Hull Shape as Keel

[oldsailor7]

Obviously not. !!!

How big is this boat with 500sq ft of sail area. ???

[Fanie]

Some pictures of the hull as to date. Keep a few things in mind when you look at this. The cat is designed to trailer, the hulls goes side by side and the max width I can make them is 1m200. The cat is 10m LOA. Sorry, typo there, the sail is 50m^2. He-he...

The hull's ratio is 11:1 and the draft is desined for 500mm but I drawed it for 550mm. It becomes fat above this quickly to the full width to work against pitch-polling. It is also shaped to deflect spray outward, hopefully one can sail without a rain coat and the wider area above the waterline adds quite a bit of space in the hulls and I like the square deck to walk on.

Inside the floor is the hull itself, so I made it flat to walk on and save the weight and cost of an extra floor, and it must be able to have a bit of a surface on the trailer as well for better weight distribution on the structure not to point load any one place. It will also beach better and not 'dig' into the sand so much, hopefully more tent to sit on it.

This whole draft area is fairly flat, mostly to get the ratio right. The only place I can round some to make tacking better is towards the stern, nobody would walk there.

So far the figures come together, wrt the weight and a few other things...Adding the daggerboards, if you could imagine them in there - could this work out ?

Please feel free to critisize anything you feel is not functional or suitable. The idea is to have a cat that works in the end.

[champ0815]

Are you sure about your calculations of the board? I calculated for a 3:1 aspect ratio board of 1m² a length of 1,64 m and a width of 0,55 m.
The board you described has an aspect ratio of 9:1.

[Fanie]

Thanks - yes Champ0815 . My calculation is completely wrong That happens when you are buzy with three things at a time. Sometimes I'm like the English say, here but not all there

Think I would have gained any with those 9:1 boards

[Fanie]

I have a few things around the sails to sort out before I will be able to calculate the board size.

If anyone would like to comment on the hull shape, go for it.

[Fanie]

Everyone has been extremely patient and helpfull, some really great informations has been laid out. Thank you for that and thanks for taking the time out. It does not always look like it, but I'm really gratefull. I know it takes time. All this is appreciated more than you would ever know.

[Meanz Beanz]

If you ever did manage to stick that bow fully underwater it would act as a bit of a plow once you over came the reserve buoyancy and it would contribute to pitchpoling. In fact the nature of the reserve buoyancy in the bow will slow the boat as it is immersed, also not that desirable. As I understand it this is why designers now favour finer deeper bows, they can carry decent buoyancy without creating drag as the bow buries. Drag being the reason you trip over the bows... I'm no designer, maybe one could comment. Not having ago, it was just the first thing that struck me about the hull shape.

Cheers
MBz

[Fanie]

Thanks Meanz, I do take note. I saw a picture of another make boat that widens out towards the front but to a much lesser degree. I will get to that in due time.

I was hoping someone would comment on the draft area wrt tacking.

[Fanie]

Everyone was ok untill I tried to get the daggerboards to the hulls

I won't hold anyone responsible for anything you suggest around this topic. In the end I will do as I think best, but I need some reasoning around this.

I foresee a lot of tampering yet. One gets stuck on your own ideas. Someone else's view opens up other ideas which brings about new possibilities...

[Nordic Cat]

Fanie, how about posting some hydrostatics? What is your Cp? It's a bit difficult to see the amount of rocker, or is it just relatively small?

I don't like the bows, same comment as MB above, also i would make the flare curve softer in the vertical part to avoid annoying wave slap.

Is each hull symmetrical?


You mentioned that there is no sole, just the bottom of the boat. Where and how are you going to add tankage?

Keep at it, it's coming along fine.

regards

Alan

[Fanie]

Hi Alan,

Thanks for taking the time out.

What is CP ? I've uploaded a jpg with the design hydrostatics. I don't know what they all mean or what ideal would be, but I get some ideas of what sould be how mostly off the forum, thanks to you guys

The bows were shaped like that with the idea to -
keep the hulls from burroughing,
it adds space inside the relative small hulls,
it's supposed to push spray down,
and adds more space on the hulls to walk right up to the bow.

At this stage I'm still playing around with the hull shape. I appreciate comments on this, and is the only way one get to know better. If anything is not going to be ok, pls feel free to critisize.

The hulls are symmetrical. The 'sole' is the boat's bottom and I've made it flat -
so one can walk on it, it
it has a larger surface to sit on the trailer,
hopefully the bows would tend to sit on the sand instead of knifing into it when one beaches,
it would be easy to get water out if any ends up in the hull which could be difficult with a floor,
It would save the extra floor's weight,
and save the cost and time to build a floor.

Tankage would have been under the berths. I have since learned about the pipe berths, which I like quite a bit since they are light, can be folded away etc, it has buggered some previous ideas up quite a bit.

Fuel tankage is not a problem though, the motors get their dedicated fuel tank compartment which would be isolated from the berthing.

Wrt the motors I have the idea to use a jetski motor in each hull, mainly since there's no prop...

[Fanie]

The floor area is 600mm wide, and tapers out another 300mm to each side to the max width of 1m200.

[oldsailor7]

FANI.

If you really want hulls with a flat bottom and a symetrical shape---------Why re-invent the wheel.

Go to Richard Woods web site and look at the copious information on design and building of the "Janus" catamaran. It will give you a whole lot of vital information which will save you hours of cogitation and wandering up blind alleys, from a design point of view. http://www.sailingcatamarans.com/janus.htm

[bobg3723]

Center of Pressure, or cP(fluid mechanics)
For a body immersed in a fluid, the point through which the resultant of the forces on the surface of the body due to hydrostatic pressure acts.

I'm not sure...does this mean exactly this?

BobG

[Richard Woods]

Cp means Prismatic Coefficient, a measure of the fullness of the ends of the boat. You should be looking for 0.6-0.64. Monohulls are normally about 0.55

I write more about hull shapes on my website.

When I design a boat the lines plan is normally about the last thing I draw. Until you have decided EVERYTHING, from mast position to galley design (and water tank placement), engine used etc etc etc you won't know how heavy the boat will be, nor where the centre of gravity will be.

And since the C of B and C of G have to line up if the boat is to float level, and obviously you want the boat to float on its marks, I don't see how you can possibly design a lines plan that has any relevance until all the details have been decided.

A boat is not a house. You can add a second story to a house, or add a spire in one corner and it won't fall over. A boat will.

A multihull is not a monohull. If a monohull is over-weight you fit a lighter keel. If it trims by the stern you move some keel weight forward. You cannot do that on a multihull.

I suggest you read all my articles and my FAQ's pages before getting too carried away with a lines plan. CAD really stands for Computer Aided Drafting, not Computer Aided Design

Good luck

Richard Woods of Woods Designs

www.sailingcatamarans.com