Sock it to me..........

[BrettM]

TROUTY,
First of all, why do you need to undo a stay from the gunwale. Surely you could use a halyard?

And Second, To get a multi off you could drag it off with a large boat and you could use some explosive or a shovel to dig it out but surely you could simply lift the centreboards?

Hey! I must be an ideas man.

[b_rodwell]

Trouty

MONO vs MULTI
I am not getting into this debate on this forum.

CHARTERING
Building a boat to survey is onerous and is required if you want to charter in Australia or USA. I don't know how important it is in SE Asia or the Pacific.

Building to survey usually adds to the total cost and, more seriously, frequently adds 'unnecessary' weight. I hate the idea of carrying around more weight than I need. I have been reviewing the USL codes and the relevant Australian Standards (AS4132.1&3). I intend to build in foam composite. I will be able to comply in strength and stiffness without a weight penalty. I am trying to work out how to comply with the penetration strength requirement. It seems more difficult and I have seen some boats where this requirement led to a significant increase in weight.

LOCKHUGHES

By canted I meant that from front on the hulls look like they are leaning inwards. See the front on rendering.

I am not convinced that assymetric hulls are an effective way to resist lateral motion. I think Hobie cats succeed despite having assymetric hulls. I do want the extra performance and I do want to be able to work to windward efficiently in storm conditions. I am told that centreboards will assist this.

[tspeer]

Originally posted by b_rodwell
...I am not convinced that assymetric hulls are an effective way to resist lateral motion. I think Hobie cats succeed despite having assymetric hulls. I do want the extra performance and I do want to be able to work to windward efficiently in storm conditions. I am told that centreboards will assist this. [/b]

I'm with you regarding asymmetrical hulls. The induced drag due to lift is inversely proportional to span^2, and the induced drag is doubled at the surface compared to lift produced at depth. So the deeper the lateral area, the better - this means some kind of board.

As for a combination of asymmetrical hull and board to generate "more lift", the total lift is determined by the loads applied by the sails. The boat will pick up leeway until the lift from the hydrodynamic side force equals that of the sail. Since the total is fixed by the rig, any side load carried by the hull is less load on the board. But pound for pound, the drag due to lift of the hull is greater than that from the board. So loading up the hull only hurts performance.

The main exception to this is that the load on the hull does extend the effective span of the board. The goal is to have the total spanload distribution of both hull and board add up to the optimum spanload distribution from the surface to the tip of the board. This optimum distribution can be calculated using lifting line theory (see http://www.tspeer.com/DesignTools/vortex95.xls). Also, see the research done by PACT, reported in CSYS #11.

So a small load on the hull can be benficial. But this may well be provided already by the symmetrical hullform.

I would be interested to see what the optimum toe angle is for asymmetrical hulls. It may turn out that by pointing the bows outward somewhat, the asymmetrical hull's bow would be more aligned with the leeway when going to weather, avoiding side force from the bow wave, and the toe-out could cancel the lift of the hull camber when moving straight ahead, thus avoiding a drag penalty due to the hulls fighting each other. This might be a more viable strategy for a trimaran ama, since the wrongly-cambered hull would be out of the water. However, it might even work with a catamaran because setting the hulls for neutral lift at zero leeway makes them comparable to symmetrical hulls (in the linear lift range) and when loaded, the leeward hull would come to dominate the boat's characteristics.

[Ric]

As to the old mono vs multihull debate. There are many pros and cons for both designs. But I don't agree with the cons mentioned. So far as running up on a reef, the shallow draft of a multi allows one to explore areas that never ever reached by a mono with 3 to 5 times the draft of a multihull.

Running up on a reef with a multihull and centerboards allows one to pull up the boards and get off. For this reason I would use two centerboards to protect both hulls. But there is no doubt that one centerboard can be made to work and is adequate in a cruiser (see John Shuttleworth website). There is a reduction of weight, complexity and an increase in space with a tradeoff off of a slight increase in leeway.

So far as flipping a cat. I would like to see the statistics of how many 70+ ft cats have actually turned over especially with a 30 plus beam and a Bruce number of only 1.45.

The canting hulls is not a big advantage in a cruiser especially in a large wide cruiser.

Windage is a concern on light and fast catamarans and should be taken into account when designing them. In fact in 20 knots of wind 60%+ of the drag of the boat to windward is from windage, not from the keel or wetted surface.

I would still have a steering station on the other side also, perhaps a small remote control station. On a large boat one needs that sometimes as one's view can be compromised by the sails. I find myself constantly walking from one side to another on cats with a single station just to ensure that there isn't someone to leeward of me.

I like the extended roof and for a cruiser the ketch sail is bonus.

Keep up the good ideas.

I think that cats make great world cruisers. The only problem may be the wide beam at marinas.

This may have already been covered:

The single centerboard will not be in the optimum fore and aft position on both tacks. A single leaward hull board should be more forward than a single windward hull board. Having 1 off center board will give you a different helm feel from one tack to the other.

I'm impressed with your skills at Rhino.

Paul

[brian eiland]

I guess I should have posted a message on this tread referencing my answer to B Rodwell's prelim cat specs.

That answer is posted under the tread titled "Big Cat, alt CB's & sail rigs"

And 'guest' please note that this 'single CB location (twin asymentrical boards) will work equally on either tack.

A truely inspired rendering of a Sydney Harbour cat cruiser