catboat minimum freebord

Here is something more sophisticated. I developed this with the guidance of AH. You can see the small notation in red.

 

rxcomposite thank you for your "support", I was wondering how the catboat could behave with a fresh breeze situation. 17 - 20 kts and waves of 1 , 5 / 2.0 mt? with its good beam ...maybe you need to work on the deadrise angle of the hull to make it deeper; are there a bit "challenging" navigation with a catboat due to the characteristics of the type of boat?

 

Oh I see, you have not activated or tried to use the spreadsheet. You just have to enable it as it is in a protected view. You need only 2 inputs plus the choice of you want it the LR or Tupper way. The answer is all there and you can choose whatever method. It is based on LR, referenced from Tupper and Principles of Naval Architecture.

As for the deadrise angle and beam variation, you should study the pdf file I posted. All the hull variations that will fit your hull is there. If this is a study, you should try to understand what the pdf is telling you.

Increasing deadrise is counter intuitive. Sure you get a deeper draught but your hull center of buoyancy goes up, the KB. It is simple geometry. An inverted triangle will have a higher center of gravity. A rectangular/bilged one will have the CG at around midpoint.

Increasing beam is much better.

 

I entered in the excel sheet the two data referring to the designed speed and the lwl to understand how to interpret the sheet, ......... example: 22 ft of lwl ........ 5 kt boat speed ........... I read a freeboard result 0.29 ??? I have to interpret it as the minimum freeboard of the boat ......... can you please explain to me how to interpret the excel sheet?
In the pdf I think that figure 3 is the one that comes closest to the "shape" of my hull.

 

Using the same spreadsheet, I fed in the figures you gave. Freeboard is 0.29 meter. AH says (he always amazes me) "So, if you have at least 300mm, you should be good to go".

Looking further down the spreadsheet, the average wave amplitude (Hsurv) you will encounter is 0.23 m. amplitude. You will have to divide that by two as it is amplitude, meaning crest to through. Way lower than your freeboard and you are "good to go" if you are sailing in calm waters and calm winds (Sea State 1). On the right side is LR rule and that is for ships. I added it so I am not limited to small boats for which Tupper is.

The spreadsheet is calculated in such a way that it considers trochoid waves as superimposed on a perfect sinusoidal wave. No such thing as sinusoidal wave as it does not exist in the real world. Wave amplitude also varies as it is measured as average, or a percentage of it occurring at a time. No two waves are the same. That is why we have H 1/3 or H 1/10/ Meaning 1/3rd or 1/10th of the time.

Now try 17-20 kts you want investigated and see what you come up with.

For the PDF, you will have to find the ratios and coefficients of the boat you designed and see where you closely resemble the figure examples of the model tested.

 

I did it for you as it is fully automated. Same length, 19 knots, you need 1 meter freeboard at sea state 3, beaufort number 2, 1.2 meter wave height.

That will not look proportional. That means you should reduce the speed of the boat in such sea state (operating envelope) or you need a longer boat. The basic rule of thumb is that freeboard must be 1/10 of the length "to look good and be proportional".

 

Ok ... I seem to understand ... or maybe I'm wrong ... the value of the "designed speed" (kt) refers to the speed of the boat? ?

 

Another old thread, but it seems like there isn't much recent discussion of catboats.
Seems to me that any 24 foot boat with standing headroom is likely to have far more freeboard than necessary. Consider that the following boat, which did NOT have standing headroom, made it all the way around the world:
#TBT: Trekka Sails Again - 48° North https://48north.com/cruising/cruising-stories/trekka-sails-again/
The working catboats didn't just go out when it was flat calm, either.
Furthermore, a traditional Cape Cod catboat, which I think is the look you're going for, has a raised cockpit coaming which in some ways acts like freeboard. Ditto the distance from the rail to the cockpit.
If I was designing this, I think I'd try to get the motor as low profile as possible, so that I could lower the floor of the cockpit, which should help a little with stability. I suppose it would mean that, beyond a certain angle of heel, the cockpit might not be self draining any more. But I don't think that's a disaster.
What kind of sailing is this boat meant for? I'm guessing it's for coastal cruising, not for crossing oceans?

Finally, what is this design supposed to do that other designs don't? I've seen a bunch of catboat designs around this size, though I don't think any of them have standing headroom.
This one's a bit smaller: 20' Plywood Catboat MADAM TIRZA https://www.woodenboatstore.com/products/20-plywood-catboat-madam-tirza
Mystic Seaport has plans for the Breck Marshal, which is 20 feet long.
Chappele's American Sailing Craft has lines and rig drawings for 3 catboats around, I think, 24 feet long.
then there's this one: Ted Brewer Yacht Design https://www.tedbrewer.com/sail_wood/capecodcat.htm
Well, I kinda think you've seen a bunch, because your design looks a lot like a Cape Cod Cat already. If you're going to go to some trouble, seems like you should go for something you can't get in plans that already exist.

 

Coastal cruising..................gentle breeze - moderate breeze - force 3/4 - wave height 0,50/1,50 mt ?? - slight/moderate