Why do props spin left or right...???? And answers...

In this thread, hoping for response from those that know..

here's what I know..or think I know..

I'm gonna talk about boats with car motors first..and what they do...

In a car the motor torques to the right, put more power and it will lift the left front wheel,.. now put that motor in the boat and it will cause the boat to list to the right.....right??...to combat that reaction...boat builders will use a right hand prop...the reason for this is to try to cancel out the motor torque with prop torque and make the boat run flat... the right hand prop will try to torque to the left and the motor torque to the right... that is why it's so important to have the right prop, more so with higher speed and HP...if the props is getting too much bite and over torqueing it will cause too much lift on the right side of the boat, all kinda weird stuff happens.. most deadly is bow steer.. and followed by barrel roll... so pretty much....you need to have a rigger that knows what to do to get a starting point.. then tune from there...most boats are right hand drive....but some are left... in some marine apps, they change the rotation of the motor... so it can get very confusing ..... that and some boats... like v drives, or velvet drives there are even more combo's... outboards have no motor torgue, only prop torque but..to keep it simple and easier to understand.. lets just talk about bravo type outdrives and normal merc power...whit all the norm... we come to the general fact... inboard boats with car motors,, most are right hand drive,, and spin right hand props

I'd say in the simplest terms, you're on the right track.

You can jump into the world of circle boat racing where you're setup to turn left and then run off the balancer, versus the flywheel on v-drives to keep the torque headed the right direction.

Now lets jump to twins. Obviously if you're going to spin one right, you have to spin the other one left, right? That way you're offsetting. How do they decide which one goes where? Spinning in versus spinning out? That all has to do with stern versus bow lift and prop style. Handling will change dramatically if you get that wrong in some cases if you're spinning the wrong way. Had a buddy deal with it on his cat. Would hop violently when he was spinning the props out. I suggested he spin them in and it solved the problem all the way through the 140 mph mark. The manufacturer was the one saying not to spin them in.

I am not aware of any newer gas engines that run backwards as you can change direction and torque twist via gearing. That being said, my big boat had a pair of 8V-92 Detroit 2 strokes in it. One ran forward, one ran backward, all to control prop rotation. I don't know of any that use that method anymore as there are better ways to achieve what you want to do and keep it simple.

I was amazed to learn this past weekend that even the SE boats are running off of the crank snout to let the engine torque help them turn... I wasn't expecting to see that set up in the "entry level" SE class...

While I am on the topic of SE boats, I want to give HUGE credit to all of the folks that hung in there and made this all happen... I heard several people say "i can see myself owning/building/racing in this class" INCLUDING the little voices in my head...

Seeing a late 60's/ 1970ish LAVEY trunk models spank the "B" boats that were running made me grin like this >>>> :wink2:

Back to prop rotation/torque... :) It is a BIG factor in circle boats...

I was talking with Phil Bergeron a few years back about influencing some kind of a class with a crate engine claimer set of rules for roundy rounds. I remember 10 years ago when there would be 3 heats of SS boats at Firebird for the finals, and then times go tough and the field shrank. Rankin had 2 boats dialed that he was running, which meant that the best anybody could hope for would be 3rd. Who wants to race for that?

Seemed like a good idea, but it would take some horsepower behind it to get going. Racing on a budget would be fun though. Sealed crate engine, inspected every race, with a claiming option at the end of the year? Keep the field level and let the best driver win, not the most expensive platform.

Yes, they'd be driving the props off the balancer for rotational purposes. :happy:

So now you have the GPS class (95mph) and the SE class (85mph)... The great Paul Grichar was one of the driving forces in developing the rules/specs for the SE class...

FC Pilot and WANNABE can tell us more about the nuts and bolts of the rules, basically its a highly restricted 350 chubby with restrictions on heads/cam AND runs some kind of restrictor plate/spacer... I heard a rumor that someone did / is running a SBF but I didn't see any out there this past weekend...

It was some REALLY great racing, and Grich's old 1970ish Lavey "trunk model" did take the overall win on Sunday...

ALSO, if you watch some of the flat bottom boats cruising at 30-40 mph you can see them "leaning into the turn" while going straight, its REALLY obvious... Once at speed they appear FLAT on the water...

I have studied the plates enough to believe/convince myself that it is the engine / prop torque that causes this lean in a "flat" running off of the crank snout (reverse)... The EL CID K999 is a really good example of this lean, once at speed the boat is flat on the water... I used to see it ALOT on the K69 but it's not so obvious anymore...??????

thanks for all the response here...love to read this kinda stuff