Draft Depth Response

I have worked on the hull of the 10m catamaran on and off over some time now.

The draft was 500mm, which I recently reduced to 300mm. The drag (delftship) is now considerably less for the same payload, so speed would be up. BUT -


Assume the draft is 0, the boat will sit right on top of the water. How will this affect stability? How about in choppy water ? How about if the wind is up ?

Assume the draft is more... ie 500mm, logic tells me the boat is more 'hanging' in the water, instead of sitting loose on top of it.

Ok, it's a cat so it should be stable, but still. If the water is choppy and you have some draft, the effect s going to be less than when it is sitting right on top of the water, right ?

The boat with more draft is going to be less windy.

Now where is the golden draft level one would be looking for ?

 

Fanie... of course we all do know well that zero draft means zero weight, so yours' is just a fantacademic question, right?
Or are you talking about the condition of maximum loading on one hull only, where the other one is just skimming over the water?

 

Just considering the extremes. Usually gives one an idea of where not to go or what to look out for. Draft cannot be 0, but would it be ideal ? Probably not.

I'm trying to determine where the ideal draft should be. I know one has to cater for weight, but it can be controlled to some extent with the hull shape.

 

From a practical point of view, if you throw a polistirene block on the water it gets blowed around quite a bit. The draft is near 0, and the only thing that keeps it from rolling over is probably the wetted surface area.

Adding a weight to it, the wetted area is increased just slightly, but now it is more stable and is not blowed around as much.

 

Consider a box immersed in water. Ignore friction and other forms of vertical drag. Say that the amplitude of it's vertical oscillations is 500 mm. That means an excursion from -250 mm to +250 mm around the static draft.
As long as the static draft is more than 250 mm, its actual value does not influence vertical accelleration, because the accelleration depends only on the amplitude of these oscillations. It doesn't depend on it's mean (static) value.
But since a cat hull is not a box with vertical walls, note that overhangs and flares above the WL will change this reasoning to some extent.

The same applies to Archimedes force. What matters for the stability is the difference between the instant and static displacement values.

P.S.
I wrote this before reading your last 2 posts. Let me check if that's what you intended...

 

Ok, now I got what you mean. What I wrote above is valid for a given shape of the box, with varying draft.
What you want to know instead is how does the stability change when the shape is changed, given the static displacement, correct?

Yes, you're right - A narrow and deep hull does accelerate less verticaly than a shallow and large one. And also sinks more for a given increase of vertical load. That's obvious.
So you have to decide first on the max. acceleration you want to obtain, than you play with LWL, B and T until you have a draft that will give you an acceptable ride, bearing in mind that the power and stability curve will change for each new configuration. I don't think anything like "ideal" in this case exist, outside of our heads.

 

Valid point. So the ride will remain smooth until the excursion reaches the bridgedeck hight, or when the excursion frequency becomes such wrt the hull lenth that it will cause the boat to dip.

 

Not sure if I understood your question. The narrower hulls go smoother in the waves, with less vertical accelerations, and less braking.
I think a semicircle has the less wet area, but it seems reasonable to me to go deeper than this at least if you are getting a length/beam less than 15 or something... I remember playing with a spreadsheet, I think it was called hullcalc.xls. Dunno where i found it. The optimum hull for a given displacement (and speed) was always terribly long, and if you put restriction on length you'd end up with a deeper than semicircular section - often more draft than beam.
Be sure to get your transoms (if you are having such) out of the water, and the bows into the water.

 

Hello Sigurd,

I would like to see that spreadsheet. The hulls in my case can only be 10m, I would love to go 12m but space allows 10m max.

Is there a disadvantage if the hulls of a cat has too shallow a draft, and can a draft be too shallow ?

The transoms are out of the water.

 

For a cat, I don't believe there IS a such thing as "too shallow" draft. But keep in mind that the really wide hulls will have a worse "fineness ratio," and faster vertical acceleration; so I'd recommend going with at least 10cm or draft unless you're wanting to build an 85kg carbon-fiber over honeycomb racing cat for single-sailing only.

I think as long as you keep your hulls realistically narrow (say, at LEAST 8:1 fineness ratio), and have plenty of lateral surface on your daggerboards/centerboards to eliminate side-slip, the shallower drafts should work out great.

Attached is a linesplan for a 22.5ft (6.9m) cat I'm building with an 0.75ft (0.225m) draft for the Texas 200 this year...I'm planning on putting a VERY shallow "full-keel" down the centerline of each hull to keep the draft to a VERY scant 1 foot (0.3m). Now, this boat was designed for just about everything BUT speed, but it shows the possibility of building a cat with EXTREMELY shallow draft...and it still displaces 0.8+ tons, even in fresh water.

 

Hi Robherc,
Just one consideration/question - if your cat is not intended for speed, then why did you choose to give it wet transoms? It will add resistance in displacement speed range. Or was it just a necessity to add volume aft and keep the draft as shoal as possible, for a given weight?
Cheers.

 

It was Malsmith (his username here) who made the spreadsheet. I've requested he send it to me or post it here.

For a lot of theory about low drag narrow ships, check out Leo Lazauskas' pages here: http://www.cyberiad.net/hydro.htm

It is not possible to have too shallow draft I think, but it is easily possible to have too wide beam! So I favor a high prismatic, low rocker, at least in the front - transoms usually need rocker. Only for quick turning would I consider to cut off some of the forefoot - making sure it is not so much that the waterline length is ever compromised when sailing or motoring.

Rob Denney proposed, but I'm not sure he ever built, a hull with bows that could be folded up for mooring in tight spaces.

I am planning to build only half the length of the hull for my proa inside, then gluing the two parts together outside.

Daiquiri - I think the "displacement speed range" is the only speed range a narrow catamaran will ever see! Putting flat or round bottom transoms at a correct height, so that it does not lift water (too high) or makes a tall rooster tail (too low) sounds very difficult!

 

I found it in his folder at proa_file yahoo group.
There are other spreadsheets there to look at as well.

 

Hullcalc Version 4.9 available at

http://groups.yahoo.com/group/proa_file3/files/DesignersTools/

 

thanks

edit: I realise there is a lot of controversy about the immersion of the transom. My 20' cat is very sensitive to trim. I get another knot or so from lifting the transom, even after the drag of the T-foil. The wake becomes much flatter. This is at about 11kt.