Bi-Plane rig on a 20m cat

I've been talking to several boat design/builders about the idea of a bi-plane rig, and here are some of the arguments against.

1) Heavier (two masts).
2) More wind drag with two masts.
3) Hobby horsing due to weight being further toward hulls.
4) Sail blanketing on a reach.
5) Poorer performance into the wind.
6) Harder to tack.
7) More difficult to reef with one boom over the water.

That's a lot of negative, enough that I have to consider abandoning the idea. However, even though I'm likely to go with a single mast setup I would like to hear from a few others about these problems, particularly the Hobby Horse effect. I've only heard that from one designer, and I wonder if he has sailed a bi-plane rigged cat, and if this is something any of you also believe would be an issue.

I have emailed a couple owners of bi-plane rig cats as well to get their opinion since they have sailed such a rig (Coolchange in NZ, for example). I'm still waiting on their replies so I thought I'd get some opinions from this forum. I do know some positive elements of a Bi-plane rig as well, but for now I'm more interested in hearing discussion on these negative elements.

So... what do you think? Hobby Horsing? True?

 

You should look ad the l'hydroptere thread.

http://www.boatdesign.net/forums/mu...lanet-61-knots-70-1-mph-peak-speed-15612.html

 

I looked, it seemed to have no relevance to my questions about a bi-plane rig for a catamaran cruiser.

 

Think about sail trim for a few moments. If you mount the rig on the perimeter of the platform (ie on the hulls of a cat), you no longer have a wide plastform underneath the main boom on which to run a traveller. The leeward rig will, therefore require some way of controlling twist in the rig without the ability to pull down from the clew to a point on the deck.
The standard fix for this issue seems to be a wishbone boom but there have also been a few fixed booms which have a sort of rigid vang that keeps the clew at a fixed height. Both these solutions are non-standard and come with their own issues. i don't know if you've considered this issue - I didn't see it in your list.

btw - I don't see how hobby horsing is increased by moving weight towards the hulls. i would have thought it would be reduced due to to the reduced overall mast height in comparison to a single masted rig.

As for wind drag - would it really increase? Freestanding masts lose all the standing rigging drag and replace it with a single larger section. On balance I would have thought this to result in a reduction of aero drag but it probably depends on the particulars of the boat.

I'm not sure that this has anything to do with hydroptere.

 

You're right, I didn't have that in my list . I agree with you on the hobby horsing, but since one boat builder mentioned it, I wanted to get some other opinions to see if anyone has any experience with this issue.

If I was going to use a bi-plane rig (not a forgone conclusion), I would probably use a fixed boom with a rotating mast. I like that solution best because I'd probably also use a self furling boom system and that would keep the sail aligned with the boom no matter what the current position, allowing me to reef the sail a little easier on the leeward boom.

At this point it's mostly the hobby horsing issue that has me most concerned, if that turns out to not be a factor, I would go back to considering the bi-plane rig. It certainly isn't for everyone, but I have some very specific needs that are not required for a standard sailing cat. Even if the boat is a little slower in some wind conditions, I might still consider using this rig. Still a lot to figure out first though.

Thanks for your response, Munter.

 

The hobby horsing is BS. With 2 lower rigs instead of 1 tall one, the platform will pitch less. I don't have experience with the cat platform but know about bi plane rigs on a tri, where the 2 rigs were mounted halfway out on the main beam - which allowed close to a balanced rigging staying base. Take a look at the rigging setup on Yves Parlier's 60 foot cat - there are images posted on this forum. The greatest advantage with the double rig is that you get a large sail area set low. The Peyron brothers had a bi plane cat and Bruno came in second behind Pajot's big Elf Aquitaine in a single handed Atlantic race ... so they can perform alright.

 

What? What!

Bi-plane tri?

Do you have info/pics, I googled such a thing but could find no evidences.

 

Here it is - plus Parlier's cat

 

Try googling "miss cindy" - basic bi masted sml cat.

 

That bi-plane tri sure is an off one, looks like a mosquito or dragonfly.

 

Wouldn't the unstayed masts be mounted further forward in the hulls to balance the C of E with the CLR. ( assuming main only , unstayed rig) Hence we have the age old problem with "cat" rigged CRUISING boats, pitching ?
---------

 

Bruce, as I say, no experience with cat platform but if you look at the Parlier cat, the rig appears to be placed in the usual fore and aft position (maybe slightly forward) and the slightly angled foils (which behave as daggerboards too) look in line or even ahead of the CoE - and the rigs are 2 unas too, no headsail) ... so what I'm saying is that you can get away with murder on spreadout multihull platforms, you can break the rules.
However if you were very worried about weather helm, you would shift the boards aft plus having larger rudders. But looking again at Parlier's courageous design, the rudders are not large but high aspect ratio with the inverted T foil sections at bottom; that designed rudder is more efficient, with the end plate effect, than standard rudders (also helpful in stopping pitching) - so that is equivalent to larger rudders anyway. Also would help in stopping pitch poling - but Parlier actually did do that - he must have buried the bows very deeply to do so.

 

I think something to think about is the apparent wind of a catamaran, and the angle the it is incident with the sails even on a broad reach, the apparent wind is moving diagonal to the beam so there isn't as much blanketing as a "theoretical" static wind condition. There is even some evidence that the "blanketed" sail gets some benefit from the incoming wind being conditioned by the other sail, similar to but totally different from the jib/mainsail interaction.

You get some other "free" benefits, for example you can reef either sail and turn it effectively into a pro-a in storm like conditions.

The center of effort for a comparatively sqft/sqm bi-masted rig is much lower than a single mast, but additionally the wide relative beam acts like a pro-a where the distance from sail to the far keel is a massive lever and you are having to lift the center of gravity as opposed to just rotating it. So, you are actually stronger, in either leeward or windward directions... Diagonally, since your CE is lower, and again, because at least part of your sailplan is much further away then a traditional design from the "corners" you are less likely to pitchpole or plow.

Not to say it is perfect, but there are some advantages.

 

Thanks for the replies. I got an email from the owners of Coolchange, they are on their way from Fiji to Vanuatu right now, but will contact me when they get there. I'm curious to hear about how Coolchange performs. Here is their web site shot of their boat.
http://www.sailcoolchange.co.nz/boat-charter-catamaran/

 

Been to that website before... that is a pretty cool cat.

The eternal question is to jib/genoa or not to jib/genoa? Or rephrased.... stayed masts or unstayed masts.