Catamaran Questions

When designing a cruising cat, do you ideally want the hulls to sit as shallow as possible (draft) or do one want the hulls to extend some in the water.

What bridgedeck clearance is the best / acceptable to have ?

Thanks -

 

Perhaps you won't like this answer... but, the answer is... it depends.

This sort of decision will be addressed in the initial design brief when the mission of the boat is described. The displacement of the vessel will need to be resolved and that will determine an envelope of hull length, width and depth. You can fiddle with these three elements to allow the boat to fulfill its mission properly.

Let's assume that the boat in the discussion is of a fixed length. If the boat needs to be moving through frequent shallow water environments, then it's going to need to be wider and less deep in the water to be able to carry the same weight. This design decision doesn't usually contribute to a speedy multihull, so you will need to be willing to give away some of the speed in order to get the shallow draft you seek. (or find a way to get rid of some of the weight you wish to carry)

If the boat does not need to be in especially shallow water, then it would be optimum, from a performance perspective, to give the hulls more draft and be able to make them thinner in cross section.

So, the solution to get both elements is to make the hulls longer while keeping them skinny and also maintaining some shallow draft potential. In this solution, you most likely will make the boat more expensive to build, more difficult to park and more expensive to berth, if and when you do find a suitable marina location.

You really need to create a list of things you want the boat to do and arrange them in some kind of order of importance. Those priorities will draw the design envelope in which you will most likely be operating for a final design solution.

 

I think you need to define exactly how big and what you intended to do with the boat first. Are you talking about a sail boat? Deep water long distance cruising? How long, how wide, how much payload, etc.

I would like a cat to be just big enough for multi-day trips, shallow draft and be able to beach it. And preferably be small and light enough to trailer it so it can stay in my back yard when I am not using it. You likely want something different.

There are a number of threads in this forum that discuss deck clearance on cats, it appears the consensus is between 2 and 3 feet min.

 

As a "for instance"
I just studied the hulls on a 26' cat runabout. The hulls were narrow but deep. The entry was razor sharp fully 1/3 the length of the boat. A very striking fine entry. The remainder of the hulls widened gradually but did not flare at the aft area. The operator assured me the boat was still quick to reach plane and was smooth through moderate seas.
It was powered by twin 115 outboards. The scum line on the hulls showed the boat rode deep while sitting or not at planing speed.
I don't understand cat hulls much I guess. I would have built a wider hull with a big flat planing surface at the stern. I'd have given the bow area a rounder entry also Apparently I'd have designed a pig.

 

I'm still fiddeling with my boxy catamaran, intending a motorised sailer. There seems to be a few things to watch out for. One of them is how deep your hulls sit in the water (draft), another problem is the deck clearance and I picked another one up as well, and that's windiness.

Seems the hulls depth in the water and the remainder out of the water determine how windy it will be. The cabin also contributes, however, if I lower the cabin I lose deck clearance.

I'm merely trying to determine the 'golden' middle way, where it would run a shallow draft without blowing away all the time, as well as avoiding getting slapped by waves.

I could lower the hulls a bit, since one basically only sleeps there, but that will make a big difference to the cabin height if the deck clearance is to be maintained. Another thing I don't want is a wave to break over a hull or the front deck in choppy seas.

I also want to maintain the access from the cabin into the hulls - nothing so unpleasant as make it to bed in the rain.

So there you have it, that's about the worst of it. I know there's no hard and fast rules and everything is a compromise. I do however know that there are deck clearances that would be ok for most conditions, or a way to reduce windyness without just about sinking the hulls.

I actually considered a pump to allow water in (somewhere) if conditions allow it. You can pump it out again when you want to move again. The idea was discarded as there's not enough room for such tanks or cavities.

 

No one gets out alive in boat design!
The tradeoffs are endless. That is why it is rare but sweet when a particular design meets the majority of needs.
IF this is your first BIG build and you want walk thru passages, multiple bunks, galley and motors (with all that alone entails), you are out of your depth as far as a safe self design is concerned.
Cats are just that,.... a breed alone. To be safe, efficient, economical and pleasurable to sail AT THAT SIZE, you need to go with either a established design or work with a good designer/builder of large cats.
I've seen too many pics of separated sponsons, trailing behind 1/2 submerged cockpits, held only by its shrouds.
If it's a big cruising cat you want, you'll need professional help.

 

Hi Ted, how about some usefull information for a change ?

Too many people going around thinking there are only a chosen few capable.

If you're going to comment, pls something helpfull, which is why I'm posting on the forum - isn't it what it's there for ? My WIFE could give me that same advice

 

Fanie, cruising cats want their hulls "in the water" so the motion will be more comfortable for those living aboard.

The more heavily your hulls are loaded the slower they rise to oncoming waves, which suggests a higher clearance is needed to prevent wave slap. For most cruising catamarans 1.0 meters is probably enough, even more than enough, in most conditions. What clearance are you thinking of?

If you want to go lower at the risk of increased wave slap you can break up and dissipate the wave's impact energy quietly by putting a series of parallel V or U shaped ribs 4-12 inches in height lengthwise on the underside of the wing deck, covered with soft closed cell foam like pool noodles or neoprene.

 

Sorry Fanie, I didn't mean to rain on your parade.
Give us some specifics please.
What is the expected displacement? Length-width? What is "shallow" to you? What sail plan? Accommodation's for how many & how long? Any example boat that catches you eye?
Are you running a design software to help you? Are you landlocked or in a "cat" area where you can peek at others designs?
We can help design for the use if we know the use.
I appologise again, sorry.

 

Fanie,

Have a look at http://boatdesign.net/gallery/showphoto.php?photo=8818

It's the complete opposite of what you are trying to achieve, but it offers accommodation in both hulls, a fishing deck and what would appear to be an easier way of assembling the hulls before launching. Less windage as well.

Pericles

 

This is what I suspected, it would improve windiness some as well. A more v-ed hull with a flatter bottom (to walk on) could be better. Someone also suggested the wider upper and narrower bottom hull, since one doesn't need very wide feet space. I'm going to have to calculate the approximate weight and the hull's volume (bottom area) to get an indication.

I had the clearance as 1m if the draft is 300mm. A hull sitting deeper in the water will lower the deck clearance of course.

I'm putting a scaled cat model together. I was wondering how true to the real thing one could simulate beaviour. Could be an excuse to go fishing

I've seen some boats have these ribs, but I suspect you would still get these bumps trying to knock the cabin upwards. I would rather have the more boxy appearance than having to deal with pounding which could last a whole trip if the weather is up to nuggets.

Since the cabin would be removable I was thinking about implimenting the ribs in any case for cabin structural strength firstly.

You didn't - no need for an appology. Just trying to get some usefull info from you clever guys.

I have freeship and delftship, and I'm using solid edge, now considering touchcad as it seems more suitable for these kind of drawings. You can also assemble the whole thing and render it for better perspective.

The design cryteria currently are -
Trailable on two trailers
Min 2 people to assemble or disassemble
Max four people accommodation up to one week - ie weekend to weekend.

The length I have currently is 8m. Cannot see anything shorter will work, lengthening it could create trailing problems. Beam 6m assembled, Cabin 5m x 3m or there abouts, 1.9m

The first drawing I made was an open design, but the cabin would be nice for socializing amongst others. You could do a lot in a cabin you could not in the hulls. The cabin stays, it also has to have access to the hulls from the cabin.

 

I realixe you need the hull cavity for accomadations, so a full swing keel is out, but a partial or swing skeg fits the shallow draft-deepwater idea
A middle hull woul allow a low windage cockpit. naybe a bubble top, fighter plane style.
Consider crawl thru tubes to access the sponsons.