Thread: Distance between mast and centerboard

I found one article that deals with half of the problem: Duckworks - Articles

The article still leaves out how to calculate a suitable lead for CE over CLR. I guess there the sheeting angle comes into picture in one way or another.

If it helps

Jim Michalak's Boat Designs/The Index

You can look at this - read the comments below my post too.
What is “lead” in sailboat design – CLR and CE. Joe self designs a rig for his Electric Quick Canoe and finds something interesting. | Michael Storer Wooden Boat Plans

It shows how each designer has a recipe. The "lead" is based on certain assumptions about the boat and then some on experience.

My boats have all turned out very nicely balanced.

These are my assumptions. They are then corrected by applying my "lead". This method will not work for boats not similar to what I design. And it is not superior to the method of any other experienced designer Even stiffer spars might move the CE further ahead than my method expects.

I ignore the hull and the rudder. Hull because for the light boats we are talking about the hull is pretty symmetrical fore and aft. Ignore the rudder because I don't believe you can calculate how loaded it will be - this is the biggest departure of my method. As you can see from the links to other designers on the page above most designers include all or some of the rudder - which moves their CLR back behind their CE. So the CLR "leads" the CE. With my method the CE "leads" the CLR.

One caution is if there is a skeg towards the back of the boat ... I would factor that in a bit too - probably using a similar method to calculating the CE of two sails or a non triangular sail.

But be really clear the physical situation of the actual boat is the same ... but our different methods are different simplifications of the same physical system. This is not real theory ... the computer guys would call it a "kludge" or an inelegant solution based on trial and error.

From experience in the size of boats we deal with I put the centre of effort about 6" behind the centre of lateral resistance - but this is a bit variable because I might have data on the type of boat ... for example the OzRacer Mk2 compared to the RV. It is going to be similar because the boat's sail areas are not much different.

So this is why you haven't found any clear information about "lead". It is just too personal and it ain't science.

MIK

This is another reason to love the balanced lug! With most of the rigs, corrections will cost money.

Whoa! An "electric" quick canoe without the engine must be fun to sail. As it is not becoming narrow aft, it will probably "plan" very easily and get faster than what the theoretical hull speed would allow.

EDIT: After a quick reading about the electric quick canoe, it looks like it is not planning (Quick Canoe Electric – a simple, cheap electric powered boat. Square back Electric Canoe | Michael Storer Wooden Boat Plans). Apparently it is just getting the nose high. Is it the rocker shape that is causing this?

This again leads me to another question I have many times been wondering about: Why do sail boats have rocker shape of the bottom? Apparently the rocker has gotten smaller for each generation of boats. Old boats have really much rocker. In some way I guess it is because the boats have become lighter and I guess the transom and the bow should be just at the water level? Is this to minimize the drag?

(I hope you do not mind this much questions, but I am a curious person that wants to understand how things works)

Originally Posted by engblom

EDIT: After a quick reading about the electric quick canoe, it looks like it is not planning (Quick Canoe Electric – a simple, cheap electric powered boat. Square back Electric Canoe | Michael Storer Wooden Boat Plans). Apparently it is just getting the nose high. Is it the rocker shape that is causing this?

It is something that happens when you stick an outboard on a sailing boat. Most hulls with rocker aft will tend to squat as they go faster - in general the more rocker ... the more the stern will drop.

So - in general all sailing boat shapes have this tendency.

It is increased with the outboard. The outboard is providing a forward force centred about a foot under the water. The hull has an equal and opposite drag force passing through the immersed part of the hull. So you get a "couple" which tends to raise the nose.

You usually don't see it with boats under sail because the couple is in the opposite direction. There is the drag of the hull and the forward force is through the Centre of effort of the sails - up in the air. So the "couple" will force the nose down. This seems to work out pretty nicely as modern fast sailing boats have moved the main planing area into the body of the boat - around the centrecase. The old style that had wide sterns wouldn't get planing nicely until the nose was well up so that flat wide stern was at a positive angle of incidence to the water - so they all planed with nose up in the air - the rocker combined with the wide sections causing them to squat. Modern boats stay in trim because the stern is narrow.

The more rocker - the more likelihood of squatting. It can even overpower the tendency of the sail to push the nose down - like with the PDRacers and OzRacers - as they start to plane the nose suddenly pops way up in the air as the stern squats. Same hull stretched 50% (OzGoose) doesn't display the same tendency at all.

This same effect has been used to make multihulls much less prone to nose diving - by pushing the rocker back in the hull and keeping the sterns a little wider the sterns pull down at higher speeds helping to keep the bow out of the water.

MIK

See how the PDR/OzRacer struggles to get onto the "plane" and the bow pops up in the air. And it drops off the "plane" easily



But the goose - same hull but stretched so the rocker is less ... so no squatting and easy transition from sub planing to "planing".



and we all know the Goat trims VERY flat when sailing - little change in fore and aft trim regardless of speed.

But under power - not much power - even two kids up in the nose don't help keep it in trim.

On small boats the lead often isn't as important as having enough area in the rig and appendages, mostly because lead can by adjusted with your butt. In larger craft, hull form, appendage choices and rig decisions can play a much bigger role. You can draw some "generalities" about leads compared to different rigs and these are well established, such as 13 - 17% for a sloop, maybe more if it's tall and fractional, maybe less if it's a gaffer or low aspect lug with divided rigs getting progressively less. These generalities assume common hull forms, appendages and rig proportions. When you stretch the envelop a bit, say with an odd rig or appendages or hull form, then you find the need to refine the lead, often simply with trials.

The are also some generalities that can be used for calculating CLP area. In most cases, if the rudder is a spade or on a very slender skeg and both have significant aspect ratio, it can be ignored in regard to lateral area. On the other hand, if the rudder is keel hung or on a substantial skeg, it's impact can't be ignored, but in most case you only need a portion of it, typically 50% of it's total area. There are other variables, in larger craft (over 18') that need to be considered, as they can affect lead and appendage placement, such as the entry angle, buoyancy in the forward sections, hull form induced quartering waves, stern fullness, buttock angles, etc. All of these could force a 15% lead on an average sloop down to 13% or up to 17%. That's a pretty big swing and certainly enough for someone to complain about an ill handling boat.

Ultimately, it boils down to experience and observation. An example might be a sharp entry, shallow, shoal hull with retractable appendages, typical of a lot of small craft. The CLP doesn't shift much as the appendages provide most of the lateral area and the fine entry doesn't affect the flow around it much as she heels, so the lead remains the same relatively. This same shallow shoal, board boat, but with a bluff, rather buoyant entry, like that of many planing dinghies, will have it's CLP shift forward when she heels over, as that bluff bow begins to force the flow around it into a very asymmetric shape, causing the boat to have a strong rounding tendency. Another example might be a short over hang double ender, who's buttocks rise sharply when heeled, so she just can't get up into weather very well or handles very poorly. Lastly, the draft in the sails can also be a contributing factor when trying to sort out lead and appendage configurations. Some rigs tend to not move their CE's as much as others, which can affect lead and appendage placement. These are all things you learn, usually after you've had to make "adjustments" and recognize why something isn't quite right. It's also why most designers are seriously balding, if not given up completely and just shave their head.

Also other factors have a big influence. Like a bendy unstayed rig can mean the CE moves back an extra 4 inches in a little boat like the OzRacer/PDRacer.

So that is where the adjustment comes in from experience. And like PAR says ... it depends on the sails too.

So if someone experienced gives you a guideline you do need to know what their assumptions are.

If you use my method on a heavy boat with a full keel it won't turn out nicely at all. But on the size of boat I usually design my method gives a very accurate balanced helm within the constraints of my assumptions. Also I have experience of quite a few boats and variations in my normal size range.

If I ever do a long keel boat or boat with a built down hull - then I would be moving more towards the generalised methods PAR mentions.

MIK

Originally Posted by Boatmik

. . . on the size of boat I usually design my method gives a very accurate balanced helm within the constraints of my assumptions . . .

I'll suspect your assumptions are based in considerable experience, which is difficult to amortize or convey effectively, though well balanced results speak for themselves.

Originally Posted by PAR

I'll suspect your assumptions are based in considerable experience, which is difficult to amortize or convey effectively, though well balanced results speak for themselves.

No more hard. Though the 6" I quote is a fudge. I actually go closer than that - but 6" would be very safe for centreboard configs with my assumptions from above

The idea of "lead" IS hard to convey because it is unscientific - a 2D approximation to a complex 3D situation. The main thing is consistency!

I did pick up your comment about high aspect rudders vs wide ones as an nice point to steal.

MIK

It really looks like a guess work for anyone wanting to understand it. MIK is not taking the rudder into account, while other do to some degree. Still I think MIK also knows that the rudder affects the balance, but he has accounted for it by experience in his initial "lead".

If a boat is having just a little bit of bad helm (either too much weather helm, or lee helm), you can actually adjust the helm by either making the rudder a bit wider or take away from it. This shows that the rudder is still having an impact.

I've addressed many boats with "bad manners", with most owners being at fault, not the design, lead or design choices made in rig and/or appendages.

The vast majority of time nothing is done to the rudder or centerboard, but the rig is tuned properly and the owner given some lessons on sail trim and setup. I can't tell you how many boats, said to have horrible weather helm actually had baggy sails and skippers they over sheeted the crap out of them, trying to get another degree or two of windward drive.

On a few occasions over the last few decades, I've cut or added area to a rudder and an equally low number of times, I've moved a centerboard pivot or cut down it's length. On many of these boats, changes to the design have accounted for the need to "re-balance" the boat. Simply put, it's quite rare, at least in my experience, to have to take a reciprocating saw to appendages to fix things.

I'm especially fond of one particular "project" where the owner had tried everything, raking the mast, moving weight around, even talking with the son of the designer, about keel modifications to solve his substantial weather helm issue. I met him at a yard, where he was about to start cutting after the haul out. I sailed the boat and agreed it had an issue, but I wasn't convinced it was weather helm yet (the boat offered a few clues). I noticed the boot stripe had been repainted and the rudder deflection wasn't offensively high. The rig showed a noticeable forward rake too. We parked in in the slip (35' old CCA racer) and I told him to call me when he had removed every single item from the boat, that wasn't physically bolted down. The next day everything from cushions to the 4 anchors he had aboard, where sitting on the dock.

The boat was originally a fairly light weight racer from the 60's, but she was carrying enough stuff to sink her. Spare engine parts, enough to rebuild the thing at sea, a whole spar rig, except for the mast, extra everything. This 7 ton boat was holding at least 3 additional tons of gear. I rigged up a running headstay and we headed out into the bay. The boat sailed like crap (again), but now was much more responsive and showing a slight lee helm. I slacked the headstay and readjusted the shrouds and backs a bit and she came alive. She suddenly became a racer again and the owner was thrilled.

As we sat on the dock, with the pile of gear before us, I simply asked, how much of this stuff do you really need aboard. He admitted he'd forgotten about much of it and "out of sight was truly out of mind". He now carries 2 anchors, not four, doesn't need a spare head for his engine and is being careful about what he has aboard, knowing if he elects to cruise her, he can bring this as needed.

I think PAR misunderstood my post. But that was a lucky mistake as it led to an interesting post from him.

I was not meaning that people should go and enlarge/shrink their rudder. I was just pointing out the fact that the rudder has its share in keeping the boat balanced even thou MIK can neglect the rudder because of experience. His initial "lead" is already having in consideration that he is not using the rudder in his calculations. From that initial lead he is adding or taking away depending on what kind of boat he is doing. Isn't it like that or did I understand wrong?

If somebody should at all do modifications to the rudder/centerboard it is the designer if he notice he had done a bad calculation and the bad helm can not be fixed in any other way. I guess such things are caught in the very first prototype/boat made, and will be corrected by most designers after that in the plans.