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Author Topic: Building a (near)Footy sailing dinghy - Keel advice needed  (Read 3096 times)

Edward Pinniger

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Building a (near)Footy sailing dinghy - Keel advice needed
« on: November 14, 2011, 02:21:49 PM »

I spotted this 13" clinker-built dinghy hull on the Mastman stand (http://www.mastman.co.uk/) at the Warwick show and thought it might make a good starting point for a sailing dinghy model! It's vacformed plastic, but with a deck and reinforcing bulkheads should be sturdy enough to sail (a bit too big for a Footy alas). I'm thinking of something along the lines of a "National 12" dinghy (I have some good closeup photos of a real one for reference). The hull will be decked over, with a well/cockpit in the middle, the floor of which can be made removable to access the radio gear.

To form the "false keel" of the hull and attachment point for the fin keel, I'm planning to make a T-shaped piece of heavy-gauge plywood - the horizontal part of the T will fit inside the moulded keel of the hull, strengthening it as well as providing a surface to attach the radio, servo and battery mounts to, and the vertical part (probably a couple of inches high) will fit through a slot which I'll cut in the middle of the hull's keel - the removable fin/bulb keel will be bolted to this.

I haven't worked out the exact rigging plan, mast height and sail area yet, but it will be about 20-24" high from keel (not including fin) to mast top, and gaff rigged. Weight in the hull will be limited to that of the RX, two servos and 4.8v ni-cad pack. The rudder will probably be externally mounted (like the real boat) and controlled via a working tiller rigged to the rudder servo.

What would the (approximate) optimum keel length and bulb weight be for a model of these dimensions? Any advice (on the keel and any other aspects of the model) would be appreciated!
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dreadnought72

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Re: Building a (near)Footy sailing dinghy - Keel advice needed
« Reply #1 on: November 14, 2011, 05:54:08 PM »

You asked for advice?  :-))

First thing I'd do would be to mark the desired waterline using a soft pencil at the bow, stern, port and starboard.

Float the hull in the kitchen sink and add blobs of plasticine wherever it's required to get it floating at the correct depth.

Using the very same soft pencil, push the hull gently sideways at various points along the gunwale until you can get it moving sideways without turning. Explain to the wife or significant other that you're not playing about, but conducting a series of essential hydrodynamic tests.

This point on the gunwale is the hull's centre of lateral resistance. You don't need to be too scientific here, as this point may well move fore or aft a little when the hull's heeled.

Take out the boat, and balance it on its keel on the now-dripping-wet soft pencil. This point is the hull's centre of gravity when loaded.

Splitting the difference between the CLR and COG, along with squinting a bit, should give you a good guide to where the fin is best placed. Mark this location with the back-up soft pencil..

Now scoop out the plasticine. The mass of the plasticine is the displacement of this hull. If this mass is less than the mass of the radio gear, things are looking good. The difference between the displacement and the radio gear is, of course, "everything else". Being a small model, I'd be aiming for maybe half of this difference (assuming there is one!) being used for a ballast bulb.

I have a question about the chosen rig: gaffers are, undeniably, the second best looking rigs for boats, but it makes for a lot of mass up top. Bermudan rigs are dull, gunter little better, but for a hull like this I'd be sorely tempted to go down the (jibless) lugsail route. A lowish, squarish, inefficientish sail that would suit the hull and not overpower it.

Best wishes with a fascinating project!

Andy



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Edward Pinniger

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Re: Building a (near)Footy sailing dinghy - Keel advice needed
« Reply #2 on: November 14, 2011, 07:02:05 PM »

Thanks for the advice. I didn't think of the method you suggested but it makes a lot of sense - work out the weight needed to get the hull down to the waterline, subtract the weight of the radio gear, mast, keel fin and other woodwork, and put the remaining weight in the keel bulb. I'll attempt to work out the centre of lateral resistance, but if all else fails will put the keel dead centre and hope for the best!

I agree about gaff rigs adding a lot of topweight, but I had the impression lugsails were difficult to easily R/C - I may end up going with a bermuda rig as that's what the National 12s usually have (even the earliest ones as far as I know).
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tobyker

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Re: Building a (near)Footy sailing dinghy - Keel advice needed
« Reply #3 on: November 14, 2011, 11:31:51 PM »

Don't you need to bring the CLR nearer the C of Effort/pressure (not sure which is the correct term) of the sail? The lead and r/c gear must balance the hull fore and aft, but the fin must balance the sideways push of the sail, so the boat neither points up or falls away. In fact I think to balance properly the clr needs to be a tad forward of the C of E, because a little bit of pointing up can be balanced by the rudder, but a keel too far aft is poison. And of course, the CLR will vary according to the rig - the CofE of a gaff rig is further aft than that of a bermudian. If in doubt make the mast adjustable fore and aft, or allow it to rake a bit, as that's easier than making an adjustable keel! Do not assume that the present CLR of the hull is anywhere near where you will need it for sailing.

I had an interesting stroll down by the marina a couple of days ago looking at the keel/lead configurations of some of the yachts out of the water - some keels have the lead "torpedoes" trailing aft of the fin, but a surprising number had the fin going down into the middle of the "torpedo". In most cases the fwd leading edge of the fin was more or less in line with the mast, but we are talking bermudian rigs with large foresails in the modern style.
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dreadnought72

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Re: Building a (near)Footy sailing dinghy - Keel advice needed
« Reply #4 on: November 14, 2011, 11:44:56 PM »

In fact I think to balance properly the clr needs to be a tad forward of the C of E, because a little bit of pointing up can be balanced by the rudder, but a keel too far aft is poison

I agree! Aiming for a bit of weather helm is good.

Quote
And of course, the CLR will vary according to the rig - the CofE of a gaff rig is further aft than that of a bermudian.

I agree again. The thing here is if the designed fin is woefully aft or fore of the "right" position (and is this likely in a 13" boat?) being a foil-mounted-on-a-plywood-spike it could be remade and moved without too much hassle.

In addition to this, the oversize rudder (it must necessarily be oversize, yes?) will affect the CLR once rigged - bringing the CLR further aft than one might think it is. Maybe this is in part why we see modern dinghies' centreboards/daggerboards right behind the mast - albeit these are balanced somewhat by a rig-with-a-jib in most cases.

Edward, I doubt you'll get it far wrong, if you stick to "tried and trusted" general locations for fin and mast. This won't be a race boat, so a bit of experimenting/inefficiency won't be too awful to undertake/live with, maybe?!

Andy
Andy
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andrewh

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Re: Building a (near)Footy sailing dinghy - Keel advice needed
« Reply #5 on: November 15, 2011, 12:49:23 PM »

Edward

AndyAndy  (so good he named it twice) has pretty much said it all :} :}

My modest observation is that this is experimental (I too, looked at those hulls) so I suggest making the keel and rig in "experimental" materials  i.e. cheap and replaceable.  For footys I normally make the keel in 6mm balsa and make it removable so they fit in a shoebox.  Then it is not difficult to make a fin with more/less rake and or move it quite a long way fore and aft without much difficulty.

Similarly sails cut from dead umbrella or a plastic bag will tell you how good the shape and size are without any grief.  THEN you can make some beautiful sails out of draughting linen or sea island cotton!

andrew
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Edward Pinniger

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Re: Building a (near)Footy sailing dinghy - Keel advice needed
« Reply #6 on: November 16, 2011, 12:04:22 PM »

Thanks to all for the advice. I'm definitely not aiming for efficiency or racing performance with this model - I'm just aiming for a model that looks nice on the water and is stable and controllable in light to moderate winds - but given my relative lack of experience with sail I thought I'd better ask for some advice first. With a sailing model this small, I'd guess that even moderate changes to the rig and ballast setup would make a big difference to its stability and sailability. If it were 3 times the size I'd probably be a bit more confident about my ability to get it sailing!

I'll definitely be using quick/cheap experimental materials (plastic bag sails and string) for the initial rig. The fin keel will be a fairly rough-and-ready affair anyway (heavy-gauge plywood sealed and painted, with lead plates bolted to either side) as it will be removable; so it'll be easy for me to make a new one. I'm thinking of building the model with a more or less scale-looking centreboard (the vertical part of the plywood "T" mentioned in my first post) which acts as a bracket to bolt the (much longer) removable keel to.
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Boomer

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Re: Building a (near)Footy sailing dinghy - Keel advice needed
« Reply #7 on: December 15, 2011, 05:12:31 PM »

Ed
Please post some photos of your progress. I am interested and I am confident others would enjoy seeing your progress. Having seen some of your other work, I am
looking forward to seeing the finished result. I just completed a V 12 Footy (Victor Model Kit) myself. These boats make a great conversation piece when displayed in
your home or office. These little guys are fun to build and should be a kick to sail - I think it is my wife's favorite. :-))

Merry Christmas to all!
Windchaser








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