Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 07-12-2010, 14:38   #16
Senior Cruiser
 
rebel heart's Avatar

Cruisers Forum Supporter

Join Date: Oct 2005
Posts: 6,190
Images: 3
Quote:
Originally Posted by unbusted67 View Post
We thought that it might be electrolysis too but the zincs seem fine and there is a definite pattern to the pitting. It is localized to the outer edges of the prop. We have been "motoring around really fast" because this is referring to the power vessel we bought in Maine not the sailboat right next to you! Stan is flying your way now as we speak.
If it's localized to the outer edges and you've been operating at a higher speed than normal, yeah, it sounds like cavitation. When are you coming back out here? I'm going to try to get a job this summer over on one of the harbor tour boats once I get my able seaman qualification.
__________________

__________________
rebel heart is offline   Reply With Quote
Old 07-12-2010, 14:56   #17
Registered User
 
tager's Avatar

Join Date: Apr 2009
Location: Vashon, WA
Boat: Haida 26', 18' Sea Kayak, 15' kayak, 6.5' skiff, shorts
Posts: 837
I think that increasing blade area will tend to decrease incidence of cavitation. Cavitation happens when the output power of the engine exceeds the maximum amount of power that can be translated through the prop. The energy has to go somewhere, so it goes into changing the phase of the water, instead of moving the boat.
__________________

__________________
tager is offline   Reply With Quote
Old 07-12-2010, 15:03   #18
Registered User
 
simonmd's Avatar

Join Date: Nov 2010
Location: Sant Carles, S Spain
Boat: 30ft Catalac 900 "Rubessa"
Posts: 876
Once some minor damage (pitting) has been done by a moderate and un noticed amount of cavitation, its effects will become more pronounced as the damage increases. The rougher surface of the now pitted props simply increases the amount of cavitation that occurs. It's like a catch 22 thing, more pitting, more cavitation causing more pitting, etc, etc.
__________________
simonmd is offline   Reply With Quote
Old 07-12-2010, 15:08   #19
Senior Cruiser
 
unbusted67's Avatar

Cruisers Forum Supporter

Join Date: Dec 2006
Location: Islesboro, ME
Boat: Looking for a new boat
Posts: 2,197
Images: 24
Quote:
Originally Posted by rebel heart View Post
If it's localized to the outer edges and you've been operating at a higher speed than normal, yeah, it sounds like cavitation. When are you coming back out here? I'm going to try to get a job this summer over on one of the harbor tour boats once I get my able seaman qualification.
Thanks everyone for getting back to me on this. We have swapped out the prop and will haul out again in April for the COI inspection at which point we might get a five bladed prop on there and reanalyze the pitch.

Erik, I am out here on the East Coast freezing for fun. I have been given this business to run so I am going to make a go of it for a year or two. San Diego is way on the top of my list though, especially after another Maine winter. That's awesome about the harbor boats thing. I think you will be surprised how much companies are starved for captains (and this may not be true for San Diego)/ABs. I'm sure you'll kill!
__________________
unbusted67 is offline   Reply With Quote
Old 07-12-2010, 23:49   #20
Registered User

Join Date: Oct 2007
Location: San Francisco
Boat: N/M 45
Posts: 236
Quote:
Originally Posted by sailingmonica View Post
So we know what it is. The question now is, what can be done to fix it: increase or decrease propeller size and/or pitch?
I suspect the correct response is: match the propeller's ability to transfer power to the water to the engine's ability to transfer power to the propeller. What you don't want is excess power in the prop that cannot be transferred to the water, as that energy will go somewhere and if the power cannot be used to push the boat forward then it will be used to boil water and you'll have cavitation.

Of the two suggestions you mention, and assuming that 'prop size' = 'increase propeller diameter', this is a double-edged sword. Increasing diameter increases blade tip speed which increases likelihood of cavitation depending upon blade twist/shape, and increasing diameter increases load the propeller can transfer from the engine to the water per rotation, and therefore usually reduces max RPM the engine can attain. It might work out that simply increasing diameter prevents cavitaion because you've slowed the engine down or increased the power the prop can transfer to the water...

Changing pitch also directly effects the ability of the prop to transfer energy, and a similar cycle ensues.

The upshot is to match the prop to the engine and boat such that you can achieve max RPM (no less, no more) that the engine design calls for at hull speed (for a displacement hull), and then operate that engine/prop at no more than max RPM (you might typically operate a diesel engine at 80% rated max RPM), and at those RPMs you should not have cavitation problems.

Anyone have a better approach/explanation?
__________________
beetle is offline   Reply With Quote
Old 09-12-2010, 08:55   #21
Senior Cruiser
 
rebel heart's Avatar

Cruisers Forum Supporter

Join Date: Oct 2005
Posts: 6,190
Images: 3
Quote:
Originally Posted by unbusted67 View Post
Thanks everyone for getting back to me on this. We have swapped out the prop and will haul out again in April for the COI inspection at which point we might get a five bladed prop on there and reanalyze the pitch.

Erik, I am out here on the East Coast freezing for fun. I have been given this business to run so I am going to make a go of it for a year or two. San Diego is way on the top of my list though, especially after another Maine winter. That's awesome about the harbor boats thing. I think you will be surprised how much companies are starved for captains (and this may not be true for San Diego)/ABs. I'm sure you'll kill!
Hauling for a COI? That's some good experience there, and you're talking about a bigger prop than I'm sure a lot of people on here were assuming.

Once I get all my paperwork in order (spring-ish) I'll send you what I have; maybe you'll need to hire an AB for a trip sometime and I'll fly out there. Or maybe (just maybe) your captain needs someone to hold the wheel while he takes a whiz.
__________________
rebel heart is offline   Reply With Quote
Old 09-12-2010, 10:08   #22
Registered User

Join Date: Dec 2010
Posts: 14
Quote:
Originally Posted by unbusted67 View Post
The title says it all.
what CAUSES cavitation is

The trailing edge of a body through water (submerged) that creates a low pressure area resulting in air bubbles forming from dissolved oxygen.

A submarines worst nightmare when going stealth. Often under water it is confused as overhead waves but it is the sound of cavitation.
__________________

__________________
BLUEwaterchaser is offline   Reply With Quote
Reply

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Cavitation at Plane michaelmrc Auxiliary Equipment & Dinghy 8 22-04-2009 16:46



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 02:48.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.