Waterplane coefficient of an older yacht.

[GILow]

Hi all,

I’ve been tracking the weight loss of the boat during the refit and I’m getting some pretty startling numbers. Now I wonder if I’m using the right coefficient for the waterplane.

I’m using 0.6 as the coefficient, does this sound about right for a Kelly Peterson 44? (Mid 70s design, three quarter keel with skeg and Brewer bite, originally drawn as a canoe stern but foreshortened by the yard at final build.)

Matt

[Pete7]

Matt,

I had hoped that this website would list your KP 44, but I can't find it, perhaps I am not searching properly and its listed differently.

What is interesting is they have done the calculations to show what happens as the weight is increased and the amount the yacht will sink deeper.

These are the numbers for our 31. So 1000lbs lowers the yacht by 1 inch.

Perhaps you can find a similar model to estimate the effects of weight on yours.

https://www.yachtdatabase.com/en/review.jsp?id=Moody+31

[El Pinguino]

Given that your yacht is an empty shell you could - using internal measurements - use Simpson's 1st Rule to calculate the waterplane area and from there the coefficient.
By memory
A = h/3( Y1 +4Y2 + 2Y3+ 4Y4 + Y5)
where h is the common interval and Y1, Y2 etc are the ordinates - of the half hull.

Failed to get the font size right... I'm sure you can sort that.

[El Pinguino]

Try this

https://www.youtube.com/watch?v=rZ6A4lj8Kac

[El Pinguino]

And then you can work out the TPCI
https://marineinbox.com/marine-exams...immersion-tpc/

[GILow]

Yes, Simpson is the man if you are looking at a big change in draft. But I’m lazy, and generally use 0.6 x LWL x BWL to figure the waterplane area at the approximate draft where the change is taking place.

(0.84 seems to be the accepted norm for modern power boats/ships.)

But I’m getting some crazy numbers on the new boat so wondered if that coefficient is too high. It’s not super critical so I’m happy to wait until I can get to the travel lift at Kettering, but it would be nice to know.

[GILow]

Originally Posted by Pete7

Matt,



I had hoped that this website would list your KP 44, but I can't find it, perhaps I am not searching properly and its listed differently.



What is interesting is they have done the calculations to show what happens as the weight is increased and the amount the yacht will sink deeper.



These are the numbers for our 31. So 1000lbs lowers the yacht by 1 inch.



Perhaps you can find a similar model to estimate the effects of weight on yours.



https://www.yachtdatabase.com/en/review.jsp?id=Moody+31

That’s an interesting site, thank you.

I found a Robert’s offshore 44 which is moderately similar. Good enough for a rough estimate anyway.

However, if those figures are to be believed, I’ve stripped over 8.5 tonnes from the boat, which is silly.

Worse than the four and half tons I originally came up with. It should be closer to three.

I don’t know, I think I’ll just have to wait for the travel lift. But I was able to figure this out for the Swanson and I was pretty well spot on. I just can’t remember which formula I used.

[Chotu]

What will be next? Are you going to build a really lightweight interior?

[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.

[Bowdrie]

It's called "Prismatic Coefficient", and a number of ~.6 is getting into power boat territory. (bluff bow/square stern).
More likely would be a # in the .51>.53 range.
The waterplane area will be used to give the pounds-per-inch immersion.

[Lee Jerry]

Originally Posted by GILow

Hi all,

I’ve been tracking the weight loss of the boat during the refit and I’m getting some pretty startling numbers. Now I wonder if I’m using the right coefficient for the waterplane.

I’m using 0.6 as the coefficient, does this sound about right for a Kelly Peterson 44? (Mid 70s design, three quarter keel with skeg and Brewer bite, originally drawn as a canoe stern but foreshortened by the yard at final build.)

Matt

0.6 seems a little low. I think you’ll find Cw is much closer to 0.7 for most sail boats. (Meaning that’s what I would use as default without further information.)

sailboatdata.com lists the immersion for the KP44 at 1785 lb/in.

If you show your work we might be able diagnose more.

[GILow]

Originally Posted by Lee Jerry

0.6 seems a little low. I think you’ll find Cw is much closer to 0.7 for most sail boats. (Meaning that’s what I would use as default without further information.)



sailboatdata.com lists the immersion for the KP44 at 1785 lb/in.



If you show your work we might be able diagnose more.

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.

[GILow]

Originally Posted by Chotu

What will be next? Are you going to build a really lightweight interior?

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

[GILow]

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.

I think you make a good point here. She’s up so high now that her BWL is much, much narrower than originally designed.

[GILow]

Originally Posted by Bowdrie

It's called "Prismatic Coefficient", and a number of ~.6 is getting into power boat territory. (bluff bow/square stern).
More likely would be a # in the .51>.53 range.
The waterplane area will be used to give the pounds-per-inch immersion.

Well, 0.5 would certainly make the numbers lower, which would help with my problem, but still not quite low enough.