Looks good. The lack of any big leaps in the numbers suggests empirically that balance should be good: discontinuities generally indicate a problem.
Yes, remember that displacement is effectively a volume which is translated into a weight assuming a given density of water. What the IOM rile measures is actual weight of the whole boat.
Build, enjoy and let everyone know how it goes.
I’m working on the shadows to build the hull. I’m taking into consideration the width of the planking and glasing. I’m also going to include some internal reenforcement pieces for the hull and deck. I’m going to be using 20 shadows spread every 5cm and then there should be about 8 shadows for the internal extructure.
I’m going to have them laser cut for more accuracy. I the hulls I have built in the past I cut the shadows and I can see that there could be a margin of error. Does anyone know a good place that can do the laser cutting? I know of a couple of places, but if anyone can recomend one it will be great
I’ll post pdf’s of everything so if anyone wants to join me on build it. Probably the first one I build will be to get all the pieces balanced. (rig, fin, bulb…). I want to make the design avalable for anyone so if any one else wants to make any comments to improve the hull shade or performance, I’ll be glad to listen and learn.
One more thing. Does any one has the data for a Rig A. What I mean by that is the center of force of the rig. This will help me in designing the deck. I undertand that that number will be and average and it changes with every boat. But I can give me an Idead of were to place the mast and build the deck around that.
Daniel
About three years ago, when I designed my IOM, I analyzed all of the designs I could find in the public domain, together with some boats I had access to, to compare the mast positions relative to the Center of Lateral Resistance. For this purpose I assumed that the Center of Lateral resistance was determined by the keel shape and position alone when upright – in other words I ignored the underwater hull profile, the bulb and the rudder, and looked only at the lateral center of area of the keel fin as a proxy. This of course was a gross simplification, but it seemed to me that:
…I could therefore use the keel shape and position alone for my calculations and be near enough. It seems that there were too many dynamics while the boat was sailing for there ever to be a perfect answer – so I figured that trying to be any more scientific than this was not going to yield a result that would be any better in practice.
Having done my analysis, I then took out the two boats at the top and bottom extreme, and considered what was left.
This is what I found, based on the keel I use on my boat (parallel, 85mm chord) and with corrections made for boats that were using different keel profiles:
The average from the center of the mast to the lateral center of area of the keel fin was 66mm. I then used this to determine the distance between the leading edge of my 85mm parallel keel and the center of the mast.
I them looked at the range both side of the average, and discovered that (again taking out the top and bottom extreme) no boat in my population varied from the average by more than 11.5mm.
Because I have so little faith in my own abilities, I decided to allow a larger range for stepping the mast - just in case. I built the boat with an allowance of 10mm either side of my target mast step – so the mast can be stepped fore and aft through a total range of 20mm. This means that the hole at the top of the mast/keel box through which I step my keel-stepped mast is somewhat larger than it needs to be, and is a receptacle for water. I tape it up for sailing.
It also means that I needed to have a wider range of attachment points for the jib boom swivel and side stays than otherwise necessary – so I use racks. This adds weight - but hey, the boat was and is intended to be a prototype and learning exercise for the design that will come after anyway. I’ve always worked on the theory that it will take me at least three iterations of design and build before I get close to a boat that behaves and is competitive (skipper’s skills aside!).
As for position of the mast/keel fore and aft – there are of course significant variations within IOMs. I took the view that my keel position would be driven by other centers, as I wanted to keep these as close together as possible. Like you, I went to considerable effort in analyzing the differences between my various centers and graphed the movement of these centers at heel increments of 5 degrees all the way up to 45 degrees.
At the end of the day, I designed my keel placement such that the center of gravity of the bulb would line up (in my case) very slightly behind the center of lateral resistance (when upright) of the hull excluding keel fin. The keel and rig placement was then driven by that. My boat is a little fuller forward than many (a deliberate design decision on my part) so my rig is also further forward than many – but certainly not at an extreme. Dan’s Vanquish design for example, has the rig quite a bit further forward than mine. The Ares design by Darren Searle, now being built by Larry Ludwig in Texas, is another example of a boat with the rig placed further forward than many.
When floated, and with every thing in the right place, I ended up with my keel bulb slightly further forward again (just talking a couple of millimeters). It is important to attach the keel bulb as one of the last parts of the build exercise. Steve Landeau has an excellent “how to” article on this subject in the USA IOM forum. But you’ve built boats before, so you are probably very experienced with that part of the exercise.
Did I get it right? Can’t say – except that the range of movement available to me for stepping and raking the mast covers all requirements. If I were to build a second boat to the original design now, I would know where the optimum positions are. The trouble is, I will not be building another boat to this design, but some time next year may build a hull which is a development of this design with lessons integrated. However before the next design hits the building board I will make the effort to take the current boat to a few regattas next year (we don’t have any IOM racing in CO.) Even if I just end up coming last - the lessons learned will be invaluable.
Congratulations on commiting to the build. The great majority of people think people like us are nuts - they’d rather spend the time sailing than designing and building. That’s fair enough - but for those of us that go through the process, we understand everything little thing about our boats. It’s a great feeling when you see you own work living and breathing on the water.
Yeah - I probably am nuts.
Muzza, Thank for the reply. I love sailing my boats but I thing that I enjoying even more the process and the idea of sailing my own design. I’m not expert at this but it’s a great process.
I have being playing with the software and moving the bulb, fin and rudder around to try to find the best spot. At least it will give me an idea. The number look good aslo with the other parts. I wanted to place the sail to get some more numbers.
On your comment about having a track for the mast. I thought about that and also adding tracks for the jib and spreader stays. I have being tryng to find places that sell spart aluminum part that I can use for a track. Any Ideas? I have bein able to find some but they have bein too large. In the past I have fabricated track with T or L aluminum 1/4" angles.
Use the Bantock tracks from Great Basin.
Cheers,
Earl
Or might be cheaper to import them from Bantock - SailsEtc. www.sailsetc.com
Lester Gilbert’s IOM has adjustable absolutely everything. See his website for ideas
Angus
I was just reading a thread on the German RC Sailing Forum here…
http://www.rc-network.de/forum/showthread.php?t=56535
Of course it is in German - but someone has helpfully set out the math for the water displaced by an IOM bulb, 212 grams. Thus my allowance of 200gr is a little light (which probably explains wht mmy boat float tested a little lower than expected) I should have done the math eh?
So allow somewhat more for the keel fin and rudder and design the hull alone to displace around 3,700 grams.
Here Are The Shadows! I Deducted For The Use Of 1/8" Planking, Sanding Down, And Then Glassing. The Final
Thinkness Of The Planking Plus Glasing Should Be Just Over 1/8". I Alson Included The Original Drawings.
Pictures before planking. I used 20 sections and laser cut 1/16 plywood. My next steps are to plack using 1/8x1/4 balsa.
I know the feeling - a mixture of delight and terror!
I’m just waiting to get the balsa and then is planking time. I have one question on laying the fiberglass after I’m done planking. What I’ll be doing is using the planking as a plug. What is the best product to apply to the wood so I can pull the fiberglass shell off easily.
I’ll be using one layer for FG S (about 5.5oz) and on top of that FG S (about 1.5 oz) After I pull the the shell I’ll use some Carbon fiber to reinforce the keel area and one long (about 2" wide) strip to go along the whole length.
For the decking I’ll use 1/32 wing cover and then one layer of 1.5oz or less FG.
If any one can comment on this and let me know if I need to add or take away waight from the FG.
Thanks you
Daniel
Fiberglass your plug with a medium weight layer of fiberglass, using polyester resin. Then sand smooth and use paste wax, and PVA (or equivalent) as release agents. When you mold the hull, use epoxy. Polyester and epoxy resins don’t bond to each other very well, so this gives you a little extra insurance, incase you do a bad job with the release agents.
Dansherman,
This will create about 1mm or more that I did not took into consideration. would this be a problem or is the increase in thickness not enough to make any difference? I us the to create a mold. llike illustrated in this link http://navi.modelisme.com/article223.html.
daniel
Hi guys
I have been reading through the various discussions on hull balance and various design theories and with all due respect to every one contributing you all seem to have ignored the work of John Lewis. John is recognised in the UK as the major influence on model boat design particularly for the larger free sailing classes such as the A. Books of his designs are available and his collaberation with Dick Priest produced the the architypal design manual in the pre computer era. John is now in his mid eighties and was actively designing boats up until a couple of years ago having graduated to MaxSurf on a Mac about 10 years ago. Who says you can’t teach old dogs new tricks. Johns’ boats all were hydrodynamically balanced ie there was liitle shift of LCB from static to heeled. This is essential for free sailing boats where the sailing behaviour has got to be predictable, otherwise you are a loser. This facility is still required today as free sailing is not dead, there is a very active and extermely competitive freesailing racing scene in the UK where A’s M’s and 36’s are the major classes. Can I suggest that you all take a look at Johns work and learn from a man that taught the vast majority in the UK how to design by providing the bench marks against which all else are judged. Johns influence as a designer may have wained of late but in his era there was no finer.
Bill
I’m a little confused,
do you have a female mold already made?
I would have to create the female mold from the planked hull.
OK,
Sand your plug a little bit more if you can, and then apply one layer of the 5.5oz cloth you have and use epoxy resin, if you have graphite powder add it to the epoxy. Take your time and use a process similar the one I outlined here
http://www.rcsailing.net/forum1/showpost.php?p=20775&postcount=25
Attached is an image of a plug I made about 8 years back using this process.
The thickness of the resin and fiberglass is only about .5mm so if you sand the hull a little you should be good to go. Even if you don’t sand the hull, the change in displacement is not significant.
When you make the female mold, use polyester resin.
Re: John Lewis
Although I may not have mentioned him specifically in the thread, I have learned much from his designs and commentaries. I have all of his books and had the great pleasure of meeting him in person during the UK/USA Challenge Cup regatta for free-sailing 36R boats and discussing the strengths and weaknesses (mostly the latter :-)) of my boat. His work is indeed worth careful study by anyone with an interest in design.
Cheers,
Earl
Bill G, I’m most interested…any specific book titles you would recommend?
Bill H