Hull Design

I have being working on a hull for a while. I would appreciate any comments, suggestions or critisism. I’m attaching a few files: Maxsuft, DXF and JPGs on different angles. I eventually will build this boat. I would appreciate if any one can comment on the fin and bulb placement. My center of gravity is about -30mm from the center of the boat.

Displacement is about 4kg
Length:1m

I made the boat beamy becuase at the pond that I’m sailing the wind changes a lot with constant heavy gusts. I kept the Cp in the mid range (.546) for the lighter winds.

Hi dsignr

Looks interesting! Would appreciate a better handle for you, however, I must confess to disliking the kind of de-personalised conversations one has with nom-de-plumes.

I assume you are designing to the IOM rules. The lines remind me of Brett’s old ‘Highlander’ design (Brett, can you get your nose out of a Footy for a moment (smile) and join us?), and I seem to remember comments from sailors in the UK at least that the radical change of immersed hull shape with heel made the boat difficult to balance and get into a predictable groove.

Lester,

Thank you for the coments and yes, I am designing a IOM.

I have being trying to maintain the hydrogrohics of the boat with minimal change as the boat heals. I understand that that is computer based and It probably be different in practice. I have noticed that the bow tends to dive leeward as the boat heals (mostly on 30+ angles).

One question: when you refer to “the radical change of immerse depth”, Do you mean depth? Could you expand on that…

Ok ,I took a look at this IOM.
Interesting,very much like my now 10 year old “Highlander” design.
My Cp was somewhat lower at around 0.5.
The general lines however are very similar.
Does your hydrostatic software tell you of the changes in the centres on heeling? That would be useful info.
Also the shape of the heeled waterline would be interesting.
From my recent work on IOM designs I can tell you that very small changes can make big diferences in the balance of the hull it seems(If the software can be belived)
My general method of designing an IOM is now as follows…
decide on desired waterline beam and general character of the hull.
Begin drawing hulls in this style,
test all the hulls drawn for centres changing on heeling and other desirable features(secrets:) )
Test the hulls with various drag schemes
return to drawing hulls and testing until happy…
Luckily the various software out there makes this all pretty painless.
Another good idea is to find guys who are doing the same and exchange files
and comment on each others work.
I have found this invaluable as you all learn together.
Thanks Bill and Rob!!!

Cheers
Brett

Hi dsignr,

That’s good advise from Brett, and although I’m sure I learned a lot more from him than the other way around, working together has really been helpful. Of course, I’m sure he keeps a “secret” or two even from his friends!

Since we’re too “thrifty” to buy MaxSurf, we’ve primarily been using Hullform. If you’d like to use it too, we could share files. The bad news is that you could get like us and have lots of ideas that never get off the drawing board and into the water where we really learn!

If minimal change at heel is your goal, I think you’ll generally find it easier to achieve with narrower-stern designs than with wide “skiff-type” sterns. Perhaps that’s part of the reason for what appears to be a return to more moderate IOM’s?

And Brett, maybe it’s time we got Lester a Footy? It might be fun to see a section of his page devoted to more challenging boats :wink:

Bill H

Brett & mudhenk27,

First of all thank you for the comments. I’m attaching the hydrographics as the boat heals. It’s a text file so you should be able read it. Question, when you refer to the center of heal are you refering to the center of balance? If you look in the file I sent the change in the center of balance (vertical and long.) is in the few millimiter range. I see a larger change in the center of force.

Another thing that I have tried is adding the rudder, fin, and bulb and stimating the waight on each. Bulb:2.5kl fin:.1 and ruther.05, then the rest of the weight I place on the hull. I can move the fin an bulb to fine tune the center of balance.

here is an open ended question, Should I reduce the draft on the hull. I can reduce the draft considerably and keep my hydrographic very close to what I have now. What are my advantages for a shallower hull?

I the wide hull…Even though the stern is wide the actual area that hull touches the water does not increase that much.

I started using Maxsurf because I was giving me more accurate info about the hull. I down loaded the educational version from their site.

Anyone out there. I would be happy to share my hull with others and should be happy to look at others too. I do have hullform so I can see those files too.

One limitation that I have is that I’m running this software on a Mac so I’m limited on the software that Virtual PC on the mac will handle.

Thanks Again,

Daniel.

FYI: My Forum Name, DSIGNR, comes from my Career, Graphic Designr. I have had that domain for many years. DR are my initials. So I have being using that screen name for a long time. I don’t want anyone to this that I consider myself a hull designer.

Hi Daniel

Yes, happens with any hull that has a wide transom beam.

when you refer to “the radical change of immerse depth”, Do you mean depth? Could you expand on that…

I was referring to “radical change of immersed hull shape with heel”. It is a situation which the computer (AFAIK!) doesn’t give you numbers for. Your design’s hard bilge is something like a chine. With “U” shaped sections, hard bilges seem to act something like chines and offer directional stability. With your “V” shaped sections, however, the hard bilge ‘wanders’ as the boat heels (best way I can describe it, sorry!) and seems to fail to help directional stability. The result is a boat which feels as though it doesn’t quite know what it wants to do…

The stability calculation performed by packages such as Hullform, which compares the heeled and upright centers of buoyancy, is correct but insufficient to produce a truly balanced hull. I learned this to my dismay when I designed and built a free sailing boat balanced according to this criteria alone. It was a disaster.

Digger deeper into the topic of balance I inevitably encountered the metacentric shelf notions of Admiral Turner. His methods are the subject of some controversy in Naval Architecture, not the least because he developed them first as a model yachtsman and only later applied them to full-sized boats. Perhaps the most even-handed treatment is by CA Marchaj in his book “Seaworthiness:”

"The main criticism directed against Turner’s concept of the metacentric shelf as the sole criterion of balance is that for convenience it involves only hydrostatics …

“Nevertheless, to quote K.C. Barnaby’s comment expressed during a discussion of Turner’s paper, ‘we may doubt the accuracy of some of his reasoning, but the fact remains that boats balanced on his metacentric shelf principle to turn out to be uncannily steady in their course.’ In practice, Turner’s theory has proved remarkably reliable in predicting yacht behaviour from lines plan, so the error introduced by [hydrodynamics] cannot be serious.”

(As an aside, sorting out the relation between the the static and dynamic effects would make a decent thesis topic for an aspiring graduate student.)

The description that follows is not, strictly speaking, what Turner called the “metacentric shelf”, but it is the more interesting of his ideas.

Looking at diagram 1, for a given section U-U’ is the upright waterline, H-H’ is the heeled waterline, A and B are the immersed areas and M-M’ is the heeled metacenter. This line marks the point at which the upward force of the water is concentrated. The amount of that force is equal to the combined areas A+B.

The location of M-M’ is found by fiddling around with pieces of paper as shown in diagrams 2 and 3; the line M-M’ is where the folded section shape balances on a knife edge. The degree of heel being analyzed is usually chosen to be “rail down.”

Once we have located M-M’ for a particular section we can measure the offset, or “arm” that the upward force of buoyancy exerts away from the center line of the boat, as shown in diagram 1. Multiplying this by the area A+B (measured by counting grid squares, or with a planimeter) gives a “moment,” or force exerted at a distance.

We can then plot the moments along the centerline of a the boat by doing the (tedious) job of finding the arm and area at each section, multiplying them, and plotting the result on a horizontal line. If M-M’ is to the right of the center line we plot it above and if it is to the left we plot it below. Connecting the resulting points gives us curves such as in diagram 4 and 5.

Diagram 4 is a curve for a badly unbalanced hull, in which, upon heeling, the moments of the forebody move to one side of the center line and the moments of the afterbody move to the other. The result is a yaw, or at worse a yaw and a dive at the same time. The particular curve shown is that for the big Victorian cutter “Satanita,” which was so hard to handle that in one race she ran out of control and rammed and sank a competitor. Note that since the two areas are of about the same size, this hull would be “balanced” under the looser criteria of CB movement under heel. A similar curve is exhibited by L. Francis Herreshoff’s J boat “Whirlwind,” which was notorious for its poor handling characteristics.

Diagram 5 shows the curve for a balanced hull. The moments for the forebody and afterbody are on the same side and the moments for the center of the hull are on the other. The ideal situation is when area A in the diagram equals area B, and area C equals A plus B.

It is not necessary to go through the whole hoopla to see if a design has a chance of being balanced. Simply locating M-M’ for two sections, one at about 25% LOA and one at about 75%, will give you a first-cut result. If both M-M’ lines are on the same side of the center line you’ve got a chance. If one is on one side and one on the other you’re in trouble. From then on it’s trial and error and a wastebasket full of paper scraps. It sure would be nice if somebody would provide a program that would calculate the moment of a heeled section from a table of offsets :slight_smile:

Since Turner’s ideas were championed by cruising yacht designers such as A.A. Symonds and Harrison Butler, there is a general notion that Turner’s system inevitably produces tubby little double-enders. This is emphatically not the case. Diagram 5, in fact, is the curve for the schooner “America,” long lines, hollow entry and all. “America”'s balance was the result of trial and error in the intensely competitive environment of the New York pilot schooners. These boats raced each other for jobs through a crowded harbor, and were often sailed short-handed. They had to be both fast and handy.

In the model yachting domain, Ted Houk’s magnificent M Class design “Rip Tide”

http://www.swcp.com/usvmyg/mclass/riptide.gif

is perfectly balanced by the criteria given above. This was intentional, as I verified by corresponding with Houk’s son. “Rip Tide” was designed in 1949, the design made the transition from free sailing to radio, and versions were still winning races and championships as late as the middle 1970’s.

It is worth noting that a balanced curve of moments is independent of both scale and section spacing; it’s the form, not the values, of the curve that counts. This means that you can take a balanced hull that was designed to one rating rule and adapt it to another without losing its balanced characteristics. I did this for the boat in the photograph, which is a “Rip Tide” modified to fit the 36 inch Restricted rules. She tracks through gusts like she was on rails.

Similar adaptations could be made to convert some of the classic (and classically well-behaved) M Class free-sailing designs to the IOM rules. Of course, to do so and claim the design as original would be the height of bad form. It would still be interesting to see how such boats would do against some of the “wedge” designs popular lately, especially in difficult venues.

Cheers,

Earl

Thanks, Earl, for your posting
I’m just playing around a little bit to make a hull design for a RG65 using FreeSHIP.
It’s my first attempt to design a hull. Your hints are very valuable for me!

[quote=Earl Boebert;35292]The stability calculation performed by packages such as Hullform, which compares the heeled and upright centers of buoyancy, is correct but insufficient to produce a truly balanced hull. I learned this to my dismay when I designed and built a free sailing boat balanced according to this criteria alone. It was a disaster.

Digger deeper into the topic of balance I inevitably encountered the metacentric shelf notions of Admiral Turner. His methods are the subject of some controversy in Naval Architecture, not the least because he developed them first as a model yachtsman and only later applied them to full-sized boats. Perhaps the most even-handed treatment is by CA Marchaj in his book “Seaworthiness:”

"The main criticism directed against Turner’s concept of the metacentric shelf as the sole criterion of balance is that for convenience it involves only hydrostatics …

Earl, unless I missed something (which does happen, I’m sorry to have to admit), the result of the method described violates a fundamental principle. The method seems to predict that many hulls will have a yawing moment merely because they are heeled. That implies that you can construct an object that will just sit there in the water and spin eternally. There can certainly be dynamic forces acting on hulls that cause them to yaw, but in this case, the only force acting is buoyancy, and buoyancy cannot cause a steady-state yawing moment. Right now, I think it can’t cause even a transient yawing moment, or any force other than vertically upwards, equal but opposite to the weight of the displaced water.

What gives?

Mike Biggs

Mike Biggs asks (in part):

“What gives?”

I don’t know, and I haven’t found anyone who does. Even an expert like Marchaj admits to being puzzled. There are two aspects to Turner’s work, the correlation between curves and behavior, and the explanation of the cause. The historical evidence for the correlation is compelling. Hulls with the symmetric “hill and dale” curve show exceptional balance. Hulls with the “crossed” curve show ugly handling characteristics. (Hulls with inbetween curves are all over the map).

The explanation is the mystery. Turner’s own reasoning is generally not accepted. My guess, and it’s only a guess, is there is some independent static/dynamic characteristic of the two extreme classes of hulls which yields balance. This is why I suggested the problem as a thesis topic :slight_smile:

Cheers,

Earl

Hi Mike

Absolutely! It is one of the mysteries of boats. Heel them and they yaw. Many yacht designers, even experienced and knowledgeable ones, don’t believe this. Fortunately the experiment to convince them is pretty straightforward in any bath or pond. Take any boat-shaped hull (leave on or take off the rudder and/or keel, it doesn’t matter, but if left on the appendages just damp the reaction), put a little weight in or on it so it heels, then push it off through the water. It’ll screw up to what we would call weather all by itself. Put a little motor in it, and it’ll circle happily all day long.

Yes, Lester, but then we are talking about dynamic forces. If the boat is sitting still in the water, it will not begin to yaw. No torque has to be applied to keep it from yawing.

Buoyancy always acts counter to gravity, or counter to whatever net acceleration is producing a change in pressure with depth. Buoyancy of parts of hulls also act counter to gravity. No assortment of up vectors will ever produce a couple to cause yaw, although they might combine to put “twist” into the hull fore-and-aft.

As soon as we introduce forward motion, there are dynamic forces to consider, including forces arising from operating at the interface of two different fluids: water and air. These forces are much more complex than simply pressure acting perpendicularly on surfaces, which is all buoyancy is. I have no trouble at all seeing how boats may do all sorts of yawing when heeled and moving forward.

I can see how there may be some sort of correlation between boats that continue straight when heeled and boats that satisfy the requirement for balance according to Turner, but I think it will turn out to be correlation, not cause and effect. Sailboats of Turner’s day tended to be double-enders, didn’t they? It seems likely to me that hulls such as were common then would be likely to satisfy Turner’s theory on balance. Right now it seems likely to me that boats that are symmetrical fore-and-aft are not likely to yaw when heeled, just because the forces fore and aft will be the same.

Until I thought about it, Turner’s scheme seemed possible to me. Now I think I’d rather see what the submerged portion of a heeled hull looked like. I’m thinking that it might be fairly apparent from such a view whether or not a given hull would yaw port or starboard when heeled. I can also sort of see how a non-double-ender that balanced according to Turner might imply that surface areas on the port side of center might be about equal to surface areas on the starboard side, such that friction did not produce much yawing. But I do not see any way that Turner’s theory on static balance can say anything at all about the dynamic forces acting on the hull other than this possibility for balancing friction - which is usually the least important component of drag except in light air.

Mike Biggs

Hi Mike

Sorry to have put you to the trouble of stating the obvious. I should have made it clear in my post that I agree, there is no possibility of a motionless hull yawing, rather than assuming this would be understood.

… boats that continue straight when heeled … boats that are symmetrical fore-and-aft are not likely to yaw when heeled … it might be fairly apparent from such a view whether or not a given hull would yaw port or starboard when heeled

We might not be talking about the same thing. I was talking about a hull only, a canoe body with or without its appendages, but certainly without its sails. I think you are talking about a rigged boat. As far as I know, there is no hull-type shape that does not yaw when heeled (artificially heeled, naturally, since it doesn’t have any sails), and does not yaw “to weather”, that is, to the side opposite to its heel.

Heck, I don’t know any hulls that yaw to leeward, either, and I was also talking about a bare hull. Not that I have any real experience with bare, heeled hulls moving forward and yawing. My wife and I generally try to keep our canoe as straight up and down as we can manage.

As I understood the original proposition presented by Earl, a boat balanced according to Turner’s theory has minimal tendency to yaw whereas a boat out-of-balance according to that theory yaws badly. The theory seems to me to be predicting a yawing moment when the boat is sitting still in the water, just because the boat is heeled, and that won’t happen. The only force the theory is concerned with is buoyancy, so I don’t see any way the theory can predict how heeled hulls will tend to yaw while moving forward, or not yaw while moving forward, except what I tried to outline in my earlier post. There are a lot of dynamic forces acting on a moving, heeled boat, and it is easy to understand that these dynamic forces can lead to yaw. Buoyancy cannot lead directly to yaw, so how does Turner’s theory manage any prediction of yawing behavior?

Mike Biggs

Doesn’t alot of this have to do with the shape of the waterline (WL) when heeled (HWL) vs when at rest (DWL)? The DWL will have a symetrical shape reflecting the canoe body’s horizontal section of the hull, which will have no yaw moment when underway. On the other hand, the HWL will assume sort of a banana shape which will create a natural tendency to yaw “to weather”. I’ve always understood this was the main reason for the “lead” distance between the CE and CLR needed to counterbalance the yaw moment induced by by the asymetrical WL shape(s) when heeled. Lester, please correct me if I’m wrong with this logic

Has anyone drawn a “Riptide” style hull in hullform? I would be interested to take a look.

Hi Bill

Yes, the heeled waterline will not be symmetrical. I’m nervous, though, about pushing the idea that the waterline shape alone creates the yaw. The HWL shape is a 2-D construction, while yaw must surely be a function of the whole 3-D submerged body volume and its distribution.

I’ve always understood this was the main reason for the “lead” distance between the CE and CLR needed to counterbalance the yaw moment induced by by the asymetrical WL shape(s) when heeled

That is my understanding as well. If a hull (any hull) didn’t want to yaw to weather when heeled, then the sail plan CE would not need to be positioned ahead of the CLR, it would be positioned over the CLR.

Hi Mike

As explained by Earl, the ‘useful’ part of Turner’s theory is to focus attention on the section-by-section analysis of how the immersed section area moves to one side or the other of the upright centre of buoyancy. It is therefore looking at the distribution of hull volume with heel, and I have no doubt that this is key. Turner’s suggestion for how this immersed hull volume should be arranged (diagram 3 in Earl’s post) is still hotly debated, mainly because it seems to work in practice but no coherent theoretical explanation can be given as yet.

In answer to Bill’s point re “lead distance” - we really do have to consider dynamic forces to explain it.

Imagine a hull which remains symetrical when heeled - just to take the hull shape out of the equation for the purposes of this explanation.

Now give that hull a nice deep keel and a high aspect ratio rig.

Now let the hull sail to windward in a stiff breeze - with plenty of heel.

Through what points are the various centers now acting?

The Center of Lateral Resistance is somewhere in the keel. It has now been pushed out to windward by the heel of the boat. In fact the keel, by providing lateral resistance is, in partnership with the wind, creating the heel. If you take the keel away and instead suspend the ballast out to weather above the waterline, the boat will just blow sideways.

So that means that, as well as creating lateral resistance, the keel is creating drag. This drag is not under the boat vertically, but is in part horizontal and out to windward.

Now what about the drive from the sails, - the Center of Effort? That is positioned somewhere within the sails, up above the boat. But as the boat is heeled, that effort is now operating through a center out to leeward of the boat. Thus it too has a horizontal element.

So we’ve got a force out to leeward driving forward, and a force out to windward draging backward.

So what do ya get? The boat has a desire to spin around a center between these two forces. We experience this as weather helm.

You don’t get weather helm when the boat us upright do you? The forces are still there, but they are vertically above and below the boat.

That, gentlemen, is why we design lead into our boats - i.e. position the rig forward of the keel.

That is of course not the whole story. The discussion about hull shape is equally valid, and why I spend so much time trying to design a hull which comprimises between maintaining a symetrical shape when heeled and yet uses form stability.

In big boats, we can move crew weight and trim our sails independently of each other to alter balance while out sailing. In 2-channel RC sailboats we can’t. For these reasons, I favor a hull design that sacrifices what in big boats we’d call a “powerful quarter” in exchange for balanced lines at all angles of heel. A rudder that is always fighting excessive weather helm acts as a brake. I want my boats to be designed and trimed for slight weather helm so that they “hunt” to windward with negligible helm input.

Just my 2 cents.