TX18 Centre stringer fit and modification

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Vundu
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Re: TX18 Centre stringer fit and modification

Post by Vundu »

fallguy1000 wrote: Thu Oct 28, 2021 11:45 am Another issue with scaling and this is just for you to think about while waiting for JM.

Suppose the stringer is not a uniform height and this one is not.

Part of the stringer is above dwl and part is below dwl. If the sole contacts the stringer, adding 10% to the entire stringer is a clerical error.


Fallguy, I do appreciate your help. I am somewhat confused, maybe too long looking at this today.

I have made sure the bulkheads are aligned horizontally at sole level both by laser and water level. The vertical centreline is plumbed and aligned by laser.

I am obviously an amatuer and after reading the tutorial on scaling and mentioning in my foam core thread I wanted to scale all dimensions equally I decided to use 10%. I then increased every dimension on the plans by 10%, length, width and height and depth. So, for example, the slots in my bulkheads for my side stringers are 10% deeper and fit the side stringers perfectly. I then spread the bulkheads +10% on my form base, my plan is to add at least 1 additional bulkhead so as to keep the hull reinforcement correct.

I need to digest what your trying so hard to explain to me, I honestly throught that by altering every dimension by the same factor I would not change the hull characteristics / angles barring the transom height and static draft.

Hopefully by the time JM gets back to us I'll have 'clicked' :doh:

TomW1
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Re: TX18 Centre stringer fit and modification

Post by TomW1 »

fallguy, Vandu did it correctly, all dimensions must be increased by the same percentage. That means every line on the plans. If he does not want the sides as tall as they come out he ccan reduce them. Vandu at only 10% you do not need to add an extra bulkhead, just spread them about 3.6" apart, this is well with in tolerances, This is why Jacques limits scaling to 10%. You will also need to cut down the center of transom for the length of the motor shaft.

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

fallguy1000
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Re: TX18 Centre stringer fit and modification

Post by fallguy1000 »

TomW1 wrote: Thu Oct 28, 2021 2:09 pm fallguy, Vandu did it correctly, all dimensions must be increased by the same percentage. That means every line on the plans. If he does not want the sides as tall as they come out he ccan reduce them. Vandu at only 10% you do not need to add an extra bulkhead, just spread them about 3.6" apart, this is well with in tolerances, This is why Jacques limits scaling to 10%. You will also need to cut down the center of transom for the length of the motor shaft.

Tom
You are mistaken Tom. He didn't scale CORRECTLY. Read what he did. He scaled from the sole. Boats are not drawn from the sole. The sole is usually a fixed dimension above the waterline. Think the constant K.

The hull dimension varies from the design waterline. Some variable X.

When the boat is scaled, the scaling needs to be done against variable dimensions; not constants. So the boat would be scaled 110% of x and no change to K.

Also...this boat would scale much easier length and beam only.
Last edited by fallguy1000 on Thu Oct 28, 2021 10:50 pm, edited 1 time in total.
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jonnymac
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Re: TX18 Centre stringer fit and modification

Post by jonnymac »

my understanding was that in a scaled boat the original design waterline needs to stay the same, but all dimensions are still scaled. otherwise the hull shape changes. so the risk is that additional weight may need to be added to bring the boat down to the correct waterline.

the displacement volume has changed, but the additional weight of the materials used for the +10% may not be enough to bring the boat to the correct line.

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Re: TX18 Centre stringer fit and modification

Post by TomW1 »

fallguy i guess I missed that. But all dimensions still need to be increased by 10%, the height of the stringers for stringers for strength, the same with the frames, only the sides can be cut down if he does not like the height of the height of them but 10% is not that much of an increase to have a good height from the deck to the rail.
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

TomW1
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Re: TX18 Centre stringer fit and modification

Post by TomW1 »

jonnymac wrote: Thu Oct 28, 2021 9:28 pm my understanding was that in a scaled boat the original design waterline needs to stay the same, but all dimensions are still scaled. otherwise the hull shape changes. so the risk is that additional weight may need to be added to bring the boat down to the correct waterline.

the displacement volume has changed, but the additional weight of the materials used for the +10% may not be enough to bring the boat to the correct line.
jonnymac the waterline is set by Jacques as he designs the boat. If you add more weight that is the waterline, if you add less that is the waterline Jacques has set the static waterline at 5" at 1800lbs. The hull dry weight is 850 lbs. The draft PPI is 495lbs and increase/decrease the design water line by 1' if you add or remove that amount of weight from the 1800lbs. You need to add 950lbs to get to 1800lbs, you will never be exactly on that. :D

Regards, Tom
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jonnymac
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Re: TX18 Centre stringer fit and modification

Post by jonnymac »

what I meant by design water line for a +10% boat is if original was 5”, now it would be 5.5” for the boat to sit the way in the water Jacques designed it.
TomW1 wrote: Fri Oct 29, 2021 12:56 am
jonnymac wrote: Thu Oct 28, 2021 9:28 pm my understanding was that in a scaled boat the original design waterline needs to stay the same, but all dimensions are still scaled. otherwise the hull shape changes. so the risk is that additional weight may need to be added to bring the boat down to the correct waterline.

the displacement volume has changed, but the additional weight of the materials used for the +10% may not be enough to bring the boat to the correct line.
jonnymac the waterline is set by Jacques as he designs the boat. If you add more weight that is the waterline, if you add less that is the waterline Jacques has set the static waterline at 5" at 1800lbs. The hull dry weight is 850 lbs. The draft PPI is 495lbs and increase/decrease the design water line by 1' if you add or remove that amount of weight from the 1800lbs. You need to add 950lbs to get to 1800lbs, you will never be exactly on that. :D

Regards, Tom

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Re: TX18 Centre stringer fit and modification

Post by jonnymac »

jonnymac wrote: Fri Oct 29, 2021 7:48 am what I meant by design water line for a +10% boat is if original was 5”, now it would be 5.5” for the boat to sit the way in the water Jacques designed it.

PPI would also go up for +10% boat because the surface area of boat bottom went up
TomW1 wrote: Fri Oct 29, 2021 12:56 am
jonnymac wrote: Thu Oct 28, 2021 9:28 pm my understanding was that in a scaled boat the original design waterline needs to stay the same, but all dimensions are still scaled. otherwise the hull shape changes. so the risk is that additional weight may need to be added to bring the boat down to the correct waterline.

the displacement volume has changed, but the additional weight of the materials used for the +10% may not be enough to bring the boat to the correct line.
jonnymac the waterline is set by Jacques as he designs the boat. If you add more weight that is the waterline, if you add less that is the waterline Jacques has set the static waterline at 5" at 1800lbs. The hull dry weight is 850 lbs. The draft PPI is 495lbs and increase/decrease the design water line by 1' if you add or remove that amount of weight from the 1800lbs. You need to add 950lbs to get to 1800lbs, you will never be exactly on that. :D

Regards, Tom

jonnymac
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Re: TX18 Centre stringer fit and modification

Post by jonnymac »

jonnymac wrote: Fri Oct 29, 2021 7:48 am what I meant by design water line for a +10% boat is say original was 5”. now + 10% boat would be 5.5” for the boat to sit the way in the water Jacques designed it.
TomW1 wrote: Fri Oct 29, 2021 12:56 am
jonnymac wrote: Thu Oct 28, 2021 9:28 pm my understanding was that in a scaled boat the original design waterline needs to stay the same, but all dimensions are still scaled. otherwise the hull shape changes. so the risk is that additional weight may need to be added to bring the boat down to the correct waterline.

the displacement volume has changed, but the additional weight of the materials used for the +10% may not be enough to bring the boat to the correct line.
jonnymac the waterline is set by Jacques as he designs the boat. If you add more weight that is the waterline, if you add less that is the waterline Jacques has set the static waterline at 5" at 1800lbs. The hull dry weight is 850 lbs. The draft PPI is 495lbs and increase/decrease the design water line by 1' if you add or remove that amount of weight from the 1800lbs. You need to add 950lbs to get to 1800lbs, you will never be exactly on that. :D

Regards, Tom

fallguy1000
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Re: TX18 Centre stringer fit and modification

Post by fallguy1000 »

He needs to draw the waterlines on the crossmembers; especially the transom and he will see where he is messed up. Most likely the transom is incorrectly placed.

The sole should be a fixed dimension above dwl. Simple as that unless otherwise drawn.
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