Hull Design

Rcsailing.net General Discussion
141

Hi Lester

Yes you are right , we are becoming obsessed with LCB and LCF movement . As you point out we know that in practice that when a boat is being heeled excessively and rounding up, that moving the mast and consequently the CE forward restores the balance .
This would tend to indicate that the boats centre of pressure has moved forward when heeled. Presumably this is a function of the centre of lateral resistance. If this is so then ignoring any dynamic effects it will be the centre of area of the heeled immersed volume ( LCR) as seen in profile.

To me it would seem reasonable to assume that when the bows pitch down , the stern rises and that this profile area will pivot about the LCB. The centre of this area ( LCR) will then move forward thus reducing the essential lead and requiring the sail’s CE to also move forward for balance to be restored.
Conversely if the bows were to pitch up on heeling , then the LCR would move aft with the opposite effect. Is this what is happening ?

Is this why by having no pitch forward on heeling with the LCB and LCF staying in the same place causes no LCR movement , and hence no round up ? Certainly it seems that boats that are balanced in this way behave well .

If this is the case I can see that a small pitch up in a gust as a boat heels would help as the effect would only be momentary and would counter the natural tendency to round up by increasing the lead distance between the sails CE and the hull’s LCR, effectively applying a touch of weather helm . Isn’t this exactly what we do to counter the problem .

Maybe by having the LCF move very slightly forward on heeling we are actually keeping any LCR movement under control and in a positive direction. In other words is controlling the movement of the LCR the real issue and LCF movement is just a means of doing this by controlling pitch on heeling ?

Anyone have any thoughts on this , or am I talking rubbish

John

142

FYI, the boat (Saint Barbara) that Eric Sponberg references in my quote can be seen at:

http://www.sponbergyachtdesign.com/SaintBarbara.htm

Cheers,

Earl

143

Hi John

I’m not too sure about the ‘wide stern vs narrow stern and balance’ debate. There is a line of argument which says the wide stern boat may remain better balanced as the boat heels… See

http://www.onemetre.net/Design/Balance/Balance.htm

I don’t know. The widest stern boat I’ve sailed is the TS2, and I found it as easy (ie difficult!) to balance in different winds as my old Ikon – which was not a wide stern design, but certainly had medium rather than narrow beam.

144

Well, I did the upright vs. heeled LCF for Satanita and it came out:

Upright: 53% of LWL from forward point

Heeled: 54% of LWL from forward point.

Now this was pretty crude, working from enlarged drawings from a book and not attempting to fair them at all, but still … Satanita is the poster girl for unbalanced hulls, both by the Turner analysis and in practice (she went out of control and sank a competitor, resulting in a fatality).

Next step is to compute the LCB shift under heel, which will take a while.

Cheers,

Earl

145

Hi,
just some thoughts about sails …

When heeling the center of effort of the sail force moves out of the waterplane area of the hull. This results in a momentum moving the bow to windward. Is balancing of the hull only a part of the game ?

146

Absolutely. The dynamic forces are much more complex than these static analyses can show. There is also the effect of waves.

Cheers,

Earl

147

Hi Joachim

Your willingness to contribute is appreciated, thanks! However, there are a couple of points in your comments which I think are misleading.

It is certainly true that the centre of effort (CE) of the sail plan moves to leeward, and the centre of lateral resistance of the hull moves to windward, with heel.

From a reference plane which is the waterplane, such movement might introduce a couple (not a momentum, sorry!), or if the couple is already present, might introduce a couple with an increased lever arm.

Thing is, if we analyse carefully the resultant force acting through the sailplan CE, this force (the overall ‘drive’ force) is always directed to leeward (not to windward, sorry!).

The drive from the sails, acting above the centre of bouyancy, is what causes the boat to heel. With heel, a component of the drive force now is directed towards the waterplane, adding to the heeling.

As you have said, I have seen it said in a number of texts that, when the centre of effort of the sail plan moves to leeward, balance is affected. But I have not read an explanation of exactly how balance is affected, or in what way, that I find satisying.

The usually reliable Larsson and Eliasson show a diagram where sail CE moves aft relative to hull CLR with heel, and then say this causes the boat to luff up. What they fail to acknowledge is that a hull luffs to windward all by itself when heeled (ie no sails are set). When sails are set, the luff to windward is very much more gentle (if it occurs at all in a well-balanced boat), meaning that the sail CE moves forward relative to hull CLR with heel…!

148

Sorry, Lester,
in this area my English is not so good. You are are right, momentum was the wrong word. What I was referring to is in German the “Drehmoment”, which is “torque” in English …

My explanation is the following: the sail force is not pointing completely to leeward, but a little bit more forward. That means the part of the force which drives the boat forward acts some where in leeward of the centerline. On the other hand the resistance of the keel acts somewhere in windward. This results in a torque to windward, which increases when the boat is heeling …

149

I think you are wasting time just looking at hull shapes for an explanation of rounding up when heeled.

Take a surf board – it steers by leaning the hull into the turn. Now make it a sail board by putting a sail on it – still steers by leaning hull into turn. Now put a centre board in it so you can go upwind – steering becomes reversed!

The large volume racing sailboards (with centreboard) that were common many years ago all did their main steering by heeling the hull. Windward side down, and you pulled away, leeward side down and you rounded up. Certainly moving the sail forward/aft helped.

The same sailboard when you rounded the windward mark and you fully retracted the centreboard steered the opposite way – windward side down and you rounded up! It did not seem to matter at all on the hull shape – flat surfboard type shape or rounded displacement type. What mattered was centreboard or not.

I cannot remember heeling the sail only having much effect on balance on those large displacement type boards, but certainly on the high speed slalom board (aft fin only) leaning the sail to windward seems to cause a slight bear-away (but I would not be sure that the sail has not also been moved forward as well).

So I think you need to look at the interaction between the hull and the keel/rudder for the reason for rounding up when heeled, not at hull shapes.

150

Hi Jon

Each to his own.

I’m not too sure how relevant sailboards are to the issue. I think we are talking about displacement sailing when we talk about balance and weather helm in yachts. I don’t really know how this works with planing bodies.

I don’t think there is any doubt that the shape and distribution of the displaced volume of a sailing hull has a profound influence upon the balance and helm of a yacht. What is still unknown and a very black art is exactly how it all works. Even today, yacht designers cannot calculate the amount of lead required in a new design, because they do not actually understand in detail the dynamic interactions involved in hull balance.

I chose to spend my time exploring this issue, and do not regard it as wasted.

151

Lester,

I guess I did not make myself clear. I was attempting to describe the habits of the large volume, semi circular sectioned sailboards that had pointy type bows and narrow transoms - much like a “normal” boat shape operating at displacement speeds, not planing.

Since your reply, I have tested a partially built rc yacht hull (A class) in the water. Without the deck on, I was able to place lead inside (I only about 8kg available). With rudder centralised, no keel, the hull went forward in basically a straight line (It was out in the open river, a very slight wind and small waves). Moving the lead to one side to give about 30 degrees heel - still had the hull going forward in a straight line. Attach the keel now, and the hull definitely “rounded up”.

Same hull, same set up, different results with keel attached.

It is possible that this hull bare - no keel, no rudder would “round up” when heeled. I did try, but it spun very quickly and with the slight wind and waves did not give repeatable results. Having the rudder in slowed any movements and gave consistent results.

152

Hi Jon

OK, great to hear about actual experiments!

The sections of the hull you were testing – were they basically circular arcs? How wide was the transom as a % of beam? You loaded it with around 8 kg of lead – what was its design displacement?

How did you propel the hull, please? Gentle push? Hefty push? Electric propellor on deck? How far did the hull travel while heeled? Four boat-lengths? 20 boat lengths? And, how did the hull trim when heeled – bows down, or stern down?

Attach the keel now, and the hull definitely “rounded up”

In which direction, please? If we imagine the heel as being due to the wind on the non-existant sails, did the heeled hull round up towards the wind, or away from it?

It is possible that this hull bare - no keel, no rudder would “round up” when heeled. I did try, but it spun very quickly and with the slight wind and waves did not give repeatable results. Having the rudder in slowed any movements and gave consistent results.

It would be really good to have an idea of what happens without the rudder!

Thanks!

153

Lester
Didn’t you say back in the earlier pages of this thread that you’d never seen a canoe hull that didn’t round up. This must have you sitting on the edge of your chair!! I know it’s got my attention.
Don

154

Lester,

I have re-floated my boat this afternoon as it was dead calm. I few differences from the other day.

This hull - an A class - is fairly narrow 275mm with a draft about 85mm. WL length of 1220.
When heeled approx 25 degrees, the beam is 300, draft 107 and WL about 1500 - :slight_smile:
I weighted to design weight - 14.2 kg - with lead inside the hull so I could move it sideways to heel it. When I had the keel on the hull, I also had a wooden plug for the lead attached (wanted to check sideways centre of resistance - I forgot to!)

The sections of the hull are more a flat “u” shape than round and for an A, the stern would be quite wide (fine and straight entry up front to bring QB measurement aft). See attached photos for an idea.

With keel/rudder attached - checked it goes straight when not heeled - then heeled it to lines on hull (25 degrees). A gentle push (GPS gave max 3 kph) it gently “rounded up” - as if it was was sailing with sails and headed towards the wind.(see first photo) It took about 15/20 metres to turn about 30 degrees from start angle. Repeated a few times - consistent.

Took keel assembly off - still with rudder glued on centre - and similar results, but in half the distance to turn 30 degrees (about 6/10 metres) - consistent.

Took rudder off - first push had the hull turning quickly the “other way”!!!
I found I had to be really careful pushing it without keel/rudder to get consistent results, as at about 2 metres travel it started to turn and then accelerate that turn so it was almost 360 degrees by about 3 metres. Done carefully, I got it to “round up” consistently.

Attached photos.
“rounding” shows boat with keel and rudder gently turning to starboard - zoomed in a little - probably about 6 metres away.
"bottom: shows a view of the hull. Wide grey line is waterline, thin grey line is heeled waterline and a few sections also.
“aft” shows view from aft - sections show a little.
“forward” …

Think I got most things you wanted - the boat has been half completed for about a year now and without deck is convenient to add lead inside!

155

Hi Jon

Absolutely priceless data! Terrific, many thanks!

What you have found is (phew!) completely consistent with experiments others have done. The heeled hull always turns, and the direction of turn is ‘away’ from the heel, always to what would be windward if it were actually sailing. As rudder and then fin/keel are added, the rounding is damped.

Took rudder off - first push had the hull turning quickly the “other way”!!! I found I had to be really careful pushing it without keel/rudder to get consistent results, as at about 2 metres travel it started to turn and then accelerate that turn so it was almost 360 degrees by about 3 metres.

Yes, without any form of direction control, the hull whips around. Others have been able to explore this behaviour with slight heel (perhaps 10 degrees) and can then easily obtain consistent results. It is interesting how ‘strong’ this windward torque of a heeled hull is.

bottom: shows a view of the hull. Wide grey line is waterline, thin grey line is heeled waterline

Now that is really useful! The asymmetry and aerofoil-like section of the heeled waterplane is very clear. The theory is that the immersed hull is like a very short, fat ‘wing’, lifting the hull to windward… Not too sure about that theory myself, but don’t have a better one. If it is true, then the shape of the hull will affect the shape of the aerofoil, will affect the lift characteristics of the hull, and will thus affect how little or how much the hull wants to torque to windward.

So far so good. It is when we add the fin that things get interesting. The first thing to remember is that, in a yacht actually sailing, there is leeway and the fin is the greatest source of lift of the hull, acting to windward. On the other hand, in the hull that you have pushed along gently, there is no leeway, the fin produces no lift, just lateral ‘guidance’, and it is the hull itself producing all the lift that we see. So what happens if the fin is particularly aft? Or particularly forward? Might we see your heeled hull torque to windward even more strongly? Or might we see it actually bear off to leeward now?

If you have the enthusiasm, what about a couple of experiments where

(1) fin in normal position, but no rudder.
(2) no fin, but rudder in the place where the fin would be.
(3) no fin, but the rudder is attached at the bows.

Great stuff!

156

Lester,

Grrrr… I thought I posted, but I must have pressed the wrong button. Second attempt.

I will try another set of tests - largely because i am interested in finding the centre of lateral resistance and comparing it to my rough calculation (balancing an outline of the underwater shape)

I will wait until suitable weather conditions ( late sea breeze arrived not long after I finished today).

Interesting today that the boat slowly turned with fin and rudder, yesterday it appeared to go in a straight line (yesterday slight crosswind breeze, under weight, …)

My guess at having a fin but no rudder will be much the same as no fin and no rudder (the fin is very close to where the hull “spins”), and the rudder at the bow would spin very early!

The “model” rocketry people have a “formula” for calculating the size of the “aft” fins to allow stable flight - looks at cg etc. I guess much the same would apply for our finless underwater shapes that are inherently unstable unless some aft fins (At one stage I was looking at constructing a water powered rocket using a large plastic soft drink container - not too different from my under water boat shape!!!)

I was thinking of trying a long fin (off a 10 rater) rather than the short short “A” fin, thinking of roughly the same area, but a longer drag “moment” that might increase the turn to windward , but OK a small fin too. (I have had an idea here - on our longish keeled boats, a sudden gust hits, the boat heels, say 45 degrees and starts to accelerate but the lump of lead, out to one side has more much inertia and is slower to accelerate. What happens now? The boat goes forward more than the lead, or rotates “about” the lead, or “rounds up”!! Is this the main cause of sudden round up?)

(Hoping to get the deck on soon and go sailing rather than continue this. Given enough time, I would be interested in trying the same with a different hull shape.

Jon

157

One of my discoveries in researching my paper was that the schooner yacht America came out perfectly balanced when subjected to the Turner analysis. This evidently was no accident, as this quote from the Lawson History of the America’s Cup (1902):

"Captain A.J.Kenealy of New York, one of the best-informed writers on yachting on either side of the water, an old sea-dog, and English by birth, thus summarizes the reasons for America’s success:

‘The model of America was designed with a special regard to stability. She was a sea-going craft, as well as a fast yacht, and with her long and somewhat hollow bow she had a cleanness of after-body which is, even at this day, worth copying … George Steers in his design of America took care to produce a model in which the centre of buoyancy was not at a ridiculous angle with the centre of the load water-line. He had hosts of imitators in England, and the result was that those who thought they had copied him were completely at sea when they tried to balance their ships — that is, to give them such a lateral plane as would bring the centre into the proper relation with the fixed point already determined. This could not be done, and the rig put on them had to be shifted back and forth until the required equilibrium was attained. There was, therefore, in several of the imitations of America, one force acting against the other, the evil effect of which became especially manifest when they were subject to heavy pressure, while in the America the harder it blew the faster she sailed. The chief defects in the English boats referred to, such as Gloriana, built by Ratsey in 1852, and Aqualine, built by Harvey in the same year, were that they were all bow, leaving nothing for the after-body, and moreover, especially short-bodied under water. Their sea-going qualities were not, therefore, the kind that a naval architect could be proud of.’"

America’s upright waterplane was nearly symmetric fore and aft, and so the center of the LWL would be very close to with the LCF. Working through his Edwardian prose I get the following understanding:

The copiers didn’t understand hull balance, and didn’t put the LCF in proper relation to the LCB. They then attempted to compensate for the “heel causes yaw” phenomenon of an unbalanced hull by playing with sail balance, that is, moving the CE of the rig to compensate. This only works (as I have learned with free-sailing models) for a limited range of wind speeds, whereas a balanced hull tracks properly (and therefore maintains sail balance) over a wide range of wind speeds. Or at least, that’s what I think he’s saying :slight_smile:

Cheers,

Earl

158

Have been spending some time getting to grips with Maxsurf, and now have a feel for the means of creating a hull . the question I have is this : for an IOM one metre so many of the normal variables are preset for us eg the sail area , displacement , maximum length and draught. Of the variables left:

beam
position of max beam
wetted surface area
cross section area
rocker distribution
planview shape of waterline
shape of cross section Vee,semicircular arc or “U”

do you guys know of any reference that gives what effect that altering these variables gives . I realise that everything is a trade-off but in trying to make a design for a given set of conditions what I would like to know is what effect on the hull’s performance does the extremes of these have .

for example a wide beam will give more stiffness but will have less draught and departing from the circular cross section will have more wetted area and form drag .

One of the lakes we sail on is in the City and is surrounded with hills and trees . consequently it usually has light but very flukey winds with severe gusts . The water is usually very flat. I am trying to develop a boat to cope with these conditions to have low resistance and drag for the light winds but have good balance at extreme heel angles for the knockdown gusts . Sometimes it seems as though the wind is blowing in three different directions at the same time and place !

My initial thoughts are a narrow deep hull of reasonably flat rocker, fairly circular sections for low wetted area with a heeled waterline that " balances" a la Turner’s method to cope with the knockdown heel without rounding up too savagely. Am trying to get a low drag easily driven hull that can cope with gusts.
Any comments would be appreciated .

Thanks

John