sailboat that planes really easily while heeled?

Discussion in 'Sailboats' started by rnlock, Nov 17, 2019.

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  1. rnlock
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    rnlock Junior Member

    Preferably plywood* and with free plans, or at least lines.

    I am thinking about making a Footy RC sailboat. Seems like all the Footy's I've seen have really high displacement to length ratios which I'm sure can't get over their own bow wave. With the light RC gear we have these days, maybe I can make one which can get past hull speed, which is very low for a Footy. There's no movable ballast, although there is a very deep keel, so the boat will have to heel some to get righting moment. I'm not sure, but I think I can get a Footy down to a displacement to length ratio of 70 or 80. I've been looking at Classic Moth designs, but maybe there's something better. I've been considering the Mistral and the Bilbon:
    Earwigoagin: Classic Moth plans http://earwigoagin.blogspot.com/2010/03/classic-moth-plans.html

    I was also considering Mini Transat designs, particularly the scow types, though since those are curvy, they'd be more work. Or I might consider the Europe dinghy. But at this stage it's probably better to keep things simple.



    *So that it can be built easily from flat sheets of material.
    Bilbon_mothLines supposedtobefast.jpg mini transat scow canting keel.jpg mistral moth hull under constructionIMG_6087.jpg y2kbug sections for mistral moth.jpg
     
  2. rnlock
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    rnlock Junior Member

    P.S. Thanks in advance to all who reply constructively. Also, please don't take this too seriously unless or until I've buit a prototype. Also, I should confess that I've been considering a reduced displacement PD Racer!
    If it's not obvious, the plan is to scale down whatever the choice is so the hull fits in the Footy box, which is 12 inches long, 6 inches wide, and 12 inches deep.
     
  3. messabout
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    messabout Senior Member

    It is going to be quite a feat to persuade a Footy to plane when heeled. I expect that you might get it to plane when flat or nearly so. All that is OK to contemplate but the bulb and keel will cause so much drag that planing is ....Iffy, at best.

    The Footy rules will not let you make the boat wide enough to generate enough RM without the bulb or other pendulous weight. The old time Cates Moth had a set of section lines similar to the ones at the end of your post. It was the way to go at the time but better designs eventually bested the Cates. That design did go well when sailed perfectly flat. Not easy for the skipper to do but doable for the athletic sailor who was also good on a tight rope.

    The extreme vee would present a broad, possibly a potential planing surface, when heeled a long way over. If heeled that far over then the wetted surface would increase a lot and the sail will have surrendered much of it's potential drive.

    Not to be a naysayer........... But..................that will be a tough hurdle to overcome. I will be most interested in your design and the result. I am also an RC modeler with some experience. I have threatened to build a Footy. So far I have been into EC12s, Marbleheads, and some non descript ones. We do have a few other RC modelers on the forum who may add to the conversation.
     
  4. rnlock
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    rnlock Junior Member

    The thing that gets me is that just about all footies are quite heavy for their length and are trapped below hull speed. Sometimes I think planing isn't necessary if the displacement to length ratio is low enough. So I waffle between a planing hull, a catamaran, and a miniaturized US1M. If I was energetic enough, I could try all three.
     
  5. gggGuest
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    gggGuest ...

    Classic Moths and the like don't really plane well when heeled, so I don't think they constitute a great model for you. I struggle to think what would be a good model though: the Mini transat types rely on the brute force of large quantities of asymmetric ballast.

    I think the big challenge in your design is going to be considering the water flow round and under the hull when heeled relative to the foils. Its well documented that modern wedge shaped planing boats don't behave at all well heeled. My gut feeling is you will need to be producing some kind of scow like shape. For what little my expertise is worth I would try drawing a straightforward flat bottomed planing hull, angle it through whatever your target angle of heel is, then mirror image it, and see if its possible to fair in the two without compromising too much. The other interesting consideration is getting it to sit at your design angle of heel, since many planing boats are very heel sensitive.
     
  6. tlouth7
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    tlouth7 Senior Member

    Have a look at the Fireball dinghy. It has large flat areas of hull at non-zero deadrise which are used as planing surfaces when sailing.
     
  7. messabout
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    messabout Senior Member

    I am with gggGuest. Make a simple flat bottomed skiff with plumb sides so that you have maximum bottom width. The extra righting moment of that kind of boat will let you use less lead for the bulb. In that case the displacement length ratio will be a little better. Such a hull will have more wetted surface but will be much more capable of approaching planing velocity. In addition to that, the flattie is dead simple to build.
     
  8. rnlock
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    rnlock Junior Member

    Do flat bottomed sniffs still pane ok when heeled?
     
  9. messabout
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    messabout Senior Member

    Yes if not heeled to extreme but probably not. Do some calculations to determine the lift of a planing surface. The lift from dynamic pressure has to exceed the weight of the boat by some factor or other or at least mitigate the displacement of the boat.

    It is fun to contemplate the velocity at which the lift pressure is equal or nearly equal to the weight of the boat. The appropriate equation is something like this.........rho x velocity squared x planing area x the tangent of the planing angle all divided by two. Rho is the density of water divided by slug constant. You may keep in mind that the dynamic pressure on the bottom diminishes progressively as the pressure gradient moves aft toward the transom. The configuration of the after run angle is a significant factor. In general the quarter buttock angle can not be less than about four degrees. Not an easy design characteristic for a boat that has a short waterline and a limitation on total displacement.

    My gut tells me that you will never achieve sufficient velocity with a Footy to get it on a plane, heeled or otherwise. The lift of a planing surface is a function of the square of the velocity. The function can be examined by plotting a Cartesian parabola.

    Sorry to be such a wet blanket here...........Keep thinking, maybe you will find a way.
     
  10. rnlock
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    rnlock Junior Member

    Certainly a footy that's anywhere near as heavy as a conventional footy won't plane. However, I'm convinced that one can be made far lighter. Maybe even 70 grams. With the dynamic pressure* at footy hull speed, that's something like 8.5 square inches. I'm sure that a real planing hull will need more than that, maybe twice, but it's still a lot less than the hull surface area of most footies. Let's say you shrunk a 7 lb US1M model to a foot long, keeping all the materials, etc. the same. It should now weigh 77 grams, and it will be massively overbuilt. Realistically, I think the radio will weigh proportionally more, maybe 10 0r 12 grams. But the other materials can be a little thinner. Righting moment will have to come from the keel (fin and bulb to rc sailing guys). Hoping that the ballast ratio can be much higher than for a Mini Transat, making up for the inability to cant it.

    *Everyone will hate the units here, but, unless I've screwed up, the hull speed will be around 1.3 knots, and the dynamic pressure, will be about 8.5 grams per square inch. My mind has been destroyed by being forced to work, too many times, with two different systems!
     
  11. OzFred
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    OzFred Senior Member

    While you might be able to build a Footy hull for 70 g or less, making a whole boat that weighs just 70 g is a practical impossibility unless you know of electronic components that are a tiny fraction of the weight of those available from traditional RC component suppliers. A rudder servo is at least 11 g and sail servo at least 60 g. The smallest practical battery pack weighs over 100 g (NiMH 4.8volt 2000 mAh). Add to that ballast for the bulb plus rig and the boat will weigh well over 250 g.

    How does that affect your calculations?
     
  12. messabout
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    messabout Senior Member

    Why work with two different systems? When I calculate the relative weights of a US1M with the footy, I get about 90 grams. The scale factor is 39.375/12 = 3.28 Cube that number and you get a divisor of 35.3.........7 pounds divided by 35.3 = 0.198 pounds or 3.17 ounces......89.8 grams if you prefer.

    Can you actually build a footy that weighs only about 90 grams? We need some batteries. Probably at least four triple As. That weight alone comes to 46 grams. Unless the sail is tiny, you will need some ballast. Probably at least another 90 grams. Then there is the weight of the hull, the rig, the servos, the Rx. The sail control servo will need some strength so a micro servo may not be powerful enough. You end up with something like the common Futaba S3004.... 38 grams not including the arm. Add all that together and there is no way, that I can envision, to have a 70 gram Footy..........300 grams maybe.....10.5 ounces.....0.66 pounds.

    The lift equation, rho x vee square divided by two , must also account for and include the angle at which the lift force is applied. The angle of incidence, angle of attack or however you wish to identify it. A planing angle of five degrees might be a good guess for a medium velocity planing angle. The tangent of five degrees , (0.0874) becomes a factor in the calculation. Draw a vector diagram to see why this is so. The dynamic lift then becomes a lot smaller that we wish it to be.

    I apologize if it seems to you that I am implying that: you ain't likely to get a Footy to plane short of hurricane force winds. That is not to say that you should not try. What the heck, we used to believe that the world is flat and now we can get people to the moon.
     

  13. rnlock
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    rnlock Junior Member

    If I was forced to use AAA batteries and standard sized servos, it would be hopeless. However, I found a busted flying model on 2.4 gHz that weighed 43 grams all up, including batteries and motor. The receiver, which has built in servos, and battery, obviously, weigh much less. I understand that people are using 9 gram servos for the winch on other footies, but in this case, for a MUCH lighter model, I'm sure I could go with much less. Particularly if I used a vane of some sort to control the sail. Thinking on a different scale is required.
    For reference, I once built a rubber powered EZB, a model airplane with an 18 inch span, at 0.6 grams. To be competitive at the highest level, it ought to have weighed less than 0.4 grams.
     
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