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How do you weigh a hull

Postby magic232 on Thu Sep 11, 2014 12:21 am

Hello.

Just trying to get some indicative weights to date. I had the boat on two horses at the back and supported at the front. I then lifted the front standing on scales and deducted my body weight. I repeated the procedure at the rear and got close to 40 kg un painted. Is this an accurate method ? How does the measurer weigh a hull ?
Cheers
magic232
 
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Re: How do you weigh a hull

Postby Youngi-from-YMS on Fri Sep 12, 2014 2:34 pm

Hi Magic,

good work with building your timber Sabre.............

weighing time.......top measurers and the ISAF norm is to suspend your whole boat from a well calibrated digital set of scales in a room with no breeze, suspended by 3 or 4 points of your boat or in slings and these ropes and slings already zeroed off on the scales......blah blah.

now to the real world. if both ends of your boat are at the same height e.g upside down on two drums of same height, then if you put your scales at one end and then at the other end, then the added total will be pretty correct (if your scales are). Most digital bathroom scales are in fact more accurate than most people would think but are usually designed for best accuracy from 40 to 100 kgs.......one end or half of a Sabre is in the inaccurate range.

so even though this may seem a little inaccurate we find this gives us exactly the same result as the any of the top "official" methods. Place your scales on a nice flat solid base with a bit of padding on top. if possible check it with 40 kilos of something on top to calibrate.Get your sabre, on edge on its gunnale and ontop of your scales and test for balance point for and aft (so scale not confused) and then the balance point towards or away from you as you hold the top gunnale of the boat. you are looking for the exact point where you can literally let go of the boat and you are not exerting any load or force on it........and that is your boat weight!

I laughed when I first saw this method.........and we have so much faith in it now that we don't bother with hanging any boats to check the weight anymore. Spring balance scales that most people to use to weigh boats are in fact notoriously inaccurate as the k-factor of the spring is effected by being banged or dropped,fatigue, temperature and humidity.

hope this helps.

regards

Youngi
Youngi-from-YMS
 

Re: How do you weigh a hull

Postby magic232 on Fri Sep 12, 2014 10:36 pm

Hey youngi.

Thanks mate re the sabre build. Taking a while ( not sure where they get 40 hours from ! I've sanded for that long !!)

Your method sounds good. I hadn't thought of that way. It does sound accurate the way you do it. The weather (wet build up) has started here so the race is on to seal the hull before it gains weight. She won't be a light boat but will be sailing when I'm pushing Daisy's.

Regards john NT
magic232
 
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Location: Australia

Re: How do you weigh a hull

Postby harold on Sat Sep 13, 2014 9:43 pm

Hi Magic
I'm Vic state measurer. Bret's method does work but for measurement we use the suspended method. Remember the minimum weight includes all fixed equipment and it does add up. Toe straps,pintels, cleats, elastic, strop in fact anything that can't easily be removed. You probably want your bare hull to be about 38 to 39kg before painting.
When I made my first boat I managed to balance it on end on a set of electronic bathroom scales but I had to be quick because they only gave a reading for a few seconds
As one of my friends said he could spend 40 hours on a centreboard and rudder.
Harold
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Re: How do you weigh a hull

Postby magic232 on Sun Sep 14, 2014 3:03 pm

Hello Harold.

It's good to know I'm not the only slow builder out there !!!
Could you tell me in your experiences what weights you've seen over the years, say lightest to heaviest in timber. As youngi mentioned on the other thread, I've been banished to the cave in the tropics so I'm going to struggle weight wise. I reckon I could build a light boat but then I wouldn't know if it would last.
On a side topic, rocker, I'm 4 mm out on the 3048 station. I can bog it to get it back into the 35 to 50 mm range (I'm 54 mm ATM) but would be playing with a nice curve. How does a measurer view this ?
Cheers
magic232
 
Posts: 48
Joined: Sun Jan 11, 2009 6:52 pm
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Re: How do you weigh a hull

Postby Youngi-from-YMS on Mon Sep 15, 2014 7:15 am

Hi John,
I'm not a Sabre measurer so my data, experience and opinion doesn't officially count in this situation and by coincidence I have a difference of opinion atm on a measurement ruling with the hierarchy atm on a similar situation with what actions I believe would fix your problem.

The precedent these recent rulings may then allow unlimited designs for the future as not limited to Plywood shapes as in the pst. One of these changes is similar to yours......i.e. a plywood plug has had a curved forward panel flattened and all you wooden builders out there would know that to do that correctly and legally on a plywood boat you need to firstly release the side seam for at least 2 feet and approx. the same amount on the centre seam. This allows for the panel edges to move out at side chine and as the centre seam cant move in as it hits other side panel, so instead they both touch and are forced upwards to find the right spot that has them in equalibrium and matching up on the centreline. This obviously produces a new rocker shape and as the new rocker in effect moves towards the measurement jig this would reduce the rocker numbers. That's only my opinion that comes from experiences like, attending and being involved at the measuring of over 15 ISAF World Championships and as the Aus Olympic Shipwright for 8 years and ensuring easy and low stress passing of the stringent Olympic Measurement at the 2003 Pre Olympics and 2004 Athens Olympics for all classes. However another builder has recently done that very process of flattening the curved panel and their website states a now "more defined 'vee' between the bottom panels with less bottom convex round, and a finer bow. The rocker line remains the same (as flat as is allowed)" . This to me defies the law of physics of "conservation of mass" .....as they've flattened a curved panel with no rocker change?! where did that extra ply go when flattened then?

so I'd love to hear other peoples opinions on this. if a plywood panel is correctly flattened what do you believe will happen to the rocker? and do you believe its possible or legally correct to flatten a curved panel with out releasing the outer and central seems?

is the Sabre to remain plywood shaped or has the beginning of unlimited shapes already occurred?

I am saying all this because if you needed to solve that measurement issue without bog, my advice would be to still release your centre seam (but not the outer seam and you probably need to release from you webframe for at least 100mm each way but most likely 200mm. if you can some how now make a 3 or 4mm gap between web frame and hull panel then you can force the hull panel flatter with steam, hot water and weights....and on the boats Ive known as you flatten that curve your rocker will rise and make your numbers reduce.

So the easy way is just add some bog in that area of 3048 and has been legally approved by measurer and committee......none of my plywood in stock has lumps of bog on it though. its just made from wood.. Sharpening that centreline v will help rise the point you need and then sand your centreline v down behind that point at your jig contact point (usually just forward of case slot). By dropping 2mm at jig contact point and building point 3048 up by 2 or more mm you reduce your 3048 rocker by about the amount needed. If you then added a mm or 2 at your transom datum point you would almost return the first 2 rocker points to original( by the "law of thirds" in this case). and as the jig basically pivots on the contact point at case front it would then reduce you rocker further at 3048 by about another 1mm......so then you'd be in easily. a mm is a mm lost. We actually have adjustable metal threads on all our moulds on the critical measurement points to ensure consistency as moulds change year round and over time.

I'd love you to release the panels from the web frame and flatten it a bit and see if your rocker does or doesn't change. There's a reasonable chance that you really had to force that ply into that tight curve and centre point as it is a tough point of multiple axis. You may need to get some kind of cutting tool to release or cut through the centreline joint,and even that small release and straightening of panel in its final extension to centreline maye be all you need. You are also allowed up to 5mm of hollow so if it needs a bit extra then you have the scope

Personally, I wouldn't do anything till you have your first 2 coats of resin or paint on hull as it may shrink a bit and rocker may differ again. Then if using minor amount of bog to do minor changes it will adhere better. Building up your rear point with thick and extra coats of epoxy resin will help the measurements but also help protect from the dreaded "bum on the ground" drag that can occur from time to time.

Give me a buzz on 0411198710 if you feel like chatting about options to drop some weight out of your boat.

cheers

Youngi
Youngi-from-YMS
 

Re: How do you weigh a hull

Postby magic232 on Mon Sep 15, 2014 1:58 pm

Hi youngi.

Wow, that was some reply. You know your boats and techniques for sure!
You lost me at hello but I kind of get your drift. For a hack like me, you just build it and hope it measures as I don't really understand all the properties of ply as you do.
I do take your point on building up the transom datum as this basically tilts the rocker reference line forward which should give me the couple of mm I need. As I was fairing with west 311 last night I added a little extra at the 3048 mm point which should also help as long as I am careful not to rub it off. I intend to glass the bottom after work also.
As for weight removal, I guess I'm too far advanced now short of doing a less is more paint system on her.
Thanks a bunch for your great advice !
Regards John NT
magic232
 
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Re: How do you weigh a hull

Postby Youngi-from-YMS on Mon Sep 15, 2014 4:03 pm

sorry about that. have a bit of a bee in my bonnet as I have seen other classes get destroyed in similar ways. The Sabre was/is the last unaffected Aus Class really as so many classes have declined while what had made the Sabre popular and strong has been suddenly eroded. While the Sabre remains plywood in shape for all construction methods it can regain its strength. The 125's let someone walk through loop holes and went from numbers in the 80's at Nationals to about 12 the next year. The Taser was actually an NS14 design and Frank Bethwaite had major concerns about its future as rules were too loose so he added a foot to his design which has evolved into the International and successful one design class it is today.

Sabre sailors and members are generally just happy people that just want to go with the flow and just sail but even though my production is in professionally built FRP boats, that only makes me more determined to stand up for and protect the Sabre we all still want or it will lose the very essence and friendly nature and character that made it popular and is non existent in most other classes. If we don't keep the design constraints in line with what amateurs can build its loses the very core item that gives this class its soul. Once an amateur builder is unable to build the same shape as one from an FRP builder there is no chance of anything "one design" or equal ever again and the assumption that they've lost the race before starting as now racing against the unknown.

speaking of "unknown" I'm not familiar with West 311? what is it?
Youngi-from-YMS
 

Re: How do you weigh a hull

Postby Aten on Mon Sep 15, 2014 8:06 pm

Not sure about West 311 but 411 is a West Systems filler additive. I guess there was a typo.
John. If the proposed sabre charter passes in its current form you will be able to ask the measurer nicely and a small error will be passed as long as it is considered no advantage. I usually find a carton of beer helps get things done.
Aten
 
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Re: How do you weigh a hull

Postby magic232 on Mon Sep 15, 2014 10:56 pm

Hey guys.

Haha, yes 411 is the filler, sorry !
With my skills or lack of, I can guarantee anything outside of the tolerances is a mistake not a go fast addition.
I bogged up 4 mm where I was low so all good now. The boat has been faired tonight and glassed from stern to web bulkhead on the outer hull. My only concern is why I was out of tolerance on one of the 3 stations as I felt it would just fall into the middle of the tolerance range as panels were cut millimetre accurate to the supplied templates ?
Hopefully painting on Thursday after I re establish the chines on Wednesday night.

Regards John
magic232
 
Posts: 48
Joined: Sun Jan 11, 2009 6:52 pm
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