Calculating keel size

[Brian MuirParticipant @brianmuir84327]

I have started this pond yacht, bread and butter construction with good quality wood that will hopefully look good in a clear varnish finish.

I am looking for help in deciding the size of the keel, I have lead weights for the ballast, and as you can see I would like a long mast on her, to give it a striking look out of the water.

Does anyone have a formula or rule of thumb on the calculation of keel size or balancing mast and keel?

Current dimesions are, L 41cm, B 9cm, D (hull) 6cm

Current mast 70cm, keel 25cm

[Brian MuirParticipant @brianmuir84327]

[Kimosubby ShipyardsParticipant @kimosubbyshipyards]

Hi Brian,

trick photography eh………..

At first glance I was seeing a monster in a very short hull length, worthy of our antipodean friends down under! I now, on second look realise that your hull intends having one mast and not two – the second is the end pole on the washing line in the garden!!!!

Not sure how you calculate such things but from the image would suggest the keel is too long, maybe shorten to 2/3 of current length, and the rudder is over sized, again maybe only half the area needed, keep length but reduce the width?

Now there is a relationship between mast height and keel length, not sure where, and then there's the ballast to consider at the end of the keel too.

I like the way you build and then ask for the rules/relationships, intriguing indeed. Kim

[Brian MuirParticipant @brianmuir84327]

Doh! Sorry about that, its a bird feeder – "bringing the outside inside" sort of feng shui thing…..

I made everything too large to allow for reduction (all just temporary fixes) I thought about using elastic bands to hold the weights on the keel and seeing how high up they can be moved until it doesn't spring back from horizontal?

I then thought I'd reduce the rudder in proportion to the keel….. and see what happens in the water!

The size of the hull was dictated by the wood I had, the mast hight is the bit of hardwood dowel I had left….. sorry no deep insights or design magic, but as always thanks for the positive advice and support. I 'll try Goggle

Edited By Brian Muir on 08/12/2013 21:14:45

[Brian MuirParticipant @brianmuir84327]

I found this ut can't really unpick it to understand it….

CSF = Capsize Screening Formula
CSF = Beam / (Disp/64.2)^1/3
The capsize screening formula is a somewhat controversial figure. It came into being after the 1979 Fastnet race in England where a storm shredded the race fleet. The Cruising Club of America (CCA) put together a technical committee that analyzed race boat data. They came up with this formula to compare boats based on readily available data. A lower value is supposed to indicate a boat is less likely to capsize. a value of 2 is taken as a cut off for acceptable to certain race committees. However this is an arbitrary cutoff based on the performance of boats in the '79 Fastnet. The CSF takes no account of hull shape or ballast location. The CCA characterizes the formula as "rough". They go on to say that "While the capsize screening formula places a limit on excess beam, which is important for good stability range, it does not control for another main determinant, ballasting. With only simple data, this is as far as we can go." Naval Architect Robert Perry calls it,"…far too simplistic to be always accurate, but it is one of the currently popular ways of looking at a boat's offshore suitability." (Sailing Magazine, Nov. 2001, p.44). Any two boats will have the same CSF value if their displacement and beam are the same. One could have a light hull with 50% ballast in a bulb at the bottom of an eight foot fin, the other could have a heavy hull with 20% ballast in a 2 foot deep full length keel. The stability characteristics of the two boats will be drastically different despite the identical CSF value.

[Amy jane SeptemberParticipant @amyjaneseptember49770]

Hi Brian.

I would suggest shortening the mast to about where the cross tree is and reducing the keel width by a third and it's depth by a half or a little deeper, and giving the rudder a little more than a third of the keel's area. If you want a very tall mast, then I would suggest using a light soft wood, and tapering the mast along it's length, to half the diameter at the top, and putting all the ballest in a lead bulb on the bottom of the keel.

[LARRY WHETTONParticipant @larrywhetton68737]

Good morning Brian,

nice bit of woodwork ,

what are the dims of hull , and where do want the water line,?..

keel blade looks to wide , rudder mass looks to much,

mast is to long and needs a taper , is going to r/c controled or a free sail,.?..

with that much timber mass you are going to need a large keel bulb….

to find keel weight cut roof lead into strips , wrap round keel dagger to you, get a happy water line, remove and then cast keel bulb, hope this helps happy sailing ..Larry….

[Brian MuirParticipant @brianmuir84327]

Hi Amy, where did you get the ratio of rudder : keel of 1/3? (It sounds good)

Larry, I have worked out the ballast for the water line, and I will use your method (but I can't cast lead yet) as I have lead semi bulbs (ready made) to fix either side of the keel, to work out how low to fix them, then cut the keel to suit.(dimensions in first post).

Its a free sail model

[Amy jane SeptemberParticipant @amyjaneseptember49770]

Hello Brian,

It's an old rule of thumb for full size boats, just a starting point. It's better to have the rudder a little on the large side with a smaller amount of travel than vice versa. (Are you putting any sort of self steering gear on her?) It's also better to have a deeper, narrow rudder than a short shallow one.

[Brian MuirParticipant @brianmuir84327]

Many thanks everyone, I'll have a go at adjusting keel size allowing for the ballast, then adjust the rudder as per the 1/3 rule of thumb.

Cheers, will put up pics of the result.

[Brian MuirParticipant @brianmuir84327]

After some tank trials, I have reduced the keel by about a third, and made the rudder a third the size of the keel and shortened the mast by a third…. see results below.

Unfortunately, due to the small amount of hull above the waterline, when the boat is heeled over 45 degrees or more then water flows into the cockpit. I may have to add a coaming around the edge of the cockpit to help prevent this, and in a future boat I would have a narrower cockpit?

Now to tidy it all up, give it a final sanding and start the varnishing.

[Amy jane SeptemberParticipant @amyjaneseptember49770]

Hello Brian

The mast height looks about right now (To my eye at least), There is an important relationship between the centre of the total sail area (centre of effort) and the centre of the boats under water area (centre of lateral resistance), the rudder area not included. This is the point that the boat pivots around when the boat turns. The centre of effort needs to be a little more towards the bow of the boat than the centre of lateral resistance. If it is not, the boat will constantly try to turn into the wind, and become unsailable. It would appear to me that the mast is a little too far aft, and the keel a little too far forward on your model. May I suggest cutting the keel with a taper, as in my scetch, which will bring the the centre of lateral resistance back, and also add a short bow sprit which will bring the centre of effort forward? The centre of effort probably only needs to an inch or two forward of the centre of lateral resistance on a model of your size.

Any way that's the way I see it, and of course second opinoins are most welcome. Sorry if I've put my oar in a little too much!

regards

Amy jane

Ps "lead" (as in dog lead, not the heavy grey stuff) is the term used for the distance between the two centres.

[Brian MuirParticipant @brianmuir84327]

That is a great help, thanks for explaining it so clearly, I agree that the keel puts the centre of resistance forward and a cut away front edge would help reduce that, and the bowsprit will allow a bigger foresail to move the centre of effort forward, I can also shorten the boom on the mainsail to reduce the size of the main sail.

Cheers, B

[Amy jane SeptemberParticipant @amyjaneseptember49770]

My pleasure Brian.

Looking forward to watching your progress.

regards.

[Brian MuirParticipant @brianmuir84327]

Coaming added, keel reduced, lead bulbs added and rudder reduced.

[Brian MuirParticipant @brianmuir84327]

And the first 3 coats of varnish applied with a gentle sanding in between applications. The wood is Meranti – recycled rainforest timber with quite a rough grain, but I think it looks beautiful when varnished, the layer of beech gives a nice waterline layer.

[Amy jane SeptemberParticipant @amyjaneseptember49770]

Nothing like the warm glow of new varnish, my next model shall have plenty, I think!

[Brian MuirParticipant @brianmuir84327]

I have run up these sails to test out the sailing qualities of the model, we only had red "bias binding" so I used it and it has quite a jolly look about it!

[Brian MuirParticipant @brianmuir84327]

The tiller uses an elastic band to return it to midships, and I have extended the tiller astern beyond the rudder post, so that a line from the boom connected to this sternmost end will have the effect of pulling the tiller to windward.

I have added a line below the boom to allow easy adjustment in place of sheets made fast to cleats, and the same on the foresail.

[Amy jane SeptemberParticipant @amyjaneseptember49770]

Yes, very jolly indeed, quite a jaunty look to her! looking forward to seeing how she sails.

[Brian MuirParticipant @brianmuir84327]

Had a go this morning, quite strong winds. No problem with stability, but still heels over and lets water into the cockpit, even with the added coaming. She now seems a little down by the head, perhaps I moved the mast to far forward?

Sailed well, especially when the wind dropped (see pic below), the self steering rig worked pretty well.

[Amy jane SeptemberParticipant @amyjaneseptember49770]

Well done, Brian, she looks like she's slipping along nicely! Being a bit bow down is probably the result of the keel being a little too far forward. a bit of lead, as far aft as you can get it (perhaps on the transom) should sort that out. The fact that the self steering works well would indicate the sail balance is about right. Perhaps you should consider putting a reef in the sails for a stronger breeze?

Any way good stuff, It's quite a thrill seeing a model you've built sailing along, You ought to be well chuffed!

[Philip Smith 4Participant @philipsmith4]

Its a bit rough and ready but there is a good photo of a scale model of an Americas cup yacht on page 76 of this:-

**LINK**

That should give you some idea of the ratios of various dimentions that you could try.

Good luck

Phil

[Philip Smith 4Participant @philipsmith4]

For that matter there is a design by Vic Smeed called Splinter on the Home page (Vintage Models Albums area), that shows a similar set of ratios.

Phil

 

Edited By Philip Smith 4 on 12/01/2014 02:22:14

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