Why hasn't someone part II

[Atom Ant 0]

OK not quite the same but another question from the uninitiated. This relates to trimarans only so all you bipod drivers feel free to pitch in with useful answers.

 

Tri's can have a centreboard in the main hull, or they can have a board in each ama, or a combination I guess of both.

 

Given the interior space limitations of a trimaran hull compared to a similar length lead slinger, I can see that the centrecase takes up space. So why don't they just have boards in the ama's and save space in the accommodation department?

[TimB 7]

AA,

Many do (leave out the centreline centreboard), eg Sam's new boat, frantic drift, Oracle BMW tri etc. However in many case the boards in the floats are asked to also provide vertical lift as well as horizontal (to stop leeway) and if optimised for lift don't do a good job of leeway prevention. The ORMA60 trimarans which could be said as developing this to an art form almost always have (had?) a very deep centreboard to provide leeward prevention.

My own experience with vertical float boards (with none in the main hull) was that the boat was very hard to tack and had gross changes in helm load, from heavy weather helm to dangerous lee helm depending sails being used. Ian Farrier has also made comments on this on a thread on SA about his new NZ8.5 design.

TB

[TimW 1]

The biggest pain about centreboards only in the amas is they need to be quite big to work properly and therefore you need to raise the windward one when not in use.

One more job for an already busy crew, and the only time the board will move is right in the middle of a tack when the boat has slowed right down.

 

BTW my understanding is that Sam's new boat, and Lucifer just have a central centreboard.

 

Where the Orma 60's ended up is a giant central centreboard to prevent leeway and then two curved foils to provide lift.

the impact downstairs can be minimised as the centrecase is generally close to the required mast bulkhead.

[Guest]

Not being pedantic - but they aren't centreboards (which fold aft) but daggerboards (which are up/down vertical or near vertical).

On the point of ama daggers and changing helm feel and difficulty to tack, I call BS. If anything, if you have ama rudders, the rudders can be smaller yet do the same work as a larger central hull one, and be more responsive (because they're set deeper in undisturbed water). On the tacking thing with ama boards, the same thing applies; as your tri flops over onto the new board, the boat is easier to get moving with the dagger out to leeward because the platform pivots there, anyway, I have never had any problems with this configuration. Also, with a more immersed leeward ama dagger (therefore more efficient than a partially aerated central hull daggerboard, or rudder) that too can be of a smaller size, yet do the same job. Also, the dagger can be asymmetrical for more gains. So you're winning with less wetted surface area, less drag, better efficiency, control and feel. But I guess it is all in the eyes (hands) of the beholder ... but I disagree with this latte mythology of ama appendages being ... difficult etc.

On the point of having to lift the windward dagger to get it out of the sea: most tris have dihedral and, with the daggers being smaller than an obese central board, only the tip will be in waves, therefore minimal effect - but in light airs, if that worries you, it is little trouble to lift it a little. certainly, in most conditions, you don't have to lift one, drop the other when tacking and gybing .. that is, if you have a trimaran, cats are different however, also three hulled cats too.

[Atom Ant 0]

This is cool. It had occurred to me that the lee-boards (if we can call them that for arguments sake) in the ama's could be asymetric thereby offering lift to windward. But the converse thought was that if it was so good then why don't Farriers for example have them. Then I also thought that simplicity has many advantages & so on & so forth.

 

I've seen the groovy lifting foils eg what Pedro has on the Recidivist trimaran (that isn't a secret is it Pedro?) along with the central dagger board.

 

The Dragonfly has a tilting board set up which certainly reduces the visual impact internally on a small tricycle.

 

DAMMIT would someone buy my boat so I can get me a tri!!!

[Hurts 6]

Hi guys - I'm quite interested in this thread too. I have bought the old frantic drift amas. I am building the centre hull without a centreboard/dagger board (or rudder) and I am planning on using the current inward-angled straight board casings with daggerboards. End result will hopefully be an 8.5 that is compeditive...

 

Was getting a little depressed at the Tim's comments but Cox has cheered me up a little again. I have also talked it over a little with the Frantic guys and they are happy with the config and continuing with it with thier new ama's. I am guessing it may be a trickier setup to get right and potentially more work if you do need to pull boards with each tack and gybe but it is going to be primarily targeted at racing so I guess it's one of the choices you make.

 

Tim B, what boat did you sail on with the gross helm loads? What was the relative posns Forward/aft of the mast base and the cases?

 

Coxcreek, Sounds like having some decent dihedral would make boat handling easier (i.e boards will automatically sort themselves out with tacks etc ) but that means to fly a hull will be harder and I'll be leaning quite a bit more and loosing grunt. I don't want to mess about with a canting rig if I can help it as mucking about with the mast support while sailing gives me the willies... Have I got my thinking right in this paragraph on the various trade-offs you make in this area? Is there other factors influencing dihedral angle choice?

[TimB 7]

Coxcreek,

You are probably right in some respects, my boat had no dihedral and both floats were immersed at rest (it was toooo heavy for the centre hull) so it was like dragging a 8.6 x 7.3m raft around. The float boards were asymetrical and the windward one made all sorts of weird noises and bubbles if it was left down, I suppose it was stalling and cavitating.

I can only comment on what I experienced and the changes in helm load were real, I always blamed the boards. If their position was wrong the helm wouldn't change from weather to lee depending on wind strength as it did. Overall the boat was not much of a success which is why it has been in a shed for 10 years waiting for inspiration.

I am currently sailing on a F8.2 and that tacks easily (without having to ease the main mostly), we never lift the single central daggerboard (sic) and even though the rudder blade ventilates at times we don't get big changes in helm load. The rudder blade is ~1.8m long with about 1.4m of that in the water. D'brd about 1.5m in the water.

[Deep Purple 511]

From the outside looking in................ the orma 60's and maxi tri's spent millions developing their boats. The settled on a big mutha central daggerboard/rudder and lifting foils. Rudders in the amas? yeah if it floats your boat, but darwin says you'll still have a rudder in the middle.

 

Don't want lifting foils? yeah put them in then amas but then you're actually sailing a 3 hulled cat. By the way...... BMW oracle was a 3 hull cat, don't be conned otherwise.

[Guest]

Lionmark, if your tri is light and you're not going for fashionable, inflated balloon-type amas, then your tri will have some dihedral. If it is light, the centre hull is going to be lifting whether you have full length floats or average ones with angled or curved or J foils. Therefore the windward dagger (sic Tim) is mostly going to be free flying ... and the more powered up the tri becomes, the main hull comes out and the windward one is higher yet. I've always thought that these cafe latee three hulled cats are going to be absolute dogs in light and drifting conditions. Okay they power up in a fresh breeze ... but so will a foiler version too.

Tim, the windward dagger if immersed, is doing nothing to help your sailing if your boat is heavy and windward ama touching. Horrible drag. On such a boat the windward board would have to come up. Down it would definitely effect your helm. Chainsaw job required, gut the boat and angle the akas up; it may be transformed.

Sundreamer, the ORMA trimarans had curved or angled lifting foils in their amas which hoisted the overpowered floats up but did not make great anti leeway devices; hence the big daggers in their main hulls. A number of them had ama rudders too, as BMWO did later but at first they had three rudders , two foils and one monster main hull dagger... but then they dumped the main dagger and rudder ... and with a few other things like wing and platform fairing, took off. Actually you're wrong about her being a three hulled cat, she was really a two hulled trimaran.

[Guest]

Why hasn't someone tried T-foils on the rudder and boards like on a Rave? (Actually someone probably has)

 

http://www.trademe.co.nz/Trade-Me-Motor ... 067902.htm

 

or just search "rave"

 

Careless Intent might make a good test bed. I heard she might be for sale. Anyone crazy enough?

[Guest]

Dan. I've had inverted T rudders on my tris for 30 years now. Here's the rough first stage for my latest version, Seditious Sid, 7.62 x 8 metres.

[Deep Purple 511]

Hey coxy, I walked down to your neck of the woods the other day and took a look at the flotilla. Nice place for gathering

[Guest]

hey Sundreamer, yes, it is a neat area at the mouth of the creek - but there is one malcontent local who hates the sight of the boats moored there (especially mine) and he's always phoning the harbour board or the ACC (or whatever it is now) ... so although a paradise, there is an unpleasant ass biter lurking behind windows there.

I'll have the repainted and repaired wing mast up soon.

[TimB 7]

Coxcreek said "Chainsaw job required, gut the boat and angle the akas up; it may be transformed".

 

Been there but not done that ...yet. Main has hull gone to the tip (was tooo heavy and tooo thin). New one, fatter and maybe lighter started. Yet to decide about foils but leaning towards central dagger. Will be a floppy tri with curved beams. Also want to make it fit NZ 8.5 rule (was too long).

[GregW 28]

Is that your boat TimB? it looks like Careless Intent style with the forward floats, but smaller in the pic. Firebird maybe?

[TimB 7]

Correct Tennant Firebird and yes forward floats which do work but now days would be the other way up.

[GregW 28]

Cool. I presume the only one built? (here anyway). Would be keen to come have a look at what your doing with it some time if convenient.

[samin 0]

OK not quite the same but another question from the uninitiated. This relates to trimarans only so all you bipod drivers feel free to pitch in with useful answers.

 

Tri's can have a centreboard in the main hull, or they can have a board in each ama, or a combination I guess of both.

 

Given the interior space limitations of a trimaran hull compared to a similar length lead slinger, I can see that the centrecase takes up space. So why don't they just have boards in the ama's and save space in the accommodation department?

 

The way I feel is its 2 x the weight and 2 x the drag unless you pull one up every time you tack which is a major pain in the ass with 3 crew on an 8.5.

 

ones guy is steering, ones trimming and one should be calling tactics and watching for shifts, if the crew who should be calling tactics is actually buggering around with the dagger boards you will get beaten by the guys concentrating on sailing the boat fast and getting the shifts.

 

On a bigger boat with more crew its a different story, as is long courses without much tacking.

 

Not only this but if there isnt a dagger case in the main hull you still have to have a bloody grunty mast step right in the middle.

 

Differnent story for you Lionmark as you have the cases already in the floats, so just run with it, some days it'l be a pain and some days on long tacks you'l prob pull away from the other guys. One thing for sure dont try using tiny dagger boards you need plenty of area for the light days in my experience.

 

I think there is pro's and cons either way but I prefer keeping things simple

[ScottiE 174]

is really that much buggering about for the 'tactics' guy to be dropping or lifting a foil on the windward side going into or out of mark. We are talking new race boats - I would have thought some properly machined acetal bearing blocks in the case and a well sanded board would be next to no effort.

[TimW 1]

is really that much buggering about for the 'tactics' guy to be dropping or lifting a foil on the windward side going into or out of mark. We are talking new race boats - I would have thought some properly machined acetal bearing blocks in the case and a well sanded board would be next to no effort.

 

don't forget canting the rig as well !