Well why are they? You can perfectly well arrange sweep-back (or forward) which might be expected to give better directional stability.
Ideas please!
UPIGHT? Please clarify this word.
No I am not getting involved in leagalisms - ever again - promise!:zbeer:
Seen in profile on the drawing board, they are generally vertical and spproximately parallel-sided.
I made one keel with a straight shape about 35mm wide and 3mm thick, and a second one about 80mm wide. It looks like the profile of a trumpet. With such a short hull length, I think that having a keel with more surface area just under the hull will benefit the boat to make way instead of leeway. Too much surface at the bottom will stress the rig in a big puff, so I chose to taper the cheeks near the bottom to allow the boat to heal easily. I also think a rudder with much surface area will help too, but since a small servo is responsible for turning it and the boat, there is a limit to rudder surface area.
thx
I was figuring on sweeping the fin either forward or aft, according to how it seemed to be needed, based on the boat’s performance with the initial fin. The bulb placement could still have a degree of independence from the fin to keep the cg in the right place. But so far it hasn’t appeared to me that there is anything wrong with the initial fin, so I never got around to making a second one.
Mike Biggs
Mike - On M class boats I’ve tried fins that were swept forward, swept back, with parallel leading and trailing edges that were vertical, and with slightly elliptical leading and trailing edges that were tapered.
Forward sweep seems to improve tracking (directional stability), but the trade-off is that you need a longer keel (with more surface area) to achieve the same ballast depth. Flow over the keel is supposed to be more effiecient, because fluids have a tendancy to travel away from a fuselage on an airfoil (thats why on airliners there are raised ridges on the wings to redirect the airflow across the wing improving lift). A forward raked fin is suposed to provide the same benefit by having flow over the lower parts of the fin redirect the fluid from the upper parts of the fin that are trying to flow longitudinally away from the hull. Tacking is noticeably slower because the fin acts somewhat like a paddle, sweeping water out of the way as it turns. This effect would not desirable in a footy.
Roger Stollery told me long ago that some keel tests indicated that a leading edge that is swept back 9 degrees or so with a vertical trailing edge appeared to be the most efficient in terms of lift and drag. This info again pertains to the M Class. I don’t know if it has any bearing on the Footy but its probably a good place to start.
I think a more important concern would be would be keel area and section shape, which again is up in the air for the Footy. Anyone out there care to provide area measurements for their appendages? Kinda sounds naughty doesn’t it? Oh well…
Mike - On M class boats I’ve tried fins that were swept forward, swept back, with parallel leading and trailing edges that were vertical, and with slightly elliptical leading and trailing edges that were tapered.
Forward sweep seems to improve tracking (directional stability), but the trade-off is that you need a longer keel (with more surface area) to achieve the same ballast depth. Flow over the keel is supposed to be more efficient, because fluids have a tendency to travel away from a fuselage on an airfoil (that is why on airliners there are raised ridges on the wings to redirect the airflow across the wing improving lift). A forward raked fin is supposed to provide the same benefit by having flow over the lower parts of the fin redirect the fluid from the upper parts of the fin that are trying to flow longitudinally away from the hull. Tacking is noticeably slower because the fin acts somewhat like a paddle, sweeping water out of the way as it turns. Slower tacking would not desirable in a footy.
Roger Stollery told me long ago that some keel tests indicated that a leading edge that is swept back 9 degrees or so with a vertical trailing edge appeared to be the most efficient in terms of lift and drag. This info again pertains to the M Class. I don’t know if it has any bearing on the Footy but its probably a good place to start.
I think a more important concern would be would be keel area and section shape, which again is up in the air for the Footy. Anyone out there care to provide area measurements for their appendages? Kinda sounds naughty doesn’t it? Oh well…
Mike - On M class boats I’ve tried fins that were swept forward, swept back, with parallel leading and trailing edges that were vertical, and with slightly elliptical leading and trailing edges that were tapered.
Forward sweep seems to improve tracking (directional stability), but the trade-off is that you need a longer keel (with more surface area) to achieve the same ballast depth. Flow over the keel is supposed to be more effiecient, because fluids have a tendancy to travel away from a fuselage on an airfoil (thats why on airliners there are raised ridges on the wings to redirect the airflow across the wing improving lift). A forward raked fin is suposed to provide the same benefit by having flow over the lower parts of the fin redirect the fluid from the upper parts of the fin that are trying to flow longitudinally away from the hull. Tacking is noticeably slower because the fin acts somewhat like a paddle, sweeping water out of the way as it turns. This effect would not desirable in a footy.
Roger Stollery told me long ago that some keel tests indicated that a leading edge that is swept back 9 degrees or so with a vertical trailing edge appeared to be the most efficient in terms of lift and drag. This info again pertains to the M Class. I don’t know if it has any bearing on the Footy but its probably a good place to start.
I think a more important concern would be would be keel area and section shape, which again is up in the air for the Footy. Anyone out there care to provide area measurements for their appendages? Kinda sounds naughty doesn’t it? Oh well…
And they( people on RCgroups) say that there’s nothing technical, or nothing innovative, to do with Footy boats! :devil3:
Not until you mates get to thinking. Been at the pints again, eh? :sly:
Pints for everybody = World Peace! :graduate: 
Here’s a keel shape for ya. This hull was behaving badly in light winds, it seemed to slip sideways and never make headway. Bill suggested that I broaden the keel some like he has on some of his Razors. So I added an extension. It behaves much better now. I’ll smooth out the rough spots and paint it later.
It might not have been exactly what Bill suggested but I happy with the results.
Agree with Niel here,I was talking to a current AC designer just before Christmas.
We were talking about keels and other stuff.he claims that 8 degrees sweepback is one of the magic numbers.
It is stuctural reasons why the AC and other big boats don’t use this sweepback.The keels just wouldn’t be able to hold the weight of the bulb on this angle without having odd section shapes etc.
8 degrees-
Isn’t that the same angle that the performance planes of WW II used on the wings?
Come off it! On longer keel to achieve the same ballast depth involving more watted area, that’s wrong. The area of the set of parallelograms with given distances between the two pairs of parallel sides is constant (http://en.wikipedia.org/wiki/Parallelogram). If you maintain a constant leading edge length, the area will fall, as will the draft.
I agree that a greater distribution of lateral area fore-and-aft will reduce the rate of turn. This may or may not be a good thing. Footys do not track well. Some are said to go about badly. Is this perhaps a result of poor wave penetration when head-on to a wave encounter, or is it actually a result of a low rate of turn?
Why can’t the people on RCGroups discuss things in this way??
Angus, I think what Niel is saying is that the leading edge will have to be longer to get the same keel depth. Righting is determined by the perpendicular distance to the bulb not the path to the bulb. I think he wanted the righting forces to be equal. Longer keel = more area.
Or am I misreading something. Typos could be confusing me.
Angus,
Is this the idea you had in mind for the topic? That’s Nigel’s hull (no offense meant, Nigel.)
no offence here man.
i was actually thinking about the potential, of a northern pike, or big-a$$ musky, attacking my footy up in cottage country.
a keel like that would bite back
I wanted to find a 10 inch chef knife, but I settled on the $1 cheapie.
My dad caught a couple pike up near North Bay once. We kids caught lotsa catfish in a brackish pond near the campsite; the bears caught a couple too.
I purposely made a fin with 7 degrees of sweep and an offset tongue so I can bring the ballast around in case it needs more weight in front or back. You can’t use the usual bulb shape in that case, so I think I will try a disc shaped ballast.
thx