Corbin 39 MkI

Discussion in 'Hydrodynamics and Aerodynamics' started by Rich Cope, Jun 5, 2019.

  1. Rich Cope
    Joined: Jun 2019
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    Rich Cope New Member

    To all of the boatdesign.net brain trust, thank you for allowing me to join your community.

    My wife and I are considering buying a Corbin 39 primarily because it is a cutter with reported seakindly motion, flush deck, sports a reputation as possibly the toughest hull ever built in a FRP cruising yacht, and is a comfortable fit in our budget. As such, I am attempting to complete my due diligence which includes determining if I can cost effectively develop a hydrodynamic solution to this vessel's reported weather helm tendency. I may be a little ocd about balance...

    I joined this forum because I have been fascinated and intrigued by the genius reflected in the conversations I have read here as I have been researching potential practical solutions to the weather helm that is reported to be inherent in the Corbin 39 MkI. Specifically, I am referring to vessels with 49' and 51' masts. In spite of having never sailed this vessel, I think I have a solid foundational grasp of sailplan and mast rake tuning that are most effective in remediating weather helm reported in this design.

    I am not convinced that Marius Corbin's MkII re-design of the deck/mast step, and addition of bowsprit were the most cost effective, efficient, and durable solution. Actually I am convinced of the contrary, and I am opposed to a bowsprit for all of its obvious inherent negative characteristics.

    Regarding underbody remedies to the C39 MkI weather helm tendency, I have read of one owner who extended the leading edge of his skeg about 6 ", as sketched by Steve Killing. He subsequently claims that this modification induced helm balance well enough to enable him to carry some roach at the head of the mains'l, full main up to 20 knots wind, and improve SOG in pretty much all conditions suitable for making way. In light of this revelation, and proximity of the standard skeg to the prop, I have been considering alternative and potentially simpler skeg or skeg & rudder modifications that may enhance adhesion of the flow boundary layer in both laminar and turbulent flow. My thinking being that if the skeg stays hooked up in the water column, this will effectively move the CLR aft an amount roughly equivalent to the 6" leading edge extension Killing sketched. Additionally, I am considering the potential for a remedy that proportionally pulls the stern slightly deeper in the water as speed increases to hull speed, and conversely allows the stern to rise as speed decreases. Thus, effectively moving the CLR aft proportionally in variable conditions.

    So far, I have come up with the following potential solutions:
    1) Adhere to the skeg (and rudder?) a full cord FRP constructed foil (possibly cambered) with integral foil-profile strakes set at an appropriate angle to enhance boundary layer adhesion while providing a downward force on the stern. Possibly include a downward lift foil-profile skeg shoe. If this is confusing, I am attempting to describe a multi-dimensional composite-foil design suitable to this specific hull and rig. One that I can simply adhere to an appropriately prepared skeg and or rudder substrate. Think of it as a form-fitting weather helm prophylactic that conforms to principles defined by Fibonacci and Bernoulli.
    2) Install appropriately designed tubercles along the LE of the skeg.
    3) Install appropriately designed turbulators.

    Potential solutions 2 & 3 could include the foil ideas reflected in 1.

    Obviously the details of such a design are of paramount importance to even have a chance at success. At this juncture, I define success as a helm that tends to balance with < 3 degrees correction and without inducing excessive drag. Excessive being a subjective term.

    Please, anyone who would like to shoot holes in or affirm my aspirations just for the fun of it, fire away. I am hoping to find a NA who can help develop this aspiration to fruition. However, if my aspiration is a pipe dream, I would like to learn that truth from those more knowledgeable than I.

    Thank you for reading and hopefully contributing!
     
  2. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Do you have a specific Corbin 39 in mind that you are interested in buying, or will you now be looking for one for sale?

    If the latter, you could try typing your constraints into Yachtworld, and seeing what other vessels 'come up' that match these constraints?

    Yachtworld currently has seven Corbin 39s being offered for sale, with asking prices ranging from US$ 35,000 - 79,000.

    They mention the weather helm problem in this review - Corbin 39 Review: French Canadian Mystery https://www.jordanyachts.com/2625
    And I would agree with their opinion that the easiest way to try to solve this is to simply move the sail area forward a bit.

    There are some photos of the hull below the waterline in this advertisement - 1980 Corbin 39 Aft Cockpit Cutter Sail Boat For Sale - www.yachtworld.com https://www.yachtworld.com/boats/1980/corbin-39-aft-cockpit-cutter-3223816/?refSource=enhanced%20listing
    One can see the large separate skeg and attached rudder. It sounds like a lot of work to do if you start trying to modify / extend the skeg forward and / or make it more aerodynamic.
     
  3. Rich Cope
    Joined: Jun 2019
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    Location: Texas

    Rich Cope New Member

    Thank you for replying bajansailor. The specific C39 I have in mind is a MkI with 49’ to 51’ mast. Beyond that basic all encompassing hydro- and aero- dynamic definition, I cannot be more specific. When I get back to my data collection, I will post some images that may be helpful in analyzing her underbody, in addition to the web sales image links you provided. Thank you.

    I dont see the application of this challenge as all that difficult. I admit my choice of “simple” was probably not the most accurate term but “efficient application of an elegant solution” is. I mean once the cad is done, its just molding, foam, frp, & application. Right? Or so it seems to me. But then I’m pretty good with conception & pretty weak on mathematics. Help!
     
  4. Rich Cope
    Joined: Jun 2019
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    Location: Texas

    Rich Cope New Member

    Couple of docs that may be helpful to analysis...
     

    Attached Files:

  5. petit bateau
    Joined: Nov 2012
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    petit bateau Junior Member

    A few points that may or may not assist:
    1. The Corbin 39 mk2 cutter steps the mast fwd at the point where the mk1 ketch intended to step the mast. The deck pattern is very clear in this respect, so basically they did get it wrong with the mk1 in this respect. You can see this in interior photos (the compression post), or in deck photos (the moulding layout, which was not changed in this area).
    2. A friend with the unadulterated mk1 is very clear that in high winds, if you do not reduce sail, then you will experience weather helm. However if you reduce sail then it is perfectly fine. This is consistent with reading through old posts on the corbin forums where some folk actually simply shortened the foot of the main to get their mk1's into better balance in a wider range of winds. So the solution is real cheap: shorten sail early - and by the time you do so you will be at hull speed anyway. Mind you I experience weather helm all the time in a Sigma 33 and it is a sign that we should have reefed the main by then, and nobody criticises that a Sigma 33 has excessive weather helm. Really I think this is something a particular and somewhat anally-retentive group got in their mind as being something to criticise the Corbin 39 design about, and whilst it is warranted, it is not that big a deal.
    3. I'm still mulling over in my own mind why they (Dufour / Corbin) got it wrong in this respect. I have a sneaking suspicion it is to do with how full the keel is, which of course is because it is an integral keel and not a bolted-on appendage. So I think they designed using norms developed for a bolt-on, but because it is very significantly fuller the flow attachment is different, and this in turn affects where the centre of hydrodynamic pressure is (or whatever is the correct term, which will in any case be a polar diagram). That analysis is in my future projects to do list. Remember, at the time they did not have FEA/CFD.
    4. I have a mk2, so I have both the advantage of the fwd stepped mast, and the bowsprit. Or as I put it, a poorly designed 42' yacht ! But one I was perfectly happy to buy with full knowledge, and would buy again. They are very nice yachts, and superb for their intended use.
    5. If you want to do a proper analysis of this, in FEA/CFD or similar, you will need original hull line drawings, or a hull cloudpoint scan. I have the original hull line drawings, and they are in one other place. However there are copyright restrictions which must be observed, and I will be observing them. I do not know of anyone who has done the analysis I have in mind.
    6. I suggest you contact other Corbin owners at Log into Facebook | Facebook https://www.facebook.com/groups/711494475930121/ . (including me). Quite a few are pretty thoughtful, and some are also academic. It will work for us all as a longer term meeting place for the benefit of all.
    7. My mk2 is perfectly balanced. So before getting interesting with the underbody, and if shortening early does not suit you, then you could do what many in-build mk1's did, and leave the mast stepped in the aft position, but add the bowsprit. In my mind I term this the mk1.5 version and you will see quite a few of these.
    8. Much more important is the price you pay, the condition your target is in, and the $/fun equation for your intended usage.
    Hope this helps.
    regards, David (BOCKRA, hull #123)

    [edit to make clear FEA includes CFD]
     
    Last edited: Oct 5, 2019
  6. Dolfiman
    Joined: Aug 2017
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    Dolfiman Senior Member

    The bow knuckle is very deep in the water, that does contribute to the lateral resistance.
     
  7. petit bateau
    Joined: Nov 2012
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    petit bateau Junior Member

    The designer either got the sail Centre of Effort (CoE) too far aft, or the hull Centre of lateral Resistance (CoLR) too far forward, or a combination of both. I am unsure as to why this might have caught out the designer when the Corbins were designed.

    What you are suggesting is that the Corbin's bow come down deep/early which would indeed bring the CoLR forwards. What I am suggesting is that, because the keel is fully moulded-in, and is quite broad, this may affect flow immediately in front of the keel more than is normal, effectively causing some blockage that also contributes to bringing the CoLR forwards. Or maybe they worked really hard to fair the wide forward/upper part of the keel in, and overlooked the effect this fairing-in would have on the CoLR. That is just a hypothesis as to what could have caught them out as they would not have had CFD to apply to the design back then to predict this. It is just a hypothesis of my own, with no evidence to back it up, and I admit it is low down on the list of things that they might have gotten a tad wrong. In all likelihood it was a combination of things.

    An alternative, and maybe more plausible, explanation is that back in the late 1970s when the design originated they might have expected that market would bias towards the ketches. I have not read any criticism of the ketch having weather helm. However the vast majority were then built as sloops or cutters, and maybe they didn't quite get on top of this issue fast enough and revise the mast location to be the fwd mounting (which was hitherto intended for the ketch).

    Something to keep an eye on when buying one of the mk1 layouts is that not all the bowsprits that you see around on many of the Corbins carry rigging. Some of the bowsprits only carry the anchor gear & serve as a diving platform. I am sure it goes without saying (but I will say it anyway) that a bowsprit that is going to carry the sail loads needs to be considerably more structural on a Corbin, and often will have a bracing wire down to near the waterline of the bow. All of the mk2 Corbins I have seen have this more structural bowsprit. But just because a mk1 Corbin has a bowsprit doesn't necessarily mean one can re-rig it.

    Anyway it is not that big a deal, and certainly should not put anyone off considering one of the mk1 layouts in my opinion.

    Hope this helps.
    regards, David (BOCKRA, hull #123)
     
  8. Rich Cope
    Joined: Jun 2019
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    Rich Cope New Member

    Thank you for the reply. It seems the biggest challenge right now is finding a suitable specimen of the C39. We inspected all of the aft cockpit units on the Great Lakes this summer and were pretty underwhelmed / disappointed in the nearly universal misrepresentation in online advertisements. Ie; sellers posting 20+ year old pictures. You know, where they have dark hair in the pictures and white hair today. That is a dead give-away...
     

  9. petit bateau
    Joined: Nov 2012
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    petit bateau Junior Member

    Indeed, or the flares & medallions.

    I think the solution to that is generally to reset the seller's price expectation very dramatically to account for the mid-life refit you would need to pay for. That is what I did when purchasing, and am engaged in now. Good luck.
     
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