Design Factors Affecting Reverse Prop Walk

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Jun 21, 2007
2,105
Hunter Cherubini 36_80-82 Sausalito / San Francisco Bay
Dear Glenn:

Thanks for your participation on the forum. I've been reading and learning!

I thought to post to you a question of mine -- and others as has recently come up on the Cherubini Hunter forum. Here is what I posted:

"When I engage reverse with the boat stopped, the prop wash (as observed from the surface) exits the boat almost perpendicular to the fore/aft axis. So the first motion of the boat is that the aft swings hard to port before any backward motion is acheived. Most people say (without guarantee) that a 3 blade prop will help. But I don't know yet. I'm waiting until my next haulout to install one. The existing prop 2 blade prop I think is original issue and has minor pitting in a few spots, so probably time to change out anyway. It's diameter is 16". Clearance between the hull and the prop is only 1.5". Less than normal. This may account for some of the prop walk issues. Changing to a 3 blade can be smaller diameter = more blade/hull clearance. Yes, I know that one can learn to use prop walk to good advantage. But in my boat's case, its too much of a good thing!"

I've read that some boats go in reverse better/worse than the norm. What are the design considerations that affect this? Attached is an aft section photo of my H36C. Thanks for your comments.

rardi
 

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Jun 6, 2006
6,990
currently boatless wishing Harrington Harbor North, MD
Google P-factor

The best example of "prop walk" is actually a helicopter but there is lots of theory about props operating at high angles of attack. What that means for boaters is prop shafts that are inclined from the horizontal have more P-factor than prop shafts that are horizontal. P-factor results in asymmetric thrust between the prop blade going upward and the blade going downward.
There is also the issue of spiral prop-wash but given the relatively smaller diameter of water props this I'm not sure this is a significant factor in prop walk.
Try
http://home.earthlink.net/~x-plane/FAQ-Theory-PFactor.html
 

ghen

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Mar 15, 2009
104
2 216 St. Augustine
The torque of the engine affects prop walk. The three blade prop will allow the boat to gain reverse motion faster so it is minimized. There is a myriad of factors that enable or disable a boat from going into reverse well. Proportional area of the rudder and aspect shape and ratio is a big factor. Keel position for and aft is a contributing factor. Hull shape and volume distribution also affect reversing ability.
 
G

Guest

Prop

Hi Rardi, our 1991 P42 came with an 18RH15 fixed three blade. When we first purchased her in 2002 it took some time for me to learn how to understand her characteristics. Knowing how she handles really helps when it came time to bite the bullet. I just love the prop on our boat.

Belle-Vie backs like a dream. I've parallel docked, backed down long narrow fairways with a 60 degree turn half way through, backed into slips and I can turn her 360 degrees literally on a dime. Most of it involves technique. Google "sailboat docking techniques" and you will find animated instructions on various docking challenges. They work!

Reading the wind and current and knowing how your boat behaves in those conditions helps you to prepare for the proper maneuver. I do not want or need a bow thruster. Most of the time I sail solo, even when the admiral is aboard. She likes being on the water but her arthritis restricts how much she can do.

Terry Cox
 
Jun 21, 2007
2,105
Hunter Cherubini 36_80-82 Sausalito / San Francisco Bay
Re: Prop

Terry:

Thanks the encouragement. Not that I am expecting that any mod's will make a boat's prop walk completely go away; just exploring if I can get it to be less severe for the case of my boat.

For the normal day's outing, maneuvering is only in/out of my familiar slip and I have been doing this just fine (to date anyway). As with you, I also view undocking/docking a solo affair, even if guests are aboard. I (almost always) remember to instruct or remind that nobody (regardless of age/fitness) is to step/jump on/off to tend the boat. With 13,500 lb. displacement and 2.5-3.0 feet height from deck to dock and with water probably in between, its just too risky to have anyone trying to fend a boat that still might be moving a bit. The fenders hanging off the side anyway do a much better job of protecting the boat from a bit of bump with the dock than people. And fenders don't get hurt!

My wife also has health considerations that prevent her from participating too aggressively. But same as your admiral, she too enjoys being on the water. And she likes to steer the boat under power in the channel, and the feel of being at the helm under sail if the wind conditions aren't too strong for her. Today's sail was a good one. Light/moderate winds, clear sky, and a favorable tidal current were perfect conditions for us to venture out a few miles past the Golden Gate Bridge towards the Pacific (and back again).
 
Feb 26, 2004
22,759
Catalina 34 224 Maple Bay, BC, Canada
Not that I am expecting that any mod's will make a boat's prop walk completely go away; just exploring if I can get it to be less severe for the case of my boat.
I agree with Terry. Most people hate prop walk and can't wait to (sorry) run away from it, as fast as they can.

I find it extremely helpful for maneuvering, and would be lost without it. We have a fixed 3 blade, installed by the PO in lieu of the old 2 blade OEM.
 
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