Pocket cruising boats

Discussion in 'Sailboats' started by Guillermo, Apr 30, 2006.

  1. Guillermo
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    Guillermo Ingeniero Naval

    So, not the same RM or RM curve, but the same area. That's what confused me. Anyhow, I'm not sure they will have the same area under the RM curve for the same size and displacement. Most probably not. Need to work on a realistic example.

    I'm also not so sure about this. We are talking unprobale boats having a 180º AVS, which will probably call for big watertight volumes on deck. Again, have to study it.

    I think it's the contrary. Read: http://www.johnsboatstuff.com/Articles/dynamic.htm


    Losting rig decreases moment of inertia, not risens it, if that's what you are talking about.....:confused:
    I think that a sailboat that has lost its mast is easier to capsize is not always true. It depends on keel weight and position. A heavy bulb will make the boat like one of those toys that you cannot turn down (How are they called in english?). Precisely this kind of boats have a very large AVS and very small area under the RM curve in the inverted position. Because of this and the big weight high over the sea level, makes very easy for these boat to turn upright again (keeping keel in its place, not breaking, of course)

    From Johnson stuff:
    "Dynamic stability controls how much a boat heels in response to a wind gust or impact of a strong wave. A stable cruising boat will resist these dynamic forces long enough for them to pass safely by. Heavy displacement helps dynamic stability, but the most important factor is the boat’s roll moment of inertia. The roll moment of inertia is calculated by multiplying the weight of each piece of the boat by the square of its distance from the center of gravity. The "squared" term makes the calculation very sensitive to how far heavy objects are from the center of gravity. For example, a dingy with two people sitting fore and aft on the centerline has a smaller roll moment of inertia than the same dingy with the people sitting side by side. Both boats weight the same, have the same center of gravity, and the same center of buoyancy (exactly the same static stability), but moving the people off the centerline greatly increases the roll moment of inertia. If two identical boats are hit by a gust, the one with the largest roll moment of inertia will roll the least."
    http://www.johnsboatstuff.com/Articles/estimati.htm

    Cheers.
     
  2. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    One interesting issue that may consern this discussion is the changing meaning of the terms "deplacement" and "ballast ratio".

    Deplacement used to mean the total weight of the boat with stores and crew, in sailing condition. Now it often is used for the weight of a new empty boat.

    Ballast ratio used to mean ballast/deplacement where deplacement is as above.

    So some years ago a boat weighing 1000kg would have a deplacement of 1500kg with crew and some food+water. Then the 400kg of lead would have been a ballast ratio of 27% while a salesperson now will say it is 40%.

    Now you often find small boats in the lower price range with an empty weight of for exapmple 2000kg and 500kg ballast. That is 25%. When the boat is in use with a deplacement of say, 2500kg the ballast ratio is 20%.

    Anyone know what the ISO is doing with this?
    I think there is a new common understanding of "deplacement" on its way.
     
  3. sharpii2
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    sharpii2 Senior Member

    After a while you get into vanishing returns.

    Especially in sail boats.

    Making my proposed design longer would increase its potential speed in good sailing winds, but, for the same given sail area, make her much slower in light winds. (10% or more) Add to that the extra matterial and the increased structure allowences for the greater length and the few extra feet doesn't seem such a bargain.

    The boat was originally concieved as a 15ft micro cruiser stretched out so she could be double ended without breaking the cheap exterior plywood she would be made of. The idea here is to make the least expensive, least complex boat possible that can comfortably cross an ocean (so much for pit stops) and be buildable by crapsmen (dangerous with a saw, deadly with a paintbrush) such as myself. She'll look like a bloated kayak with high sides and a vane tacked to the stern and a single ballanced lug sail over her deck. Which, by the way, will be so narrow that it would be dangerous to venture out on. Hence, all the sail handling will be from below.

    If you can imagine das boote with sails, you'll get the picture. Ballast was once going to be water, but is now going to be the somewhat denser sand. And it's all going to be internal. Water stores will be in 2 liter bottles stowed low so they can double in effect as ballast. Each bottle will be refilled with salt water as it's emptied. Water will make up about 60% of the stores.
    Everything else will be rank by heaviness and stowed accordingly.

    The idea here is to consolidate the forces acting on her (sails above/ ballast below moved as close together as practical) and distribute loads widely. This, I understand, makes for a long lived structure.

    She will look and perform nothing like a yacht, so little effort will be made giving her that kind of finish (the main reason boats are so expensive). She will therefore be seen as a pleasure boat. A work boat that doesn't have to pay for itself (nor impress anyone). She will creep accross the ocean at 2 to 4kts, sometimes reaching speeds up to 6kts when the wind is just right.

    My mantra is: "simple to build, cheap to maintain." Especially for entry level blue water boats.

    As for the disproportionate payload, with all proportions kept the same, boats go up in displacement with the cube of their length. Therefore, a forty foot version of my little beasty will displace eight times as much. So, unless I'm planning on eight crew on a forty foot version, I can afford to cut displacement. And, with a likely two person crew, payload would be the first place I would start that cutting. The boat would end up lighter, but would. proportionately, have the same rig. She would therefore be disproportionately faster.

    In your conversion of a steel design to a foam fiberglass version, have you raised the price as you cut the displacement?

    Bob
     
  4. Guillermo
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    Guillermo Ingeniero Naval

    ISO 8666 defines masses for recreational craft, as follows (simplified):
    - Light craft mass, mLCC
    - Mass of the craft when loaded on a trailer, mT
    - Performance test mass, mP
    - Maximum load mass, mMTL

    and loading conditions:
    - Test condition
    - Ready-for-use condition
    - Fully loaded ready-for-use condition

    ISO 14946 defines maximum load as the "manufacturer's recommended maximum load"

    Too detailed and taking too much time to write it down in full here :(
     
  5. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    Thanks Guillermo!
    It's quite interesting talking to the sales persons on boatshows. Ask them about the displacement and they will give you the mLCC :)
     
  6. Windvang
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    Windvang Yacht Designer

    Is your design a bit like the Paradox cruiser?, it sounds like an interesting experiment!. Raggi what is your idea abouth the increase in price?, as you are a builder.

    Yes, the fiberglass hull is more expensive in materials than the steel one. I think up to 25% especially as we used Kevlar. But it had a lot of advantages:

    a: It was very easy to build using foam battens around chipboard frames. The foam was cut by a knive then hot-melted to each other and the frame. It took 2 men abouth 4 days to close the hull. Remember the Colin Archer is a complex shape.

    b: We could just paint the inside and did not have to insulate and finish it.
    c: The boat will have less maintenance in the future.
    d: we could use a smaller engine, and a smaller sail area, saving quit a bit of the price back.

    Arthur
     
  7. sharpii2
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    sharpii2 Senior Member

    Yes. It is very similer to a Pardox, but its concept pre dates it by about two decades. There are other differences too. A Paradox is intended for shallow water sailing with an occasional (but perfectly safe) venture into deep water. My Lola is intended for the oposite. Above the WL they will look almost identical. Just shows you how design principals dictate design. If you want a big range of stability in a shoal draft boat, the hull section is going to have to be tall for its beam. Or it's going to need an awful lot of ballast (50% or more).

    The most striking difference between the two is going to be below the waterline. Paradox has a flat bottom: Lola has a 23 deg 'V'. She also will have boards extending down from her sheer to the depth of her 'V'.

    Parodox, on the other hand, has a 'L' section running parallel to her chine. She can sail in extremely shallow water with this device and doesn't need any boards. I haven't heard how well this works to windward, but niether have I heard of any complaints. Maybe she has adequate windward ability, which is all she really needs (the relentless search for more and more windward ability has, in IMHO, done great damage to sailboat design).

    For the same length, Lola will be about 50% heavier, but have just about the same amount of sail (no races, please, unless they are either dead to windward or across an ocean:) )

    I hope to hear more about your project. Especially when it is ready to sail. It may be very instructive to see how the sail away price of your version compares with that of the steel version (as for myself, I wouldn't bet either way;) )

    Bob
     
  8. Vega
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    Vega Senior Member

    You are right about the roll moment of inertia, I was thinking upside down, regarding the meaning of the concept. I was thinking in bigger roll, when what it means is bigger inertia to roll.

    About which boat is easily capsized I am not sure you are right, see this:

    “On the one hand, in a 'static' sense, more ballast lowers the center of gravity, and should therefore be beneficial. It is obvious that for sail carrying, yes more ballast is beneficial. For comfort though, it is not. For resistance to being rolled in actual dynamic conditions, it is not.
    A light weight vessel having a large concentration of ballast will have a much lesser 'roll moment of inertia' so will be much more easily put in motion and therefore will be more likely to experience large roll angles due to wave action.
    While the 'ballast ratio' may have some utility as a measure of seakindliness (i.e. more equals less), it is in fact quite meaningless as a measure of either stability or seakeeping ability. Why?
    We know nothing about stability without considering the distribution of weights ( the vessel's actual center of gravity), and the shape of the boat.”
    ROLL MOMENT: While the amount of displacement has an effect on a vessel's motion, it is the distribution of displacement that has the greatest effect on roll motions. We can improve comfort, and we can actually improve safety by increasing the "roll moment of inertia" of the vessel. This is accomplished by spreading out the various weights aboard rather than having them highly concentrated. This is very much counter to the usually assumed requirement for a specific 'ballast ratio' on sail boats, or that there must be a certain amount of ballast present for the safety of a power boat.
    For the most basic understanding of this, we can say without doubt that an object with its mass distributed toward the perimeter will have a higher resistance to changes in motion. Therefore, it will be more dynamically stable. This can be intuitively thought of as the ?gyroscope? effect. "

    http://www.kastenmarine.com/beam_vs_ballast.htm


    About this I think that there are few doubts. There are many studied cases of boats that were very easily rolled after having lost their rigs . The evidence from the Hobart race is clear on that point, one of the few where it was possible to take conclusions.

    "On boats, it has been well proven that the very distribution of weights athwartships and into the rig are in fact favorable to stability in a 'dynamic' environment. We have observed that boats that have been dismasted are much more likely to be rolled over. This is due to the lack of inertia (lesser roll moment of inertia) and the relative ease with which a heavy roll can be suddenly induced. This cannot be demonstrated by static analysis as one would normally expect, since the dismasted boat obviously has 'more' static stability without a mast - in the actual ocean though, it is the opposite."

    http://www.kastenmarine.com/beam_vs_ballast.htm

    About this you are absolutely correct. This is in fact a useless analysis about almost impossible boats, in particular the light one.

    I would propose a much more useful exercise. Let’s find two really good boats of this size, one heavy the other light, and lets have a look and compare their stability curves and roll inertia moment and try to see their overall safety regarding oceangoing cruising.

    I will see what I can find.

    Um abraço
     
  9. Guillermo
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    Guillermo Ingeniero Naval

    Maybe not so incomplete. Let's see:

    Transportable cruiser. (So beam has a limit)
    LOA=8.00m
    LWL=7.44m
    B=2.25m (Aha...!)
    DSPL=1270kg
    SA=43.8m2

    As B is a limiting factor, due to transportability, I think designer has gone to a relatively narrow boat for the size. Lwl/B = 3.31 is a clue, confirming this.
    SA/D is almost 38, which is too high, even for a high performance racer. This indicates this area is including an big overlaping genoa, instead of the 100% J, as it should be. Anyhow we can asume sail area is pretty high, probably (because of the sporty look at the image) in the range of racers, let's say 22.

    D/L ratio is 86,01, so very light. But most probably stated displacement is in lightship condition (RaggyThor...? :eek: ), so numbers above will be probably incorrect, as full loaded displacement with 2 to 4 people (max), fuel (25 lts for the outboard), sails, anchor, warps, provisons, etc, will most probably be in the range of 1600 - 1750 kg. Let's say 1675. D/L come now to 113,44 still in the band of light boats.

    With this displacement and the SA/D ratio of 22 mentioned before, we obtain a sail area (100% J) rather in the range of 31 m2 instead of the 43.8m2 stated

    Big sail area, light displacement and relatively small beam calls for a retractable keel (transportability) most probably with a bulb. This will make a nice enough AVS, probably over 115º as per a quick estimative. She has a nice GMo too, but not very high, due to the limited beam. So accelerations are probably not so unconfortable because of this. Capsize ratio is 1,19 with the corrected displacement, so good enough and supporting the thesis on stability exposed upwards.

    Engine power (outboard) is probably between 8 to 10 HP, as 6*HP/D varies from to 12.99 to 16.23 and we need more power than this absolutely for nothing. I'd bet for 8 HP, due to the light concept of the boat.

    So, as you can see, we may at least guess many things from those basic data. If I'm right, we may have now a quite close idea on the design concept; if I'm not right, at least we may reach the conclusion designer is using 'cheap' commercializing tricks. RaggyThor, what do you think? ;)

    And....what do you think on my lucubrations, Paulo?

    Cheers :D
     
  10. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    Guillermo, I think you have figured out a lot :)
    The sail area is very large, compare with Didi 26
    (http://www.mboats.no/Didi26/didi26.htm or
    http://www.dixdesign.com/26didi.htm)

    LOA 8.00m (26'3")
    LWL 7.55m (24'9")
    Beam 2.47m (8'1")
    Draft 0.5/1.7m (1'8"/5'7")

    Displ to DWL 1500kg (3306lb)
    Displ light 1000kg (2204lb)
    Ballast 435kg (959lb)
    Waterplane area 9.87sq.m (106sq.ft)

    Immersion rate 101kg/cm (565lb/inch)
    Wetted surface 13.8sq.m (149sq.ft)
    Sail area (main + jib) 31.9sq.m (343sq.ft)
    Sail Area/Wetted Surface 2.31

    Sail Area/Displ 24.6
    Displ/length 97
    Prismatic coef .54
    Block coef .40

    Fineness coef .66
    Righting Moment @ 30 deg 815kgm (5895ft.lb)
    Righting Moment @ 60 deg 848kgm (6134ft.lb)
    Righting Moment @ 90 deg 458kgm (3313ft.lb)

    I 9.08m (29'9")
    J 2.70m (8'10")
    P10.50m (34'5")
    E 3.90m (12'9")

    Mainsail (excl roach) 20.48sq.m (220sq.ft)
    Jib 11.46 sq.m (123sq.ft)
    Drifter 15.88sq.m (171sq.ft)
    Powering 10hp outboard
     
  11. Windvang
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    Windvang Yacht Designer

  12. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    Wow :)
    The T8 has more draft and more ballast, I suppose you also need the crew...
     
  13. Vega
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    Vega Senior Member

    hum, I doubt about the retractable keel, I bet it has a bulb, and I think it will have a draft of at least 1,8 m. The Avs is probably between 120 and 125 and will not carry more than about 90l of water, and about 50 liters of fuel.

    But that are only guesses.:D

    What I don’t understand is why you want to compare this boat with the Vancouver 28.
    One is an ocean going boat, long range cruiser, the other is a fun and transportable coastal water protected craft; a weekend cruiser. What’s the point?:confused:

    Comparing the Vancouver with another real cruiser, not a weekender, other class A boat of completely different design, like the Dehler 29, would seem more fit and useful to me.

    http://www.dehler.com/

    But that’s your thread ;)
     

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  14. Windvang
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    Windvang Yacht Designer

    In that category I nominate the Hunter Channel 27 and Pilot 27. They sail perfectly well, are seaworthy and quite fast. I actually have won some races with the 27 twin keel version! :) The interior space is amazing for their size. O.k. their looks might not be their biggest plus. :rolleyes:
     

  15. Guillermo
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    Guillermo Ingeniero Naval

    Neither do I....:p
    No, seriously, the idea is to investigate small cruisers' design criteria, and I don't want to tight to my personal opinions (I'm in the line of Vancouver 28 and the like), but would like to hear other people supporting other design concepts, because lightness may have its advantages, as been able to run out of trouble quickly, reason many times used to favour fast passagemakers rather than seaworthy ones, i.e.... That's why I introduced this boat.

    Explain to me how it can be trailerable with a 1.8 m draught....

    I've refined numbers: AVS something like 128 or over. 90 l water seem somewhat high to me. Probably water is carried in portable plastic containers or bottles. Also fuel is all it can enter a portable can of 25 l...;)

    Following Raggi Thor's post, yes I agree: probably SA/D is more something like 24. I figured out things without consulting other designs, just imagining....And comparing with Dix's design, I was close, wasn't I?
     
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