Upgrading sailplan for an Oceanis 300 - CLR vs. CE

Discussion in 'Boat Design' started by Nachtvlinder, Oct 12, 2014.

  1. Nachtvlinder
    Joined: Oct 2014
    Posts: 5
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: Netherlands

    Nachtvlinder Junior Member

    This is my first threat here and is about upgrading the sailplan on my Beneteau Oceanis 300. I'll try to explain my ideas so far and the issues I'm struggling with.

    First some backgound.
    The Oceanis 300 is a detuned version of the First 310, the unstayed section of the fractional mast being cut-off while an in-mast furler and below-deck furler were supplied as standard. FYI these hulls, as well as the First 31.7, were all based on the Figaro 1 hull (a Finot design).

    As I don't need/want the in-mast furler and feel under canvassed below 9 kn wind, I am planning to replace the complete rig and revise the sailplan. The goal is to increase sail area, minimize drag and weight while not disturbing the balance of the boat too much. If possible a smaller, easier to handle head sail would be welcome.

    I am not starting from scratch as I have the First 310 and First 310S ("S" is the racing rig) as references, however I'm stongly thinking of having a main with extreme elliptical roach or even a squarehead because of the "auto-twist" property and increased efficiency compared to a triangular shaped main. The rig (swept spreaders) will then require top and/or fractional runners to control head sail sag and stabalise the mast.

    A performance comparison between my current boat against a First 310 and First 310S at 6 knots TWS is below (from the ORC database):

    I summarised the rig dimensions below (the first column "Aurora" is my boat). The difference in J measure is because the Oceanis was standard provided with a below-deck furler. It may be possible to relocate the head stay on the bow if needed. You see my boat shares the same fore triangle with the First 310:

    Underwater there are more interesting differences. I have the very rare "twin swing keel" option, comprehensing two hydraulically operated high-aspect foils (1.85 m draft, 1100 kg total ballast, bottom 40 cm of the foils are twice as thick).





    OK, you will get the idea by now! (how can I reduce the size of the pictures?)

    The First 310(S) was available with the following (single) keels:
    1.80 m draft, 890 kg bulbed finkeel, or
    1.30 m draft, 1050 kg bulb

    I would think my boat would at least have the same RM30 as the 1.80 finkeel version (deeper, heavier and still -sort of- bulbed)

    I'm now in the stage of iterating various sailplans while evaluating (geometric) CE of the sailplan vs. CLR (quarter-cord method).

    While this is all rule-of-thumb, having read a lot in this forum, I understand that more accurate methods are not feasible for an amateur.

    So, I could design for the same lead as the First 310(S), however my experience is that my boat is having an utterly neutral helm (sometimes a bit more pressure at the helm would be welcome) up to 25 degrees of heel.
    I do not understand this since my boat is quite beamy and flat bottomed in the rear. I would rather expect a significant disturbance of the under-water trim, shifting the CLR to windward when heeled, introducing massive weather helm. This is certainly not the case!

    Could it be that the twin swing keels and double rudders do a special job here i.e. the leeward keel wil point straight down in the water when heeled (effective draft over 2 m), while being on the leeward side of the (then asymmetric) underwater-body? Could that mean the 3D CLR would shift to leeward, just as the CE of the sailplan when heeled?

    So, am I allowed to use the same rules-of-thumb for lead by back-engineering the Firsts and try to get there or are these rules not valid for my keel configuration? Would reducing the lead be a good idea? This stuff is all too expensive to do some trial runs obviously and should be "first time right"

    I also realized that a square head main has that unique "auto-twist" feature such that CE would not be effected so much in stonger winds or gusts because the sail would twist away. Would it be allowed to assume CE a bit more forward compared to the geometric CE? How much? ;)

    In general, how would you approach this project? Do I focus on the right things? At some stage I need to start talking to a rigger, preferably one who is also a sailmaker.

    Thanks for your feedback,
    regards from Holland, Maarten
  2. philSweet
    Joined: May 2008
    Posts: 2,550
    Likes: 319, Points: 83, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Hello and welcome Nachtvlinder.

    Your description of your boat is excellent. It is not very often that we get so much to work with.

    There are a couple of more questions though. Some of your statements are giving me a bit of a confused picture of where you want to go with this project. What is your intended use for the boat - Buoy racing, offshore racing, or just fast cruising? And what sort of crew do you have - do they have an inexhaustible supply of energy and talent, or are you looking for a somewhat self-tending, well mannered rig for social sailing. How you plan on minding the rig - maniacally obsessive or "I'd rather not have to" - is going to have a lot to do with how you go about this.

    Learning all you can about this stuff is going to help the pros quite a bit, but in the end, you will need to hire professionals to get the engineering correct if you actually expect to see significant overall improvements.

    The very first thing I'd do is replace that prop with a folding racing prop. The prop shown may be responsible for as much as 1/4 of the total drag at low loads and speeds. It would be worth nearly a knot in light air on a reach or run and it will change the balance of the helm a bit, so do that first and check the helm.
  3. Nachtvlinder
    Joined: Oct 2014
    Posts: 5
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: Netherlands

    Nachtvlinder Junior Member

    Thanks for the welcome and very valid questions!

    I'll eleborate a bit. Your questions make me think as well...

    First a reaction on the intended goal/aiming for an overall improvement: above 9-11 knots of wind the boat is sailing fine and predictable. At least "it feels OK". My frustration is light-air performance. I like to still be sailing where I'm now starting the engine because I have no speed. On the other hand, this boat is a good surfer so having more power downwind when the wind picks-up seems like a lot of fun as well.

    Although I have about 10 years experience with my previous boat (I'm sailing for 20 years now), a late 70's 27' modest IOR racer ("Loper" in the polar), I only own this boat since last spring (and was aware of the low SA/D just above 16).
    Could it however be that I haven't found her handles yet? I don't know (can't imagine - not intending to be arrogant). This boat in light air is "just not going". I think you know that feeling. You can't sit still because something feels "wrong".

    I am not interested in racing this boat in any kind of competition, however when sailing I am continuously trimming, looking, feeling (and since shortly) measuring against polar. I just want to go as fast as possible and like to spend some effort in that. For me that's the fun of sailing. If possible I set the spinnaker. Anyway I would not classify myself as a sit-back sailor.

    The crew varies from solo (30%) to (lazy) family cruising (20%) to "experienced friends" (50%). Most are weekend trips on the "big lakes" we have here, however with the "experienced friends" crew we do multiple day passages/North Sea crossings as well.

    Note this polar is based on the current propeller. The polar itself does not look too bad I think, however I'm not sure how accurate that polar is, as I don't think the ORC VPP software has models for these kind of keels? At light air, I'm way-off in any case.

    One of the pictures shows the keels are completely covered with wraths of rust till 1 cm in thickness. I guess that introduces additional drag en destroy the flow around the foils? Would this be significant you think? Could it be that the lack of pressure at the helm is because the foils are not working/lifting as they should, therefore moving CLR aft?
    The plan for next winter is to get the keels sand blasted, epoxy faired and coated. I will also install a feather (Kiwi) prop.

    Agree regarding the dimensioning of the rig en designing the sails, but how would the professionals approach such a project? Do they use advanced software, ask better questions or is it just their experience to fill in the unknowns? Note that the First 310 rigging would fit as well (same mast location, schroud positions etc - underwater is different however)

    Maybe my first goal should be to try to make the polar, what would you think? This should be possible with a clean hull and polished keels right?

    This project is not scheduled for the next weeks btw! It seems like a lot of fun as well, although costly if the fun/price ratio is low.

    Thanks again,

  4. Nachtvlinder
    Joined: Oct 2014
    Posts: 5
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: Netherlands

    Nachtvlinder Junior Member

    I realize this post is well in advance of the discussion so far however it illustrates some ideas I have.

    I made some initial sketches and tried to do the lead calculations. I say tried because the lead calculations for the twin keels must be far off: even the current rig shows no lead. Seen the fact that my boat currently is more towards leeward helm than weather helm this makes no sense (unless the effect of the heavy prop (Phil) and fauled hull is that big).

    I used the quarter-chord method as described in "Principles of Yacht Design". Extended leading and trailing edge towards CWL, drew a line through 25% of all chord lengths and selected CLR at 45% down from CWL to bottom of the keel on that line. I am doubting if this is valid for the twin keels as they are swept back almost 50 degrees, which is I think unusually high. Does anyone have ideas what could be wrong? The rudders do have a larger wetted surface as normal, however the keels do as well. What should be done here?

    See below the comparison. Note as I have no official drawings to scale I had to use a sketch from the brochure which I scaled in both directions :mad:


    Compared to the fin keel version, the CLR for the low-draft keel is 16 cm forward and the twin keel CLR would be another 19 cm forward. I don't understand why Finot would design like that. He probably didn't ;)

    Below are some initial sketches:

    Regarding sail plan and planform, I know one should design on "design wind", however I expect all these rigs except A should be reefed from 15 knots on, making CE to shift forward anyway. Then there is the "autotwist" of the square head, which might delay the reefing moment a bit? Seems hard to quantify and is dependent on masttop bending moment, trim etc.

    A, B and C are existing boats. The leads for B and C are in the expected ballpart (are they?), however my current rig (A) shows no lead at all. Hmmm, this should result in a very unbalanced boat, which is not the case.

    D and E are the first options I have in mind. The masts being similar height as the standard First mast. I tweaked between mainsail and headsail area, trying to fit a square head main. The combined CE for both option D and E are very close to A, with (as expected) non-realistic values for the resulting lead.

    As I have a brand new PEX-10 laminate genoa, option D and E maintain the current fore-triangle (although the head stay is relocated to the bow). The choice would then be to

    -only use a small overlapping genoa on a furler (a few turns reefable - large wind range) (D) or
    -keep the current genoa as well for light conditions (option E).

    If you compare option D with already well performing B, what gains may I expect due to the increased efficiency? Total area is similar.

    I mentioned earlier that the goal would be a larger area but especially a more efficient sailplan. That's why I became enthousiast about the square head. The overlap and sheeting of the head sail will probably also influence the efficiency of the two sails working as one, I'm still studying on that. Is there any consensus on the optimal slot for light air?

    Ealier Phil made a sharp remark about conflicting statements I made (easy to handle while having to handle running stays at the same time). This is true, however I do not mind trimming from the cockpit. What I don't like is to have 3-4 different head sails. In theory this is nice but in reality I only use 1 or 2 (on my previous IOR boat). With that repect I like the "106% on a furler" idea: do most with the mainsail, less with the head sail.

    Pfff, a lot of things which need to settle out.

    First I would like to understand the lead issue however.
    What's going on here?
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.