~~PACIFIC BEE~~ FOILS & SAILPLAN
First of all, thanks, Sven, for always answering our many, many questions. Your advice is much appreciated.
So my question is; what is the difference in upwind performance between using the daggerboard in the ama compared with putting the front rudder dagger down a little. Both change the CLR. Is there a performance difference between those two options? What is it?
I want to build as few foils as possible, but I also want to get to windward well.
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Chris, you don’t have enough lateral area in that front rudder sticking down only a bit , so you need that daggerboard somewhere.
Even two rudders sticking fully down, both steering won’t work, especially in a round body vaka , leeway will be huge , unless one fools himself by steering the contraption to windward.
I don’t advocate that the leeway preventer area , has to be in the ama, for lateral reason is OK in other places as well , other guys [Laurent for instance ] us lateral area on the windward side of the vaka.I am experimenting with different ama and/or beam end foils , but I do not have a conclusion made up for myself. it fully open design territory . the 53 will get a multiple foil system on the end of each beam, to experiment in practice , we simply don;t know what is best yet .
I like the vector foil a lot , but render it not practical in the real offshore condition, might be worth a trail though.your question; building only a few foils….
My answer is simple , for the same reason you will have multiple sails onboard to suit the different course/conditions. you need different foil options to combine sails & foils to a proper setup/balance. so that means also different lateral area setting.
on Pacific Bee I mosttimes sail without the daggerboard in, that is, say whenever, I can crack the sheet [ bear away some 8 to 10 degrees] the foil goes up totally.
When I have to make a headland ,sandbank or whatever object , It goes down fully. the upwind gain is some 4-5 degrees higher than a racing monohull. this is also because the slender hulls go through the chop way easier than any monohull.
Cheers Sven
FIN RUDDERS
Fin rudders (hope I have the correct term) that is a single blade on a shaft, are now used by just about all racing boats and assumed to be the most efficient form of rudder.
Cheers / Jzerro PB etc all use a rudder hung on the rear of a board (skeg), which is deemed to be less efficient. There can be a loss in efficiency due to the gap between rudder and skeg, I think Sven has answered this one earlier, that the gap is very close, so no problem.
The skeg mounted rudder is, I assume, the best choice for an amateur builder, being much easier to fabricate without access to an engineering workshop. A fin rudder requires a very strong shaft and machined bearings.
Sven, no doubt the engineering is not a problem for you. Is there a good reason for sticking with the skeg hung rudders?
Mark
Apologies for earlier version of this post, in a hurry and got my terms all wrong. Must remember to check before hitting the submit button.
FIN RUDDERS
Fin rudders (hope I have the correct term) that is a single blade on a shaft, are now used by just about all racing boats and assumed to be the most efficient form of rudder.
Cheers / Jzerro PB etc all use a rudder hung on the rear of a board (skeg), which is deemed to be less efficient.
Aloha Mark, Yes you are right, a deep blade is proven efficiency by today’s standards.
However the Pacific Proa is a different story, we have a to design-in some extra features.
- 1. as it is a Pacific Proa, [draft 4 feet rudder down, 1,5ft rudder up ] we all like to go places where monohulls simply can’t go. i.e. we like to hug the shallow waters, beach, dryout. and in doing so we gently inpinge ground with the rudder on a regular base.
with a freehanging blade un-supported by a strong skeg , you really asking for trouble, as one day it will bend/brake the stock.
I normally drop my anchor in 8ft of water, sail slowly up to the beach, and pull the rudder full-up as soon as the skeg is about to scrape the sand.
The last bit I steer with the sail, than I walk over to the ama, and step overboard knee deep , to drop the other anchor on the beach. Job done.
-2. One day you going to hit a big log, or worse , really bad luck you miss out on that growler container in the night. a skeg rudder may save the day, a fin rudder might likely fail.
-3. when damaged [see earlier post] as its repair is explained.
-4.The performance of a Pacific Proa is already ridiculously fast, I wouldn’t bother to much, if I do 17.84knots or 17.88 knots. i.e. the extra performance of a fin rudder is there, but who cares , you outperforming most around you anyway., till today the only guy that could beat me was Phillip 60ft Proa Des Jours Meilleurs, no monohull , and no cruising multihull. A 40ft Dragonfly trimaran,[finfoils & 4ft longer waterline] was passed upwind with a speed difference of about 4 to 5 knots , a hour later we could barely see him over the stern.
you surely don’t beat Seacart’s but that is hardly a cruiser , besides we are build of wood ,and that 30 years ago.
5. Last but not least, the rudder at small angles & skeg form an a-symmetric shape by itself, generating lift. as such contributing to the balance of the platform. I know for some this is a debatable point, but from life testing this, we noticed a constant slight [some 1.5 to 2.0 degree max.] rudder angle to give an absolute neutral helm. letting the autopilot do its thing, you literally don’t hear the autopilot work for a minute at the time, before giving a very minimal correction. [and this no matter how the ama is going through the waves, concluding that it is not drag of that ama that turn the vaka into the wind. [back then,I happened to be a bit neurotic about ama drag, till I found out that there wasn’t much force to pull the platform into the wind.]
As soon as the ama or net is overloaded [vacation stuff, fuel tanks,dinghy’s ,etc. the platform direction of travel needs to be corrected
you have to correct the proa by steering down . This proofs that a heavy Ama is no good for performance, no matter what.
Proa’s with heavy ama’s , ............better build a catamaran.!!
Cheers Sven
Sven,
as ever, thanks for the detailed reply.
Lying in the bath after posting yesterday, I realised the collision damage problem is far worse with a fin rudder. The impact point being in front of the pivot point means the blade will turn and be pushed back, so a real big jamb and lots of damage, ouch!!. Certainly a very good reason to stick with the skeg rudders.
Yup: you have all the answers!
Good to hear your sailing reports. Beating a longer Dragonfly - excellent. Also good to hear Des Jours Meilleurs is still doing her stuff. Would love to see a few photos of the action.
thanks again
Mark
POST # 35 sailplan 53ft “UNAMA”
Mastlength 53ft 16meter above deck
Airdraught 58.4 ft 17,80 meter above CWL
1th pic. full main & genoa
2th pic. blade jib only
3th pic. 2e reef & blade jib
4th pic. 1th reef & working jib
5th pic. full main & flat reacher code1
POST # 36 Rig Impression
Reaching with full main & reacher code 1
Dear Sven,
Like the images - a picture is worth a thousand words, but then raises a few queries.
- Outer hull. Like the wave-piercing shape. What is the cross section? (my thought being that it should give minimum wind resistance / overturning force, so an inverted teardrop / airfoil section)
- Foils. Shown hooked inwards. Is this a good idea? Whilst giving more downforce, when the boat heals, there will be a more sudden release as they leave the water. If hooked outward they will give some lift to the outer hull, which as you point out in regard to its weight, can be of benefit. Racing = hook in, Cruising = hook-out?
- Auto-pilot. You better have a good auto-pilot, with that sassy-bird onboard, you will not have much time for helming!!
POST #36 53ft “UNAMA”
upwind 1e reef , working Jib
Dear Sven,
Like the images - a picture is worth a thousand words, but then raises a few queries.- Outer hull. Like the wave-piercing shape. What is the cross section? (my thought being that it should give minimum wind resistance / overturning force, so an inverted teardrop / airfoil section)
- Foils. Shown hooked inwards. Is this a good idea? Whilst giving more downforce, when the boat heals, there will be a more sudden release as they leave the water. If hooked outward they will give some lift to the outer hull, which as you point out in regard to its weight, can be of benefit. Racing = hook in, Cruising = hook-out?
- Auto-pilot. You better have a good auto-pilot, with that sassy-bird onboard, you will not have much time for helming!!
Aloha Mark,
Outer hull; the crosssection is deep V, because the displacement needs to gradually become less, as such you notice your remaining richting moment.
Foils; I designed the daggerboard boxes for multiple options, trails will tell what works ,what won’t.
No, I not worried about anything like sudden release, because 99.95% of the time the ama is in scimming mode.
Autopilot; spot on Mark , I like to enjoy nature to the max. So I love autohelm technology.
Cheers Sven
Hey,
there’s an additonal benefit to the skeg rudders which hasn’t been mentioned yet, and that’s that skeg rudders can go to a much larger angle of attack (and thus produce a much larger rudder moment) than spade rudders, before stalling. There’s a section on it in Marchaj’s “Seaworthiness”.
When a rudder stalls, you have a sudden loss of lift from the rudder, the steering goes totally mushy, and the boat is pretty much out of control until you decrease the angle of attack to a really low value so the flow can reattach. If you’re in very rough water, and the boat and flow of the water are going every which way, the rudders with even a relatively small skeg infront of them (or a full length keel) are much more likely continue functioning normally than the spade rudders.
Cheers,
Marco
Sven,
Another question…
On your rig, your stays are not to the top of the mast, and you have a square top main sail anyway…
It is the stays that provide anchorage for the mast on the lee side, as the mast step is significantly to windward, right?
So how do you deal with the back stay when you want to sheet in the main all the way for going upwind. The back stay has to be in the way of the sail, right?
I see on your pictures that there is a line attached at the stay, mid-height. Is it a “pull line” to bring the stay to windward so it allows the mainsail to move more to windward?
How does it work? Does it mean that the stays are adjustable in length,so you can “bend them” upwind when they are a back stay?
I have noticed something similar on the Bieker proa.
Regards,
Laurent
Hey,
there’s an additonal benefit to the skeg rudders which hasn’t been mentioned yet, and that’s that skeg rudders can go to a much larger angle of attack (and thus produce a much larger rudder moment) than spade rudders, before stalling. There’s a section on it in Marchaj’s “Seaworthiness”.
When a rudder stalls, you have a sudden loss of lift from the rudder, the steering goes totally mushy, and the boat is pretty much out of control until you decrease the angle of attack to a really low value so the flow can reattach. If you’re in very rough water, and the boat and flow of the water are going every which way, the rudders with even a relatively small skeg infront of them (or a full length keel) are much more likely continue functioning normally than the spade rudders.
Cheers,
Marco
Aloha Marco,
Note that; a proa at speed barely uses its rudder, maybe 2 degrees at the most. because Its tracking in the groove.
As written earlier, the only moment you use full rudder is ; coming out of a shunt. and harbour manoeuvring.
Flow seperation never occurred as far as I can recall.
cheers Sven
Sven,
Another question…
On your rig, your stays are not to the top of the mast, and you have a square top main sail anyway…
It is the stays that provide anchorage for the mast on the lee side, as the mast step is significantly to windward, right?No , the mast is attached to one big shackle , on which ; 3 smaller shackles are attached , the shrouds run to both bows & 1 to the ama.
The mast sort of “hangs” to leeward one that 1 shackle.
So how do you deal with the back stay when you want to sheet in the main all the way for going upwind. The back stay has to be in the way of the sail, right?Because of necc fathead mainsail twistoff , the backend of the sail barely hits , only if you want to really squeeze the last few degrees of height, you need to pull the “Brown Fiddler” as we call it [ Credit for Russell the inventor] . with that line to the ama you can pull the backstay away from the mainsail leech . On Pacific Bee its 220mm. on the 53 it will be 620mm
on the 53ft UNAMA I donot use those 3 shrouds to carry sail on , so the Brown Fiddler is permanently attached to the shown point.
I see on your pictures that there is a line attached at the stay, mid-height. Is it a “pull line” to bring the stay to windward so it allows the mainsail to move more to windward?
How does it work? Does it mean that the stays are adjustable in length,so you can “bend them” upwind when they are a back stay?
no, the bow shrouds remain at there fixed length , for safety , when you pull the Brown Fiddler , you take that slack out of the windward shroud. the change in shroud length is barely anything.
I have noticed something similar on the Bieker proa.
Regards,
Laurent
Hi Sven-
In some of the pictures there seems to be a leeward shroud that attaches to the center of the leeward pod (I assume to support the mast when backwinded.) How does one shunt with a stay like that?
Keep up the lovely work.
Mitchell
Hi Sven-
In some of the pictures there seems to be a leeward shroud that attaches to the center of the leeward pod (I assume to support the mast when backwinded.) How does one shunt with a stay like that?
Keep up the lovely work.
Mitchell
Aloha Mitch , Hawkeye !! well,2 running backstays . just a consideration for long offshore tracking…....... have to see if its needed, or only peace of mind. Cheers Sven.