Vertical motor trim tab on twins?

[onoahimahi]

I'm wondering what is the point of vertical trim tabs on twins. :hmm I'm talking about the little vertical trim tab underneath the anti-ventilation plate aft of the propeller designed to adjust for veering caused by sideways prop force. They are necessary on single engine installations but redundant on twins since the counter-rotating props cancel each others sideways forces.

If they are not required, I'm thinking they should be removed since they are just one more thing causing drag and effecting fuel economy. They could be replaced with some kind of cap or cut off and ground smooth. At the very least, they should be adjusted to the equivalent of 0 degrees to minimize drag and not try to produce any sideways forces.

I made the video below to see the position of my AV plate (for a lightly loaded case). Once the boat is planing you can see the extra little roostertail kicked up by the trim tab just port of the center of the AV plate so there is definitely an effect. In the grand scheme of things it is probably pretty minor but every little improvement helps...

I'd be interested in others thoughts or opinions on this.

Thanks,
-Scott

https://www.youtube.com/watch?v=9XBsCt_9bgc

 

[DennisG01]

I agree. With counter rotating engines, I don't see the need - or at least I don't understand why there would be a need. With stern drives (that have power steering) it's also not needed. This is with a single engine, too. Instead of a "fin style", they use a flat-style - same idea as what you're thinking of with cutting it down. You may be able to simply purchase a flat-style in the future. As far as extra drag, I think it's extremely negligible at best. I certainly wouldn't do any changes based on hoping for fuel savings. Probably used up all the savings in fuel with the "electricity time" it took to write the post! :mrgreen:

 

[lime4x4]

On my offshore with twin outboards they both have the fin. One engine is normal rotation the other engine is counting rotating. They were on when i bought the boat

 

[bayrat]

I agree. With counter rotating engines, I don't see the need - or at least I don't understand why there would be a need. With stern drives (that have power steering) it's also not needed. This is with a single engine, too. Instead of a "fin style", they use a flat-style - same idea as what you're thinking of with cutting it down. You may be able to simply purchase a flat-style in the future. As far as extra drag, I think it's extremely negligible at best. I certainly wouldn't do any changes based on hoping for fuel savings. Probably used up all the savings in fuel with the "electricity time" it took to write the post! :mrgreen:

this ^^^^^^^^^^^^^^

 

[onoahimahi]

Yeah - probably just wishful thinking on my part. I have read, however, that raising the engine height can result in significant savings and performance improvement for cases where the motor is mounted too low, and that dealers tend to mount them too low so as to not risk prop blowout. That's the other rope I am pushing at the moment.

 

[bayrat]

Yeah - probably just wishful thinking on my part. I have read, however, that raising the engine height can result in significant savings and performance improvement for cases where the motor is mounted too low, and that dealers tend to mount them too low so as to not risk prop blowout. That's the other rope I am pushing at the moment.

My new boat had the motor mounted too low and I had to raise it 2 holes to get the anti vent plate up to where it was not buried under water. I was also told that when they rig a new boat they err on the side of safety rather than performance (read as avoiding lawsuits) as far as engine height.

 

[g0tagrip]

I asked two respected engines guys the question. Here is what they both said. The vertical engine trim tabs main purpose is to protect the lower unit from electrolysis damage. Those trim tabs are zincs. In twin engine boats the tabs should be positioned one notch outboard. Port engine zinc tab one notch port, starboard engine zinc one notch starboard. Both my engines came set like that from the factory installation. According to them, most boats now have hydraulic steering and twins have counterrotating engines so you do not notice the resultant forces on the boat due to the rotation of the props. One said with a good hydraulic steering system you could do with out the fin, but you would still need the zinc for electrolysis protection.

 

[onoahimahi]

I asked two respected engines guys the question. Here is what they both said. The vertical engine trim tabs main purpose is to protect the lower unit from electrolysis damage. Those trim tabs are zincs. In twin engine boats the tabs should be positioned one notch outboard. Port engine zinc tab one notch port, starboard engine zinc one notch starboard. Both my engines came set like that from the factory installation. According to them, most boats now have hydraulic steering and twins have counterrotating engines so you do not notice the resultant forces on the boat due to the rotation of the props. One said with a good hydraulic steering system you could do with out the fin, but you would still need the zinc for electrolysis protection.

On my motors the trim tabs are not anodes. They are aluminum and painted white to match the motors. I found out that Evinrude makes a trim tab cover #336284 that can be used to replace the tabs. They are ported for water intake. There is a photo of one here:

http://www.ebay.com/itm/Johnson-Evinrud ... 0968647151

I'll put this on my list but it will stay near the bottom for a while.

 

[DennisG01]

Just for clarification, I was referring to the trim pieces as "anodes" as well, when I referenced stern drives (Mercruiser, specifically). But, the main point is still the same - I just didn't want to be misleading in any way.

Ono - are you sure the opening in your trim piece isn't for exhaust? I only ask because I've never seen an intake there - but I have seen exhaust outlets there. But, I am not familiar with your engine, either.