TX18 Centre stringer fit and modification

[Vundu]

Hi Guys, a little help please. Maybe I can't see the wood for the trees!

Assembling my form for my TX18+10% build. All bulkheads side stingers and transom appear to align correctly. When it comes to the centre stringer it aligns at Bulkhead C then rises above D by 5mm and then above MW by 12mm, but flushes perfectly at the transom with both side stringers and the transom V. I searched the forum and found a post from 2011, but it seemed to result in no conclusion.

The plan diagram for Centre stringer shows a curved line below the straight line marker--- see pic. This looks like it should fit to the bulkheads and align to the transom, but there is no offset measurement. The diagram also shows notched where the stringer meets the bulkhead, but if I cut these to make the sole side match the bulheads then the stringer will be at least 12mm low on the transom.

note curved line below top line and notched on bottom

I am worried about changing the angle of the hull incorrectly as the inverted V becomes a slight V at the stern and I assume the transition is important. Any help would be appreciated.

transom aligns on side stringers and centre, MW 12mm low

[jacquesmm]

It must be straight. That is why I wrote "straight above that line. I don't know why there is second line with a camber superposed on it.
I will check my version of the drawing later today: I am on the road now.

[fallguy1000]

Point of clarity...

The transom cannot grow by 10%, yes or no? A 21.5" transom cannot be built to 23.65" high.

The waterline in a 10% bigger boat remains the same and the baseline at the transom remains the same, yes or no?

If you make the boat bigger by 10%, then there needs to be some way to resolve the transom back to the original height. (This is not done!!!)

Based on looking at your pictures, it looks like you are trying to scale the height and I believe this may be causing the issues?

[fallguy1000]

You cannot scale the height.

Doesn't this picture show you attempting to do just that?

You would create some hook or rocker in the hull as drawn and the boat would perform badly.

[fallguy1000]

It must be straight. That is why I wrote "straight above that line. I don't know why there is second line with a camber superposed on it.
I will check my version of the drawing later today: I am on the road now.

See my comments. I see some notes on the pictures that concern me.

[fallguy1000]

Also, the boat should be getting built with the waterlines level to the world.

I am having a hard time believing your waterlines are level with all the variation going on...I could be wrong here...

...but I think the height scaling is throwing things off as well...

The good news is if you made everything too big; you should be able to repair them. Wait and see what JM has to say, but please post back what you did with the heights.

[Vundu]

Point of clarity...

The transom cannot grow by 10%, yes or no? A 21.5" transom cannot be built to 23.65" high.

The waterline in a 10% bigger boat remains the same and the baseline at the transom remains the same, yes or no?

If you make the boat bigger by 10%, then there needs to be some way to resolve the transom back to the original height. (This is not done!!!)

Based on looking at your pictures, it looks like you are trying to scale the height and I believe this may be causing the issues?

HI Fallguy, thanks for the input.

I am making the forms from MDF and the hull sides for half the hull from cheap crate plywood as I wait for the marine ply etc to arrive. I would rather modify and adjust cheaper easy to obtain materials till I get it right.

I have scaled all measurements by 10% and used the sole line as my base line as per the drawings. So I measured up and down from that baseline and left and right from the centre line. I increased all measurements by 10% so as to keep the scale correct. As I see it if I only increased the scale of the length of the stringers then all bow curves would be wrong. I cannot see how I can scale some components and not others.

I agree the transom height cannot be +10%. My plan was to rectify by measuring from the V of the transombottom up (down when inverted) to get the 20 inch finished transom height. As I wanted higher gunnels I was going to cut down for the transom forming a U, leaving the sides higher.

[fallguy1000]

Point of clarity...

The transom cannot grow by 10%, yes or no? A 21.5" transom cannot be built to 23.65" high.

The waterline in a 10% bigger boat remains the same and the baseline at the transom remains the same, yes or no?

If you make the boat bigger by 10%, then there needs to be some way to resolve the transom back to the original height. (This is not done!!!)

Based on looking at your pictures, it looks like you are trying to scale the height and I believe this may be causing the issues?

HI Fallguy, thanks for the input.

I am making the forms from MDF and the hull sides for half the hull from cheap crate plywood as I wait for the marine ply etc to arrive. I would rather modify and adjust cheaper easy to obtain materials till I get it right.

I have scaled all measurements by 10% and used the sole line as my base line as per the drawings. So I measured up and down from that baseline and left and right from the centre line. I increased all measurements by 10% so as to keep the scale correct. As I see it if I only increased the scale of the length of the stringers then all bow curves would be wrong. I cannot see how I can scale some components and not others.

I agree the transom height cannot be +10%. My plan was to rectify by measuring from the V of the transombottom up (down when inverted) to get the 20 inch finished transom height. As I wanted higher gunnels I was going to cut down for the transom forming a U, leaving the sides higher.

This is incorrect afaik.

Boats, (like this one) when scaled are only scaled in the beam and length. Consider your plan wrong until you hear from JM. If you want the sole 30mm higher; it is added straight as 30mm to all BH tops.

If you want a higher sheer; that is done with the hullsides; never bulkheads or stringers unless you were going to set the gunwhale on the edges of a bulkhead. And then you'd only adjust at the edge for the sole to remain same if sole is bh top.

I am very certain you have scaled wrong and I saw it on the drawing.

[fallguy1000]

Also, if you only scale beam and length; the inverted vee is only wider and longer as expected.

also, you would never scale in the height from the sole; the boat is drawn on the waterline usually

Wait for JM to get involved here. I am only book smart and the scaling tutorial here is not very good. Scaling an inverted vee is a serious adventure. Glad you brought up problems early.

[fallguy1000]

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.


Use this stringer for an example.

2000mm long
Transom depth of stringer 100mm, top of stringer is sole.
Forward bulkhead depth of stringer 200mm, ""
Design waterline at transom 70mm from baseline, 30mm from 'sole'.
Design waterline at forward bulkhead 170mm from baseline, 30mm from 'sole'.

Scaling from dwl 10% (still an error because transom is affected)
Transom stringer height 77mm plus 30 to sole. Total 107mm
Forward BH 187 plus 30. Total 217mm

Scaling from sole (worse error because sole is affected and not level)
Transom 110. Less 70mm dwl to baseline, sole is 40mm above dwl.
Forward BH 220. Less 170mm dwl to baseline, sole is 50mm above dwl..??? Bad

Do you see how the waterline, also the sole, is no longer level in the height scaling from the sole?

The design waterline must he laid out on all your parts and shot with a laser or a level of some type.