Changing a boat's characteristics

Discussion in 'Stability' started by Strudders, Sep 13, 2010.

  1. Strudders
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    Strudders New Member

    My wife and I sail a Legend 29.5 mainly inshore and a several times a year longer passages say to France or the Channel Islands - we are based in Portsmouth, England. We sail her all year round and often in windy conditions quite happily when others shy away.

    The boat has two characteristics that we don't like. She rounds up in gusts and can be very bouncy / bumpy - on these occasions my wife starts to feel unwell - this doesn't affect me. I am a very experienced helmsman and have raced dinghies and yachts, so I often work hard on the traveller and mainsheet to reduce the rounding up - but I can't stop the bounciness. And the autohelm simply can't cope in certain sea states and I have to manually steer or de-power the rig which slows our cruising speed.

    We love our boat, and like a lot about her. Logic says that we should simply change boat design and by a more traditional cruiser - and we have ridden aboard Island Packets, Najads and HRs with friends that give us more the ride we would prefer, but it's a huge step up in price for us, and I just wondered if one could change the boat by perhaps changing the keel, adding more weight inside for example, that might achieve our aims.

    We appreciate that this may cost several thousands of pounds but we are talking a gap of many tens of thousands of pounds to step up to a different boat - and actually we have invested a lot in our boat on electronics and heating etc.

    I hand it over to the forum for some ideas - thanks
     
  2. Petros
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    Petros Senior Member

    It seems to me a redesign of the keel will help both issues. Though that is a pretty drastic (and costly) change.

    Two things that you can try that would not cost much is to move the center of effort of the sail forward relative to the hull. A simple test would be to adjust the rigging to remove any aft rake in the mast, and perhaps even rake the mast slightly forward. Larger rudder might also be called for here as well.

    The other would be to add weight below the floorboards down low in the bilge, make sure the weight is secured. This is not an uncommon "fix" for production sail boats. Lead or iron blocks bolted down would be preferred. You do not want lead bricks flying around the cabin in rough conditions.

    You also might talk to an experienced sail designer about the problem, sometimes they can resew your sails to improve control. Or the next time you order new sails have them alter the design of the sails as appropriate.

    have you talked to other Legend owners about these issues? Some may have already found inexpensive fixes that work.

    Good luck.
     
  3. Perm Stress
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    Perm Stress Senior Member

    Rounding up and bouncy ride are typical for any flat-bottom, wide-stern sailboats.

    Bouncy ride could only be changed, when hull is changed (i.e. different boat of different design).

    For rounding up, normal sequence is as follows:

    1 Boat heel so much, that top of rudder is lifted out of the water; for boat with wide, shallow, flattish stern this could be as little as ~20 degrees in flat water; in waves this could momentarily happen with less heel, while top of the keel is usually still reliably covered with thick layer of water.
    2 Rudder force is about halved because of loss of end-plate effect of hull and suction side ventilation. (What this mean: dramatic change of balance - center of effort of underwater forces jump far forward, boat want to luff, to exaggerate luffing tendency due to heel. You could see this change of balance in the side view drawing of your boat by simply covering half of the rudder with paper.)
    3 With now changed balance, boat turn in to the wind, is heeled more, rudder comes out of the water entirely...

    To avoid all of this, twin rudders were invented.

    With twin rudders, the above sequence is changed:

    1 Boat heel so much, same ~20 degrees, that top of windward rudder is lifted (partly?) out of the water, while lee rudder is depressed deeper. BOTH top of the keel and top of lee rudder are still reliably covered with thick layer of water.
    2 Unlike single rudder boat, underwater center of effort and balance is not changed much; lee rudder is still fully effective both for balance and for steering.
    3 If really strong gust heel the boat further, than top of the keel is uncovered BEFORE the top of lee rudder; now force of keel is almost halved due for loss of end-plate effect of hull and suction side ventilation, while lee rudder is still more or less covered and end-plated. Underwater center of effort is shifted aft. Balance of boat is changed so that now it want to bear away, to counteract the luffing tendency due to heel. (With rudder still effective for steering.)

    To sum up: while the same boat with single rudder will lose her steerage way at about 25-30 degrees of heel in flat water, twin rudder boat will remain under control to about 45-60 degrees

    There is drawback: with twin rudders and single propeller, maneuverability at slow speeds will suffer. I am also told that with twin rudders, tack happen somewhat slower.

    Installation of twin rudders is not a cheap proposition, but still a lot cheaper than Halberg Rassy or Najad.

    Regards.
     
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  4. Strudders
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    Strudders New Member

    Perm Stress: Twin keels or longer keel?

    I agree that twin rudders and or any of the solutions to be considered are not cheap, but probably the cost of these modifications will be a fraction of the difference between selling my boat and then buying the HR, Najad, Malo or whatever, so that's why I am seeing if it might be worth it.

    I hear and understand about the rudders, and in fact I see that the new Southerlies made in the UK are now often supplied with three rudders to counter the disadvantages you mentioned

    I had however thought that replacing the shallow ( 1.2M ) wing & bulb with a longer, slightly heavier and slightly deeper fin ( say 1.5 / 1.6M ) - perhaps with a wing would also give the stability through the water?
     
  5. Perm Stress
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    Perm Stress Senior Member

    I f new keel will be both smaller in area and with lower VCG, maybe.
    Increasing the area of keel in any way (short from making it full length of hull) would increase luffing.
    Shifting the keel aft would help, but I doubt if it is technically possible.

    Now on cheap options.

    Increasing area and depth of existing rudder would help. Strength of rudder stock and it's bearings should be checked, however.
    Any means for trimming boat more stern down.
    Check, if the boat is not overloaded in general -if weight is significantly over the design value, stem is much deeper in the water as designed, and tend to increase weather helm and luffing.
    New sails -main with max belly almost on the mast, generous roach and "flat as a table" exit would improve luffing problem somewhat. By the way, if your mainsail is in-the-mast furling, with hollow leech, changing to mainsail with full-length vertical battens and positive roach would help -I know it sounds illogical at first.

    ______________________________

    From direct experience:
    I did design a new rudder for the Elan 340 here, in Klaipeda.
    She lost the original one due to poor manufacture, and needed a replacement. In the process they wanted to solve the bad luffing problem: upwind in Force 3, only completely loose mainsail would prevent her from broaching. New rudder was twice the area and 1.3 times depth (~10centimeters /4inches above bottom of the keel). Rudder stock also was solid instead of pipe (owners decision). They also added some 80 kg to the bulb. With new rudder and additional ballast, luffing and broaching ceased to be a problem.
    Initial sources of the problem was:
    1 cruising interior inside of racer hull - lots of extra weight, much of it forward of amidships and above the WL i.e. hull deeper in the water and trimmed bow down, with decrease in stability;
    2 furling mast -extra weight high up and forward -more bow down trim and more decrease in stability, already compromised with heavy interior.

    Regards.
     
  6. Strudders
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    Strudders New Member

    Rudders and keels

    To Perm Stress:

    Yes we are similar to the 340; loosen the main right off is the only solution - of course you lose speed though! We have slab reefing and fully battened main.

    Yes I like the sound of changing the rudder, and agreed strengthen the rudder stock which is only fibreglass bearing on fibreglass - it had to be remade when we bought her due to wear on recommendation of surveyor. ( fine when checked recently ) This would be a fairly straightforward job.

    She is always loaded with water and fuel and we have heavier batteries than standard but it's a cruising boat for us! However, it is trimmed more aft. The bow is normal waterline - stern down by 60mm - but we can't change that. Also it is no different if we take the weight out.

    Of course luffing is not the only problem as I really want to improve the ride and I thought a heavier longer keel or more wing or something might go some way towards that.

    There is plenty of scope to move the keel aft as there is a big gap back to the saildrive and I reckon we could get more bolts in - see pic at this link http://www.selene.me.uk/photos/20060909_InTheYard/slides/DSC02102.html and this pic of a craning in http://www.selene.me.uk/photos/CraningIn/slides/DSC00728.html

    I was surprised you said smaller area - not larger for the keel?
     
  7. Perm Stress
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    Perm Stress Senior Member

    From the photos it appear that increasing area (not depth -there is almost no room left) of rudder is the obvious first step to try. And also most easy to make.
    If rudder stock is intact, it would be not wise to replace it with something much stronger -in case of heavy impact or whatever, instead of breaking the rudder, you will break its bearings out of the hull -with all the consequences.
    In increasing the area, it should be done so that 4/5...5/6 of extra area is aft of the stock and 1/5...1/6 forward of it to maintain balance of rudder blade. For first try I would make a foam plug around existing rudder, glass it over, seal, fair, paint, etc.. and go for sail trials.

    Ride in sense of bumpy motion is governed by the shape of hull and cannot be changed.

    Moving the keel aft is not so easy as it sounds:
    1 if you have a fiberglass sump (sort of stub keel) to which actual keel fin is bolted, the only way is to order new keel with same bolts and bearing surface as original one.
    2 if the keel is simply bolted to "flat" bottom of hull (I doubt it), there are still VERY HEAVY reinforcements both around all bearing surface and especially where bolts are located, and structural floors arranged to take the loads. Moving the keel would mean tearing apart most of the saloon furniture and whatever is placed inside/below it and make new reinforcements for new locations of bolts.

    Increasing the area of keel would only help if it is extended aft almost to the rudder, leaving only a few feet between them, making it effectively full-length keel.

    If it is to remain a fin keel, only significant reduction in area will change balance in such a way that boat will stop luffing. Keel looks like it is really carefully designed; changing/modifying it should be one of the last options, when nothing else works. If so, significant reduction in keel area would be in order.

    To sum up:

    1 Least intrusive, most simple and cheap first step towards improving balance would be to increase the area of existing rudder, building additional structure over existing blade.
    2 If that is not enough, than installing two standard rudders on the quarters and as far aft as possible, would be the next step. Only THEN I would start to contemplate any modifications to keel fin.
    3 For bumpiness of ride, nothing could be done. This is a function of hull form, and no modifications to appendages will change that.

    Regards.
     
  8. Strudders
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    Strudders New Member

    All understood

    Many thanks for your comments.

    Yes I understand - and will certainly look at modifying the rudder in the way you describe. It does make sense.

    And you are sure that adding more weight to the bulb or inside below the waterline will not improve the ride - that the hull shape is the hull shape and that is it?
     
  9. Perm Stress
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    Perm Stress Senior Member

    Adding weight to bulb or inside of hull will increase stability; increased stability will increase boat speed upwind and wind strength at which you start to reef. Generally nothing more.

    If under "ride" we understand the way a boat react to waves, hull shape is the governing factor.

    There is nothing radical that could be done to it.
     
  10. gonzo
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    gonzo Senior Member

    Increasing the ballast will put more stress on the rig and hull. That can cause major structural damage.
     
  11. BATAAN
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    BATAAN Senior Member

    Raking mast slightly forward often cures many ills. Increasing ballast increases effect, therefore quicker recovery, or "snappier" roll maybe. Changing mast rake is a very cheap tool to reduce weather helm when overpowered.
     
  12. BATAAN
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    BATAAN Senior Member

    Increasing ballast increases its (the ballast's) effect I meant to say
     
  13. BATAAN
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    BATAAN Senior Member

    Usual cures for weather helm are move trim ballast aft, reef earlier, bigger jib or... change mast rake if none of those work. Agree a lot of "ride" or comfort level has to do with hull shape and how heavy the boat is. A light boat bounces around a lot more than something with some hull mass to it and there's nothing you can really do except explore other vessels. The wide flat shape of high performing modern types gives a quick and snappy motion that can be really tiring.
    Traditional UK workboat designs are available in many modern materials and developed into excellent performing practical small yachts, so different than the usual uncomfortable, marketing designed, mass consumer yacht product you seem to be sailing on. If you're not racing on these trips, why have a race boat? Even if they call them "racer-cruisers" all the marketing is speed and efficiency and nice winches, not comfort and happiness.
     
  14. gonzo
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    gonzo Senior Member

    If you look at traditional working designs, they are not always well behaved. For example, the Luggers- a typical local working boat- had the mainsheet attached to the rudder blade to compensate for the ridiculous ammount of weather helm they had. Strudders is complaining about the behaviour of a design type. That is how they sail. You can spend a lot of time and money in modifications and still end up with a badly behaved boat, just more expensive. Reducing sail and living with a slow boat is the most sensible option, or buying a boat that was not based on a racing formula.
     

  15. philSweet
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    philSweet Senior Member

    Two things come to mind-
    using a jib with a shorter hoist for offshore work. This will substantially effect light wind performance, but may improve the gust response enough to justify it. Try borrowing a jib with 4' less hoist and see. You want to be able to keep the main powered up in order to dampen roll. If that works, you may be able to make a short pennant for the jib's tack that lets you fly it near the masthead on nice days.

    Repack the boat for offshore work. No water in v berth tank (if that's where it is). Use jugs. Basically fill the bottom of the salon with all the stuff in the v berth, bow lockers, and lazarettes. Mostly a matter of self discipline and accepting that this is part of going offshore in that boat. Once you have reached your cruising ground you can unravel the mess and repack. I clear out each cabinet into its own duffel bag. I leave nothing stowed anywhere above the waterline on my boat (a Catalina 38). You'll have to make some mods to secure anchors, buckets of chain, tool boxes, and the (empty) dinghy outboard and portable generator.
     
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