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Author Topic: keel position  (Read 11641 times)

stringer

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keel position
« on: April 01, 2013, 11:40:27 am »

Hi everyone, I have designed my own yacht and have placed the keel in the same position as the real yacht I am copying from photographs, now that I have all the bulkheads and keel glued in place, the keel position does not look right if anything it looks to far forward,I have slottted another keel behind it ( ref attached photo ) which looks better, but I don't wish to continue until I know its in the right position.
I know when balancing an aircraft by holding the wing tips it should be level,
Can anyone help me locate the keel relitive to fore and aft, is there a formula, or do I have to work out the area of each bulkhead below water level and find the average using all bulkheads, I am looking at the forum here for some help please
regards Geoff
 
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malcolmfrary

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Re: keel position
« Reply #1 on: April 01, 2013, 12:03:47 pm »

As you have noticed, there is a limit to the similarities between a boat that sails in water, and a plane that doesn't.
I have worked with the idea that the centre of effort of the sails should be a little forward of the centre of lateral resistance of the underwater bits.  Determining these for me is usually a case of making a card cutout of the sailplan and balancing that on a pin, then doing the same for the side view of the underwater parts.  After that, informed guesswork creeps in, since the centre of effort moves with the sail settings.  Dont forget that theres a rudder involved, as well.
Since mine have tended to shallow, but long fore and aft fins, effectively the centre of the fin has been the centre of resistance, but the guesswork calculation is less critical.  Probably.



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roycv

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Re: keel position
« Reply #2 on: April 01, 2013, 12:18:55 pm »

Hi I agree with Malccolmfrary about position of keel.  There is another way to sort out CLR.  Use a card cut out as Malcolm suggests.  Get a piece of black thread (easier to see) and afix a weight to make it hang straight. Hang thread from pin, the pin should pierce the card and the card should be able to pivot from the pin.  So you have the pin with a hanging thread and the card cut out all on the axis of the pin. 

Draw a line where the black thread traverses the card.  Do the same again on the other side of centre of the card and draw another line as before.  Where the lines intersect is the CLR. 
It is much easier doing it than describing it.  This method enables you to use less than stiff card as per cornflakes packets.
I do not think you have to take into account the rudder area.  Anything within 1/2 an inch is OK the rest can be trimmed out with the sails.
I have used this method and it works.
good luck,
regards Roy
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tigertiger

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Re: keel position
« Reply #3 on: April 01, 2013, 01:50:10 pm »

Isn't that the centre of pressure (CofP) of the sail area that has been described?


I thought that finding the CLR could only be done after the keel is in place.
The placing of the keel is not critical unless you are racing, some boats (Balmain bugs for e.g.) have a sliding keel.
What is important is that the placement of the keel bulb has the boat floating straight and level.




To find the CLR I put the complete hull/keel/bulb in the water and then push against the side of the boat. I find the point where the boat neither swings here nose forward or aft, the boat just moves sideways. That point is the CLR.


Another thing that might work, is to complete the hull so that it is water tight. Find the CLR without the keel. Then design the keel and bulb so that the combined centre of area, is in line with (vertically below) the CLR without keel. This will only work if the keel and bulb are fairly evenly shaped (flat keel, torpedo shaped bulb), however an egg shaped bulb would have some effect, but not shift the CLR that much (mm).


Just my two cents.
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tigertiger

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Re: keel position
« Reply #4 on: April 01, 2013, 01:54:20 pm »

Looking at your photo upside down, I have a few questions.


Have you double checked your measurements?
Have you double checked the angle between the sheerline/deck and the keel fin?
Is the fin on the original boat dead straight?


Is comparison with a wing tip fair?
What does the wingtip test show? Weight distribution?
A buoyant boat is perhaps weightless, and are we only measuring balance of lateral forces through the medium with a boat?
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stringer

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Re: keel position
« Reply #5 on: April 01, 2013, 03:19:46 pm »

Hi TigerTiger do not fully follow the two previous replies with cotton and carboard as they are talking sails, where as you are talking about the hull, as you can see by the photo the keel is cut as part of the centre of the boat which all the bulkheads are attached to, my layout of the keel placed it where it is, and to replace it anywhere else would mean cutting it off, as for making the hull water tight, it is not on, as its secured to the build table, If its wrong I will need to cut the keel off, , and plank up and then float it as you suggest, I was hoping I could find the correct location at this stage of the build, the problem of taking measurments from Photo's are they are not always true lengths. to answer your questions
Have you double checked your measurements ( yes )
Have you double checked the angle between the sheerline/deck and the keel fin? (you will have to explain this)
Is the fin on the original boat dead straight? ( yes )
Its not only the boat that will be out of its depth, I think that I am as well, but it does keep me busy.
Thanks for replying
Geoff
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Netleyned

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Re: keel position
« Reply #6 on: April 01, 2013, 04:04:49 pm »

Geoff
In your photo the aftermost keel looks right and normally
something that looks right is right.
All my yachts have the centre line of the keel fin in the
side view just aft of half the hull length.
The bulb will have to be placed in the correct position
on the fin to give the correct waterline.

Ned
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malcolmfrary

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Re: keel position
« Reply #7 on: April 01, 2013, 05:13:17 pm »

Quote
I thought that finding the CLR could only be done after the keel is in place.
The method that I was originally given for finding the CLR was to put the hull in water, then using a finger, push it sideways.  When the spot that caused it to go just sideways was found, that was the CLR.  Without a floating hull, a silhouette of the side view below the waterline will do the job by finding the point of balance.  Racing type fins tend to be much twitchier in their placement than cruising types.  Usually, the fin will be the major influence in deciding just where the CLR is.  In much the same way, the lead ballast position will be the thing that has greatest influence in where the fore and aft centre of gravity will be.
As Ned said, if it looks right, it probably will be.  If the fin is fairly near, there will hopefully be some adjustment that can be done with the sailplan. 
On my first try at a self design yacht, I decided that as it was becoming obvious that I had a lot more to learn, it would lose the yachty bits and gain a motor and superstructure.  It gave great pleasure for several years as a freelance pilot type boat.
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JayDee

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Re: keel position
« Reply #8 on: April 01, 2013, 05:32:33 pm »

Hello Geoff,
What length is your hull, it looks to be around 30 inches long.
IF, that is correct, you have way too many bulkheads for such a small hull.
Taking half of them out would make a lot lighter boat.
John. :-))
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stringer

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Re: keel position
« Reply #9 on: April 01, 2013, 07:01:17 pm »

Hi JayDee
the distance between each bulkhead is 2.5" and the overall length is 53" with a beam of 15", I am happy with the bulkhead layout, its the position of the straight keel which I need to place, the hull will be planked with 3/8" x 1/8" wood when I can find it, and then fibre glassed inside and out, I cut and make my own planks from sheet, glueing and pinning at each bulkhead, the first 3 1/2" of the bow will be solid wood
Geoff
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dreadnought72

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Re: keel position
« Reply #10 on: April 01, 2013, 09:52:30 pm »

Geoff,

Thinking laterally (maybe not a good idea in a fore-and-aft issue like this!) could you:

Drill vertically through the keel and mount four(ish) long steel tubes, each a set distance apart, to be braced by the frames at/near their tops, well above the w/l.

Add two parallel steel shafts to the top of the fin, threaded at their tops to accept a nut, the shafts positioned to work with any pair of the tubes.

This would allow the fin some fore-and-aft adjustment during trials.

Once you're happy with a position when sailing, finish the fin off to mate properly with the keel.

Andy
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stringer

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Re: keel position
« Reply #11 on: April 01, 2013, 10:49:45 pm »

Hi Andy
This sounds an alternative to think about, can you give me a sketch of your idea please, it would give me a means of removing the keel for transporting the yacht.
 Geoff
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mrpenguin

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Re: keel position
« Reply #12 on: April 01, 2013, 11:00:25 pm »

@Stringer:
Geoff - that is going to be a BIG boat....
The Centre of Lateral Resistance (CLR) could perhaps be determined by looking at the cross section (below waterline) from one side and finding the centre of that area. Then add your keel centred on that. Lateral resistance is directly related to area while the boat is level. However, as soon as it heels, that area changes and all bets are off.... that is beyond what I am capable of calculating...
 
One matter that has not been mentioned by others yet is the sail plan to be fitted. Comments thus far are probably assuming that you are intending to have a pretty standard sloop rig on a single mast... Perhaps if you could clarify what is planned for the rest of the boat (and a photo of the original boat perhaps) you may get some more useful feedback. As it stands, you could be planning a couple of masts and gaff rigs or anything, bowsprits etc. Something like a bowsprit moves the Centre of Effort (COE) of the sail plan a long way forward and your original keel placement may well be valid.
Does the original boat you are working from have a drop keel like this? These larger models often have a more traditional keel.... also keep in mind that the boat as you are building it will be pretty heavy and may have a significant draught - finding suitable water to sail it in may be an issue..... also transporting it.... and launch/recovery...
 
As it stands from your photo of the build, IF you are planning a standard sloop/Bermuda rig, then the aft keel "looks" about the right position to me. Assuming a sloop rig, the (single) mast position would be about level with the forward edge of the keel fin in that configuration... that should put your COE just ahead of CLR, resulting in slight weather helm which is pretty easy to sail with and the amount of weather helm can be tuned to be more or less with sail adjustments...
I have probably provide more questions that answers, sorry.... Hope this helps...
 
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dreadnought72

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Re: keel position
« Reply #13 on: April 01, 2013, 11:50:07 pm »

This sort of thing...


... but built considerably better than my artwork.  %%

Andy
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stringer

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Re: keel position
« Reply #14 on: April 01, 2013, 11:53:07 pm »

Hi mrpenguin
the model yacht I am building is as near as I can get to a volvo VO60 ocean racing yacht, the real dimensions are
 
AttributeMetricImperialLength Overall
21.5 m
70.5 ft
Beam
5.7 m
18.7 ft
Draft
4.5 m
14.8 ft
Mast
31.5 m
103.3 ft
I have attached a photo of the yacht
regards Geoff
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stringer

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Re: keel position
« Reply #15 on: April 02, 2013, 12:07:36 am »

Hi dreadnought72
I like the idea, my centre support is 12mm 9ply marine ply and is an inch high at the top and bottom of the bulkheads, it could take the load of the keel if I added some offcuts each side of the centre section where the keel bolts would go through, will do some tests on some scrap.
thanks for the idea
regards Geoff
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stringer

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Re: keel position
« Reply #16 on: April 02, 2013, 12:12:49 am »

thanks for all the comments, and I will try and sleep on them, this is without doubt the best forum
I am off to bed
Regards geoff
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tigertiger

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Re: keel position
« Reply #17 on: April 02, 2013, 01:45:18 am »


Have you double checked the angle between the sheerline/deck and the keel fin? (you will have to explain this)



Check angle 'a' (see below). The deckline may not be horizontal when afloat.
Looking at your picture, it appears that the keel is canted forward. It appears that the keel is perpendicular to a tangent of the hull at that point.


Looking at your picture of the original, the keel could in fact be in the correct place. The double rudders would allow the keel to be more forward than on a standard single rudder boat.


When you are designing your rudder tubes, I also assume that you know that these rudders are probably not (if memory serves) vertical, when viewed head on.
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roycv

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Re: keel position
« Reply #18 on: April 02, 2013, 08:41:27 am »

Hui Stringer, sorry to contradict but I was talking about hulls.  It seems that some of the advice offered is very complicated!  A card sillouette of the under water section either balanced as Malcolm suggests or my black thread method works very well and is accurate.  The finger and hull method is also good(ish) but only after the hull is finished!
If you want to know how to balance the sails there are several of us that can explain further.  It can all be done on paper as well!

If you are constructing a scale model of an existing sail boat and have not made any adjustments for displacement then the fin should be quite workable in the same place as the full size.  I suspect it will not have enough area though. 
Good luck with the project.
regards Roy
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malcolmfrary

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Re: keel position
« Reply #19 on: April 02, 2013, 10:11:37 am »

Just had a thought - is it going to replicate the original sailplan (overlapping Genoe type rig) or be simplified into a self tacking one as per Marbleheads etc?  The overlapping rig, especially if the fore is a masthead rig, might well move the centre of effort, and thus the sweet spot for the CLR.
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stringer

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Re: keel position
« Reply #20 on: April 02, 2013, 01:44:38 pm »

Hi roycv and malcolmfrary
lets start with you roy, I have marked out a downsized silhouette of the side view of the below waterline and balanced it, this gave me station 6 which is bang on where the plan shows the keel to be, but, does it take into account the width of the yacht which is wider going aft of of 6 and narrower to the bow, I had no Idea what effect the width of the yacht would have on the position of the water line relitive to the profile of the yacht, the plan I have drawn looks ok, but the build makes me feel the keel should go back more, I'm now after listening to all the comments I should cut the keel off and plank the yacht up, seal the hull, and float her to find the right position.( I did not sleep alot last night thinking about this)
Next to Malcolm,
Hi thanks for your reply, not a Genoa, it will be a self tacking one, as you suggest like a marblehead, I may change my mind as the build gets to the sail making stage, but I will be putting in line control in front of the mast prior to fitting the deck just in case I want to control the fore sail, and if things alter dreadnought72 idea might just be the way to go for the keel.
I can feel another sleepless night coming on!!!!!
Regards Geoff
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tigertiger

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Re: keel position
« Reply #21 on: April 02, 2013, 02:28:57 pm »

I think you really need the fin in place, as this would affect the lateral resistance.
You could however, make it removable, so that if you are not happy, you can change it later. Or if you are happy, there is not need for retro-fitting a fin.
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roycv

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Re: keel position
« Reply #22 on: April 02, 2013, 02:53:51 pm »

Hi TT we are deciding where the fin should go.  You are right it will have a big effect on CLR.  But when the sail c of e is worked out and the mast position agreed then the keel fin can be tweeked fore or aft.
regards Roy
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tigertiger

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Re: keel position
« Reply #23 on: April 02, 2013, 03:39:39 pm »

I know where you are coming from.
It might be worth considering what the final aim is. Racing? A model that looks right and sails OK?


It might be easier to tweak the sail area, and this changes depending on wind conditions anyway. And to come back to the point again, unless you are racing (or sailing to a class rule) the dynamics of sail boats are very forgiving.
The racing boats we model are, in the real world, bound by class rules. There is a whole science dedicated to squeezing out every last knot of speed. To try and replicate the science for models (where scale effect negates a lot of this) is overkill. Unless we are designing racing hulls (e.g. IOM competition boats).

It is too easy to get hung up on the detail and 'science'. As someone once said on here, 'generally if it looks right, it is'.

Where I live there are steel hulled boats, with the basic shape of a grand banks dory, most are punted, but some have sails on a bamboo or steel mast, and no keel. The sails are no more than old fertiliser sacks stitched together into a small square centre to the mast. The boats would not win any races but they make good headway.
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roycv

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Re: keel position
« Reply #24 on: April 02, 2013, 04:23:03 pm »

Hi all, I think we are into Scale Sail here, this boat does not fit into any class rules.
I think Geoff wants a boat that looks good on the water with a nice turn of speed.
A little bit of planning will help a lot.
regards Roy
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