Stem and rocker design on slender hulls

Discussion in 'Multihulls' started by Will Fraser, Sep 8, 2015.

  1. Will Fraser
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    Will Fraser Senior Member

    I would like to learn about the trade-offs and design considerations involved as applied to plumb-bowed hulls typical on small trimarans i.e anything between, say 10' and 17' - or thereabouts...

    At these short lengths, while keeping the waterline beam limited, getting the necessary displacement seems to require quite a bit of rocker - that is, unless the stem is submerged quite a bit.

    Seaclipper, Littletri, Kurt Hughes' 12ft cartopper and a host of others seem to prefer the rocker option with the stem only just touching the water, while I see Mike Waters' W17 has a deeper stem and less rocker.

    I tried to compare the different designs by using the Michlet program to get a drag estimate. The deep stem, low rocker showed a good deal less drag at speed-length ratios of 2-2.5 while only giving slightly more drag at sub-hullspeed. A pointy, submerged stern also showed less drag at high speed than the popular squared-off transom meeting a swept-up bottom panel at the waterline.
    I left the trim and lift set to zero since I have no idea what would be reasonable values or how they would vary with speed. Advice on this would be appreciated.

    The only theoretical verification that I know of which agrees with my Michlet results is that the submerged stem design has a higher prismatic coefficient which is generally agreed to be more appropriate for semi-displacement speeds.

    Are my assumptions and results even vaguely on target, and if so, what other aspects of design would be responsible for the popularity of rocker and up-swept ends?
     
  2. Doug Lord
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    Doug Lord Flight Ready

    With a submerged stem and/or transom you will probably have reduced maneuverability because they will drag as the boat is turned. Small boats like to turn quickly so its something to consider.
    Another consideration is using a wider hull IF the power the boat will have is enough to get the main hull to plane and/or fly the main hull in light air.
    Good Luck!
    My test model uses a relatively wide planing hull as did the 20 footer I designed and built in 1971. Also designed and built a 14 using the same concept. The other two boats below Banshee Ambulance and Bethwaites HSP both use planing main hulls:

    Pictures,L to R-1) Banshee Ambulance using a Cherub main hull, 2) Bethwaites HSP, 3)& 4) Fire Arrow test model, 5)& 6) my 20' tri:
     

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  3. Will Fraser
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    Will Fraser Senior Member

    Thanks for the examples Doug.

    The Italian 10ft "development" class, Diecipiedi, has a nice variety of designs with the Uffa 10 trimaran also employing the planing vaka. The gentlemanly, laid back nature of the typical participant and aversion for taking a dunk however means it will most likely never be sailed in conditions that supplies the necessary power to get the vaka planing. It is therefore also not surprising that most other trimaran entrants (it is open to cats and mono's as well) prefer the deep, slender hull design. This includes the current record holder, Slim, at 9.6kts.

    It is based on these figures of displacement, length and speed that I modelled the hulls in Michlet. The argument for rocker on the vaka makes sense. I see your tri also has significant rocker on the amas - was that also a design choice based on manoeuvring?
     
  4. Doug Lord
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    Doug Lord Flight Ready

    ================
    No, at that time I was concerned with the main hull planing-not flying the main hull. And my thinking on the ama was to keep it from tripping while handling waves well. That was 1971 and I've learned a lot since then. My test model illustrates the type of (two stage) ama I think is best for small tri's these days. Whether it's a full foiler or not I think a planing ama with foil assist is a good way to go. And I like having a wand controlled* daggerboard foil to allow the main hull to fly before it would due solely to wind pressure. The main foil allows a wide tri to fly the main hull in light air while having much greater pitch control than a "normal" tri. The rudder T foil and main foil work together controlling pitch and the ride angle of the ama/ama foil.
    * it's possible that a surface piercing main T-foil could work instead of a wand controlled foil

    Concept model of a 12 footer with wand controlled main foil and rudder T foil with planing amas(same basic concept as the larger Fire Arrow) :

    click for larger view:
     

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  5. Doug Lord
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    Doug Lord Flight Ready

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  6. Will Fraser
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    Will Fraser Senior Member

    Dough, while I do have an interest in trimarans and their potential as a platform for extreme performance, foiling or otherwise, my other (and more immediate) design interest lies in applications that get "good" speed with very little sail area.

    It is, in fact, the sail and rig design that is of most interest to me. I have an overly curious mind and more "what-if" ideas floating around in my head than could be considered healthy. I think you can relate.
    I would like to build and try out a few of my designs to verify some assumptions, calculations and simulations. The design-build-test-tinker-test-repeat will be an end in itself.
    Since I have no actual speed goal in mind, I will most likely keep hull length somewhere between 10' - 12' and sail area somewhere between 40-60sqft. This should keep costs down due to limited load and material requirements. Given these constraints and my 220lb, a slender hull seems to be the best way of softening the drag rise at hull-speed and will hopefully make it easier to detect small changes in rig performance.

    There is one very particular aspect of a wide, flat transom that I like a lot, especially when being pushed beyond hull-speed, and that is the sensation of speed created by the wake. I don't care if it is truly planing or not, for a day-sailor just out to have fun, it would certainly be up there among other important requirements!
    And while on the topic of blunt transoms and planing - is there a drag benefit in reducing LWL by means of bow-up trim as seen on a number of fast skiffs and even keel boats that ride with their bows right out of the water on fast runs and reaches? Or is that more a control-driven requirement?
     
  7. Doug Lord
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    Doug Lord Flight Ready

    Flat planing/"humpless" planing hulls

    I wouldn't think it is a control driven requirement-it's a natural result of dynamic lift. However, you might be interested in Frank Bethwaites "High Performance Sailing" where he discusses "humpless" planing hull design.He called his hull shape "flat planing". Many planing type boats sort of squat stern down , bow high as they approach the hump in the transition from displacement sailing to planing. Bethwaite says his design doesn't do that.

    PS-forgot to mention Bethwaites second book "Higher Performance Sailing"-there is a lot more on his "humpless" planing hull designs.........
     
  8. ThomD
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    ThomD Senior Member

    I don't think you are really seeing designers manipulate rocker, they are manipulating displacement which on a small boat may require you to build relatively stout hull that in profile view will look like a boat with a lot or rocker. But unlike a white water playboat, the resultant rocker is something you have to contend with not a design objective for manoeuvrability.

    Basically I start with a hull that is normally about half beam in draft, the bow immersion is 40% ish, and the transom is just kissing the water. That would be the normal operating displacement with the capacity to get displacement for a cruising load or whatever.

    Starting with a length is the wrong way around. You need to start with a service and then come up with the best envelope for that service. Of course if you are planing on racing in a length class you do have to take that into account, but it may mean that you end up with a bad boat. Getting as much information about the weight of everything before you start, the rig for instance, will reduce the guesswork one might otherwise end up suffering for.
     
  9. daiquiri
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    daiquiri Engineering and Design


  10. Leo Lazauskas
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    Leo Lazauskas Senior Member

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