Keel skeg addition to traditional sailboat

Discussion in 'Sailboats' started by bpw, May 10, 2012.

  1. bpw
    Joined: May 2012
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    Location: Cruising

    bpw Senior Member

    Hello everyone,

    A question on a modification to hull shape...

    I am currently off cruising in an 28 ft William Atkin Inga, drawings here:
    http://www.atkinboatplans.com/Sail/Inga.html

    pictures of our boat here: https://picasaweb.google.com/anna.b...&authkey=Gv1sRgCKmoqMm936L8Dw&feat=directlink

    You can see hull shape in the pictures of her dried out on the beach, scroll down a bit.

    28ft long
    8' 6" beam
    3'6" draft (but closer to 4' loaded)
    12,200 lbs as designed, at least a few thousand pounds of cruising gear aboard.


    She is a lovely boat in many ways but going up wind is not her strong suit. Over the course of 10,000 between San Francisco and Valdivia Chile I have begun to think a bit more keel would be quite helpful.

    First the problem I want to solve:

    Upwind she stalls easily and has trouble powering into waves, bow gets knocked off very easily. Everything is OK when we have not too much wind and un-reefed sails, good balance etc. But as we reef down center of effort shifts forward even more than is normal because the boom is so long. By the time we get to a trysail it is almost impossible to keep her head into the wind.

    Sails are new (or where new when we left), full batten and since we are engine-less we work hard to sail the boat well. Keep things trimmed well, reef when needed etc.

    We also are near fanatical about keeping weights low down and having nothing on deck to add windage or weight, anchors go below offshore, no jerry jugs on deck, folding dinghy, Very minimal spray hood instead of a dodger etc. Bottom always kept clean. Have already filled the aperture where the prop and shaft used to be so nice smooth hull.

    So not much room to increase upwind performance that way.

    What I have been thinking of doing is adding a fixed skeg about 8-10 ft long and adding about 12 inches of draft. Since the skeg would be forward of the lowest point of the keel the added skeg would be closer to 18 inches in height forward and tapering aft to create a level run.

    Plan is to center the new skeg on the center of lateral resistance so would shift CLR slightly forward (since skeg would be deeper forward than aft)

    Minimal ballast on the new skeg, maybe 200-300 lbs as a steel plate bolted to the bottom, looking for lateral resistence more than increased righting moment.

    Rig is overbuilt so should have no problem with slight increase in righting moment.

    My vision is basically a centerboard that is fixed in place, the extra depth is no problem in Chile.

    Construction would be of laminated wood, 5-6 inches wide and then shaped to create a foil. Current think is that an 8-10 foot length 5 inhces wide will provide plenty of surface to glue the skeg to the keel with epoxy an no need for keel bolts. Boat is ferrocement so bolts get very tricky.

    Also like the idea of being able to cut the whole thing off with a chainsaw if it doesn't work out or we decide to cruise the Bahamas some day.

    Now the questions:

    Does my idea of centering the new skeg around current CLR make sense? I want to slightly increase weather helm.

    What sort of foil should I look at, this is a slow boat so 3-4 knots. And the foil needs to be pretty wide for strength, 5 inches or so.

    Was thinking of making the steel plate on the bottom of the foil hang out the sides about an inch to create an end plate, good idea or bad?


    the tricky one:
    I have been assuming highest loads would be when we dry the boat out on her side and the new skeg takes side loads of a significant part of the boats weight. I used a beam strength calculator to test an 18 in long 8 ft wide 5 inch tall beam and saw loads of around 500psi holding 12000 with a single fixed end. This would be beyond worst case since it is hanging the entire weight of the boat from the keel. Am I doing this right? not an engineer so may have missed something. But seems epoxy would have no trouble holding it together.

    Also worse case the skeg falls off and the boat is no worse than it was before. I can afford to experiment.


    Problems I see.

    Extra wetted surface will hurt in light air, but would be a good trade for increased upwind performance in the type of sailing we are doing.

    Boat will be a little less happy running down wind at speed, might have to slow down a bit sooner to keep steering easy for the wind vane.


    ??????


    Construction is no problem for me, and will be cheap to do so not out much if it doesn't work.

    Would love any feedback or suggestions.

    Thanks

    Ben
     
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Purely a function of her built down hull. She has poor "penetration" ability, a nearly nonexistent appendage in sectional form and little to prevent lateral skid, except the mass of her flanks. Coupled with her relatively deep fore foot and low aspect appendage (figuratively) , she's screwed going to weather.

    Your idea of adding depth to the keel could work, but its position should be worked out with your sail plan, not the CLP location.

    Wetted surface with this particular hull is not an issue. The modest amount you'll add will make no noticeable impact on vessel preformance. Simply put, she has so much wetted area now, you''l never notice any increases.

    Filling in the prop aperture, wouldn't offer much help at all. The real problem is that this is a very old design from Billy and it's got most of the things wrong with it as other plump, relatively shallow, steep buttock double enders do. The rig has typical for it's era proportions too, none of this is helping your wishes for more helm and ability to weather. The fair body and keel are too thick, not enough power in her aft sections, etc., etc., etc. Now, as far as Billy was concerned, this "lifeboat" style of hull form was just right, as it can take a beating in the roughest of weather and stand back up. This was the designs goal, not the ability to "hang on" upwind.

    This particular hull shape doesn't have the "hard mouthed" double ender habits, often seen in short over hang double enders. Most hulls shaped like this, need to have the ends drawn out a little, to prevent them from rounding hard, once they're heeled over a bit in a blow. Billy changed the volume distribution in this hull form, to ease this tendency on a quick curve, double ended hull form. So, she needs to be sailed as up right as possible. This is difficult because of the lack of power in her midship and aft sections. What she'll want to do is flop over to about 12 degrees, then she firms up a lot and becomes very steady. Unfortunately, most of the lateral area becomes dramatically less effective, at angles beyond this amount of heel, so she skids to leeward. As winds build, heeling angles increase and effective lateral area decreases, so the helm goes away.

    You might have better luck with a redesigned rudder, that had more area down low. You can also consider raking the mast aft as much as practical or recutting the main with a shorter foot and possibly a mizzen. She'd look very nice as a ketch, maybe with more yawl like proportions. I'd place the mast at the very aft end of the foot well or deck stepped, on the forward end of the aft deck. You'd have the option of flying a mizzen staysail too, which is a real work horse in light to moderate air.
     
  3. bpw
    Joined: May 2012
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    bpw Senior Member

    Thanks for the reply Par, was hoping you would chime in here.

    You described how she sails exactly, Well aware this hull form is a long ways from ideal but it is what we have at the moment so trying to see what I can do to add a little bit more bite going up wind. Not so much concerned with speed as being able to hold ground when trying to claw our way to weather. As she is now she is lovely offshore where we have lots of sea room, but heading south from here we will not have that luxury until we pop out again into the atlantic. Have already been through every weather helm increasing trick I know,raked mast, main sail shape etc.

    Have thought about a mizzen a few times but would be difficult because of the wind vane, tiller etc. Also would be very difficult and expensive to do here in Chile. Bit beyond what I want to get into on this boat.

    Keel would be cheap and easy with the materials I can source here. No hardware needed, just wood, epoxy and some time.

    As far as working out keel position I was using CLP instead of sailplan because I figured since she already was already close to what I wanted (almost neutral helm slight lee helm when reefed) moving CLR forward would give a slight increase in weather helm, since the rig will be staying the same. Effectively would increase lead, probably come to same result as going from sail plan, but I will go through calcs based on sail plan to make sure.

    Any advice on how much I should increase lead to give her a bit of weather helm without going to far? Better to get a bit too much than too little since I can easily add headsail area or rake mast forward a bit. Should I calculate lead based on reefed sail area to double check how she will behave?

    I was thinking of adding a small balanced portion to the bottom of the rudder since the new skeg would be able to protect it. Would hopefully make steering easier. Easy to do while I am working down there. Only downside is possible complication when drying out. Would need to make sure she would lay on the skeg not rudder when on a beach, probably not too hard given where the weight is in the hull. Good idea?

    I will try to work up a couple simple drawings in the next couple days and post them here.

    Thanks,

    Ben
     
  4. messabout
    Joined: Jan 2006
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    messabout Senior Member

    Going south from Chile and popping out into the Atlantic implies that you are going to round the horn. Small boat, mean water.....Yikes! Is that your intent?
     
  5. bpw
    Joined: May 2012
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    bpw Senior Member

    Almost, but not quite, planning to go through Canal Beagle. Spits us out just east of Cape Horn so won't quite be rounding the Horn.
     
  6. alan white
    Joined: Mar 2007
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    alan white Senior Member

    Have you thought about leeboards? Possibly a way to save a few bucks without the major surgery. Should improve windward performance without any wetted surface penalty when off the wind.
     
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    This boat has huge amounts of wetted surface Alan, so any additions wouldn't be much of a concern. Also adding lateral area in the forward portions of the hull will not do much in regard to moving the CLP. Make a cardboard cutout of the underwater profile and find the CLP. Use only half (vertical cut) of the rudder profile for this balancing act. Then make a new cardboard cutout with various keel modifications that place more area forward. What you'll find is it take s a lot of area to move the CLP very much. To make maters worse is the long, very rounded off and faired keel appendage. Long keels tend to nullify dramatic changes in CLP locations, which is one reason the hull tracks so well and has an easy helm.

    I think you could install a keel extension that might improve her bite to windward, but don't get to excited about the improvements, as there are many things working against this hull shape. The logical thing to do is make the extension drop down at about a 45 degree angle, starting at station 3. Draw a line parallel to the LWL, but even with the very bottom of the stern post and fill this triangular space under the keel in. The keel will not have "drag" any more (you don't need it), it'll be deeper forward, flat bottomed so she can take to ground (if stoutly built) and you'll define the leading edge a little better.

    The crude drawing shows basically what I mean. It doesn't need to be deeper, just fill in the area, flush with the bottom of the stern post. Also the 45 degree angle should be more like 60.

    While doing the CLP drawing, also figure out what your current lead is. I'll bet it's on the low side, maybe about 12% - 13%. You might want more, say in the 15% - 16% range.
     

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  8. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member

    Another thought might be adding bilge fins.

    As the boat heels and the main 'keel' becomes less effective, the leeward bilge keel will become more effective.

    In theory, that is.

    I would design them, so they could be easily broken off, without putting a hole in the boat. And I would center their area under the the Horizontal Center of Area (HCA) of the main, with its last reef tucked in.
     
  9. bpw
    Joined: May 2012
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    bpw Senior Member

    Thanks for the ideas everyone,

    Thought of leeboards but actually would be more difficult and costly to build, same with bilge keels. The bottom of the current keel is a big chunk of cement and lead so really strong and easy to attach to. Bilge keels would be really cool in that they would keep the shallow draft but would mean internal reinforcement of the hull, a major project. Keel addition is basically gluing up and fairing a big block of wood that then gets glued to the hull, pretty easy and almost all the work can be done away from the boat in a dry shed.

    Par, your drawing was exactly what I had in mind, but I was thinking of adding a little draft (12") while I was at it between say station 4 and 8 roughly, but sounds like might not be worth it? Draft is a non-issue in our current cruising.

    Would adding draft to protect a rudder extension make sense? Perhaps bring the rudder down 6-8 inches and forward of the rudder post to create a bit of balance, hoping to make life a bit easier on the steering gear when running fast.

    I have a lines drawing of the boat showing CLP and CLE so will pull them out and play with them and some cut outs of new hull shapes.

    I have been thinking that it is not so much an issue of moving CLP as giving the hull more "bite" she is pretty good in flat water (but could use a touch more weather helm) but has trouble keeping the bow up when its rough, so a small change in CLP may be all we need if there is more keel down below to help prevent stalling and getting shoved sideways.

    Thanks again everyone, this is really helping to refine ideas.
     
  10. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    More depth will help, the new sketch shows some drag added to the keel and several inches of depth, but you start taking away from what you've added forward, if you add some aft as well.

    I wouldn't start your new keel addition at station 4, but at station 3, raking down to station 4, as shown. This is because station 3 is still the hull, but station 4 is into the keel sections and you'll have to start with wide, massive sectional shapes to fair it in, but at station 3 you can begin the fair body fair fine. In other words, it will be much easier to shape if started at station 3 then 4 and the new keel addition can have a finer entry, which will help.

    Also note the rudder has about 30% more area, all down low where it will be most effective. Do not add balance to this type of rudder, as it'll just act like a big brake and dramatically slow the boat, especially in maneuvers. This type of rudder is about the least effective you can have. This rudder has had some depth added but is left a few inches short of the bottom of the new keel, which will protect it fine.

    Lastly, don't round over the bottom of the new keel extension, like the old one is (as shown in body plan). Just round over the leading edge, but leave the bottom squared off, with crisp edges. In fact, don't taper the sides either, just make the extension straight sided, all the way down to a perpendicular bottom plate. This one thing, will make a big improvement in decreasing leeward skidding.
     

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  11. bpw
    Joined: May 2012
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    bpw Senior Member

    thanks, think it is all coming together,

    Very good advice about the rudder, makes sense now that I think about it, and about the keel shape, I would have been tempted to shape the bottom so it would "look right"

    2 more quick questions...

    Better to end the keel at station 8 or 9 instead of going whole way aft?

    Should I put an end plate on the keel? Maybe leave the steel ballast plate an inch or so wider on each side than the keel?

    Thanks for the help, Looks like this will be happening in the next few months, going into rainy season here so will have lots of time for indoor work gluing up and shaping the keel, then haul out in a few months and stick it on. Will be sure to put some pictures up as it comes together.

    Edit: Sure appreciate the professional advice, its amazing what the internet can do, and how free people are with their time.
     
  12. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    An end plate wouldn't hurt, but you're never going to notice the difference with or without it installed, so why bother. It would be better to bring the addition all the way back to the stern post. Other wise if you terminate (between station 7 and 8 would be best) the rounded over, original keel will then be there offering it's crappy leeward resistance. You're better off with a "slab sided" new addition, which will add bite, because of it's crisp lower edge and flat sides.

    Is the rudder just a slab of edge glued wood too (typical for Atkins)? You may want to refine the shape a little, if adding some meat to it's trailing edge. It doesn't have to be knife edged, but a tapered trailing edge will also help.

    Free? Send me a six pack if it makes you feel better (it would me :rolleyes:) . . .
     
  13. bpw
    Joined: May 2012
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    bpw Senior Member

    So the new keel is in place and we are back in the water, pictures as promised are here:

    https://picasaweb.google.com/bwedlo...authkey=Gv1sRgCN7AhPa66qO3qgE&feat=directlink

    Still have to finish painting the decks and put the rig back in (with our new Dynex standing rigging!) so no chance to try it out yet.

    The steel plate on the bottom is about 150 lbs and should just about cancel out the added buoyancy of the keel. It sticks out an inch on each side giving a small end plate, was easy and let us put a little more weight on the keel without going to thicker steel. The keel is bolted together but attached to the hull with only epoxy, about 6 square feet of gluing surface so we expect in will be strong enough.

    Added draft is 6 inches.

    Due to epoxy being just about impossible to find in Chile we kept the keel a bit shorter than planned so as not to use up our entire glue supply. The planks of the keel are bedded in sika 221 adhesive and the only epoxy used was on the hull joint and to lay some glass over the leading edge. The gallon and a half that we have actually came down checked luggage with my Dad! As it was we used about a half of a gallon doing the joint.

    Thanks for all the help and ideas, hopefully she sails well!
     
  14. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Welcome back, don't forget to post performance reports, as you get a chance to test her out.
     

  15. BATAAN
    Joined: Apr 2010
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    BATAAN Senior Member

    Wow. Great thread, and PAR's design thoughts seemed to me very well thought out, as were the poster's questions and workmanship. Very much looking forward to seeing more of this little craft on a very interesting trip so please keep us posted!
     
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