Effects of offset mast or leeboard – Lee helm and weather helm.

Discussion about centreboards and leeboards and where to place them, the effects of having offset centreboards and leeboards and offset masts with some sailing technique tips at the end.

More discussion about the design of the Oz Goose here.
Oz Goose plans information here.

Centreboards on a scow or multihull that is sailed heeled

Sometimes scows and some multihulls require that the centreboard  be moved a bit further forward compared to a conventional monohull.

The reason is that most of the resistance of the vessel comes from the most deeply immersed part of the hull/hulls – which is the leeward side. This unbalanced resistance tends to make the boat want to bear away so the board/s need to be moved forward to create an opposing moment in the opposite direction.

I was “reasonably” sure the position of the sail and centreboard would be OK and we had some latitude to change the mast position if there were problems. However it ended up being a moot point – the PD Racer hull just doesn’t like being heeled – because of the straight sides any heel just makes the corners of either or both of the bow and stern transom dig in slowing the boat badly.

So the boat is never heeled enough to move the drag centre to leeward as Bob was suggesting.

Bob Wrote:

Thank you for your letter. I didn’t know if you had any experience with straight sided scows.

I learned to sail in one. And that ones designer put the leeboard where it would > usually go in a pointed skiff. It would not sail upwind at all until the board was moved forward.

Hi Bob,

I learned to sail in something quite similar by the sound – though with a centreboard – if you took A OzRacer cut it down to a minimum hulldepth scow with 3′ 6″ beam and 40sq ft of sail you would have a Northbridge Junior. Hullweight was about 45lbs

Here is a pic of a modern one – it has started to sprout most of the expensive trappings of a conventional racing dinghy.

Mine was quite a crude instrument – for example had a big rubber band as a boom vang – a particularly silly idea – didn’t work very well – a bit of rope is much more effective:-)

A decade and a half later I spent a couple of seasons sailing an International Moth around – 11ft with 10ft wings, 85sq ft and a hullweight of 60lbs (it was heavy by the then standard of 40lbs).

Leebooards don’t go in the same position as a centreboard.

Leeboards are a bit more complicated. When we draw the sailplan and centreboard or leeboard position to figure out the balance it is a 2D drawing.

But in reality the force from the sail is not lateral, it is mostly lateral but a bit forward. Typically around the 10 degree sheeting angle of the front sail of the boat. The leeboard needs to be along that line of action.

If using a single leeboard on one side this means something curious. The leeboard needs to be forward of a centreboard position on the leeward side or behind it on the windward side.

A single swinging leeboard can take care of a mast on the centreline of the boat. . With a fixed leeboard it makes more sense to set it up to make sure there is a very balanced helm on one tack and a bit of weather helm on the other.

A humbling lesson about lines of action from a very simple boat

There were a few 8ft ducks built with biplane rigs – a mast on each side of the boat and fitted with a single leeboard. The  one on the right by Dave Grey the guy who supplies materials to make sails of polytarp – Polysail International as well as completed sails that save money.

In stronger  winds one of the masts and sails could be removed. When the sail was on the same side as the leeboard it was fine  to sail. But when the sail was on the opposite side to the leeboard the line of action was too far forward on one tack relative to the leeboard and too far aft on the other. It was very difficult to tack and very difficult to make sail properly.

If the centre of the rig moves aft then the centreboard or  leeboard needs to move aft.

Bob: BTW.

On multis don’t they move the mast further aft as well as the board?

Exactly. It doesn’t matter where the centreboard/Leeboard is in a boat – the rig just has to be sorta in the right place in relation to it. The “sorta” is where the experience comes in – as you so rightly point out.

We’ll have our boats in the water in a week and a bit – so … proof of the pudding … that on an 8ft boat it is OK to have a centreboard a long way back in the hull. Necessary because of the sail size.

And just be aware that I have never been wrong before 😉

(In the end I wasn’t)

The Oz Goose sailboat, neutral helm and waves

Initially people sailing the goose can feel that it has a lee helm. It is an interesting observation, but some special and interesting things happen with a boat that has very neutral helm.

The correct sail depth to dial in a little weather helm

Sail depth in the foot is critical to give weather helm and feel.

The  boat is designed around this sail depth at the foot.

Read this to the end … There’s something important.

Don’t flatten the bottom of the sail until there’s no other way to survive.

The Tiller does interesting things in a boat with very neutral helm – but that is OK

What you feel is that when a boat is truly balanced the rudder will orient to the direction of actual travel. This includes leeway.

So tiller end will point to leeward.

I was very concerned about this in the early days until I put my GoPro clone at the top of the mast and say the rudder spent most of its time quite central.

But the point where tiller loads makeyou feel the boat is changing from steering up to steering down is with the tiller to leeward.

Some have tried more weather helm using the back mast step but found it slow so moved back to the normal forward position.

There are three main advantages to a very neutral helm on a sailing dinghy.

The two design reasons for the very neutral helm.

1. One upmost in my mind was that when I used the aft mast step Steering around the worst of the wave peaks in strong winds upwind was really slow. With a neutral helm it easy to click the bow to leeward. Aft mast hole there’s too much rudder movement needed and the boat slows.
2. Second reason is the very balanced helm means the boat will steer nicely in any situation. Heeling, not heeled, planing fast, sudden deceleration ploughing into a wave going downwind it is easy to steer. Fingertip control and balanced helm unlike the tiller fighting with most dinghies.

Sometimes the tiller will point crazy to leeward

Now this is the important bit….

As I said the tiller will indicate the true direction the boat is heading.

So if tiller end is well to leeward it means that is where the boat is going. Cherrie had that happen in this  photo. See the tiller pointing so far to leeward so she can go straight.

A tiller pointing to leeward to go straight means you are making too much leeway. It is confirmed by the wake of the boat – see the wake angling to windward  (to the right side of the boat) rather than trailing straight behind the boat.

Here is a boat with the centreboard working nicely with central tiller and wake streaming out directly behind the boat

Two reasons are

1/going too slow. Ease the mainsheet to point a bit lower, get the speed up. Leeway reduces and tiller comes back to the middle.

Or

2/ there’s a bag or weed on the centreboard.

It’s most commonly the first one. Ease the sheet 2 feet to  get the boat moving well and the tiller will come in central.

If it doesn’t then there is a bag or clump of weed on the centreboard. Ease the sail or loosen it completely, pull the board up out of the case almost then put it back.

More discussion about the design of the Oz Goose here.
Oz Goose plans information here.