Underwater hull form for sailing catamarans

Richard my only useful VPP is for propeller driven vessel.

For the WFC estimate above, I made a spreadsheet to balance forces on the cat at different angles to the wind assuming a hull speed of 20kts. I then determined sail and keel sizes required to generate the forces. It was a laborous and iterative process. Hence why I narrowed it down to the limit case of just flying one hull.

My interest is in propeller driven boats where you do not have to contend with large force couples that upset trim. I also work on the basis of operating in smooth water.

Rick W.

 

OK cheers Rick. If you find that your test models perform well in waves as well as smooth water, then I guess you've got the tools you need. I think it would be nice to at least have a VPP for waves (other than the waves generated by the hull). Then you could simulate what it would be like to be surfing the biggest waves nature is likely to offer.

 

Ahhh...those would be rougue waves and the simulation would denote massive amounts of bowel movement in the water and stained shorts to match each floater. That is called a "pucker" moment.

And no, they are not as few and far in between as most would think them to be. Is a matter of fact, satellite images have shown actual "hot spots" where these phenomena occur pretty regularly. I'm referring to the rougues...not the feces.

J

 

Looks good - I have a question (or two)

How high is your hulls? and how did you get to 4000kg's ?
The DWL indicates 1650mm ?

The video's I looked at of sailing cats suggested the hulls doesn't get pushed that far under as you've indicted, unless they're very slim.

 

Fanie
I just chose 4000kg as a displacement figure for a 40ft boat that was designed to take serious loads. Not as light as you could make but capable of taking the required loads and with some internal space for maybe bunks. Treat it as a target weight for first iteration of a hull design.

The DWL is the static draft with boards down. Actual hull draft at design speed and trim is 495mm.

The forces to get it to do 20kts are significant. We are talking about lifting the 2 tonne windward hull clear of the water sailing 35 degrees to a 30kt breeze. Apparent wind is around 20 degrees. The keel required to react the side force is large and this adds extra drag. Also the stern is canoe shaped so does not develop buoyancy rapidly with draft.

I determined the CoB would move about 1m from static trim to sailing trim at design speed. This is due to the overturning pitching couple between the hull/keel drag and the thrust high up in the sail. So to get it to level out at design speed it will sit bow up under static state.

I have not seen a 40ft cat doing 20kts to windward so I have no observed reference. On little cats you certainly like to keep weight aft when lifting a hull and their trim moves bow down when underway.

If you look at a windsurfer you will see the sailor moves weight aft as they pick up speed to keep bow up. This balances the sail force and moves the CoG aft.

Rick W.

 

http://www.youtube.com/watch?v=lsTXGhN3rLU&feature=related

You probably saw this, interesting one can see the hull shape, but note the leeward hull doesn't get pushed under... it seems the forces is converted into speed rather than down forces.

Maybe if you calculate with some weight and sizes closer to home you may be surprised. Such a large hull compared to yours would be significantly more heavy, accumilating weight quickly in building materials to stand up to the loads.

 

Fanie
She is sailing quite free - almost beam on to the waves. Apparent wind probably around 40 to 50 degrees. Sail loads are considerably lower than I was trying to achieve. The windward hull is still carrying load as well.

Also I chose an overall beam of 8m so I could get very powerful righting moment. The picture would be different if it was narrower but it could not point as high at the design speed.

Even so in the clip the lee bow is being pressed. Look at how much the sterns are lifting clear of the water. Have you got a photo of the same boat just sitting in the water to compare it with.

Rick W.

 

Catamaran lines drawings.

Round sections may be good for low wetted surface, but v-shaped sections avoid pounding, and flatter sections aft make for faster boats. So, most boats go from a v to round to a flattened round as you move from forward to aft. It is possible to use a shallow v in all sections, though, and get more reserve buoyancy forward, and keep some v aft, and still end up with buttocks with a shallow exit angle for speed.

Some v aft helps avoid slapping at anchor, too. There is a sailmaker who actually sells a removable bustle to put under your stern overhang to quiet slapping at anchor!

For ease of building, combined with lower wetted surface and no slapping, you can design a radius chine hull. You pretty much never see a sailboat with no rocker, so there is probably more too the rocker question than ease of turning.

For my take on all this, see my lines drawing at http://www.dunnanddunnrealtors.com/Catamaran.html

 

Fanie
Here is a much better video that shows what happens when a cat is pressed:
http://www.rawnerveracing.com/user/RawNerve.mpg
Look at what is happening to the lee bow just befor she rounds up. This is not a little cat it is 56ft long.

So if you are intending to press you cat make sure you allow for trim change as things load up.

Rick

 

Very narrow hulls !

 

And not long enough.

We are all learning here and I for one, am very grateful for the generosity of all who contribute their time and expertise here. Thank you one and all.

Regards,

Perry

 

Fanie.

That boat is a poor example. I for one would not like to sail on her, even in those far from extreme conditions.

The pitching is excessive and would tire the crew very quickly----but the badly designed bow is constantly producing a VERY wet boat.

Compare that to the hull shapes of the trimaran IDEC.

These must be as near to the optimum shapes for fast sailing in rough water as you can get.