Trimaran Turning Radius: Ama Position

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Board position does not necessarily mean the boat is balanced but assuming for a moment that both #2 and #4 are balanced what is the effect on turning of the shorter distance between the rudder and daggerboard in #2 and # 4?
What effect does that distance have on the ama assuming both amas(#2 & #4) are similarly immersed at the start of the turn?

 

Surely you know how to work out the answer, so why ask the question. comments like 'wrong', 'not significant' sugest you already know the answer.
The reality is there are so many contributing factors that will change how the said vessels turn, that the relevance of the question comparing two very different multihulls is what ?

 

Well, I'm interested in what others opinion is of these two design choices. I feel like there is plenty to learn here. Also, I don't think the "answers" are cut and dried.

 

Hey, that does make more sense. When you're tacking, windward ama. Jibing, leeward ama. Or maybe not.

 

I withdraw my last post, I should wake up before posting.

However your assumption in bold above is mistaken. If you look at the plan below I have drawn perpendiculars from the arc, due to it not being circular the lines do not all meet on the centre line at the same place but none of them are near the dagger board trunk.

I think what has happened is the designer has moved the turning circle aft and just shown a part of it to give the customers a feel for its manoeuvrability. If he was to draw it as per sketch 2 you would have a large circle with a smaller boat in the middle.

It just proves that a stripped out racing machine turns better than a 6 berth cruiser.

It will be easier to discuss if we use a racing trimaran for both examples one with a centre board further aft than the other.

My thought is that the force the rudder can create will be the same in both cases but the more the centre board moves aft the less leverage the rudder will have.

At some point the maximum leverage will become less than the couple needed to turn the boat quickly and will become less manoeuvrable.

The position of the centre board is governed by the sail plan.

 

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No, its not. The daggerboard is much further forward that you show the radius on drawing 4. Look at the Sailplan/profile view and then you may see where the board is. I drew the radius line as accurately as I could using paint. It's not very good , but its somewhat close. The outline of the circle shown was supposed to represent a circle with a radius equal to the distance from the daggerboard to the rudder, It did not work out accurately but its the best I can do without spending a whole lot more time. To get better accuracy you canblow up the picture and hold a ruler up to the screen ,measure the distance between the cl of the rudder and cl of the daggerboard and move it around.NOTE: to be very clear I drew the sketch, not Kurt Hughes. The radius error is mine not his.
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If you had two equal sized rudders moving at the same speed that were turned the same amount the force developed inititally would be the same at the rudder. But the force transmitted to the boat would be greater on the one further away from the daggerboard.

Illustrations(Kurt Hughes Design) showing the daggerboard location-Same as pictures 3 &4 in post one. Radius is based on the daggerboard position shown in both drawings. I drew radius lines in that are supposed to be the same length as the distance between the rudder and daggerboard. I just checked a second time and they're still off. Thats all I'll do-having too much trouble working with paint.
click on image:

 

It is a interesting topic you raised Doug.

About the interaction of the center board the distance between the float and main hull as well as the drag from the float during a maneuver.

Were you also referring to an over square trimaran as part of the discussion.

That is one of the problems of using paint to form the radius, being limited to four points to work from. You can only do the best you can with it.

 

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Sure, it could be-I think there are some interesting considerations involved here. Take ,for example USA 17: no board in the main hull-very forward location of the lee board(and only the lee board is used at any one time). And the Rave and Osprey foiler-no foil in the main hull and both tack well but the boards are quite far forward-and both daggerboards are always in the water.

 

Are you sure the distance between rudder/board and it's radius is even relevant to the turning radius?

 

I was fully aware where the centre board trunk was but was working on the assumption that it was the designer who had added the approximate turning radius. I printed off the plan drew the right angles on which worked out the centre of the circle well behind the trunk, then re-scanned. Using Paint is beyond me, much happier with pencil and set square.

Why?

Your theory seems to be that the distance between the rudder and the centre board equals the turning radius irrespective of the design, which seems very implausible.

You say initially, how do you see it changing with time?
The force is the same but I agree the effect is greater i.e. the moment of the force due to the extra leverage.

 

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1) Because there is a difference in behaviour during a turn depending on this distance( and other factors such as weight, ama immersion etc.)

2) As best I can tell ,with the greater distance between the rudder and daggerboard the boat is less likely to stall and should slow down less thru the turn.

 

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Yes, fairly sure.

 

Gooday fung. Thanks for your many & varied questions.

Why can't you stay with 1 simple question at a time - instead of 'mudding-the-waters' with several other questions ??? Whilst the 'other questions' are related - they are - only somewhat distantly related - as you well know. Discussion for discussions sake - is only of value - if we all learn something from the discussion at hand. 'Drifting' is something that should remain the province of other forums & for late afternoon - rum races around Auckland harbour ??? Yes/no ???

"Doug if you know how foils work" - Hey there - fung - if you don't think that a vessel will turn in the circle that Doug has drawn (for the sake of the discussion) then please fung - you draw a circle that you extimate that the vessel will thrn in.

Your reference to 'gliders' or 'sail-plains' as they are - may be relative in a horizontal plane but surely not vertical plane ? Yes/no ??? (I see a 'LS4a GKP @ $55K NZD) however I 'WANNA-GO-SAILING' on the water - can't get to SE Asia in a 'sail-plane, darn ! !

Next - what does a c/b - being - off-set - have to do with this discusion - or in fact anything else ???

Your post today - starts with - "What are you really asking" & then goes on to 'tell' me what - is being asked. HA - I think - NOT - If you can't go back to the beginning & read what has been spoken about - from the beginning of this 'thread' the I suggest you do & it will answer your question - as to what is being asked & discussed & what we all are positively dtalking about & discussing. I - personally - suggest you try it - you'll find it works.

Another next - if sailing balance is totally devoid from turning-compromise & the overall sailing ability of a comfortable sailing vessel - I would ask you to please explain - as I've not experienced these 'matters' that you allude-to.

Looking forward to your explainations so I may further my learning in these matters before I return to cruising, Thanks. Ciao, james

 

Gooday Wassa - I don't understand your last sentence. Please explain as I'm a bit thick & inexperienced & don't understand the other matters you raised - either. Thanks for any explainations. Ciao, james

 

Doug, what is the point of this discussion? ... because, as I read it, you're implying that the trimaran has stopped, yet continues to pivot on its (dagger?) with the rudder prescribing an arc around this dagger, pivot point. You know in the real world this does not occur. There are a large number of variables all acting out their parts.
For example, you go into a tack on your 12m x 12m, high performance, fine bowed trimaran (that actually prefers to run straight, doesn't like turning much), leeward float begins its WIDE turn, main hull begins it less wide turn, windward hull is flying clear, already you've travelled what, 30 metres forward and to starboard and you still haven't come head to wind, okay crossing through now, leeward float lifts and windward float begins to drop - but at this moment both floats are in the air, but fine main hull bow has been digging a slow turning groove through the sea and now hits a wave further slowing the turn, you've gone another 10 metres forward and to starboard, boat is slowing right down, you look back at your wake, the radius of your turn looks like what? - whatever, very wide, your wing mast rotates towards central, and you've just freed the main sheet so as there is absolutely no chance of weather cocking, crew eases headsail, helm is now hard across, further acting like a brake but still keeping the tri slowly turning, maybe 5 metres, if it stops you're going to be cursing under your breath, because you're going to go into reverse and will have to reverse the helm to back the *** upwind (what is your turning radius now? - must look kinda strange on a plan drawing) good, doesn't happen, what was formerly the windward hull comes down and hits water, digs in a bit, slowly you come round to new board, travelled another 20-30m forward and right (just for a rough estimate), now you slowly bring in the main again, headsail being sheeted in, you look back at your wake, huge bloody turning radius looks like 100 metres... sure wish we could stop and spin on a penny like Doug's drawing - oh, well, you've got a wide tri, can't have everything, eh?