Aka attachments to Nacra 5.2 hulls as amas

Hello all,

I'm converting my foiling trimaran to a "normal" trimaran after too many structural failures while foiling. I have a Supercat 20 hull for the main hull (vaka). The ends of the aluminum tubes used for the akas are 4" o.d.. I have 2 Nacra 5.2 hulls to use for the amas. The Supercat and Nacra aka saddles don't line up. Thought I'd use the forward Nacra saddles, but the amas are too far forward then. Using the rear Nacra saddles makes the amas too far back. So, I need to fabricate mounts for the AL tubes to meet random points on the Nacra hulls. Am pondering various ways to do this. I could buy or make 4" i.d. fiberglass tubes and laminate them into the tops of the the Nacra hulls at the correct location, but that's a lot of fabrication, epoxy and cloth to ensure a strong enough connection. Have also considered laminating a 4" i.d. aluminum tube to the epoxy for this (wet sand AL with epoxy before laminating for a strong connection). Have also considered welding AL plates onto a 4" i.d. AL tube and bolting or riveting the plates to the insides of the Nacra hulls. Any thoughts on the easiest/strongest way to do this?

If you're interested, here's a video of this boat when it was a foil boat earlier this summer:



-Jim Gallant

 

You could use the existing saddles as molds,..lay plastic over them then layer glass, remove and reposition and glass into the new spots that line up with the supercat. I think you probably want your amas bow nearly as far forward as the aka bow. Interesting project , good luck.

 

Hi Jim, My "best guess"is that the nacra hulls are quite big relatively to the main hull, and not all three hulls should be in the water while sailing. So I think you need to raise the nacra hulls a bit. Maybe you can mount the beams through the hulls like this (example picture from harryproa built with unidirectional carbon fibre ). I dont know if the uni tow is also on the inside of the laminate - probably not.
You might already have thought about this solution and rejected - but just in case. Good luck with the build, and I want to say I really like your inventive builds.
Cheers, Hans

 

Begin by removing the foredecks.
After that you can easily build up the necessary reinforcements from the inside.
Here is how to remove the deck:
Inside early Nacra hulls :: Catamaran Sailboats at TheBeachcats.com http://www.thebeachcats.com/pictures?g2_itemId=82844
It also depends on how the waterlines of the aka and amas line up. Do you have to sink the tube into the hulls or could you possibly just add two saddles on top.

But as Zilver says the amas might need be to be higher relative to the aka. Then just pick up the hole saw and go ahead, both at the rear and the front.

 

Thanks much for your thoughts Trip the light, revintage and Zilver. I'll know more about how high the amas should be mounted in relation to the vaka today. Will position them so only one ama will be in the water at a time. I suspect that using the existing front saddle would be about right based on the height of both hulls at that point. That thread showing the insides of the Nacra hull is really informative.

I originally thought I'd use the front saddles as is, then reinforce the Nacra transoms, and bolt on aluminum assemblies that a friend of mine could weld with a 4.5" tube to slide over the aka ends like so:



Seemed like a pretty simple solution until I saw how far forward the akas would be (bows aligned). I could stick with this plan and add more hull onto the rear of the akas so they're longer. That's a bit more work than I was hoping to do.

 

I would say in Trimaran terms using cat hulls as Ama's is that you would want the amas as far forward as you can get, cat hull design puts the main flotation area back behind that of a Trimaran ama. If you think that the heeling motion is more a diagonal force in a trimaran rather than a cat which is more a mixture of sideways and diagonal.

 

nwguy,

Great project, good video.

How about using the forward ama mounts and moving the aft vaka mount forward to line up with the aft ama mounts.
Less work, more functional, and it would look a lot better than the other options.

But you're going to want lots of adjustment for testing first.

 

I really am looking forward to this boat.
The ama's need to be raised so all the hulls are not in the water at the same time.
Raise up the fwd Vaka mount to raise the amas. Perhaps with a metallic structure/ beam cradle.
Move the aft Vaka mount fwd, and raise it to match the fwd one.

So I'm suggesting you have the ama fwd of the Vaka bow is a better solution due to the diagonal tripping potential.
Of course you really don't need that much volume in the aka's.
Could you see your way to taking out some height in the ama by cutting a slice out parallel to the waterline?
Lots more work and not easy to make it work right.
But less weight.

Are you going to trailer this boat?
If so will you disassemble it to trailer?

 

I don't plan on trailering this boat.

OK, I attached the amas to the forward aka and propped them up with buckets. See pics below. I think the fore-aft positioning is fine. The distance from the bottom of the forward aka the bottom of the hulls at that point are 23" for the vaka and 19.5" for the amas. So the bottoms of the amas are 3.5" higher than the bottom of the vaka. The waterline when the boat is unloaded, based on discoloration of the bottom paint at that point, is about 7" deep. I don't know what the waterline will be with this new boat due to the redesign changes. I suspect the new boat will weigh close to the same as the foil boat version, so maybe it'll be close to 7". This will put both amas in the water at the same time (not good). One ama would probably fly when sailing with a decent wind, but I think they need to be raised.

I'm loath to move the aka mounts on the vaka. I added bulkheads below those saddles with significant hardware to attach the water stays and rear diagonal support struts. To move them I'd have to make new bulkheads with their hardware, which means cutting open the deck of the vaka. My hope was to minimize the composite work in this conversion. I'm a bit burned out on composite work after making foils, the T-foil, amas, and all the other mods for the foil boat.

So, the question is, without changing the height of the amas by slicing out the middle (!) per Upchurchmr's suggestion, how much should the amas be raised? If I do what Zilver suggests, hole sawing through near the tops of the amas so the aka fits through the ama, that would raise the amas about 4", which would probably be about right. So maybe that's the way to go. Given that there's a bulkhead directly below each saddle, I'd probably want to cut a hole for the aka either directly forward or in back of the existing saddle/bulkhead. I think I'd go for just in front of the saddle/bulkhead. I could wet sand a foot long, 4.5" o.d. aluminum tube with epoxy and wrap in in fiberglass. Cut holes in the amas to fit that, laminate carbon and epoxy around the holes on the inside and outside, then laminate the fiberglass-wrapped aluminum tube into that hole. Securing the amas onto the akas would only need a bolt or pin then. Then for the rear I'd make the aluminum assembly shown in my sketch above. I think this would be the least amount of work with the optimal results.

Upchurchmr, are you suggesting that the ama bows should be forward of the aka bows?

 

I was just suggesting that Ama bows fwd of the Vaka bow is better than Ama bows well behind!
There were some daysailing tri's from Australia (Crowther?) that were fwd to prevent lee bow tripping on a reach.
The reports said that worked good, but no other comments on any negative aspects.
This was in 1970's. Actually it was not Crowther - can't remember the designer, right now.

The one thing I wouldn't want is ama/ aka connections close to the ama waterline - due to waves hitting the fwd aka and slowing the boat down.

Are you going to put a lot more sail area on the boat? Looks like the extreme beam will make it more difficult to get the vaka out of the water.
That is assuming you don't move between windward and leeward ama's to allow heeling - but I'm probably being silly, given the extremes you are willing to sail.

Is there any way you could make a curved rear aka? That would allow you to use the rear beam as a curved track for holding down the boom, allowing you to minimize trim changes and keep the boom under control.
It probably would not be a perfect match given the extreme beam.

IMHO, the only reason to suggest slicing the ama's is that the 5.2 hulls have a geometry which actually "might" make this relatively easy. Still a huge amount of work, without knowing it would significantly improve the boat.

 

The new hull positions look pretty good, the rear beam and elegant[curvy] looking brackets is a good solution I think. There's scope for some adjustment on ama bows down[preferable I think]] which would be nice for sea trials A trimaran that flops side to side at anchor is annoying, but it really depends upon use and the average wind speed you expect and how much weight you will carry. If the windward ama is just touching in light wind, move your weight, you will spill a little wind but not as much as raising both amas and sailing in more wind..if you get my drift..? You could cut the old rear position saddles off and put in an inspection cover, if you want to tidy up the look.

 

Before you cut a hole in the side of your amas.... Give a little thought about what will happen when the ama is pressed into the water. I think the leeward beam end may be dragging in every wave that passes causing a ton of drag as well as lots of spray. The best approach is to keep the beam on top of the deck and create a curved or angled beam. It is more complicated but well worth the effort.

 

You're actually not far off having the Amas at the right height with just simply bolting them in. Perhaps it maybe quicker to simply extend the rear of the Ama back to the beam and think of swinging your rudders from there, with that sort of beam you are only going to have to fly the main hull a few feet and your rudder will be out.

Once attached do a water test to see where the Amas touch the water. In a well powered up Tri then its not going to need much heel to have the Lee ama out of the water.

 

Marlow,
Did you mean "to have the Windward ama out of the water"?

 

Hello waynemarlow, about rudders do you think it will be harmful to have the them placed at the transoms of the 5.2 hull amas even if the main hull protroudes rearwards? About rear beam, I would move it forward to the 5.2 fittings. Wouldn’t be to hard to add new rear beam fittings to the aka.