Is it worth increasing keel weigth on particular boat?

Discussion in 'Stability' started by cuorefocoso, Dec 22, 2007.

  1. cuorefocoso
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    cuorefocoso Junior Member

    Hi
    I was running some forums to find it out, but worthless.
    I own a keelboat Inferno 26, with modified keel - it was changed from classic one to kind of bulb. Anyway, according to info what I have collected, the original keel was approx 800kg with 1.5m draft; what I have now is ~450kg at 1.8m draft. And there are the pictures of how the boat sails in a bit heavier air; it is under number 8238.
    I had experience it goes mostly same comparision with other boats - I need to reef first out of the fleet.
    So, I am heading to increase keel weight. As bulb structure is like "half-of-the-pear", I am going to add non-equal second half, something like 100kg.
    Can anyone tell the possible affect? I know the boat will be heavier, the loads on bottom, rigging etc will increase. But performance and so on in general? Nevermind the cost, will it be little easier to sail in medium to heavy weathers?
    I have IMS certificate data if it would help someone to think about it...
     

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  2. Eric Sponberg
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    Eric Sponberg Senior Member

    Generally, whenever you add weight to increase stability, you will always gain more power and sail faster. It more than makes up for the added weight.

    A gross check on the design shows that you have lost about a third of your ballast stability:

    800 kg x 1.5 m = 1200 kg-m

    450 kg x 1.8 m = 810 kg-m

    This is why you are reefing sooner than the others. For your current keel depth, to get up to the same ballast moment, you can go to 667 kg total weight:

    1200 kg-m/1.8 m = 667 kg.

    So you can likely safely add 217 kg (667-450 = 217) to get to the same ballast moment as originally designed. The boat and its rigging should be able to take this. If you want to go beyond this, that is another matter and the structure in the bottom of the boat and the mast and rigging would all have to be checked.

    This is just a very rough guideline, you may want to check with a professional on the actual details of the boat.

    I hope this helps.

    Eric
     
  3. cuorefocoso
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    cuorefocoso Junior Member

    Thank you Eric
    As far as I understand you have used 1.5 and 1.8 just as rough digits? I mean that it is not exact position of theoretical keel weight; it is max draft.
    I was trying to get drawings of the boat, but there is only paper copy, and it is complicated to get it scanned and sent.
     
  4. Eric Sponberg
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    Eric Sponberg Senior Member

    Correct, you really need to check the CG of the weight instead of the draft. That is why I called it a "gross check". The best bet, if you can, is go back to the original designer for advice. He will likely have the technical information at hand to advise on how much extra weight you can add for the keel that you have. As you surmized, one has to be careful about overloading both the boat structure and the rig when adding keel weight.

    Eric
     
  5. tom28571
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    tom28571 Senior Member

    One question is, do you know how stiff the boat was with the old keel? What was the rationale behind changing it? In general, a tender boat will rarely give back enough in rating to make up for the loss in sail carrying ability and performance. In the first photo, all that rail meat is barely outside the CG and doing little good. Much better to have that weight in the keel where it can get some moment arm.

    For whatever reason the boat looks much too tender to be successfull in in those conditions. But it's not last so there is hope.
     
  6. cuorefocoso
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    cuorefocoso Junior Member

    The boat with old keel (as the other same boats) was pretty tender - I mean it was light wind sailor. I don't know exact reason for changing it to new type of keel, unfortunately I had no chance to reach the guy who did it.
    Actually now the boat is sailing pretty nice with exception of it heels very easy. It is easy to beat 30 and 33 feet boats even with not biggest sailplan.
    The case is what Tom was trying to touch: I think lead is better than 1 man on a rail. What are other concerns?
     
  7. Joakim
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    Joakim Senior Member

    I don't know why you did not give this background information from earlier discussions: http://www.inferno29.org/foorumi/viewtopic.php?t=114&postdays=0&postorder=asc&start=0

    I quess that was one of the worthless ones?

    When you want to calculate the rough changes to RM, you should refer to metacenter not waterline. http://en.wikipedia.org/wiki/Metacentric_height

    Adding weight to near the waterline also helps RM considerably!

    The metacenter is about 1 m above the waterline with this boat (at small heel angles). If we asume that the new keel has it's CG 1.7 m below waterline thus 2.7 m below M and the original 0.9/1.9, we get

    2.7*450 = 1215
    1.9*800=1520

    To get equal RM, you would need about 100 kg more in the bulb, as I told you earlier.
     
  8. alan white
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    alan white Senior Member

    Some boats do heel a lot without adverse effects. They generally don't develop a bitchy weather helm even when heeled down to the rail because the hull has balanced ends, usually meaning a narrow stern and fuller ends.
    The heeling in and of itself is not a problem except if it causes an unsafe condition or it slows the average boat speed down.
    In your case, you say the boat is not suffering for speed. If no unsafe condition is apparent, why worry? A lighter boat needn't necessarily be stiff so long as enough other properties are enhanced by the hill shape and rig design---- like minimum wetted surface area and a lower mast/rig height.
    I'm reminded of the old yawls like Albert Strange's designs--- balanced narrow hulls and moderate rig heights made them well-mannered and fast boats in a lot of conditions. They heeled and still sailed well in spite of very (by today's standards) unimpressive ballast ratios.
    Generally, you'll reef sooner but a narrow and balanced and lighter hull won't slow down so much when reefed due to good rudder position and better hull symmetry when heeled.
    I don't know if these characteristics apply to your boat. It may be that it would be better to work with the rig rather than the keel. You might lower the mast a bit and thereby reduce the working plan 10%. You probably won't miss the extra area insofar as speed goes as you can always pile more sail on with a genoa and other sails.
    There's nothing inherently faster about a boat having both a large sailplan and a heavy ballast keel when compared to a same-length boat with half the weight and half the sailplan. What is most important is that the rig is matched to the hull under the most conditions. If reducing the sailplan achieves this, it seems to me it makes more sense than changing the keel by adding ballast.

    Alan
     
  9. cuorefocoso
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    cuorefocoso Junior Member

    It might be a case, Alan, to reduce the rig.
    From the other hand, it seems to me wiser to add when you can remove it later rather than to cut something without a chance to comeback.
    I would say the boat has a style you told - the bow is pretty nice, and transom is wide and planning I would say. It goes very nice downwind with current sailplan. In adition, it has only 26 feet lenght, so it is not comfortable to have some 4 or 5 men on a rail. These are my issues why I am looking to increase the bulb weight. The reason I am looking for theoretical help is no-one did this kind of job around; I am not suffering in how to do it, because I find the situation thankful.
    So, any suggestions are welcome.
     
  10. Guillermo
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    Guillermo Ingeniero Naval

    Hi Rimantas!

    Conceptually I'm rather in line with Eric's analysis, as center of buoyancy should be used rather than metacenter for these comparative purposes. As we may asume CB as being at 1/3 of body draught from WL level as a rough approximation (which in this boat's case is only a few cm), we may also use WL just to have an idea of what to do.

    If we use CB for comparisons, we get:

    1/3*body draught = 0.1 m (roughly)

    so:

    800 kg x 1.4 m = 1120 kg-m (1.4 = 1.5-0.1)

    450 kg x 1.7 m = 765 kg-m (1.7 = 1.8 - 0.1)

    now

    1120 kg-m/1.7 m = 659 kg.

    and

    659-450 = 208 kg


    Anyhow, as the COG of ballast is not at the tip end of the keel (so this estimative can be grossly inaccurate), you now should contact (and pay!) the boatyard, the designer or a NA to perform a proper calculation for you.


    Here some numbers for your boat, based on the data you sent to me:

    Length/Beam Ratio (2Lwl + Lh)/3B = 2,88
    Lwl/Bwl Ratio Lwl/Bwl = 3,40
    Length/Draught Ratio Lh/T = 4,38
    Beam/Draught Ratio Bmax/T = 1,38
    WL beam/Body draught Bwl/Tc = 7,14
    Ballast/Disp Ratio W/Disp = 0,40
    Displacement/Length Ratio D/L = 177,41
    Sail Area/Disp. Ratio SA/D = 20,60
    Sail Area/Wetted surface SA/WS = 2,73
    SA (metric)/ Power (Imp.) SA/HP = 3,57
    Power/ Disp. Ratio HP/D = 2,04 HP/ton
    Hull speed HSPD = 6,33 Kn
    Potential Maximum Speed PMS = 7,64 Kn
    Velocity Ratio VR = 1,21
    Capsize Safety Factor CSF = 1,99
    Motion Comfort Ratio MCR = 17,65
    Heft Ratio HF = 0,80
    Moment of Inertia I = 63996,07 Lb/ft2
    Roll Period T = 2,14 Sec
    Roll Acceleration Acc = 0,12 G's
    Stability Index SI = 0,86


    Cheers.
     
  11. cuorefocoso
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    cuorefocoso Junior Member

    Guillermo,
    could you please give short comments to the digits you supplied, or would it be something extra?
    Ref to calculations - don't know no-one to help me around. Would you be interested in doing such calculation?

    Happy New Year for everybody!
     
  12. Guillermo
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    Guillermo Ingeniero Naval

  13. terhohalme
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    terhohalme BEng Boat Technology

    Hi, saw my keel here!

    This keel was originally designed for a much lighter boat, Päre, her displacement is only 1400 kg. Her keel is 520 kg cast iron. I send the keel model to one guy (can't remember his name now), he was first planning to use lead. Made from lead the weight should have been enough for Vital 26 which is much heavier boat than Päre. Evidently, the keel was too thin for lead or for a some other reason he made it from composite.
     

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  14. cuorefocoso
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    cuorefocoso Junior Member

    Hi
    I guess the guy was Pekka Lamsa,was he?
    Do you have any digits about the keel?Drawings,skethes?
    My keel is GRP in the fin,8 stainless steel bolts that go thru from lead bulb to the inside of the boat.To my information it is approx 350kg the bulb and 80kg in the fin.
    Maybe you have any pictures from how the keel was attached and fastened inside the boat it was designed for?
    By the way,how was the performance of that boat in a picture?Looks very attractive to my point of view.

    BR
     

  15. terhohalme
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    terhohalme BEng Boat Technology

    Sorry, too many PC changes and black moments...

    I'd remember 8 stainless bolts first 1 in front then 3 pairs and then 1 in back. Wide between bolts was propably 100 mm and length between 200 mm. Mine bolts was drilled and threaded to the cast iron, So yours can be different. To me your keel is too light for Vital 26.
     
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