Trimaran ama angle question

[smallyachtsailr]

I'm working on a new 14' sailing trimaran design and I have one question. I know some designers position the amas parallel to the main hull, and others advocate angling them inward (in plan view) a degree or two on the theory that when the leeward ama is submerged it will generate more lift to windward than it would if mounted parallel to the main hull. Does anyone have any opinions or thoughts on this question?

[Doug Lord]

I would think something like a gybing daggerboard would be more efficient than angling the hull. You can also mount the daggerboard in each ama and set the trunk toed in slightly-but I'd keep the hulls parallel. The hull is not as good a lift producing foil as the daggerboard. Try posting this in the Multihulls forum. http://www.boatdesign.net/forums/multihulls/

[smallyachtsailr]

Thanks. The boat will have a centerboard in the main hull. I'm new here, will repost over in the multihull section--didn't notice it!

[Silver Raven]

Originally Posted by smallyachtsailr

Thanks. The boat will have a centerboard in the main hull. I'm new here, will repost over in the multihull section--didn't notice it!

Gooday 'new-be' - 's-y-s' - - Great to see another new face - join our - wonderfully diverse mob - of slightly bent (oh my gosh - that's a big understatement) bunch of 'sailing, designing, cruising, & other' guys & gals - in these - very wide-ranging 'forums' discussions.

A big Welcome to you ! ! ! Stick around & enjoy - we don't bite (we do get a tad bit - 'heated under the collar' from time to time) but 99% of it is all in good natured friendship - so a bit of lateral tollerence IS required & in general we're an O.K. bunch.

Enjoy & contribute as you feel comfortable to do. Ciao for now - from down-under, james

[tspeer]

Originally Posted by smallyachtsailr

I'm working on a new 14' sailing trimaran design and I have one question. I know some designers position the amas parallel to the main hull, and others advocate angling them inward (in plan view) a degree or two on the theory that when the leeward ama is submerged it will generate more lift to windward than it would if mounted parallel to the main hull. Does anyone have any opinions or thoughts on this question?

The total side force is going to be the same, whether the ama is toed in or not, because the total side force is determined by the sail trim, not the hull/board design. (Think of the difference in lift between sailing upwind and dead downwind - the underwater shape is the same, but the lift is vastly different.) The leeway angle will adjust so the side force from the hull & board(s) equals the applied aerodynamic load from the rig and topsides.

The question really is will the drag be reduced by shifting some of the lift from the main hull & board to the ama? Since the induced drag due to lift is inversely proportional to the square of the span, and the ama has a much smaller depth than the board, I would think shifting lift from the board to the ama would increase the drag.

However, if the boat has weather helm, getting more lift from the ama bow may help to balance the boat better and reduce the drag of the rudder.

You need to do some drag bookkeeping, accounting for each source of drag and how it would be affected by the difference in the orientation of the ama.

[tom28571]

I think Tom Speer has the answer and having the amas at a different angle from the main hull will definitely add drag. I doubt this is a good idea and a jibing board is likely no better than one on the centerline either.

[Silver Raven]

Originally Posted by tom28571

I think Tom Speer has the answer and having the amas at a different angle from the main hull will definitely add drag. I doubt this is a good idea and a jibing board is likely no better than one on the centerline either.

Gooday Tom-28. Read what Mr. T.S. said - please. With all due respect - "I think Tom Speer has the answer - etc etc"

That NOT what he said - YOU'VE taken T.S. - out of context.

I'm not trying to start a 'deep & meaningful' arguement - however - Please let me assure you that - monohull keels with 'trim-tabs' fitted on the trailing edge - - c/b or ama-fins &/or rudders that have 'trim tabs - DO WORK repeat after me - DO WORK - - They worked 45 years ago - they work today & they will continue to work tomorrow. This IS NOT ONLY SOMEONE'S OPINIOPN - it is known by all the people who - really - were out on those boats & did the sailing - c/w a wet ass, hicking to windward, doing 5 hrs - harbour races - - racing 200 miles & yes 500 mile races - in monohulls, catamarans & trimarans.

This continued - "foarmation of conclusions from incomplete evidence" is very sad, extremely misleading & a total bunch of "trivial, or pretentious talk/writing".

Designers such as - A. Payne, B. Miller (B. Lexan), P. Cole, L. Crowther & several others - knew - back in the 60's that 'increased camber foils' work.The continued - lack of acceptance of this 'well-known' fact - does do you no credit & in fact reduces these 'forums' to the lowest 'common denominator'

Go sailing - try a wedge in a 'board' - oooch the rudder 2 degrees - see what happens & then come back - wet-ass & all & let us know 'just how bad it really was'.

Next - 'break through' you'll try to - tell-us - doesn't work - is 'canting mast' are a total waste of time -YEA - - RIGHT - don't think so. Not this week. jj

Ciao, james

[philSweet]

I can't see any possible advange when sailing steady. Any chance that it helps when tacking?

[tom28571]

Originally Posted by Silver Raven

Gooday Tom-28. Read what Mr. T.S. said - please. With all due respect - "I think Tom Speer has the answer - etc etc"

That NOT what he said - YOU'VE taken T.S. - out of context.

I'm not trying to start a 'deep & meaningful' arguement - however - Please let me assure you that - monohull keels with 'trim-tabs' fitted on the trailing edge - - c/b or ama-fins &/or rudders that have 'trim tabs - DO WORK repeat after me - DO WORK - - They worked 45 years ago - they work today & they will continue to work tomorrow. This IS NOT ONLY SOMEONE'S OPINIOPN - it is known by all the people who - really - were out on those boats & did the sailing - c/w a wet ass, hicking to windward, doing 5 hrs - harbour races - - racing 200 miles & yes 500 mile races - in monohulls, catamarans & trimarans.

This continued - "foarmation of conclusions from incomplete evidence" is very sad, extremely misleading & a total bunch of "trivial, or pretentious talk/writing".

Designers such as - A. Payne, B. Miller (B. Lexan), P. Cole, L. Crowther & several others - knew - back in the 60's that 'increased camber foils' work.The continued - lack of acceptance of this 'well-known' fact - does do you no credit & in fact reduces these 'forums' to the lowest 'common denominator'

Go sailing - try a wedge in a 'board' - oooch the rudder 2 degrees - see what happens & then come back - wet-ass & all & let us know 'just how bad it really was'.

Next - 'break through' you'll try to - tell-us - doesn't work - is 'canting mast' are a total waste of time -YEA - - RIGHT - don't think so. Not this week. jj

Ciao, james

For a post with no "deep and meaningful argument" you certainly came bounding off the wall. And you took me out of context by misunderstanding what was said. A fixed foil, not an asymmetric or articulated one, is going to generate the same degree of lift on a boat whether it is jibed or not. There is no mention of an articulated foil in my post. As T. S. said, it can however affect the heading of the boat and therefore the set of the sails to the apparent wind. What happens to the effect on the hull and rudder is another matter. Read a bit more carefully before flying off the handle.

[Silver Raven]

Originally Posted by tom28571

For a post with no "deep and meaningful argument" you certainly came bounding off the wall. And you took me out of context by misunderstanding what was said. A fixed foil, not an asymmetric or articulated one, is going to generate the same degree of lift on a boat whether it is jibed or not. There is no mention of an articulated foil in my post. As T. S. said, it can however affect the heading of the boat and therefore the set of the sails to the apparent wind. What happens to the effect on the hull and rudder is another matter. Read a bit more carefully before flying off the handle.

Gooday Tom Lap - - nice to see you here & to hear you again. After 1683 posts - I'm very inpressed - that you'd even bother to reply to me.

I'd like to bring to your attention that - "d & M' - - "bouncing off the wall" is a tad - - over excenuated - - I M H O - - You're - far more experienced - than I am. (maybe).

I did NOT 'misunderstand you ' what I did was read what your DID write - my mistake ??? I think NOT. I'm trying (very hard) to understand what you - - said - he said - cause as a sailor - of very fast m,ultihulls - I totally bissed IT totally. s- I ,uxt have.

I've sread - what you've written - & I've NOT 'flown off the handle' at all

I did - challenge what you - claimed what Tom Speers - said - cause I didn't read it that way. If YOU choose to do that's your "problem' NOT MINE -

I did in my - 72 years of life - - 66 years of sailng - - make a great mistake - I read what you said - & read what you said TomSpeers said - i'm not sure what else you would reasonably ask ot me - nor - at this point - do I care or would I know. I'm been running my own 'marine business' for a few days - at this stage.

You are very welcome to your opinion - However I'm not having to share i & am not likely to get in that 'long' line.

I've been 'playing with' multihulls, monohulls, wing-masts, canted keels, trim-tabs - keels & rudders - - since 1966. If you have a problem with the truth - please be considerate enough to acknowledge that - I did - not - - bounce of the walls - just - the other day.

You have a great day - - do enjoy your sailing - but please leave me - well & truely - - OFF - - your future - mailing-list. Thanks, james

[tom28571]

Originally Posted by Silver Raven

You have a great day - - do enjoy your sailing - but please leave me - well & truely - - OFF - - your future - mailing-list. Thanks, james

That's easy. But please go and take your medications. Bye.