Wooden boatbuilding

[wietzehuitema]

Hi everyone,

All the best wishes for an enjoyable and healthy 2015 !

I have a question concerning building an OK-dinghy in wood. Old boats in the Netherlands are made from 7mm plysheet for sides and bottom of the hull. Sometimes the deck is less thick, sometimes its 7mm as well. Sometimes there is a) some additional sheeting on part of the cockpit floor for reinforcements, or b) stringers than run only for 95% of the length of the cockpit floor in for-aft direction.

In the classrules there is a statement about hull thickness:

8.3 Construction of the hull, with the exception of stringers, shall be of approximately even thickness (within 10%) and density longitudinally and no attempt shall be made to concentrate weight near mid-length, or at any other point. If it is suspected that this rule is being broken a NA may order test holes to be drilled in the skin or structure.

Does that mean that I can use different thicknesses for the bottom, side panels and decks as long as each panel has an even thickness on its own longitudinally? So for example would an OK with a bottom of 10mm sheet (Is that what the boats build by Tony Tresher have got ?), sides of 6mm sheet and a 4mm deck be within class rules? Or should all the hull panels be of the same thickness?

During the AGM there has been discussion about thickening only the cockpit floor and the grey area of stringers that have to run the whole length of the hull… Do you think stringers that are only on the cockpit floor and nog the entire length would be illegal for a new build OK?

Hope someone can help out. Perhaps it’s just my lack of fluency in English reading…

Bye,

Wietze

[davecooper]

Hi Wietze,
Happy New Year!

Yes, you can definitely use different plywood thicknesses for bottom, side panels and deck.

In rule 8.3, it says “…with the exception of stringers…” so your stringers don’t have to run right through the boat, although I should think you’d want to run them past bulkheads 1 and 2 (that is forward and aft of the cockpit) to spread the load further through the boat.

I don’t know about Tony Thresher’s boats, but I’m sure he’d be happy to tell you all about it if you drop him an email – he’s a very nice guy! If you click on uk/hulls on https://www.okdinghy.co.uk/php/suppliers/buyselect.php you’ll find his email.

Dave Cooper

PS That’s with the class rules as they are. Always a problem if the class rules change while you’re building your new boat!

[wietzehuitema]

Hello Dave,

Thanks for your answer !! Than I can make some changes in my design of the hull to get a strong and stiff enough cockpit floor…

And I will drop Tony Thresher an email. I hope he’s got some useful tips for me that he’d like to share with me…

Wietze

[Finn]

The agenda for the AGM suggested that a rule was being voted on that would mean stringers would need to run full length and be of consistent cross section.
Apparently the AGM happened on 29/12 so the should be an answer is out there somewhere.

[davecooper]

Yeah, well this is what the minutes say:

C. Hull Construction
Rule 8.3.1
Construction of the hull, with the exception of stringers and framing, shall be of approximately even
thickness (within 10%) and density longitudinally and no attempt shall be made to concentrate weight near
mid-length, or at any other point.
If it is suspected that this rule is being broken a NA may order test holes to be drilled in the skin or structure.

Reasoning: Over the years many boats have been built with frames supporting the centrecase and other areas.
This simply removes the grey area of legality.

8.3.2 All framing and longitudinal stiffening, including stringers, shall be consistent throughout the boat.

Reasoning: Again this removes a grey area on whether stringers have to run the length of the boat or not.

Carried: 8 for (NED, POL, FRA, AUS, NZL, SWE, GBR, GER), 0 agains

I’m not sure that I understand 8.3.2. If it means ‘Stringers must run the whole length of the boat’, why doesn’t the rule say so?

Dave

[wietzehuitema]

8.3.1
Does this mean the centreboardcase supporting elements are allowed? Even if that is extra weight in a specific part of the boat?

8.3.2
My thoughts exactly. I’m not sure what the rule says right now…. If it’s not along the entire length of the hull, what does ‘consistent’ mean…

Wietze

[Bob Bourne]

Hi guys,

Yes, it’s not particularly clear (and never has been – looking back), I’ve always played safe and if I’ve used stringers, made sure they run the full length of the boat, They don’t weigh that much in the ends anyway.
The rule about ‘concentration of weight in the middle’ (or anywhere else) is open to abuse by builders, but it should be up to measurers to report such abuse, and refuse to issue an approved measurement form, in my view.
All of this doubt could be overcome with some detailed guidance from the Tech Committee, as to what is and what is not acceptable. At the moment it is left to some strict (and courageous) decisions by event durys and measurers to bring these things out into the open, as in the Svendsen case at the Worlds.
Nobody wants to have doubts about the legality of their own boat, or anybody elses for that matter.

Cheers,
Bob

[davecooper]

It isn’t up to measurers to refuse to fill in measurement forms, Bob: measurers just measure boats. If they think there’s anything weird going on they include a note. It’s up to the National Authority to decide whether they want to issue a certificate or not. It’s also the NA who decide if they want to drill holes in the structure to see if its all the same thickness, so unless the measurer has superhuman powers he doesn’t have any way of telling what’s going on in there. If we did a swing test to check the pitch inertia of the hull it would fix the whole problem. Finns do it, europes do it, it’s something you can measure easily and you could say that as long as a hull weighs 72 kg and passes the swing test all the details like framing and stringers and material thicknesses are completely free.
As it is, although the rules say you musn’t centralise the weight, it isn’t clear whether (say) building the centreboard box out of thicker material is centralising the weight or just structural common sense (considering that centreboard boxes quite ofter fail near the front). Same goes for including extra deck beams close to the cockpit where the helm’s going to park his bum when it’s light or windy – it’s got to be structural sense, but it might be illegal under the proposed rule.
Dave

[wietzehuitema]

Do you think it’s legal to have extra framing (like an extra station) between the transom and station 1? And between station 2 and station 3 to support the shape of the hull?

My old wooden 1971 OK dinghy has a total of 4 diagonal ribs ( you could say stringers) on the bottom panel to keep the panel into shape (both starboard and port, between transom and station 1, and between station 2 and station 3). Or would it be better to thicken the bottom plates and have nog stringers or what so ever?

Wietze

[Bob Bourne]

Hi Wietze,

I don’t think there is a problem if the frames in the boat are reasonably evenly spaced, (even extra ones, like maybe near the mast for instance). And as Dave says, if there is a good reason for a frame for structural reasons. Be careful about adding too much weight though, our boats are pretty tough, even when quite lightly built. Make everything as light as you can.
Fore and aft stringers, going full length through frames are also good for preventing the dreaded ‘bumps’ in the hull shape at the bulkheads and frames (you only notice them when the boat is nicely finished and painted!).
Good luck with the refurbs.

Bob

[bradjohanson]

Quote:
8.3.2 All framing and longitudinal stiffening, including stringers, shall be consistent throughout the boat.
Reasoning: Again this removes a grey area on whether stringers have to run the length of the boat or not.

[attachment=1:2q3h3tug] Stringers.JPG [/attachment:2q3h3tug]So these two common ways of laying out hull or deck stringers either parallel to the centreline or parallel to the chine could be considered no longer legal as they do not run the full length of the hull? How do deck stingers run the full length of the hull with a cockpit in the middle?

Alternatively this arrangement could be considered legal as it has a rule that applies consistantly to the full length of the hull. A poorly worded rule.
Brad Johanson
USA-1010 and CAN-500

[Bob Bourne]

Hi

I have a copy of the building plans (from 1965) and there are no floor stringers drawn in at all……
There is though, a strip of 6mm ply shown, and I quote, ‘glued to the inner skin’ of the cockpit floor. This is about 150mm+ wide and runs parallel to the centreline between Stn1 and Stn2 bulkheads !!

I guess the Rule was developed to prevent people starting to use excessively heavy/thick ply here to centralise weight. So, a stringer or ply strip, if fitted, is to run the full length.
I assume that means Parallel with the centreline, because it’s Really difficult to get 4 or more stringers a side to meet, on the bottom panel right in the bow. (I’ve tried it!).

Bob

[JEFFH]

Hi All
Has anyone seen this link http://www.yachtsandyachting.com/news/181459/The-return-of-the-woodie
Could generate some new boats at very reasonable prices.
JH

[Hoppetosse]

I have had a set of OK plans since about 1975 but haven’t got round to building one yet. With to much time on my hands I have recently been thinking about building one but have been worried that it might not be a ‘fast’ boat. So following on from the item above and talking to OK representatives at the Dinghy Exhibition I contacted Dan Leech in New Zealand and got the following reply.

****
Hi Andy,

It is a very simple construction designed for the home builder. No hand cutting required.

The design fee is $700 NZD which includes computer files to cut the female hull jig/mould out, and get the boat cut out. All the parts of the jig and boat are cnc cut, so no hand cutting required.

The $700 NZD design fee is so you can build one boat for yourself. Any more boats that are built from your hull jig, the design fee is $500 per boat.

The design package includes the computer dxf files to get all the plywood for the hull jig and boat cnc cut out, and 3D CAD images showing the assembly sequence.

You source the plywood, take it to a cnc cutting company in your area, and they will cut your boat out for you. This saves on freight cost from NZ.

The plywood required:

3 sheets of 18mm – good grade plywood for the jig.

4 sheets of 6mm Gaboon marine plywood, and 3 sheets of 4mm Gaboon marine plywood for the boat.

A standard rate to cnc cut a sheet out is around $25 / sheet (to give you an idea).

Regards,

Dan.
*********

With the New Zealand Dollar around 50p to the pound, by my rough calculations that would make a first boat built around £700 to £800. If anyone was interested in building from the same Jig then subsequent boats would probably be less than £600. Obviously that is without rig or fittings but still a very economical way to get a world class boat on the water and according to the article they only take a couple of weekends to build.

Andy

[davecooper]

Sounds really promising!
If there’s some appetite for this, I wonder if the UK class assoc should shell out for the jig? It’s not as though many people will want to build more than one boat (apart from addicts like Bob Bourne, obviously!). It might kick-start the process, and I know Jules and Paul are just desperate to spend some of the association’s money!
Dave

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