Mast-aft Footy Daft?

[DarnonKhan]

Has anyone ever done a mast-aft footy? I'm thinking of something like this (http://www.runningtideyachts.com/sail/) .

I was thinking this would be a super simple rig and could possibly have quite a large sail area. I'm not even close to competitive Footy racing so couldn't give a rats :censored: for class rules and so forth.

Would it be very difficult getting the rake of the mast correct, and if I were to put the keel on the guesstimated COE how wrong could I be? I'll make sure that I make the rake of the mast adjustable, at least until I've got it tested. I make sails from flower wrapping so cost and effort are minimal for testing.

Should I try and do you have any tips or suggestions?

[martin richards]

I seem to remember Canadian Marblehead plans in the old Model Boats magazine - it must have been the late 70's - with a chine hull and a mast aft rig. I think one of the advantages touted was, with the mast being out of the way, sail aerodynamics were impoved. It did however have a "normal" sail plan. If it were being done now, an "A" frame mast would probably be suggested, probably with three sails to lower the CE.

[mfr02]

The thing to remember is that the sail is an aerofoil, and to work usually needs a rigid leading edge. Conventionally, the mast does this with the main.
When a sailboat is working, the one place that the wind doesn't come from is in front, so a mast at the front, is more out of airflow than one at the back. Just a thought.

[Gregg28]

The thing to remember is that the sail is an aerofoil, and to work usually needs a rigid leading edge...

How does a jib work? Or for that matter, a spinnaker? The mast just needs to supply a tension point to pull the sail luffs against. Having one in the back does get the mast out of the mains airflow entry, but it does put it at the exit. The A-frame puts one side in the relatively free air above the main, but the other side is in the jib's exit flow. While having a conventional mast is not perfect, it is the most widely used because there does not seem to be a configuration whose added efficiency outweighs it's strength and simplicity.

Also, the aft mast will require a lot more structure at its attach point and must be stiffer. A Conventional mast is under mainly compressive forces. An aft mast will also be under bending forces, with the fulcrum at the deck.

[Earl Boebert]

On the other hand, having the mast aft of the longitudinal center of flotation (LCF- the point around which the boat rotates when pitching) means that the down pressure from the mast inhibits any tendency to dive when running.

Cheers,

Earl

[DarnonKhan]

I was hoping that on the small scale of a footy the mast could be rigid enough to keep tension on the leading edge of the sail without requiring lots of support.
My previous footy did have a slight tendency to dive in gusts of wind so the mast aft could have a slight effect on this.
The mast will obviously still add drag.
This would be such a simple thing to do, I'm going to try anyway. The great pity is that I don't have anything to compare this sail performance to. I don't race other footys so if I were to say "It goes really great!" there is only so much weight that could be attached to my assessment.
I do have a concern that the sail will have too much luff when trying to sail upwind as there will be no boom.
Like I said, it's very quick and easy to try so I'll keep you posted with a report and pics.

[rcgroupie]

On the other hand, having the mast aft of the longitudinal center of flotation (LCF- the point around which the boat rotates when pitching) means that the down pressure from the mast inhibits any tendency to dive when running.

Cheers,

Earl
I have trouble visualizing this. Where does the down force originate, from the stays, or from aerodynamic loads?

[Earl Boebert]

I have trouble visualizing this. Where does the down force originate, from the stays, or from aerodynamic loads?

It appears to be from the aerodynamic loads. In the 1930s there was a long and heated debate in the UK Marine Models magazine as to whether down force really existed or not. WWII ended it without conclusion :-( I sail a free sailing boat, fixed LOA like a footy, with a sliding rig. The rig is racked all the way back on the runs, and you can definitely see the difference in the pitch axis versus the times when I get tired and forget to adjust the rig :-)

Cheers,

Earl

[rcgroupie]

So the downward force is generated by the sail? That would make sense since any downward thrust generated by the stays on the mast would have a net zero force against the water - sort of lifting yourself by the belt.
If it's the sail generating the force, it is the effective center of the sail where the thrust is being applied to the boat, is it not? Thus would it not be the location of the sail, not the mast, that determined pitch?

[Gregg28]

The "downward force" is a rotational force. It is down in the bow, but up in the stern. It is generated at the center of effort and rotates around the center of buoyancy. The mast position really has nothing to do with it. as rcgroupie points out.

[Earl Boebert]

The "downward force" is a rotational force. It is down in the bow, but up in the stern. It is generated at the center of effort and rotates around the center of buoyancy. The mast position really has nothing to do with it. as rcgroupie points out.

I guess we'll have to agree to disagree on this :-) I might note, however, that the CE on a run, when the boom is all the way out, is close to the mast position. Also, the hull rotates around the LCF, (calculated as the geometric center of the plane flotation, i.e., the shape made by the LWL) which is not necessarily the same as the LCB.

As a point of general interest, useful drawings of hull hydrostatics can be found at:

http://www.oneoceankayaks.com/smhydro/hydro.htm

Cheers,

Earl

[Gregg28]

I might note, however, that the CE on a run, when the boom is all the way out, is close to the mast position.

This is true when the luff is attached to the mast. However, its location is a function of sail position, not mast position. On a run, the CE moves forward regardless of mast position.

Also, the hull rotates around the LCF, (calculated as the geometric center of the plane flotation, i.e., the shape made by the LWL) which is not necessarily the same as the LCB.

That is true. There is a moment component.

[DarnonKhan]

A lower COE (which is supposedly one of the benefits) would theoretically result in a lower rotational force because of the shorter axis?
I have a Razor hull already built and the angle of the transom looks ideal for mounting the mast. I'll give it a go and let you know. I might have to use carbon instead of my usual aluminium.

[mfr02]

How does a jib work? Or for that matter, a spinnaker?
The jib has a leading edge supported by the forestay, or something similar. A spinnaker is a downwind only device.

[Gregg28]

The jib has a leading edge supported by the forestay, or something similar...

Sorry. I must have misunderstood your original post. A rigid leading edge as supplied by a mast or a taught forestay, as you say.

[martin richards]

Doesn't your RG-65 have a carbon fibre rod jib forestay, Earl?

[Earl Boebert]

Doesn't your RG-65 have a carbon fibre rod jib forestay, Earl?

Yes, and a jib radial as well. Jib is 50% of the sail area, to get the CE forward and the mast aft of the LCF.

Cheers,

Earl