Given a marblehead fibreglass shell.

I have been given a marblehead hull, as i dont have much experience with the class (IOM sailer)im keen to get some pointers on how to set it up, I would like info on where keel should go, It has a reinforced pad but it seems much further forward than my IOM in relation to its position compared to hull length. The mast step I am going to track as well as the stay position and jib position, what size carbon tube should i use. Should I go for a flat deck or some sort of raised fore deck sunken aft deck. more questions to follow im guessing


Check out the Aussie Marblehead forum and website - you’ll find it linked off the Australian Radio Yachting Association website.

See if you can get our hands on a little information about the hull you have:

  1. What design is it?
  2. What is the designer’s target weight for the all up boat?
  3. Is the design intended for swing rigs (especially the A rig) or conventional rigs?

With regard to the mast position, be aware that within the total sail area, you can come up with any mix of jib:main ratio. There are two common flavors - and the forward maststep on your boat might suggest that the designer had in mind a small jib/large main combination. On my boat, the mast step is about 45mm long, so I can move the mast through a wide range of fore and aft positions. I have a sliding through-the-deck mast chock to accomodate this range of movement without having a large unsealed hole in the deck.

If the boat is to be rigged conventionally (non-swing) then you have the choice of a stayed mast or an unstayed mast - the latter is very common. Your choice of rig will help you decide the mast type and section you will be looking at. It is quite possible to build an M mast with three (or even four) lengths of straight carbon tube of progressively smaller diameter - such tubes being available from kite suppliers. Alternatively you can obtain one-piece tapered carbon tubes to various specs - but you may expect to pay a higher price.

Also check out (Alan Hayes’ NZ site) and see the link to the Ultralite spars under NZ Businesses on the links page.

The cool think about these boats (apart from their performance) is that, unlike the IOM, you have a pretty free hand in terms of how you rig them and set them up - they make a great compliment and diversion to your IOM (ironic really, given that the IOM was a diversion to the Marblehead while they were getting established).

I’ve been going through a similar process to you - having recently acquired an M hull for a bit of fun and to compliment my other boats. Compared to my IOM, the hull of my M is very light and fragile. Weight is critical in this class -especially over hear where our average windspeeds are light.

If you have an active fleet of Ms near you, check them out.

Hi Muzza,
Im having trouble finding the design on the net, ,It appears to be styled on a volvo 70 or other wide beam ocean goer, the transom is nearly flat, it doesnt have a deck so a lot of the info i need isnt readly “visible”. Im fairly sure it is a conventional rig as no keel box is fitted either. Im not going to compete in a marblehead only class, our club sails a mixture of boats with a handicap system. I can get an idea of the displacement, just have to float it.

Certainly sounds curious for an M, at least an M younger than 15 years old.

The hulls have been getting progressively narrower over the years. 190mm - 220mm seems to be the range for many of the modern Ms.

Here is a pic of the hull

Just float tested the hull, it needs about 3.7 kgs to get stern and bow to kiss the water in fresh water.

Nick -

with that shape of hull and if it is an “M” class size, you might consider being a charter member of the 1/10 class idea being floated here on a different thread. Just find a boat that has a moden design that appeals to you and consider dressing it up . Probably boats in the 39 - 41 range would be close to this scale size - and perfect lines of a hull would be hard to decipher. Make the deck “look” like a big boat, have some “stand-off” scale equipment and you might find it would be more fulfilling than trying to “force” it to be in the “M” class. I agree with Muzza in that most modern “M’s” are pretty narrow.

This one hull you posted has a bit of a modern sport boat look to it - and might be a scale of an IACC or Open series boat - maybe at 1/18 or 1/20 scale?

Just an opinon.

Dick’s suggestion is certainly an option, especially as you’ve mentioned you are not planning to race it as an M, but if that 3.7kg was added to nothing but the hull shell, you are still looking at quite a light boat and therefore a potentially competitive M. If the hull shell itself weighs, say, 500 grams max (given that you do not have the weight of the deck and interior fitout), you are looking at an all up sailing weight of only 4.2kg - that is a light M.

My boat weighs 612 grams without electronics. The keel and bulb together are 2861 gr. The rudder is 56 gr. Electronics add just under 350 and the rigs (I’m building new rigs) will not be lighter than 300 - or about the same as my IOM rigs. So that suggests a sailing weight of 4.2 kg approximately. That’s a comptetive weight for the light North American conditions. Antipodean and European Ms are often a little heavier - you’ll see plenty in the 4.5 - 4.8kg range. A competitve M of 15 years ago - say the 1992 era Walickis, were around 5.2kg.

What ever you do - if your boat sits on its lines with just 3.7kg added to the hull shell, you are going to need to build as light as you can if you are not to sacrifice bulb weight.

(Translation for Mr Lemke - this boat will have a sailing weight of only 9lbs 4 ozs. :wink: )