Swept back fin

[Jamestj]

I am trying to design an IOM and have received lots of good advice from this discussion board, even when my questions showed my complete lack of understanding of boat design. Could someone help me with this question please?

Airplane and full sized yacht fins usually sweep back so that eddies are encouraged to move to wing tips. Eddies are then encouraged to go away by tapering the fin / wing towards the tips. (Birds, I understand, split the flow at their wing tips by spreading out the feathers at their wing tips like fingers and achieve a similar effect.)

The fins on IOM's usually hang straight down. I can imagine that this would reduce wetted area and be an advantage in ghosting conditions but would this not promote eddies if the wind picked up?

A sweped back fin would also allow the mast to be stepped onto the fin giving engineereig advantages which could lead to weight savings.

Finally, the protruding bulb and straight fin would seem to be an ideal shape to catch debris which would slow the yacht down.

Has my understanding of Physics let me down or have I missed something in the class rules?

[Arvin S.]

James,

I am not a boat designer, nor know a lot about physics, I just know how to sail

But a short answer to your questions is:
That is the nice thing about IOMs, the rules are open to hull development (with some restrictions) that you can actually find out for yourself

For instance you can get one or two IOM hulls but use different appendage (fin) designs and test them in different conditions. Cool huh?

One thing that I understand you can't do to IOM coz of the rules is to use "wind tips" or hydrofoils on your appendages.

The long answers are:

Do also understand that there is such a thing as scale effect, which from what I understand means that what works on full sized boats might not work on an IOM. Case in point, you won't be able to make an actual scale model of the lastest America's cup boat sail the same way. Another example was one that was discussed in a different topic in this forum, when the 3M "shark scale" sticker for hulls will not help an IOM go faster coz it will create drag, though it worked for the Cup boat it was used on before.

Also I think you just saw "some" of the IOM designs when you said that IOMs use less swept back fins, I have seen some IOMs that have radical designs even a front swept fin designs.

Check out Lester's "Chairman" technical site for more info on physics of IOM sailing:
http://www.onemetre.net/Technicl/Technicl.htm
http://www.onemetre.net/Technicl/Fillets/Fillets.htm (this site deals with Fins)

We need (like) thinking and analytical people like you and lester

TTFN
Arvin

[Steve Landeau]

Two simple reasons for parrallel fins as opposed to double tapered fins on an IOM:
1) A parallel fin is simple to make. More IOM builders are home builders, not commercial.
2) Based on what designs are winning, or at least fast enough to be capable of winning, the double taper does not seem to be "better" (or worse...).

I have tried both on my Patriot, and have not seen any significant difference. Making the DT mold is either very hard by hand or very expensive on the pocketbook.
Theory shows a DT fin to have advantages in stiffness and drag, but so far it has not shown on the water.

[Jamestj]

Models are used to predict the performance of full sized hulls and therefore copying a full sized hull would logically work on an IOM. This is clearly not the case since large changes to a hull may not cause an obvious change in performance.

Is it a case that sail design and sheet control systems determine IOM performance?

I am working on a school project and would like advice on how the pocket money should be spent so ideas would be most welcome.

[Arvin S.]

Is it a case that sail design and sheet control systems determine IOM performance?

I am working on a school project and would like advice on how the pocket money should be spent so ideas would be most welcome.

James, technically sail design is the one thing that is very restricted in IOM class. However I do agree that sail design does affect the sailing performance of a sailboat. But keep in mind that as I said IOM rules on sails is very restricted and thus experiements on sail design should follow these rules. (If your school project wants to concentrate on IOMs as a class).

Sheet control system is not too important in my opinion, thuogh it does help with peformance in some situations. Eg. a fast sail servo makes it easy to sheet in and out and "flip gybe". While a very weak winch or servo might not have enough power to take in the sails in Strong winds. But again I dont' think there is much development left in sheeting system.

Please expound on what you want to acheive with your school project? Then maybe other sailors will be able to help you with it. IF your project's aim is to make a winning IOM design, I think a lot of people are also working on this project too and unfortunately it is not an easy one.

Good luck!
TTFN
Arvin

[Jamestj]

Thanks Arvin
I am trying to make a winning IOM but I dont want to copy other peoples designs. However I do appreciate ideas from other people. I would also like to know why IOM's are designed as they are. All ideas are welcome.

[Arvin S.]

James,

I wish you luck with your endeavor.

Some notes which shouldn't be considered "law" at all.

1. I understand IOMs are designed with specific sailing conditions in mind and thus they vary a lot in design. Meaning some designers come up with boats that do great in light winds (canoe types) while others make boats that are great in strong winds (like skiff types) and a lot of boats in between.

2. I found reference to a boat (unfortunately can't remember which site it was), which showed a very narrow wooden boat that is almost all home built, it won a major race in it's area. Which goes to show that you can win with home built boats given the right conditions and sailing skills.

3. Of course my generalization about canoe and skiff types is just that I am pretty sure there are some skiff types that do well in light winds but that is generally what I was told or have found out.

4. So you want to make a race winning IOM design? Either make one that is designed to the prevailing conditions where you will be racing or make one that can do well in light and strong winds (but this is probably the holy grale of IOM or boat designs in general). Heck even some America's cup boats don't do well in 20knots of wind, while some offshore boats are just getting settled in 20 knots.

TTFN
arvin

[Steve Landeau]

James,
GENERALLY (certainly not the rule),
It is easier to make a narrow(er) boat go well in the heavy than it is to make a wide boat go well in the light. It's not as related to wetted surface as it is to balance changes as the hull heels.
Unless you are using a computer design program, or have time to read a good book (Principles of yacht design by Lars Larsson & Rolf E. Eliasson), it will be tough to draw a great design on your first try.
Good luck, and keep us posted.

[Jamestj]

Many thanks for the replies, guys.

I still don't know why a skiff type should be good in a blow. The bows are so fine that they don't look as if they would lift the boat out of the water if running downwind. I would think that it would have terrible lee helm if caught broadside in a gust and weather helm when it accelerates. Would this not make a skiff type difficult to control?

The rules allow 3mm flare in the bows but designers tend not to take advantage of this, in fact Triple Crown is quite rounded in cross section, all the way through.

Perhaps the opposite of a skiff would be a scow which has a wide and flared front end. They are designed to be sailed on their ears and plane easily. The underwater shape does not alter so much when sailed at a large angle of heel so steering would be easier.

Why is the scow design is not used in IOM designs please?

[Chairman]

I found reference to a boat (unfortunately can't remember which site it was), which showed a very narrow wooden boat that is almost all home built, it won a major race in it's area.

Hi Arvin

Michael Scharmer's boats are all home designed and home built. His latest placed 4th in the Euros 2004, and is the very narrowest possible IOM that meets the class rules.

[Chairman]

The rules allow 3mm flare in the bows

Hi James

This is commonly thought. In fact the rules allow unlimited flare. What is limited is any "hollow" when measured parallel to the water line. Section H of the IOM Class Rules gives a good diagram of this.

[Jamestj]

Thanks , Lester. This gives me some more ideas to ponder.

It seems that the consensus is that a slim hull works best. A slim hull would need less epoxy and my weight problem would be solved.

Are there any owners of beamy boats such as Boxkite who could claim that their design was superior?

[Chairman]

It seems that the consensus is that a slim hull works best.

Hi James

Hmmm... IMHO it is still horses for courses. If you'll usually sail in light to moderate winds, then yes, narrower beam would probably be better. If you'll usually sail in heavier winds, then beamier is probably better. If you are going to travel around and enter events in a variety of locations, compromise and a moderate beam is indicated.

But the jury is still out on this, since so much depends on the (unknown!) fine detail of rocker line, stern flatness, bilge hardness, and the distribution of volume in the hull. At the Euros 2004 Scharmer's ultra-narrow was locked in a 7-day battle with Borin's (wide-ish) Ikon and at the end there was 1 point in it at an event which was mainly top-of-A-rig. At the Worlds 2001 Cameron's (ultra-wide) TS-2 was locked in equally tight battle with Bantock's (narrowish) Italiko in an event which was mainly moderate, middle-of-A-rig conditions. Byerley reckons a couple of millimetres makes all the difference, but like the wasted 50% of any advertising budget, wishes he knew exactly where ... (smile)

[Arvin S.]

If the conditions you sail on has relatively smooth water with very strong winds the wide beam boats will do very well. It would be better if you try to search for the IOM worlds results and see which boats did best and what designs they were, don't just look at the top 10 though because a good skipper with less than optimal hull can still beat a bad skipper with the best hull for that condition.

If we talk about big boat designs consider that all the round the world ocean racers have wide beams.

But then again I do agree with Steve that moderate to narrow beams give more flexibility when you know that the wind might change from light to strong, while it would be harder to make a wide beamed boat to work in very light winds.

My two cents.
TTFN
Arvin

[Patrick Bigand]

Maybe a different twist on the same topic - pun intended - I built a fin and I have about 3 cm of flex or curvature if I hold the fin horizontal from the box end with the 2.4kg ballast attached. Is there any performance issue or impact directly linked with fin flex? How much flex is tolerable?

[Muzza]

James

Nobody seems to have addressed your question about scows yet (which I though was a pretty interesting question). I remember some Marbleheads that were scows in the early '80s. I think I've even got a photo somewhere.

Without sitting down and working it out, my gut feel is that the 4kg minimum sailing weight for an IOM may be a little too heavy for the scow concept. I'd be surprised if someone, somewhere, hasn't already tried the concept on an IOM. Anyone know?

[Nigel28]

Patrick
I look at it this way:-
Energy is a simple thing, you can't make it or lose it you simply transfer it from one thing to another, ideally from the wind into boat speed, therefore your boat speed can only be reduced if some of the rigs energy is being used to flex the fin or hull ( remember one result of either of these flexing will be a small increase in heat, the energy for which must come from somewhere ) In my opinion stiff is good.

[Arvin S.]

Replying to basically two topics here:

Re: Scow
I agree with Muzza, scows are designed as flat light boats that use crew hiking out hard to compensate for heeling. Scows in my opinion have a big SA/D (sail area/displacement ratio) which if scaled won't be IOM class legal. I haven't seen an IOM that is really flat like a scow though, so it might be worth it to experiment with Not sure but a scow IOM might end up looking like a skiff type IOM that is a bit narrower, now don't know if that will mean it would be great for light winds or strong winds. Some scows have 2 rudders coz of their heel, this is not allowed on IOMs, so you might have to compensate with a longer rudder.

Re: Keel blade sag

In my opinion the problem with a flexible keel blade would be more important when the boat is at near maximum heel, in that case the keel system will not be giving maximum righting moment because of the sag. In light winds I doubt if this is important, heck it might even be a bit of an advantage. Consider that in big boats they are actually doing the opposit of what you have, they are "canting" their keels to windward to reduce heel, this is of course illegal for IOMs or any other RC class that I know of.

I agree with Nigel's conclusion though, Siff is good. But if you sail in moderate to light airs a bit of flex won't hurt you too much I imagine.

TTFN
Arvin

[Jamestj]

[quote]In fact the rules allow unlimited flare. What is limited is any "hollow" when measured parallel to the water line./[quote]

Thanks for this information, Lester. I have re-read that part of the rules and it is now obvious to me.

Perhaps the waterline shape of a Triple Crown with wide flared bows would work well. It would stop the boat going nose under and it would be easier to steer

[Patrick Bigand]

Thanks for the answers. Since I molded the fin box over the fin, increasing the fin thickness is no longer an option. I'll call my first IOM a light airs boat and build myself a moderate to stiff winds boat next time. So it's not a bug, it's a feature.

[Jamestj]

In reply to Arvin and Muzza

I have recently been told that the scow shape originated from the days when the rating was based on waterline length. A boat with a long waterline would have a speed advantage over a short one. This resulted in long overhangs. By making the bow very flared, the effective waterline increased when the boat was healed.

This effect would not work with the IOM since it is the overall hull length which is measured so stem and stern are upright to maximise waterline length. However, IOMs have a tendency to submarine when pushed hard downwind. I understand that measuring the mast from a low part of the deck can reduce this effect because:
1) The wind is weaker close to the water
2) The turning moment is less if the centre of effort is lower.
I learned from these forums that many IOMs have concave aft decks so that the mast height above the waterline will be less.

I cannot understand why the turtle deck forepeak in such designs are often used as an atachment point for a mast jack. This is used to lean the base of the mast aft. My instincts would lead me to pull the lower part of the mast forward so that the back stay would take the mast into a gentle back curving er.. curve. If you used a mast jack and the mast was prebent forward, would you not get an 'S' bend in the mast with a mast jack?

Since some sailmakers have good reutations for the cut of their sails, are you supposed to design the rigging to set the mast to conform to some standard bend? If you didn't the sail would be baggy in the wrong places.

Since I have not got an IOM yet I might have missed the plot somewhere so where have I gone wrong?

Woops, have gone away from the subject of the original thread.

[edmorales]

james,
i believe that the mast ram controls lower mast bend. i use the jib topping lift as a bend reference when adjusting the mast bend.i line up the jib club amidship,next to the mast and use the topping lift as a guide when adjusting the vang and mast ram. you will notice how these 2 adjustments can effect the mast bend, and you can clearly see if your mast is straight, or slightly bent aft, or s-shaped
ed
edit: n.b. this works on my boat which has a deck stepped mast.i assume it would work with the keel stepped designs as well

[Chairman]

I cannot understand why the turtle deck forepeak in such designs are often used as an atachment point for a mast jack. This is used to lean the base of the mast aft. My instincts would lead me to pull the lower part of the mast forward so that the back stay would take the mast into a gentle back curving er.. curve. If you used a mast jack and the mast was prebent forward, would you not get an 'S' bend in the mast with a mast jack? Since some sailmakers have good reutations for the cut of their sails, are you supposed to design the rigging to set the mast to conform to some standard bend? If you didn't the sail would be baggy in the wrong places.

Hi James

Getting the right bend into the mast is quite a challenge. Get it wrong and you'll set an "S", as you say. And, it is not uncommon that the bend you worked hard to get just right for sailmaker "A"s product will yield a badly set sail from sailmaker "B". (Not uncommon? Well, for me, a certainty...!)

The idea of starting with a pre-bent mast (bent forward, that is) is that you achieve higher backstay tensions (and the point of *that* is to get higher forestay tension) when you then bend the mast in the right direction. With a carefully set mast ram, you can contol (constrain) the bend of the lower part of the mast, and achieve a useful hike in backstay tension in doing so. Then, you will set up your shrouds and spreader so that they also contrain the mast bend, so a further hike in backstay tension is achieved. To do this, you will likely have the chainplates (shrouds to deck attachment) set aft of the mast step, and the spreaders will have that amount of "V", and will be set vertically on the mast, so they fair out the mid-mast bend nicely. Finally, your upper mast bend is now directly controlled by the backstay, and this is the region where you will have concentrated the majority of the mast pre-bend in the first place.

To revist where you started, the high peak of the foredeck is the ideal place to put the mast ram for maximum leverage and sensitivity in lower mast bend control. Also, the jaws add vital lateral stiffness, and it's always been my view that this lateral stiffness is the most important part of this design, though little understood, 'cos we're not on the boat to see what's happening when it is heeling and the mast is bending laterally.