Excess Helm Pressure Under Power

[malyea]

How much helm pressure from prop wash do you feel on your boat when motoring at 2800 RPM? We’ve owned the Breeze for 6 years (‘87 Irwin 43 MkII). Usually when leisurely motoring/motor sailing we typically run 1500 to 2200 RPM - gives negligible to light helm pressure from prop wash. We now have reason to run at 2800 RPM couple times each month - and we’re quite surprised how much more helm pressure from prop wash is evident. I’d call it almost excessive - trying to make sure it’s “normal”. Yanmar 4JH4-TE right turning 3 blade fixed prop. Boat pulls right going forward with significant left helm pressure required to hold heading. New Foss Foam rudder 1 year old. Diver just inspected it and says it’s AOK….Thoughts?

[Bill O]

Can you reach full motor rpm (3,200) while underway? If not, possibly over propped.

Did you change the rudder shape or chord length with the new rudder design?

[malyea]

Originally Posted by Bill O

Can you reach full motor rpm (3,200) while underway? If not, possibly over propped.

Did you change the rudder shape or chord length with the new rudder design?

Yes and no.
Thanks

[malyea]

Soooo….How much helm pressure from prop wash do you feel on your boat when motoring at 2800 RPM?

[Bill O]

Our Perkins 4.236 is a continuous max of 2250 and when we rarely do ~1,900 rpm (similar proportion to max continuous), we do not have any helm pressure.

Did the helm pressure only happen after the new rudder?

[malyea]

Originally Posted by Bill O

Did the helm pressure only happen after the new rudder?

Thanks, good question - Can’t say for sure, see my original post.

[Sailmonkey]

I’ve been on a number of boats that require lots of pressure to maintain a course under power, some way worse than others.

Our own boat will get a very heavy helm if I exceed 1600-1800 rpm, but we’ve also got a huge constellation rudder that exhibits all sorts of wonky traits.

[C420sailor]

Beneteau 361 with a Westerbeke 30B and Autoprop H5.

Max is 3600rpm I think, but the boat makes 5-6 knots in the 2000-2500 range, so we don’t see the need to stress her. I don’t think I’ve ever pushed her over 2800.

Zero pressure from prop wash.

[Martkimwat]

Have you increased your prop shaft length that has resulted in the prop being closer to the rudder? ( fitting a rope cutter or even changing to a feathering prop may do that ) As others have said, a larger diameter prop may also cause that even if the diameter/pitch calculation is correct for your boat (ie not overpropped). I unless I missed it, you don't say whether your boat has a long keel with a cutout in the rudder and sternpost for the prop or whether it is a separate skeg and keel arrangement. the former is more prone to excessive rudder interaction from the prop than the latter but for example, even on my Parker 27 which has a transom hung rudder with the prop at least 14" forward of the leading edge I can get a bit of rudder kick from the prop at near full revs and that is with a balanced rudder as well.

[Cadence]

My guess would be your rudder does not have any leading surface ( surface before the rudder post). This may be advantageous under sail but a negative under power.

[Finalee]

My thought is its normal. Sailboats are designed to have a natural motion thru the water. Prop wash upsets that.

[Bill O]

Originally Posted by malyea

Thanks, good question - Can’t say for sure, see my original post.

Malyea,
Would discuss this w/Gene Gammon (Mr. Irwin), since he knows the whole Irwin line well to see if this was normal for your MKII.

[PineyWoodsPete]

Originally Posted by malyea

Yes and no.
Thanks

"Propeller demand" - the HP absorbed by the prop - varies with the cube of the difference between peak HP RPM (3200) and a given lower RPM.

At 1500 RPM, PD is only 10.3% of peak HP; at 2200, 32.5% of peak HP; and at 2800, 67%. So there's a logarithmic increase in prop-thrust pressure as RPM's increase rather than a linear one. Totally non-intuitive!