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Author Topic: scaled down marblehead  (Read 1935 times)

knoby

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scaled down marblehead
« on: January 27, 2013, 11:45:21 PM »

Have been thinking about building an RG65 & was considering scaling down a marblehead plan i have to 650mm loa. I understand this will make the displacement 25% of the marblehead. Is there anything else I have to take into consideration when doing this?
I know there are RG65 plans available, but just fancied a smaller version of my marblehead if it was possible, but since i dont know a lot about yachts, I just wondered how easy it would be


Thanks Glenn
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roycv

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Re: scaled down marblehead
« Reply #1 on: January 28, 2013, 09:26:11 AM »

Hello Knoby, Couple of things.
Firstly scaling down from your 50 inch loa Marblehead to the RG65 or 650mm which is 25.6 inches.  Consider if you were going down to 25 inches or half size then the calculation is 1/2 x 1/2 x 1/2 which is 1/8 or 12.5 % volume or weight.  So half what you were thinking.
You would end up with perhaps a less able yacht to compete with the RG65 class.

So, which Marblehead plan do you have?

There are a few designs called half M (Marblehead).  Model Maker (which continued as Model Boats and others) magazine tried to introduce this size and they do look elegant.

The best looking one I have seen is a half size Betty, total old fashioned elegance.

kind regards Roy

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knoby

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Re: scaled down marblehead
« Reply #2 on: January 28, 2013, 09:19:47 PM »

Thanks Roy, i was going over the figures in my head last night & had a feeling i had them wrong.
The marblehead plan i have is a cumilus. Tonight I did a quick scaled down drawing & whilst it looks cute, it does appear very thin, which gives me doubts about its performance.
The half marbleheads sound interesting, will look into them further. If not I will try to find an RG65 plan that i like the look of
thanks Glenn
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knoby

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Re: scaled down marblehead
« Reply #3 on: January 29, 2013, 07:20:29 PM »

Finally got my brain into gear tonight with regards to scaling things down.
RG65 sail area is about 33% of a marblehead
RG65 ballast seems to be around 22% of a marblehead
So that wont work will it.
Only way forward seems to be increase beam, say 10%, & decrease rocker by around 20%, but thats starting to get complicated.


Looks like the best plan is going to be build from an RG65 plan for first boat & gain some experience in the class with that boat.


Looked for half size marblleheads but couldnt find any info on the web, can you give me any pointers where to look?
thanks Glenn
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tigertiger

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Re: scaled down marblehead
« Reply #4 on: January 30, 2013, 02:47:17 AM »

The other thing to consider is sail areas.

The hull will be 1/8 of the original by volume. The sail area will be 1/4 of the original. Race boats are usually finely balanced designs changing thinfs this much may lead to another imbalance.

As I am no expert, I would use a plan for a boat of the size I wanted to build, where somebody has already done all the maths. But that is me.
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knoby

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Re: scaled down marblehead
« Reply #5 on: March 08, 2013, 08:01:38 PM »

After a long think, have decided to build a racing sparrow rg65. Like Tiger said, someone has already done the maths on this one &, reading various reports, it seems to be a great sailer.


Cheers Glenn
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