Friday, February 7, 2014

What's a spin anyway

Boat talk. I've always liked boat talk. In RL is is usually about sailing faster or repairing something. In SL it is often about making the sailing experience better. Right now sunshine is on the top of my list, but having both sunshine and boat talk at the same time... Doesn't get much better. I caught Craig "J-man" Ktaba tuning his lovely Ktaba 20 Teleri somewhere in the North Sea. It wasn't exactly a clear blue sky, but a piece of blue is also good.
Clear blue sky and boat talk
I wasn't really looking for blog-stuff, but I just had to snatch the picture above. Especially as february carries on with the sunless days we saw in january. As mentioned in the former post: January 2014 was  the darkest month in 45 years.

The North Sea is a pretty sailing area, but this won't be a sightseeing post. Craig had a really interesting issue to discuss: How does the spin-trim - or lack hereof - affect the boatspeed at different windangles. Now, Craig knows perfectly well what a spin is. The real question is: How to make a good approximation of all the wonders of spin-trim in SL. That's a question that won't fit into a short happy-go-lucky post.

Let's delve into it. Say you already have a boat that sails pretty much according to a polar. How does the spin then affect performance? Easy. Compare two polars from the same boat; One made without spin and one made with spin. The only real challenge is to get polars from a J-Class, but we can take a spin from a smaller similar boat and check that out. The IOM for instance. In low winds, going close to the wind, the spin can double the boatspeed. In strong winds most boats will be doing max hull speed. Unless the boat can plane or foil - and a J-Boat can't - it is impossible to cross the max hull speed. (See this for details. Scroll down).
Gulls and seals
Then there's the matter of trim. The polar may show boatspeed at certain wind angles, but usually the polar assumes optimum trim. This makes it a bit tricky. On a dead run you'd have a pretty rounded spin; Not too round cause it should also be wide enough to catch as much wind as possible. Not too wide though, cause then the wind would just glide off. Let's call that medium round trim. It's easy to find on a dead run. When the log maxes out you'r there. But... How do you graph that?

On a reach the spin works entirely different. Reaching means getting the wind from the side at some angle less than 180, and that means the boat will heel. If you heel too much, the keel will have less effect and the spin will just drag you sideways. So max speed on a reach doesn't neccesarily mean max power. You may very well need to depower in order to reach max speed. Not only is the position of the spin important. You'll also need to change the shape.

On a dead run you'll want a nice round spin. The airflow is kinda like a bag on a stick. We all did that as kids, right? Bamboo-stick, piece of string and a plastic grocery-bag. Now, on a reach you'd want to stretch the spin - flatten it - to make it work more or less like a giant genoa. The airflow gradually changes (with the apparent wind angle) from kite-like to wing-like. (See my post on telltails).
Let's go for a swim
The smaller the windangle the more flat you need the spin to be, or it'll just heel you over and drag you sideways. At around 100 AWA a normal spin stops working. A little less in low winds, a little more in strong winds. That's why real racers have many spins for different angles and windforces. So the boost from the spin depends on windspeed, boatspeed, windangle apparent windspeed and trim and some of these parameters are (very) interdependant. Now, all we need to do is to convert this gibberish into math - et voila - we'll have a great simulation of the spinakker.

In other words: We're looking for a formula that takes windangle, windspeed and leeward and windward sheet positions and produce a power factor which we can convert into speed and add to the speed coming from main and jib. I think that's what they call a VPP. Have a great weekend...

PS: More on the Ktaba 20 updates in a future post. Here's a hint...


  1. OH, you mean a Spinakar ('spin' for short), as opposed to a spin around in the boat being a little jaunt around the local harbor as in: "I think I'll take a little spin." , or a spin as in the penalty turn in racing as In : "we had the lead until that spin on the downward leg" the New Ktaba boat looks like great fun.

    1. Yup! That's what I mean... a spinaker :-)


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