tansomed vs double ended amas

[peterchech]

I am redesigning the ama on my tacking outrigger canoe. It was originally designed as a shunter, so the ama is double ended. However i want to replace it now with a higher buoyancy one (it is currently %100 of unloaded displacement). I was on an f-27 the other day, and noticed that the amas were transomed, like most amas. My friend, who owns the boat, told me it is possible to bury the amas in an f-27. So why the transom? A submerged transom can cause a vacuum and slow the boat down. Especially in light of the apparent trend for low rocker amas and highly rockered vakas on trimarans, which can't help keep the transom out of the water...

My current double ended low rocker ama just slices the water when it is over pressed, partially submerged. No transom when sumberged. So should i bother building a "modern" transomed ama or will a double ender work?

[garydierking]

There are differing requirements between a small outrigger canoe and an F27. The fore and aft trim of a small canoe will change radically depending on whether you have crew up forward or if you are sailing alone. A heavy skipper sailing alone and sitting aft will drag the stern of the ama. A double ended ama's drag is unaffected by this, but dragging a transom is a drag.

[peterchech]

Got it gary thanks... i do notice that alot. Guess ill keep the double ended shape then, just build in 6 more inches of sheer to the current, dory style hull. It is currently very narrow, about 5 inches wide at the waterline (though it rarely stays there) and 16 feet long. Maybe ill do ur ama nui instead. How many pounds displacement is that ama if i may ask?

[catsketcher]

Gday Peter

One idea of a transom is to reduce drag. Strange as it may seem an immersed transom can have less drag than a double ender AT HIGH SPEEDS. If you want very straight aft buttock lines (like a planing dinghy or speed boat) then putting a transom on can help you keep volume to the ends and keep the buttocks straight. If you put a double end on you either need to reduce volume (which you may not want) or curve the aft run (which is often slow at high speeds)

Your boat won't be that high speed so as Gary says the double fine stern is a good approach for many reasons.

If you look through trimaran history you will see transom amas on great boats like Toria (Kelsall always liked transom amas) and modern ORMA style tris. Earlier Irens tris had a Shuttleworth style upsweep on the keel line which kept the amas wide aft but reduced volume. Now they just are big all over.

You can find lovely boats with no transoms on floats too so it all comes down to the design spiral.

cheers

Phil

[cavalier mk2]

Newick uses mainly double ended amas. Some designers have taken into account the time spent is various speed ranges. An improvement in the speed range where you spend most of your time, usually lower, should increase your overall averages.

[peterchech]

Originally Posted by cavalier mk2

Newick uses mainly double ended amas. Some designers have taken into account the time spent is various speed ranges. An improvement in the speed range where you spend most of your time, usually lower, should increase your overall averages.

Good point.

What do you mean by "high speed" if I may ask? >10 knots? >15?

[keysdisease]

What about some strakes or rails to provide some dynamic lift for like zero weight?

Wouldn't weigh anything, would cost very little, would only come into play when hard pressed, might make the boat a little drier

I believe it was "Twiggy" that had a designer mod lifting strake on the main hull after one pitchpoled and you sometimes see them on ocean kayaks.

Steve

[peterchech]

I searched "lifting strakes" but they come up mostly in terms of powerboats. How would this be done on a skinny ama?

[catsketcher]

It has been done on both Twiggies and other tris. A US designer did a powerboat like strake ama in the 80s. He built one boat (about 30ft) and I never heard of another. It was called the Warp or something. Strakes must be inefficient if you try to get them to develop lift as they are really very low aspect foils and so inherently bad news for lift. They would also be a bugger to make.

Going for dynamic lift from strakes and ama shape is probably not the best idea. The best way to get dynamic lift is with an efficient canted foil. Even Dick Newick stopped using the new moon ama shape.

Two or three Twiggies had a little foil thing on their bows - about 500mm out of the water. Ian Johnston put foam on the original one too. The problem with both modifications was that if they put themselves underwater they could actually trip the boat quicker.

cheers

Phil

[Corley]

I can recall Ian Farrier mentioning that one of the reasons for his transomed floats was because most crews tend to congregate in the cockpit and on the back of the floats particularly when racing and the extra volume is required to stop the boat dragging it's rear. It's also important in case you get caught during a tack with full sail up there is a remote possibility the boat could pitch over rearwards and the extra volume in the rear of the floats helps prevent that from happening. I must admit from an aesthetic viewpoint though a canoe stern ama can look very attractive.

[cavalier mk2]

A canoe stern ama can be made longer to preserve the volume aft and keep a flat run if you want, the only penalty is weight which shouldn't be much.

[keysdisease]

I would envision a spray rail / lift strake similar to this one:

http://www.tacomarine.com/item--2-1-...-V21-0874.html

Starting just below the deck at the bow and curving aft and down to just above the static waterline about 1/3 back from the bow.

This would reduce spray and provide dynamic lift when pressed, more as the ama submerged and more lift came into play.

Cheap, light, easy, might even work.

Steve

Originally Posted by catsketcher

It has been done on both Twiggies and other tris. A US designer did a powerboat like strake ama in the 80s. He built one boat (about 30ft) and I never heard of another. It was called the Warp or something. Strakes must be inefficient if you try to get them to develop lift as they are really very low aspect foils and so inherently bad news for lift. They would also be a bugger to make.

Going for dynamic lift from strakes and ama shape is probably not the best idea. The best way to get dynamic lift is with an efficient canted foil. Even Dick Newick stopped using the new moon ama shape.

Two or three Twiggies had a little foil thing on their bows - about 500mm out of the water. Ian Johnston put foam on the original one too. The problem with both modifications was that if they put themselves underwater they could actually trip the boat quicker.

cheers

Phil

[yipster]

Originally Posted by Corley

I can recall Ian Farrier mentioning that one of the reasons for his transomed floats was because most crews tend to congregate in the cockpit and on the back of the floats particularly when racing and the extra volume is required to stop the boat dragging it's rear. It's also important in case you get caught during a tack with full sail up there is a remote possibility the boat could pitch over rearwards and the extra volume in the rear of the floats helps prevent that from happening. I must admit from an aesthetic viewpoint though a canoe stern ama can look very attractive.

i also recall Ian Farrier mention his boats plane, some stuborn forum members did not belive that and wanted him to prove that in extremes. pitty, otherwise he might still be around on the forum. so.. when a boat -or ama- planes than the hard stern gives less drag, yeah, not planing it might ad drag etc. but think thats one important consideration on transoms. came acros another post on amas with some good links, jim antrim you have to google for tho

Originally Posted by tspeer

Shuttleworth discusses ama shapes in general in his papers. Jim Antrim has a very different take on the ama design for the Antrim 40.

Basically you have to look at the requirements - and there are a lot of them for an ama. Piver's designs were intended for easy construction in plywood, and he believed that he could get away with no or reduced keel size by using lift from the hulls. Modern designs have a more integrated understanding of the factors affecting ama design.

The ama moves up and down a lot and even when completely out of the water is still subject to wave impact. A somewhat V'd bottom reduces the slamming loads on the ama. But you don't need the entire section V'd - that increases wetted area and provides a flat surface that can be slammed from the side. Farrier uses a fairly rounded section with deadrise that is a good compromise. Antrim used elliptical sections for minimum wetted area. Dick Newick has also gone to rounded sections with deadrise, as seen here:


The topsides need to be a compromise between windage, reserve buoyancy and practical considerations. Farrier's topsides have rounded gunwales and some flat area in the center. Antrim went with elliptical topsides. In Hoerner's "Fluid Dynamic Drag" you can find empirical data on the drag of blunt shapes that are relevant to the crossflow drag of an ama in the air. You get the most benefit by a radius that is 20% of the height of the section. Compared to a sharp edged cylinder, that cuts the drag by 80%. However, as you make the section more rounded from there, only a little additional drag reduction results. So I see little reason to go to a completely round deck on the ama.

A practical consideration in the deck shape is the crew getting access to the ends of the ama when underway or docking. You'll want secure footing to place fenders, fend off the dock, or place snatch blocks for spinnaker sheets and twings. My Chris White designed Hammerhead 34 has flat decks which could be improved from the point of windage, but I feel significantly more secure going to the ends than I do on an F-boat. I would hate to have completely round amas like Dick Newick's Traveler trimaran - beautiful, stiff, minimum area, but I'd feel like I was walking a tightrope every time we docked.

Your question concerned sections, but I want to put in a plug for sheer. Straight or even negative sheer seems to be the style these days. But if you consider the worst case loading of an ama, it's providing diagonal stability. In that case, the bow is down 5 - 10 degrees and the boat is heeled to submerge the ama.

When the bow goes under, the center of flotation shifts rapidly aft, and you lose pitch stability. With straight sheer, the bow is the first thing to go under. If you build in positive sheer, you can design the ama so the deck is horizontal for this worst-case condition. That preserves waterplane forward so the ama still does its job of providing stability. So I think a reasonable design criteria is for the slope of the ama sheer at the bow to equal the pitch trim of the boat at it's diagonal capsize condition. What you get from this is much like the sheer of the classic Newick designs.

[peterchech]

Plenty of food for thought here... im def gonna do tortured ply here, which would make a round bulge difficult, but eliptical fine. I guess a symetrical double ender will suffice here... maybe extend the lengthfrom 16 feet to 20 to reduce the disadvantages of a double ender

[cavalier mk2]

On another note, a few years ago a F boat capsized bow over stern in gusty steep wave conditions. The boat jumped a wave with the bow up and the wind force on the wide webbing tramp nets flipped it backwards with one person losing their life. A net problem, not really the amas but it shows how many things need to be taken into consideration.