Surprising speed difference 12ft Oz Goose vs 8ft OzRacer

The 12ft Oz Goose was originally designed as an “extra hull” to take the rig and foils of the 8ft Oz Racer to take the family out, we were expecting around a 20% increase in performance but we found much more than that.

A resizing that worked beyond all expectations and taught us something about the limitations of conventional thinking about “what works in boat design”.

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First of all a Caution. This article is not permission to change the length of a boat plan.

Please consult the designer of your boat.

I have written articles before talking about the dangers of thinking that boats can be stretched or shrunk. They often cannot unless one understands what corner of design the boat inhabits. A very short Goat Island Skiff was a disaster for one amateur builder.

First Shock – 8ft Oz Racer handles 81/89 square feet mainsail easily all wind strengths

There are dangers in shrinking and stretching designs.

One way of avoiding dangers is to know which corner of design the starting boat inhabits.

In the article link above shrinking the Goat Island Skiff didn’t work because it is already a very small boat for its length and beam. Narrow waterline, light weight, minimal construction and big sail. So making it smaller was a disaster.

The 8ft Oz Racer was different.

We originally designed the 8ft Oz Racer to sail with an 8ft Duck class. Most boats had sails around the 40 to 45 square ft range with one or two boats with a bit more.

We decided that because the sails were so cheap (Polytarp) we may as well go big – as a sort of nationalist Joke.

After all it is an Australian Tradition.

What we were not expecting was that the 8ft boat fitted with well designed and made foils and sail/mast interaction would handle so very well even when the wind was strong. This was not without some struggle as we went through 4 mast designs and four sail designs to get the gust response we wanted.

The secret is the enormous stability of a rectangular platform boat. Any cutting away of the shape to make it more curvy or pointed reduces the stability very quickly.

The difference is radical – how many 8ft boats sail and handle well in all winds with a 90sf lugsail?

This is me coming in after a record breaking sail at 9.1kn. Not so fast, but this was a day at the Goolwa Wooden Boat Show where no other boats were on the water because the wind was going through severe gust cycles.

You can see I’m a bit damp from spray!

Why the Goose was designed – an extra hull for familysailing

I originally designed the goose as an extra hull for family sailing using the OzRacer rig, foils and rudderbox. Occasionally a plan would be sold.

Then around Ian Henehan started putting up videos – 10kn, then 12, then 12.9.

This is the 12.9 video. It turned out these speeds were not uncommon for the goose. And that led us to start building a fleet in the Philippines – now 50 boats with another 20 by the end of this month.

Wrong – our underestimate of Oz Goose performance.

Being a 50% scale up we expected a performance jump mathematically related to the increase in waterline length. I know many of you know this.

Roughly 22% greater speed.

But it didn’t work out that way.

That was our expectation. So the Oz Goose was designed around 2008 but it wasn’t until we saw Ian Henehan’s Videos in 2014/2015 that made us realise there was something unexpected happening.

Qualitative performance jump for the Oz Goose

I’ve written articles about hullspeed and particularly “planing” are such mushy definitions in practice that they are kind of misleading. What happens in reality?

When we first sailed the 8ft Oz Racer we found that while it would plane, the speed always wanted to quickly deteriorate back down to roughly 4 knots.

There was a really feeling of speed fluctuation up and down. The speed would quickly build in a gust and then quickly drop down with the slightest drop in gust strength.

As a measure of how seriously I sailed the OzRacer – my record of 9.1knots agrees with the best peak speed in the Duck class by Kenny Giles of 9.0knots.

The goose feels totally different – you get the speed up easily and it does not deteriorate – it is happy at the higher speeds. A slight drop in wind pressure and the boat speed drops only slightly, a slight pressure increase and the boat accelerates slightly. More wind is more speed.

This is the PSA (National Team) giving a great demonstration of the easy speed in fairly moderate wind – it is easy to see the acceleration with the gusts even with two aboard.

This is Not disparaging the 8ft boat which has some practical advantages and probably a performance advantage in very light winds where the Goose is not a slouch either.

Speed is not the only criterion – handling in strong winds

Both the Goose and the OzRacer have excellent handling in stronger winds. In our club racing fleet we frequently have beginner/intermediate sailors complete all their races in fairly strong winds without any capsizes.

That huge stability makes the boats very forgiving.

Additionally the high volume of bow and stern – and that they are so similar – means the boat has no vices when heavily heeled. If it sticks its nose into a wave in stronger winds it will still go where the crew points it – very different from a pointy nosed dinghy.

Thus the “wow” from the commodore who is filming my bearaway to avoid the committee boat.

Numbers on boat performance Oz Goose Compared to Oz Racer

OzRacer Performance

So a big qualitative difference as we have numbers from GPS readings over time. What about the numbers.

I spent a day in light and moderate winds and smooth water sailing the Oz Racer around with a GPS.

The big sail pushes the small hull up a bit over hullspeed even in light winds. See a lot of speeds around 3.7 plus or minus 0.2knots.

Downwind is much the same. With significantly stronger winds the 8ft boat will burst up to 6 or 7 knots but quickly drop down to around 3.7 plus/minus with just a slight change in pressure.

Just before the first of our masts breaks we see the somewhat slow planing speed of the 8ft Oz Racer.

With ideal speed conditions of winds over 20knots and good waves it is possible to see occasional speeds higher, but it is very hard to get much faster than 9 knots.

Oz Goose Upwind – Light and Moderate conditions

We have had 5 years of racing the Oz Goose now in fleets. We have also had other crews come from other classes of boats and occasionally hand our butts to us – because they found something new. Also we have crews that have made speed breakthroughs within our group.

When winds are light and the crew is just sitting upright on the side deck speed is typically pretty much in line with the 8ft OzRacer – around 4 to 4.5 knots.

A bit more wind and crew goes out to full hiking and the best upwind speed seems to centre around 5.2 knots plus or minus about 0.2.

This is a good speed to practice sailing at for solo practice if enough wind for full hiking – try to sail as high as possible while retaining this speed range.

You can actually hear the difference … the thump thump sound of the boat hitting waves changes to a cush cush sound.

It is also important to know the idea of “luffing in gusts” upwind is an old and slow technique. It is no longer used in dinghy competition. If a gust comes … don’t luff – just ease sheet to keep the same angle of heel. This works because the boat keeps moving fast at the target speed. If you practice with GPS you will see that speed drops and the boat makes more leeway every time a gust hits if you try luffing.

Planing upwind in the Goose

More wind than this and the boats with two crew start to have another option that the solo sailors don’t have. We were taught this by the first PSA crew to join us. First race of ’18 Nationals was pretty strong – around 16 to 20knots of wind.

I don’t have the speed readings for how fast they were going but they were about 25 to 33 percent ahead of our best sailors. This gives an upwind speed of around 6.5 to 7 knots. Around 7 is the speed where the Goose starts to feel Loose in planing mode.

Why is this possible?

With two people aboard the righting moment has doubled but the increase in overall displacement has gone up somewhat less – maybe only by 50% – but the boat must be sailed very level by constant sheeting.

If the water is not too rough this is a very effective mode. Big waves can hamper this approach.

Downwind in the Oz Goose

We saw that the smaller boat mostly sailed a bit above 4 knots and required significant wind to get up to 6 or 7. This puts average speeds very firmly a bit above 4 knots

When the wind is lighter, sailing performance of the 12ft goose is probably around the same.

But a bit more wind and the Goose starts spending a lot 7 to 9 range where it comes alive and feels like it is doing something. It is starting to feel exciting and is quite effortless.

Once it is there it likes to be there and more wind means significantly greater speeds. Averages are much higher – 7, 8 or 9kn is possible for leg averages. Getting up towards double the OzRacer’s numbers.

Peak speeds (all on tideless lakes) have `12, 12.9, 13.8 and 18.0 (for a full 2 seconds and three GPS points).

SUMMARY – Design is always interesting when performance is not as expected

This was one of the huge surprises to me as a designer. Same cross section, same sail, same rocker, same righting moment, same foils.

A theoretical speed around 20% greater, – yet utterly utterly different.

This doesn’t mean that any design will benefit from extra length. For example I doubt adding more length to the goose would improve performance much.

But in the case of increasing the 8ft OzRacer hull by 50% we had a hull

• with exceptional stability from the rectangular planform (which means power)
• a very large sail for the hull length and width
• overloaded with two sailors aboard

The Goose gives us

• Much higher average speeds
• A real sense of planing performance downwind
• Upwind planing with two crew in stronger winds
• Excelling light wind performance even with three adults aboard (instructor and two students)