Author Topic: Lifting keel extra weight  (Read 5461 times)

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Offline claudiof

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Lifting keel extra weight
« on: January 07, 2007, 19:15:28 PM »
I saw on the web only very few protos with a lifting keel. The Didi mini (www.dixdesign.com) proposes one for the cruising version of their mini design. The dum question is, why is the ballast weight so high for a lifting keel? Definitely it must be higher that a fixed keel if it is not as deep, but for the didi mini for example the "cost" is 110kg more. Is this due to the Classe mini rules?

Offline claudiof

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Re: Lifting keel extra weight
« Reply #1 on: January 15, 2007, 09:18:55 AM »
Re-reading myself I notice how bad the question is stated! I'm much smarter now (yeah) after I read lots of post on this forum, so let's try again.

It is quite clear that the Didi Cruise-mini lifting keel results in lower performance than the fixed keel option: 1.6m draft instead of 2m (hence extra bulb weight) and probably less area. If you look at the Transat mini and Cruise-mini options on the Didi site (dixdesign.com), it's basically like comparing a proto against a production mini. The point is, I would like to build a mini myself but with some of the production boats characteristics, because it will be a (small) cruiser-racer.

Now, can someone comment on how this difference translates in a typical mini race, beyond the podium places? An interesting point is that the Didi mini has a very light hull (I heard 165kg, but I'm not sure at what stage of building), so the 110kg extra weight difference is somewhat reduced compared to the heavier protos. But what is the tendency of the protos racing out there? All carbon or will there be other heavier protos I would have a chance to compete against?

I know it's all about compromises. Just trying to figure out the impact of the different options (see also other posts on windage).

Offline NicoG

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Re: Lifting keel extra weight
« Reply #2 on: January 15, 2007, 09:27:12 AM »
i think 110 kg is a lot, thats 10% of the weight.
I have not hte time to search all the dix info, next time some direct links would help too :)

Just ask your question to Dix.

And forget the windage thing.

Offline claudiof

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Re: Lifting keel extra weight
« Reply #3 on: January 15, 2007, 20:25:56 PM »
Right:

Didi cruise-mini: www.dixdesign.com/didiminicruise.htm
Didi standard mini: http://www.dixdesign.com/didimini.htm

Dudley Dix mentionned in a previous e-mail exchange that the lifting keel results quite evidently in less performance due to the extra weight. I'll dig this a bit further with him and post the answer.

Not sure what you mean by forgetting the windage thing: Forget a higher roof (and extra windage) or don't worry about the performance impact?


Offline NicoG

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Re: Lifting keel extra weight
« Reply #4 on: January 15, 2007, 21:10:23 PM »
dont worry be happy for cabin...

Offline windswept

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Re: Lifting keel extra weight
« Reply #5 on: January 16, 2007, 03:35:46 AM »
  The typical proto keel bulb weighs 300kg plus what ?50 kg for carbon shaft. So 350 kg. Almost all
the sieres have a 450kg total, with 275-300kg in bulb and  150-175 in steel foil.  So 100kg or 110 kg
is typical weight change. Sieres needs 450kg total to meet 90 degree rule with reduced draft and
steel fin requirement.  For all thier attempts to keep the cost affordable for production boats I find it strange there going to allow lithium  batteries. You could buy a carbon spar for less money!!

Offline GDesign

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Re: Lifting keel extra weight
« Reply #6 on: January 16, 2007, 19:49:50 PM »
if you use a carbon composite structure for your keel you dont need more than 10kg of laminate there (and you can do with much less if you take it to the limit).
Most protos still use metal structure, Steel, Weldox.. etc.


Offline claudiof

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Re: Lifting keel extra weight
« Reply #7 on: January 17, 2007, 08:44:20 AM »
Here is DD's answer to the question:

"The Didi Cruise-Mini is aimed at people who want to cruise or to race under handicap systems in areas where there is shallow water. It is not intended to equal the performance of the Didi Mini. The Cruise-Mini keel is a lot less efficient than the Didi Mini keel for various reasons. It is shorter and wider, so it has a lower aspect ratio. The keel will not develop as much lift because of this, so the boat will not sail as high to windward. The keel is also thicker, so it has more frontal area, surface area and volume, which gives it more drag.
 
The bulb is heavier mostly because the keel is shallower. It also has to overcome the greater buoyancy of the bigger keel. Righting moment with the lifting keel will be a bit more than with the fixed keel but it is not much. This is fine because it does not have the advantage of water ballast.
 
If I was to design a lifting keel for the racing version, it would be made from aluminium or carbon, not glass covered wood. This would allow it to be close to the fix keel in size and shape but parallel. It would only lift inside the cabin with a short box to about 300mm above DWL. It would bolt down onto this with a flange and butterfly nuts. I may do that one day but don't have the time available to do it now."

Offline NicoG

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Re: Lifting keel extra weight
« Reply #8 on: January 17, 2007, 10:39:32 AM »
oh, it was shorter :)
normally a fixed keel or liftingkeel difference is about 15cm more keelfoil and an endplate on top (alu), but you can laminate the box a bit lighter. So its about 15kg heavier, and really strong.
(like the Zero)

Offline claudiof

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Re: Lifting keel extra weight
« Reply #9 on: January 17, 2007, 18:00:02 PM »
The zero has 1.6m draft isn't it? I wonder what is the boat overall performance compared to protos with 2m draft...

Offline NicoG

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Re: Lifting keel extra weight
« Reply #10 on: January 17, 2007, 21:03:38 PM »
slower :)

Offline claudiof

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Re: Lifting keel extra weight
« Reply #11 on: January 18, 2007, 20:06:13 PM »
Hin hin, you just concluded on my various questions on the forum. I'm (almost) sure now that I will build a Didi mini with "race" options. Plus a trim tab on the keel. Just feel like it: lots of unknowns, not sure what the result will be... anyways Minis are for the craziest sailors, isn't it?

Offline NicoG

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Re: Lifting keel extra weight
« Reply #12 on: January 18, 2007, 20:18:32 PM »
good luck, forget the trimtab. spend it on something smarter :)

Offline windswept

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Re: Lifting keel extra weight
« Reply #13 on: January 18, 2007, 22:24:42 PM »
    Have you looked at francois lucas  cp 650  for multichine, looks like it has a more powerful hull shape
 than didi if your thinking in that direction.  www.fr-lucas.com/cadreus.asp

Offline claudiof

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Re: Lifting keel extra weight
« Reply #14 on: January 19, 2007, 21:08:25 PM »
I got details on the Mini CP already. It's a top design and Fran├žois is a respected designer and sailor. See also his Class 40 with a measured top speed of 32 knots!!! The Didi mini has more building options but the Lucas Mini CP is definitely on my list.

The list by order of preference is currently the following:

- Didi Mini (www.dixdesign.com/didimini.htm)
- Lucas Mini CP (http://www.fr-lucas.com/site/realisations.asp?detail=27#liste)
- Villenave Proto 650 (http://asso.abv.free.fr/bateaux/p04turb650.htm)
- Finot 650 (http://www.finot.com)
- Julien Marin 650 (http://www.julienmarin.com/_fr_newslire.asp?num=4)
- Selway Fisher 650 (http://www.selway-fisher.com/Yacht2024.htm#TRANSAT)

Tough call! I'm switching back and forth between the study plans and changing my mind every day or so...  Hope to fix my choice really soon :-)