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  Why is my boat doing this?

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Author Topic:   Why is my boat doing this?
Whalerdan posted 02-13-2002 10:35 AM ET (US)   Profile for Whalerdan   Send Email to Whalerdan  
I have recently put a '96 Evinrude Ocean Pro on my '85 Montauk. When I have the trim (actually tilt) set tight to keep the nose down it pulls hard to the right. I have put the trim tab straight and fully right but it does it in both positions. When I trim the motor up the pulling goes away. Should I try taking the trim tab off completely? As best as I can tell the motor looks like it's mounted square on the boat.
Bigshot posted 02-13-2002 11:02 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Every boat will do that. You are stuffing the bow and it wants to track one way or the other. The torque of the engine pulls it one way. The key is to trim it up so the steering gets light....unless you like getting bad gas mileage, wet, slower speed, and a heafty upper body workout:)
Whalerdan posted 02-13-2002 11:28 AM ET (US)     Profile for Whalerdan  Send Email to Whalerdan     
My old motor didn't do it. It might not have had as much tilt range as this new one though.
Bigshot posted 02-13-2002 12:54 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Bingo! I can stuff so far that it is almost impossible to steer.
lhg posted 02-13-2002 06:19 PM ET (US)     Profile for lhg    
As mentioned this is normal. However, the purpose of this anodic trim tab is to counter those forces. If the boat is regularly run trimmed in, the tab should be rotated to point to the same side that exhibits the steering pull. On almost all single engine boats, operated with engine in normal trim position, the tab should point toward port when viewing the boat from behind. This will help eliminate steering pull.
Whalerdan posted 02-14-2002 07:47 AM ET (US)     Profile for Whalerdan  Send Email to Whalerdan     
I'm going to try taking the tab off. There are stops that keep me from rotating to point the direction I need (to the right). Also, the tab on this new motor seems to have more of a cup to it than the old, and the cup "points" to the left making the pull worse.
lhg posted 02-14-2002 05:13 PM ET (US)     Profile for lhg    
You should not remove it. You need it for lower unit corrosion protection. Just set it straight back, then it will have no effect.
Tom W Clark posted 02-14-2002 05:59 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Whalerdan,

Do not remove the trim tab. It is there for a reason (though not for anti-corrosion purposes; OMC trim tabs are not sacrificial zinc anodes, those are mounted elsewhere on the motors). I do not think you are experiencing anything unusual. Let's go through it again:

First of all, let's dispense with talking about left and right since they are relative terms and I suspect are also the root cause of our confusion. Instead, let's use port and starboard.

If you, or anybody else, trims their motor all the way in (bow down) it is expected that the motor will pull to starboard, that is to say, if you let go of the wheel the boat will tend to steer to starboard.

If the motor is trimmed all the way out (maximum bow lift) it should pull to port.

When the motor is trimmed in the position it is most frequently used the motor and steering should be neutral. The trim tab is used to achieve this.

The trim tab has some curvature, cup as you say, to it. It is curved to starboard or in other words, the trailing edge of the tab lies to starboard of the center line. It is designed like this to counteract the always present P-factor or propeller torque. From your description of your trim tab you suggest it is not like this but I suspect what you are calling left is, in fact, starboard. Do I have this correct?

If your motor is pulling to starboard, the solution is to move the trailing edge of the trim tab to starboard, not port. If the motor is pulling to port then move the trailing edge of the tab to port. But do not remove it.

Whalerdan posted 02-15-2002 08:25 AM ET (US)     Profile for Whalerdan  Send Email to Whalerdan     
Tom,
Right! If I let go of the wheel, with the motor trimed (tilted) in, the boat will pull to starboard and the curvature of the fin is trailing edge is to starboard. I tried adjusting it so that the trailing edge was fully starboard but it didn't help. Its pulling so hard in either position it's hard to tell a difference. I guess it's something I'll just have to get use to and keep the motor trimed higher.

You're also right about that the tab isn't an anode. There's no way it could be, it's painted!

Tom W Clark posted 02-15-2002 12:36 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Whalerdan,

The question I have is: Why are running around with the motor trimmed all the way in? The only time I ever did this with either of my two Montauks was when it was very choppy or when pulling a skier up, in which case the motor was trimmed out again as soon as we were on a plane.

If you do run with it down, then go ahead and move the tab all the way to starboard. this will do what it can. Your other alternative is to switch to a no feed back type steering be it hydraulic or Teleflex NFB. But I suggest you just trim the motor out to where it's supposed to be.

lhg posted 02-15-2002 04:09 PM ET (US)     Profile for lhg    
If I remember correctly, when a single engine boat pulls to Starboard (ie steers hard to Port, easy to Starboard) when trimmed in, the trim tab should be pointed in the direction of the hard steer, in this case Port. I think you've done it backward, making it even harder to steer.
Tom W Clark posted 02-15-2002 04:25 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
I may have it wrong, but I don't think so. I have gotten it wrong before. It is hard to keep all this straight within a verbal description so perhaps lhg makes a good point:

If it doesn't work on one side, try it on the other. The problem with this discussion may simply be semantics. Nothing wrong with experimentation. It's how we learned what we know.

lhg,

Just for the fun of discussing it, I am recommending that Whalerdan move the trailing edge of the trim tab to starboard to counteract the pull to starboard (boat turns to starboard when he let's go of the wheel). You suggest "the trim tab should be pointed in the direction of the hard steer, in this case Port". Well, if the tab is "pointed" to port, doesn't that necessarily mean the trailing edge is to starboard?

Here's what is happening: With the motor trimmed (Whalerdan, why do you keep saying tilted?) all the way in, the propeller blades have a greater angle of attack on the down sweep (starboard side) than the up sweep (port side) thus there is greater thrust on the starboard side of the motor's center line. As a consequence of this the motor wants to swing to starboard, i.e. the gearcase wants to move to the starboard side of center. When this happens the boat turns to starboard.

By moving the trim tab's trailing edge to starboard, you create a rudder that pushes the gearcase back to port thus equalizing the torque steer.

hauptjm posted 02-15-2002 04:40 PM ET (US)     Profile for hauptjm    
I think this should simplify it:

If boat steers to port, trailing edge of trim tab should be moved to the starboard side.

If boat steers to starboard, trailing edge should be moved to port.

a "rudder" steers a vessel to the trailing edge. Hence, a rudder's trailing edge to port, steers to port. If you're looking to counteract a tendency to steer to port or starboard, you have to have the trailing edge in the opposite steerage.

Tom W Clark posted 02-15-2002 05:17 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
hauptjm,

Not quite. The trim tab steers the gearcase not the boat. Gearcase steers the boat.

You are correct, "If you're looking to counteract a tendency to steer to port or starboard, you have to have the trailing edge in the opposite steerage." BUT, it's the trailing edge of the gearcase that need to move to port in order to overcome a tendency of the boat to steer to starboard. This is accomplished by moving the trailing edge of the trim tab to starboard which pushes on the trailing edge of the gearcase to port.

kingfish posted 02-15-2002 05:45 PM ET (US)     Profile for kingfish  Send Email to kingfish     
I'm still working on getting a clear mental visualization of the various forces at work here, but for the record, here is a direct quote from the BW Owner's Manual for 18 foot -- 25 foot Models:

(under "Trim Tab Adjustment")

"...For example, if the boat pulls to starboard, move the trailing edge of the trim tab to starboard..."

kingfish

vdbgroup posted 02-20-2002 10:07 PM ET (US)     Profile for vdbgroup  Send Email to vdbgroup     
If after all this discussion you have not gotten rid of that nasty prop torque. Try this: Once you are at running speed, let go of the steering wheel, stand-up on your seat, raise your arms parallel to surface like wings on a plane, then counter act the torque by banking your arms, like an airplane turning, opposite the way the boat is steering. For added effect make a prop or jet noise. (Sorry, I could not resist!)
Whalerdan posted 02-21-2002 08:10 AM ET (US)     Profile for Whalerdan  Send Email to Whalerdan     
Good one! But if I did that I really would be flying, at least for a little while.
smirkless posted 09-02-2003 01:11 PM ET (US)     Profile for smirkless  Send Email to smirkless     
All this is great unless you have a no-feedback steering system. I lost my trim zinc a few weeks ago; bought and installed a new one at the angle i thought the previous one was at. Don't feel any pull, trimmed in or out but at high speeds the boat now skips back and forth on port & starboard sponsons in an unsafe way. Nothing else has changed that I know of.

Question is: How can I tell what angle to set the zinc with no-feedback steering?

Taylor posted 09-02-2003 03:10 PM ET (US)     Profile for Taylor  Send Email to Taylor     
smirkless - when I pulled my lower unit the instructions I was following suggested marking the position of the trim tab before removing it. (Turns out that the instructions where incorrect about the need to remove the trim tab, but that's another story). When I went to make a mark, I noticed some other scribed marks already there in the case near the trailing edge of the trim tab. You might just have a close look and see if you can find anything like that. That would get your trim tab in the neighborhood.
Sal DiMercurio posted 09-02-2003 07:56 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
This is right out of Evinrudes mouth.
A single trim tab adjustment will relieve steering effort under only one set of speed, motor angle, & load conditions.
No single adjustment can relieve steering effort under all speed, motor angle, & load conditions.
If the boat pulled to the starboard [ right ], move the "REAR" of the trim tab slightly to the "RIGHT", if the boat pulls to the left [ port ] move the "REAR" of the trim tab slightly to "port" [ left ].
Retighten the trim tab to 40 in lbs.
With power steering, set the tabs dead center.
Sal
Moe posted 09-02-2003 09:25 PM ET (US)     Profile for Moe  Send Email to Moe     
The higher the engine is mounted, i.e. the higher above the bottom of the hull the AV plate is, the less of the trim tab that's in the water, and the less effect the trim tab will have. There isn't much tab left 2/3-3/4 down from the plate, because it isn't nearly as wide there as at the top. At at that point, its effect might be negligble.
--
Moe
Sal DiMercurio posted 09-02-2003 10:32 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Moe, the trim tab is only about 1 1/2 nches long & 1/4 inch thick.
Sal
David Jenkins posted 09-02-2003 11:07 PM ET (US)     Profile for David Jenkins  Send Email to David Jenkins     
I believe that what Tom Clark is saying is that the trim tab's effect is the opposite of the effect that an oar would have if you stuck it in the water behind the boat.

If you angle an oar off the starboard side of the boat, it will pull the boat to starboard.

But the trim tab works on the gear case, not the boat. So if the trailing end of the trim tab is to the starboard side, the resulting force will try to swing the gear case to the port side of the boat, which will pull the boat to port.

So if you have the engine correctly trimmed and it is still pulling to starboard when you take your hands off of the wheel, you need to more the rear end of the trim tab more to starboard. That will have the result of pushing the gear case to port which will make the boat want to turn to port, which will neutralize the force which was trying to pull it to starboard.

Moe posted 09-02-2003 11:43 PM ET (US)     Profile for Moe  Send Email to Moe     
Sal, that's pretty small if you're talking 1-1/2" down from the AV plate. I'd think mounting the motor with the AV plate 1-1/2" above the bottom of the hull would have that tab completely out of the water.

Smirkless, FWIW, mine came with the aft end of the trim tab one notch toward starboard from the center.
--
Moe

John O posted 09-03-2003 08:43 AM ET (US)     Profile for John O    
Point the front of the trim tab slightly in the direction of the side the boat wants to turn. If boat tends to go right turn tab to the right.

Why is this so confuising?

John O posted 09-03-2003 08:47 AM ET (US)     Profile for John O    
Hell, try it both ways until it works. I would only adjust it 1/16 of an inch at a time.
Whalerdan posted 09-04-2003 05:36 AM ET (US)     Profile for Whalerdan  Send Email to Whalerdan     
A 1/16 of an inch! Man you'd have to be the worlds greatest boat driver to notice a 1/16 of an inch. At least on my boat you would.
John O posted 09-04-2003 04:21 PM ET (US)     Profile for John O    
1/4" to begin. Not 1/16 sorry. My point is to move it a small amount and test. Do not just move it 2" and go.

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