Build a plug and mold instead of stripplanking

Some thoughts on building some 30 to 34 foot narrow symmetrical cat hulls.

I was originally looking at strip planking with juniper and than epoxy with a layer of 1708 on the outside and inside but the price and scarcity of Juniper is making me rethink.

The hull sides are going to be very slab sided with a semi cylindrical bottom .

So than I started thinking plywood ugh and pricing up plywood panels for the sides and I would strip just the bottoms .

I would still want to cover the outside in a layer of 1708 for protection so that cost is still present.

But of all the plywood construction I can't believe the amount of time you have into frames and stringers and than have to live with all these little crevices to clean and keep dry.

And when you are all done you still have the stigma of a plywood boat to the uneducated non builder.

So my new thought is to just build a quick and dirty plug out of sheets of cheap luan and plank the bottom with some clear pine or perhaps place blocks of foam and than just carve out a bottom and have templates for shape.

Cheap polyester filler and shoot the whole plug with gelcoat and sand and finish it enough that it is straight and true but not necessarily a perfect finish.

Than spray the plug with multiple coats of PVA release agent and layup a mold over the plug.Add a little bit of steel tubing fiberglassed to the outside to keep the mold in shape and be able to support it .

After I pull the plug out of the mold go over the mold with 400 on a Da and pva it too before I build a set of hulls'

The hulls would be gelcoat and 1708 on the outside with a 3/4 inch balsa core and a another layer of 1708 on the inside .Hit the inside cloth weave with some fairing putty while the cloth is still green and spray on some gelcoat and the insides are finished.

Pull a hull out of the mold and give it a wetsand and buff and call it good.

The advantage of this system i think will be the quick and cheap building of the plug.

And once the plug is done all you have to do from there is layups.

Saves buying expensive wood and epoxies along with expensive urethane paints and have the advantage of a hulll that is fiberglass and not wood even though it does have balsa in it.

I was all against balsa but after seeing Lagoon is building with it and its proven to be very strong and reliable if kept dry and it is a little less pricey than foam cores. and does not necessitate vacuum bagging.

Sure seems like a lot less work than hand constructing 2 presicion hulls and all the finish work that goes with it and when I am all done I have a mold if I ever want to build another out of it.

Cost of a 55 gallon drum of quality non vinlyester is just over a 1000 bucks .

I dont think you would have a hard time getting a mold and 2 hulls out of a 55 gallon drum.

5 gallon bucket of gelcoat is 200 and that would get you 2 hulls in and out another 200 for a couple gallons of tooling gel.

All that is left is your cloth and balsa expenses.

Thoughts?

 

Sounds super easy if you read it fast!

 

Tommy, no way will 5 gallons of gel do two hulls. Do your homework.

A good quick and dirty mould is to build it in female frames using glossy melamine mdf sheets.

Perhaps Derek Kelsalls KSS method might appeal.
The materials in a quick and dirty glass mould would be better spent in a hull.

 

Redrueben you are correct i will need at least 12 gallons of gelcoat to coat inside and out

Figuring roughly 500 sqft of surface inside and out at 20 mills per a hull works out to roughly six gallons per a hull . Forgot to add in the coverage for the inside of the hulls, my bad'

The target thickness for gel coat is 0.020-inches.
To estimate the quantity of gelcoat needed:

--compute the surface area to be covered;
--multiply the surface area by 0.020 to get the cubic volume;
--convert the volume to quarts/litres
--purchase at least that amount of gel coat



Example:

Surface area to be covered is about 100-ft-square or 14400 square inches.

Multiply by 0.020-inches = 288 cubic inches


Using the back page of a Merriman-Webster dictionary, one cubic-inch is equal to 0.016387 liters, so

288 cubic-inches X 0.016387 = 4.719 liters.

One quart = 0.94635 liters, thus

4.719/0.94635 = 4.987 quarts

Figure 5-quarts of gelcoat for every 100-square feet of surface area.
This gives you a 20-mil coating, and not much margin for error.


I have also followed quite a few builds using the KSS system and it is has you dealing with large un weildly panels that require several hands to assist you . This is a one man job I am doing here.

 

Plus deck area. Centre cockpit ? Hard decking between hulls.

The main issue is not the materials required for a hull but how you get that hull. One off build or plug+mould+hull.

 

Sounds to me tommy that your ideas are still brewing, don't rush into anything just yet !

 

I have pretty much ruled out a radical deviation from the norm by trying to incorporate some planning aspects into the hulls after reading thru quite a few threads on here and answers i received on my own thread.

So the design will basically be a Tornado times 1.7 with some added freeboard and maybe a little more volume added to the hulls.

I still believe I can build a quick cheap plug with a ton of filler in the rough areas and be an animal with an air file to carve it out .

The money I would have in the plug would be very little.particle board framing set on a couple 30 foot i beams i have and insert a couple stringers and than sheet with the luan.

Small pine strips for the bottom to get it close and lots of filler for the parts that are not close.

I believe i could put together a plug ready for a mold in 50 hours,

Next onto the mold

So a few hours into spraying the plug with pva and letting it set and than an hour spraying the gelcoat on and maybe another four hours to lay down a chop skin coat for the mold, I have a dump gun sprayer and a hand chopper gun so that process would go quite fast.

Next day take a half a day and lay on another coat of chop and a layer of roven

Do the same the next day and the day following so you have 3 layers of roving and 4 layers of heavy chop.

A day of welding up some tubing and glassing it to the mold.

So lets sat another 40 hours into the mold process.

A day to pop the mold and get on it with a Da and some 400 to 800 and maybe a few necessary spot repairs and maybe some polish.

Now onto the hulls

A couple thin coats of pva sprayed over the morning with a layer of gelcoat following later in the day.

Laydown a skincoat of chop the next day in a few hours and let that go off until the next day

Following day spend the day laying in a layer of 1708.

Next day bog in your balsa core - no vacume bagging i dont think 50 lbs is going to kill me here.

Following day another layer of 1708 over your balsa core and 2 additional layers of 1708 in the bilge

Next day lay in your stringers and bulkheads where needed and a shelf to build the deck on and than fill the cloth weave with some filler

Next morning run over the filler with the da and blow some gelcoat on it

So another 40 hours per a hull so roughly 170 to 200 hours to build 2 hulls that are more or less finished with paint inside and out.

So could you build two 34 foot hulls in that time with paint on them out of plywood or strip, or the KSS method that requires tons of fairing ?

 

I really dont want to rain on your parade here, but the times in your last post just seem so unbelievably optimistic to me that I had to say something.

Have you had much experience in a glassing workshop doing this kind of work? The times you have listed would be extremely difficult to work to with a large team of experienced laminators, if you are planning on those times as a single worker your dreaming. 34 ft hulls are bloody big.

Just as a sample of times for you I will add a couple of pictures of a mould I have just finished building.

A 2 part mould (inner and outer) of a bowl 1000mm diametre by 400mm deep, (maybe 4 or 5 square metres of surface area total) so quite small compared to a 34 ft hull!! Building and shaping the plug alone is almost a weeks worth of work (not including the high quality finishing that adds another few days) So for the plug alone it was a 7 days work at best. Then you are looking at atleast another week to build the moulds, all for a (relatively) small bowl.

I understand you are not looking for the quality of finish that I work to, but I really think those numbers you have looked at are unrealistic at best.

By all means, keep continuing with your project, but you have to be realistic with time and costs or you will end up with a massive mess

 

How do you get that ? There is little fairing in his technique, and certainly a lot less than the method you are considering.

Making a plug to make a mould to pull two plugs out of is way harder than kelsalls method.

If you cant hire a couple of capable lads for the big jobs, you are not going to be able to organize a 30ft + boat build.

 

I grew up and worked with an older brother building a 19 foot hi performance deep vee of his own design that went on to become a very popular design . Yes I watched him struggle for perfection on the plugs . Finally settling on pulling a mold off the original strip planked hull plug and than building a fiberglass plug out of that mold and than further adding lifting strakes and perfecting the shape as the wood plug was moving too much.

Painfully slow operation.

Than I spent a year building custom 65 foot juniper planked fiberglass epoxied sportfishing boats on the Outerbanks.

These boats are all built by eye no plans exist and though a few may look a little crude most of them would shame a Rybovich.

But the most imortant thing I learned there is keep moving and don't get hung up on the details . It will work its way out. Unbelievable how quickly a boat progresses with that attitude.
So to get back to your question of do I have any experience in this field and the answer is yes.

I had a small fiberglass manufacturing Company that was building scoring consoles for bowling alleys back in the day. I will admit It takes a ton of time to work on a small detailed part that requires proper draft and such especially when your plug will be the mother of 16 molds .

Out of these molds for 2 years I pulled roughly a 1000 parts before getting out of the business.

What i am talking is a large area that should be close to fair by layout and design.

This is a not a mold for perfection but maybe add another 30 hours to get it perfectly fair.

Still not going to get a perfect shine as that would add another 100 hours easily to the finish process.

Hey i appreciate all feedback good, bad , or otherwise.

 

The detail on that mold is exquisite my hats off to you.Was that cnced out originally ? If not wow there is some skill there my friend . May I ask what this is the mold for?

 

I have not read many very good reviews of how easy the KSS method is and the restrictions imposed on the design are a little tough.But to each his own.

 

I have build 3 strip planked multis - the first two in cedar where the cost is now prohibitive and the last one in Paulownia. If you haven't discovered this timber, I suggest you do more research on it, especially availability in your area.

Paulownia is a Chinese species now grown commercially in many parts of the world. It is lightweight, straight, fine grained, rot resistant and with exceptionally high fire resistance. It just loves epoxy as a bonding medium. Best of all, the cost is less than half of cedar. It is easy to work with sharp tools and a good sander. 440 GSM glass cloth both sides makes it very strong

Seriously recommended

Alan

 

Aussiebushman thanks for the input .

Love you Aussies by the way and thanks for all your inputs on my threads all of you from half a world away. You guys make boat building look easy over here compared to what restrictions and materials cost over there.

CHEERS to you all.

,
Currently working on a good friends 43 Choeylee these days who is one of your transplants over here now .Great Chap.


Actually nothing is affordable these days .

Juniper that I used to build the big sportfishing boats with is thru the roof and by the time you get done with the epoxy the price becomes golden.

That is why I was leaning towards a polyester conventional cored hull ,for one i thought the materials would be a little more in line ,but I still believe I could save time in the building process of the hulls.

And yes I do know that the hulls add up to maybe 20 percent of the job at best.

But after the vendor got back with me on 5 to 6 dollar a sq ft for balsa core i am re figuring once again.

Unless this Paulownia is priced accordingly the best bang for the buck is still the best plywood at 80 bucks a sheet with a thinner outer cloth and epoxy .

But god the hours into precision fitting frames and stringers and than have to look at that busy crap down there for the rest of my life is a little hard.


Is there a recent thread on here that anyone knows of that does a cost comparative for strip plank with epoxy sheathing vs a high quality plywood with epoxy sheathing and also includes cost factors of a polyester balsa cored hull compared to a foam cored hull ?

Once again thanks for everyone's advice If we aren't building them it's still fun talking about building them.

 

Thanks for the compliment No cnc was involved in the construction. I start with a dirty great lump of foam and start shaping and sanding until I'm happy with the shape. I could have spent more time on the shape of this plug, but there was a deadline I needed to hit, and the shape was good enough.

The mould is for a GRC (Fibreglass Reinforced Concrete) bowl, used usually as water features. The GRC is sprayed onto the fibreglass mould and both parts of the mould are joined while still wet. The moulds cop a bit of a flogging as the GRC is sprayed at approx 80 PSI onto the mould, so alot of maintenance is required

Pics below are of the bowl once de moulded (has not been trimmed and no coating applied) Pretty amazing the shine on the concrete. Little off topic

Im glad you arent taking any of the advice the wrong way, I'd hate someone to give up on a project on any of my advice, I just dont want to see people rush into a project without thinking every aspect through thoroughly. Good luck with yours, and keep asking questions, building more knowledge is always a good thing