Foil Ratio

Here is 1983 Split Enz with the four boards/two rudders. Didn't Tom Blackaller's twin canard/rudder AC 12 metre come out around that time. The Canting Board Twin Rudder design is fairly recent I believe - also it is different; the keel cants, therefore not quite the same. Here also is an 1898 Logan Brothers' Patiki with twin rudders and going further back to the early 1850's, the NZ Scows had multiple boards plus rudder. And Dick Newick was also trying interesting things with his 1968 proa Cheers. Nothing is really new. The first canting keelboat was Jim Young's Fiery Cross, launched in Auckland in the late 1950's.

 

I apologise,but I have had no joy with the pics.None of the pics that I took at the mooring are on the camera.The pics of the DB and rudder at home cannot be put on the commputer for some reason.I am going to have to buy yet another camera after the holiday is over.My apologies once again.One thing I can tell is that the DB is 186cm x 50cm.103cm of it projects beneath the keel of the boat.The DB case is offset to Starboard.The rudder is not a regular rectangle,but drawing one outside it gives 87cm x 40 cm.This projects beneath the hull.When I was trying with the photos. I laid the rudder on top of the DB and there is very little difference in the size of the 'immersed area'.The immersed area of the DB is 5150 sq cm,and the rudder is 3480 sq cm.If the ideal DB/Rudder ratio is 3-1,then either my DB is way too small or my rudder is way too big and this is the problem.
Regarding the rudder,the centre of the rudder stock is 12 cm back from the leading edge.I think that this is excessive.If I cut back the leading edge of the rudder to the point where it matches the edge of it's cassette,this will loose me 5cm x 87=435 sq cm of area ahead of the rudder stock,and must help to reduce the servo action on the rudder.
Can I please run this by you all as a plan of attack?I realise that not having seen the items discussed that you can only generalise,I appreciate that.
1.Measure the sail area.If I have to roll the sail out on the local carpark and measure the sail that is no massive hardship.
2.Use 2% of the sail area to give me the size of DB immersed area required.
3.Adapt/make new DB to suit figure found in 2.
4.Adapt/make new Rudder 30% of the area of the DB,as found in 2.
I've got to repeat that I'm really miffed at not getting some pics sorted.I feel that we are making progress on this,and I really appreciate the input from you all.I am on the case for a new camera.
Twin keels-some of the very old Pulling and Sailing RNLI Lifeboats had tandem centre boards along the keel line.More recently,(1950-1960?)I think a mono called an 'Atalanta' had side by side twin CBs?This was a boat apparently designed to be dropped from an aircraft.I am enjoying absorbing the theory discussed on this thread.A couple of things I am stuck on are-
RHough-what is 'Decalage' please?
OS I was with you on the earlier post 37,except-what is CB,CF,CL(centreline?) and CD please.
On the subject of luff tension in the main.I have a good downlaul,8-1 IIRC.I generaly downhaul enough to just put the luff 'to sleep',more is required when the wind increases.When trying to figure out the problem with the boat,I even tried to sailing with the mast central to try and move the draft of the sail aft,rather than have it rotated.
Cheers

 

Here are some pics from my 'library'.The first one is the oldest pic I've got of the boat.The second one is as initial set up to the witness marks on the rigging.At this point I stopped thinking about how to rake the mast back further,and started to think about moving the mast foot aft.The third one shows the present set up.The foam packer goes at the back of the DB case and holds the board forward.The mast socket on the deck,lies directly above the centre of the box section compression post.The top is adjustable.The forth one shows to deck at present.If you save as a bitmap and zoom in,the front edge of the mast base in just under my thumb.Fifth one shows the DB in the case.Sixth is general view.last one is rudder.
I hope that this gives a bit of an idea of the boat layout.I will specifically target the DB and hull positions ASAP.
Cheers
Number 2 and 5 will not load.
Cheers

 

Number 2 and will not load for some reason.Rudder,the white lines show roughly the position of the stock.
Happy new Year to one and all.'Blwyddyn Newydd da Chi' as said in Wales.
Cheers

 

All this leads me to think, the ultimate config for multihulls is two foils of similar size? allowing for max aspect ratio for given depth.


3 foils could never have less drag than 2, for required force. -- paraphrasing the guru.

 

measure!

Caiman, designers got by long before there was an internet or cameras Measure the hoist and foot of your primary sails, (long tape measure on the boat, you don't really need the sails) and measure the distance from the mast to your dagger board- front and back. I think with the size of your board that you already have, that would be enough. It really sounds as if your board is undersize. Your rudder is probably not quite right, but it is not the "problem", just tends to snatch out of your hands when you turn Happy New Year everyone! B

 

Neat looking trimaran. The rudder is a little overbalanced and could take over so reducing the chord by removing material (say 200mm) from the leading edge should reduce it doing the unwanted servo trick. You have not complained of this but it could be the answer to your problem.
I presume your dagger board is vertical, not raked aft with your foam packer. If you can rake the board either forward or aft, then test sail, you should probably notice quite a difference in helm feel. Then take it from there. There is nothing wrong with your dagger being set to one side of the hull, effects nothing, nor either raking it forward (except for catching weed). I'm with Bruce; your dagger could be deepened and extended for more area.

 

Caiman.
The area of the Jib is not important as most Jibs overlap the mast to some extant. The projected sail area is:-

Forestay triangle + Mainsail area.

Easy to measure.

 

rig position

don't don't know if it is the picture perspective or actual, but the whole rig seems very far aft on the boat. It would seem that would contribute to weather helm, not lee- but it does "look" a bit odd, and if the jib "J" is particularly large, it could contribute to the lee helm, especially if the jib is blown out. The board trunk looks about right for the mast position- most tris have the trunk right behind the mast, and this one seems normal. Is the board under-size to make it removable from the top? I can't tell from the photo. I think Gary has called your rudder correctly- and I would suggest it is probably easier to make a new one than to correct yours. You will be amazed at how much better the helm will feel, and it is seperate issue from the "lee" helm, but they work together to make a boat a pig to handle. B

 

More boards

Idkfa, I am sure two boards do have less drag, but on many tris, tacking would become much harder. Most faster (racing) tris have very little of their float boards in the water at the middle of a tack, and without the center board, they will stall. I noticed that the AC tri had a center dagger, and I know they were very sensitive to drag, but thought they needed the extra board to tack reliably. I also noticed they stalled anyway I guess it is practical versus theory. B

 

Hi All.I apologise up front if my spelling,grammer or consistancy is at fault but alcohol does that to us.I borroeed the camera from work at dark'o clock this morning to take some pics.All has gone well.upon returning the camera to work this afternoon.I've bumped into the 'straglers' from the NYD swim,while trying to transfer the pics onto my memory card thing,I was forced to drink Budwieser,and John Smiths Bitter.There we be.
more pics to follow.
Cheers

 

Duh! That is 20mm off the rudder leading edge, not my 200mm typo.
Still think that should be done first, rather than build a new rudder.
Bruce, as I see it the board is Forward of the mast, in second photograph, see the black main beam forward and mast support behind board case- aft rig, forward board, that boat should have weather helm. The headsail must be a spinnaker to blow the bows off. If the case went through the deck, I'd imagine the board would have to be raked aft for correct rig/helm balance. This boat is a bit of a mystery.
By the way, AC33 BMWO removed their central hull dagger (and their central hull rudder too) and relied only on their deep float foils and float rudders for racing.

 

More pics

 

Ah, new revealing photographs. That rudder is horrible, Caiman, make that 40mm, maybe even 45mm off the leading edge - that is your first job. The dagger is definitely not deep enough either, even pushing it down flush with the interior case top - how do you keep spray out at speed?

 

more pics