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Apr 12, 2007, 10:11 AM
Registered User
RhysPorter's Avatar

Keel Fin IOM

Ok, ages ago you might remember me asking some questions about building an iom... Well ive done it... and my Hull is complete :-D

But now i need a keel and rudder fin :-(
One of the guys on here used a helicopter blade for his Century 750's keel fin, and i'm just wondering if i could use a bigger one for the IOM?

Also, does anyone know the average weight of a bare IOM hull without the keel, electrics or rigging? I'm interested to see how mine stacks up seeing i built it with strength in mind...

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Apr 12, 2007, 10:26 AM
rcminiman's Avatar
Hi there, Your looking at around 400-600 grams for a bare hull, minus the deck. Im not a 1M guy (YET!), but I was reading up about them recently. Anyway, its built, and you can only ballast it to the designed weight.
I dont know about using a heli blade for a 1m. My Marblehead has a balsa cored fin and rudder, skinned using thin plywood glued to the balsa using epoxy. Id do the same on a 1m, using 1/32 ply instead of the 1/16 I used. The fin is tough, cheap and easy to make.

Apr 12, 2007, 10:43 AM
Registered User
RhysPorter's Avatar
SWEET! Mines 500 grams with the foredeck...

Hmmmm, dont really think i could be accurate enough making my own fin... would much rather a carbon one too
Apr 12, 2007, 11:24 AM
rcminiman's Avatar
You could use a balsa core and cover it with carbon cloth and epoxy. Thats how the fin for my M was supposed to be made.
I would watch that the heli blades are fully symmetrical sections before use if you do go that route though.
You can buy ready made wooden/carbon fins, but the price puts me off every time! I built a complete M for half the price of a carbon rudder. Perfect!!
Apr 12, 2007, 02:48 PM
Registered User
Have you built the keel box yet? I'm assuming that you built neither the keel box nor the aft deck at this stage?
Last edited by Just Lurking; Apr 12, 2007 at 02:50 PM. Reason: Really bad typing skills.
Apr 12, 2007, 09:36 PM
Registered User
RhysPorter's Avatar
Thats right i havnt made the Keel box or Aft deck yet, was planning on doing that this weekend.
I was planning on just glassing a solid balsa box into the hull, then drilling and sealing 2 holes for keel bolts?

I can pick blades up for $40 on ebay? how much would the carbon cloth be etc?
Apr 12, 2007, 10:19 PM
Registered User
search carbon fiber cloth on ebay. They sell tons of it for cheap. I wouldn't use it on a full sized boat, but they'll be WAY under the working load here, so you'll be fine. Also, if you want REALLY light stuff, google CST composites. They sell it all the way down to 2oz (99.999% of what you find on ebay will be 6+oz's which is overkill and a half for a cored keel fin.
Apr 13, 2007, 09:38 AM
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RhysPorter's Avatar
hmmmm.. $40 for one sheet of the 2oz carbon. Thats the same as what i've paid for the hull so far (I got a few freebies like the glass)
I might just use normal fibreglass for it if i have to do the core thing, I have some left from the hull and its pretty thin stuff.

Might ask for a damaged heli blade in the Heli forums just to give it a test... try both and see which looks/sails better i suppose.

Thanks for your help :-)
Apr 13, 2007, 12:27 PM
Registered User

If the carbon is a little out of the budget range for now, I'd go with the ply envelope over a core. If you are looking at the heli-blades, make sure you have enough chord and area. I don't think I've seen a heli-blade that will do the job for an IOM - but they may be out there. As for dimensions - the simplest is to stick with a parallel fin of 85mm chord. That will give you sufficient area.

Your proposed method of fixing the keel to the hull is fine - but keep in mind how you are going to attach your mounting bolts to the fin, and also how you are going to attach the keel bulb to the fin. Make sure you have a plan in mind before you take the next step.

Most of us (and I think every professionally built IOM I've seen) use a keel box which comes up to deck level, with a single bolt in the top of the fin wich extends through the deck to take the restraining nut.

The advantage of this system, apart from the fact that the fin is easily removable (check the rules) is that you can use the keel box as a strong point and build the loads to it. On my boat (own design and build) - the chainplates (and therefore rig loads) are tied back to the base of the keel box. The mast box is tied to the keel box. The main bulkhead (there is only one) is tied to the keel box. The sail servo mount is in part tied to the keel box. Further load bearing units are tied indirectly - viz - the deck level mast partners and the mast ram (via the main bulkhead) and the deck itself - across the top of the keel box - which further spreads loads from the rig and main bulkhead. As a result, much of the internal structure you see in some homebuilt IOMs is completely unnecessary, and the rest of the hull and deck just needs to keep water out.

The easiest way to build a good keel box (IMHO) is to build your fin first, then mould the keel box in fibreglass around the top of the fin. If you do this, you get a perfect fit. I recommend running such a keel box through the hull, rather than mounting it to the inside of the hull and then cutting the through-hull hole to size. In my experience, it is easier to get accurate alignment, and a good, strong, watertight seal by taking the box through the hull. I'm sure you've already worked out that you will want to have the keel fin inside such a box before you mount it in position - and then use a jig, spirit level, plumb bobs and a good set of eyes to get things lined up. My approach, once I'm happy with alignment, is to hold the box in place with temporary balsa and a couple of dabs of CA glue. If more is needed to hold things in line, you can use a removable glue (such as with a glue gun) and temporary braces across the top of the keel box. Then go head and glass the whole lot in place permanently, after removing the fin of course.

I hope this helps.

Apr 13, 2007, 06:23 PM
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RhysPorter's Avatar
Thanks heaps :-)
Shall use a lot of that info and make some changes to my keelbox :-)
(It was only a 15min job with balsa bits of ply sandwiched between)

But first i gotta make this fin....

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