Balancing Design Layout

Questions about boat repairs with our resins and fiberglass: hull patches, transoms and stringers, foam, rot etc.
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MSRiver
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Balancing Design Layout

Post by MSRiver » Mon Aug 26, 2019 9:32 am

This question comes from the other thread 1981 20' Dolphin build. Stringers are complete and I'm starting to fine tune where the gas tank and compartments will be. With that I have a good idea of where the original tank location, but never got to ride in it so I don't really know how it rode in the first place. The whole layout is changing from what it original was, and you can refer to that post to see my new proposed layout.

Let me know any additional information one might need to help out. Roughly have 16' running surface on bottom, gas tank will be roughly 336lb full original location for point load is 13.5' from 0 running surface, 150 mercury is roughly 425lb and will be on a 8-6" offset jack plate. Im planning to have tabs on it. The back bulkhead will be full length at the front of the live wells in the back, I was planning on have 2' of room (which can me less or more, not quite sure of how much room i want back there) then 16" from the front of the seat would be my console. Last pic i put up is in that set up. Ive looked up Center of gravity and mass and all that but not knowing where on the longitudinal axles it should fall im just not sure about.
Any help is appreciated.



Fuzz
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Re: Balancing Design Layout

Post by Fuzz » Mon Aug 26, 2019 2:59 pm

I have a thought but it might not work. Move the boat so the 40% point is over the trailer axle. You will still have some tongue weight so put a scale under the support and get a weight. Now place weights in the boat to simulate things and see how it works out. This way you can move stuff around as needed.

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Re: Balancing Design Layout

Post by DAYTRIP » Mon Sep 09, 2019 12:55 pm

You have three theoretical points you are dealing with. 1st is CG = theoretical point the boat weight acts through. COB= center of buoyancy which is point the buoyancy of the boat acts at rest. Then the last is COL=center of lift which is the point the lift from moving acts through. Ideally the all line up. I did a Bertram that I took the inboards out and put outboards on a bracket. On that I did a very simple balance calc to figure out how moving the weights around impacted the balance and how to adjust. I had no idea of the COB or COL. I just compared the before moments to the after moments and made them match as close as possible. Boat runs awesome. For example, use the transom as the reference point. Multiple the weight of a object/piece of equipment time the distance from the transom in the original configuration. Do that with any component that has changed location in the new layout. if the equipment is aft of the transom subtract that weight time the distance. See what you get for a number. now do that for the new configuration. the numbers should be the same in order for it to balance the same.

so for your example, 336lb full original location for point load is 13.5' from 0 running surface, 150 mercury is roughly 425lb and will be on a 8-6" offset jack plate. Im planning to have tabs on it. The back bulkhead will be full length at the front of the live wells in the back, I was planning on have 2' of

336x13.5-425x.5 = 4323.5 lbs-ft new configuration
lets say the engine was at the 0 location previously and the tank is 12' forward
336x12-425x0 = 4032 lbs-ft old configuration

basically the balance has changed. to balance you need to add (4323.5-4032) = 291.5 lbs-ft of counterbalance. So lets say you add that 18' forward of the 0 point you need 291.5/18 = 16.19 lbs of weight.

No idea if this is what you are asking and I tried to simplify but think it is now more complicated than need be. The center of lift is moved with the trim tabs dynamically. Ignore all if this isn't what you were asking for. :doh:

fallguy1000
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Re: Balancing Design Layout

Post by fallguy1000 » Mon Sep 09, 2019 9:05 pm

here is another thought

Install fuel tank and helm station later.

Use a 1-3 gallon tank. Get in an run it...Use jerry cans of water for estimates.
My boat build is here -------->

viewtopic.php?f=12&t=62495

TomW1
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Re: Balancing Design Layout

Post by TomW1 » Wed Sep 11, 2019 6:32 pm

DAYTRIP wrote:
Mon Sep 09, 2019 12:55 pm
You have three theoretical points you are dealing with. 1st is CG = theoretical point the boat weight acts through. COB= center of buoyancy which is point the buoyancy of the boat acts at rest. Then the last is COL=center of lift which is the point the lift from moving acts through. Ideally the all line up. I did a Bertram that I took the inboards out and put outboards on a bracket. On that I did a very simple balance calc to figure out how moving the weights around impacted the balance and how to adjust. I had no idea of the COB or COL. I just compared the before moments to the after moments and made them match as close as possible. Boat runs awesome. For example, use the transom as the reference point. Multiple the weight of a object/piece of equipment time the distance from the transom in the original configuration. Do that with any component that has changed location in the new layout. if the equipment is aft of the transom subtract that weight time the distance. See what you get for a number. now do that for the new configuration. the numbers should be the same in order for it to balance the same.

so for your example, 336lb full original location for point load is 13.5' from 0 running surface, 150 mercury is roughly 425lb and will be on a 8-6" offset jack plate. Im planning to have tabs on it. The back bulkhead will be full length at the front of the live wells in the back, I was planning on have 2' of

336x13.5-425x.5 = 4323.5 lbs-ft new configuration
lets say the engine was at the 0 location previously and the tank is 12' forward
336x12-425x0 = 4032 lbs-ft old configuration

basically the balance has changed. to balance you need to add (4323.5-4032) = 291.5 lbs-ft of counterbalance. So lets say you add that 18' forward of the 0 point you need 291.5/18 = 16.19 lbs of weight.

No idea if this is what you are asking and I tried to simplify but think it is now more complicated than need be. The center of lift is moved with the trim tabs dynamically. Ignore all if this isn't what you were asking for. :doh:
A clarification. The measurement for COG, COB and COL are all taken at the transom at THE WATER LINE. Jacques uses a number between 40 & 45 percent for his various boats depending on use for the COG. Good luck on your rebuild :D
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

MSRiver
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Re: Balancing Design Layout

Post by MSRiver » Wed Oct 02, 2019 11:57 pm

Thanks Daytrip, and yes that what i was talking about. And see really with a load distribution that point load can only go to 10.5'. But since then i figured I've got a good idea of where the original tank was and scooted it as forward as i could go. Im going to put below deck storage or fish box might serve as both plus i need some storage, at this point i don't plan on putting back a casting deck yet. So that is kind of the only hindrance, smallest fish box that would make since is 32". So i got to move the gas tank about a foot forward from original. In the end ill have trim tabs and I've already got most everything else more forward than it was before anyways I'm sure she'll still float by the end of it.

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