Waterplane coefficient of an older yacht.

[Chotu]

Originally Posted by GILow

Yep, I’ve used pvc honeycomb panels. The interior is now 90% built forward of the cockpit. Incredibly lightweight build.

Ooooh!!! this is a very interesting project.

Can’t wait to hear what you come out with for performance!

[Lee Jerry]

Originally Posted by rslifkin

Depending on the hull shape, the pounds per inch of immersion will start to change as the boat gets higher in the water (as the waterplane area shrinks). So starting from the design waterline, you may have to add, say, 2 tons to sink the boat 2 inches, but removing 2 tons would raise it 3 inches.

Originally Posted by GILow

Ok, that’s giving me pretty much the same result as my own calculations. Which is just insane.

Conservatively, the boat has come up 200 mm, more like 250 mm really. She’s out of trim fore and aft right now, with the bow right up and the aft only up 100 mm, so I took a reading midships where the boat is nice and close to the dock finger.

This is giving me six or more metric tonnes removed. My records are telling me it should be closer to three.

I think that between these two bits of info, you have your answer:
(1) The immersion TPI is not constant and decreases as draft decreases. That’s why it’s “per inch” (or centimeter) and not “per foot” (or decimeter).
(2) The immersion TPI is for level changes. The bow has come up a lot more than the stern which means a lot less volume than you are calculating / assuming by using the average draft change.

This easily gets you down to the 3-4 mt range you think it should be.

[GILow]

Originally Posted by Lee Jerry

I think that between these two bits of info, you have your answer:

(1) The immersion TPI is not constant and decreases as draft decreases. That’s why it’s “per inch” (or centimeter) and not “per foot” (or decimeter).

(2) The immersion TPI is for level changes. The bow has come up a lot more than the stern which means a lot less volume than you are calculating / assuming by using the average draft change.



This easily gets you down to the 3-4 mt range you think it should be.

Ok, point 1 makes good sense.

On point 2, I figured choosing the midship level change would average things out. But perhaps not, when I take into account the different waterplane coefficients of the bow and stern.

Well, when I strip the stern cabin I HOPE to get the boat back in trim. That is another half ton to remove at the very least.

[GILow]

Originally Posted by Chotu

Ooooh!!! this is a very interesting project.



Can’t wait to hear what you come out with for performance!

I’m both excited by the prospect AND a little nervous. (I have contingency plans for adding some lead ballast if needed.). But she’s scoring two salt water ballast tanks in alignment with the cockpit against the outer hull sides, which should help, up to 900 litres each side, though 600 litres per side should be adequate. I don’t want to overstrain the rig, which isn’t really designed for this sort of stuff, unlike a cat rig.

A few pieces of the puzzle to go yet, a lighter engine to install for a start, pus stripping the aft cabin, she’ll get lighter still.