About that little fin above the propeller....

[stevens]

Quietcat,<br /><br />In a different thread about trim, you just said the following:<br /><br /> "There is a vertical tab/fin at the end of your outdrive. Sticks down towards the prop and slightly behind it. It is mounted to a horizontal plate that "covers" the prop from above (anti-ventilation plate). This fin is there to help you turn the wheel and counteracts the propeller's desire to push the drive one way (without it your boat would steer very hard one way and too easily the other)."<br /><br />Now, when I switched to the Ballistic propeller on my Alpha drive, the new propeller would hit this little fin, and it was therefore removed and replaced with a flat anode instead.<br /><br />I haven't noticed the asymmetrical effect on the steering though, that you describe above. The boat seems to steer just like before, equally well to both sides and at all speeds.<br /><br />What do you think; would having hydraulic steering counteract or mask the effect you mentioned? Or is there another explanation for this?

 

[mattttt25]

Re: About that little fin above the propeller....

yes. the hydraulic steering is countering the effect.

 

[cuzner]

Re: About that little fin above the propeller....

If you have power steering you don't need the zinc tab (what mattttt said). The prop torque is still there, but you should'nt be able to feel it.<br /><br /> Jim

 

[umblecumbuz]

Re: About that little fin above the propeller....

Don't agree. Sorry fella's.<br /><br />That trim tab counteracts the torque that is exerted by the prop. For torque, read strain. If you can't feel the strain, it doesn't mean it's miraculously vanished. It's still there, straining the expensive steering parts.<br /><br />It was put there for a purpose, and having steering that masks the effect doesn't negate its original purpose.

 

[craze1cars]

Re: About that little fin above the propeller....

I do believe Mercruiser sells a shorter trim tab that is specifically designed to eliminate this problem with certain props. You might ask your dealer, or post this question on the Mercruiser thread where more people know these drives well.

 

[stevens]

Re: About that little fin above the propeller....

If you look at an Alpha-drive from the side, the area of that little fin accounts for how much of the total area below the anti-ventilation plate?...maybe 5%?<br /><br />

<br /> <br />Wouldn't that mean that this little fin would only increase the rudder area by, say, 5% ?<br />And, it would effectively only increase the ability to counteract the sideways propeller torquing power by the same amount?<br /><br />The dealer installed an original anode without the downward pointing fin on it, which is supposedly standard on drives with power steering. I guess it is not crucial then, to have this fin. Or am I missing something here?

 

[chuckz]

Re: About that little fin above the propeller....

You're missing something. The amount of force is related to the distance the fin is from the pivot point and the velocity at which the water if flowing over it. That little fin is as far away from the pivot point as possible in order to generate maximum forces.

 

[prockvoan]

Re: About that little fin above the propeller....

L.I. hit it on the head.I got one for ya'll,try this.<br />Take a bearing away from a landing at a idle speed.Hold the sterring wheel in place,don't move it.With in 1000 yards,your boat will be heading to the right big time compared to the bearing that you started out from.By removing that little tad,it will incress 4 fold.<br />All boats that are single engine will drift off to the right.That is a fact!Tryed it to many times to think other wise,proved it to others with a little money on the side!

 

[mattttt25]

Re: About that little fin above the propeller....

never said it wasn't needed. simply said his hydraulic steering is countering the effect he would feel if he didn't have hydraulic steering.

 

[Dhadley]

Re: About that little fin above the propeller....

Hydraulic steering does not counter the torque. It will mask it or hide it from the operator. Its still very much there.

 

[umblecumbuz]

Re: About that little fin above the propeller....

What Dhadley says...<br /><br />The "little fin" is often assymetrically shaped for this purpose, and it might also be set at an angle. As it's at the extremity of the lower unit, its influence is far greater than its size might indicate.

 

[ALAG3]

Re: About that little fin above the propeller....

I just had this problem when I changed my prop...went to West Marine and they had an entire section of these anodes, found one to replace original, but only set back a little further. It cost like $11.00. Simple fix after some frustration about new prop not fitting!!!

 

[QC]

Re: About that little fin above the propeller....

Sorry Stevens missed this yesterday . . .<br /><br />You guys are missing something though. On power steering models the fin is replaced with a flat and flush with the antiventialtion plate zinc. I have had a 5.7 TBI Alpha that was setup this way from the factory and my current 6.2 MPI Bravo is the same. No steering trim tab skeg. I have a lot of experience playing with that fin on an old OMC, however, I can't find any reference to it in the Merc manual that I have. IMHO it is to be adjusted to counteract torque that you feel in the wheel. It is not there to save the drive from stresses. Theoretically those stresses would be higher on a 5.7 than a 3.0, so I doubt very seriously if it is a protect the drive thing.<br /><br />So it is my positon that it is absolutely unnecessary if there are no steering situations that are unsafe or uncomfortable. This assumes that it is replaced with some sort of zinc anode.

 

[wvit100]

Re: About that little fin above the propeller....

My boat has the alpha 1 with a 5.7. It has the flat anode and it came that way from the factory.

 

[mattttt25]

Re: About that little fin above the propeller....

bingo. and my use of "countering" could be replaced with masking or hiding. i meant to imply the same thing. he's not feeling the torque because he has hydraulic steering. now if he had cable steering, i would bet he'd feel it.

 

[chuckz]

Re: About that little fin above the propeller....

The rotating prop creates a force trying to turn the boat to starboard. The "fin" creates a force towards port to CANCEL OUT the starboard force. As in the force is no longer there. Less force means less stress. If a motor comes with the "fin" or not is inmaterial to the laws of physics. If you can fit the outdrive with a "fin" and adjust it properly, there will be less stress induced in the outdrive.

 

[bluewater19]

Re: About that little fin above the propeller....

How can you ajust the "little fin" correctly if you can't feel the torque due to your no feedback steering?<br />I have a starcraft outboard that came with the fin. Both my glastron and bluewater did not come with the fin. It is my understanding that you ajust the fin until you don't feel anymore torque steer. How can you ajust it if you don't feel it in the first place?

 

[QC]

Re: About that little fin above the propeller....

Agree LI, but my point is that it doesn't matter for safe operation OR outdrive life.<br /><br />bluewater, <br /><br />With power steering you would not feel it, without you definitely would IMO. On my old boat that had a fin and did not have power steering, when I adjusted it, the RPM at which you might feel torque one way or the other would move, I found a spot where the combination was best for the boating that I did, never touched it after that. Could be a pretty strong pull at some RPM usually toughest when coming onto or off of plane.

 

[stevens]

Re: About that little fin above the propeller....

OK, now I am confused. Should I try to find some smaller fin compatible with my new propeller, should I go back to the old propeller and put the original-size fin back on, or should I leave the new propeller and flat anode on as they are, and not worry about it? I'm leaning towards the latter...

 

[qaztwo]

Re: About that little fin above the propeller....

Like alag3 said they make a set back fin for high rake props. Or just take a file a shave off where its hitting the fin.