Boating Straight

Tom.B · 08-23-2010, 08:42 PM

I'm curious, how hard is it to keep your boat going on a straight course? When I first bought our 35' Senator (single screw) she handled ok. It wasn't really good or bad at keeping on a straight heading but was rubbish at slow speeds. After a little snooping, I decided to have the rudder extended by 4" aft and 1" fore (that was the exact same ratio as the current measurments of the stock rudder). It was actually a little more than I had originally wanted, but I was limited by the size of the size of the metal stock at the yard.

Anyway, my point is that while the slower speed handling is better, the cruising speed handling (7-7.5 knots) SEEMS worse. It's quite a task to keep her going straight ahead for any lenth of time. I tend to wander her back and forth some (and when the First Mate takes the helm... fugettaboutit) I* worry about it whenever others follow us and they might be laughing at my inability to keep her in the ditch. It takes a bit of Zen to get her to a hands-off helm position... even for a short time.

Are other Taiwanese trawlers like this? Was it like this prior and I was just too new to feel it? I wonder if the forward edge of the rudder is too close to the screw now? It's not close enough to hit, but I am not an expert on fluid dyanmics to know. DUH!

Opinions? I'd hate to think I really botched it, but I see some trawlers with HUGE rudders and I thought, of course, that bigger is better.

Daddyo · 08-23-2010, 08:48 PM

It sure sounds to me that your leading edge is too close. You might want to take some off fore and aft on your next haul out or spring for the extra bucks and have an engineer run some some calculations.

Tom.B · 08-23-2010, 08:56 PM

That might be easier said than done, but I wonder if that's a real issue? I mean, the rudder being too close to the prop. Would that really cause this problem? What type of engineer would I talk to?

Nomad Willy · 08-23-2010, 09:49 PM

Gonzo,The first question I've got for you is is your steering hydraulic or push-pull cable or (not likely) cable through pulleys and/or sheaves? If it's the latter you may benefit from adjustable springs or long rubbers to center the rudder. With single cable there's not much one can do except go hydraulic. One can put wedges on the trailing edges of the rudder to help w directional stability and also give additional turning force a bit like a flap on an airplane. The turbulence coming off the trailing edge of the keel may be getting into the act and as I recall the wedges help stability. With hydraulic steering I don't see how you could be getting the problems you describe. On smooth water I set my hyd steering in smaller and smaller adjustments until I'm only making 1/8 to 3/16" changes (on OD of helm wheel) and very soon I can leave the helm for a minute or two (even more at times) without input and the boat goes straight. Most of the time I can snake along w no more than 1 or 2 degrees variation. In one or 2' seas * *..about 5 degrees. My (fairly new) hyd steering may be better than the Simrad AP we had on the last boat (w cable). Basic steering effort is a bit higher though.*About your mod * *.. adding too much at the leading edge will give good power and extended trailing edge gives stability. What is your rudder made of? Should be easy to whack down w a cutting torch if it's ferrous. Perhaps your additions aren't perfectly straight. An error in one direction may give you what youv'e got and the other way could be even better than straight * * ..don't know. If you let go the helm does the boat always turn in the same direction? The forgoing is obviously thinking out loud but may give you ideas.
I have read and liked a great many posts by you and this is the first time I think I've addressed you directly. I sure hope one of us helps you into the right direction as a boat that steers badly is hard to like.

Eric Henning

**Peter B** · 08-24-2010, 03:30 AM

Gonzo, It's possible, even probable the addition to the leading edge in particular is making the steering too twitchy because it has brought it closer to the prop flow, and because it is ahead of the fulcrum, the rudder stock. This would make it more responsive, but also more sensitive to water movement - maybe too much. Maybe just taking some off this would help, leaving the extra on the trailing edge to give more turn effort when really needed, as it will do little when at the straight ahead - other than keep it straight ahead, I would think.

FF · 08-24-2010, 03:45 AM

The most common number is the lead in front of the rudder stock is 17% of the total rudder length.

Tom.B · 08-24-2010, 04:42 AM

It's hydraulic steering. Bronze prop 24/17.

Steering input adjustment CAN be small ones, but they range wildly. Sometimes it will drift off and it takes a half-turn or more to catch her wandering. Then a full turn back and the process starts all over again trying to find her center. Maybe I'll record a video of it this weekend.

The welder did a good job of keeping the additions straight. What DID have to happen was the bar stock he used was slightly thinner gauge than the original rudder material. Maybe 1/16th or 3/32nd than the material the rudder was. We figured that at the slower trawler speeds, it shouldn't have an effect.

FF... Is that a rule of rudders? I can't tell you exactly what the final measurements were.

I am constantly kicking myself for this. I jumped too soon to try and fix something that may not have been broken in the first place and while fixed one issue, made another one worse. I hope that the leading edge can be removed cleanly. I made a big deal with the welder to make it look as good as he would on his mother's boat. Trying to cut 1/4" or so off would be a huge challenge with a plasma cutter and grinding wheel.

I'd like to talk to an expert about this, but I don't know who to call. A boat builder? Some kind of nautical engineer?

Thanks for the help y'all.

**Baker** · 08-24-2010, 05:09 AM

Are we sure that your hydraulic system is air free? It may not have been noticeable with the less sensitive rudder but magnified by this one?

Steve · 08-24-2010, 05:14 AM

Maybe the connection between the hydraulic cylinder and it's mount or at the rudder arm loose or sloppy
Steve W

albin man · 08-24-2010, 05:32 AM

Mine tracks straight as an arrow, since I installed the autopilot.

hollywood8118 · 08-24-2010, 07:04 AM

Quote:

albin man wrote:

Mine tracks straight as an arrow, since I installed the autopilot.

I second this one.... who hand steeers anyway!. I almost never hand steer since owning boats with autopilots.
Have the Admiral turn the helm about 5 degrees from center to each side while you watch the steering gear do its thing above the rudder inside the boat. This way you can rule out any unnecessary play in the system, then look at the dynamics of the rudder install.
By the way, my boat wanders a few degrees to each side even on auto pilot, I have learned to let it go!

*

-- Edited by hollywood8118 on Tuesday 24th of August 2010 08:05:14 AM

Codger2 · 08-24-2010, 07:59 AM

Quote:

hollywood8118 wrote:"By the way, my boat wanders a few degrees to each side even on auto pilot, I have learned to let it go!"

Ditto!

*

Marin · 08-24-2010, 08:48 AM

Quote:

GonzoF1 wrote:

I'm curious, how hard is it to keep your boat going on a straight course?

Water comes off a prop blade in a curving motion* If you've extended the fore and aft dimensions of your rudder it's possible that it is now being affected by water movement that wasn't affecting it before.

I've only run one single-engine trawler-type boat and that was a GB36 that we chartered.* While it had an autopilot we never used it and I don't recall having any steering issues at all.

Our own GB is a twin.* We removed the autopilot as soon as we got it but it tracks as straight as a boat will track given the influences of wind and waves.* It does not hunt back and forth at all, but left to its own devices it will very gradually (or very rapidly depending on the conditions) shift off in one direction or another.* It was much less directionally stable with the props as they were when we got the boat but a few years ago we had them reworked and found that they had been horribly set up in California.* Re-pitching and balancing them made a significant difference in how well the boat tracked because it eliminated the uneven thrust.* But that's not an issue for your boat.

Others have talked about issues with the steering system itself and that can certainly be a possibility.** But if you've done nothing to the boat except change the rudder and the problem has arisen then I would think it's being caused by the changed dimensions.*

Perhaps it's got nothing to do with the water coming off the prop, but the fact you've changed the dimensions means you've changed the balance and also changed the power of the forces acting on it.* So now it's overly-sensitive--- when it's moved just a little bit the extended dimensions generate much more force than before and the rudder is much more easily turned by the water flowing over it.

There may be things you can do to it--- trailing-edge wedges or tabs as Eric suggested--- or perhaps you need to return the rudder to its original dimensions.

*

-- Edited by Marin on Tuesday 24th of August 2010 09:52:22 AM

Nomad Willy · 08-24-2010, 08:50 AM

Gonzo,I know where you can get the expertise you seek.

http://www.boatdesign.net/

This is a huge site with members all over the world. There's on there that know very little about boats and many many others that are naval architects with vast knowledge. On this site you will often see "2 viewing" but on design.net you'll frequently see 30 or more viewing. They don't discuss anchoring or how to back up a SS boat but design, propulsion, building methods and materials, Projects and much more are dealt with extensively. Your question should probably go on the "powerboats" thread. It's a bit like going to the library.
One can spend time there and at least one of our members (other than me) is on there.

Eric

-- Edited by nomadwilly on Tuesday 24th of August 2010 09:55:51 AM

Tom.B · 08-24-2010, 09:18 AM

TBH... I haven't checked any hardware issues like the hydraulic connections or the like. I will do that, but it really did start when she was dropped in the water in December after the modification.

Do you guys have any pice of these "wedges" you are talking about?

I'll nose around the boat design site and see what I can see. Thanks Eric.

JD · 08-24-2010, 01:15 PM

Quote:

GonzoF1 wrote:

TBH... I haven't checked any hardware issues like the hydraulic connections or the like. I will do that, but it really did start when she was dropped in the water in December after the modification.

Do you guys have any pice of these "wedges" you are talking about?

I'll nose around the boat design site and see what I can see. Thanks Eric.

Gonzo,

Talk to Kenny Bock and tell him what you did and see what he thinks.* Just like Bert Ferebee is the guy in our part of the world for AC, Kenny*may be*the guy*for*your problem.*

Bock Marine*(252) 728-6855* Down Adams Creek at the Core Creek Bridge.

*

skipperdude · 08-24-2010, 02:04 PM

There is something called a Reynolds number.
*A basic concept of fluid mechanics.

R=V*L/KV

Where** V is velocity
************* L is length ( fore And Aft of the foil)
************* V is velocity
************ Kv is kenematic viscosity

Kv =~10-5 ft2/sec for water.* thats 10-5 squared/sec and ft sq
** You may have changed the semmetry of the foil or rudder If you could check the angle of attack (AOA) in relation to lift and drag of the rudder.*
Just by lengthening the rudder for and aft you may have changed* both numbers

AOA is 0 IF no other forces are in effect. The same for drag.
Your keel acts as a rudder if your moving straight the AOA is*0.**Wind or current can change the AOA*+ or-
Drag slows the boat. lift on the rudder is what makes the boat turn. it's a foil much like a wing. (What say ye Marin)
*
This is way to complicated To solve for Newtons laws of motion to design a rudder that works at all speeds.

Buy a new Rudder. Or check with a Hydrodynamicist or aerodynamicist.

Just curious did you do any computations before you redesigned the rudder?
Or just guess?

SD

-- Edited by skipperdude on Tuesday 24th of August 2010 03:10:21 PM

-- Edited by skipperdude on Tuesday 24th of August 2010 03:32:09 PM

-- Edited by skipperdude on Tuesday 24th of August 2010 03:36:57 PM

Tom.B · 08-24-2010, 02:14 PM

Quote:

skipperdude wrote:

There is something called a Reynolds number.
A basic concept of fluid mechanics.

R=V*L/KV

Where** V is velocity
L is length ( fore And Aft of the foil)
V is velocity
Kv is kenematic viscosity

Kv =~10-5 ft2/sec for water.* thats 10-5 squared/sec and ft sq
You may have changed the semmetry of the foil or rudder If you could check the angle of attack (AOA) in relation to lift and drag of the rudder.*
Just by lengthening the rudder for and aft you may have changed* both numbers

AOA is 0 IF no other forces are in effect. The same for drag.
This is way to complicated To solve Newtons laws of motion to design a rudder that works at all speeds.

Buy a new Rudder. Or check with a Hydrodynamicist or aerodynamicist.

Just curious did you do any computations before you redesigned the rudder?
Or just guess?

SD

-- Edited by skipperdude on Tuesday 24th of August 2010 03:10:21 PM

My head just asploded!

It was a total guess. So that guess, I guess, was a poor one.

*

I'll give Bach Marine a call as see what Kenny has to say. Thanks JD.

skipperdude · 08-24-2010, 02:45 PM

Sorry Bro,
Fluid dynamics is just a little hobby of mine.

Not doing anything with it. Just trying to understand the math.

SD

Datenight · 08-24-2010, 06:31 PM

Gonzo,

I feel your pain! I have had the same problem for 13 years on our Sung Yung Marine 37' sedan. We have a Wagner 700 series hydraulic dual station system with a teleflex ram. I have to constantly correct to starboard in heavy weather. It will not track in calm (1'-3') seas. We have bled the system more times than I can count. Last summer while in the slip I had a friend turn the wheel 1/6 or one spoke while I had a hand on the cylinder. I could feel the motion so there seems to be no "play" in the steering

Solved the problem this spring by finally installing the auto pilot I have had for six (!) years. It still hunts a little but I don't have to correct by hand.

Rob
37' Sedan