Corbin 39 MkI

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

  1. Rich Cope
    Joined: Jun 2019
    Posts: 3
    Likes: 0, Points: 1
    Location: Texas

    Rich Cope New Member

    To all of the 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
    Posts: 181
    Likes: 13, Points: 18, Legacy Rep: 37
    Location: Barbados

    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
    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 -
    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
    Posts: 3
    Likes: 0, Points: 1
    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
    Posts: 3
    Likes: 0, Points: 1
    Location: Texas

    Rich Cope New Member

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

    Attached Files:

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.