TX18 Centre stringer fit and modification

[jonnymac]

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

why? I ask because I see so little details on this forum about the why and that is as important as doing it.

[Vundu]

Good afternoon Guys.

I am just a layman, and appreciate all the comments and assistance. That being said I am trying to learn, and will be asking questions till I understand, so please be patient with me.

When I wanted to scale the TX18 I posted in the core material thread and mentioned scaling up in all directions. I also said I wanted to lift the sole a few inches (3-4) and the gunnels similarly. On this JM did not think there would be a problem as long as I stayed within 10%.
MY engine is heavier than standard, and I wanted to add livewell and 20-30 US gallon fuel tank, and cooler.

There are 2 issues as I see things:
1) I have scaled every dimension by 10%. The base line on the TX18 plans is also the underside of the sole. I now understand that I am being advised to scale only from the waterline, and to scale beam and length not depth below the sole. I have attached below a diagram showing Bulkhead C, A- standard (Purple), B- Green- adding 10% to beam only, and C- adding 10% on all dimensions (which is what I have done). I have not arbitrarily lifted the sole, or the gunnels, it is all part of the 10% increase.

Am I correct in saying I should scale the beam as per picture B?
Considering that the TX18 is designed to rise above the water when on the plane is the static draft an issue?

2) The curved line on the plans for the centre stringer, and the centre stringer lifting above Bulkhead D and MW but flush with the transom.
When I built my first TX18, which was not scaled I recall having the same problem, and instead of asking for help I cut out the offending material making a straight line from Transom to MW and then another from MW to Bulkhead D. This created somewhat of a ramp effect between MW and Transom, and I do not think this is good, especially with a larger engine planned for.

I have scaled my plans on the computer and compared them to what exists on the workshop floor. The straight line of the centre stringer sits 12mm above the bulkhead MW and 5mm above bulkhead D. When I compare the distance from the curve to the straight line on my computer I find they are 11 and 4.5mm respectively. So, if I created the curved line as per the plans I would not have the problem at hand. All this being said, the plans clearly state to follow a straight line. Hopefully JM will be able to shed some light when back from his break.

[TomW1]

Vandu you are correct in scaling every direction by 10%, that means every line on the plans. It doesn't matter where the base line is just scale the lines by 10%, there will be a new base line for the scaled boat. The stingers and frames to support the extra weight of the sole aad the wider spacing of them. I don't know if you have read this but it definately says scale all dimensionss: https://boatbuilderceontral.com/support ... -plans.pdf Since the TX18 is such a specialized boat I would post a note for Jacques in the Questions Before *** above this about Scaling The TX18 10% you may get a faster answer that way. As for the transom yes scale it but cut down the motor well to the designed height probably 21 1/2", other wise the sides of the transom will not meet the height of the sides of the boat.

Good luck on your build, Tom

[fallguy1000]

If you had this problem on the unscaled version; then there is something in the plans or your interpretation that is incorrect.

But not drawing the waterlines is not helping because you ought to be able to reference details.

The associative property of math allows multiplying each or combined by a common amount. Disregard my comment saying you cannot. However, if you want to maintain the same sole distance above dwl, then I am right.

So...

1.1 • (k + x) where k is the sole above dwl and x is the dimension from dwl to hull is same as 1.1 • k + 1.1• x,
but if you want k to remain the same, then scaling is done as 1.0•k + 1.1•x..... verstehen Sie?

Why would you want to raise the sole or change the depth of the bulkheads and stringers? They are within their limits at 10% and require no increases!

But if you know the error is 12mm at mw, then you ought to know the error at transom.

This is all for the designer, I have tried to help, but have not. If the issue was apparent in the unscaled version; it is either the same error repeated by the builder or in the plans.

[rick berrey]

There is no cabin , so wind is not much issue if you scale above the waterline , no real safety or performance downsides . I would go with C , and lift my sole if I wanted to , transom cut like Tom said would have to remain the same from waterline to top of cut . Waterline might change a little due to scaling , add an inch , put it in the water and trim as needed .

[Vundu]

Thanks guys for the input. I will keep going with the +10%. Once I figure our the centre stringer quandary.

Fallguy, To answer your question about why to raise the sole. I wanted to add some space for storage. Having seen whats available with the 10% increase I won't be adding any more to the sole.

I am going to find some scrap and cut bulkheads C, D and MW as well as the centre stringer without scaling and see what happens. I'll keep you posted.

I know JM is away for a short while yet, I'll try and get his input when he's back and settled.

[fallguy1000]

Vindu-you should be able to determine the following..

length of center stringer
Distance from sole to dwl all stations @ center stringer (transom,mw, etc).

Distance from dwl to hull all stations @ center stringer.

Then you should be able to graph the distances at each station and determine if they are indeed a straight line.

No need to cut anything.

Send me the numbers in a private message. I'm not splashing Jacques plans. I'll graph it out and tell you what I get/find.

[fallguy1000]

Sorry, Vundu...yours is not an American name and I am an American. Will work on spelling it right!

[Vundu]

Sorry, Vundu...yours is not an American name and I am an American. Will work on spelling it right!

No Worries Fallguy! Vundu is a type of large catfish we get out here . I was gone before your kind offer to help came through.

I left home early today and and went off to my cave to see how much of a 'numpty' I really am. I drew out the centre stringer without any dimensional adjustments on MDF, extrapolated the curve on the plan, and cut a few pieces of MDF the exact size of the slots in the bulkheads and put it all together.....

Note, I used the .pdf drawings from BBC and a free online very basic drawing program to do it...... The results are below:

full size mock up

The height of the slots in the bulkhead appear to follow the curve better than the straight line.

closeups std.docx

(8.86 MiB) Downloaded 5 times

I then drew the curve on the full size Stringer and obtained similar results:

Centre stinger to Bulkhead C and B fits perfectly. Stringer is proud to A by 8mm. I will check the curve and measurements monday.

I do not think the curve on the plans is a mistake.

[Dougster]

I'm thinking the curve is supposed to be there too. It is a twin V hull with a complicated center stringer to fit I suppose. Here's a pic of the bottom of the hull in construction before stringers from DaveTx's build. Scroll down to the bottom of the page for the pic:

viewtopic.php?t=15242

Dougster