28' sail cat

Discussion in 'Multihulls' started by mihari, Sep 4, 2009.

  1. mihari
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    mihari Junior Member

    (can somebody tell me how the multiquote button works?)

    The pop top sounds like a great idea. Its max height would still be at the boom, and the top wouldn't help as a structural element to tie the boat together. But, I think it is a great idea. (I was aware of that pop down bottom, I didn't know it was Woods'). I will look into that later on.

    The .34m2 cross section is the actual cross section of this particular sketch if the hull is immersed 40 cm under surface. It is not calculated that this is the needed depth of immersion of this particular design. I have done a couple of other models though for the same hull length, and had found that the needed draft was a bit less than 40 cm. This being wider than the previous hulls will probably need less draft at mid ship. So, it is just an indication of where waterline might be (could be off by 15 cm). It is just that I know that I want bridge deck clearance to be at least 50 cm.

    Also, both inner and outer flares are at the point where the hull meets the flat water if the boat is geometrically tilted around where the central axis of symmetry meets the water, to where the opposite hull is barely touching flat water surface. Maybe this is a bit difficult for me to explain in words, so I will try a sketch.
     

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  2. boat fan
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    boat fan Senior Member

    [
    If you look through the thread , you can hit the multiquote button , ( it will be highlighted now ),
    go to another message in the thread and again hit multiquote etc then
    press reply / go advanced /...and you will see the messages you selected
    pasted in your reply window in the order you selected them.

    To remove some or all messages you selected just hit the multiquote button and it will no longer be highlighted so will not appear in your reply window.
     
  3. boat fan
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    boat fan Senior Member

    No , true , it would not help to strengthen the bridgedeck structure.
    However , if you engineer the boat and cross beam to function like any open bridgedeck cat it becomes a non issue.
    You will find most saloon roofs do not do a lot for strength anyway.They need to be lightweight and are inherently relatively flat .
    The main forward beam needs to do the work , in particular it has to handle the substantial downward thrust of the mast and rig trying to push its way through the bridgedeck.
    ( assuming a bermudan fractional sloop rig now ) that rig seems the simplest with relatively good performance ...that`s a whole other issue .......Making those assumptions ,
    your saloon will be there just to keep the elements out.

    Even a tent would do that if it doesn`t get blown away :D

    Are you using freeship ?
     
  4. mihari
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    mihari Junior Member

    Not atm, but will soon probably.

    These are the results I got from that excellent dimensioning tool by terhohalme. Note that it is a bit longer than 28' (9.10m Lwl).
     

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  5. boat fan
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    boat fan Senior Member

    I noticed on the spread sheet thread by terhohalme you had some questions.I`m not a designer , so I cannot give the answers to some of your questions without doing a lot of work beyond the scope of time available.

    Some of the questions below area little easier to answer :


    1 .
    Height of lateral area: what is it? I saw in your "manual" that it is below waterline, but to where?

    -------

    CENTRAL LATERAL PLANE
    The immersed longitudinal vertical middle plane of a vessel.

    CENTER OF LATERAL RESISTANCE
    The point through which a single force could act and produce an effort equal to the lateral resistance of the vessel. It is ordinarily assumed to coincide with the center of gravity of the immersed central longitudinal planes. But not always.


    All hulls have an underwater profile. The area of that profile is calculated and its center is plotted .This is your CLR or CENTRE OF LATERAL RESISTANCE.It is below the waterline because only the underwater area is calculated.Sometimes expressed as a negative number , if the DATUM line is set at Zero , at the designed waterline (DWL)

    A "rough and ready" way to determine CLR is to lift the shape off your profile drawing , cut it out on card stock and balance the shape on a knife edge. Mark the vertical line at which the profile balances .If you place a pin near the centre along this line you will locate the theoretical point at which the shape will balance.The rudder is usually included.(Some designers only include half the area of the rudder ?...)
    You now have your CLR.

    Software like freeship will do this better for you of course, but you get the idea......


    -------



    2 .Height of fore triangle - the height of center of effort from waterline?
    Is it for fractional rigging?
    Height of mainsail - the height of center of effort from waterline?

    --------


    Theory is similar to CLR that is , the concept of a centre of area, but
    this time as applied to sail area.Applies to ALL sail plans , weather fractional sloop or gaff or lateen ...whatever ...


    http://www.google.com.au/url?sa=t&s...QJ--zuqDrFhTfQtUw&sig2=fkUGJeSiAxE14iI9tNrQ5A

    http://www.google.com.au/url?sa=t&s...EhTWK7RS7MqI90jxQ&sig2=NgLa1nLCDkG1MTTvmzvg_w




    The effect of the relationship between CLR and CE of the sails are briefly discussed here :



    http://www.google.com.au/url?sa=t&s...SQO9nzg-bwXyYMp1A&sig2=FXYYuB9cqAshjwb8g6m-gA


    3...Design righting moment - how does this help me?

    Essential design consideration.

    There are a number of reasons for depressing the leeward bows of multihull craft. The ratio of the lateral stability against the longitudinal stability has an effect on the pressure that is applied to the leeward bow.
    [SIZE=+1]
    Boats with greater lateral stability will be able to sustain greater sail forces before tipping over sideways, therefore allowing more driving force (and counterbalacing resistance) to be transmitted into the boat and thus resulting in a greater pitching moment depressing the bow.


    This is a VERY basic theoretical comment.


    4. Limiting righting moment - how does this help me?

    ---------


    This has far reaching effect on stability , motion , pitching , and more.

    Limiting righting moment reduces the stress on your rig.


    At a very basic level , it works like this :

    For multihulls , the wider the boat , the more INITIAL static stability .
    Or it has a large righting moment. Naturally if you make the boat wider , the more it will be resist lifting the windward hull' and capsize.

    As a result the boat has a huge righting moment.Some times referred to as being a " STIFF " boat . ( Low righting moment referred to as TENDER.)

    Wind pressure on the sails acts to try and tip the boat over , ( overcoming its righting moment ), and also driving the boat forward.

    [/SIZE][SIZE=+1]As a boat heels some of the forces generated by the wind acting on the rig are[/SIZE][SIZE=+1] transferred to immerse the hull.This reduces the stress on the rig.

    The overly stiff boat develops larger stresses on its rig because it remains upright and does not depress its leeward hull as much as a more tender boat.The rig has to withstand much more force to remain upright.

    LARGER RIGHTING MOMENT REQUIRES A STRONGER RIG.

    A modern multihull is subjected to large vector forces . These are complex and non static as the boat sails , particularly in variable wind / sea conditions.
    [/SIZE][SIZE=+1]

    Hope this helps:)
    [/SIZE]
     
  6. catsketcher
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    catsketcher Senior Member

    a few comments

    Gday all

    A few quick notes

    I got it wrong when I said the flared hull shape was hard to get right with a "high immersion rate" I should have said "low Immersion rate" A high immersion rate will sink slowly.

    Pop tops - I have only seen them on one cat - the Crowther inspired (flop moulded and copied I think) Osprey 32. It was not a success. I tried a semi pop top (hinged at front and up and down at back) on my cat and hated it. I just put a new cabin on it. A 28 footer will not have standing headroom. Get clever - Stand up behind the cabin and put a roof over the cockpit. See if you can incorporate this for hull to hull travel. ie don't use a pop top but a removable roof covered by the cockpit dodger going forward over the cabin. You only need standing headroom in the middle. After a few head hits you will be fine stooping to enter and exit the hulls and all you need is one spot you can unwind.

    Longitudinal stability and athwartships stability. Do not think that you gain more stability going wider only. The reason cats are not square is that if you make a rig that flies a hull - going to windward or on a reach- on a very wide cat it will nosedive very easily downwind. This is doubly bad because you can ease off sheets to windward or on a reach but doing so on a run is useless. You should have much more fore and aft stability than sideways but this is tricky. If you put a large fordeck on a bow it will trip the boat better than if it had a smaller foredeck (and less fore and aft stability)

    cheers

    Phil Thompson
     
  7. boat fan
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    boat fan Senior Member


    Agreed .

    I woud be a little more conservative regarding the rig.No need to tempt fate and fly a hull on a cruising boat ....:D

    If I was building a 28 ft cat , I would put 36 ft hulls on it anyway. Length is cheap . And fast.



    As to the pop top on the saloon , I know the girls like that saloon ,I think it could be done , but I would opt for a hard top myself , with drop down clears. Less windage lighter and airy. Terhohalme`s boat comes close to what I would have.We are blessed with our weather here in OZ , this boat would suit my cruising grounds. ( far right ) . Nice .
     

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  8. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    Phil,
    What do you mean by a "large" or a "small" foredeck? Would you help me understand how the size of a foredeck affects a cat's propensity to trip.
    Thanks,
    Imaginary Number
     
  9. boat fan
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    boat fan Senior Member

  10. catsketcher
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    catsketcher Senior Member

    pitchpoling

    A multihull pitchpoles (capsizes over its bows) when the force from the rig going forward is greater than the drag created by the hulls. Of course at a constant speed both drag and rig force are equal.

    If you push the bow of a hull under water then the drag increases greatly usually at the same time as the rig force forwards increases too. This will produce a torque unless the hull is free to accelerate.

    If you have a large flat wide fordeck under water then the boat can't accelerate (in fact it will decelerate) and you will have the recipe for a pitch pole. This is why small cats like the A class have very fine bow decks. Racing trimarans have floats that can go under water with little increase in drag.

    I once asked Nigel Irens how much stability a multi should have fore and aft relative to sideways stability. He said because of the drag question it is hard to give a quick answer. I think the best way is to think- Will this bow ever go under water? If it will then make the deck rounded and small. If it won't then you can have flare and a flatter deck.

    I was able to demonstrate this phenomena when sailing my 31 ft Crowther tri. I could stick the bow in a wave when sailing at 15 knots plus and feel no difference. Nice thin float with rounded deck.

    cheers

    Phil
     
  11. boat fan
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    boat fan Senior Member



    The Alinghi hulls look very fine; is there enough forward buoyancy?


    Nigel Irens: They are fine and in multihulls it’s always been a bit of a cleft stick. The layman’s view is that you must have lots of buoyancy to stop the boat capsizing forward, but to have that buoyancy you have to make the waterline a bit blunt and the result is that you get more drag than you get lift, so you actually cause the boat to trip over. That was the lesson learned from the mistakes of the '70s and '80’s though, interestingly, the Tornado – a most successful boat - was designed in the '60s and the man who designed it, Rodney March, realised then that you don’t create the capsizing force if the bow is fine, so you don’t need the buoyancy.


    The BOR boat seems to be a lot longer than it was and a lot longer than Alinghi. How much of an advantage is that going to be?

    Nigel Irens: I can’t say how much, but - apart from the obvious advantage that, in basic terms, longer waterline means faster boat - there is some advantage in that a longer boat is less sensitive to longitudinal trim issues. Clearly, if the sail plan is trying to force the boat fast in one direction the limiting factor is when it wants to cartwheel forward and a boat that is longer is going to have a bit more resistance.


    Oracle has now gone to wave piercing bows, can you explain the advantage?

    Nigel Irens: Well, I think the bows it had before could be thought of as more suitable for offshore sailing, where there are big waves. As I’ve said, you must keep the bows very fine, in order to reduce the tripping forces, but having sufficient buoyant force there is important. So, in recent years, people have elected to keep the entry fine but make it taller, so that if you are tearing along in the ocean and go into the back of a wave, you haven’t slowed down too much but you have considerable correcting force. In flatter water that is not quite the same and, if you put a big area of hull forward, there is quite a lot of structure involved, so quite a lot of weight and the bigger it is the more of a target it is to waves coming from the side. So, by making it a wave piercing bow, you reduce its side profile quite a lot and therefore the windage and the structural loadings. I’d say they’ve changed from a classic offshore bow to something that is more associated with inshore sailing.
     

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  12. boat fan
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    boat fan Senior Member

    Pitchpoling......

    Common sense does a lot :

     
  13. catsketcher
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    catsketcher Senior Member

    pitchpoling can occur on a cruising boat

    Thanks for the Irens quotes - I am a huge fan. I once bought him a beer - big highlight in my life. I got to talk to him when writing an article on a big tri built in Gosford.

    Pitchpoling can occur on flat water. Lots of small cats do this - tearing along and then the bow dips and over you go.

    It can also occur on cruising cats. A Lightwave 10.5 pitchpoled on the Wide Bay bar a few years ago. It surfed a wave which is normal on a bar. In fact it usually is safer to surf a cat than to try and slow it down. The problems may be that the Lightwave has a very uneven bouyancy distribution - it has very fat sterns and little bows. In a wave these powerful sterns can overpower the less bouyant bows and dig the bows in

    I prefer a boat to have more balanced bouyancy distribution but then I was not asked to draw a boat that had lots of room in the ends of the hulls like the designer was.

    cheers

    Phil
     
  14. boat fan
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    boat fan Senior Member


    Yes I like Irens too...seems everything he does is quality.

    I stuffed a Nacra 5.8 in relatively flat water years ago :D

    If the seas are really short and steep , surfing is not that smart . If your boat accelerates too much ..........it all depends on the conditions and the boat really.

    I agree with you on the bouyancy distribution thing too , that`s why I would put longer hulls on a " small " cat ...leave the ends empty....just water tight bulkheads.

    Oram likes small cats on longer hulls for the same reason. I looked at the first Mango he drew years ago , only small boat, and he told me to go longer ( only ) on the hulls . All makes sense to me.
     

  15. mihari
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    mihari Junior Member

    Sorry it took me so long to respond... just needed some time to digest all the information... and do a little of research.

    Boat fan, you are coming through for me again. Thank you for helping an ignorant, boat designer wannabe, to become less ignorant. I received a lot of attitude by posting those questions in terhohalme's thread. I hadn't realized that my questions can be that offending. Wouldn't it have been easier for everybody not to have responded to my questions?... Well anyway. Retrieving back to my little cave of darkness...

    Apparently I have a huge gap with marine design terminology. For example: the "Height of lateral area" seems very vague to me. Then "LATERAL PLANE" and finally "CENTER OF LATERAL RESISTANCE" are more self explanatory. Its just so hard to make the connection if you don't know the terminology. That is all I asked for in the first place. Thanx again boat fan.

    Phil, thank you for posting that about hull deck form. A lot to think about.

    Pitchpolling is a condition that I think I will never have to face. Mediterranean seas are a lot calmer than the oceans... though less predictable. Pitchpoling can occur only in quite extreme conditions and being a leisure sailor, and not a racer, I will probably never need to worry about that. However, a boat that can deal with pitchpoling better than another, has better overall stability in diverse conditions, thus making it a safer boat.

    Moving along with the design, since only 2 doubles and a single bed is what I was aiming for, at the length of 28' (to 30' max), there is plenty of space for buoyancy/watertight compartments. According to terhohalme's spreadsheet, the full load displacement is calculated to 2634kg (I am fine with this), empty boat to 1846kg (that seems too heavy unless I am making a terrible mistake in my rough caclulations). With a rough estimate on all the weights apart from bare boat, I came up with 1300kg, and that includes dinghy, sailors, rigging, provisions, batteries, water... everything I could think of. So unless "empty boat" has some extra weight other than bare boat mass, I have to start looking at a different hull design. I was hoping that bare boat was about 1300 - 1450 kg.
     
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