scaled down marblehead

am new to sailing & currently sail a marblehead. However our lake suffers from lack of depth at times durring the summer making it difficult to sail marbleheads. I recently saw an RG65 sailing & thought it looked interesting & would give me something to sail when the water gets low.
I have plans for a Cumilus marblehead & was thinking of scaling it down to 650mm. My question is would this work? I understand the displacement would be about 25% of the marblehead but would this cause problems?
Also is there anything else to consider when scaling down plans?
Many thanks

The RG65 is basically a “Half Marblehead” so it is possible to scale down plans. You will have some issues with your displacement weight as most RG65’s are running about 1 lb. (16 ounces) as an average weight. Visit the class page on AMYA web site to verify class rules.


welcome. the RG65 is lots of fun and there are plenty of build logs here and plenty of options for hulls if you choose not to build your Cumilus…

take lots of pics…

Most definitely you can scale down a Marblehead to an RG65. You may have to do a tiny bit of tweeking to get the displacement that you want. But this is one of the few, if only, pair of classes that this works for.

Is this true or did you mean 1kg?

“BULB Weight”
0.45 kilograms … = 16 oz or 1 lb.
Purchase lead fishing weights from Cabella’s Sporting Goods
two 1 lb… weights were $7.00
Use them as they come - or melt and re-pour into a thinner/longer bulb.

my current albacore is going for the heavy weight award… 650gms for the bulb… hull with electronics is 350… rig is another 50. so just over a kilo…

Hi Gregg28,
I do have a class M (Marblehead ) that weight 4350g and the sail plan is 7200cm² with a bulb of 2950g attached to a 48cm long Fin.
What will be the corresponding values once scaled down to 650mm length ?
BTW for what I knows and from the web, I reckon that RG65 weights are varying from 850g up to 1100g while the sails area shall be max 2250cm²
Thanks a lot

Ah! OK.

As you know a straight reduction gives the following table:
127cm Factor 65cm
Disp (g) 4350 cube 0.134 582.9
Ballast (g) 2950 cube 0.134 395.3
SA (cm2) 7200 square 0.262 1886.4
Fin (cm) 48 linear 0.512 24.576

As noted, the rule allows 2250 cm2 sail area.

However, since the stability scales by the 4th power while SA scales by the square, a straight reduction leaves a very tender boat as indicated by the atypically low displacement. At this point I would increase the displacement by stretching the shadows in a single direction. If you were to pull the rocker down by 20% you would be up to a displacement of 698g. then increase the width by 10% and you would be at 768g.

The point being that since the M and the RG65 design rules are essentially the same with the only difference being the LOA, the design goals are the same. They differ greatly from an IOM, which dictates a minimum weight, maximum rocker etc. Which is why scaling an IOM down to an RG65 is not as simple.

well…a marble head is 127 cm. an rg is 65cm so for easy math we’ll say an RG is a half scale marblehead which means your scaled down marble head is going to be about 4" wide…

keep in mind though that the RG has a max sail area of 2250cm which is 30% of your 7200. and on a heavy side an RG bulb is 650gm which is only 22% of your 2950.

the thing you have to remember about scaling is that we can’t scale down the water or the wind…

if you look here, (thanks claudio d for the plans)

you can compare plans of the Fuxia marblehead with the fuxia 65 to get an idea of how well some stuff scales…

Hi Gregg,

The Ballast of 2950g is already part of the full weight of 4350g

The displacement for the RG65 will be thus 582g but that include the bulb of 395g therefore the margin left for the construction is 582-395 = 187g
Do you really think that is possible to build a full RG65 with 187g ?

Maybe you could read the two paragraphs that followed?

I read it !

But my conclusion is very different : the Class M cannot be strait scaled down to RG65, unless you modify entirely the hull shadows shapes and the rocker curvature !

Much easier to use a dedicated RG65 plan that proved already to be OK in the water !


I think that we agree on that.

Firstly, thank you all for your fast a comprehensive replies. I was aware that it wouldnt be possible to scale it down exactly because of the displacement. However you have given me plenty to think over. looks like the RG65 sail area would be too much for the hull without serious adjustments & at this stage I lack the knowledge to do those alterations. As much as I fancy a miniature version of my marblehead, I think it may be prudent to first build a recognised RG65 design to gain experience in the class.
What would you all recommend as a first build for an RG65? Although new to yachts, I have been building powered boats for around 40 years, so am capable of tackling reasonably difficult designs, although I would rather keep it simple & make it in wood & not go the composite route yet.
Many thanks for your help

The RG65 design “JIF” is a balsa sheet, hard chine (corners) that is a pretty fast build. There are several build logs on this site. Also check on the “Round Ranger” which is available from our member C. Huzway (spelling ???) and there are a bunch of free plans available on the net and via the Yahoo Group for the RG65 (look at Blue Splash as another well performing, good looking boat that can be strip built for a rounded bottom instead of flat) - all of them able to be built with balsa, and not to worry too much about fiberglassing if you simply coat the interior/exterior with a good marine epoxy (I recommend WEST Systems). Good luck and hope to see photos of whichever design you elect to build. These are not (for the most part) technical masterpieces and most are just good, decent sailing boats that will be competitive for local club races and perhaps even some National regattas. Only if you decide to seriously race National and International events will you need to look at serious designs and be careful with the total weight as you build.

I’ve done 4 for family - one for son, one for grandson, one for son-in-law and one for myself. They sail well, are easy to transport, and costs are minimal. You will need (when ready) two winches - one for sail control and one for rudder, a receiver and a transmitter., Look to Hobby King for the transmitter/receiver in 2.4 gHz for a cost under $50 and you can make your first set of sails from ripstop nylon from local fabric stores. Carbon fiber mast (tube) and booms can be purchased from kite supply stores and mast at 48 inches (you need to shorten per rules) can be found on-line for less than $10.00 - so in the end, with a careful build, you will have an extremely well sailing competitive boat at about 25 inches in length.

Lots of guys on this site to answer questions and give advice.

Good luck, Dick

You can easily scale M’s or IOM’s to RG-65’s. An IOM scaled to 65% has a displacement of ~ 1kg, right in line with most boats. To scale a M by 1/2 it needs to be an older M design that had 16+ lbs of displacement. Those end up right about 1 kg displacement, no changes to the frames needed other than making them 1/2 size. FWIW I am building a vane M based on the Doppler RG. It is double the size of the RG since the M rule I am building to is a 15 lb minimum weight.

Finally took the plunge & built the cumilus at 650mm. Increased the beam & hull depth by around 10%, using the very technical ‘it looks about right so that will do’ method. Its built from scrap & odds & ends that were lying around, all I bought was some dacron for the deck patches. All up weight is 1.1kg with a 675gramme bulb. Only sailed it twice in light winds, but it seems to sail well & it looks pretty too. Will try to get some pics on here this weekend. Unfortunately this is the only RG at my lake at the moment so it is hard to say how good it is compared to others. I dont expect it to be competitive with the current RG’s, but I am happy with the results. Thanks for all the help & encouragement given here.

Photos ???

I overlooked this thread somehow. Sorry for my comment, comin a bit late

It is true that a M cannot simply scaled by 2 to get a good RG. However, a M design might be a good starting point to make a well performing RG. Several boats have been build by more or less scaling down a M und blowing up the shadows a bit. The Little Jive and the Crazy Tube Too are two examples doing well here in Germany. My own design No.3 has started with the scaled lines of the Prime Number, which I modified according to my ideas. The JethRace, one of the best designs (or even the best) in the moment, was inspired at the beginning of the development by the M boats, too.

Stability is not an issue! A RG65 carrying full sail area and having something around 600g lead on a 25-30cm fin is usually performing well.
About 1kg displacement has proven to be a good compromise. Too less weight causes difficulties in stronger winds, but also in waves and unstable, light winds. In addition, you will need a very long fin to compensate for the low ballast weight, because hull, rigg and RC components require some minimum weight.

There was a very lightweight design some years ago at Lake Konstanz in the southern part of Germany. Under special condition it was extremely fast, but is was not usable as an allround boat. The idea of extreme lightweight boats was completely abandond since then.

Some inertia helps to let the boat run smoothly, but too much weight causes too much inertia in light winds, the boat will not accelerate because of weight and wetted surface. In heavy winds a heavier boat sometimes may have some advantages

Hi - Good to hear from you and thanks for posting. Hope all is well. Dick