I've been looking at iceboats for a few years now, and I've got a fundamental (I think) quesiton. What is the deal with the mast down pressure and where does it come from? Why would the iceflyer have any more down pressure than any other iceboat?

I'm not talking about the force on the mast from sheeting etc, I'm asking about the down pressure that I've seen discussed as moving the center of gravity, and making the runners stick to the ice better. I guess I don't get it. someone with some knowledge please explain.

thanks

I'm sure others can explain this better, but i will try. The sails on iceboats act like the wing of a plane. Wind creates lift in the sail which translate into side force. Hopefully, your runners are sharp enough so you do not go sideways, but forward. Since not all of this energy is released in the forward motion, it creates downward pressure on the mast. I know when my DN catches a puff of wind, the entire boat sinks downward.

Since the Ice Flyer uses a wind surfing rig, I'm not sure how much downward force there is on the mast. Especially since the carbon fiber masts are so flexible, a lot of energy is released as the mast bends.

Wally...please jump in here.

Roger L.

Roger, I think your descreption of why you get the down force is good.

(here is the long "winded" answer - my opinion only, could be wrong) but I think in general, you also need to consider that there can be two mechanisms of down force.

The first is what Roger described and its simply because the sail is constrained at the bottom so the lift vector of the sail ends up putting a "torque" on the sail and results in a down force at the mast base and to some extent an up force at the sheet location of the sail.

The second down force doesnt really apply to boats which use a windsurfing sail "rigged like a windsurfing sail" but on a conventional sail, part of the sheet force is setting the shape of the sail and this also puts a static down force on the fusalage where the mast base contacts the fusalage. Since a windsurfing sail is rigged with all the tension - and it is a LOT of tension - for setting the sail shape "internal" in the downhaul before the sail is put on the boat, you still get some of the this second force with the Iceflyer since the sheet has a down force vector but it is significantly reduced compared to a conventional sail.

I have had the Iceflyer is some just fricken wild high winds (because of any liability I might have, I dont recemend this to anyone) and I cant say that Ive yet experienced its wind limit. I think this is partly because the twist of the windsurfing sail which lowers the ce of the sail and also spills some wind but I believe its also because the down force of the mast is about 4 inches behind the boats static center of gravity. So no matter what the mast down force is, it will at worst shift the boats center of gravity by 4 inches. Why is this good? First, the needs to be a balance between the sail center of effort and the boats center of gravity and because of the stable location of the boats center of gravity, the balance with the sail center of effort is also stable.

The other benifit of the stable center of gravity is that you want the boat to remain with "understeer" where the rear runners have equal or higher down force than the front runner. Ie, its stable if the front runner slides first in a turn, not stable if the rear runner slides first. Exactly equal wieghting gives maximum turning capability while remaining stable which is the way the IceFlyer should be balanced. So even in high winds, the CG of the boat never shifts forward creating an unstable oversteer condition (which I believe is probably the main cause of "flicker" or spinouts in ice boats).

Not related to the mast downforce but the other thing that allows the boat to sail in high winds is that there is a lot more wieght on the front runner and the front runner stays on the ice and in control when you are going fast and let of the sheet so that the sail is only generating drag and trying to lift the front end.

Of course, the benifits of the stable CG on a small boat have a down side and that is that the pilots weight is further forward on the Iceflyer so the pilots wieght is less useful in keeping the boat from hiking which limits how hard you can sheet - which in turns limits how much power you can get from the sail. The "fix" for this is an aerodynamic down force rear beam (and a heavy beam also doesnt hurt) but the aerodynamic rear beam is not as efficient as pilots weight to the rear for sheeting the sail - but seems to be fairly close. But the pilots weight to the rear creates all sorts of control problems in higher winds so there are tradeoffs either way.

The following is a cut and paste from iceflyer.com

"The sail mast downward pressure location (where the mast attaches to the fuselage) is placed as close to the boats cg as possible. The reason for this is that the mast can exert a huge downward force on the fuselage and being a force just like the pilots weight, it can shift the boats center of gravity towards the mast down force location. By placing the mast down force near the boats cg, the "dynamic" cg of the rear sail boat shifts very little during sailing."

My point here is that the mast force as explained here is not just like the pilots weight and does not do anything to effect the boats center of gravity. The pilots weight significantly effects the location of the CG, while the vertical mast force does not. Since the center of gravity is not changed, the force on the runners to the ice does not change. It has to do with your point of reference. the pilot's weight is an external load, while the vertical mast force due to sheeting is an internal load. Further, the

I agree there are 2 reasons why there is a downward mast force: one from tipping, and one from sheeting. neither of these shift the center of gravity fore and aft, the sheeting does not shift the CG at all. It could be argued that the tipping force may shift the "dynamic" CG laterally along a line perpendicular from the static Cg to the line drawn through the front and rear runner bolts (or Hiking line). In that case, as long as the dynamic CG is inside the hiking line, the boat is on all 3 runners. When the Dynamic CG is outside the hiking line, then the boat goes up on 2 runners. Either way, the net vertical force on the runners remains the same. There is no increase because there is no net increase in vertical forces. Granted, the windward runner's vertical force goes to zero as you hike, but the leeward runner's vertical force increases appropiately.

The upshot of what I'm saying here is that the vertical force on the mast is an internal load and does not contribute to increased performance. It has to do with your point of reference. The pilot's weight is an external load, while the vertical mast force due to sheeting is an internal load. Further, the force on the mast due to tipping does not result in a net increase in vertical forces because it is offset by an equal and opposite upward force on the windward stay.

I think the highly flexible mast and the sail's ability to twist off and subsequently lower the CE is the main reason why the iceflyer has such a high wind range.

That is just my .02 cents. It does does not change the boat's performance, it is just an academic arguement for what is going on. As with anything else, my .02 cents are subject to critique.

I think that certainly the down force on the mast created from a conventional sheet tensioning the sail to create shape only creates internal tension to the boat and does not effect the location of the center of gravity. Similar to the tension on a windsurfing sail put in with downhaul - it has only effects on the internal tensions of the sail. I also think that stays will affect the location of internal tensions on a boat but on overall boat CG, stays or unstayed work the same.

But.. You would have a hard time convincing me (you still might be correct - but just would have a hard time convincing me) that the torque of the sail as its creating a lift vector which is not in line with the direction the boat is traveling and some component of that lift vector is contrained by the lateral resistance of runner or to some extent forward drag of the boat - is not creating a down force at the mast and an up force at the sheet. Ie, Im pretty sure it is creating these forces and they also act on the boat just the same as pilots weight and will shift the boats center of gravity. The pilots wieght creats a force because of gravity. The mast creates a force because of the wind. But they are both forces acting external to the "internal structure of the boat" and affect the location of the cg.

Assuming that the sail does create a torque on the boat (which I think is correct), we have to recoginize that a torque is more complicated than just a mast down force and actually a torque could shift the center of gravity up even forward of where the mast is located! However, as you pointed out, the windward runner lifting will tend to result in the weigth distribution of the Iceflyer being on just the two runners and without considering the effect of the sail forces, would result in 2/3 weight on rear runner and 1/3 weight on the front runner (a change from equal weight on all runners with no sail force). Ie, this would try and shift the CG backward from what it is with no wind. So to some extent, these two effects compensate each other and the simplified explanaiton considering only mast down force doesnt really seem that far off based on what seems to actually happen with the boat.

??????????

woops, made a mistake. It is the rear down force foils which will dynamically shift the CG towards the back and not the windward runner lifting (which will only shift the CG "laterally". Dave, the lateral shifting of the CG you described is an interesting concept..

One thing I have never understood on this boat is that even without the down force rear foil, the boat was always good in high winds and was almost less likely to hike than in light winds even when doing linked down wind turns in high winds.. I think there is something else going on that Im missing.

"It is the rear down force foils which will dynamically shift the CG towards the back"

I'll buy that. The forces created by the foils are in the vertical direction, and serve it increase the net downward vertical forces on the runners. Same concept as race cars use to keep the tires on the ground.

To Anonymous:

The resultant force vector produced by a sail is generally accepted to be in the horizontal plane. I don't see why the iceflyer rig would be any different. The newer DN rigs with their bendy masts complicate that assumption, but for a typical rigid mast/wing, the vector would be in the horizontal plane. That resultant vector acts at the proverbial Center of Effort of the sail and is pointed slightly forward and significantly to the leeward side of the boat. The forward component of that vector gives you the go, and the leeward component gives you the tendency to hike. Since the runners would need a vertical force to increase their resistance to sliding, and the resultant force vector produced by the sail is essentially horizontal, I'm saying the wind in the sail does not increase performance in the way of making the runners stick to the ice in turns. Bottom line is if you sum the forces in the vertical direction, the sum is the same as it was and did not increase. Look at a catamaran. They do not sink into the water any more due to the sail in the wind. They hike, or rotate about the CG. Part of the boat goes up, and part goes down.

Either way, the iceflyer is pretty fun to sail. Especially on beam reaches in high wind. I've experienced the tendency to hike in lighter winds more than high winds too. One of the best days of sailing was on smooth ice with about 8-10 mph of wind steady. I was able to hold the hike for 30 seconds or more. I could sail for what seemed like forever on 2 runners. What a blast. I think the higher winds tend to twist off the top of the sail, and bend off the top of the mast. That part of the sail is not doing anything but creating drag at that point, so a smaller sail may be better. When the sail twists off (as it's designed to do for windsurfing) the CE drops way down, and the hike is avoided. 15+ years ago, I used to sailboard with an old 5.0 meter camber induced windwing with a very stiff mast. In gusts, I would literally get thrown forward (launched) at times. The newer masts and sails do not do that since they bend and twist off. That's great for wind surfing, but I wonder if we'd rather they not twist and bend so much on the iceflyer. I wish I still had that old mast a 5.0 because I think it might be pretty powerful on the iceflyer.

Anyway, Sail safe and have fun.