Trimaran Turning Radius: Ama Position

If the turning radius falls inside the ama, it means the captain will be very pleased as the boat will be able to turn in almost its own length and it won't act like a bull in a china shop whenever it enters a marina.

However, the turning radius has nothing to do with the circles drawn on the planforms diagrams.

As a first approximation to the turning radius, one might draw a line perpendicular to the daggerboard and another line perpendicular to the rotated rudder. Where the two lines intersect would be the turning radius.

That, however, does not take into account the fact that the board and rudder have an angle of attack, and therefore are not tangent to a circle about the center of the turn. If you assumed an angle of attack of, say, 10 deg, then you could draw the local velocity vector at the board and rudder(s). Lines perpendicular to those velocity vectors would then intersect at the center of the turn.

What you'll immediately see from this is that it's not just the distance between the board and rudder that matters, but also the angle through which the rudder turns. I can tell you from personal experience that having a trimaran with a somewhat limited amount of rudder deflection can make it a real bear to maneuver in a marina.

 

Yes, Doug, Tom has pointed out the most important aspect of a wide, light multihull making a narrow (as is possible with such a platform configuration) turn: the rudder must not stall, be of excellent hydrodynamic shape and area AND be able to turn to near right angles with the main hull or platform ... and, did I say, not stall. Because you've got the very widely positioned ama (plus maybe a foil) executing a wide turn radius before you get head to wind, not to mention the fine main hull bow crabbing round - that rudder has to be right to handle all this.
Hey, by the way, Camper, absolutely stripped and gutted of all excess weight, strutted her stuff and stomped the fleet in the last leg.
Lightweight ... can't beat it - except in a multihull going through a tack in a seaway.

 

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You got the part where he said:" it's not just the distance between the board and rudder that matters...." ,right? Of course you did........ Camper...in the Volvo-cool!

 

... "but also the angle through which the rudder turns."
Referring more to that, Doug, considered it more important ... because there always is a decent distance between dagger and rudder for leverage.

 

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Oh come on......Congrads to TNZ! Why can't you beat Jimmy? Sorry I couldn't help myself......

 

Gooday you mob. Heaps of factors & many more matters to consider - for sure - I M H O - so cause I haven't got it all back together as yet - I'll try several short (ha ha) posts - 1 to each part of the subject.

Big - Please - here from me especially - Doug, Gary, Tom, Eric & some others - please stay with this rather & convoluted subject - please. - I'm a very slow learner & extremely ancient also. We need your collective knowledge with this subject.

So - here I go; might some persons in this discussion - agree that this 'changing heading' elipse (a good - quick manouver - is not qa circle or part there-of) - of say 90* has many many governing factors; some might be considered to be; weight - especially at the ends - - rocker (& the correct balance of that rocker) - - centralized sail plan - drag/lift of the sail-plan - - not withstanding the position of the C of LR including & not excluding the c/b(s) & rudder(s) & or any other foils but considered seperately.

Two observations here; Question is - what do Eric, Gary, Doug & Tom have to say about the position of & the rake of BP5's foils ??? - c/b + hull - draft 19' (5.8m) - - I do admit - BP5 - is not light - Disp 23 tons (for its size it may be) - it's not short (@ 131' - 40m) - it's rather wide (@ 75' (23m) (for its size ???) & @ $20mil Euro plus - it sure isn't cheap - BUT - IT IS FAST very bloody fast ( top speed of 47.15 kts - new world 24 hr record of 907 kt miles - - new Tarnas-Atlantic record & as Pascal Bide'gorry, Loick Peyron, Frank Cammas & others agree - - BP5 sure does have much more speed to show us all. I'm waiting - - Gawd - if only we were Rockerfeller, Wennergren, Kennedy & could afford to build a bigger, newer, better, faster, one - then I could live for another 100 years on just the 'adreline' alone.
Anyway blokes - what do you think about the c/b placement & angle ???

Now - again - to you all - (but addressed to Gary) - an observation - that is a known fact; [Addressed to Gary - cause he has sailed on said 40' cat (Sundancer) many times].

Back in 1967/8 - a 40' cat - did tack for many miles & tacks - (weekend after weekend) - going up a harbor - out-tacking 90% of the -'gung-hoe' race boats in the monohull fleet - sailed faster, sailed higher to windward, tacked quicker, tacked inside the mono's & just - out-tacked, out pointed & sailed away from them. Overall race committe - came on board - after the race (several of them) & spent an hour - looking for an engine - - ha ha - there was not any engine on board for many years after it was built. Question is; if we could do it back in 1967/8 - why are most people having trouble doing it or realising it can be done - now that we are so very advanced in 2011/12/13 ??? Thought & comments - please. I - obviously just don't understand ! ! !

OH - & Go - Go Go Camper. Just gotta-love them light boats - mono or multi. ! ! Ciao, james

 

James, Sundancer always had the legs on monohulls (and multihulls too in those days) so that was not a big deal being able to beat them. I remember the dancer making mincemeat of maxi Buccaneer in one Balokovic Cup Gulf race. Because Sundancer was years ahead of anything else around; very light, decent rotating mast rig, not wide overall beam and some rocker too in the narrow but round bilge, cross section hulls (sorry too lazy to look up figures), good daggers and deep transom rudders (and a big sweep angle of the latter .... Doug?).
BPV's foils are just big curved and angled slightly forward, but her dagger is in typical French fashion, acutely angled aft from forward of mast to get the CLR into the right area; the rig is set well back from bow too. Not at all being critical but if you watched her tacking, it was a slow process ... but you're talking about a multihull monster ... rather than some little models ... Doug!

 

Gooday 'Cocoa Beach' kid - You are from the USA - right ??? Easy now there (ha ha ha) - Careful of the numbers on the other side of that comment - real carefully tread there - hay ??? I can't find a 'huge' laughing face & still don't know how to use the 'icons' - aren't you lucky ??? Ah - ha ha ha

Where was 'jimmy' born ???
Where does 'jimmy' call home - in the real world ???
Where do 'most' (at least 35%) of the AC boys come from - gained their sailing expertise ???

There's a song that some of us sing - about where do we call home ??? - I wonder if it's "I come from a land down-under" maybe - he he ! ! !

I'm sure I can rest my case - - so back to you - why would we think it all that important to beat - one of our own ??? eh ??? Please ??? - ciao, james

 

Nah, James, Doug is just being an agent provocateur - as if Aussies and Kiwis loathe and detest each other ... What? ... ANZAC's, no way, Hosea (Gear?). The latter will confuse him.

 

Gooday Gary & others. The real reason I brought up the subject - yet again - is to see if ohters might wish to try to learn from David & his designs.

There's not doubt that 'dance' was quick - not only in 1967/8/9 - but also until she was - as they say 'de-commissioned' by the bloody jerk - that owns her. Bloody stupid man. Personal ***** & I'm allowed ! ! !

Light - yes @ 5300 lbs (2400 kgs) back in those days - everything else around weighed 9000kgs & we couldn't even talk Lock into building any lighter - at least for the next 6 to 10 years. Great learning times though.

That 'rotating mast' (the very first in the world for an ocean giong multihull) took us at least 6 months of intensive homework - with the best mast chaps around in those days, but thanks to David's genius - we got it right the first time around. There was some truely ground breaking design work in that task.

At only 17' (7.7 mtrs) wide & extremely stiff (to sail) along with the - symmetrical-hulls - canted out at 7* c/w - pre-tensioned (pulled-in hull sides) & top-hat foan sandwich ribs from keel to gunnel - the hulls were very stiff indeed & light for those days. Oh - what we could do today -WOW plus !

I didn't/don't think - that at 36" (.915 mtr) max beam at the waterline - I don't seem to recall that - that was all that thin ???

No need to look the figs up - I have the drawings right beside my left elbow - (a brand new set) c/w the lines of another one super-imposed on top - thank-you David. Must phone him - been putting it off until I feel better though.

The c/b's & rudders - were like the mast - many many hrs of development & mostly in the right direction. Only to be superceded by all the work that MIller, Cole, Payne, Crowther & others put into c/b's, rudders & keels - some years later - & funny that not 1 person has picked-up on all that development - not even today. Go figure eh ??? Those developed foil shapes are at least 20% better if not more ! ! ! Less wetted surface area, more efficient, closer winded, not hard to build. light & far less resistance (less angle for a bigger effect) than anything around today. I M H O - of course.

Thanks for that info on BP5 - I honestly hadn't noticed that 'she' tacked slowly - must be the only thing that 'she' does slowly - me thinks. Has anyone noticed how the new 70's have positioned their c/b & rudders = other than - usually flying in the air - of course. Very impressive but not on this weeks allowance - DARN ! !

Sure wish someone would -fess-up - & tell me what section to use for a wing-mast or at least a couple of educated guesses. Anyone out there - care to draw a section or offer some advice ??? All & any info gratefully accepted ! !

I see somewhere there is a subject about models yachts - Gawd - don't start me in on that subject - we built 100's of them - to do a lot of research - with the above mentioned luminaries of the design world - which were a great assistance. Each one of us (some 20 guys in total) had at least 8 each. Mostly built in my living-room over a 5 year period (big room though & got extended 3 times). We built all 3 classes - Marblehead - 10 sq raters & 'A' class - all radio controlled & with 10 & more channels. What great fun that was & a real vertical learning curve. Gawd - back then - it was 24/7 boats - boats & more boats.

Ciao, james

 

James why don't you post some of those developed foil shapes? It would be good to see what everyone has forgotten.

 

Post what ???

'cav - PNW - Just how many - 'SUPER STARS' do you think that would drag out of the wood-work ??? Just in loose 1000's will do foar a guess ! ! ! Now there are some really smart chaps in here - but 2 hands-full would pull up that total number. Like Tom Speers, Nigel Irene, Loick, Pasca, Cammas, Rob Denny, Eric & maybe a few others - but - if I chose to - why would I want all that - self-inflicted pain ???

I've got a 15 plus acre farm to run; breed Australian tropical parrots - that spend more time in the house than us, sometimes; trying to find this guy named - 'health'; getting ready to go th SE Asia & get 'toy; keep loving my 'green-eyed' Angel (that's the most important item); sell this bloody property so we can - move on in life - & - now you think I'm going to put myself up for the likes of all the 'really smart' chaps in here top use me as target practice - non-stop 24/7/52 for the next year - just to inflate their own egos - Ha ha - I THINK NOT ! ! My friend (if I may) - I'll keep taking your perscribed medicine - on toast - but that suggestion IS just never going to grow legs - not ever. You want it - you ask for it. Enjoy your day - Oh - did you see that Canuck, in here - that said - in his bio heading that - where he was - spending B.C. in the summer - - ha ha - Can't remember that I ever recall a 'propper' summer in B.C. - not in all 20 years of waiting. Ha ha. Good one to y'all. Ciao, james

 

My lips are sealed James, the gubs hope Oz is less wet than Canada and that their umbrellas aren't needed

 

A lot of chaff to sort from wheat here.

But the following three videos might help your discussion. They are all of a real boat, a Strike 18 trimaran. It has a rudder on the main hull and daggerboards in the outriggers

In the first video you see the boat sailing by itself in a straight line with no one steering. I just let go of the tiller, it's not lashed.

http://www.youtube.com/watch?v=lhDim5EJC84&feature=plcp

So that proves the boat is directionally stable

In the next video you can see that at 8 knots boat speed I have the end of the tiller balanced on my thumb.

http://www.youtube.com/watch?v=JOYhtdFwTpU&feature=plcp

So that proves the boat is easy to steer

In the final video, after a couple of tacks I pushed the helm hard to starboard and kept it there while letting the boat sail round in circles. It would have done that for ever, but I got bored (and I'm sure you would get bored watching it as well). But you can clearly see the turning circle is little more than a boatlength

http://www.youtube.com/watch?v=pECFWJvXAow&feature=plcp

As I say, I hope that helps

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Thanks Richard for putting up The you tube clips, it probably helps to be able to see it in action. Rather than trying to describe it in words.