seacat leans left down right side up

jasn

Cadet
Joined
Nov 29, 2012
Messages
6
Hi, I am trying to figure this out maybe someone has experienced this.
I have 2 regular clockwise rotating outboards on a 26foot seacat . As both motors are trimmed evenly and running the same rpms the right side raises higher. Its more noticable at speeds greater than 3000rmps and with the bow up. I am guessing it is either a prop issue or a torque tab adjustment that could fix this. Does anyone know what position the torque tabs should be placed at with 2 props both rotating clockwise. right now the back ends of the tabs are slightly closer
 

Georgesalmon

Lieutenant Commander
Joined
Apr 14, 2012
Messages
1,793
Re: seacat leans left down right side up

The tabs are for counteracting the torque of the prop rotation. They should both be the same usually slightly to the port side. They will only effect the steering torque and with no feedback or hyd steering they are not needed at all. Larger, newer outboards don't even have them when shipped from the factory anymore. I doubt adjusting them will do anything to help your situation. Changing props won't help either. You need to look at weight distrubution but I doubt that will help much either. Think about installing trim tabs for the best solution.
 

jasn

Cadet
Joined
Nov 29, 2012
Messages
6
Re: seacat leans left down right side up

Sea Cat: Catamarans for Fishing, Family Fun, and Charter Boating Phone (321) 268-2628

They should be able to help you. I suspect it might have something to do with propeller torque, with both motors being right hand rotation.

Does the boat list to port at rest, or does this happen only while under power?

only under power, unless both trims are tilted almost all the way down. With them both down its not so noticable but it still leans. my props are less than perfect right now
 

Frank Acampora

Supreme Mariner
Joined
Jan 19, 2007
Messages
12,004
Re: seacat leans left down right side up

If you analyse the situation it becomes clear What is happening: A clockwise spinning propeller has a reaction with the water. It is separate and different than the torque reaction that wants to turn the engine to one side. There is a lifting moment because one blade is traveling down into "clean" water while another is traveling upward near the surface.Water coming off the blades can react more strongly with the deeper blade than with the shallower running blade. Thus, the propeller wants to lift the engine and tilt it to port.

Now, with twin engines the effect is magnified BUT on a mono hull, since they are close together, it can be mollified some.

ON A CAT, where the engines are far apart. the port engine it trying to lift the stern and and tilt towards port BUT-- it has the full weight of the other engine at the other side of the stern. The starboard engine however not only has no leverage working against it but has the port engine helping to lift it. Thus, you see a tilt to the whole boat--

When you trim the engine all the way in, the propellers are deeper in the water so this reaction is lessened.

If you don't already have them on the engines you can try cupped props which lessen the water spun off the blades and MAY lessen the reaction--no guarantees though.
 

jasn

Cadet
Joined
Nov 29, 2012
Messages
6
Re: seacat leans left down right side up

If you analyse the situation it becomes clear What is happening: A clockwise spinning propeller has a reaction with the water. It is separate and different than the torque reaction that wants to turn the engine to one side. There is a lifting moment because one blade is traveling down into "clean" water while another is traveling upward near the surface.Water coming off the blades can react more strongly with the deeper blade than with the shallower running blade. Thus, the propeller wants to lift the engine and tilt it to port.

Now, with twin engines the effect is magnified BUT on a mono hull, since they are close together, it can be mollified some.

ON A CAT, where the engines are far apart. the port engine it trying to lift the stern and and tilt towards port BUT-- it has the full weight of the other engine at the other side of the stern. The starboard engine however not only has no leverage working against it but has the port engine helping to lift it. Thus, you see a tilt to the whole boat--

When you trim the engine all the way in, the propellers are deeper in the water so this reaction is lessened.

If you don't already have them on the engines you can try cupped props which lessen the water spun off the blades and MAY lessen the reaction--no guarantees though.

Thanks alot, it makes sense. And i bet your right. I will try it. After all my props are beet up a bit.
Do you know if cupped props have an effect on rpms compared to the same pitch w/o?
 

Frank Acampora

Supreme Mariner
Joined
Jan 19, 2007
Messages
12,004
Re: seacat leans left down right side up

Yes, rule of thumb states that a cupped prop will lower RPM about 200, given the same design, diameter, and pitch.
 

jasn

Cadet
Joined
Nov 29, 2012
Messages
6
Re: seacat leans left down right side up

Yes, rule of thumb states that a cupped prop will lower RPM about 200, given the same design, diameter, and pitch.
I think read a similar formula about the pitch also effecting the rpms X200 per #inches of pitch or something like that... With this in mind, and trying to accuratley pick a set of props.. what would the difference in performance be between 15x19 that is uncupped or a 15x17 that is cupped.. right now I have 15x17damaged cupped props and e115 engines they have no problem pushing the boat and easily getting around 6000rpms... but not trying to blow anything up i back the throttle down..I am sure its not good running these damaged props.
 

jasn

Cadet
Joined
Nov 29, 2012
Messages
6
Re: seacat leans left down right side up

I think read a similar formula about the pitch also effecting the rpms X200 per #inches of pitch or something like that... With this in mind, and trying to accuratley pick a set of props.. what would the difference in performance be between 15x19 that is uncupped or a 15x17 that is cupped.. right now I have 15x17damaged cupped props and e115 engines they have no problem pushing the boat and easily getting around 6000rpms... but not trying to blow anything up i back the throttle down..I am sure its not good running these damaged props.


If i were to use 15x17 cupped would it be close to using 15x19 uncupped? Or does the water flow altogether differently from the cup shaped prop?
 

jasn

Cadet
Joined
Nov 29, 2012
Messages
6
Re: seacat leans left down right side up

ok i figured it out. on the beat up prop also had a slipping rubber hub so i got less torque from the one side but at lower rpms i still had some torque. so at high rpms and two uneven props and hubs it made it lean a little. now i see why i lost some speed and was able to rev to higher rpms. this seacat boat rides around forty mph great with new 15x17props
 
Top