Author Topic: Boat building  (Read 27462 times)

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Offline ross_burkin

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Boat building
« on: May 21, 2008, 04:14:49 PM »
Seeing as the 700 idea has now gone way off topic, I thought I would start a new thread.

Matt Harris wrote
Quote
would it be possible to build a wooden hulled boat that would take the strains from an 05 rules cherub?  i know that time wise this could take longer and be a lot more complicated, think i would be more inclined to throw a £100 or so at a wooden design hull as a bit of a laugh rather than spending many pennies on a carbon foam boat.

Roland and I have talked about different ways of building and I had a good chat with Will and Lucy last night about building boats.

My plan was to build something similar to Aqua Marina (a racked P7) but with a ply hull skin. A carbon tube (or something similarly strong) space frame is then fitted inside the hull which takes all the rig loads. The racks are incorporated into the space frame. My idea was that this would reduce the hull cost massively by not having to buy 40-50 meters (a figure I got from the nationals) of carbon cloth and then a load of foam to build the hull skin. Both of which are very expensive when compared to ply. I have been sketching away at home (at college atm) and will post up some scans later if you want.


Obviously the hull is only a percentage of the costs when building a boat when you have to get spars, rigging, sails, fittings, foils ect.
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ghislain_devouthon

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Re: Boat building
« Reply #1 on: May 21, 2008, 04:33:04 PM »
Just give you an idea of NBS costs :

Hull / boards : 3500 €
Fittings : 1200 €
Sails : 1500 €
Masts, boom, proder, pole, racks : 2000 €
Others (trailer, trolley, covers) : 1000 €

Offline daryl_wilkinson

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Re: Boat building
« Reply #2 on: May 21, 2008, 08:29:26 PM »
Hi Ross,

what ever you do building a boat costs a lot of money. if you basically skimp on the hull you end up putting good gear on to a very compromised hull.

The most economical way and still competitive way is to find and / or build things like the pole, mast, foils, stock etc and get a shell made and finish off the rest yourself.


ghislain_devouthon

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Re: Boat building
« Reply #3 on: May 22, 2008, 10:05:59 AM »
Quick precision.

I don't want to afraid anyone with NBS cost.

I was not in a cost reduction mood.

There are plenty solution to reduce costs :
- Vynilester,
- Glass instead of carbon on some parts,
- Non epoxy Paint or monocomposant,
- Cheap fittings,
- Cheaper foam than Airex C70
- Cheaper carbon than Sicomin's ...


However, I'm not certain that building it's own pole and boom lead to a cost reduction.

Considering :
- that a C-Tech boom is about 250 £ (without shipping OK).
- you need at least 10 ply of carbon which means 10 sqm of cloth for a 10 cm  diameter boom * 2.5 m
- carbon is around 20 £ / sqm
- you need some epoxy ...

mathew_harris

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Re: Boat building
« Reply #4 on: May 22, 2008, 10:25:27 AM »
Hi Ghislain,

Not neccarily arguing with the facts just a bit of the maths,

0.1m (10 cm) diameter * length of boom (2.5m) = 0.25m2

you need 10 layers

0.25 * 10 = 2.5 m2 of material needed @ £20/m2 = £50

factor in maybe £50 of epoxy (which is a lot of extra epoxy)

total of £100 which is a good saving on a £250 c-tech one before you factor in postage and packaging.  I admit i could be very wrong but just based on what you have said thought i'd query it.

Offline Tom K

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Re: Boat building
« Reply #5 on: May 22, 2008, 10:52:34 AM »
Matt, don't forget you need the circumference x boom length to get the area you need. Currently you have enough carbon make a stringer the length and width of the boom.

0.1m x PI = 0.3142m (circumference)

0.3142m x 2.5m = 0.79m^2 (boom surface area)

10 layers of carbon - approx. 8m^2 of carbon.

At £20/m^2 that works out at £160.

Offline Phil Alderson

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Re: Boat building
« Reply #6 on: May 22, 2008, 10:55:29 AM »

A boat needs a hull skin to be tough enough to take some kicks and stiff enough to hold its shape when impacted by waves. By the time you get the strength in the skins to take this abuse you are close to having enough strength to take the rig, foil and wing loads. You then add some framing/bulkheads to the hull to  give all the strength you need. Properly designed the framing strengthens the hull and the hull strengthens the framing.  If you isolate the framing from the hull into a space frame you duplicate structure and weight. 

If you look at the way gantry’s are built, they do not need any skin to work so most are built using tubes, however they can be built just as stiff and just as light using foam panels. This is a case where monocoque  is as good as space-frame even if the skin is not required.

It is bad if the hull deflects when you pull the rig tension on, but also bad if the skins are deflected by the waves, this uses energy and if significant changes the shape, probably in a bad way. It is not too difficult to build/reinforce a ply boat in such a way that it will take the rig loads, however the skin stiffness is more problematic.

The stiffness of a panel is a function of the cube of its thickness so doubling the thickness of a panel makes it eight times as stiff, Typically you would build using 4mm ply against 8 or 10mm foam sandwich. Foam is softer and more flexible than ply but once you have put glass or carbon skins on it becomes incredibly stiff and the weight is low because the heavy but strong skins are held apart by the foam. A layer of glass on a 4mm sheet of ply will only make a small difference but an extra layer on 10mm foam sandwich sheet will make a large difference for the same extra weight in both.

If you can’t get the panel stiffness through skin stiffness you need to get it somewhere else, one way is panel curvature if you take a piece of paper it will bend in every direction, hold it so it has a curve in it and suddenly it is quite strong in one direction. This has been done in ply boats like the recent [link= http://www.internationalcanoe.yachting.org.au/?Page=24752&MenuID=How%5Fto%2F13919%2F0%2CHow%5Fto%5FBuild%5Fan%5FIC%2F13920%2F0] Hollow Log[/link] DC and a Moth by the same builder. He uses very low density foam blocks to shape the hull stiffen the panels in areas where there is low curvature. If you can abuse the ply into a double curvature then you have loads of stiffness, think Morris Minor bonnet.

A different way of getting panel stiffness into a hull is to use a large number of panels that are joined together along curved edges, the panels are curved along the length of the boat forming the shape. The joints between the panels give good stiffness along their length and the panels are not that wide so do not have a large unsupported area. Think Enterprise or Wayfarer both originally designed to be built in ply. This can only be done to a certain extent in a cherub as there are restrictions on the placement of chines.

If you do not use panel curvature or multiple chines to stiffen you panels you can simulate skin thickness by using frames and stringers and battens (think aircraft fuselage) each frame or stringer reduces the un supported span of the skin panel and so stiffens it, this, however takes time to do and adds weight.

A well designed structure will use parts of all of these techniques to get the required skin stiffness and hull strength and will do this whether it is built from strip plank, ply sheets, Glass/carbon/Aramid foam sandwich or carbon nomex or something new that no one has thought of yet.
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Offline phil_kirk

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Re: Boat building
« Reply #7 on: May 22, 2008, 06:04:46 PM »
Oh look what we started.

Since older boats were generally built with glass skins and carbon reinforcements in the high stress areas and still appear to be quite stiff you could save a lot by replacing 1 layer of carbon inside and out with glass.  that would give a cost saving.  you may wish to add extra carbon reinforcements around bulkheads rack tubes etc.  Slippery is mostly a glass hull.

Ross one way to add a space frame is lay strips of carbon tape along the chines and pannel joints.  You then have a monoquoque space frame.  Phil A is right that pannel stiffness is important and that curved pannels have more inherent stiffness than flat pannels.

It is not to hard to make flat foam pannels for the bulkheads.  and these could be built into a hull of any wood/ foam construction to take a lot of the loads.

my initial idea was to keep the shape simple to make it easier and quicker to build not neccesarily cheaper.


Offline tim_unerman

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Re: Boat building
« Reply #8 on: May 22, 2008, 07:50:16 PM »
Just remeber you can still have a low carbon boat that is under weight. Pocket Rocket has only 1 layer of glass and 1 layer of aramid either side of 8 mm foam on most of the hull with only a small amount of carbon in high stress areas. The bulkheads are all carbon which is the important bit. Which if you work hard can be made quite cheapely using out of date prepreg at a fraction of the cost of normal carbon.
But if you also look at the weight calc for Pocket:
When first measure 2006 nats 50 kg
now with new deck and wheelie bars 52-53kg est.
Replacing the ex RS 600 mast tubing in the wings and replacing with lighter weight option (6kg saving) 46kg.

So having a glass / aramid or even an all glass hull if still a light weight option

Offline ross_burkin

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Re: Boat building
« Reply #9 on: May 23, 2008, 12:42:07 AM »
Tim, I remember you saying that something about Lost in Space's construction which was a little different to the norm...
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Offline tim_unerman

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Re: Boat building
« Reply #10 on: May 23, 2008, 08:35:46 AM »
I only now limited information about Lost In Space, but basically she was mainly glass fibre over nomex core with carbon/armaid in high stress areas with an extra space frame under the foredeck and around the shrouds of carbon tube. Thus it was and still is very stiff even compared to a new boat. I think that you could get rid of a lot of the tube to save weight if you were clever. Using nomex core is more difficult and expensive than normal foam but does increase the stiffness a lot. Something I would consider if I was building bulkheads for example.

Tim

iain_christie

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Re: Boat building
« Reply #11 on: May 23, 2008, 09:32:03 AM »
Ross I really would not bother.  As has been said before, the difference in actual material costs of the hull in the grand scheme of things are pretty insignificant.

Foam is not that dear...have you seen the price of good quality marine ply these days?  And as has been said before, you can use different resins and glass.

I think that the skills required for building a decent looking wooden boat are far, far in advance of what you need to put a good looking foam boat together, and you'll need to spend a fortune on tools to be able to work the wood too.  Plus, a wooden boat will eventually rot, or warp, or delaminate, or something else horrible when you've got bored of painting it every year...

If you were seroiusly thinking of building a wooden boat, do so, but don't make it a Cherub.  A short highly loaded boat with panels curving every which way is not going to be easy.  You are better to start with something that was specifically designed to be easy for a home builder to make, such as a Mirror or a Fireball, with simple curves, and no bow.

I'm not "anti wood" (I would love either a Swift Solo or a Bonezzi Contender" but this would have "Unfinished Project" written all over it.

If you want to understand foam construction a bit more, then feel free to pop up one weekend when Simon and I are doing the Slug, and I think you'll be surprised just how simple it is compared to wood.

Interestingly, I have a book about building wooden boats the stitch and glue method, sizes from rowing tenders to twin screw motor boats, and guess what it recommends as an additive for filleting?  Flour!!!!!  Have you felt the weight of a bag of flour???!!!! :o :o :o

Offline daryl_wilkinson

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Re: Boat building
« Reply #12 on: May 23, 2008, 09:57:54 AM »
sure it don't mean 'wood flour'?

Offline JimC

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Re: Boat building
« Reply #13 on: May 23, 2008, 12:26:41 PM »
If you want to understand foam construction a bit more, then feel free to pop up one weekend when Simon and I are doing the Slug, and I think you'll be surprised just how simple it is compared to wood.
I really really strongly support what Iain is saying... I've had (and still have) several wood Cherubs, and as well as all the extra skill in building them, the maintenance overhead really isn't pretty. Read up some o the old Nationals reports and see how hull failures used to come up from time to time - split chines and the like - and have a read of some of my pages on Queenie SJB, maybe starting here to appreciate how much work goes into a good wooden boat... http://www.devboats.co.uk/qsjb/constructintro.htm. Queenie gets one or two outings a year and usually needs some maintenance work after them. Currently she needs the hull cross beam put back together - must remember to do that!
Here's a construction schematic. Consider how much more work this is than a foam boat, and you need damn near as sophisticated a building jig as woith a foam boat too.

On materials and carbon, one thing to consider is that we tend to use basic grades of carbon cloth, whereas we used to use expensive grades of glass. Halo and LFC, for instance, were built from 45/45 biaxial glass, and R glass too IIRC. This stuff is a lot more expensive than standard glass cloth, so the difference between carbon and glass isn't as much as you might think.
« Last Edit: May 23, 2008, 12:30:50 PM by JimC »

Offline ross_burkin

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Re: Boat building
« Reply #14 on: May 23, 2008, 06:37:43 PM »
Just remeber you can still have a low carbon boat that is under weight. Pocket Rocket has only 1 layer of glass and 1 layer of aramid either side of 8 mm foam on most of the hull with only a small amount of carbon in high stress areas. The bulkheads are all carbon which is the important bit. Which if you work hard can be made quite cheapely using out of date prepreg at a fraction of the cost of normal carbon.
But if you also look at the weight calc for Pocket:
When first measure 2006 nats 50 kg
now with new deck and wheelie bars 52-53kg est.
Replacing the ex RS 600 mast tubing in the wings and replacing with lighter weight option (6kg saving) 46kg.

So having a glass / aramid or even an all glass hull if still a light weight option

So how much is a fraction? Can prepreg carbon can be used in the same way as standard carbon?
« Last Edit: May 23, 2008, 06:40:43 PM by ross_burkin »
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