89er

[JulianB]

Hi all, couple of sideways questions so I will answer here because your probably all thinking them.

I have been at the Basalt thingy now for about 15 years and there are basalts and there are basalts.    Super interestingly, I spoke about been warned off SMS by some people hi up in the industry and being given someone’s name, to ring which I had yet to do, well he rang me yesterday out of the blue, and we spoke for about an hour, sure, he is not a fan of SMS, but more importantly he is in the composite industry and he also agrees with most of the things I have found out about basalt.    In his job he uses it extensively and has travelled to China many times to see it being made (along with std glass manufacture.)     Some hi end military applications you can’t use Carbon.

Some of you have gone to Wikipedia and 2006 contribution suggests that the MPa of Basalt is between 2-300MPa, where as the stuff I am using is about 480MPa.    Just FYI I happen to be using material that is made in either Russia or the Ukraine.     I do have some of the Chinese stuff and I am using it also in localised areas, but you need to do your research and you need to test your ideas.

To be honest whether it’s 300 or 480 is a little irrelevant to me.

We are talking about a 1 tonne boat, with a max un-supported span of 600mm and we are using a 25mm thick laminate (Chine downwards).

I have picked a laminate weight and composition a) because it’s about right, b) because it’s highly likely to be impervious (to water) without much trouble, c) it along with P115 outer core should give me way beyond ample “idiot factor” for that inevitable smack up against the dock (or another boat) so I have picked it knowing that structurally I could well ½ve it and still be pretty safe holding shrouds in the boat, rudders on the back and Fin not overly bending.

Most Sportsboats have 6mm shrouds, BS of 6mm 1:7 “dyform” is  3550kgs, very few boats use 5mm who’s BS is 2440kgs, No one carry's 7mm (BS of ½ tonne) so you can pretty safely say that if you stress the boat to carry 3550kgs you should be OK,   If you want double that to be conservative, great.    That’s a point load, normally on a 12mm (  ½ “ pin).   If you assume that only the top ½ of the pin is taking the load (the shroud is pulling upwards) then it’s a π(R/2)² equation/loading, so assume the chainplate is 4mm thick SS plate, you have 75mm² taking 3550 kgs so every 1mm² is handling 47kgs.

Go out 1 dia your at 150mm² and load per mm² 1/2ve’s to 23kgs.

At 36mm dia your down to 15kgs/mm², 48mm Dia, your down to 12kgs and so on.

200mm diameter away from the centre of the pin your loading is down to 2kgs/mm² and a bit of toilet paper impregnated with Epoxy is more than capable of managing that load without notable elongation.

So holding the mast up and the shrouds down is not really a big ask and some very well placed Unies, double, tripled and quadrupled 150mm away from your Chainplate pin is more than capable of managing the load.    Mast step, same story, max load down = max load up on the shrouds, you can argue its doble because punching into a wave at speed you could get to BS on both shrouds, but that’s still only 7,000kgs, so 100mm away your down to 4kgs/mm².

My point is this, if you are ever going to get anywhere near max elongation of the fibres, and to do that your probably need to ½ ve even your craziest lightest laminate it’s only going to happen within say 50m and at a max 100mm from the pin.    Max elongation of Carbon is about 1.2% x 50mm = 0.6mm, if you use Basalt, and you double the % then you get 1.2mm movement.  That’s assuming absolute minimum laminate, only just strong enough to hold the shroud in the boat.

 WRT the choice of material for use in a Sportsboat, with a well supported mast structure and well enclosed spans, with adequate (yes excessive) material then the numbers for Carbon will be 1/3rd maybe and likewise for Basalt.      Add another 1kg of either material all within 200mm of the pin you won’t be even able to measure it!

Salvation is in skin thickness and reducing un-supported spans.

 

[JulianB]

Sailhmb, we keep 6 Pac in the same dry storage as Yorky's Reo-speedwagon, and where as we decided to do maybe 18 events a year, just about all of which would be judged on PHRF, Yorky, possibly because he is the Pres of ASBA has gone the other way and fully down the SMS route.   So it's fully optimised for SMS.   And good luck to him, it's not what I want, and the boat looks grossly under rigged.   In a breeze is amazing, tiny rig, lots of power, it goes up-wind about the same speed as everyone else in terms of VMG, but 10 deg lower and 2 knts faster, down wind, its doing 20+ knts, where as Tasty (the gun Super 30 boat which is a modified Hick 30)  is probably doing 12-13knts.    Yorky's big problem, and he may have just managed to solve it is to get everyone to satisfy Cat or ISO Cert, so we will all watch with interest as they actually go and do that (it's a physical actual test where they have to fully capsize the boat sails up, and get it back upright in 5 mins.

Looks like it highly possible that Don't Panic may get a sister ship, talking about borrowing the plugs, and if that happens that owner will go the opposite direction to Yorky, even bigger than what I am contemplating, because he says, he sails in predominately lighter winds.

Horse for courses.     Super 30 stretched the rule downwards to allow boats like Yorky's to compete as it's very much under length, and in a breeze, it win's, but it aint that often.

Jethrow, if I may give you a little picture.    And it is to approx scale.

View attachment 469004

White represents the S-Glass 200gm

Brown represents the Basalt 370gm

Red represents 3.5mm P115 PET outer core

Blue represents the 20mm P80 PET inner core

I would make the P115 outer core thinner if I could, optimum would be around 2.2mm, if I was a purest, but it's not feasible.

Think about it another way, your going from Water at a bit over 1000kgs/m-cu, to S-Glass/Basalt and Epoxy, possibly 1300-1500kgs/m-cu, then we are quite deliberate about the bog, the interface between the Basalt and the P115, it's probably 450kgs/m-cu, then to the P115 at 115kg/m-cu, then to the P80 at 80kgs/m-cu then to basalt/epoxy at 1300-1500kgs/m-cu.     It's very obvious where you may get sheer, and we do things like only use 75-80% of a Bar to suck it all together so we ensure that the bog remains viable and goes deep into the pores of the foam.    Also the mixture of the bog includes Aerocell/Carbosil along with Q-Cell, so it dose not all get sucked out under vacuum consolidation.

Not the first time we have done this and it won't be the last.   Also pretty common in the gliding industry to have feather laminates.

                           jB

Jethrow,  have been reminded that in 1982-3 we built the Prime mk3 18teen which had a hull weight of 45kgs (100lbs) and in that boat, I used 8mm strip-plank balsa as a core on the cockpit floor and we sucked down 1mm aircraft play where the "2" crew ran about (with a domestic vacuum cleaner).

So the balsa would have been 65-70kgs/m-cb and the aircraft play, think it was beech would have been 400+kgs/m-cb, so this is not the first time I have done this and Prime mk3 was still sailing 20 years latter, mostly as a 3 hander and the cockpit floor was still pristine. 

Sorry guys, been crazy few days, try and get you a update later today.

                    jB

 

[Jethrow]

Thanks Julian, I remember the Duracore style stuff with the balsa inside the timber as another example. Your mentioning using a higher density foam sandwiching a lower density foam got my mind thinking about another project where this might be a low cost solution to a problem...

 

[JulianB]

Duracore was developed by Arnie Duckworth, and he did exceptionally well out of it.    It was Arnie that pestered me so much that I sold off a 1/2 built PVC core Prime mkii (to Jermmy Sharp) and then went on to build the balsa strip-planked Prime that ened up on the front page of the Boston Globe, and had the first assymetric spinnaker system, back right before Australia won that cup.

And yes, that was a twin Core density system that has been responisble for many a fine boat.

 

[JulianB]

I said a bit back that the 1st thing you do and what I believe the most import part of a boats design process is a spreadsheet.   In the old days we did this all longhand, and if you designed 18teens, or Tasars or 49ers you did a lot of empirical testing.   So, you laid up a test panel.  I spoke of the Prime mkiii, and I remember well going to the local model shop, buying a few lengths of 8mm thick balsa and coming back to the factory and using a domestic vacuum cleaner to pull some skins on, then weighing it, and bashing it, to test the process.

But I lie, it’s not the first thing you do, the 1st thing you do, is what I have expressed previously, you work out what it is you want, what it is you trying and achieve and in my case that has included using things like basalt, PET, and racing under PHRF.    But (the spreadsheet) it is still the most important thing you will do!

2nd thing I do (in this world of 3D computer programs) was to take Vivace and stretch it to 8.5m, plug a rig on and look at what’s there.    This is super important from my POV, because it gives you areas and spans, and intuitively that allows you to effectively “allocate” loads.

3rd thing is generating the spreadsheet.

4th, 5th 6th 7th ---------20th thing you do is refine the spreadsheet.

So I have attached my spreadsheet for Don’t Panic.

I probably first did this in July, and last time I re-fined it was just before I posted it.

To digress momentarily, I have had people say I am crazy for doing this, why would you give your competitors access to this information, and there are 2 reasons,


[SIZE=11pt]1st , my dad said, best place to hide something is in plain sight, and the 2nd, Paul Elvstrøm who I had the very great privilege of meeting on a few occasions, “you have not won unless the guy you beat believes you have won.”    Most classes are riddled with people fiddling and not necessarily being up-front about what they are doing.    And a few classes are the antithesis of that.    I would like to think the 49er is one of those, (that don’t/can’t hide) surprisingly so is the 5o5, and I really like it.     I hate “black boxes”!   [/SIZE][SIZE=11pt]So 6 Pac’s SMS I posted on the web site some months back with GT’s blessing, and I have no issue being very public with this.[/SIZE][SIZE=11pt][/SIZE]


OK, to explain some of the spreadsheet,

Vivace had a 230kgs bulb and that was to satisfy Cat 5.    Pretty obviously the single biggest “opposing” force to the bulb is the rig.    Vivace started life with a 90mmID, approx. 2.6mm WT so 95.2mm OD carbon tube.    In Vivace’s case that tube weighed approx. 24kgs, and that’s very much in-line with what 6 Pac’s mast weighs, it was 12.3m high (off the deck).

So part of the refining process has been to look at masts, and yes I will more than likely use a 90mmID carbon tune, 2.6mm WT, 95.2mm OD but what is really surprising is that this mast now weighs 1.2kgs/m below the hound (taper) and 1.1kgs/m above the hound.   Now this new mast also has high stiffness cof, and because I plan to use a well-proportioned square head main I don’t need the length, so I am likely to come down to 11m off the deck.   I don’t have to decide this just yet, so I am not other than to be in the ballpark to do the maths.    8 x 1.2 + 3 x 1.1 = 12.9kgs.   Almost ½ ve.

Then you can see the maths WRT rigging, this boat will have all synthetic rigging, so saving 7-8kgs in that.   You can see the weights of the spreaders, they have ½ ved also, and this was true when we switched to Carbon spreaders on the 49er also, all that weight gone in the rig mean that we don’t need anywhere the weight in the blub.    Hence ringing up Chris Mitchel and buying a pre loved 140kgs SKUD bulb off him.   (plus it’s real cheap, and finished, (I’m lazy)).

When I did the initial displacement of Vivace in it’s stretched form, it came in at 1200+kgs.

So I needed to lose ¼ of a ton displacement.

This is where the magic comes in, this is where you incorporate all those things which you have learnt in the last 20 years to take a stretched Vivace to a refined Don’t Panic.

And no, I’m not going to tell you what that magic is, it may yet be a crushing failure, time will tell.

I know I am going to be asked, why target a weight less that what you expect to carry!

I learned this back in 18teen days, Prime mkiii (which was designed as a 2 hander) sailed extraordinarily well as a 3 hander also and became the route design for all the B18 series.

Took dad and me a bit to work it out, but at zero knts, if you weigh a tone, you will displace 1 tone.

As you start to move, some of your displacement will be carried dynamically, call it quasi planning, and at 5-6 knts (a 18teens hull speed [HS]) you still want full waterline length, and you want to be sailing on your marks.      If you initially design the boat to displace 1 tone at rest, then at HS your ends would be out, and you have lost full water line length, right when it’s most critical.

So we initially design our 18teens to be 19ft long and chop the back 1ft off.   The AAMI’s and the Nokia’s you see in the mid 90’s, where all designed to be about 22ft long, and we chopped the back 4 ft off.      It was interesting watching the max chine beam march back ad back and back.

Dad being dad, work all this out in ft/lbs and CoL’s of the hull, and we now have a formula, and we discount the displacement so as the boat will be optimum LOA at HS. 

Finally, I said “4th, 5th 6th 7th ---------20th thing you do is refine the spreadsheet.” Each time we do a part, we weigh it, and then go back and see if it agrees with what we calculated, In column N you will see some bold measurements, these are actual’s these are just those bits that we have made and gone back and seen it they agree with what we expected.    Basically, we are on target.

~~~~~~~~~~~~~~~~~~~

We did the initial laid up of the Cockpit on Tuesday, today we prepped to do the 2nd lamination that will happen tomorrow.    Monday or Tuesday that whole plug, with laminate on will go outside and be wrapped in a tarp.   It was 31c today, so we know inside the tarp it will get to 60-70c, maybe even as high as 80c.    Near perfect post-curing temp and very much at the right price both in terms of $$$ and cost to the environment!      Probably about Thursday, we will weigh the cockpit, refine column N again  and see where that sit with our calculations.

That’s the end of the little bits, hull plug gets starts Thursday also.

Hopefully rotate the hull early Dec, be out of there Mid Jan (I must spend most of Feb in Europe)

               jB

View attachment Weight Oct 2021.xlsx

 

[Gone Ballistic]

Julian, 

Thanks very much for sharing all this info in such detail. It's going to be a very educational thread. 

You referred the 'the magic' of refining Vivace to Don't Panic, I guess this is the bit that you'd really be crazy for giving away!? Are you willing to answer if 'the magic' is specifically with respect to design details or is it global hull shape too. You describe this time as stretching Is it ever a blank sheet of paper effort? 

Cheers

Andrew 

 

[Kenny Dumas]

Thanks Julian, my nephew is studying composites in college and your empirical approach is instructive. 
Can you elaborate on the interface adhesion between foam densities (how are they bonded) and the bog used for the skin attach process?  Our local big boat shop, Schooner Creek, punches reinforcing threads through the laminate before skinning but maybe it’s not an issue with smaller spans or better materials / processes?  I’ve found that the cheap spray adhesive (Elmer’s) roughens foam enough to get good adhesion (after having some skin blister in the hot sun) instead of sanding. 

 

[Sidecar]

Now this new mast also has high stiffness cof, and because I plan to use a well-proportioned square head main I don’t need the length, so I am likely to come down to 11m off the deck. 

Presumably single spreader rig? How much SA are you planning to hang off the mast?

 

[JulianB]

Gone Ballistic, pretty hard to think of a situation where you don’t draw on previous experiences in designing something for the future.    In the case of Don’t Panic, a lot will come from the 49er/29er lineage as well as Vivace, the SKUD and looking around the fleet.

And Martin (Billoch) is keeping me in check~

I think it was Van Goh who said something like “big thing come from a series of little things done well”.

Maybe when we did the speed boat for Mahindra (India) or when we do the new PE/RM coach boats for PlusOne (China) we where very much blank sheet of paper.   The single hander we have just done is very much a bank sheet of paper approach, so that will be interesting.

But Don’t Panic is about 5 (really 4 and one young guy) old farts wanting to go sailing and see this fabulous country and drinking way too much Van Rouge!

 Kenny Dumas,  the late great Dave Ovington always thickened his initial coat of epoxy with 10% of Aerocell, because it stuck to the Polyester gelcoat better.     If you go to the experts they will tell you that you can never get Epoxy to stick to cured Polyester, I think 3,000 49ers later, those experts look a little stupid (and about 100 18teens)

He and I, and later Chris Turner, who brought Ovigy’s, have done a lot of tweaks, and played around a lot with different formulations, so absolutely, don’t trust your supplier because he is only reading the crap he is given and probably never got his hands dirty.

Theory has to equal fact!    How often are we told Fact must = Theory and normally by people who have never actually tried.     For me empirical testing is everything!

Sticking to PVC, and then there was this stuff “corecell” and now PET has required playing around to get additives in the right ratio, and not all epoxies are the same either, by any stroke of the imagination (+ Polyesters are a lot thinner so require a lot more additive.

We are presently putting about 4 expresso cups of AeroCell with 4 expressor cup of Q-Cell into 800gm of mixed epoxy and that seems to stay put, bind to the PET and also act as a good interface.

SideCar really don’t like the tree trunk masts and the single spreader rig, that a bit too much of the sailmaker and mast maker, say, hell you dumb-shit, you know no better, so this will do.

Now I may be a dumb-shit, but I like “well proportioned square heads, because empirically Dad and Simon Watin worked out how they work”, and I like to be able to move cambers and placements around, so I will have a double spreader rig, not to dissimilar to a 49er rig, Cap shrouds running to the deck and adjustable, along with primary shrouds, D2’s and D1’s.    Jury is out on a ram vang yet, it’s biggest advantage is it give the fwd-hand un-fettered access to get the spin down, and it lifts the D1’s and D2’s so you end up with better control of the mast.

But the plan is to use a 90mm ID base tube because it’s pretty resilient, at 2.6mm WT pretty idiot proof (remember 5 old guys, we don’t want to have to put off drinking and story telling time while we re-rig a mast) and to be honest the weight differential is only just over 0.1kg/m, so we may loose 1kg in total, (that’s a 80mm ID tube with 3.2mm WT), I think we are looking at a 12m² jib and a 29m² main.    Can’t carry a A1, the boat will be too fast for that in every thing above 5knts of WS, so probably a A1 ½ and a rope luff flatty as a #2.

But I will get to that in about a month, that whole decision making process is in-fact quite numbers driven and whether you chose to be a 95% boat or a 110% boat.

 

[WCB]

Julian, love the thread so far but do you have any pics? I'm a huge fan of the building process. 

Your note about the sharing of information is so important. My father and his co-owner and crew used to tell everybody how they won their races. It builds the fleet and people appreciate the open discussions. It's also no guarantee that your competitors will know what to do with the information anyways so you look better simply for offering it. I have been and will be again a part of the 505 class and their post race debriefs are great. We also hold chalk talks after racing in our weekly Laser/ILCA series simply to bring the entire fleet up so that everybody is having fun.

 

[eliboat]

Yes

 

[JulianB]

Pictures,







3D renders of what we are building, 1/2 model pretty obviously, this is probably the 3rd or 4th refinment.

You can see the reduction that happened when I took 1/4 tone of displacment away, the dark blue is the breast hook pre reduction.

Red & grey are the areas where we will have 3.5 + 20mm so approx 25mm skin thickness, Green and everywhere else it will be 3.5 + 10mm so approx 15mm skin thickness.

And yes we are toying with DSS, Martin's partner, in Benous Aries is Joaco Zerbo was involved in the Prada program and specializes in foils, so hopefully I will get to get some perals of wisdom from there also.

BTW, the boats is 3.5m wide, so we will do what Vivace and just about all the 79ers, and S8's did and effectively run the boat though a bandsaw, 1.2m off the CL and then bolt on "winglets".   So trailer towing width will be 2.4m (2.5m is AUST limit) while sailing width will be 3.5m.

Actual production photo's, most are on Alex's phone (my apprentice/son) and he is 4hrs nth of Syd with his brother and cousins right now, fishing on some river and swagging it out at nights but.



Foredeck mould starting, mould is 2600mm wide with 100mm flangs, you can also see the probably 30 year old vacuum pump directly behind the mould which so far has been faultless.



Alex plating the mould, the one thing differently I would do next time is plate with 4.7 or 6mm MDF, (this is 3mm) but this was a 1 shot mould when we built it, looks like it will now be a 2 shot mould, so we would have been more liberal with glue (PVA) also, Cockpit, we used a lot more glue.



Cockpit mould taking shape.



Alex gelcoating the mould so we get a vacuum level of airtightness.   We initially glass the MDF using CSM and cloth (Polyester), then gelcoat, and that has been very effective.



Partially inspired by Martin, this is him (standing) sailing his new creation on the River Plate in BA, it's IRC designed and he is doing very well with it.

 

[sailhmb]

no rake angle on the keel?  why not?

 

[JulianB]

Why??

 

[WCB]

Pictures,

View attachment 470486

View attachment 470487

View attachment 470488

3D renders of what we are building, 1/2 model pretty obviously, this is probably the 3rd or 4th refinment.

You can see the reduction that happened when I took 1/4 tone of displacment away, the dark blue is the breast hook pre reduction.

Red & grey are the areas where we will have 3.5 + 20mm so approx 25mm skin thickness, Green and everywhere else it will be 3.5 + 10mm so approx 15mm skin thickness.

And yes we are toying with DSS, Martin's partner, in Benous Aries is Joaco Zerbo was involved in the Prada program and specializes in foils, so hopefully I will get to get some perals of wisdom from there also.

BTW, the boats is 3.5m wide, so we will do what Vivace and just about all the 79ers, and S8's did and effectively run the boat though a bandsaw, 1.2m off the CL and then bolt on "winglets".   So trailer towing width will be 2.4m (2.5m is AUST limit) while sailing width will be 3.5m.

Actual production photo's, most are on Alex's phone (my apprentice/son) and he is 4hrs nth of Syd with his brother and cousins right now, fishing on some river and swagging it out at nights but.

View attachment 470491

Foredeck mould starting, mould is 2600mm wide with 100mm flangs, you can also see the probably 30 year old vacuum pump directly behind the mould which so far has been faultless.

View attachment 470492

Alex plating the mould, the one thing differently I would do next time is plate with 4.7 or 6mm MDF, (this is 3mm) but this was a 1 shot mould when we built it, looks like it will now be a 2 shot mould, so we would have been more liberal with glue (PVA) also, Cockpit, we used a lot more glue.

View attachment 470493

Cockpit mould taking shape.

View attachment 470494

Alex gelcoating the mould so we get a vacuum level of airtightness.   We initially glass the MDF using CSM and cloth (Polyester), then gelcoat, and that has been very effective.

View attachment 470495

Partially inspired by Martin, this is him (standing) sailing his new creation on the River Plate in BA, it's IRC designed and he is doing very well with it.

Thank you so much for posting those pics...what a treat

 

[JulianB]

no rake angle on the keel?  why not?

Why??

Sorry, a bit abrupt,

The fin dose 2 things, most importantly it is only one of 2 things driving the boat fwd, it and the sails (with a tad of input from the rudder if the boat is well balanced) are the only 2 things that make the boat go, so it should be 40-50% at least the focus of your attention.

The other thing it does is it's a structural beam, and by that, it needs to resist side force, and if there is a bit of lead hanging off the bottom, it needs to resist a canterliver force that the lead, and resulting crew members (in the event of a capsize) will exert.

The camber (read section) of the foil is pretty well defined, due to expected HS and therefore RN [Reynolds Number].

Area can be very well defined due to RM [Righting Moment], it is surprisingly small, and you can do empirical numbers based on similar boat doing similar speeds.

AR [Aspect Ratio] so the ratio of span (length) over cord (width) really does not factor into it, because you are going to effectively endplate the keel at the hull and also if you are using a "especially" T bulb then you will very effectively end plate there also.    When you end plate "well" you double AR, so you have a 2 x endplate so you have a 2 x 2 increase in AR so a say 1m span x 500mm cord = an effective 4m span over 500mm = 8:1 which is huge, think a glider wing.

But the other reason you make the fin longer is to get more RM from the bulb, but this boat is likely to be sailed flat (+/-10°) so on say 1m span you gain 173mm movement, 2m you get 340mm movement, far far better to get your crew to hick just that bit further out and you will get far more benefit from the shift in CoB of the hull.

As to Dihedral or Anhedral (and yes for-n-aft rake is also the same term) you can make a far greater argument from raking your LE fwd, “anhedral” than aft “dihedral”.    The major reason to rake it aft is to get weed to come off with greater ease, but if you actually do that experiment, and you only need look at an Optimist rudder to get your answer, there is a sweet spot around 27-30° were weed starts to move and your way outside the +/-7° minimal effect rake.

I have just about always put my fins in with the TE (Trailing edge) perpendicular to the keel-line aft, the rational being that as the nose lift once you start planning the LE dose not go past vertical, so it dose not go critical.  The reality is, its 3-4°aft.

But the Moths have put paid top all that, all their foils are raked 5-10° fwd to stop ventilation, so what do I know.  

Maybe, we make it vertical!

Get into the maths surrounding fin’s in a bit!   Need to position the fin, in the next 10 days, as we make the hull plug, and that could start as early as Thursday!

 

[sailhmb]

Sorry, a bit abrupt,

The fin dose 2 things, most importantly it is only one of 2 things driving the boat fwd, it and the sails (with a tad of input from the rudder if the boat is well balanced) are the only 2 things that make the boat go, so it should be 40-50% at least the focus of your attention.

The other thing it does is it's a structural beam, and by that, it needs to resist side force, and if there is a bit of lead hanging off the bottom, it needs to resist a canterliver force that the lead, and resulting crew members (in the event of a capsize) will exert.

The camber (read section) of the foil is pretty well defined, due to expected HS and therefore RN [Reynolds Number].

Area can be very well defined due to RM [Righting Moment], it is surprisingly small, and you can do empirical numbers based on similar boat doing similar speeds.

AR [Aspect Ratio] so the ratio of span (length) over cord (width) really does not factor into it, because you are going to effectively endplate the keel at the hull and also if you are using a "especially" T bulb then you will very effectively end plate there also.    When you end plate "well" you double AR, so you have a 2 x endplate so you have a 2 x 2 increase in AR so a say 1m span x 500mm cord = an effective 4m span over 500mm = 8:1 which is huge, think a glider wing.

But the other reason you make the fin longer is to get more RM from the bulb, but this boat is likely to be sailed flat (+/-10°) so on say 1m span you gain 173mm movement, 2m you get 340mm movement, far far better to get your crew to hick just that bit further out and you will get far more benefit from the shift in CoB of the hull.

As to Dihedral or Anhedral (and yes for-n-aft rake is also the same term) you can make a far greater argument from raking your LE fwd, “anhedral” than aft “dihedral”.    The major reason to rake it aft is to get weed to come off with greater ease, but if you actually do that experiment, and you only need look at an Optimist rudder to get your answer, there is a sweet spot around 27-30° were weed starts to move and your way outside the +/-7° minimal effect rake.

I have just about always put my fins in with the TE (Trailing edge) perpendicular to the keel-line aft, the rational being that as the nose lift once you start planning the LE dose not go past vertical, so it dose not go critical.  The reality is, its 3-4°aft.

But the Moths have put paid top all that, all their foils are raked 5-10° fwd to stop ventilation, so what do I know.  

Maybe, we make it vertical!

Get into the maths surrounding fin’s in a bit!   Need to position the fin, in the next 10 days, as we make the hull plug, and that could start as early as Thursday!

I should have added to my original post that conventional wisdom around here prefers 6 degrees of dihedral to reduce induced drag.  I have never seen the data supporting this.  Your response was thought provoking.

 

[JulianB]

At the end of the day, that's roughly where I have my fin, but I can't see why that would reduce induced drag.

Love to hear the thinking on that, I get that you already commented you have not seen the data, but it would be interesting to see why someone thinks that?    Normally such things have a element of substance behind them, ofcourse Moths have anhedral which is therefore counter intuative even to me, and as I said before, what do I know, I have always just put my TE perpendicular to the keel line aft and worked around that.

 

[neuronz]

I should have added to my original post that conventional wisdom around here prefers 6 degrees of dihedral to reduce induced drag.  I have never seen the data supporting this.  Your response was thought provoking.

Do you actually mean sweep? Dihedral/anhedral would be a canting keel in my opinion.

Anyway, conventional wisdom over here says straight wings have the lowest drag as sweep promotes spanwise flow which increases induced drag. There are some arguments for a forward swept fin promoting spanwise flow in the opposite direction and thus reducing induced resistance, but I do not think the concept ever got traction.

 

[Sidecar]

^^^^ This.

Forward sweep has been tried many times on aircraft because theoretically it is more efficient for a given wing area, you are effectively preventing span wise flow because of the large fuselage/hull end plate. It also theoretically makes the wing and whatever it is attached to more manoeuvrable, but it tends to be more unstable and subject to structural flutter. Anhedral wings can have a similar effect.

And as previously mentioned, Moths have some forward sweep to prevent/minimise ventilation down the leading edge.

Sweep at large angles, can also help with longitudinal stability.