Crowther Kraken 33 - Crossbeams

Hello,

I purchased a Kraken 33 last fall (I used to own a Buccaneer 24).

My is demountable crossbeams, that is uncommon, made of channeled aluminium (why, I have no clue). The crossbeams are pretty heavy and I am think of switching marine ply or structural foam box crossbeam.

The reason I am thinking of this is I want to open headspace in the cabin with the forward beam and for the aft beam reduce the high the middle (attached, remove what is in orange).

What do you think? I will add some pics.

Thanks!

 

Duane. This may help, the following is Twiggy foam glass beams. Twiggy is a Crowther 32 x 30 foot sailing tri. The Twiggy beams were upgraded to foam glass beams which were several hundred of kilo's lighter than the original timber beams. The foam glass beam has a 25 mm divnylcell foam web with 5 layers of 500 gsm cloth on each face. The top and bottom flanges are 25 x 40 mm of uni directional glass in the centre tapering to 25 x 15 mm in the ends of the beam. When doing these unidirectional flanges DONT lay them up in one go. They need to be progressively laid up over 24 hours as one thick lay up builds up to much heat as it cures and often fractures the layup. Also each uni directional layer need to be tensioned to ensure the fibres are straight. Finally use epoxy in the flanges as it is "stronger" than polyester etc. On the front of the beam is a triangle of 12mm foam triangle with 2 layers of 500 gsm on each face. The triangle acts as a torque box.

Whilst talking about Twiggy's foam glass beam I will talk about other features of the beams. The full cross beam comprises of a top flange, which is mainly in compression. Yes, I have spoken of tension lbs/square inch figures when comparing top flanges but that was to emphasis safety factors and glass resin combination capability. The bottom flange in a crossbeam is normally in tension. In between the top and bottom flanges is the web faces and in the case of foam glass beams a foam separator. The foam has to be good quality (corecell, airex etc 5 lbs/cubic foot 80 kg/cubic meter) but basically is a former with a low compression of 7500 lbs/sq inch and only has a shear of 160 lbs/sq inch. The web faces carry a lot of the load when combined with the foam core. In the case of Twiggy's beam the web faces are S-500 orcoweb, an aircraft quality unidirectional S glass that weights 275 gsm. The web faces near the center of the beam are 6 layers of s-500 laid at 45/45 degrees to (in short building 3 layers of biax cloth) and from mid beam to the outer end 8 layers s-500 laid at 45/45 degrees. The web faces are about 3 mm thick. Please understand this entire beams flanges and web faces glass content is S glass with epoxy, which is 20 to 30% stronger than normally available E glass. Please also study the variation of thicknesses of the glass flanges along the length of the beam as this will give a hint of load characteristics at various parts of the beam. This takes a lot of calculation to get right. These beams need to well built hopefully with vacuum bagging to get high glass to resin ratio's which again improves strength.

 

Thanks! Twiggy is a nice design. I will look through this and again thanks oldmulti.

I love Lock's designs. My Kraken came with the full plans, written corresponce between the builder and Lock also built with a foam core. Here are some more pics (Needs paint and a change to the name).

 

Gonna need engineering.

I’m a little confused because aluminum channel is generally stronger than glass pound for pound.

Detail the current beam for us.

 

Good to note. Here are some pics aluminium beams.

 

Duane, I don't think they will need engineering - as the twiggy & kraken are very similar.
In fact if anything, the twiggy's were higher performance - so could be more than you need (good)
The only thing is, how much weight loss/stiffness gain are you going to achieve via this upgrade ?
Is it worth your time, money & effort ?
Nice idea, if so.