A question of lead…

Discussion in 'Boat Design' started by Ja guar, Nov 14, 2014.

  1. Ja guar
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    Ja guar Junior Member

    As in balance, not ballast.

    I recently purchased a light weight fractional boat with overlapping headsails and in-line spreaders.
    I plan on converting to a sweptback rig with lappers.

    From Principles of Yacht Design (Larsson & Eliasson) the lead on a fractional fin keel sloop should be between 2-6%. This is based on using the extended keel method to find CLR and the geometric centre of the main (P&E) and basically 100% jib (I&J) to find CE.

    I realize the amount of lead is as much art as science, but the amount of lead they indicate for a fractional would, I assume be based on a boat with a 150% genoa, even though the lead is based on a 100% jib.

    If you only run a 100% jib, the CE should move forward relative to the same boat with a 150%, yet the calculated CE position would be the same in either case, using the traditional method of calculating CE.

    So, my question is how much lead will be required for the new rig set up? Because CE moves forward with the new rig, I assume lead has to be reduced from the numbers quoted in Principles of Yacht Design, but how much?

    As a side note, in an old Skenes Elements of yacht design, a lead of 14-19% is used, but while the CE calculation uses the same method, CLR is calculated by the geometric centre rather than the extended keel method (which generally places CLR further forward than geometric centre, which is why the amount of lead indictated by Skene is greater than that proposed by Larsson & Eliasson)
     
  2. PAR
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    PAR Yacht Designer/Builder

    What make model and year yacht are you working with, as we can discuss generalities all day and get nowhere.

    The boat's CE is calculated with the area of the fore triangle, not the jib type you're flying. There are also several factors that can affect the lead, including entry type, general hull form, rig type, appendages, etc. So, lets nail down what you have and go from there.
     
  3. Ja guar
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    Ja guar Junior Member

    Okay, here are the details:

    Type: Dash 34
    Designer: Laurie Davidson
    Year: 1981
    Design Displ: 5900 lbs
    LWL: 30 ft
    B: 10 ft
    I: 35.5 ft, J: 11.5 ft, P: 38.5 ft, E: 15 ft
    Draft: 6 ft
    Ballast: approx. 2300 lb

    Following are some measurements I've taken:
    BWL: 7' 10"
    Keel span: 62 "
    Root chord: 54", Root Section 64008
    Tip Chord: 18", Tip Section 64012
    Rudder span: 56"
    Root Chord: 19.5", Root Section 63010
    Tip Chord: 12", Tip Section 63010

    Note: the keel is stock, but the rudder is a more recent custom build.

    I've created a fairly representative set of lines using Delftship and have come up with the following:
    Half Entry angle: 7 degrees
    PC: .57

    A few things to note:

    The stock boat could use more helm. Several owners have gone with a large roach main to obtain more helm and several have been turboed to good effect. The stock boat is known to be underpowered for the conditions of the Pacific North West. The boat currently has a fathead main and fairly complex set of aft stays: check stays, runners, and top mast runners .

    My plan is to make the boat easier to short hand, so my goal is to ditch all the aft stays and go with a sweptback rig with lappers (moving the shrouds/chain plates aft and outboard to the rail.

    Now the boat is already underpowered, so to just sail with lappers would make a bad situation worse. The forestay is 1 ft aft of the bow stem, so to gain back lost SA, my plan is to move the forestay to the bow stem and move the mast back about 2 ft. Combined with about 25 degrees of spreader sweep, this adds about 5 ft to the foot of the lappers - to the point I could probably cut down the foot of the existing panelled genoas by only about 2 ft and the clews will be forward of the shrouds.

    If it is any help, I've attached some photos of the boat on the hard.
     

    Attached Files:

  4. PAR
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    PAR Yacht Designer/Builder

    Those are the dimensions of the tall rig, but the big question might be, is this the original rig? Does it have jumpers for a masthead kite? Runners? These where known to have a fairly flimsy rig, particularly above the jib tang, so a lot of folks used jumpers and other contrivances to hold them in column. Also, how much rake is in the stick now?
     
  5. Ja guar
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    Ja guar Junior Member

    The TM version has an I of 38 and a P of 40.5. AFAIK only one was built, and that was a custom build, with a Farr 30 carbon rig, bumped up about 18" with a pyramid mast step as I understand it (must add a fair amount of loading to the deck).

    The boat in question does not have a MH kite, although the spin sheave is about 3 ft above the jib hounds. No jumpers on this boat. Original double spreader Kiwi rig, can't recall the builder - Yacht Tech or Yacht Spar - something like that.

    With ditching the runners, I'd thought about jumpers to get forestay tension, but am now leaning to an adjustable forestay lead to a cabin top winch.

    Not sure of the rake. Not much but haven't measured it yet.

    As far as lead goes, going with lappers pulls the CE forward, and moving the forestay to the bow stem pulls it forward even more. I'm trying to offset this by pulling the mast aft. The question is how much?

    Using the method outlined in Principles of Yacht Design, I've calculated the lead on the Stock Dash (with genoa) to be about 6.1%. With the revised sail plan and the mast pulled back 2 ft, I come up with about 3.4%.

    My gut feeling is that 3.4% should be okay since the actual lead should be somewhat greater than the calculated method indicates (3.4%) perhaps 4-5%, but will be less than the stock Dash (6.1%) and less than the upper range (6%) indicated in Principles of Yacht Design and almost certainly greater than the lower range of 2%.
     
  6. philSweet
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    philSweet Senior Member

    I think you are greatly overestimating the change in lead caused by loosing the genoa. I'm sure you've sailed it without a genoa and it still worked fine. Moving the headstay is a big deal, but I think you can nearly ignore the loss of jib foot behind the mast. Back there, it is just a question of the distribution of load between the main and jib. The total load at a given transverse location will not be changed much. It will be shifted inboard to the mainsail, which has a small effect.

    Moving the headstay would probably be best offset by a longer boom and a new mainsail, which is probaby part of the plan anyway. Can this boat sail on just a reefed mainsail in a blow? I ask because my boat is pretty much hopeless without a jib, and you need to decide how important that is to you. Do you want a race boat optimized for light air and full sail, or do you want the balance optimized for survival conditions and then cut your light air sails as best you can?
     
  7. Ja guar
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    Ja guar Junior Member

    Actually, the boat has been on the hard since I bought it a couple of months ago, so I haven't had the opportunity of sailing it yet.

    No plan for a new main since it came with a fairly new fathead 3DL. As mentioned, my plan is to move the mast aft about 2 feet to hopefully slightly more than offset the effects that sailing with a lapper and moving the forestay forward will have on the location of CE.

    A person more intimate with this particular design believes the stock boats should go to a large roach main and move the mast aft about 6 in. In other words, he feels the boats need to have CE moved aft.
     
  8. waikikin
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    waikikin Senior Member

    Hi Ja guar,
    based on the above I reckon that sailing it as presented for a while would be warranted, play with the main trim, backstay etc, that's what you do on a fractional rigged boat..... then when you develop some as sailed concrete behaviours then consider chopping the boat/rig......

    Jeff.
     
    Last edited: Nov 15, 2014
  9. PAR
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    PAR Yacht Designer/Builder

    Agreed, don't do anything to the boat until you've had a chance to rake the main aft a wee bit and determine what she really needs. Most of the time, skipper complaints of weather or lee helm are sail set and rig tuning, not the need to move the stick or wires. If you do think you'll need to move the stick, you'll have a lot of structure to add to the boat, to handle the new rig hard point locations, mast compression and lateral loads, etc., so be careful what you wish for.

    Before doing any of this, but after several hours (20 or more) of sailing in various wind strengths and sea states, make a scale drawing of the rig and the hull. Find the actual centers and calculate an appropriate lead for this sail plan/appendage configuration. On that boat, you'll want a pretty generous lead, so aim at the upper end of typical recommendations, of course after comparing the current sail plan CE with the originals.

    What I do with these types of situations is, first empty the boat of everything that's not bolted to something else. Every cushion, anchors, empty every locker and storage bin, then I take it out for a sail, as if it was just delivered naked from the factory, with tanks 1/2 full. Then I start with adjustments and more evaluation sails. I mark the actual helm deflection on a ~10 knot, close hauled course and might even scale the pressure, if the helm seems excessively heavy. Noting the difference with each rig adjustment, usually "sneaks up" on a solution or at least and "avenue of pursuit".
     
  10. Ja guar
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    Ja guar Junior Member

    I agree completely with what you guys are saying. However, the situation with the boat is that it is on the hard and will likely be there for at least the next year and a half until I can find suitable moorage – such is the moorage situation here in Van. So I figured now would be a good time to do any mods.

    So out of necessity, I am pretty much flying blind with any mods, so unfortunately I have to rely on quantitative rather than qualitative design methods.

    With that in mind, I’ll go back to my original question about whether there is a lead adjustment for boats with 100% sails since I believe the published lead values are based on boats with genoas. Kind of like how Skene suggests adjusting for beamy boats or keels with sharp leading edges.

    If such an adjustment exists, some guidance on that would be much appreciated.

    I have come up with a reasonable facsimile of the hull lines and sail plan for the stock Dash as well as keel geometry and location. Using the extended keel method as outlined in PYD for locating CLR and 100% sail plan, I`ve resolved that the stock Dash has a lead of 6.1%. Applying the same methodology to the revised sail plan, I come up with a lead of 3.4%. However, I would think a 100% sail plan would have a CE further forward than a boat with the same rig dimensions but using overlapping sails. As a result, I suspect the CE is effectively further forward than calculated using the PYD method. In other words, the lead on the revised rig is probably greater than 3.4%, just wondering how much.

    According to a local sailor who is more intimate with the Dash than I am (having modified one himself, albeit differently and for different reasons) indicates the design could use more helm, i.e. CE should be moved aft. On the other hand, I don`t want to stray too far from the amount of lead LD put into the design in the first place. But I feel I`m pretty close.

    My fear though is that as B Perry pointed out, it is usually easier to adjust a boat without enough weather helm than one with too much.
     
  11. PAR
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    PAR Yacht Designer/Builder

    That's a bit like having facial reconstruction surgery, when you seemingly look fine, isn't it?

    You have to compare apples to apples. The area center for the headsail is calculated from the fore triangle, not any actual sail you have. The area center for the main is from the hoist, foot and diagonal dimension, not counting roach. The CE is calculated from these two points, again not from the actual sails. This will provide the CE originally intended for the boat, assuming mast rake is to design.

    With the real CE in hand, you can place it over the LWL and figure out what the relationship is to the CLP (ignore the rudder area for CLP calculation). It'll likely be in the 15% to 17% of the LWL range. With the square headed main, calculate the main area center using about 1/2 the roach. Now, you'll have the original and the likely further aft CE of the current rig and reasonable assessments can be made. All this only requires a sail plan and a hull profile. No 3D models are necessary, just an accurate underwater profile and sail plan.

    Given your situation, I'm sure there are hundreds of things you can do to this boat, as you wait on a mooring, aside from moving the rig. Find out what you really need, do the things you can, fix what needs to be addressed, then access the boat's abilities on the water, when this opportunity presents itself.
     
  12. Ja guar
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    Ja guar Junior Member

    But suppose you don`t look fine.

    In the case of the Dash, it has: checkstays, running backs, and top mast running backs, which I feel is too complex a rig for the single and short handed sailing I anticipate.

    For now, forget about what I said about ditching the aft stays, moving the mast and various other bits of hardware.

    Let`s just say I am going to leave everything exactly where it is. If I take the boat out for a sail, will it balance the same whether I have the jib or genoa up.

    The standard lead calculations would say yes since the CE (as you mentioned) is based on rig dimensions only - the overlap area is irrelevant.

    If this is true, then fine. I just felt the Centre of Pressure would move forward with the jib up relative to having the genoa up.

    On the other hand, if this is not true, are there any known or proposed methods of determining how far the centre of pressure moves when switching between the genoa and jib.

    And apologies if I have used the terms CE and Centre of Pressure somewhat interchangeably, the same applies to CLR and CLP.
     
  13. PAR
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    PAR Yacht Designer/Builder

    A nearly blind wife can help the first issue.

    Yes, there are ways to predict where the centers will be, but these centers are really irrelevant for the most part, assuming a reasonable lead is used. Yep, stuff changes with different sail combinations, points of sail, wind speed, etc., but you're still working within the sail plan's physical dimensions, so it doesn't matter (to the appendages) what kind of jib you've got flying (from a geometry point of view). The main has moved the CE aft a touch (not as much as you might think BTW), so moving the headstay forward to the stem may be all you need, if you've raked the mast and it's still a soft helm.

    As to designing a B&R retro fit for this boat to make it more solo sailing friendly, well this is a considerable undertaking, with everything needing to be resized and arranged, including the structural considerations within the boat, to absorb the new load paths.
     
  14. Ja guar
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    Ja guar Junior Member

    Thanks for the input PAR.

    Not technically a B&R rig since there will be no reverse diagonals. Yes a lot of structural re-work I reckon. Some have said I should be able to relocate the chainplates by adding knees, but the bottom on the main bulkhead is a bit punky anyways - so I figure better off to do it right and yank the old one out and build a new one in the revised location. While I'm at it, I'm contemplating building E-Glass chain plates for the relocated shrouds and forestay.

    Perhaps I should start a new thread because I have some questions about sweepback.
    The more you put in, the more secure the mast longitudinally and the more forestay tension you can dial in, but you lose control of things like forestay tension and mast bend (I would think). I could put in runners, but then I'm back to square one and you get more side bend since the shrouds tend to slacken.

    Is there any ideal angle for sweepback on backstayless rigs. B&R rigs seem to be around 30 degrees, but I'm wondering if 20- 25 is more appropriate.

    As for forestay tension, I'm contemplating an adjustable forestay, like a 1D35, except mechanical rather than hydraulic. They seem to have a fair amount of spreader sweep. From a visual inspection, they seem to have more than 20 but less than 30 degrees of sweep. On the other hand, one of the dangers I foresee with an adjustable forestay is the very real possibility of overtightening the shrouds. Like most issues related to boats, there is seldom a clear cut solution.

    Again, I'm wondering if I should start a new thread here.
     

  15. terrnz
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    terrnz Junior Member

    why don't you keep the mast were it is, sail the boat for a bit, then see about changing things around.
    If you want to do away with the checks, runners, sweep the spreaders 25 to 30 deg, use a running topmast for forestay tension, it won't be critical so short handing is ok.
    If the boat still has the original keel why not look at changing that.
     
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