Introducing 'Erica'... and a few questions...

[John B]

It's a really nice shot and there's some good information about sheeting arrangements for your headsails. In particular, there appears to be two parts to each sheet before they run through the fairleads on deck. What's the go there?

It's nice to see the position of runners on profile shots too. It's apparent from most of the pictures & illustrations gaff rigged boats I've been on/seen the pendant should be aimed at about the middle of the foot of the main.

Runners .. well you need to decide whether they're on levers or on a purchase. The big plus with levers is they are very positive and set the rig to a predetermined tension every time, and thats quite a lot of tension. The big minus is that they're on or off and when they're off that leaves a great bight of runner hanging out the side of the boat and potential to wrap the spreaders( and unrig the boat ). The big plus with runners on a purchase is they can be let out just enough each time so lessen the wrapped spreaders problem.
If they're on levers then you have a problem because inevitably you will have to unhook them for a run, a totally unacceptable thing to have to do.
You can rig up a system of blocks on the deck to beat that ,which is what I did. I have never seen it replicated on another boat to this day surprisingly enough.
The classic runner landing point is the same behind the mast as the staysail is forward but thats flexible. Further aft and you get better geometry but they'll be in the way on the wind. Further forward you don't quite get the best geometry but you do get to leave them both on on the wind and short tacking.
To me , what you do is make a rig which has enough drift in the lowers etc so that it is essentailly able to stand without runners, but you have them there to pull on over 18 knots or punching a sea say. I'd often let mine off on a dead flat run in moderate air in case of a gybe.

The jibs were set up on our size boats with a bullet each block at the clew or on pendants. So the sheet looped around the fairlead, up to the bullet block and then back through the fairlead to the sheeting point for a 2:1. They are extremely dangerous set up that way. The way to solve that is Murrays.

[nessboat]

methinks...its about time you started to think about buildiing...and all the detail that involves.....you have the hull design sorted and 90% of the rig..Having gone through the same process I know well enough the desire to perfect everything on the drawing board but in my experience ...even if you spend the next 2 years designing it to death...it wont be perfect and you will still have carry out modifications....hopefully minor.!
I would hate to see all the efforts you and all the other contributors have put into this project ....not to end up with as a mere "dream"..
This boat .....IMHO....should be built ......go for it.....

[Duncan Gibbs]

When the topsail isn't set there are only two lines leading aloft as vestige that it's used at all and one of those, the sheet, can be unrove completely when the gaff is lowered. When I want to use it I can. Every account I've read of topsails is that whenever its able to be set it gets set and the sailing gets better.

"Men looked aloft, swore at them, fiddled with them, vowed to get rid of them, and then ordered bigger ones."

[RFNK]

I'd love to see all those accounts of short-handed boats sailing successfully with topsails in the temperate zones of the South Pacific or Tasman. Should be very entertaini ng
Rick

[Duncan Gibbs]

WBM #131 for a start! The Carrs on the subject.

Shopping then shed building so I have space to stick the keel when it gets cut soon!

[John B]

What I typed up before and didn't post was that this topsail as first reef business might have been the go of neccessity back in the day, but now the best approach is to make the rig so it performs well under the lowers. Keep the topsail for the light crewed day, the regatta or race day .... Treat it as an extra in other words. That means peaked up as high as you dare, in practise about 30 degrees, and plenty of mainsail area.
That gets you your cake and you get to eat it as well.

[John B]

I came across a sketch of the system from the WB article that the carrs used. I used it and it worked well.


However, the more I think about this boat and the way it'll be used ( its not a racer) the more I think you should have a tall pole mast ,( near marconi as the last of the gaff boats from the late 1920's were) with steepish jib luffs and a 30 degree peaked mainsail of decent area . I'd also keep the spreaders high and with the crane intregrated into them like I did with Waione , to get you a max luff length for a trysail and very simple geometry. That way you get a gaff rig with fisherman topsail or yardless jackyarder( have both), and a manageable option to go to sea with in 25 or 45 knots.

[Andrew Craig-Bennett]

I agree with that.

[Peerie Maa]

What I typed up before and didn't post was that this topsail as first reef business might have been the go of neccessity back in the day, but now the best approach is to make the rig so it performs well under the lowers. Keep the topsail for the light crewed day, the regatta or race day .... Treat it as an extra in other words. That means peaked up as high as you dare, in practise about 30 degrees, and plenty of mainsail area.
That gets you your cake and you get to eat it as well.

This reminds me of a mate from Andrews neck of the woods who set his yard topsail over his reefed main in order to maintain a long luff.

[Duncan Gibbs]

I came across a sketch of the system from the WB article that the carrs used. I used it and it worked well.
[...]
However, the more I think about this boat and the way it'll be used ( its not a racer) the more I think you should have a tall pole mast ,( near marconi as the last of the gaff boats from the late 1920's were) with steepish jib luffs and a 30 degree peaked mainsail of decent area . I'd also keep the spreaders high and with the crane intregrated into them like I did with Waione , to get you a max luff length for a trysail and very simple geometry. That way you get a gaff rig with fisherman topsail or yardless jackyarder( have both), and a manageable option to go to sea with in 25 or 45 knots.

I like that sketch. I think you posted it on a thread by a chap who had a 24(?) footer who loved his topsail he recycled from an inverted jib. His boat sailed better on all points with the topsail from his account, but I'm not sure he'd used the halyard as jackstay system. It's precisely the system I want to use. The eye on my mast sits about a quarter of the way up from the throat to the topsail halyard sheave, so I could still set a small fisherman with a reef in the main as Nick's mate is wont to do.

The peak I have as drawn is 37 degrees (in practice it will probably vary +/- 5 degrees as Ed pointed out) and I think will work well with the roller gear so I'd be a little cautious of heading any higher. The spreaders are now integrated with with hounds cheeks and the crane for the throat. If I did go for a full pole I'd probably keep the spreaders at the throat crane and run the caps higher. I'd also think about putting that third shroud back into the picture. It all kind of adds up and I'm liking my rig that can be cut back if I start hitting winds above 10 or 12 knots and really snug down when it starts blowing (or I cross Bass Straight, or the Ditch). The average wind speed around here along the coast is around 8 to 9 knots.

[Andrew Craig-Bennett]

A point about topsail yards is "what do you do with the .... thing when it is not set?"

There are those who will say "stow it along the boom in the lazyjacks" which is nice if you have points reefing and lazyjacks.

The merit of the yardless topsail is that it can easily be set over a reefed main and can (most important point!) be easily handed.

I practice, cruising, I seldom bother with the topsail.

[Duncan Gibbs]

I have a nice, big toerail on the foredeck where the yards can be stowed and will out out of the way of moving around deck, handling lines, anchoring, foresail sheeting points and so on. I have the capacity to carry it, and a smaller yardless one set over a reefed main. Under main and foresails alone I still have 52.9m2 set and the lead only increases slightly.

I think my rig is feeling okay at this stage, so I have something to put weights to. I know this discussion will most likely continue and I won't discourage it (even if my pedagogue is bored ), but I should really turn my attention to weights and measures and hull scantlings.

John (B), if you've found any of that data you had at some stage (wall of info' Patrick referred to it as) I'd be most obliged and appreciative.

[Hesp]

I had a pole mast on my gaff cutter. Yes, there are no awkward topsail spars to stow, but more, the topsail was 'permanently' stowed in a bag at the foot of the mast, and 'permanently' hanked on to a jackstay with the halyard, downhaul and sheet attached to it ready for action. To set it I opened the bag, hauled the sail up chock-a-block (watching the sheet), tensioned the luff with the multi-part downhaul and sheeted in.

Are you going to be sailing the boat on your own sometimes? :-)

John

[John B]

John (B), if you've found any of that data you had at some stage (wall of info' Patrick referred to it as) I'd be most obliged and appreciative

What have I forgotten to do Duncs?

[Duncan Gibbs]

I think I pea-emmed you a while back there John, but I can't merember if you said you still had all that hull scantling data on a few of the Logan/Bailey boats or not. If not, is it in Ivor Wilkin's book?... Harold Kidd, or Jase have any of that kind of data?... I know I'll be using SE Queensland/Northern NSW timbers but the differences can be accounted for in a comparative analysis of the structural properties. I'm guessing though that there's not much data on the Pohutukawa. I certainly only found a smidgen on the intertubes... Enough to give me an idea of it's strength, but nothing particularly definitive with swell/shrinkage percentages or hardness values.

I want to produce a comparative chart or two of the different boats' scantlings and pop Erica in there to see where she sits. It should be an interesting exercise even without Erica plugged into the mix.

Sounds like the consensus is moving toward full pole eh?

[Hesp]

A few of us on this thread seem to like the idea, but I'd say the consensus out there in the real world isn't for the pole mast. I just thought I'd throw in how mine worked to fill in the gaps. (I didn't mention it before to avoid cluttering the thread).

Don't take to much notice of us (I mean "me" really). Yes, you've had a fantastic amount of advice from various people, and Ed in particular has given you some highly valuable advice, particularly on hull design where the technicalities can be a bit daunting; but don't lose sight of the fact that you wanted to design this boat, you didn't want the WB forum to design it; and the boat should be the boat you want, not the boat somebody else says is better.

I don't design many boats, but I do design all the time (and I know you do), and perhaps one of the most interesting things about this thread to me is that it has tended to force a linear approach to the design process, as opposed to the more usual "spiral" approach used in design. I guess that's the nature of web forums. I like to step back and look at the whole picture and slightly refine an area, step back, refine another area, slowly building up detail like an oil painting. "Two cheek blocks or a double for the jib" - well I guess I got sucked into it too, but not without being aware of it. You'll have plenty of time to decide on what sort of topsail you'll have, and you'll probably change your mind several times. If you can't bring yourself to make a decision on something which isn't going to happen for five years, but it's necessary to design the "foundations" for it (say - frame sizes and positions for different mast types) try and design in for both eventualities.

At the end of the build you've got to be able to say "I got what I wanted".

John

[Duncan Gibbs]

Thanks for the wise words John! As I've said the half pole/full pole can wait. I really hope to get back down to Tassie and sail on a few more different boats to give me a better appreciation of the pros and cons of various arrangements. I'll be able to get on with building the boat itself in the meantime, which I'm sure will evolve as it gets built as well. I appreciate the spiral approach. I still have a vivid memory of Luke drawing me such a diagram on a restaurant table in Saigon after some excellent Vietnamese nosh! (Something I hope to repeat one day again Luke!)

Part of the reason I want to look at weights now. I'll then have a pretty complete picture and can start going back and tweaking lines, rig, scantlings and the interplay of all three.

As an environmental designer I'm all too aware of the catastrophic design approach of trying to please everyone and ending up pleasing no-one, and turning out mediocrity made concrete. This is, indeed, my boat and I want it to work for me. I'm certainly not expecting to turn out a cruising classic, but just something solid, reliable & reasonably simple, enjoyable to sail in most conditions and pretty to look at.

[Ed Burnett]

Believe it or not, my hope is that we will be spiralling with this one. We just haven't completed the first loop yet so it looks linear!

As Duncan now suggests, although we have had some entertaining diversions into rig detail, we have achieved what we need to continue with the structure etc. so it now makes sense to move on.

It is also absolutely the case that Duncan is going to have to make some judgements about what he wants, and these judgements need only be justified to himself. I am trying to guide in a manner that leaves the decision making to him, and gives him the tools and the benchmarks he needs to make the various assessments. I hope that I am being at least partly successful in this - it's hard to put one's own ideas to one side sometimes!

Anyway, I went quiet because I'm busy, and because I do think we have said enough on rig (especially the funny triangular things at the top) for this loop of the spiral.

[Hesp]

I think you're doing a wonderful job Ed, and we're all getting a lot out of it. I hope you're getting something out of it too.

John

[John B]

I think I pea-emmed you a while back there John, but I can't merember if you said you still had all that hull scantling data on a few of the Logan/Bailey boats or not. If not, is it in Ivor Wilkin's book?... Harold Kidd, or Jase have any of that kind of data?... I know I'll be using SE Queensland/Northern NSW timbers but the differences can be accounted for in a comparative analysis of the structural properties. I'm guessing though that there's not much data on the Pohutukawa. I certainly only found a smidgen on the intertubes... Enough to give me an idea of it's strength, but nothing particularly definitive with swell/shrinkage percentages or hardness values.

I want to produce a comparative chart or two of the different boats' scantlings and pop Erica in there to see where she sits. It should be an interesting exercise even without Erica plugged into the mix.

Sounds like the consensus is moving toward full pole eh?

I don't recall that .. I don't really have solid data on a range of the kiwi boat's scantlings.. I have a boat though , which means simple things like stringers etc can be measured. Did I send you the deck plan I sketched for Steve when he was doing Ngatiras deck?
I wouldn't be too fixated on pohutukawa though. It was a source of timber for naturally grown knees and floors etc but it doesn't have the longevity of Kauri typically. Its protected now.

[Duncan Gibbs]

Anything you can send John is most welcome and I will be much obliged. I'll PM you with my email so you send the sketch. Patrick sent me a bunch of data for Rogue, plus I have the original contract spec's for Viking and some data from that WB article on Rona. Nick also sent me some data on another GL Watson design, Vivid, built in Scotland, although she's actually framed as well. I could send you the Excel sheet so you can get a feel for the kind of data I'm after. Maybe some of the other Logan/Bailey owners you know could add some data as well. Charts can then be produced to see where each vessel's scantlings sits in comparison with others. It think it should be an interesting exercise all round.

[Peter Vella]

Thanks for emailing me the parametrics spreadsheet, Duncan. It is interesting seeing how your design evolved. It does seem to me that you have done it the long way by drawing everything out in 2D and fairing the lines from that. I beleive that would be the traditional way of drawing a boat. As you know, I have been using Freeship to design my boat, and I dont think I could have got as far as I have from scratch without it. There are some annoying quirks to Freeship, but overall I have found it very useful to have the ability to apply small changes and have all the lines and hydrostatics updated in real time. I don't think I would have contemplated drawing a hull from scratch without it. I know it may be too late to be useful to you now you have your lines finalised, but this has got me thinking about making a Freeship tutorial thread, or at least going through step by step showing how I drew my hull.

I think it is good advice that someone posted here, that you shouldn't strive for perfection on the drawing board or else you might never actually build the boat. There has been alot of discussion on the rig here, as there has been already on my thread, but I think as long as you have the fundamentals pinned down as in those details which will effect the hull design most (rig type, scantlings and expected weight of the rig, position of chainplates etc), then going over details like running backstays, or the how the topsail is going to be set can be done while you build the hull. At least thats how it seems to me, and you should probably take anything I say with a pinch of salt as I am a complete amateur. I just know from other projects that it can sometimes get bogged down in going over fine details that are not absolutely necessary to get the project rolling. It is a judgement call as to what constitutes fine detail, but I would say that much of the rigging details falls into that category. Similarly I have caught myself scouring ebay for an anchor windlass, although realistically I wont actually need one for at least 2-4 years!

Robert666 posted a comment on my thread saying that he has been designing a fishing lugger for years but has never got past the stage of tweeking the lines. I could see myself doing that quite easily. I can see how you have been through the 'design spiral', it is the thing I think that makes boat design such a challenge, that it is not a linear process. Ed's advice is definately both valuble and re-assuring, and makes very good sense. I have sometimes thought that I am off my head attempting to design a boat myself. It is heartening to see people with so much knowledge and experience taking the time to offer constuctive criticism for a project like this.

I'm looking forward to seeing your build thread for Erica.

[Duncan Gibbs]

Thanks Peter!

I've cut a fair amount of timber for my build already, but have to try and sort out my keel timber as it went from being pretty straight off the saw to developing a big bow along it's length. It's something I haven't really begun to try and grapple with yet, although I do have an idea, or two to try out. I have a reasonable idea of the weights and measures that need to be applied to the design, but, as always, there have been other things getting in the way. Once I have my shed finished I should have a fair bit more time up my sleeve to look at this aspect of the design, along with another look at those lines, where I think I might improve them to make the design more builder friendly.

Thanks for the bump!

[Peter Vella]

In his book 'Boatbuilding' Chapelle recomends that the keel timber be boxed heart, ie; with the pith running though it. I can see the logic of that because there is more chance that the internal stresses of the timber will remain roughly equalised and the keel won't bow after being sawn, but I think that would work well only for timbers that are not overly prone to heart shakes and checking. I don't know what the white mahogany you have is like, most tropical hardwoods tend to have an interlocked grain, so Chapelle's suggestion is ideal when using them. I may end up using oak for my keel, which is very prone to checking, but hopefully I might be able to source some good elm instead.

In my early iterations of my design I had very hollow garboards and a tight tuck towards the stern. These have gradually been softened as I reworked the design thinking about how I am going to have to bend the frames and planks into place, and also thinking more about accomodation. I had got to the stage where I had something that looked good and would probably sail well, but could it be built with the minimum of broken planks and cursing?

What you have looks builder-friendly enough to my eye, but I don't have any experience working with the Aussie hardwoods that you will be using for the frames. I have heard that Aussie boats have commonly been built with double or even triple bent frames, like a glueless laminate, which suggests to me that your local boatbuilding timbers are not as easy to bend as the oak which is commonly used over here.

[Duncan Gibbs]

Spotted Gum is commonly used for frames and not infrequently for planking and other major structural members, is what I have for the keel. I wouldn't dream of using boxed heart in Spotted Gum as the heart is very pithy and would most likely rot away quickly. The heartwood, OTOH, is very durable.

I'm planing on using the Scottish/NZ Ashcroft system of planking:

A Question for the Kiwis

The beginning of Erica's journey into life can be read about here:

Erica Underway

[Duncan Gibbs]

It's time to bump this thread and it's content along. Way overdue some may say, but life has a way of throwing up challenges and additions to one's compliment listed as 'to do.' But, I feel I must return to the progress of designing Erica like some begotten ship's captain returning home to his lonely family after a year at sea! And what do I return with? Why! A cargo hold full of questions!

Well, maybe not a cargo hold... Just two really...

To begin with I've knocked up these two spreadsheets dealing with the scantlings; firstly for vessel data, which is a far way from looking complete, but hopefully a few blanks will be filled in by some of the NZ forumites, and another dealing with timber properties. So I hope I'm on the right track here:




NB: If you right click the charts and open in a new tab/window you'll be able to zoom into the data to really read the charts clearly.

I have to say upon a preliminary review of the figures for a rather bigger boat like Viking and the comparisons for the timber properties I feel reasonably confident in my initial scantlings. But I may yet be disabused of early illusions by voices wiser than mine. In fact I expect it so!

The second question relates to how one actually conducts the weight calculations (LCG) with regard to long items such as the hull planking and the keel. I understand the way it works for smaller (more compact) things such as floors, mast steps, anchors & rodes, water tanks, engines and so on, but not for these longer or 'all embracing' items like the skin and backbone. There's a nice method explained in Gordon Trower's book on yacht design which boils down the tedium of these calculations to a rather nice chart where one can plug in the data for the mass and distance from a fixed point, in this case the bow to obtain the horizontal moment and a replication of this for the VGC calculations from the base line as the fixed point.

Does one simply take a long item, such as the keel, and measure segments of it to replicate this process and arrive at an aggregate figure for said long item?

[Duncan Gibbs]

Bump!

John? Ed? Nick?

[PeterSibley]

I knew this was a bump! How much does a piece of wood weigh?

Just go through them one by one and work out a volume x density, something to fill in the evenings .

[Duncan Gibbs]

I knew this was a bump! How much does a piece of wood weigh?

Just go through them one by one and work out a volume x density, something to fill in the evenings .

I do know how it works, aside for things like the hull skin that I need advice on.

Anyhoo, stop your winking at me Peter! It's most disconcerting!

[Hesp]

"The second question relates to how one actually conducts the weight calculations (LCG) with regard to long items such as the hull planking and the keel."

I just click on the "Area Centroid" icon in Rhino, then the hull skin and it tells me. For the keel I click on "Volume Centroid".

Seriously, I said right at the beginning of this project to do it in 3D CAD, it'd be the best investment you could make for this boat and your career. I still say that. But in your current position (I know you use 2D CAD) I'd -

For the hull skin - In 2D CAD, at each station take the length of the half section (just the length of the hull skin) and plot them vertically, spacing them station spacing apart. Draw a curve through their ends and use CAD to find the area under the curve (area of stb side hull skin) and the centre of that area.

For the keel - Take the areas at each station and plot them as before. The area under the curve is the keel's volume, the centre of the area is the centre of gravity of the keel.

For scantlings use "The Elements of Boat Strength" by Gerr, or you should find Lloyds rules in the back of Pardey's "Details of Classic Boat Construction".

John

[Duncan Gibbs]

Thanks John! I'll follow up on this and post some results... Soonish! In fact I'll put up some preliminary results to show the process.

I really do see the total benefit of doing all this in Rhino or similar, particularly in terms of time. However, as Rick (RFNK) has pointed out, I'm a Mrs Beeton sort of a chap: "For beef pie, first slaughter your oxen..." I best learn computer programmes in a similar manner as well: Hands on with a teacher. Being out in the countryside doesn't help. It would be great if there were someone nearby who could teach me the fundamentals of Freeship or Rhino, figuring out the rest becomes reasonably easy from then on. I've had a go at Freeship but can't seem to grasp the basics. Add an ageing Mac and an ageing PC into the mix and it doesn't get any easier.

I actually have Lloyds Rule and Regulations (1979) to refer to as well. I'll plug in some of the data from that as well.

[RFNK]

I know all about Rhino. If one comes at you, climb a tree - quickly!

Oh, and they like to stamp out fires!

Rick

[Ed Burnett]

Hello Duncan,

John's advice re doing stuff graphically is spot on. There are a lot of things you can do this way right on the drawing.

For example, if you have a constant siding sternpost assembly, simply query the profile area of the whole lot and there you go. Assuming you have set your drawing up with the zero point at your weights and centres origin, hover over the area centroid and the coordinate readout gives you LCG and VCG for that part.

Don't forget you can often do framing as a "smeared thickness". Siding x Moulding / Spacing gives you the thickness that regularly spaced framing would represent if that quantity of material were smeared over the surface. Given that you have already worked out the area and centroid of the planking this saves a bit of time.

Set up your spreadsheet so the weight of any one thing is calculated as the product of two numbers, ie an area x kg per m^2. Or a length x kg per m. That way as you iterate and revise scantlings you can quickly see the effect on total weight.

The most important thing with weight estimates is to have a number down for everything. There isn't much point getting worked up about accuracy in any one part if a whole other part doesn't feature at all! Also, be aware of the numbers you come up with. Lift your head every now and again and ask yourself if you believe it!

At the end of the process your general confidence determines how big a margin you put on. In my weights sheets I have a code letter next to each entry - k for known or verified, c for calculated, e for estimated and hopefully not too often g for guessed! The more k's there are, the lower I am prepared to set the margin.

Not the most thrilling of tasks but worth being thorough over.

ED.

[Ed Burnett]

Forgot to mention on scantlings....

Be a little careful with the old Lloyds rule and your chosen method of construction. It will be a good start for backbone scantlings, and probably deck structure, but the rule draws it's basis from plank on frame hull construction with frequent transverse frames. Appropriate plank thinkness for this may well be different to that for your chosen method, and frame scantlings are going to be pretty meaningless. That's where your data from other boats is of value.

What the Lloyds rule may well be useful for is the method of correction to account for variations in spacing and timber density. This could serve as a means for making sense of the scantling data you have.

[Peerie Maa]

'Cos Erica is a shapely boat, you may need to factor in the girth of the skin around the topsides. The actual distance covered between each station is further there than at the keel. Big ship design offices used to do this on the shell expansion, but I don't know if yacht offices need to do so. Perhaps Ed can advise.

[Ed Burnett]

That's a valid point Nick. I expect if one simply measured around the DWL in the plan view, and corrected the planking area in the ratio of this girth to the waterline length that would be a sensible enough adjustment. LCG is unlikely to be affected.

It's not a problem for those of us that model the hull surfaces in 3D!

Who would have thought that weights and centres could be so much fun....

[Hesp]

That is an interesting point Nick. I suppose this difference would be greatest where the hull skin isn't parallel to the centreline or LWL of the boat - ie the bow and the stern. As this is a double ender I imagine this error will balance out?

Anyway, it'd be interesting to know what the error would be as a percentage.

John

[PeterSibley]

That's a valid point Nick. I expect if one simply measured around the DWL in the plan view, and corrected the planking area in the ratio of this girth to the waterline length that would be a sensible enough adjustment. LCG is unlikely to be affected.

It's not a problem for those of us that model the hull surfaces in 3D!

Who would have thought that weights and centres could be so much fun....

If only, we computer illiterates are in trouble .

[Peerie Maa]

That is an interesting point Nick. I suppose this difference would be greatest where the hull skin isn't parallel to the centreline or LWL of the boat - ie the bow and the stern. As this is a double ender I imagine this error will balance out?

Anyway, it'd be interesting to know what the error would be as a percentage.

John

The more rigourous way to do it is to set out the stations of the hull expansion with the spacing equal to the local girth around the waterline you chose as most appropriate, set the girths of the stations out at these locations to draw your shell expansion and then use a planimiter to measure the area. Or in the absence of a planimiter erect new stations at equal spacing on your stretched shell expansion and use Simpsons Rule.

[Hesp]

I suppose if the most appropriate WL is taken to be the LWL the angle of entry might be 20 degrees so the hull skin could be 1/cos20 = 1.06 times as big. Or say 6% to big from sta 0 to sta 1, 5% from 1 to 2, 4% from 2 to 3 etc. That could add up if it weren't balanced by a similar discrepancy at the stern, which I'd say this boat will have. And I take it the boat will have some internal ballast to take care of things like this.

Anyway, I'd say it's probably best to run through the weights quickly at first and then use a more refined method if it's felt necessary. (I'll leave it to Duncan's imagination as to why).

John

[Duncan Gibbs]

I was thinking about this and realised that if you use the lines plan as a grid reference you'll end up with a whole series of slightly curvy rhomboids (like the face of the Earth) which aggregated as a sub set of the larger weights calculation will (should) be pretty accurate.

I'll try all the methods and see if there's much agreement over them all, both in terms of mass and moment.

I've put the spreadsheet together but haven't managed to sit down and start the crunching.

[Ed Burnett]

Ok, interesting enough, but don't forget this is and will remain an estimate!

Timber is hardly the most consistent material anyway, so don't spend more time on this than you have to, or let a tedious process lull you into a false sense of security. This is a time for sensible and efficient estimates so you can get it done and move on to the more enjoyable stuff.

[RFNK]

TWWNS

Rick

[Duncan Gibbs]

Afla?

[RFNK]

No

Rick

[Duncan Gibbs]

Another Five Letter Acronym?

Okay! What's TWWNS mean? I'm in the dark on this one, so throw me a bone!

[Hesp]

I think it's an obscure mathematical term who's rough meaning is "the most accurate answer so far".

[RFNK]

Mr (?) Hesp has it in terms of gist, but with that mathematical reference, only gist.

Truer .......

Rick

[Duncan Gibbs]

Clunk!

I'm a bit jiggerd from the last few weeks. At least that's my excuse and I'm sticking to it!

[RFNK]

Root of all evil descending doth toll - behold!

Rick