
Eodman
10-03-2007, 11:23 AM
Ok please explain center of gravity, someone mentioned 32% of the hull from the transom --- my question is does that change when you mount outboards to the transom? You are actually changing the overall length of boat when you add the outboard -- if I use the 1st CG then the batteries have to be all the way forward in the sponsons(remember I'm building a cat), 32% of 29 is 9.25 inches!
If I use the back of the outboards for CG then I gain about 3 inches back towards the transom and the batteries are right under the hatch cover!
Putting it on a broom handle I can balance the boat fine in the second example!
I know this will change somewhat when I add the electronics, but I don't think it will change alot!
IMO the best way to do it is ballpark it on your bench first. Be sure to leave room to slide your packs back and foward. Run the boat and make the ajustments as needed. Be sure not to run the boat wet or bow heavy for to long. It causes heat build up.
Flying Scotsman
10-03-2007, 12:00 PM
An outboard motor does not change the centre ballancing point. Take your measurements from the transom with the OB installed. If you have a multiple stepped hull ....take the measurements from the last step
Douggie
Doug is measuring it out like that what we should do every time we setup a boat? I have never put much thought into it really. I just tossem in and move the packs. Am I skipping a step here?
Flying Scotsman
10-03-2007, 12:32 PM
Doug is measuring it out like that what we should do every time we setup a boat? I have never put much thought into it really. I just tossem in and move the packs. Am I skipping a step here?
No question, the ballance point is critical. Dsmock has some great ideas on this point. Please note that, as I prefer to call it, the ballance point, is different for hull types.
Douggie
Eodman
10-03-2007, 12:43 PM
Thanks guys, I guess once i get this thing almost ready to go .... the cover to the hottub comes off for a test float! To late for the pool mI'm closing that uo tonight!
I'll post pictures and any other questions that I will come up with over on the Here kitty kitty thread!
Please remember this is our first build! Any suggestion to improve our build as we go along feel free to say something!
As of now Darin Jordan is putting 2 outboards on a Force 22 and he also can help you out!
Darin Jordan
10-03-2007, 02:00 PM
Please remember this is our first build! Any suggestion to improve our build as we go along feel free to say something!
My biggest suggestion would be to have EVERYTHING that is going in the boat on hand BEFORE you start glueing and cutting and figuring where things will go... Mock assembling everything is the only way to know for sure where things need to go, or can go...
With all this in place... try to get the CG set to around 30% or so, and then work from there... I have VERY few boats that like the CG much further back than that, but it depends on the hull design and how much power you are putting to it...
DO post pics! ;)
Flying Scotsman
10-03-2007, 02:45 PM
My biggest suggestion would be to have EVERYTHING that is going in the boat on hand BEFORE you start glueing and cutting and figuring where things will go... Mock assembling everything is the only way to know for sure where things need to go, or can go...
With all this in place... try to get the CG set to around 30% or so, and then work from there... I have VERY few boats that like the CG much further back than that, but it depends on the hull design and how much power you are putting to it...
DO post pics! ;)
Darin is dead right. I have made the mistake of installing components, prior to finding the ballance point. Move the compenents and when you find the ballance point, then install your hardware. This includes all components including batteries. The batteries should be adjustable..fore and aft..to allow for weight adjustments.
Doug
Ok so say if you placed the boat mocked up, on a flat surface, on a dowl it would balanced @ the CG correct? So then I can do the math and put the weight where it needs to be never having seen water yet.
Darin Jordan
10-03-2007, 03:00 PM
So then I can do the math and put the weight where it needs to be never having seen water yet.
No... but you'd have a good standard starting point that will be very close...
30% is just a guideline... every boat... every hull, every prop is a little different... You'll fine-tune from there...
But it's a MUCH better way (been there, done that) than getting it all assembled and painted up, only to find out you mounted the motor 2" too far back and don't have room to move the batteries any further forward to make up for the weight... Or that the batteries you had planned on using just won't quite fit now that you've added shrink wrap and velocro to the equation...
It's never cut and dry is it.....lol
Flying Scotsman
10-03-2007, 03:13 PM
Ok so say if you placed the boat mocked up, on a flat surface, on a dowl it would balanced @ the CG correct? So then I can do the math and put the weight where it needs to be never having seen water yet.
Yes, as long as all compenents within reason are placed in the hull where you want them and you have YOUR CG point. ...the one component that is hard to figure is the strut and rudder. Move them to the back of the hull and figure a few mm to the rear of the found CG point. Batteries that are moved fore and aft will compensate for any discrepencies.
Doug
Flying Scotsman
10-03-2007, 03:37 PM
Boy, Steven your site is slow at times on updates. Darin, your response is correct. Mine came in later.
Steven, do not get upset at my speed comment. My typing speed is probably the problem!! Thank you for the great site:smile:
Doug
Doug Smock
10-03-2007, 05:17 PM
My biggest suggestion would be to have EVERYTHING that is going in the boat on hand BEFORE you start glueing and cutting and figuring where things will go... Mock assembling everything is the only way to know for sure where things need to go, or can go...
With all this in place... try to get the CG set to around 30% or so, and then work from there... I have VERY few boats that like the CG much further back than that, but it depends on the hull design and how much power you are putting to it...
DO post pics! ;)
Douggie and Darin are right,.... floatation and the whole nine yards.
I like to set my monos up so that I can adjust the balance point (CG) between 27 and 31%. If you can do that you won't miss. I like to run most of them around 27% or so,(except in extreme conditions) loose is fast but you HAVE to be quick OFF/ on the trigger!:w00t: LOL
Good luck and post some pics!
Doug
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