Windward improving foils on cruising catamarans

Zonker

Super Anarchist
9,726
5,686
Canada
We will be heading down to Georgetown from Marsh Harbor on the 15th so be there on maybe the 19th.
Look for my friends on their St. Francis 44 "Majestic" from Annapolis. I think they're in the Bahamas about now.

Doug and Cindy are awesome people and their kids are nice and full of energy.

 

boardhead

Anarchist
It all adds up. 
Very True, 

One area where big gains can be had on older cats is the plexi glazing on deck hatches which can be replaced with cored panels - good for as much as ten pounds a piece and also shutting off the greenhouse heating and need for covers, or, if the you like the lighting they provide just use thinner lexan.

 

boardhead

Anarchist
Fair enough. How bout end plating the main?

Russell suggests that Jzerro sails well to weather because the main is to weather of the headsail. Ever try tacking the headsail to leeward?
My understanding is that lift is a function of luff length so I lengthened the headstay to improve the taller blades contribution. 

Tacking the headsail to leeward of centerline would yield a finer entry for that sail while maintaining the same slot to help mainsail lift. Not going there due to the complex engineering (the headstay holds the rig up) for the jib. Could be done for lighter sails on the sprit end which could easily be adjusted off centerline but adding area ahead of the forestry doesn't work, upwind. I wonder if the AC foilers did any research on this one - worth a look.

Jzerro benefits from reduced wetted surface and lower parasitic drag compared to a trimaran but suffers from reduced righting moment also BUT there are a lot of specific variables in there.

Don't see a lot to be gained end plating the main which won't add to luff length and the boom is set low anyway on the Saint Francis. 

 

MultiThom

Super Anarchist
1,748
393
Benicia, CA
My understanding is that lift is a function of luff length so I lengthened the headstay to improve the taller blades contribution. 
Curious to know what the fraction was and what it currently is.  After the change did you have any issues balancing the boat?  

 

Steve

Anarchist
563
77
duluth, mn
Very True, 

One area where big gains can be had on older cats is the plexi glazing on deck hatches which can be replaced with cored panels - good for as much as ten pounds a piece and also shutting off the greenhouse heating and need for covers, or, if the you like the lighting they provide just use thinner lexan.
I replaced the 28" square x 1/2" thick acrylic lens on a big old Bomar cast aluminum hatch with a 1/2" honeycomb panel for the weight savings as well as shaved off the struts and holdowns as i didn't need them in the location i was installing it. If you wanted to keep the light transmission  on a large hatch a PP honeycomb cored panel with polyester resin would work well. It's not really practical to use a thinner acrylic lens to save weight as it would not be stiff enough and polycarbonate is worse as it is much too soft. Probably ok on small vent hatches.

 

Sidecar

…………………………
3,138
1,501
Tasmania
My understanding is that lift is a function of luff length so I lengthened the headstay to improve the taller blades contribution. 

Tacking the headsail to leeward of centerline would yield a finer entry for that sail while maintaining the same slot to help mainsail lift. Not going there due to the complex engineering (the headstay holds the rig up) for the jib. Could be done for lighter sails on the sprit end which could easily be adjusted off centerline but adding area ahead of the forestry doesn't work, upwind. I wonder if the AC foilers did any research on this one - worth a look.

Jzerro benefits from reduced wetted surface and lower parasitic drag compared to a trimaran or catamaran but suffers from reduced righting moment also BUT there are a lot of specific variables in there.

Don't see a lot to be gained end plating the main which won't add to luff length and the boom is set low anyway on the Saint Francis. 
Fixed. People also overlook the fact that proas also have far less windage.

Agree about luff length.

Jzerro’s foresails to leeward are more about separating them from the mainsail, rather than trying to get them to act together as one foil. A foresail to leeward also has the potential benefit of acting as a windward canted sail, ie lifting the bow up. In any case, Jzerro had no choice. You have.

Interesting your thoughts on endplating the mainsail, bearing in mind you endplated a very stubby keel. The point of endplating is to increase the apparent leading edge/luff/aspect ratio of a foil. But I can see that there may not be much scope on your boat.

Have you got a square top mainsail?

 
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PIL66 - XL2

Super Anarchist
2,740
835
Stralya
I replaced the 28" square x 1/2" thick acrylic lens on a big old Bomar cast aluminum hatch with a 1/2" honeycomb panel for the weight savings as well as shaved off the struts and holdowns as i didn't need them in the location i was installing it. If you wanted to keep the light transmission  on a large hatch a PP honeycomb cored panel with polyester resin would work well. It's not really practical to use a thinner acrylic lens to save weight as it would not be stiff enough and polycarbonate is worse as it is much too soft. Probably ok on small vent hatches.
Poly carbonate is tuff... It may flex a little but it will not break, Use UV stabilized tinted PC... bullet proof

 

boardhead

Anarchist
Curious to know what the fraction was and what it currently is.  After the change did you have any issues balancing the boat?  
The taller headstay allowed a 47' 3" luff compared to the 41' 4" luff on the abandoned lower. Sail area increased from 287 sq ft to 355 sq ft. With the leech battens the new sail carries a 10" roach. 

Also moved the existing, wonderful, Fiberspar battens higher on a new mainsail increasing the area higher up without resorting to a square top design,which I have no time for, so no issues with balance.

 

Sidecar

…………………………
3,138
1,501
Tasmania
You don’t say how much weight you have saved compared to a “standard” St Francis 44, nor how much extra sail area you have squeezed on. On the assumption they are both reasonably significant, they alone will make the boat go faster, and make the keels more “efficient”. Give us a clue.

http://www.multihulldynamics.com/images/cme/St Francis 44 - 44MKII CME.pdf

I would be interested to know why you have no time for a square top mainsail?

 
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boardhead

Anarchist
Show me a Standard Saint Francis on a load cell at 15,500 and I will believe that!

Mine weighed 16,000 after the diet and does float 5" higher at the sterns than before.

 

boardhead

Anarchist
I would be interested to know why you have no time for a square top mainsail?
I always focused on luff length with the help of my "Full Hoist" headboards. That trailing corner on the square top mainsail does not generate any lift and the broad head does not have any difficulty generating twist but I don't have to f--k around with a gaff batten raising and lowering the sail.

 

Zonker

Super Anarchist
9,726
5,686
Canada
My 40' cat was about 15,000 lbs. Fully loaded.

When we moved back to land, the boat rose about 5" - about 4000 lbs of stuff that a full time liveaboard needs. (this included semi permanent gear like the dive compressor and scuba tanks which we sold separately). Just my wife's shoes were at least 25 lbs of that.

 

Zonker

Super Anarchist
9,726
5,686
Canada
Course my tools must be over 300 lbs or so. Never dared weigh them. Hmm now that I have a bathroom scale I could....

 

Sidecar

…………………………
3,138
1,501
Tasmania
Show me a Standard Saint Francis on a load cell at 15,500 and I will believe that!

Mine weighed 16,000 after the diet and does float 5" higher at the sterns than before.
So what did she weigh before the diet, what was the sail area before hand and what are the SA numbers now?

 

Zonker

Super Anarchist
9,726
5,686
Canada
I had 3 sanders... (5" orbital, 6" orbital, triangular detail sander). I don't count the angle grinder.

 

boardhead

Anarchist
I spent 13 years modifying and rebuilding - totally nuts!

0 sanders, ditto grinders - NOT working along the way now, been there, done that.

No generators, compressors, ac from 1500 and 1200 watt inverters with a small rechargeable drill. Decent amount of hand tools for cutting, filing and shaping, wrenches and spares so in the region of 100 pounds.

Not intending anywhere near as long distance cruising as you, mind, Zonker.

 

boardhead

Anarchist
So what did she weigh before the diet, what was the sail area before hand and what are the SA numbers now?
Bearing in mind the boat was owner (and helpers) finished in South Africa and set up for live aboard and fully crewed charter:- I would estimate she weighed four or five thousand pounds more than when relaunched since when my cruising gear has added 1,000 pounds.

I would say she weighed well over 20,000 and now weighs 17,000 pounds.

Sail area change is more accurate - Mainsail was 485 sq' now it's 782 sq' Headsail was 280 sq' now it's 355 sq' Total upwind was 765, now it's 1,137 sq'

 

Sidecar

…………………………
3,138
1,501
Tasmania
I would say she weighed well over 20,000 and now weighs 17,000 pounds.

Sail area change is more accurate - Mainsail was 485 sq' now it's 782 sq' Headsail was 280 sq' now it's 355 sq' Total upwind was 765, now it's 1,137 sq'
So assuming an effective waterline length of ~ 42 ft, the Base Speed numbers are:

Original configuration: ~ 9.35 knots

Current configuration: ~ 11.11 knots.

if you added/extended (effective) sugar scoops just under ~ 2.5’ long and kept overall weight the same, Base Speed would go up to ~ 11.41 knots.

Interestingly, the previous sail area was also way under the designer’s info?

I also had a quick look through the OMR, MOCRA and Texel ratings and couldn’t find any St Francis cats. Rating certificates are the easiest way of finding reliable info.

 
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