Shaft and Rudder alignment

Kiffa · Post by **Kiffa** » Sep 24th, '12, 10:29

Hi,
I think I may of mentioned on here some time back about my boat kind of nose diving and behaving weird if I don’t apply some negative tab.
After hauling out this time for the 1st time I realized that my port shaft is not directly in front of the rudder as it is on the starboard side.
After measuring on my boat and also on another 31' it seems that somewhere along the way the strut was not put back in the exact same location. The engine is lined up to the stern tube and strut, it’s just of by 1.5 inches compared to the other side and other boat.
I spoke to Capt Pat and Danny at High Tide last week and both came up with basically the same advice, that I should just move the rudder over to align it straight behind the shaft.
My question is has anyone seen this before, and also do you think this would cause the bow drop and sluggish steering when running?

Chris

CaptPatrick · Post by **CaptPatrick** » Sep 24th, '12, 10:39

Chris,

Measure from the aft corner of the chine, where it meets the transom, to the centerline of the rudder shaft on both sides and let us know what those two measurements are...

Br,

Patrick

Kiffa · Post by **Kiffa** » Sep 24th, '12, 10:46

Ok i wil go do that now.

kiffa75 · Post by **kiffa75** » Sep 25th, '12, 09:12

Hi Capt Pat,

I took the measurements and it seems I have a double whammy. Both the strut and ruder seem to of been put back in different locations.
The Distance from the chine to the centre of rudder re : Port 28.5 inches Stbd 29 inches. Distance from outer part of Strut to chine : Port 29.25 inches Stbd 28.5 inches.
So I think I’m correct in saying that combined that would make it just under 1.5 inches off from being aligned straight behind the shaft.

Chris

CaptPatrick · Post by **CaptPatrick** » Sep 25th, '12, 09:58

OK Chris, while your rudder/shaft offset on the port side might make a bit of difference in how the boat steers, (less active prop wash hitting the rudder blade), that seems to be more of a left to right issue than a bow up or down issue. I'm thinking that relocating the rudder port or re-aligning the engine, (or both), isn't going to cure your nose dive.

What engines, horse power, and transmission reduction are you running? Your strut bases look to be pretty flush to the hull without any severe wedging, so your shaft angle should be close to the stock 15º.

At what speed and RPM does the nose dive start? How much stuff do you keep loaded in the bow/V-berth area? Proper load distribution on the B31 hull is 60% aft of the forward engine space bulkhead and 40% forward of that bulkhead. Do you keep your fuel tank pretty much full? What capacity is the tank?

Br,

Patrick

kiffa75 · Post by **kiffa75** » Sep 25th, '12, 12:33

Hi Pat,

She is powered with 250 hp 6BTA with 1.5:1 gears.
Once you past 18kts the steering gets mushy an you can see the wake start to change then bow down.
This boat used to do this way back when she had the Chryslers in her.
The tank is the smaller original fuel tank. Since the rebuild she is quite heavy for a Bahia Mar since we used 3/4 Sapele Marine ply that we totally encapsulated with fiberglass before installing. HEAVY. The travel lift showed her at 14000lbs with about 40 gal on diesel on board and nothing else, not even engine boxes which do have the soundown installed. HEAVY.
We fill her up every week and burn the fuel throughout the week with no change.
I have attached some pics of her running and also sitting in the water with and without engines.

CaptPatrick · Post by **CaptPatrick** » Sep 25th, '12, 13:17

Chris,

Depending on what all you rebuilt using the encapsulated plywood and bringing the weight up by approximately 2,000 lbs, you may have upset the fore to aft balance ratio... But then you say the nose dive issue was even there before the rebuild and repower?

Br,

Patrick

Carl · Post by **Carl** » Sep 26th, '12, 07:02

Maybe a stupid question...but are the trim tabs set correctly?
Do they retract all the way back to home position?
I think home position should have end of tabs 1/2-5/8" higher then hull bottom.

Are they mounted flush with hull? Down a little, up a little as they should be,

Any chance someone glassed in a set of wedges?

How are props...any chance they are thin and flexing?

Rudder and tube not flexing...is rudder mounted straight downwards?

Capt Pat...as the prop works most in the 3-9 O:clock position...could the wash be hitting and lifting the rudder deflecting some wash downwards adding lift. It's a stretch...but wondering.

Need to edit that...wheel pushes the other way on rudder.....any chance of a paddle wheel effect as the wash is not directed aft by the rudder?

kiffa75 · Post by **kiffa75** » Sep 26th, '12, 07:35

Hi Pat,

Yes she always did this even before any refit and with the Chryslers. She was stock back then and was light.
Sim not stupid questions at all.
I will double check the tabs, but I’m almost certain they retract above the hull bottom. When a slight negative tab is on the portside the problem stops.
The props are new.
I will double check the udder to see if they are straight.
It may be a stretch but I too kind of think that it’s the prop wash pushing down on the ruder on that port side, especially since when tab is applied on that side things improve,

Chris

CaptPatrick · Post by **CaptPatrick** » Sep 26th, '12, 08:03

Capt Pat...as the prop works most in the 3-9 O:clock position...could the wash be hitting and lifting the rudder deflecting some wash downwards adding lift. It's a stretch...but wondering.

Good observation Carl. With the rudder offset to the outboard side the 3 o'clock, (inboard), face would be getting a much greater percentage of prop wash which could be causing some lift.

So it looks like we've maybe eliminated all other possible variables outside of the rudder/shaft offset.

Well, Chris, you have a project for when time allows. Hope that after the job's done, the problem is solved. Keep us updated...

Br,

Patrick

Harry Babb · Post by **Harry Babb** » Sep 26th, '12, 08:06

Sounds to me like there is a "Hook" in the hull.......but the negative tab don't support this idea......unless may be your having to add tab to only one side.......even that don't add up......

According to the pics your running angle looks pretty good to me.

hb

CaptPatrick · Post by **CaptPatrick** » Sep 26th, '12, 08:24

Harry,that begs for a better description from Chris as to just how much "nose dive" at full throttle is being perceived...

With adequate power at full throttle the bow will lower significantly from the position it takes at cruise throttle, due to the shaft angle being reduced in relation to the water surface.

Chris, when you start to perceive the nose dive, and then from then, on through the throttle range, do you continue to gain speed? Is there some point that even with a higher RPM you loose speed over that of a lower RPM?

Br,

Patrick

kiffa75 · Post by **kiffa75** » Sep 26th, '12, 09:12

Hi,

It dips quite a bit even at cruise and instead of the bow proud angle she runs pretty flat. From cruise up to full throttle the boat speed increase all the way and to me quite normally.
In all these pictures she has a slight tab applied, if it were to be removed after a few seconds the problems occurs.
One other thing I just remembered was that if I were to go hard to one side then to the other the bow raises and the wake flattens out but it will occur again if no tab.
I am kicking myself for not taking a video before hauling out.

CaptPatrick · Post by **CaptPatrick** » Sep 26th, '12, 09:32

kiffa75 wrote:From cruise up to full throttle the boat speed increase all the way and to me quite normally.

OK, that tells me that your weight distribution and power transmission are pretty much good. A true nose dive to me is when the bow drops so much after a certain point in the RPM range that it begins plowing water and no more speed is gained even with a higher RPM.

The fact that you're getting a positive speed increase throughout the RPM range is as it should be. So we'll not use the term "nose dive" anymore...

And I think that we're back to my previous post that says go for the rudder re-alignment...

Br,

Patrick

kiffa75 · Post by **kiffa75** » Sep 26th, '12, 09:37

Ok Pat I will no longer use that term! I will proceed in aligning the rudder and take it from there.

Chris

CaptPatrick · Post by **CaptPatrick** » Sep 26th, '12, 09:42

Chris,

Good luck and call me if you need to...

Br,

Patrick

JP Dalik · Post by **JP Dalik** » Sep 26th, '12, 13:55

If you just want to try something, drop the rudder plug the whole with a bung and take her for a spin. Just joy rig the tie bar so it wont flop around

Steering won't be great but she will steer

Be kind of a shame to do all that work and be wrong

CaptPatrick · Post by **CaptPatrick** » Sep 26th, '12, 14:46

JP,

Chris has my rudders, so it wouldn't be that bad either... Add to that, a twin engine boat really doesn't need rudders at all. I blew a hydraulic ram seal on a 48' Ocean Yacht SS once and had no problem finishing that leg of the delivery steering only with the throttles and gears, both at full speed and slow. About 8 miles form the inlet to deep into Tom's River. Your neck of the woods.

Keith Poe · Post by **Keith Poe** » Sep 26th, '12, 23:07

Hey Chris sorry to hear your having problems.

The picture where the bottom is being cleaned looks like a lot of barnacles on the ground.

Is the performance noticeably different before and after cleaning ? does it affect the problem in any way you have noticed ?

.

.

kiffa75 · Post by **kiffa75** » Sep 27th, '12, 07:19

Plugging the whole and going out with one rudder does seem like a good idea. Im going to be ashore for at least 2 weeks as im off to FL next week so il probably do this when i return.
Yep these barniclas were from her, but it was when she sat for 3 monhs waiting for engines, i never ran her wih this much growth.
I should also say the problem is the same with just myself on board or 12 on board.

CaptPatrick · Post by **CaptPatrick** » Sep 27th, '12, 08:17

I should also say the problem is the same with just myself on board or 12 on board.

Another good indicator that everything else, other than the rudder placement, is in proper configuration...

Bob H. · Post by **Bob H.** » Sep 27th, '12, 18:45

Check the bow area for any ballast that may have been glassed in place somewhere down the road, there is alot of boats with lead shot used to trim out poorly balanced rides. My buddy had a boat with three wheel barrows of sand glassed in the bow. You could also try adding barrels of water to the cockpit to see if the fuel tank size difference really makes that big of a difference. I moved my tank and engines back by at least 6 ", you really need to keep the ass end planted on the 31s. Thanks Mr Lico...BH

Keith Poe · Post by **Keith Poe** » Sep 28th, '12, 02:34

Sounds like to me you have to much weight in the front i would strip everything out of it i could and put 4 guys on the transom or swim step as far back as i could safely then lift the tabs all the way up and go for a sea trial and see if it makes any difference.

Would be great to get a video from another boat.

Bob H. · Post by **Bob H.** » Sep 28th, '12, 17:45

Another idea is the prop rake can change the running angle of the boat, Harry told me about a 65 Resmondo that ran bow down after a major repower, they tried everything and in the end a prop shop added positive rake which pushed the nose up enough to gain the lift needed. I dont understand how the postion of the rudders could make her run nose low, if they were offset evenly would it do the same? Ive seen many custom rigs set up just offset so shafts could be changed without dropping rudders. Maybe Capt Pat and Sim could set me straight on this one. BH

kiffa75 · Post by **kiffa75** » Sep 29th, '12, 08:21

Bob, I had this boat to a bare hull, and there was no ballast anywhere.
We are running now 19X24, we used to have 19X 23 cupped and with the Chryslers 17X18 ??????? so I doubt the props, what do you think?
Keith I will take videos from the cockpit and also from another boat once I’m back in.
As per Pat's advice, she is not nose diving, her running angle decreases and the steering gets a bit mushy. The wake starts with the nice flat 31 wake, then its gets distorted.
More and More I think it’s the rudder to shaft placement. I will decide if I’m going to try JD method and plug the whole or just line them up once and for all.
Either way I will be taking videos and hopefully I will solve this problem!

Bob H. · Post by **Bob H.** » Sep 30th, '12, 20:15

Chris, Cup and rake are two different prop variables, cup is for better bite, positive rake leans the blades back towards the transom which will raise the bow, stock props may have very little or no rake at all, find a prop shop that understands the rake dynamic. Id mess with that before I cut out and moved the rudder post. Bob Lico is the prop expert here so I suggest pm him to get his input. You have narrowed the variables to rudders and props in my mind, rudder drop as JP suggested will eliminate one more. BH

Carl · Post by **Carl** » Oct 1st, '12, 08:10

In my experience, it's always been pretty standard to have rudders offset a tad to ease pulling shafts without dropping rudder.
But both rudders are always offset the same amount in opposite directions keeping effect (if any) equal.

Rudders are not toed "in or out" more then a 1/2" or so at the trailing edge....right?
They should just be "a bit off straight" back so they have some tension and don't vibrate themselves to death.

Front to back, rudders are 90deg to hull?

I am not a prop guy...but I deal with several good ones. The again it's 90% recreational inboards...so mostly straight forward new or recon work. So no advice here...just ramblings and wonderings.

Positive Rake on wheels has been explained to me as condensing the prop wash...tightens it up, reducing amount of downward wash which adds lift to the stern.

Many prop guys don't touch Rake on inboards...and as you said our inboard wheels normally come with little if any rake as it is.... so our worse case is rake was added which should be of benefit bringing that nose up.

Downside...if adding rake is the fix( bring the nose up)...it's sound's like it would be more of a patch to mask a problem.

Bob H. · Post by **Bob H.** » Oct 2nd, '12, 17:20

The prop is the final piece in your drivetrain, taking all that torque and hp and converting it to thrust, the rake condenses the thrust which gives more bite from the entire blade area sending it downward( 15 degree shaft angle ) and lifts the bow NOT the stern. I respectfully disagree with Sim on the prop as a patch for the problem. Check out running pics of Bob Licos Phoenix, the wake leaves the boat at the engines, not the bow area, that was done by proper weight distribution and prop tuning until you get the desired running attack. BH

bbtiller · Post by **bbtiller** » Oct 3rd, '12, 20:59

Ok, I've read all these posts and have a question. I'm removing my fuel tank (222gals? she's a '76 FBC) in order to gain access to replace the deck supports and bulkhead just forward of the tank; since it's going to be out anyway, should i tab it back in a little more aft of the original position? I have not checked but I think it could slide back 6 to 10 inches or so. I don't have tabs on the boat now but plan to at some point. Thanks.

bob lico · Post by **bob lico** » Oct 3rd, '12, 22:15

first of all thank you bob for the "atta boy" in reality i don`t know how to express myself on this subject unless i am blunt ,tell it like it is not making friends but sincerely trying to help. that boat is running terrible! the water should be breaking under the outriggers on your particular boat. look you don`t understand i am not here to brag i have complete use of oakdale yacht facilities.i borrow props,steal props, confiscate props whatever and experiment plus i can go in the back of the prop shop and do my own thing ok, enough said.look at your boat and imagine a 5 foot wave directly in front of you now look at mine running attitude hitting the same wave.i have decrease my wot. speed two knots but increase my absolute economy speed to 27 knots at two miles per gallon! new wot is 33knots. normal cruise is 30knots all day long with somebody else paying for fuel.forgetting the splash look at this photo with old windshield now boat rides even better after changes. look at the angle of attack about 30knts.

bob lico · Post by **bob lico** » Oct 3rd, '12, 22:20

here is the operation;

bob lico · Post by **bob lico** » Oct 3rd, '12, 22:31

you have a enomous amount of drag between front of hull and water certainly decreasing fuel economy . the boat was originally designed for big block gas engines so the 250hp cummins with lightweight zf gears is only about 150 to 200 lbs heavier-------perfect because the weight is aft of the imaginary center of balance point in bahia mar which would be the forward most point of engine hatch.do you have access to a travel lift in your mariner?

Carl · Post by **Carl** » Oct 4th, '12, 08:16

Bob H. wrote: the rake condenses the thrust which gives more bite from the entire blade area sending it downward( 15 degree shaft angle ) and lifts the bow NOT the stern.

I respectfully disagree with Sim on the prop as a patch for the problem.

Check out running pics of Bob Licos Phoenix, the wake leaves the boat at the engines, not the bow area, that was done by proper weight distribution and prop tuning until you get the desired running attack. BH

Bob H and Bob Lico- No need to be respectful...at least not to me...tell it as you know it to be...and let us learn. Props are not my area of expertise...I'll give "Props' (pun intended) to Bob Lico for forgetting more about Props then I'll ever know.

the rake condenses the thrust which gives more bite from the entire blade area sending it downward( 15 degree shaft angle ) and lifts the bow NOT the stern.

I agree with this statement, I mis-spoke.

But as to adding rake being a patch...my thinking is our boats don't nose dive at at just beyond 18 knots with standard wheels. Unless prop shops removed rake from wheels or....that nose should be riding higher at 18knots...shouldn't have mushy steering...something is bringing nose down...or something is lifting the stern. Could it be the boat has hook in hull, wedges laid into the stern, trim tabs engaging. Could be the boat is trimmed wrong...water laiden bulkheads and cabinets up front...big water tank up forward with a holding tank..too much chain in rope locker, deck replaced with lightweight cored material, fiberglass tank removed with small light alum tank, motors too far forward...as I believe these questions have been asked and answered.... maybe the rudders non- stock placement has something to do with the problem.
Pulling Rudder out and running is not a bad idea. Worse or better at least you have some direction with little effort.

A Patch.
My thought is you go to stock first...a known performance baseline for the boat with that power and running gear.

Kinda like your car starts to run like crap and lost power. You don't add a Nitrous System to give it some umph back...you first give a full comprehensive tune up. Then when running good...you tinker away from that baseline.

bob lico · Post by **bob lico** » Oct 4th, '12, 08:27

Exactly Carl and that is why I want to put that Bahia mar in a work slip and take ONE strap from the travel lift and place it under the boat in line with the front of the engine hatch and lift slowly till you see the hull dip one way or another like children on a see-saw. That will tell me if I have a balance problem or something else. Like oh " I have a 100 lbs. anchor in the anchor locker" serious it could be a real fast remedy.

CaptPatrick · Post by **CaptPatrick** » Oct 4th, '12, 09:17

bbtiller wrote:should i tab it back in a little more aft of the original position? I have not checked but I think it could slide back 6 to 10 inches or so..

Brad,

Don't slide the tank any further aft... It won't change the loading enough to even notice a difference, but will severely limit your access to the aft center bilge where you need room for an aft bilge pump, possibly wash down/livewell pump, bilge alarm switch, tie bar connection, etc.

Br,

Patrick

kiffa75 · Post by **kiffa75** » Oct 4th, '12, 14:27

Hi Guys, thanks for the fed back.
To answer you Sim, I can see no hook in the hull at all or any wedges.
This problem is only with no tab, slight tab corrects it. They don’t extend below the hull when retracted.
Bulkheads have zero water. They were encapsulated in fiberglass so heavy but not water laden.
The cabin is the basic Bahia mar, no extra shelves or cabinets. In fact everything in the bow is made of Nidacore so very light.
$0 gal of water in plastic tanks, under helm seats. IV run with them full and empty with no difference.
No holding tanks.
150' of rope and 10ft of stainless chains. One fortress anchor.
Aft deck is solid and heavy deck from Alex at 31 Bertram Specialists. from the engine boxes forward is ultra light Nidacore.
Fuel tank is original Bertram. In original Bertram location.
Bob I think the pictures I put up may be a bit misleading because of her running into the waves. The water does break closer to the outriggers and that is even with the slight tab applied. Tab is applied in all these pics.
With a clean bottom, these little 250 will push her to 31kts and she can cruise at 26 very easily.
I attached another pic showing her running and also her sister 31' running angle, this boat has no steering issues.

Bob H. · Post by **Bob H.** » Oct 4th, '12, 18:11

Chris, Eliminate the potential rudder problem, if you can measure the other boats rudder position and compare to yours could be an instant answer. Check for any damage, after working on my bahia mar I found a poorly repaired strut and a rudder shelf that was rotted to crap, got to hold those rudders still, does she wobble or snake back n forth before it starts to act squirelly?
Sim, Im far from a prop expert as well, I do SEE how other boats run and ask alot of questions...A boat is like a see saw as bob lico said, must balance fore n aft as well as side to side..Ive tried to keep the weight as low as possible in my 31, Did slide engines and tank back a touch..
That second set of pictures shows a better running angle...straight edge the bottom and check all strakes and running surfaces to makes sure thay are flat and true..
With the power of the sandbox Im sure you will get to the bottom of this issue..BH

bob lico · Post by **bob lico** » Oct 4th, '12, 21:02

Bob I was thinking more like your high school days with physics classes and the law of levers .so the guy with a 16 lbs anchor on the bow pulpit and the effect it has 31 feet back to the transom .the side to side is easy.in water the tell tale boot stripe tells all. I have generator on port so I put porcelien head and holding tank opposite still not enough so will reinvent the wheel for a 31 Bertram with a split a/c unit. Compressor in head vanity at extreme aft. Lines up pillar and air handler in salon ceiling. Yea man like the real thing ------ cold air drops and warm air rises -------- I think Obama has not change that yet!,,,

bob lico · Post by **bob lico** » Oct 6th, '12, 08:35

Kira the wot.speed as well as cruise tells me everything is working ok. So now we are back to steering component problem .follow capt. Patrick advice but look for another 31 Bertram on the hard and compare rudder/ struct/ shaft measurements. Especially equal dimension from top of shaft taken just after cutlass bearing and front of prop. Hub to bottom of boat. Boat may have been hard grounded in past with rookies doing the replace/ repair struct job.i would love to see photo at 30 knt. With no tabs apply.------------ bob

Kiffa · Post by **Kiffa** » Nov 13th, '12, 11:12

Hi everyone, I have some updates.
I finally got back into the water. Whilst I was ashore we decided to bite the bullet and move the rudder to the location of the other one and placed it right behind the prop. We also gave the 1/2" toe out and made sure everything was nice and tight.
Hit the water and the boat is 80% better. She is steering better than ever before and I can run longer without needing to apply a little negative tab.
But 80% is not 100%! All my measuring between my boat and the other 3 31' were done on the port to starboard alignment. After double checking strut placement and ruder placements I now see the difference between my boat and the others.
My rudders are placed further to the stern than the others. So far that the edges are protruding beyond the transom.
On the 2 sport fisherman from the 70s era have the center of their rudder shaft 13" from the stern. One express from the late 60s has it 8" from the stern. Mine are 5".
The express used to have a similar problem to mine but not as bad. It was solved when they placed the trim tab right over the rudder, I guess acting like some sort of cavitation plate?
This first video shows the wake start to distort at the 1:12sec mark. At this point the steering does not respond as quick and the bow lowers. The boat was running at 25kts.

http://www.youtube.com/watch?v=Cljo54jI ... e=youtu.be" onclick="window.open(this.href);return false;

Would I be correct in thinking that because my rudders are to far aft causing them to cavitate?

Rawleigh · Post by **Rawleigh** » Nov 13th, '12, 14:19

Just to make sure that we have all of the info, which way are your props rotating? I know it is basic, but I want to eliminate all possible variables.

Kiffa · Post by **Kiffa** » Nov 13th, '12, 14:37

Rawleigh they turn outward.

Carl · Post by **Carl** » Nov 13th, '12, 16:14

Only time my wake looks similar to that is when I'm low on fuel and forget to fully retract tabs.

glad your at 80%

Bruce · Post by **Bruce** » Nov 13th, '12, 18:20

The video shows a clear problem with shaft, rudder geometry.
You have two distinct prop washes that never meet, kick up and then splay out.

neil · Post by **neil** » Nov 13th, '12, 18:32

We have a 67 Bahiamar and it has the same problem. It steers great until we catch a wave, or turn and the bow kicks down. Here is a picture of the wake with the bow down.

The boat has 270 cummins.

Rocket · Post by **Rocket** » Nov 14th, '12, 01:35

Kiffa, you say that your rudder posts are only 5" to the transom whereas the SF models are 13" and another express is 8". Is the relative distance from the propeller to the rudder the same in all these cases or is there a greater distance between the props and your rudders?

Carl · Post by **Carl** » Nov 14th, '12, 09:40

Centerline of motor and tail end of shaft same distance from keel on both sides?

JP Dalik · Post by **JP Dalik** » Nov 14th, '12, 14:19

I've watched this thread for awhile, I've been on Neil's boat when she goes into cant steer mode and that's with Capt Pats rudders. The one consistent is the larger distance from rudder to prop. I think in Neil's case he's trying to steer in froth. I can't imagine it's a weight thing,,, when I look at Chimera I can't think of a more bow heavy b31. Generator water tank refrigeration tv microwave air onditioner cabinetry and 2400ft of 1/2 with 20ft of chain in the bow all in front of the engines.

The boat steers dead balls anywhere you turn it.

Distance from Front of Rudder to back of shaft is just enough to get the wheel off

I believe the fix has to be to move the rudders forward, it's the only different thing. Neil's old 31 benefited by dropping the dolphin rudders down which put them directly in the prop thrust Nd it steered well.

If there is room to lower the rudders I would try this first not sure how your shafts are laid out

Carl · Post by **Carl** » Nov 15th, '12, 09:07

Carl at Elite installs transom mounted rudders, does he relocate strut and props right back to rudders too?

Could motors be too far forward
Different shaft lengths
Different angles

JP Dalik · Post by **JP Dalik** » Nov 16th, '12, 18:28

More then one boat affected. Perhaps more than 2.