TX18 centre of gravity

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Vundu
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TX18 centre of gravity

Post by Vundu »

Happy New Year all!

I have just taken my TX18+10% (all dimensions) off the strong back and have it on a trailer frame.

As I go forward I would like some advise on where the centre of gravity should be. I was thinking around station 'C' which is also the point of widest beam. That being said, I have no idea, and would be grateful for any advise.

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Re: TX18 centre of gravity

Post by fallguy1000 »

Just seesaw it on a 2x4
My boat build is here -------->

viewtopic.php?f=12&t=62495

Vundu
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Re: TX18 centre of gravity

Post by Vundu »

Thanks Fallguy.

I have made a pivot point to cradle the hull, however, what I am trying to get some help with is where exactly the COG should be in relation to the length of the hull.

From what I have managed to read a rule of thumb is approximately 1/3rd of the LWL measured from the transom. I do not know if this is correct, or not, and if it applies to inverted V hulls.

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OrangeQuest
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Re: TX18 centre of gravity

Post by OrangeQuest »

Vundu wrote: Sat Jan 14, 2023 9:22 am Thanks Fallguy.

I have made a pivot point to cradle the hull, however, what I am trying to get some help with is where exactly the COG should be in relation to the length of the hull.

From what I have managed to read a rule of thumb is approximately 1/3rd of the LWL measured from the transom. I do not know if this is correct, or not, and if it applies to inverted V hulls.
I was told 40% of length from transom, but I would suggest motor instead, if using a jack plate. But a lot on small boats can change that from gear, batteries, live wells and so on.
"that it isn't just an ordinary sort of boat. Sometimes it's a Boat, and sometimes it's more of an Accident. It all depends." "Depends on what?" "On whether I'm on the top of it or underneath it."
A. A. Milne

TomW1
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Re: TX18 centre of gravity

Post by TomW1 »

When Jacques and I discussed this topic, he normally made the COG abput 6" behind the console. This is taken from the hull and not the deck. Looking at the study plans I would go with this. The COG never changes you make weight adjustments forward and aft of it An adjustment for the jack plate and motor weight is made later. A 6" jack plate and a 70HP Yamaha motor would require a weight movement forward of the COG of 123.5lbs. which is half the weight of the motor, I don't know the weight of the jack plate. This doesn't take much move the console and 1-2" forward or add some other weight in a locker.

Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

Vundu
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Re: TX18 centre of gravity

Post by Vundu »

Thanks Orange Quest and Tom, thanks for your advice.

From a quick look at my scale drawings and the advise from you both it looks like the 40% forward is fairly close to the advise from Tom. I will follow Tom's advice and adjust accordingly. Thanks again!!

TomW1
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Re: TX18 centre of gravity

Post by TomW1 »

Vandu, I want to get a little more specific with you and balancing weights. Let's say you add a bait well of 12 gallons at the stern bulkhead, this will weigh about 120lbs and be about 7' from the COG so you must move an equivalent amount of weight forward of the COG and at an equal distance or at various distances that equal the 7' x 120lbs = 840lbs equivalent weight. You can move one thing or several things to meet this requirement.

The best way to do this is set up a spread sheet as you will have things both fore and aft that will offset each other.

Please ask if yoy have any questions.

Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

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OrangeQuest
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Re: TX18 centre of gravity

Post by OrangeQuest »

You can use a simple lever/fulcrum formula to figure how much weight needs to be in front of the COG. COG being the fulcrum. The motor and things aft of the COG are the load. All weights forward of the cog it the work force or counterbalance. And where you have movable weights, you can move them closer to COG or farther away from COG. Calculate the weight and distance each will be and add them all up to counter the fixed position of the motor. Make sure you measure the distance the motor is from COG.
https://www.engineeringtoolbox.com/levers-d_1304.html
https://www.vcalc.com/wiki/KurtHeckman/ ... Calculator
"that it isn't just an ordinary sort of boat. Sometimes it's a Boat, and sometimes it's more of an Accident. It all depends." "Depends on what?" "On whether I'm on the top of it or underneath it."
A. A. Milne

TomW1
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Re: TX18 centre of gravity

Post by TomW1 »

OQ there is no need to take the distance of the motor into account as Jacques has already taken it into account a general sized motor to establish the COG and placement of the console except for the calculation I already did for him. He needs to take all other measurements from the COG line and offset the weights he puts in the stern by an equal amount forward or vice versus. He doesn't have to move the same amount of weight an equal distance he just has to get an equivalent amount forward. For example, if you move 10lbs aft you need to move an equal amount forward, 10lbs x 5' - 50lbs equivalent. Now you can add that wight forward in one or several pieces. One would be 10lb 5ft forward of the COG., 2 would be 5lbs 5ft forward of the COG. or 2- 6.25lbs 4ft forward of the COG. This is the one thing your lever formula can't do OQ I believe.

Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

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OrangeQuest
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Re: TX18 centre of gravity

Post by OrangeQuest »

Maybe I need to post a spread sheet with the formula as to how to figure where a known weight needs to be placed at the correct distance from COG and you can adjust the load weight, load distance from COG and then do the same on the effort side of COG. But I don't know how well that formula works without entering the distance a given weight is from COG.

EDIT: I made the spreadsheet but don't know how to post without giving access to my cloud drive.
"that it isn't just an ordinary sort of boat. Sometimes it's a Boat, and sometimes it's more of an Accident. It all depends." "Depends on what?" "On whether I'm on the top of it or underneath it."
A. A. Milne

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