Prop rotation vs helm position

[dsaville]

I have a old 23ft cabin cruiser with a Volvo Penta 270 outdrive. Prop rotation on this drive can be switched from left hand to right hand rotation. The current prop has a left hand rotation and the helm is on the left (port) side. When the boat accelerates it often takes a sudden hard turn to the right which can be corrected with trim tabs, but initially that sudden turn to the right can be rather scary.

I have been researching prop torque and prop rotation and have read it different ways on different web sites. One forum says the helmsman's position/weight is used to counter balance prop torque and if the helm is on the left(port)side you should use a right hand prop. However another forum says exactly the opposite, that if the helm is on the left(port) side you should use a left hand prop. So which is it? Before I buy another prop, which prop rotation should I be using?

[TollyWally]

I have a little larger probably heavier cabin cruiser with a left hand wheel and a starboard helm. I don't have the issues you do under acceleration, but then again my boat accelerates fairly slowly. However in slow speed manuevering the starboard is the favored side with a straight shaft and a left hand wheel. Kicking it in reverse will suck the boat to starboard when docking. I prefer to have the helm on my "strong" side when engaging in dockside ballet or picking up crab gear. Prop shops will sometimes let you make arrangements to borrow different wheels to test pitch etc. I don't know how involved it is to change the gear rotation but maybe you could try one and see. Or buy a cheap used prop and experiment. Good luck.

[mark775]

I was goig to write what TollyWally wrote... Manuverability trumps acceleration isues for most of us, then again, anybody that typically has steering control issues while accelerating probably just gets close enough for a boathook anyway, waterski pullers the notable acception. I would quit boating if I had to use a left hand prop on a port steered boat or vice-versa. Sure way to have docking accidents and look like a dork doing it.

[gonzo]

Are you talking about the boat heeling over or changing course? A left hand propeller will lift the port side which is where you have more weight.

[baeckmo]

Sorry guys, you are barking up the wrong tree! First: the Volvo drives have the "rudder shaft" at 102 degrees angle to drive- and propeller shafts, not 90 degrees. This results in a vertical torque; the engine torque trying to rotate the drive to port, while the reaction from the LH propeller is rotating the unit to SB. As the propeller torque is greater than the engine torque by the gear factor, the final outcome is a tendency to steer to SB with a LH propeller.

With a RH propeller, both engine torque and prop shaft reaction pull to port, resulting in a strong unbalance, forcing the drive unit into a port turn. So, in single installation, the preferred propeller rotation is LH in order to minimize the torque effects. In your case I would check that the trim tab aft of the propeller is intact, and adjusted to push the drive slightly to port in order to counteract the inherent tendency to rotate to SB! In addition to that, you should have your entire steering mechanism (including the yoke bushings in the drive) carefully controlled for unacceptable play.

Second: Regarding the use of helmsmans positionfor balancing the propeller torque; forget it, it's nonsense in a normal size cabin cruiser. If you have an engine with, say 250 hp @ 4500 rpm, it will have a maximum torque of some 490 Nm @ 3000 rpm or so. The gear ratio would be ~1.6:1, giving a propeller shaft torque of 784 Nm. That corresponds to the heeling moment of one person of 80 kg standing one meter off center.

[gonzo]

Outdrives don't have a set shaft angle 102 or otherwise. They move side to side and up and down.

[baeckmo]

You missed the point, Gonzo. The important issue is the "rudder shaft" angle, not the vertical secondary shaft. With anything but 2 x 90 degrees, and any gear ratio but 1:1, there will be a net torque in the "rudder shaft" direction, see pic attached.

If you feel an itch to continue arguing without arguments, first read Volvo Penta publication no 7731045-6

[dsaville]

Thanks for the input. In reply to some of the posts, this is a Volvo Penta 270 outdrive, the engine is a V-8. The trim tab aft of the prop is centered at the moment. I have adjusted that tab in the past with little or no effect to this paticular problem. The steering system from the wheel to the final connecting linkage is new and tight. I replaced the entire steering system to eliminate that as a cause. I do know that this engine and drive is not original to the boat and has had this issue since I bought it.

What happens is this: I accelerate the boat slowly (it is a heavy boat) and as it reaches a point around 15-20kts the bow takes a hard turn to starboard. The wheel does not move, there is no loose play in the steering. The boat just does a hard right as if it was suddenly steered in that direction, and my reaction is to immediately cut power and/or steer left to compensate. This is not a random event, it happens all the time, and I use the trim tabs to compensate and stop that behavior. However it is obvious that something isn't right.

I've eliminated loose steering, loose outdrive components, weight distribution. Now I am down to the prop and think it may be the wrong prop and rotation for the design of the boat. Changing the prop rotation on this particular outdrive is a a simple linkage change. Takes a few minutes. However I wanted some input from those more experienced than I before I invested the money in a new prop. I'll take all the info I can get!

[apex1]

You should trust on Baeckmo in this case, he keeps his mouth shut when he is not sure about the issue!

Richard

[baeckmo]

Still, in order to have minimum steering forces, you should use a LH propeller. Since you already do so, and have checked all possible sources for play (grabbed the aft end of the drive and pushed hard sideways??), the misbehaviour has other roots.

Has the boat any kind of keel strake, continuing too far aft? Is the hull symmetrical, or has it seen any sloppy bottom repairs? Is the drive leg reaching deep enough? If the upper part of the propeller disc is operating in the wake of a keel, it may produce a side-paddling effect; the bottom half of a LH prop paddling to port. The same occurs if the leg is too high on the transom. You said "heavy boat"....then the bottom of the anti-ventilation plate should be ~½ - 1 inch below the bottom line.

When the turn is initiated, does the boat heel, and if so, which way? What happens to the trim angle during acceleration and when it is diverting from the steered course?

[dsaville]

I'll do some research and let you know what I find. Thanks for helping me with this.

[tuantom]

Baekmo,
This is interesting.
Just to clarify this for me a bit - Do you want the engine and the propeller to have the same or opposite rotation?

[apex1]

Tom he describes that very clear in his post above! #5

[gonzo]

The problem looks more of a hydrodinamic one. I think so because it doesn´t happen when you accelerate,which would be the effect of torque. There is no such thing as "vertical torque", it is the turning force. Maybe we are loosing something in the translation. Volvo manuals used to be really bad in English. I suspect the bottom is asymmetrical. Lay a straigh edge on both sides and compare if possible.

[apex1]

Are you a bit pis.ed at present Gonzo?
Of course there is vertical torque, and Baeckmo told us precisely why!