Boxy Fisher Catamaran idea

Im not a rocketist so I'm taking some of the rocket's rocker out. Thanks for bringing it up Richard. It was an accident...

Same question still, what's an acceptable bridgedeck / cabin clearance to have on a cat ?

 

It seems the more rocker I put on the hulls the faster delfship rates it

 

Fanie,

Bridge deck clearance is mentioned here.

http://www.boatdesign.net/forums/showthread.php?t=19139

Hope it helps.

Pericles

 

I am inclined to feel that formula is somewhat conservative for Saffa & Au oceanic seas. Locally something between 10 & 15 % of either beam overall, width between hulls of distance between centrelines of hulls??? It all depends - to whom are you addressing when asking :?:

Try Delft with total rocker disk on edge - you may be on to something - flying saucer & light speed? or the going rapidly in ever decreasing radii till disappearing up some fundamental hole/error?

 

I would not take a lot of notice of the drag results from Delftship. I have found them to be next to useless.

Rick W.

 

There, Pericles, Richard said it himself... standing headroom in a small boat. Te-he... my, my, seems we are exactly the same

Bridgedeck... no one seems to give any figures... just mention low or high. I had it as 1000mm, can maybe drop to 700mm, but that already looks too low.
Numerous articles mentions 500, 600, 700mm fully laden and other does go to 1000mm. None mentions if deck slapping occurs on THEIR boat or if it is an inconvenience. All manufacturers of course mention the plusses, never the negatives.

Ah, thank you Rick ! Phew ! For a while I was worried the boat would not even need an anchor it has that much drag

 

Hay Fanie, I gave you the opportunity to choose 6 different figures, I am all about self choice and no one is "right" or "wrong" unless it does not work for you!

If you are going for REALLY skinny hulls, then I guess a higher bridgedeck clearance may be more the go, with more flair (above the waterline) to give extra buoyancy in a heavy seaway...

Go to the seaside and look at waves in the "worst condition you would go out in". Measure the wave height - from trough to crest, and the distance from crest to crest, the shape, then use your model to see how it would be in that condition. then ................? You will know.

 

I did !! Worst case seems I have to make the hulls about six meters high and about 100m LOA and a 3m500 deck clearance

Seriously, I actually have. Onder normal and under somewhat less normal weather conditions I think the rig so far should be ok, which is the dimensions I went for. The only drawback is windyness. I've never sailed a cruising cat, so I'm not really sure what to expect - which is what I'm hoping to find out from you guys here but what I do expect is to catch a heck of a lot of fish from it

Guess I'll stick to my current measurements for now. I'll do some assesments and see if I can get around the windy part of it.

Thanks for the comments guys - it really helps.

 

I guess Fanie, follow your own feelings. I have been advised that I should keep the design sleek & low profile as windage will slow you down, and in almost the same breath say "Mostly I sail downwind" - so my current thinking is why should I have to crouch to move around the bridgedeck and suffer discomfort for no consequential gain, when cruising I doubt I will be going faster downwind than the wind (I may tack faster than the true wind-speed but that is another story.)

The only time sleek bridgedeck lines will be noticeable is when I am motoring directly, (or close to), into the wind.... or at anchor! - my sacrifice is no flybridge!

 

I like the mostly sail downwind... I myself may want to go back sometime...

I don't expect a super duper fast planing cat that can go right into the wind under sail. It would be a bonus though... but I do expect to be able to reach anywhere under sail even if not that fast.

I have read about what I can and have tested some of the unknowns. I guess the next step is to put things together and begin to refine and do some more of the interior planning.

Collect and build/make the accessories which seems like an endless list.

 

Hi Fanie, I have been dead asleep while you were asking about bridgedeck clearance. Time zones....bit of a pain. For a 10m cat, 2 feet from waterline to bridgedeck sole is considered good, according to major yachting mags and various sources on the net. A lot of Aussie built cats of that size are around 2 to 3 feet clearance...and they are designed to sail in the rough.
If you plan to do a lot of rough weather sailing, I wouldn't recommend less than 2 feet.

 

Masalai, quote "going rapidly in ever decreasing radii till disappearing up some fundamental hole/error? "

LOL

 

My deck clearance is less than 2 feet....but it uses tramps to absorb the slapping. I have placed less emphasis on comfort during those conditions. My reward is low weight and low centre of gravity.

 

So, how big are your feet Richard ? Are you guys in .nz finally in the process of switching to the mertic system, mixing your feet and meters

I think 600mm seems a bit skimpy, I'll have to see what I can fit in and how.

How big a problem does windiness really creates ? I know it could make tacking difficult. Any problems one should take note of ?

 

Hi Fanie, us kiwis work in metric. When it comes to building, I find it easier to imagine distances in my head, when I am thinking with imperial. It's surprising how many things are one foot wide. And many things are 2 feet wide. And many things are 3 feet wide. So you can multiply things with simple numbers. Working with feet makes everything easily divisable into thirds and quarters. The metric system should never have been invented.

If you want to get really obsessive....we shouldn't be working to the power of 10. We need to invent 2 extra digits, so we can work to the power of 12.
No problem for all the kids...but utter hell for us guys!


Anyway....regarding your bridgedeck clearance, if you use naciles like Rick showed in his demo example of your boat, you can get away with a minimal clearance. When a big wave hits the nacile, the boat is lifted over the wave so the wave doesnt get a chance to slap. The nacile creates a convenient place to put stairs and adds a whole lot of strength to the hull/bridgedeck connection. Only down side is that it can add some drag in 'normal' chop, when slapping wouldn't occur anyway.

If you design your cabin as close as possible to a dome, you will have maximum strength and minimum weight and minimum windage. The chairs/sofas could be positioned against the perimeter, with a coffee table in the middle. You would have standing head room when you stand in front of your chair, but not directly above the chair. The stairs would have standing headroom. The kitchen/galley could be in the hulls, but exposed to the central cabin, so you can still talk to the cook. You could have a stove top on the coffee table (making the coffee table a bit like a 'camp fire' with everyone sitting around it). Just an idea.