TW28 Scantlings

[jacquesmm]

The answer by Fallguy was correct: no need to change the plywood thickness or fiberglass layers.
The deck panel panel span will be fine, no changes there either.

I am still surprised by all the features you put in just a small 10% increase. Years ago, I sketched a 34' version of the TW28 and was not able to put that much in there but I kept a large cockpit. You live in a cold country and give priority to the inside, here in Florida, we prefer large cockpits.

Rick is a goo example to follow. He did a good job on his boat.

[Rum Runner]

The answer by Fallguy was correct: no need to change the plywood thickness or fiberglass layers.
The deck panel panel span will be fine, no changes there either.

I am still surprised by all the features you put in just a small 10% increase. Years ago, I sketched a 34' version of the TW28 and was not able to put that much in there but I kept a large cockpit. You live in a cold country and give priority to the inside, here in Florida, we prefer large cockpits.

Rick is a goo example to follow. He did a good job on his boat.

Thanks for verifying the scantlings. The layout is just a rough idea of what we would like. It is based on the long pilot house version, stretched 10%. Ideally I would probably like to place the rear pilothouse wall centered in between stations G and H. Best compromise, I think.

[fallguy1000]

I'm not sure what you have ahead of the helm station, but by moving the head back to the opposite side of the kitchen, the entire settee can get better views, and you can probably make the bed bigger.

If the roof slopes forward some, the shower further aft is also more height and you don't need height in the settee.

The i beams are made from 4# corecell with 1700db each side. Three pieces. Two 2" and one 1" iirc. Glue them together with pb epoxy, then fillet and glass some 1708 on each side; like a 2" piece. The one incher is in the middle, of course.

I can stand on them at like 30" pieces. I did not calc the modulus..

[Rum Runner]

The answer by Fallguy was correct: no need to change the plywood thickness or fiberglass layers.
The deck panel panel span will be fine, no changes there either.

I am still surprised by all the features you put in just a small 10% increase. Years ago, I sketched a 34' version of the TW28 and was not able to put that much in there but I kept a large cockpit. You live in a cold country and give priority to the inside, here in Florida, we prefer large cockpits.

Rick is a goo example to follow. He did a good job on his boat.

From reading through the forums, my understanding is that to scale up, you have to scale all dimensions by 10%, but I also came across this statement in the study plan

"Some will ask to enlarge the boat. Lengthening is possible to maximum 10% but the builder will have to recalculate
most dimensions. Scaling her up in 3 dimensions would require new complete hydrostatic calculations and
scantlings, we do not support those changes."

Which one is correct?

[fallguy1000]

Boats are scaled in length and beam; not height.

In order to change the height of a boat; you must make accomodations for the increases by reducing weights up higher.

Otherwise, the boat gets heavier on top which can result in many issues..instability, rolling in a seaway, pitching, etc

[TomW1]

Boats are scaled in length and beam; not height.

In order to change the height of a boat; you must make accomodations for the increases by reducing weights up higher.

Otherwise, the boat gets heavier on top which can result in many issues..instability, rolling in a seaway, pitching, etc

Fallguy you are correct up to a point. You can only increase the height by a proportional % of the increase of the length and width. This is in the 3-4% area. This also keeps the proportions of the boat proper. Well that is what I remember from my courses.

Tom

[Rum Runner]

Boats are scaled in length and beam; not height.

In order to change the height of a boat; you must make accomodations for the increases by reducing weights up higher.

Otherwise, the boat gets heavier on top which can result in many issues..instability, rolling in a seaway, pitching, etc

Ok. I understand the concerns about being top heavy. I see some of the builders have increased the height of the pilothouse and cabin a few inches, and I have not read any concerns about instability. If I increase the beam, but not the height, this changes the shape of the hull. Although the difference appears minimal to my untrained eye, I have no idea how this will affect the performance and/or stability.

Your comments are greatly appreciated!

[fallguy1000]

If a boat is 108" wide and 28' long and 108" deep; it is not scaled 10% n all directions because t would then be nearly 120" tall and the gains would be in the cabin top and you'd be adding 11" to cabin heights which is silly.

But Tom's point is not invalid either. If you expand the cabin by 5.5" to starboard; the cabin wall bulkhead would move 5.5" out. There is certainly no reason to shorten the cabin wall to retain the head height at the center. So, say you have a rounded roof, you would probably take the roof up by an inch or two simply by extending the same radius to a wider chord.

There are practicalities here. No way for us to see them all, but the 10% is not scaled in height.

The same is true for bulwarks. If the bulwarks are 36" with say a 14" rail, there is really no reason to raise the 22" bulwark by 2.2" to then install only a 11.8" rail!

[fallguy1000]

It is likely that scaling will result in some errors. The best way to scale will be to spend a lot of time reading and reviewing the plans and knowing what will change before you enter the build phase.

You need to anyhow for your materials order. So, once you get the plans, really think through how they will change and something like wall heights you'll need to pay some close attention to...

Try to drive the errors at planning and not build.

The idea of 'adding a few inches' will bite you unless you understand the relationships well.

If one cabin BH is say 74" and another one is 76"; you can't just raise them both by 3" and expect things to work....they may work or that dimension change in the 74" may be 3.08" on the 76". You jist have to think through carefully. I also used some 10' straightedges in my build to make sure my plans made sense before glue.

[Rum Runner]

Yes, makes sense. That would be an unnecessarily high pilothouse roof. That is why I was trying to verify the statement in the study plan that recommends 10% increase in length only. I have the plans and have been working through them to understand all that would be involved. If it is just a matter of recalculating the dimensions and nesting. I think this is within my capability. I just want to get a confirmation from Jacques as to stretch length only, or both length and beam.

Thanks,
Trevor