Doubts about proper keel design

Discussion in 'Boat Design' started by andy47, Nov 6, 2017.

  1. Earl Boebert
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    Earl Boebert Senior Member

    Good observations. I considered recommending a full keel, but thought the advantages of being able to adjust the CLR by moving the keel was better. Thinking about it some more, the combination of forward overhang and the acute angle of the keel LE may indeed be better. I don't think you'd need quite as much overhang as the Universal Rule boats had, but taking the Rip Tide hull and adding (say) 15 cm overhang plus a long fin keel should maintain the balance properties of the original canoe body and give the ability to slide over kelp and junk.

    The Canterbury J class of model yachts have the form shown in the above post and they are popular in areas with weed-filled ponds.

    Cheers,

    Earl
     
  2. rwatson
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    rwatson Senior Member

  3. andy47
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    andy47 Junior Member

    Hi, here are the pictures of the design as promised! Any feedback is appreciated.

    side.jpg

    bottom.jpg

    front_orto.jpg

    side_orto_clp.jpg

    In the picture above, I calculated the CLP (Center of Lateral Plane) of the keel and then the CLP of both the rudder and the keel. The keel's CLP is exactly below the mast which is close to the CE (Center of Effort) for my free-rotating wingsail. When I take the rudder into account, the CLP is a little behind the mast – about 5% of the boat length, so the rig is slightly positive. Do you think it's fine?

    Also I'm trying to figure out how additional side floats (like a trimaran) may affect the boat dynamics. The boat is designed to work without floats as advised, but I have fittings on both sides of the hull to attach floats just in case I find it useful some day (another function of the fittings is to lift the boat with ropes). Imagine the boat is at a specific angle (see the picture below) where one of the floats is digging into the water and the center of effort is vertically projected between the hull and the float.

    side_float.jpg

    I intuitively think that the float would create drag that would slightly turn the boat to the side and this turning effect would have to be compensated with a rudder (just imagine an extreme case – a float that's very far from the hull). Is my assumption correct? The turning effect that makes navigation more difficult would be a hard argument against side floats. A good thing about side floats is that a shorter or lighter keel is more robust in the ocean conditions and robustness is a priority over speed.
     
  4. rwatson
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    rwatson Senior Member

    That's a much better attempt. Your negative thoughts on the floats are totally correct. You should indeed lose them. Make the hull wider if you are worried about buoyancy.

    Also, the deck area that is not swept by the sail ( eg the Foredeck) should be raised and curved by several inches, to assist righting after a capsize. Also, you should build in a lot of curve on the sheerline. A sharp edge on the deck makes for bad aerodynamics, and since you don't have any crew to walk on it, build in lots of tumblehome for better sailing performance.
    I cant stress enough, AGAIN that the boat WILL be turned turtle during its voyage, and with Floats, it may not become upright.

    Make sure you read that link to the failed attempt in the previous post, which failed with the rudder.
    I think your current rudder design will be too prone to damage. It needs to be better protected and supported, which is why I am keen on the full keel.

    Finaly, the "bargelike" shape of the hull is going to make for inefficient sailing. The hull needs a slow rise to the stern. The sharp draggy transom will slow things down a lot, as well as being a problem for following seas.

    Have another good look at the photo of the old model yacht I posted. Its not that much harder to build, and it solves a lot of the problems in you current design.
     
  5. andy47
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    andy47 Junior Member

    I noted a few posts back that the wingsail is buoyant, so the boat won't capsize. It displaces 13kg of water which is even two times more weight than the keel bulb.

    The problem with modifying the edges on the deck is that I can't bend the solar panels (well, they are "semi-flexible", but better not to try it...), so the only way of increasing the radius would be increasing the hull width, slowing it down...

    My rudder design is unique – it avoids entanglement and reduces the power needed to steer the boat at the same time.

    24210288_1278790825559358_7300942857401337774_o.jpg

    I think if the guys from UBC used a balanced rudder instead of a protected rudder, the boat would go further, because the rudder stuck at one position was the first issue (like many other boats by the way). It's the motor that must handle the force, the water kicking the rudder from side to side and a lot of cycles at the same time. It's a much weaker element than the rudder itself (if something breaks, it will be the motor or mechanism first, then the rudder). Another option is using a super-expensive actuator of a small size that can easily handle that force and millions of cycles - one company gave me the quote $4000.

    By improving the stern, do you mean this?

    side_orto_fix.jpg

    I can cut this edge. You see the hull has the greatest vertical size in the middle and it's slowly rising from the middle to the stern and bow. It's symmetrical, because it's preferred that the mast is vertical when the boat is balanced (unlike hobby sailboats where the mast is not vertical).
     
  6. Earl Boebert
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    Earl Boebert Senior Member

    Excellent progress. I would agree with rwatson, drop the floats. I would also consider rake on the rudder.

    Cheers,

    Earl
     
  7. JosephT
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    JosephT Senior Member

    Good progress on the autonomous boat. A couple of points:

    1. Make sure the rudder post is sealed and well lubricated. Microscopic sea life will penetrate the tiniest slots and orifices and seize up moving parts.
    2. Why such a long rudder? This increases the likelihood for grounding and provides more surface area for barnacles, plankton and other sea life to attach, grow and slow down the boat. Go with a shorter, heavier rudder that is tapered. You've made a smart modification by not protruding the bulb forward. This will minimize snagging debris.

    The rudder below is a design I've sailed in myself for thousands of miles. Zero debris snags. Very robust design.
     

    Attached Files:

  8. rwatson
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    rwatson Senior Member

    Sterns.png
     
  9. rwatson
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    rwatson Senior Member

    "It's symmetrical, because it's preferred that the mast is vertical when the boat is balanced (unlike hobby sailboats where the mast is not vertical)."

    A symmetrical hull is not necessary.
    The mast can be moved along the hull to achieve balance.
     
  10. andy47
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    andy47 Junior Member

    I see. Thinking out loud, having the stern very thin would significantly reduce robustness, i.e. I would have to remove some part of the inner skeleton where the rudder and wind sensor rod are attached. But what I can do is increasing the height of the hull in the middle which would increase the slope from the middle towards the stern. I found that the hull height/width ratio on my hobby sailboat is 2:3, so I designed it with the same ratio. Maybe I can increase it?

    6.jpg
     
  11. rwatson
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    rwatson Senior Member

    You have built in a few problems there.
    I understand you like to have a "module" for testing the system outside the hull, but having a skeg extending into the keel is not a good idea.
    The hull should be supporting the whole keel/skeg, and you should be able to slide the rest of the mechanism fore or aft to achieve the required balance.
    The rudder could be on some sort of telescoping arm of course.

    Ideally, the hull should be built as a "monocoque" for lightness and strength, and the support for the mast mechanism can then be supplied by the hull, which you can bolt the removable "module" into, not just a dedicated single spar inner skeleton.

    Josephs comments on the rudder are on the mark as well.

    Re the solar panels.
    I am surprised you don't have some on the solid mainsail. You are going to need as many as you can get.
    Angling the ones on the deck a bit will help with solar efficiency, as well as promoting runoff to avoid covering with dried salt.

    Finally, I know you keep saying that the mainsail will provide sufficient righting moment. BUT your craft WILL get rolled, many times, solid mainsail or not. My suggestion to test it in the surf still stands.
     
  12. andy47
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    andy47 Junior Member

    The balance will be achieved later by manufacturing the keel after I build the whole boat (without keel). Now I have a rough estimate about the center of gravity and I will measure it precisely later. There is some sort of freedom to design the keel shape and bulb weight while keeping both the correct CLP and the fixed upper dimension where the keel is attached. The keel shape also allows to hide the end of the mast.

    The reasons I didn't attach solar panels on the sail are: 1) increased weight of the sail 2) additional rotary connection needed to connect the solar panel which can be a weak point (I will already use a twisting cable for powering the actuator, but that's another long story).

    The skeleton will be the heaviest part of the boat. The reason for it is storm survival. Without it, a hurricane like Irma will tear the boat apart. With the skeleton, it can be found somewhere in one piece. My previous boat was destroyed by small cyclones coming from Matthew.
     
    Last edited: Dec 6, 2017 at 11:48 AM
  13. Earl Boebert
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    Earl Boebert Senior Member

    A few more suggestions, if I may. A free-sailing model yacht (which is the way I look at your project) needs to have what the old designers called a "balanced" hull (not the same as the "balance" you mention in the post above). The "old balance" definition was that when going to windward the boat would not have a tendency to yaw when it heeled. The visible manifestation of this property is that the boat will maintain its track all by itself when a gust hits. I think that "old balance" is important for your project because it would greatly reduce the wear and tear on your control system.

    There are several ways to achieve "old balance," described in the attached paper. One thing the paper fails to emphasize enough is that hulls that exhibit this property almost always have their LCF coincide with their LCB, and this would be the minimum I would strive to achieve. The next level would be to do the "in wedge/out wedge" analysis described in the paper. I think those two would get you a decent hull.

    Cheers,

    Earl
     

    Attached Files:

  14. andy47
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    andy47 Junior Member

    Interesting point about the yaw. It estimated the water level and made a quick check on my CAD model. It seems that LCF is close to LCB. I will the paper further.
    On my boat, I don't expect too much heeling. The angle of heel I calculated will be 27 degrees at 50 knots. If the wind is too strong, it will automatically adjust the flap to 0 degrees which is the same as taking down the sails, because the boat would be uncontrollable in rough sea conditions. So I expect the angle of heel between 10-20 degrees most of the time.
     

  15. Earl Boebert
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    Earl Boebert Senior Member

    Right. But remember, it's not the angle of heel, it's rate of change. An unbalanced hull will yaw in 5 kt if hit by a 10 kt gust.

    Cheers,

    Earl
     
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