Deck shape

[Jamestj]

I have yet to make an IOM so my question might seem a little trivial so please excuse this possible waste of forum space.

I understand that sail dimensions are measured from deck level and the wind is usually strongest the higher you go. Why do most IOM's have the mast on the lower deck and not on a cabin roof as full sized vessels are usually constructed (the sails would catch the wind more effectively)?

I am finding it difficult to design a boat close to the 4kg mass. The hull would weigh about 1kg so this is where savings have to be made. A full length turtle deck would be stronger, lighter amd easier to make than a concave aft deck / truncated fore turtle deck.

Please, what is the advantage in measuring from lower down the mast and why are many IOM decks so complicated?

[Chairman]

please excuse this possible waste of forum space

Hi James

No space wasted by any serious question...!

I understand that sail dimensions are measured from deck level

Not quite. Measurement of the *rig* is from a mark you make on the "deck". You can place this mark in more than one place, and so chose to specify where you want your "deck" level to be. You might need to look at a skiff-type cockpit to see the point I'm trying to make, however... The *sail* dimensions are measured in relation to the sail, rather than the deck.

the wind is usually strongest the higher you go. Why do most IOM's have the mast on the lower deck and not on a cabin roof as full sized vessels are usually constructed (the sails would catch the wind more effectively)?

Yup, and would catch the wind rather too effectively towards the top of the wind range of the rig, particularly no.1 rig. The jump to no.2 rig is quite large, so most designers figure they want to be able to hang on to no.1 rig for as long as possible -- hence they lower it as much as possible, and shrug their shoulders if the event features light airs...

I am finding it difficult to design a boat close to the 4kg mass. The hull would weigh about 1kg so this is where savings have to be made.

I'm not a builder, but yes, a light hull is a very good idea. Ideally so light, in fact, that even when fully equiped with batteries and radio gear, you still need to put in 100 or 150 gm corrector weight.

[Jamestj]

Thanks for the encouragement, Mr. Chairman.

I have made a mock up out of builders foam sprayed onto a plywood former. I thought that a high foredeck would reduce the chance of the hull burying its nose into the water when running downwind. It would also divert the wind to blow into the foresail. I also formed a raised aft deck. This could also allow a better sheeting angle for the main sail without the need for a long bit sticking up aft of the mast. The high aft deck also masks the height of the foredeck. I have a well deck behind the mast about 60mm deep so that the kicking strap does not catch the aft deck. The shearwater has a pronounced curve to help the water to run out of the well. The whole effect looks very realistic if the object was to build a pretty sailing yacht.

I cannot find any pictures of similar deck shapes. Would this arrangement be contravening the rules or are there any disadvantages that I have not thought of?

The mock-up is very rigid and weighs just 750 grams despite being solid. It is a shame that foam is not allowed in the construction since it was cheap to make and quite easy to make when I knew how to do it.

[Roy Thompson]

It is not unusual in some parts to use this type of foam core as a male mould from which to take a hull. It's a bit like how they hand make surf boards but you will remove the foam 'core' after moulding the hull outside it (opposite to a normal mould). By preparing the hull carefully, (normally removing the thickness of the finished grp shell from the male foam plug), you can get a reasonable hull from it. You will of course have to sand down the hull (outside) to get a good finish before painting (same as a wood hull though) and carefully clean all the releasing agent from inside before attempting to fix in the finbox etc, but I have made hulls weighing approx 300 grams (without deck) which are stiff enough for their wt. And that's without fancy rollers or vacuum bagging etc. two layers of glass, with extra reinforcing in strategic places and bingo, instant hull. Especially if you've already done the work on carving the hull from foam.
The hard part is fitting it out, keel box and alignment etc... but that's another story.