Questions On How Keel Design Affects Lying Ahull Or Heaving Too

Studied the David Vacanti article again while awake and the taper in the keel he was referring to was not cross sectional but longitudional, Thought I had this taper in the cross section answered. Man getting detailed info on long keels is like finding hens teeth but i'm slowely getting there. I've got a good handle on most aspects now with exception of this cross section taper and what effect it has on performance. Ya I know performance on a long keel whats he talking about it doesn't exist, well thats the general consensus however it doesn't mean one shouldn't strive to get as much out of it as possible. Afterall it is still is the best rated keel for open ocean work. Thus far here's what i've learned to gain the most out of the full keel family
#1 break them up into a long keel--a cutout--and a skeg
#2 Forefoot area is an important factor in the ability to "heave to."and "tacking"
#3 The cut out area is an important factor in ease of tacking
#4 If the long keel section is too long to build in a foil then go with a foil leading edge- a parallel mid section and a foil tailing edge.
.#5 My unanswered question, what effect does a tapered cross section have on a long keel-- Is cross section taper between the root and the tip good or bad. I suspect it has a negative effect but can't get any info on it. Question answered see posts (20 & 21 below)

While Davids article in Professional Boatbuilder doesn't address my question as I thought it did I'll post it anyhow for general info.

 

This engineering and design query has been presented for almost a week now(also on another thread) beginning to believe the unbelieveable that maybe this "cross sectional taper" has never been considered as a factor in long keel design. What about foils, is cross sectional taper a factor in foils ?--Geo.

 

I'm not sure if I'm right in this but what comes to long keels like old pilot cutters (and also to slender hulls without keels and daggerboards like outrigger canoes me thinks) it's better to forget scientific theories about air foils etc. Instead the major influence affecting leeway comes from the narrow beam which resists leeway by it's form according the "formulas" of hull speed, in this case affecting just to sideways. - Maybe some experts can shoot me down in this or not?

 

No doubt those factors play a role, prime example the Fisher Motorsailer. It has a very small full keel area in relation to what one envisions on that size hull. Then again windward performance certainly is not one of it's assets and rightly so as it is in the motorsailer class not intended to do so. What baffels me is the amount of longitudional dimension modifications that have been made to the full keel to improve it, why wouldn't they have looked into the other dimension "cross sectional". Other than rounding the sharp corners not much more seems to have been considered. It's not as if they were suddenly dropped in favour of their foil counterparts. Full/long keels are still in vogue today because of their great seakeeping qualities. One would think there would have been or should be research to further improve it by looking into the cross sectional taper and on how it affects performance. Example does taper in the cross section re direct the flow downward along the length of the keel fore to aft as the vessel moves thru the water. If so, reasoning says this will increase the crossflow and result in more drag. However there could be another hidden factor playing a role here that more than compensates and offsets or even neutralizes this. That being by re directing the flow downward it enteres into a deeper and cleaner flow pattern along the keel creating less crossflow and drag. So by manupilating the cross sectional taper we can possibly further improve the long keels(non foil) performance. But which way do we go and how much, increase taper, reduce taper, combine a root taper and a straight section. If this research has not been looked into and I can't believe it hasen't then why not ? I would dare to guess that 75% of the long keel owners out there have pondered how to improve their windward ability with some actually posting on this forum their intention of cutting off their keel and replacing it. My question is and i think it's a bonified one under "boat Design" can the long keel be further improved by controlling cross sectional taper. The longitudional manupilations seems to have reached their limit what about the cross section.---Geo

 

Viking, The questions you're asking about tip-to-root thickness ratio are legitimate. However, the answer is not so simple as it might seem (as usual).

If you consider just the keel with no hull (which becomes an equivalent of a wing), your conclusions are correct, and the reason is laying both in the spanwise flow components and in the fact that fatter foil sections have a bigger form (zero-lift) drag. For example, a NACA 2408 (8% thickness) foil has a minimum drag coefficient equal to Cd,min = 0.0055, whilst a NACA 2424 (24% thickness) has a Cd,min = 0.0075 (from Abbott and Doenhoff "Theory of Wing sections").
So if you have two hypothetical keels, the first one straight and made with 8% thick foil, and the second one tapered from 24% at the root to 8% at the tip, you will get the result that the second keel is more draggy because foil sections it is made of are more draggy near the root.

See the attached calculated polars (second picture) for a low AR keel (no hull) moving at 2 m/s (3.9 kts). It is for visualisation purpose only, the real flow around this type of keels is more complex than the software I use can handle. However, it does show a qualitative difference between the two cases. What you can read from the graphs is that a thickness-tapered keel will give less lift for the same leeway angle, will have more drag for the same lift and will have the point of maximum efficiency shifted towards higher leeway angles.

Now, the above considerations are valid for isolated keels (no hull) only. Once you have a keel-hull combination, other drag forms have to be taken into account.

The first one is the interference drag at the keel root, where it intersects the hull. If this intersection is not faired (which means that the keel has to widen towards the root and gently marry with the hull) you will have a strong vorticity generation along the line where the two intersect and form a corner.

Another additional drag when considering keel-hull combination is due to an increased wetted surface of an unfaired, or untapered keel. You can see it more clearly in the first attached pic, where the difference in wetted girth for 3 types of keels is visible. The case number 3 in the pic is an extreme one, put there just for comparison. The point is - a higher wetted surface means a higher friction drag.

Finally, the hull motions and the resulting wave system will invalidate some important common assumptions used in the theory of finite wings (where most of our knowledge comes from). The flow field superimposed on the keel will not be steady and rectilinear, for example, so the wing (or isolated keel) analysis techniques have to cope with some additional numerical complications.

Resuming the above facts, you have two well-known counteracting effects here:
1) a straight keel will have a smaller form drag but a higher friction and interference drag when mounted in a hull.
2) vice-versa, a tapered keel will have a higher form drag but a smaller interference and friction drag when put on a hull.
and perhaps several others which are less known.

Hence, the optimum will be somewhere in between (as it usualy is), probably shifted towards the straight keel.
But don't ask for numbers, because there's probably no person around who can give reliable numerical estimates of these effects.

Cheers!

 

Daiquiri, I got it 100% thank you-- This morning I basically gave up on the possibility of an answer within the time frame i was working with, i had to go onward with building the mold so I made a compromise and built all my mold bulkheads with both a taper and a straight section. The long keel( not a foil) is 8ft. long by 24in. deep. The cross section is 91/2 in.at the root tapered to 71/2 in. half way 12 in,down and run straight for the other 12 in. resulting in 71/2 in. wide also at the bottom(tip). My thinking was if after i had the keel built it was discovered that a full taper from root to tip was more beneficial I could simply fill in the hollow between the two with foam and glass it over.If it proved that a straigh cross section was more benefical, well at least i had it 50% right. Well while not pre engineered to be it looks like I might have the best of both. The other feature I was trying to build in was the ability to experiment with a scheel style and design bolt on wings to experiment. The flat section makes this easier to do. Ok thats good, once again thank you, I'll have my very good friend Sammie Gueridano* on Sicily drop by with a nice bottle of wine --Geo.
P.S. should have added this note, While I do have a good friend from Sicily, This is fictionious name.--Geo.

 

Tell him I'm very picky when it comes to wines.

And make that fillet around the root abundant. The bigger the radius, the better.

Cheers!

 

Will do, thanks again my friend--Geo

P.S. knowing Sammie as I do the quality would never be questioned However come to think of it he might be in Toronto this time of year so it could be Canadian wine ???

 

To those following this thread re info on full keels I will do a followup ( in a few months)on my final design on my build thread under Boatbuilding (The Nancy G---) once my designer has fine tuned my proposal--Tnx. Geo.