# Trimaran Turning Radius: Ama Position

Discussion in 'Multihulls' started by Doug Lord, Jun 27, 2012.

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### smallyachtsailrJunior Member

Lots of variables to consider here, but personally I think redreuben hit the nail on the head. I always like to bring things all the way back to Newton's Laws. Boats have mass and inertia, and also water resistance from things such as boards and amas and fine bows. From my experience, not only with sailboats, its mainly mass and hence, inertia that has to be overcome with a force imparted by the rudder. If as redreuben says, your lever is short, or your rudder is too small or inefficient (or stalled!), you won't have much force to change the inertia of your boat, and you'll continue moving forward faster than you alter your course--and so, a large turning radius. Conversely, a large lever (large separation between rudder and turning point (board position or combination board/fine bow etc.) and efficient rudder will turn you faster. If you use Newton and go through the calculations (f=ma), you'll find that even a lighter weight boat has a heck of a lot of inertia to overcome.

Total boat mass isn't the only thing--keeping the ends of the boat light makes a huge difference in maneuverability.

So, I guess I'm saying, sure, underwater profile and turning drag is of importance, but IMHO its a simple force vs. inertia problem and putting your rudder far from your pivot point (board) gives you more force.

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### Gary BaigentSenior Member

Sure, all short and narrow multihulls behave like sailing dinghies when tacking ... how about we turn our attention to lightweight, square multihulls of at least 10 metre length and beam, weight around 600kgs - ah oh, not so fast turning now, eh?

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### Doug LordFlight Ready

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Unless I misread what he wrote, he was referring to the same length boat with a long or a short distance between the board and the rudder?

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### Richard WoodsWoods Designs

Well, a Hobie doesn't tack as fast as a dinghy.

I thought AC45's tacked fast enough. OK they aren't 10m long

my Strike is half the length of a Gemini, but wider

Sorry, I thought videos of a real boat being steered under three different conditions was going to help you

Richard Woods

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### warwickSenior Member

Hi Richard. I suppose that is one of the problem you have to face, one person would like their boat to track straight and another would prefer it to round up gently. Would the issue be able to be trimmed out, either way to suit the owner.

Then your next problem would be if the boat is single handed or crewed which come to Doug's issue (which he is entitled to bring up).

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### Silver RavenSenior Member

Who can sail what ???

Gooday Richard & Good-on-ya ! ! ! What a very self explanative set of videos !

What a SUPA little tri & QUICK AS & EASY TO SAIL as well.

Thanks so much for posting. Great to see the 'old phobias' - that multihulls can't 'tack' or 'jibe' - finally put rest. Thanks again.

What's even better I M H O is to the the 'gerri-hat-trick' - older fossile - - out there doing it all - so extremely well - without effort - as 2nd nature & with a big smile on his/your face . WOW - that's the part that really grabs me & I sure hope sell lots more plans.

There wouldn't be 1 family - that I know of - that wouldn't want to - enjoy having that much fun with their kids - sailing a super little - cost effective tri just like that.

Again - well done & a mighty thanks, Ciao, james

PS. What did it cost all-up to get to that sailing stage ??? Very reasonable - I'd think ???

OH - & didn't anyone say to you - that it's not 'cool' to burn-off much bigger multi's especially sailing through them to leeward ??? Well done - like your style !! jj Wish I'd been there. Where is that lovely spot - care to tell ??

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### Gary BaigentSenior Member

Er, Richard, Hobie 16's have long, thin, deep hulls, no boards ... and isn't that what the thread is about, rudders and boards? Those thin, asymmetric hulls act just like a finebowed multihull; slow to tack, is that not so?
Also I've seen the proportionately not very wide AC45's (narrower than the X40's), stuck in irons many times.

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### Silver RavenSenior Member

Gooday bloke - you've got some very big 'irons' in NZ to "iron-out a 'flat-tack' for an AC 45 - I'd like to see an AC 45 with hulls the shape of a 'Hobie' - Ha ha ! !

Now while - I'm sure we'd all admit the AC 45's are very - very quick - there's no way that they are the quickest 45' cat's available - for the money spent. That's just not what they were ment to be ! !

I'd still like to learn all I can about blades - be it - wing-masts - rudders - c/b's trim-tabs - either leading or following ones. That's the subject in this 'forum' - other subjects - for other 'forums' Please

More information about the selected subject matter & less wandering around inj the bushes & in circles - that is - unless you want to - look around 15.5 acres in Far North Queensland & the BUY-IT. - So I can go sailing ! ! !

Ciao, james

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### Doug LordFlight Ready

Richard, I'm not so sure about the "directionaly stable" video-wouldn't that behaviour be dangerous if the boat was singlehanded and the crew fell off? Would it be better if the boat rounded up and stopped if the crew lets go of the tiller? Thanks for contributing to the thread!

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### WhatsisnameNew Member

About how far forward are the boards in the outriggers from the rudder? Knowing that distance some of the theories from the thread can be tested somewhat, since we can see an approximate turning radius from the videos.

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