Jump to content

Mast Rake vs Centre Board Rake


GregS

Recommended Posts

Afternoon All

I'm a relative newbie to PT's, and would be interested in hearing some feedback on the below if anyone has some pearls of wisdom...!!

My PT is set up currently with a 110mm mast rake (measured using the halyard / front beam / rear beam method). Centreboards are maybe 220mm wide, long straight ones (but not as long as the McKenzie ones you can get), already raked back (angle unsure). The steering balance feels nice at the moment, slight weather helm.

I'm looking at experimenting with the mast rake, taking it back to say 60 or 70mm, and I understand in order to compensate for this I'd be looking at adjusting the centreboards.

Now my question is: Is is preferable to increase the rake of the boards back to maintain the balance I have, or is it better to have them maybe at the same or less rake (more upright) and just positioned further back? Put another way, assuming balance is good, what are the thoughts between raked boards or upright boards?

Cheers , Greg

Link to comment
Share on other sites

G'day Greg,

I would only experiment with mast rake at this point.

If you are getting weather helm, then you may want to rake the mast back a bit further. As you have said it is at 110mm, so try for 90 - this will usually just be another shackle at each front chainplate (or up the mast) and adjust rears accordingly.

See what happens. You will also notice that the boom will be closer to the deck upwind.

If not, try 70-80mm. You will find a balance point on your boat somewhere - rake on average is between 140mm and 60mm.

The other thing to look at with the weather helm is to check when the rudders are down to see if they are either vertically inline with the pintles or slightly in front - ie: if you place a straight edge lining up the top pintle with the bottom pintle and extend to the bottom of the board, does it touch the line or in front? If it is behind the line, then that's where the weather helm is from.

Let us know how you go.

Regards,

Neil

Link to comment
Share on other sites

> Is is preferable to increase the rake of the boards back to maintain the balance I have

Probably

> or is it better to have them maybe at the same or less rake (more upright) and just positioned further back?

They should be as far back as the rules allow: "6(1) Rear of slot at keel to outside of transom: min 1375, max 1405."

Typically the case is put as far back as possible during construction. Changing centreboards usually requires filling around the board at the bottom of the case, to stop water squirting up around it, and packing foam into the space in the slot to help the board slide and reduce the water you're carrying around with you.

> Put another way, assuming balance is good, what are the thoughts between raked boards or upright boards?

The most rake I've measured (off photos of the leading sailors) is 60degrees. I'm guessing maximum you can achieve will be less than this.

I've used 4 different sizes of boards, and every time maximum rake was best.

After consulting with leading sailors, I set my mast at 90mm, with loose upper stays and very loose lower rear stays. It's going well. Bruce Rose has a very different setup,a nd fine-tunes his rake and stay tension to maximum performance. Not sure how that works...

My understanding is different to h20's: more rake = more weather helm, as it moves the centre of sail effort further back. This makes the boat more like a weather-vain that will point into the wind if you let go of the steering.

Weather helm is good; it means the rudders generate lift to windwards, which helps pointing upwind.

Link to comment
Share on other sites

Hi Greg,

Where do you sail the PT? Are you involved with an established PT fleet?

What is your boat's name and sail number? We just like to ensure our records are kept up to date.

Please let us know if you require any further assistance. You might like to check out the PT website at www.papertigercatamaran.org as well as the Australian PT newsletter at http://aptca.papertigercatamaran.org/index.php?option=com_content&view=article&id=60&Itemid=53

Regards,

David Stumbles

Secretary

Paper Tiger Catamaran International Association

Link to comment
Share on other sites

Thanks Dave,

I'm based in NZ, and a regular visitor to your website. The recent article by Ian M got me thinking, hence my first post... He seemed to be thinking about lessening CB rake. And thanks to Tony and Neil for their replies. I'm entering an expermental phase now after the nationals, should be interesting...

Cheers

Link to comment
Share on other sites

Yeah its all a bit about trial and error huh?

I just started thinking about this after raking my mast back somewhat, then thinking about the effect of balance on the rake, then wondering why a number of other PT's having a reasonable aft rake on the centreboards when by shifting the CB's back you can presumably achieve the same effect on helm balance while have deeper CB's. And this would presumably increase the aspect ratio of the foils improving the lift vs drag relationship of them.

Or I could just improve the way I steer the thing haha. Oh well I guess I'll find out....

Link to comment
Share on other sites

Absolutely. At the recent Australian National Titles, the top ten boats had a wide variety of centreboard types, sail cuts, stay tensions and hull manufacture. Speed differences on the course came down to better sail settings for the conditions, yet most of the top 20 had near equal speed. The results came down to decision making: starting strategy, upwind strategy, buoy tactics, etc.

Having finished 19th, I'm hitting the sailing theory books again!

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

×
×
  • Create New...