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Author Topic: False keels  (Read 4710 times)

Nordsee

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False keels
« on: December 04, 2009, 02:08:57 PM »

Hello, got a couple of queries. My new boat, in build, is of a North Sea Fish Cutter. 1.06 meters long  (Hull) and Gaff rigged. I want to fit a removable false keel, about 4 inches deep and the full length. This will carry the extra Ballast and increase underwater area.Being removable will make the transport of the model easier. The questions are ; Will that work? do I need to have it full length or just the rear part? If this is the case, how long? I would appreciate any constructive hints from you Clever people!
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tobyker

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Re: False keels
« Reply #1 on: December 04, 2009, 09:57:52 PM »

The longer the keel the less manoeuverable the boat will be under sail. So if you want to go about quickly, have a daggerboard with a lump of lead on the bottom, but you will have to pay more attention to the steering. With a full length keel the boat will tend to stay on course until you turn it, but will need a bigger rudder  to turn. Looks nice. I think I'd have a keel about half length in the middle of the boat, and the rudder a bit deeper than scale.
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tigertiger

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Re: False keels
« Reply #2 on: December 05, 2009, 01:33:11 AM »

A full length keel would indeed reduce maneuverability.

If the keel was at the back it would act like weather vane, and the boat wouldnot turn at all.

You need to have the centre of area (the centre of resistance) of the keel and bulb assembly, just behind the centre of lateral resistance of the boat. This will give you a little bit of weather helm (nose tends to turn slightly into the wind) which is desirable.

I am in a bit of a rush now and must go out.
But try using the search facility on this site and search 'centre lateral resistance', I think it has been covered 3 or 4 times now.

A keel of about 6" wide and 9-12" deep should do it. I would also guess you are going to need about 8-10 lbs of lead.
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tigertiger

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Re: False keels
« Reply #3 on: December 05, 2009, 05:42:52 AM »

I am no expert, but here is what I have been able to learn.
Sorry if I am teaching you how to suck eggs.

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Nordsee

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Re: False keels
« Reply #4 on: December 05, 2009, 10:27:07 AM »

Thanks to you all, got lots to think about now, and some solid facts and suggestions. I did rather want to avoid the dagger board and lead bomb , that is why I asked about the long keel. I think when she is ready to sail, then I will make a semi long keel with ballast and try all the options. She needs to be able to turn reasonably swiftly, but I am not expecting spinning on her axis! She is supposed to be a Fishing Boat after all, also a stable boat that keeps her course is relaxing! Thankyou all again, will keep yu posted! Next real job is either making the sails or having them made, HHHMMMM !! It is so difficult here with an almost total lack of Model Shops or local people with interest.
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tigertiger

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Re: False keels
« Reply #5 on: December 05, 2009, 11:14:19 AM »

I also wanted to avoid the keel fin, but it seems to be the most common solution on smaller models. Perhaps with good reason.

My current build project (working boat Louis Heloise) is 1.1m hull. In the original plan the keel is hollow, and filled with 4kg of lead. However the model has a reputation as being a light airs boat. Some have overcome this by using 10-15kg of ballast. This means she won't heal over in the wind as much, but this will increase drag a lot and really slow the boat down, and maybe full stop in light or inconsistent airs. There is also the additional cost of the extra lead, and transporting it all.
That is why I opted for the drop keel, to get the ballast much lower in the water.

You could try a steel bar of about 2" width, but I think 4" would act too much as a friction surface/sail. You would require a much larger rudder. I have seen bars of about 4" but these are on 2m hulls.
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Greggy1964

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Re: False keels
« Reply #6 on: December 05, 2009, 01:18:11 PM »

Lovely ship Nordsee.

I love the shape of these old boats O0

More photos please :-))
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tobyker

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Re: False keels
« Reply #7 on: December 05, 2009, 11:50:57 PM »

Mind you, my fairly scale Thames Hoy (24" hull length) has a full length keel of 3/8" wide by 2" deep mild steel and that sailed very well with a weighted rudder, and seems to answer to the helm fairly well now she's got radio rudder. The rudder is extended downwards about 7/8". So if your sailing waters are not too restricted, and the rig not too tall maybe you would get away with something similar. People seem to go for the lead mine on the end of a steel rule, but I guess its not always necessary. I'd post a picture, but the rudder's off and the bottom scraped for a refit!
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tigertiger

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Re: False keels
« Reply #8 on: December 06, 2009, 03:00:06 AM »

I tend to think that 2" is about the limit. But I am no expert. But I do think that 4", as originally suggested, would cause problems.
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Nordsee

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Re: False keels
« Reply #9 on: December 06, 2009, 11:05:02 AM »

Thanks once again, I have asked the local Heating Firm to fold me some sheet steel, about1mm thick, galvanised, in a U section. Wide enough to slide over the existing Keel . They are making the legs 8cm long, as the original keel is 2 cms at it smallest point, I can adjust the depth by the well tried "Cut andtry it" without lots of expense. When satisfied I can make a smart job of it!
  The rudder is already plus 125% in area, of which the depth is increased by 2 cms.
This seems to have created some interest, I am pleased! Will keep you informed and take some more photos as I progress.
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Brooks

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static vs dynamic CLR
« Reply #10 on: December 06, 2009, 08:05:41 PM »

Tiger^2 's diagram is for finding the Static CLR (hull not moving forward). That's a good start to finding the location of your false keel, but your job is not finished yet :-) The hull's CLR will move forward as the boat starts to develop lift off the hull. The leeward drift of a sailing ship hull (leeway) sets up a positive angle of attack, turning the hull itself into a wing - this is the true reason we can sail upwind, the dynamic lift off the hull that resists the wind's push to leeward, as opposed to the "pinched watermelon seed" explanation you might have seen. The Dynamic CLR will be located forward of the Static CLR. So, centering the keel on the Static CLR may not give you the maneuverability you desire. That is, you may end up with lee helm or excessive weather helm, depending on how far the Dynamic CLR deviates from the Static CLR. As a rule of thumb, the Dyamic CLR of a wing is at about the 1/4 chord position from the leading edge. This rule will apply pretty well to a false keel of plate or of symetrical airfoil section. The movement of the CLR for the hull itself is harder to calculate.

Because of the movement of the CLR, I don't try to precisely calculate the false keel's position. I believe sail testing is the best way, let the boat tell you, rather than you trying to tell the boat :-). For my models,  I design to make the keel location adjustable, fore and aft. I will test sail with the keel in different positions until I find the sweet spot that gives me the maneuverability I desire.

On my boats, I have an aluminum L screwed to the keel of the boat. I hang the false keel off the L. By drilling several holes in the L fore  and aft of the calculated CLR (static) I now have the ability to move the false keel and try different positions. For some of my models, I have to move the false keel forward of the static CLR, for some I have to move it aft :-).

The reason I have had to move the keel on different models different directions (relative to the Static CLR) is that it is the Interelationship Between CLR and Center of Effort (summation center of lift off the sails) that is critical, not the CLR just by Itself. Depending on your sails (cut, shape, which ones are flying), the CE will shift and may not be in the same place as the real ship. So, just adding a full length false keel may not do the job.

Any method that will let you move the keel to temporary locations while you test sail will work.



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Nordsee

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Re: False keels
« Reply #11 on: December 06, 2009, 09:04:31 PM »

Wow! So many ideas and opinions! Just to show the lines of the boat better here is a photo when the decking was finished and she had a coat of Primer..........
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tigertiger

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Re: False keels
« Reply #12 on: December 06, 2009, 11:44:53 PM »

Further to wht Brookes has said

It is a good idea to be able to adjust your keel forward and backwards for trim. You wil see it on some racing boats.
But again this is not a racing boat.
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Brooks

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Re: False keels
« Reply #13 on: December 07, 2009, 02:56:32 AM »

My keel ballast is lead shot in a PVC pipe. If the shot does not fill the pipe entirely, I can shift the ballast fore and aft, for trim purposes like Tiger recommended, by stuffing packing in one end or the other of the pipe. For testing, I'll make up a pipe with a solid cap end and a removeable cap end, all fittings found in the plumbing supply part of the local hardware. Once I've figured out the optimum configuration, I will make a replacement ballast pipe with solid caps at each end. If you sand off the cap edges to smooth them into the pipe itself, it seems to be pretty drag-free, at least for non-racing models
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Nordsee

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Re: False keels
« Reply #14 on: December 07, 2009, 02:23:51 PM »

I thought I ought to explain why I don't want a dagger board and bomb keel. I sail on a lake that was a Gravel Pit. It is now a leisure centre and as such has gently shelving beaches, so launching a boat with a deep keel requires wading in. No matter how deep the model is I will need wellies or in the summer bare feet. But not upto my knees! Also a heavy boat and Tx are awkward to handle and I do most of my sailing alone (All together now, "AAAAAAHHHHHHHHHHHHHHHHHHHHHHHHHHH! Shame!!"
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tigertiger

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Re: False keels
« Reply #15 on: December 08, 2009, 10:36:05 AM »

Ha ha.
The boat will not be any heavier with a drop keel. It can in fact have less ballast to keep her upright, as the lever effect of the righting moment is increased. I know that problem well.

Also, even with a drop keel, she can still have a draft of about 9-10". Easy for wellies.
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