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Author Topic: False Keel Attachment  (Read 3318 times)

Richardjm

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False Keel Attachment
« on: August 13, 2013, 05:15:35 PM »

Hi Everyone


I have not been here for a while, that *** DIY gets in the way.


I am building a 1:20 sailing trawler and have just measured the necessary ballast at about 12kg (5 bricks) and I know the approx location of the C of G.


I now realise that a fairly robust mounting method is needed. I don't want to obstruct the internal hull too much due to the sheet mechanisms and of course the result must be watertight. I also want to be able to fit the keel pond side to ease transport problems as I have experience of hauling around a Robbe Atlantis with 14 kg ballast.


The final requirement is for both the keel and the ballast weight to be separately adjustable for fine tuning (just in case my calculations are a bit off!!)


Any advise would be greatly appreciated


Richardjm

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Sir_Gorg

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Re: False Keel Attachment
« Reply #1 on: August 13, 2013, 08:43:28 PM »

Do maybe something like on the drawing? False keel housing has to be alu U-profile, finished at the back ant the front with adequate angles and "closed". Assembly to the U-profile fixed to the hull by 4-5 M6 stainless bolts alongside of the profile.
 
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mrpenguin

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Re: False Keel Attachment
« Reply #2 on: August 14, 2013, 12:37:44 AM »

Many larger racing yachts (A Class for example) have a threaded stainless rod (about 3/8" diameter) right through the boat from bottom to top in a sealed tube. The bottom end of the threaded rod is a permanent part of the keel.... You finish up with a nut and washer on deck. This method makes the keel easily removable for transport as well.
 
For your situation, might there be a (removable) cabin or such situated more or less above the centre of gravity of the keel or such that might conceal the nut?
 
 
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Richardjm

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Re: False Keel Attachment
« Reply #3 on: August 14, 2013, 11:58:32 AM »

Thanks for the ideas. I think the U section channel semi-permenently attached to the keel may be a good bet. The loss of static scale appearance will be offset by the ease of incorporating fore and aft adjustment. Also I may be able to devise a display stand that uses the same connection points as the keel.
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tigertiger

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Re: False Keel Attachment
« Reply #4 on: August 14, 2013, 03:28:58 PM »

This first set up below would allow a more streamlined keel to be fitted.


But the second similar set up with a narrow wooden keel and keel bulb would work better.
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Richardjm

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Re: False Keel Attachment
« Reply #5 on: August 14, 2013, 06:17:52 PM »

Thanks, this is what I will do with a lead bulb at the bottom. I am thinking a keel depth of about 200mm (the hull is about 1000mm long) with 1100gm (measured requirement) lead ballast on the bottom will be a good starting point, any thoughts on this?


I really appreciate the help I have received.
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Brian60

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Re: False Keel Attachment
« Reply #6 on: August 14, 2013, 06:36:05 PM »

The method I am going with is two bolts epoxied through the ships keel spaced about 150mm apart for to aft on the keel. These protrude far enough downwards to allow a stand to be bolted to the keel when the boat is in storage. When in use on the water the stand can be substituted for a false keel. This can be fashioned in the shape of a 'T' the arms of the tee fastening to the bolts with wingnuts or ordinary nuts. The leg of the tee carrying your weights.

tigertiger

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Re: False Keel Attachment
« Reply #7 on: August 15, 2013, 12:50:14 AM »

Thanks, this is what I will do with a lead bulb at the bottom. I am thinking a keel depth of about 200mm (the hull is about 1000mm long) with 1100gm (measured requirement) lead ballast on the bottom will be a good starting point, any thoughts on this?




As the masts are not tall (compared with a Bermuda rig) the keel does not need to be very deep. You may even be able to get away without a keel. As the original vessel would have been relatively low and slow, compared to a race boat. But bear in mind the more weight you can get lower down, the better.
On one of my models I am preparing to have an external keel of about 5cm. The boat is a similar size. Rational of this? Reduce the drag as much as possible, and the I will use a bolt on keel I can alway make it longer later.

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mrpenguin

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Re: False Keel Attachment
« Reply #8 on: August 15, 2013, 07:40:33 AM »

Thanks, this is what I will do with a lead bulb at the bottom. I am thinking a keel depth of about 200mm (the hull is about 1000mm long) with 1100gm (measured requirement) lead ballast on the bottom will be a good starting point, any thoughts on this?
@Richardjm:
The longer the keel fin, the less weight you need on the end for the same righting moment.

Here is a real world example of a model boat I own that sails quite well in most weather including a blow - the Shunbo Monsoon is 900mm long, mine has a 1700 gram keel bulb. The keel fin measures 400mm from the waterline to the bottom of the keel bulb. Overall height of the boat is 1800mm from bottom of keel bulb to top of mast. Sail area (standard Bermuda rig) is about 0.4 square metres.

Pictures of this cheap Chinese boat at: http://www.hobbyking.com/hobbyking/store/__8309__RC_1_8mtr_Monsoon_Sailboat_90pcnt_RTR.html
I can tell you that the recommended 1300 grams keel weight on that website is NOT enough for this boat; it makes it very tender to sail in anything but very light weather.

Depending on your sail plan height and area, your proposed 1100 gram bulb on a 200mm fin may be a bit short and a bit light. The above figures may serve as a guide if nothing else
 
Keep in mind that as you lengthen the fin, the leverage increases and you finish up with a lot of stress between the hull and the fin. With the Monsoon, the keel fin extends right up through the boat to deck level to spread the loading.
 
Hope this helps...
 
 
 
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Richardjm

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Re: False Keel Attachment
« Reply #9 on: August 20, 2013, 01:16:32 PM »

Thanks for advise. I am somewhat daft as the measured required weight is actually in the order of 11.0kg, NOT 1.1kg. This seems a lot but is actually equivalent to the 5 bricks i had to use to bring the hull down to the correct waterline. Even using lead, that is a substantial bulb to hang off the keel And I do want to keep some liveliness in the handling.


Richardjm
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tigertiger

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Re: False Keel Attachment
« Reply #10 on: August 20, 2013, 01:55:09 PM »

as yours is a working boat, you only need enough keel (length) and bulb weight)  to affect the righting moment to counteract the wind on the mast.
You can also have a lot of weight below the waterline in the hull.


In some scale sailing models (working boats), the design is to have no external keel, but to have all the lead set into the keel itself. Others have a solid steel plate bolted to the keel along the full length of keel. Maybe keel width (1/2") fat and 2" deep.


There is an Amercian company producing large scale models of the HMS Surprise, and other ships. They have a keel extension but not a keel fin.


One of the reasons for the drop keel on scale models is because of the effects of scale. The bigger the model, the smaller the effect.
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Richardjm

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Re: False Keel Attachment
« Reply #11 on: August 20, 2013, 04:52:33 PM »

Posted reply earlier but the website seemed to be down and it disappeared so will try again.
I may reduce proposed keel depth to 100mm which will also help with pond weed problem. I will build in as much adjustment as possible and then fine tune by trial and error at the pond side. I am surprised how large the lead "bomb" need to be. I do want to keep the ballast external as this is my first scratch build. I need as much internal hull space as possible as I have not yet worked out the controls but am assuming 3 servos, rudder, main plus mizzen and a third to pull the overlapping foresails across.


For interest, I am building 1:20 scale Lowestoft trawler Excelsior (google it) using drawings purchased from the current owners. They were drawn at the time of the restoration work.


Richardjm
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JerryTodd

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Re: False Keel Attachment
« Reply #12 on: August 20, 2013, 09:19:39 PM »

My Pride of Baltimore has what amounts to a dagger-board trunk with a removable fin.  The trunk is glassed inside and fully sealed.  The fin slips in from below and is held with a brass screw and nut.

Constellation has a PVC pipe full of lead shot that weighs some 42 pound.  It's held to the hull by two stainless threaded rods in tubes that run from the spar deck through the keel as shown in the diagram below.  They will have the head of capnut welded to their tops.  An alternate version of this, and probably better version would have the threaded coupling embeded in the keel and tubes in the ballast torpedo - a pair of stainless bolts some 4" long would hold it on from underneath.  This would make removing the ballast easier from under the model instead of reaching through all the rigging of a square-rigger.

Mind you these are large scale models.

tigertiger

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Re: False Keel Attachment
« Reply #13 on: August 21, 2013, 01:33:56 AM »


The constelation is a nice set up.

If there is a problem with weed, avoid a bulb that protrudes forward of the fin. It will act like a hook. A fin that has a leading edge that tapers backwards is better ([size=78%]See image below)[/size]
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Richardjm

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Re: False Keel Attachment
« Reply #14 on: September 05, 2013, 10:34:10 AM »

Sorry for delay in reply, been on holiday in the caravan, no internet.


The hull planking in Pride of Baltimore looks really great.


I am going to attach the keel from the outside as I suspect the inside is going to be too crowded with the sheet mechanism.


The weed issue is very relevant where I sail but I will have to compromise between the length of the required ballast and keeping to a reasonably short keel for manoeuvreabilty, a bit of a head scratcher!


Richardjm
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