australian direction

[peter]

The Australian multihull seen is improving. The boat designs that we are currently using are quick and very responsive in medium breezes. What we need to do is design a boat that works well in light winds, but can have weight added to help it when the breeze comes in.

This new design needs to be built light, and have a small wetted surface. I'm wondering seriously about whether the "tunnel hull" idea that I spoke about with you awhile ago would work for this. Narrow boats are good in light air, but they trip over very quickly when the breeze comes in.

The idea of having two boat's one for light stuff and one for medium to heavy winds is something else to be considered. The main problem with this however is that we sail our club championship over 6 months so the weather conditions change and you can't sail one boat in a heat and the other in another heat.

So that is the dilemma I am faced with at the moment. Any ideas?

Peter

[ABC]

Maybe the rigs need to be more tuneable so that they depower (especially up top) more in heavier breeze?

Adding some weight to the boat should help as well but I wouldn't imagine you would need a great deal. Maybe a fishing sinker or two on each hull??

What part of Aus are you in? I'm in Canberra.

Andrew.

[hoj]

If you measure the surface area of a tunnel hull you will find that it is double that of a circular x-section. It think the next step would be to experiment with 1/2 angles. If it is the same as monohulls( which it appears to be) 10 degrees would be an average boat and then make it thinner for a light air boat till you get too much nosedive. Also the position of the widest section. Ernst seems to think 40 % for the amas. I have not got the Nightmares finished. They are all quite different. Will report as soon as I start trialing against each other. I do think that circular hulls are the way to go. Look at all the 60' tri.

[MultiFast Is MultiFun]

Bill -

based on what I am seeing in the "A" Class cats, I tend to disagree a bit.

The newer cat designs (which are really kicking ass) seem to have very narrow and "V'ed" with maximum buoyancy taking place just behind the mast. Underwater, the hull sections seem to go flatter - perhaps to induce planing.

The "A" Class boats have a minimum weight of 168 lbs. (all up sailing plus skipper) and use a mainsail only.

The reason I disagree, is that these cats tend to be more closely related to our lightweight models, than the Open 60's which must be overbuilt - and usually are in order to meet conditions found at sea (wind and waves).

Our "little' boats just don't seem to need to be as overbuilt, and thus less displacement (weight) means less surface area under water, less drag and significantly better performance acceleration.

While 1/3 the size of an ocean going 60, it seems based on Peters experience with weight factors, the lighter weight, less "draggy" type of boat (hulls) would be preferred - simply for acceleration alone - if for no other reason. Again, I look to the underwater sections between the FREIGHT TRAIN and GHOST TRAIN (same designer) and see where he went with less underwater size for hulls and floats on the last incarnation (Ghost Train). Perhaps to reduce wetted surface and drag?

[hoj]

I was replying to the statement that a tunnel hull would give less drag ( wetted surface ). I said that circular sections give the least drag. I would like to experiment more with x-sect. and whether egg shape fat up or fat end down is the best way to go. The Freight Train and Ghost Train x-sect. are the opposite of each other and the amas are the opposite of the main. I think they should be the same amas and main. I have been reading that multi's are switching to fatter bottoms and narrow tops to promote planing.

[MultiFast Is MultiFun]

Peter -

is there any way you can update us on the weight difference between your local boats and the lighter weight "outsider" ?

I'm trying to get a grasp on what constitutes a "heavy" boat, and by how much the difference might be?

I can appreciate the lighter boats ease of acceleration, but wonder at what point weight begins to become useable in lulls, and when tacking. Somewhere the graph lines will cross, and I think developing around that specific and optimum point would be worthwhile and a good place to start.

Here in the US we can have drifters one day and howlers the next. Usually summer is a mixed bag, but fall and spring usually see fairly steady and strong winds here where I live. Meanwhile in the south, light winds and very fickle seem to be the common type. This is what is making multihull development here a hard project - since we are spread so far with such a variety of winds.

Would be so simple if all boats lived and raced within a 100 mile radius ! ;D

[peter]

Andrew first of all to answer your question I'm in Brisbane.

The boat that we sailed against that blew us away weighed 2.2kg on the water.

Our boat's presently weigh somewhere between 3.5 and 4.5 kg on the water. A heavy boat is 4kg.
I think now to get down to 3kg is the way to go.

The extra weight will hurt in light air but will hold up better when the breeze picks up.

I am now working on new design number three for this year, as I haven't yet come up with a boat that is working properly in our conditions. I've got the sails and the rig working really well now, so the platform/hull shape is the thing to work on

The rudder and centreboard design is very efficient.

Peter

[hoj]

What length, area, thichness, and shape are you using for ctrbrd, and rudders

[peter]

Rudder's are 8 inches(240mm) long. They are 3 inches(75mm) wide at the top and taper down to 2 inches(50mm) at the bottom. They are about 1/4 inch(5mm) thick which taper as well.

Centreboard's are 14 inches (350mm) long. They are 4 inches(100mm) wide at the top and taper to 3 inches(75mm) at the bottom. The centreboard's are 3/8 inches(8mm) thick and taper down to 1/4inch(5mm) at the bottom.

On rudder's we fit "winglets" to assist in stronger breezes.

Both rudder's and centreboard's are made from class or carbon. They are fitted into the bottom via 3/16 stainless rods that fit into tubes fitted in the boat.

The shape of the rudder is critical on multihull's as it must not cavitate and must always have "contact" with the water. You not want a rudder that stalls out.

Peter

[MultiFast Is MultiFun]

Guys - I just added a couple of photos to the MultiONE area for my board and rudder - including basic dimensions for reference and comparison.

Am trying to keep sections grouped together so there is logic to where we are posting.