Liquid tie bar alignment theory on catamaran

[rslifkin]

It might be worth aligning them underway at a few different speeds, both under power and eventually under sail. See if they align differently under different conditions. And then consider a setup like Chrisr described to allow future static alignment based on how they line up under what you consider to be your most common running conditions.

[Chotu]

Originally Posted by rslifkin

It might be worth aligning them underway at a few different speeds, both under power and eventually under sail. See if they align differently under different conditions. And then consider a setup like Chrisr described to allow future static alignment based on how they line up under what you consider to be your most common running conditions.

Getting into the theoretical here, how can there be a difference?

Each rudder is on the centerline of a hull that is 19.5 feet apart.

It doesn’t matter what you do. They both are going to point in the same direction if you are moving through the water and they are free to rotate.

I’m not seeing all of this extra stuff people are talking about. Even if the boat was moving sideways through the water at a direction 90° from the bow, they would still line up correctly. Perfectly parallel.

They have to because they are both in the same flow.

My feeling is that any of that other stuff is just over thinking. If you change it from a perfect alignment, I think you are overthinking the problem. Åkerman excepted, but Ackerman is nothing anyway. Because you don’t turn a boat that sharply at speed anyway. That doesn’t even happen. Rudders only move few degrees from center line unless you’re docking.

[rslifkin]

Originally Posted by Chotu

Getting into the theoretical here, how can there be a difference?

Each rudder is on the centerline of a hull that is 19.5 feet apart.

It doesn’t matter what you do. They both are going to point in the same direction if you are moving through the water and they are free to rotate.

I’m not seeing all of this extra stuff people are talking about. Even if the boat was moving sideways through the water at a direction 90° from the bow, they would still line up correctly. Perfectly parallel.

They have to because they are both in the same flow.

My feeling is that any of that other stuff is just over thinking. If you change it from a perfect alignment, I think you are overthinking the problem. Åkerman excepted, but Ackerman is nothing anyway. Because you don’t turn a boat that sharply at speed anyway. That doesn’t even happen. Rudders only move few degrees from center line unless you’re docking.

I'd expect they probably line up pretty straight to the hulls on a cat. But depending on wake interaction between the hulls at higher speeds it's possible that the water flow at the rudders may not be exactly parallel to the hulls.

[Chotu]

Originally Posted by rslifkin

I'd expect they probably line up pretty straight to the hulls on a cat. But depending on wake interaction between the hulls at higher speeds it's possible that the water flow at the rudders may not be exactly parallel to the hulls.

Hmmmmm.

This needs to be figured out.

I’m very curious. I have presented my arguments but yours also make good sense.

It appears that the bow wake meets aft of the boat. I have noticed this in calm conditions.

[rslifkin]

Originally Posted by Chotu

Hmmmmm.

This needs to be figured out.

I’m very curious. I have presented my arguments but yours also make good sense.

It appears that the bow wake meets aft of the boat. I have noticed this in calm conditions.

You have the ability to let the rudders feather into the water stream, so it should be easy enough to see if alignment other than straight is enough to be worth worrying about.

[s/v Jedi]

Originally Posted by ChrisOwens

Cross current shouldn't matter; the hull, keel, and rudders can't tell whether or not the water is moving relative to the seabed.

I am worried about the interaction of current with apparent wind and the mirrored shape of the wetted part of the hulls. It seems to me it’s easy to see different flow at each rudder.

[Sailorichiban]

Hello all,
I’m sure you have mentioned your general information (specs) concerning your boat. If so, I don’t know how to access them, how could I do that or could you post again?
What kind of performance are you expecting under power and sail when you are “completed “?
I realize that if you are like me then a boat is always a work in progress.

[Chotu]

Originally Posted by Sailorichiban

Hello all,
I’m sure you have mentioned your general information (specs) concerning your boat. If so, I don’t know how to access them, how could I do that or could you post again?
What kind of performance are you expecting under power and sail when you are “completed “?
I realize that if you are like me then a boat is always a work in progress.

No problem. I’m supposed to put together a little website about it for this purpose. My fault for not having done that yet, so here you go:

Structurally, a KH design, but heavily modified from a styling point of view. Lines were changed, etc. Ripped off Gunboat and built a “poor man’s Gunboat “. It’s basically a Gunboat 48.

50ft LOA with reverse bows and added length in the sterns. 25ft beam. Weighs somewhere around 12,000lbs but the rig isn’t on yet, nor the interior done. I’m on my way now to get the rig on.

“Every pound counts” build philosophy. Vacuum resin infused hulls and beams then fillet/cove jointed the parts together to make the boat. Decisions were made to keep weight off, such as outboards vs diesel.

Used to do 6-7 knots under power with a single 30hp outboard on (I run one at a time). Now is heavily fouled and needs a haul out and bottom job.

Boat was built with myself leading a small team that worked at Hinckley, Catalina and some help from a guy that worked at Tartan. I personally mixed every drop of epoxy and did the resin infusions. I was also quality control. The hull was built perfectly. With complete perfectionism. Eric Sponberg and Richard Woods deserve credit for the help they gave me, as does one quote from Chris White: “it takes just as much time and money to build a crappy boat as a good one - might as well build a good one”. Also I have asked a bajillion questions online to make sure I’ve thought of everything. I’ve had a lot of help from members here and on another more build focused forum.

Build philosophy: No through hulls below the waterline. Simple, off the shelf systems that are easily replaceable so you don’t have to fuss with making a “my refrigerator broke again” or “my generator doesn’t work” or “my air conditioning water pump is dead/clogged again” thread and project. And of course the “every pound counts” philosophy.


The bad: my first attempt was using the KH cylinder mold method. It’s flawed. Not a good way to build a boat. I can explain more if anyone wants to know about that. I built 1/4 of a hull and it had to be cut up and put into a dumpster. Luckily it was fairly cheap. Once I switched over to corecell, Fiberglass and epoxy, everything went fine.

I’ve also had some pretty bad help on the boat. I had to kind of hire whoever I could toward the end because I can’t work with epoxy anymore. And there were definitely epoxy jobs to do. The boat has sustained some damage from those people doing a bad job. And not caring about the boat at all. Doing the the bare minimum and breaking things/destroying things.

Personally, I also have had probably more setbacks than the average boat builder. Divorce, which is very common for boat building, heart attack, and pretty extreme problems with chemicals. Severe epoxy allergy. Anaphylactic shock and what not. Getting over these things has delayed the boat build.

Was hit by 3 (qty) category 3 or higher hurricanes and took heavy cosmetic damage from one of them. The rest of the cosmetic damage is from careless workers.

Currently, I am having one of the world’s foremost experts put the rig and plinth (sail control table) together and get it on the boat. Very excited. No idea how I found this guy. I am on my way up there to get it done right now. Right next to the old Gunboat factory.

Ohh yeah. Upgraded to Hydraulic steering as seen in this thread.

Currently trying the boat out under sail to make sure I want to spend more years of my life finishing the interior and fixing all the damage from hurricanes and bad workers.


Answering the performance question:

I don’t know. There is a sister ship out there who has posted great info on VMG and wind speed. I’m looking to be able to sail when others would be motoring and reef when it’s windy.

72ft air draft. Have a main and self tacking jib as well as spinnaker. Need to add a screecher/sprit later.

Regular passages in the teens should be normal for speed I hope. Should be able to hit 20 under absolute perfect conditions. But I built it so i could sail instead of motor in light winds which is what is typically the problem sailing.

[Tupaia]

Twin rudder indicators and a remote bypass valve.

[Zanshin]

@Chotu - NACA profiles for water? I would have thought that the Re-numbers would be completely different?

[mark_morwood]

Originally Posted by Zanshin

@Chotu - NACA profiles for water? I would have thought that the Re-numbers would be completely different?

Yes, NACA profiles are commonly used for modern foils on boats - rudder, keel, foils etc. I don't know enough of the math to explain why.

[Chotu]

Originally Posted by Zanshin

@Chotu - NACA profiles for water? I would have thought that the Re-numbers would be completely different?

Speaking of Eric Sponberg:

http://www.ericwsponberg.com/wp-cont...der-design.pdf

There is an article with all the math needed to discuss foil shapes for control surfaces. The latter half of the article gets into the NACA shapes.

Basically, they have less drag and better lift, while not stalling as easily, I think.

Same reasons they were used on aircraft. Same application, really.

Of course mine are symmetrical profiles and not flat on one side.

They are also kickup rudders and balanced with no skeg and the rudder shaft placed correctly for good balance

[Chotu]

Incredibly frustrated looking at that picture!!! Need to get new bottom paint on asap.

[Sailorichiban]

Thank you Choto!
Your reply to the questions concerning your boat was terrific.
Very interesting and informative.
One of my favorite threads , I have read it many times , it keeps giving new insights.
Perhaps you could provide the correlating timeline? Undoubtedly you have already satisfied many of your readers with so many details of your journey, but still I am wondering when did you start thinking of this project? What steps at what time have brought you to this point?
I really appreciate your statement concerning spending years to finish .
Thanks again!

[motion30]

In actual function a small miss alignment between the rudders makes a very little difference in performance