The more I study this hull the more it intrigues me.
Attached is a graph plotting the LCB vs LCF on heeling.
I think this may just be the reason for the good balance.
The LCB and LCF are nearly in the same place as she heels,I have never seen these two centres as closely matched as this before.
I have also rescaled the lines to IOM length and displacment…interesting
and even down to a footy.all the hulls have the same characteristics as the original.
I also wonder how a slimmed down version of Riptide would work as a modern M?
with a deeper fin and reducing displacment to around 5kg it could be interesting.
I am just a dummie when it comes to this stuff, but I feel like I am learning something just by “listening” to you guys discuss this. It is very interesting. Is there an actual picture of this boat RipTide that I could see, to better understand all of this?
Bob
Hi
Nice looking hull! Reminds me a lot of the Martin Firebrace designs, ‘Ericca’, ‘Eureka’, and so on. How did you go about developing the sections? Immersed parts look close to circular arcs, so how did you go about developing the change of volume with section station? Can you post a curve of areas (maybe a screenshot from Maxsurf) to show how the bows are fuller?
I have completed my study of this hull in hullform.
I traced the section view that Earl kindly put up here into AutoCad,then imported it into hullform,carefully traced the hull,then went back peice by peice editing the sections from the offsets provided by Autocad.
My hull lines are now almost exactly those of “Riptides”
The final tuning of the design has not changed the various parameters or nature of the hulls behaviour that I mentioned earlier.
I think I now have a better idea of what is needed to design a balanced hull form.
Earl mentioned in a earlier post that he designed a hull in which the LCB did not shift on heeling,he noted the hull was unbalanced in practice.I to have been thinking that this was a desirable feature.Riptide has made me think that the LCB and LCF should have a closer relationship,this is much harder to acheive.
shoot me a e mail anyone if you want a hud file for “Riptide”.
Hi Brett
OK, I’ll bite! So, do you now think that a balanced hull form requires LCB and LCF to have a closer relationship? If so, what is the nature of this “close relationship”?
While I find these discussions of “hull balance” interesting, and I’d certainly like to understand hull design better, it seems to me that what we are actually looking for is a boat, not just a hull, that tends to track straight in spite of variations in the wind. A slight tendency to yaw to windward is useful in gusts, but mostly we want the boat to go straight until we tell it to turn.
Once the boat heels, the large distance between the center of drag on the hull and the center of effort on the sails produces a substantial couple in the boat to yaw to windward. What we’d like, therefore, it seems to me, is a hull that has a tendency to yaw to leeward when heeled, in order to counter the general tendency of any sailboat to yaw to windward.
Whatever, it looks to me like some are on the right track, which is to look into design parameters of boats that track well. Maybe the look just needs to be broadened to include the fin, bulb, and rudder. (Just… he said. Hee hee hee. I am serious about what we need, though.)
Mike Biggs
In considering the factors contributing to the balance of a boat such as “Rip Tide,” it is worth noting that the vast majority of the high-end American M’s of this period used a sliding rig. Therefore you can assume that the sailplan is perfectly positioned for the conditions of the day, and the principal effects on balance are those contributed by the hull.
Cheers,
Earl
Hi Mike
I am always very happy to be corrected, but I wrote earlier:
The problem with current conceptualisations of the mechanism of balance and weather helm is that they do not take this property of any hull into account. Given that this is what a hull does all by itself, its natural tendency to weather helm is “controlled” by the lead of the CE of the sail plan ahead of the hull CLR.
If a hull was somehow designed so that it did not show any tendency to yaw to weather when heeled, or even had a tendency to lee helm, nothing much would change, except that the lead would be reduced to zero or even driven negative. That is, we would still be left with the problem of balance, because this is a problem of what happens as heel changes, not of what happens at a given heel.
The conventional explanation of weather helm with increasing heel has us thinking that the drive force / drag force couple increases with heel, leading to increasing weather helm.
But, given the increased tendency of the hull alone to weather yaw as heel increases, it now follows that in a balanced boat the drive force / drag force couple must actually decrease with heel… The boat becomes unbalanced when the couple “fails” to decrease. In mathematical terms, the couple must in fact be negative, and become increasingly negative (ie decrease more!) in order to balance the increasing yaw of the hull.
So a different approach needs to be taken to the issue of balance. If this is true, the real issue is how to have the canoe body tendency to increasing weather helm with heel exactly match or keep pace with the ‘decreasing’ (negatively increasing!) couple. In mathematical terms, the issue is not the amount of yaw or the size of the couple as such, but the rate at which these two values change with heel. A well-balanced boat is one where the rates at which these two values change are perfectly matched.
Heck, Lester, I can’t believe I overlooked the couple between the hull’s lateral resistance to leeward motion and the lateral forces on the center of effort of the sails. Nothing like leaving out half of the really important forces and resulting moments.
The forces on the sails may be resolved into leeward and forward components, and the flow forces on the hull may be resolved into windward and aft components. For calculation of yawing moments, only horizontal distances between the forces count. So if there is no heel, there is no yawing moment due to the couple between the sail drive force and the hull drag against foward motion, because there is no horizontal distance between the two. As the boat heels, this distance increases (because of the elevation difference between the two resultant forces), but the magnitudes of the forces on both the sails and the hull are also affected by the heeling, so we do not know from these thoughts whether the moment increases or decreases.
It appears from the above that balancing the boat implies that the mast and fin must be located such that the CE of the sails is slightly forward of the CR of the hull (including rudder and fin), so that there will be a leeward moment to offset the windward moment when the boat heels, as well as any tendency of the bare hull to yaw to windward. Which is what you said. Seems like I’ve even read that before.
Looks like it will take some thought for me to get a handle on how the magnitudes of the forces vary as wind speed increases, increasing heeling. Right now it seems unlikely that the usual velocity squared relationship applied to the diminished apparent area of the heeled sail would be all that accurate, as there would be an increasing flow of air from foot to head on the sails. This could contribute to heeling, and to the lateral force on the sails, but not to driving force. However, such an increase in lateral force without an increase in driving force would tend to produce a leeward yaw rather than windward. Which is why it looks like I’ll have to give it more thought.
To summarize, there appear to be three somewhat independent moments at work: the hull itself, especially when heeled; the couple between the lateral force on the sails and the lateral resistance of the hull; and the couple between the driving force in the sails and the drag on the hull. All we have to do is to keep them balanced over wide-ranging wind speeds and our boats will track true.
Mike Biggs
Hi Lester,Im not exactly sure of course,but think I may have a better understanding from studying a hull which apparantly has good balance.
“Rip tides” lines show that the LCB and LCF are nearly in the same place,and that both of these centres move fwd at about the same rate as the hull heels.
We have also established that all hulls create some yawing effect to windward as they heel due to the change in shape of the waterplane etc.
So heres what could be happening in “Riptides” case.
The boat heels over,creates this yawing effect to weather,but LCB and LCF are moving fwd at the same time somewhat canceling out or controlling the yawing effect???ie,the hulls CLR for want of a better term (I am completely ignoring keels /rudder and rigs here) moves fwds negating the weather helm somewhat,result is a hull that wants to track straight as it heels.
Now add the keel/rudder and rig and the problem changes again,but if we start with such a hull then perhaps we are off to a good start??
BUT…is a hull that shows this kind of balance actually fast?or are more unbalanced hull forms actually faster???are other speed producing factors more important than this holy grail of hull balance?
I think for a radio yacht this type of balance could be really usefull,the boat would be easier to tune and I would have more time to look around the racecourse whilst the boat looks after itself.
Though I’m well over my head on the science of this, Brett’s view seems to make intuitive sense to me. I’ve looked at some of my own designs, and the ones that seem to sail more easily seem to be closer to this congruence of LCB and LCF. I wish I had enough real experience to prove it, since it would save time in future experimentation!
For future Footy design efforts, I will try to keep LCB and LCF close to each other, and changing at similar rates. Hopefully, I’ll be able to report some conclusions.
Though there may be some who don’t take Footy design efforts seriously, I really think that Footys represent a greater challenge than some larger boats, as small differences represent much greater proportional change. I’m coming to the conclusion that for Footy design, a difference of one or two millimeters is really significant. Trouble is, I’m not sure I can build (and I’m not bad) to the same level of accuracy that Hullform allows me to design.
Bill Hagerup
OK,
I designed an IOM last year ,A freind of mine is building it but it is not yet finnished.
Anyway,I reviewed the lines and made some small changes to bring the LCB and LCF into a closer relationship.
Attached are some screen grabs showing this design for discussion.
Brett
If some of the readers are getting lost in all the terminology, good tutorials can be had at:
http://www.oneoceankayaks.com/smhydro/hydro.htm (no math)
http://web.nps.navy.mil/~me/tsse/NavArchWeb/1/module4/basics.htm (much math)
Cheers,
Earl
Great links,
Thanks a lot
lester,
I’m attaching the history of my work.
There are three sets of files: v1, v2, and v3.
All three include JPGs of the plan and JPGs of the curve graph, as well of the maxsurf files for them.
v1: is my first port which was a while back
v2: is the file that you requested the see the screen shoots.
v3: is a modified version that I’m working on right now.
I’m not sure if I’m going in the right direction right now. I did reduced the Beam and draft on v2 and v3.
The numbers in general on v2 look better but I’m still working on v3. On v3 I added more control points for the curves. I’m trying to have a better shape other the WL. But its taking a lot more time to make any changes.
I’ll probably re post v3 later again.
here is the link for the files
This post is in reply to one of series that seems to have vanished into bit limbo, which is just as well, since I put out wrong info in one of the posts. Anyhow, Dan Sherman had a set of lines and wasn’t getting a reasonable looking moments curve out of it, and I volunteered to take a shot at running the numbers by hand. In one of the responses, I talked about the arm of the moment as being measured from the hull centerline. This wrong (post in haste, repent at leisure/people my age shouldn’t try and expound from memory :-)). The arm is measured from the whole-hull heeled CB, which is arrived at by balancing the stack of all sections. Anyhow, here are the numbers I promised Dan:
Scale: heeled beam at waterlne 10.4 in, draft down centerline from w/l 2.5 in. I counted 25 sections and did 2, 5, 13, 19, and 22, pretty quickly so these are approximate numbers. The whole-hull CB came out 1.58 inches to windward. The numbers I got were:
Section 2: -.82 (arm) x 2.3 (area) = -1.9
Section 5: -.7 x 9.0 = -.63
Section 13: -.06 x 22 = -1.3
Section 19: .57 x 12 = 6.8
Section 22: .75 x 6.3 = 4.7
Yielding the classic unbalanced curve attached.
Cheers,
Earl
Thanks Earl, that looks just like the one i came up with. And i can tell you that design is not ballanced at all, because she has been built and sailed. :scared: I’m going to try and make the next boat a little more balanced.
Sailing Anarchy is tackling the stability issue too:
http://www.sailinganarchy.com/fringe/2006/yacht%20design%20101%20stability.htm
Gio
I just finished a design for a 30" LOA free-sail boat to be used in a youth program. After carting out about a garbage bag full of cardboard scraps 
I finally got the metacentric moment curves to come out right. Guess what: LCF dead on LCB. Is this a necessary or sufficent condition for “Turner Balance?” Methinks there may be a learned monograph (as Sherlock would say) in this result, especially if somebody dug into the FREE!Ship code and put in the metacentric moment calculations. In any case it would be interesting to see if one could devise a hull in which LCF = LCB but the metacentric curves were bad.
As an aside, the boat uses a 24oz torpedo sinker for a bulb (cheep! cheep! he chirped). Led me to think about a TS (for Torpedo Sinker) class: you get a sinker, a piece of 6 in by 1 in by 1/8 in aluminum for a fin, and off you go. Just kidding, folks
Cheers,
Earl