4 blade vs 3 blade torque

[Stinnett21]

Considering a 4 blade for next season on my 21’ I/O and thought I would pose a question. There is one thing that bothers me about single engine right hand prop boats and that’s the fact that prop torque creates a port side list while underway. This is why most boats are controlled from the right side so the weight of the driver counteracts this condition. It has also been my experience in many years of bow rider boating that whenever guests come aboard they tend to load to the left adding to the problem. Of course on warm days the windshield lays open to the left. Also when you consider the “keep right” rule most man made waves approach from the left and if the boat is leaning to the left it puts the flat part of the hull against the waves creating a rougher ride. So my question is would a 4 blade create more torque (twist) than a comparable 3 blade thereby adding negatively to the condition?

 

[Chris1956]

IMO, your premise is incorrect. Right hand props will pull right when trimmed down and less to the right, when trimmed up. Usually you can find a neutral spot.

Also, most I/Os have power steering, which negates prop torque for the most part.

In addition, list is not caused by props. Tracking errors are caused by single prop drives. That is why torpedoes have counter rotating props. In your case, with a single prop drive, the dummy behind the wheel is responsible for fixing that.

JMO...

 

[Scott06]

In a word no, main difference is in holeshot and top speed. If you are looking for better loaded down holeshot and towing performance you may prefer a 4 blade

 

[ahicks]

I get what you're saying, the reason the boat's driver is placed on one side or the other.

To answer your question, it's nothing I'd worry about. I doubt seriously it's anything most people would ever notice.

 

[jimmbo]

Number of blades is not the only thing involved. Blade Shape and Area are other Factors, as are Rake and Cup, Angle of Attack in regards to Direction of Travel, how much of the Hull is out of the Water.

 

[Stinnett21]

Thanks for the replies. Chris, you are correct that tracking is part of it. We correct with rudder in a subtle way that goes unnoticed by most boaters but I disagree that props don't cause list. List and tracking are related. A right hand prop will "twist" the boat down on port up on starboard. This is the same reason helicopters have tail rotors. If not for the tail rotor they would spin in circles. I believe the center console crowd deals with this more than guys like me due to the weight being centered. This is why lots of center consoles come standard with trim tabs. Yes I'm probably overkilling this but what the heck it's cold out and I like thinking about boating situations haa.

 

[Chris1956]

OK, In theory, props do cause list, however, unlike a helicopter rotor, the effect of the prop on most boats is nil. Helicopters operate in air, which is much less dense than water. The water does not allow list nearly as easily as air. IMO.

I suppose, if you had a real light high performance hull, with a very large motor, I would expect some small list due to the prop, but on the vast majority of boats, the effect is nil.

 

[Texasmark]

On this argument........then why have counter rotating props on dual installations? Why, with most props these days being right hand turning, which tends to put the Starboard side of the boat deeper in the water, did they (somewhere back there....30's, 40's) decide to move the helm from Port (where it countered the RH prop rotation) to Starboard?

 

[QBhoy]

Trimmed anything neutrally to downward, a big diameter prop will almost always pull to the right. It’s just perhaps that you counter act this naturally by steering to the left. Wind direction has a fair bit to do with it too.
trimmed further out or up beyond neutral should sort it out.
4 blade props of similar pitch to a 3 blade are almost always larger diameter. If it helps answer your thought

worth saying that the above is usually only an emphasised and a real issue if you don’t have power or anti feedback steering.

 

[Starcraft5834]

4 blade will move more water= more push.. simple concept.. your top end speed will come down a mph or 2. Heavy V hulls will perform better with a 4 blade.

 

[JimS123]

I don't understand some of the opposing comments here.

A RH prop will make the boat list to port. It will also make it hard steer to stbd.

Sure, with power steering the operator won't "feel" the hard steering but the list will always be there.

The switch from port to stbd steering was done to make the boat run better. That's not the issue. Why it was on the port side in the first place is the issue. I suspect it was because most auto steering wheels are on the left.

I have no experience with a 4 blade prop, but then again I don't need low end performance so there never was a need. Why not switch to an Enertia prop? I did and wholly crap does this thing perform!!

 

[ahicks]

On this argument........then why have counter rotating props on dual installations? Why, with most props these days being right hand turning, which tends to put the Starboard side of the boat deeper in the water, did they (somewhere back there....30's, 40's) decide to move the helm from Port (where it countered the RH prop rotation) to Starboard?

On this piece, I have to wonder if the position is affected by the prop rotation? I agree most of today's stuff use RH props, but when it gets right down to specifics, transmissions and V-drives, along with the use of standard rotation automotive engines, may have something to do with it. I can share that my V-drive equipped Sanger for instance, was equipped with a left hand seat and controls.

 

[Stinnett21]

The list is somewhat subtle. My point in the original post was that it's aggravated by human habits (guests wanting to sit on the left) and the nature I suppose of a bowlder such as center sections of windshields laying open to port. I have a Bryant 214 and that center section has some weight to it. I realize moving weight to the right is the most effective cheap fix. QBhoy hit the nail on the head with my question. The 4 will likely have less pitch more diameter and may result in more list. Not that this would drive my decision. I may experiment with a cheap aluminum and return it to Amazon if possible. I think I read a post on THT where a guy bought a 4 and returned it due to increased list. Maybe he didn't know what he was doing but kinda scared me. Yes I've heard great reports on the Enertia but not in a position to drop $700 on a prop at present. Maybe Santa could bring a belated gift this June when the debris clears and I can work with a prop shop on exchanges. Hmm something to consider.

 

[Chris1956]

Gee, has anyone actually had a noticeably or significant list to port due to RH prop torque? I never had one or saw one in 60 years boating in low and high performance hulls. I think this thread is much ado about nothing.

Also, there are many theories about why most boats have RH steering. A lot have to do with "Steerboard" which is thought to reference the steering system of early boats. It is thought that we call that Starboard.

 

[QBhoy]

4 blade will move more water= more push.. simple concept.. your top end speed will come down a mph or 2. Heavy V hulls will perform better with a 4 blade.

Jim on my outboard powered cuddy. It has a command thrust gearbox so takes the big merc props. Unfortunately it has no power steering so you feel every effect.
It has a 14x20” Enertia prop. If I let go of the steering wheel at anything trimmed down to neutral or anything below 25-30mph...the boat will violently throw itself to starboard for certain. It’s a RH prop.

 

[JimS123]

I also have a 60 CT lower unit. It came with a trim tab on the anode. With a little trial and error I was able to dial it in to eliminate any thrust at "normal" operating speeds and tilt angles. Of course, by adjusting one way or the other you could make it throw violently to either side.

 

[QBhoy]

I also have a 60 CT lower unit. It came with a trim tab on the anode. With a little trial and error I was able to dial it in to eliminate any thrust at "normal" operating speeds and tilt angles. Of course, by adjusting one way or the other you could make it throw violently to either side.

Excellent Jim. Interesting to hear that.
my issue is perhaps that the 115 CT I have is bigger again (I think) and taking the largest of the props, there isn’t room or clearance for a trim anode. I may be able to cut a slot in one I suppose.
I’ve heard that there is a cut down version for this reason, but I’ll be damned if I can find one here in the U.K. for sale. Off the shelf at least.
I need to do something. It’s a hell of a force to try and counter at slow planing speeds. It demands that I cruise at about 3600/3700 rpm at least before it is anything like neutral feeling. Sometimes that’s a little to fast in poor weather really. Other than that...should the apprentice young skippers want a shot at driving...there is no way they could hold the wheel from firing off to starboard. Not sure even the good lady could hold it easily.
perhaps anti feedback steering is the way to go. Not cheap though. Might be the answer. It’s a big lump of an engine to not have anything at all.

 

[JimS123]

Excellent Jim. Interesting to hear that.
my issue is perhaps that the 115 CT I have is bigger again (I think) and taking the largest of the props, there isn’t room or clearance for a trim anode. I may be able to cut a slot in one I suppose.
I’ve heard that there is a cut down version for this reason, but I’ll be damned if I can find one here in the U.K. for sale. Off the shelf at least.
I need to do something. It’s a hell of a force to try and counter at slow planing speeds. It demands that I cruise at about 3600/3700 rpm at least before it is anything like neutral feeling. Sometimes that’s a little to fast in poor weather really. Other than that...should the apprentice young skippers want a shot at driving...there is no way they could hold the wheel from firing off to starboard. Not sure even the good lady could hold it easily.
perhaps anti feedback steering is the way to go. Not cheap though. Might be the answer. It’s a big lump of an engine to not have anything at all.

Mine DOES have anti-feedback steering, and as received it would hold course without the wheel turning, but you could still feel the torque when turning. The adjustment eliminated the hard steer.

My 150 has a flat anode, so no adjustment possible. It also has hydraulic steering. You can still feel a difference port to stbd, but the force to steer is close to the same.

I have seen cut back tabs for bigger props. In fact I had one on my last I/O so I could run a custom prop. They weren't really "cut back", but rather 2 different sizes.

Mercury has a good customer service hotline. Maybe give them a call. All you need is the right diameter and thread size and you can fabricate any tab size you need.

 

[QBhoy]

Will do. Thanks again Jim.

 

[Texasmark]

Merc makes 3 types (I guess after Jim's comment about a flat one). One is full width, the second is about half width and effective for high rake prop clearance like you would have with a bass boat and the other is flat....per Jim.

I ran a TP 14" diameter 3 blade wheel on my last engine (2002 2 stroke 90ELPT) for a test with the half thickness anode and just cut a chunk out of it for the prop to clear.

i had NFB steering on that boat and liked it....it's my avatar. Current boat apparently doesn't and steering is very sensitive to trim position, being Port favoring above mid trim, pretty well compensated at mid trim (tab all the way to Port....which isn't but 10-15*) and Starboard favoring below. With my engine now down on the transom, more of the LU is in the water and as a result, the tab position is more pronounced. When up in the 4th hole I just ran it amidship as I wasn't in the water much.

Thinking about where I am now with finding the clogged fuel filter, I might jack that sucker back up to the 4th hole, put that Ballistic 22XL back on there and see if she will top 50. Slip on the TPS is running around 5%. GO-Fast PSC says that 50 is a piece of cake with it.....but once I prove the point (that the PO did in fact run the Ballistic as his every day prop), it would be back down on the transom and the 17P 4 blade for my soft ride in a good chop at a moderate speed. Yak yak, gab gab. Bed time. Bye!