Here are pics of my SKA by Brad Gibson in balsa strip.
As of last week I managed to gash myself nicely, having a boxcutter blade break, swipe accross the top of my hand and take a tendon with it requiring surgery so the build process is on hold for the moment.
The planking is 3mm basla and its glassed externally with 120gsm cloth, a bit heavier than id like but all I can get my hands on easily. Its aaalmost stiff enough as is but i plan on glassing the inside. Peelply is fantastic for removing resin and getting a litgh good finish. As it is the hull is 260gms so I guess im aiming for a 350 target for the shell. I my order some very light 25 or 50 gsm cloth but it seems to be that all the weight is in the resin ratio not the glass.
Excellent progress and sorry to hear of your injury - it’s incredibly frustrating to be held up, more so from avoidable accidents.
In terms of overall weight when using epoxy and glass, as a rule of thumb whatever the glass weighs will also be the mixed weight of the epoxy. As an example, if your hull had a total external surface area of 0.2m2 and you were using 120 gsm cloth then the weight of the cloth would be:
120gsm x 0.2m2 = 24g
which would then require 24g of epoxy to wet it out properly. It’s always a good idea to mix a little extra, there’s nothing more infuriating than running out near the end of a job.
As an aside, instead of calculating the surface area of the hull it’s much easier to drape the glass cloth over the hull dry, trim where necessary and then weigh the piece(s) of cloth you intend to use to determine how much resin needs mixing.
Hi Barbera, sorry to hear about your accident & wishing you quick recovery.
Balsa sandwich hull is the most diffulcult to keep light, you will find the balsa absorbs quite a lot of epoxy especailly in warmer ambient temperatures.
One alternative you have now (with nicely planked male mould) if you wish too, is fair the plug to final hull shape and then apply a release on the mould and hand lay-up something like 4 x 80 gsm layers of twill fibreglass cloth, then you will have a very light shell to start with.
Thanks for the feedback guys particularly regarding glass weight. Ive thought about going the male mold route only to think that the amount of fairing would be the same as the balsa hull but the resin usage is a bit creepy.
The layup with the 6 overlapping sheets is the Peelply…fantastic stuff suprised about how much resin is being lifted off the job.
Id really like more feedback re layup schedulus for a male mold please chime in.
I’d be inclined to go with Alan’s recommendation of 4 x 80 gsm for the hull. As you’ve already been using 120 gsm cloth & assuming you’ve still got sufficient, you could always go with 3 x 120 gsm. Might be a bit heavy but for a first build (& steep learning curve!) I wouldn’t be overly concerned. I’m sure others will have recommendations but I’m convinced that for a ‘newbie’ keeping costs down is pretty important. What’s more, it’ll probably work out lighter than a sheathed balsa hull - as has already been mentioned, it’s quite difficult using glass/epoxy on balsa owing to the way balsa soaks up the resin.
The best method for the beginner is getting on the water asap - sheathed planking can look absolutely stunning (when unpainted) but requires alot of experience & attention to detail which doesn’t really go with speedy construction!
Which ever way you decide to go I wish you good luck & also a speedy recovery from your hand ‘injury’.
The key to a good final hull shell shape is dependent on the amount of work you put into the mould, you are right in that either way you decide to go both requires the same amount of fairing.
If you decide to go with mould lay-up, when you have finished fairing you need to prepare the plug with suitable release agent (so epoxy does not adhere to the mould) use paste like release wax that you wipe over the plug and let dry and then buff it to a polished surface, here you need to apply min 10 coats @ about one every 30 mins. (or depending on instructions of wax being used)
Buff each layer between coats …but be careful to only lightly buff between coats otherwise you can rub of the wax layers you have just applied. You can also apply a PVA release liquid that you spray onto the surface and let dry before laying up.
For IOM using Twill Fibreglass, the only difference between Plain and Twill weave is that Twill is much easier to drape over shapes, particularly when you use it on the 45 degree bias, that will add strength to the laminations layers.
Cut the cloth so that you have 2-3 cms extra cloth as it drapes over the plug, Twill frays pretty easily, one trick is use masking tape on the cloth shape before you cut the cloth, then with scissors cut down the middle of the masking tape and it will prevent annoying fraying. If you have enough try and cut the cloth so you have 45 degree bias for each layer, as mentioned before this will add strength the final cured layers.
My rule of thumb for the amount of epoxy that is needed is to weigh the cut cloth and use the same weight in epoxy final mix + 10-15% for safety & wastage. I generally use resins that cure at room temp of 25 C, higher temp resins require higher controlled environments, you can use a cardboard box with a light bulb inside to increase the ambient temp but pays to have the air moving so you don’t have heat spot inside and you need thermometer to keep track on the temp.
Check the epoxy data sheet and for the pot life, ideally you want 30 mins plus so you have enough time to apply all layers before the resin starts going off, for best results should be apply each layer to the plug wet-on-wet. If you don’t have enough time to apply all layers with one mix, just mix enough resin for each layer and give yourself sometime between layers.
Generally pin-holes are the enemy with hand lay-ups, this is where the epoxy does not fill the cloth weave pattern and leaves microscopic air bubbles trapped in the cloth weave, to help overcome this problem I personally like to use low viscosity epoxies (consistency of runny honey) and sometimes even warm up the epoxy to lower the viscosity which helps it wet-out the cloth thoroughly and reduce pin holes.
I remember the first time, I was nervous as hell that something would go wrong and I was like cat on hot tin roof checking everything as I went along, but it is very easy. When you have thoroughly mixed the resin with the hardener, let it sit for 5 mins to let the air bubble rise to the surface before using it with the cloth. Pays to use those disposable rubber gloves when comes to laying up the cloths on the plug.
With light cloth like 80 gsm I like to first apply coating of epoxy to the plug surface evenly with paint brush, then gently lay the cloth onto the wetted plug, there should be no creeping of the cloth this way and will soak up the epoxy very quickly and you can work out any air bubbles. Then apply next layer of resin down and apply another layer of cloth until you have finished.
After the final layer is down, use peel-ply to soak up any surplus resin. Then leave the job to go off and cure as per data sheet instructions, under no circumstances let the wet lamination sit in the sun or heat it up as any tiny air bubble trapped in the weave with expand with increased temperature, causing bigger pin holes or even bubbles in the lamination, I’ve got that tee-shirt !
After the hull has cured use plastic wedges to pry the shell from the plug gently, assistance of air gun helps also.
Epoxy take several days to completely harden, Refer to data sheet for info and it may suggest a post cure heating (annealing) a cheap way of doing this is to put the shell on the plug again and slip it inside a black plastic rubbish bag and leave it in the sun to post cure.
After post curing don’t forget to wash the inside of the shell thoroughly with vinegar to remove all the wax residue, otherwise when it comes to bonding fittings inside hull, it won’t take properly to the hull surface.
There are varying opinions on how to do this, this is my way and works perfectly for me.
The latex method looks great, I wish I had come across it earlier (I also like to avoid sanding where possible). Do you know of any text or thread that describes, compares and contrasts the different approaches to making hulls? I’m sure that all approaches are described in one or another thread on this forum, but a thread that links them all together would be really useful for those of us who are just starting.
Barbera, sorry for the slight diversion from your thread.
I finally got the ok to remove the splint from my arm this week from the surgeon yay. I still have to go easy and cant race my Laser for a while yet.
While I was out of action I wasnt entirely out of action. Not being able to do much on the IOM I built a mold and shell for a Frank Russel Goth RG65 in glass.
Im concentrating on the SKA for the moment though and I have just layed on what I hope is the final slurry of Q cells for fairing. Im also cut out frams and cut out the transom for the skiff deck.
What deck type to go with has given me sleepless nights so now that the transom is butchered its decided.
I have a keel and keel box on order, the keel is slightly used apparantly but is a "220 Obsession foil for less than half that. Im also doing a lot of searching to secide what sail winch to go with. Id love an RMG but im a poor uni student. At the same time there is no alternative it seems, the HiTec 785 or whatever is hoperlessly slow. Im trying to work out how to fit an arm winch into the area under the foredeck.
Has this been done before? any advice appreciated.