So much prop wash I can’t steer the boat

[Ward H]

Hi Matt,
You posted in the C30 forum. Seems like the right place to me.

This is my first season with my 1993 C30 with the M25XP Universal. This is my first boat with an inboard motor. Previous boat was a tiller steered, outboard powered O'day 25. So, the previous posters have loads more experience than I do but I do have a C30.

This is good because even turning 3000, I can’t get more then 4.5 knots out of it. Is this normal?

My prop is a 3 blade marked MI/FED Sailer, 13RH10. Pretty sure I read where this was the standard 3 blade.
On the sea trial last November the surveyor noted WOT was 3500 rpm 6.8 knots and a cruising speed of 2300 rpm 6 knots. The speed came from the boats knot meter and this was on flat water.
I changed the alternator over the winter so I believe the tach is off a little and the knot meter stopped working after the first week so my speed is now coming from the GPS.
What I see now is: WOT is 3200 RPM for 6 knots and I run between 2500 - 2800 rpm for 5.5 knots.

There is so much prop wash when motoring it takes 2 hands and a tiring amount of work to keep the boat from turning to port.

I keep a straight course under power with one lazy hand on the wheel.

Over the winter I read every thread on this Catalina 30 forum. While there has been discussion on different props, pitch and size most were related to reducing prop walk in reverse and balancing forward speed under power and under sail. I don't recall anyone noting a change in a prop causing problems holding a course in forward.

 

[CrispyCringle]

Identified in engineering terms I agree.

I used the term in the "feeling" a boater has when experiencing the effects of prop wash.

In Aeronautical terms the fluid dynamics active when a prop is turning and moving air across the airplane body and tail is identified as:
Prop wash
A propeller pushes air not just horizontally to the back, but more in a twisting helix around the fuselage (clockwise as seen from the cockpit). As the air whirls around the fuselage it pushes against the left side of the vertical tail (assuming it is located above the propeller's axis), causing the plane to yaw to the left. The prop wash effect is at its greatest when the airflow is flowing more around the fuselage than along it, i.e., at high power and low airspeed, which is the situation when starting the takeoff run.

The prop on a boat acts similar when examining the fluid dynamics against the hull and the rudder.​

After reading Peggie Halls message "If you can't explain it to a six year old, you don't completely understand it yourself." --Albert Einstein I am leaning towards less technical detail and more communicative terminology in an effort to communication. The walk or movement to port feeling you experience might resonate more easily.

Your thoughts?

I disagree somewhat in your theory (Isnt it great how everyone becomes an expert in threads like these? My turn.). Ive got many thousands of hours flying turbo-prop planes. Prop walk in boat speak is the same as P-factor in airplane speak. Prop walk and/or P-factor are caused by the axis of the prop being different from the direction of motion. For example, an airplane angled nose up for take-off but still rolling on the ground, or a prop shaft that is angled down and not parallel to the surface of the water. It results in the prop blades on one side, left or right depending on the rotation direction, having a greater apparent pitch angle than the other side. Therefore, greater thrust on one side over the other. It creates a yaw moment, and causes the drift in steering. Thats my explanation, and Im sticking to it. Next.

 

[Benny17441]

I know the symptoms, a real heavy pull to port and significantly reduced top speed, have seen them before and they led to a fouled prop. At first engine performance was suspect and fuel filters were replaced to no avail. The prop was cleaned and the sacrificial anode was replaced and problem went away. I could surmise that a damaged prop could yield the same results.

 

[Tally Ho]

I have an O’Day 322. When at speed under power, I can hold the boat straight with very little pressure on the wheel. HOWEVER, if I let the wheel go, the boat will veer very hard (I haven’t paid attention if it always goes the same way). Will have to check that out next time I am out.

Greg

 

[smokey73]

I know the symptoms, a real heavy pull to port and significantly reduced top speed, have seen them before and they led to a fouled prop. At first engine performance was suspect and fuel filters were replaced to no avail. The prop was cleaned and the sacrificial anode was replaced and problem went away. I could surmise that a damaged prop could yield the same results.

Seems this is the direction that you need to look before you think about changing a prop. Based on the fact that you say you have to put a lot of effort into keeping it from turning to port rather than a "light hand" as other C30 owners have noted (but only under power) would seem to indicate a prop poblem. However, a 2 degree change in prop pitch would probably not cause such extreme symptoms so it would be reasonable to suspect a fouled prop and or damage to the prop itself.

I know when my prop is very fouled I cannot get over 4 knots and only about 2800 to 3000 rpm and it vibrates the wheel and is "heavier on the hand" to keep the boat in a straight line. With a clean prop its 3600 rpm at wide open throttle, easy crusing at 2600 rpm, smooth as silk and only a very light hand to keep it straight when motoring. Awhile back I could only get to 3300 rpm at wide open and adjusted the pitch (I have a Maxprop Classic) by 2 degrees and can now do 3600 rpm but I didn't notice any signficant impact on turning to Port when under power or on prop walk in reverse for that matter. That is why I think changing the pitch by 2 degrees would not seem to be the fix for the heavy hand to steer straight when motoring.

Focus on fouled prop or damage to the prop before you drop bucks on a new prop (unless you just want a folding or feathering prop of course), but I'd sort the problem with the heavy hand with the prop you have before I spent big $$ on a folding prop just in case the prop is not the problem.

 

[NoAgenda]

Matt
You won't be able to fit the same size fixed blade prop there isn't enough room. With a folding prop the blades are offset aft that allow for the larger diameter prop. I bought it a year ago January for boat show pricing which was basically the same regular price of the 2 blade prop. The wheel will always pull when motoring on the Catalina 30. I have been on many of them and they all do it to some degree.

 

[rpell1@msn.com]

All good points. The more I think about my own symptoms, I suspect I have a bad or incorrect prop on the boat. A clean, fresh bottom and running gear made no difference. It does not happen under sail, and I am over revving and under speed. Since each degree of pitch equals about 200 RPM, and I'm running at 3000, I would guess I need a prop with 2 degrees more pitch then I have. It could very well be I am throwing a ton of water at the rudder with no thrust. I could see how that could cause some interesting things to happen down there.
So, if anyone knows what size Fixed props they have on the 25 universal that works great please share, a prop change will be my next step.
No Agenda, I see you have a 15"x9 Gori. First, I'm jealous, that things probably worth half the value of the boat! second, I wonder if the proper fixed size translates to the same size in a folding prop, I would love to not drag a 3 blade fixed prop all the time.
Thanks!

Hi Guy,
My experience with my previous boat, a 1991 Cat30 MKII has shown me that I really have a choice of only 2 sized props. The 13x10 three blade is the OEM on this boat. I tried a 13x12 three blade and actually experienced a little more speed and just a bit less walk. The differences are truly minimal, but if you are under power as much as I am in order to clear the mooring field and very busy channel, you can feel these differences. Both of these props are "Mickey-Mouse" eared. The latest boat I now have is a 1997 Cat20 MKIII with an OEM 13x10 three blade, but has much narrower blades. I have yet to experience any under power maneuvers, but I can tell you that speed is an advantage and there seems to be less walk. Good luck and Happy Sailing.

 

[Coriolisbc]

Matt, my boat pulls to starboard, only at higher rpm, and I can counter the pull with two fingers on the wheel, I have the big ears 3 blade prop.