Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 14-05-2008, 09:54   #1
Registered User

Join Date: May 2008
Location: Port Townsend, WA
Boat: 37' Samson, Ferro
Posts: 69
Images: 9
prop backwards or bent?

I have a 10hp Saab propulsion setup in my 10 ton sailboat. An auxiliary power unit to say the least. My speed is pretty slow. Last summer I had a diesel mechanic glance at my prop and say the blades are backwards. Its a 2 blade feathering Saab prop. Now I have two situations:1. the prop blades were installed backwards (the number didn't line up), 2. the blades have a 10 degree bend (not pitch) to them, not like a traumatic damage type of bend but rather a deliberate manufacturing type of bend. if you imagine an animated cartoon of a boat traveling super duper fast and the blades are bending backwards from the extreme speed of the boat then that is what mine remind me of (I actually wondered for a moment if that happened). I have looked at 4 other Saab feathering props in the yard and they are all straight.

So, to set things right I switched the blades around to line up properly with the numbers but then the bend in the blades are facing forward. I shrugged my shoulders and put the boat in the water hoping that maybe i remedied a problem the previous owner was unaware of. However, the boat is agonizingly slow and I have no knowledge of its performance before I changed the prop.

Anybody have any thoughts about the bend, performance, cause, solution?
__________________

__________________
Borealis is offline   Reply With Quote
Old 14-05-2008, 09:57   #2
Moderator Emeritus
 
David M's Avatar

Cruisers Forum Supporter

Join Date: Sep 2007
Location: San Francisco Bay
Boat: research vessel
Posts: 10,152
Weird to say the least. Have you contacted Saab?
__________________

__________________
David

Life begins where land ends.
David M is offline   Reply With Quote
Old 14-05-2008, 14:23   #3
Senior Cruiser
 
Alan Wheeler's Avatar

Cruisers Forum Supporter

Join Date: Dec 2003
Location: Marlborough Sounds. New Zealand
Boat: Hartley Tahitian 45ft. Leisure Lady
Posts: 8,038
Images: 102
A common cause of bending prop blades is to ground in mud. The effort exerted on the blades will bend them like Bananas instantly. It is possible the boat has been aground and the prop has been in something harder to move than water.
At a ratio of 1Hp/T don't expect wonders in acceleration either. That is a very small engine for 10T of boat.
__________________
Wheels

For God so loved the world..........He didn't send a committee.
Alan Wheeler is offline   Reply With Quote
Old 16-05-2008, 21:53   #4
Registered User

Join Date: Apr 2008
Location: sydney australia
Boat: samson c-shell
Posts: 52
Hello again Borealis, I cant comment on your prop shape , but ten horse power diesel in in that boat is way to small eg my boat same design 48 hp with a fixed 3 blade prop.feathering 2 blades wiil not be very efiecent.I used to have a rorc 32 ,7 ton and that had 12 hp motor which was embarrasing when motoring.
__________________
4900bruce
4900bruce is offline   Reply With Quote
Old 16-05-2008, 21:56   #5
Registered User

Join Date: Apr 2008
Location: sydney australia
Boat: samson c-shell
Posts: 52
just remembered ,there are hull speed calculators and prop sizing calculators on the internet which will tell what speed you should be travelling at and what size prop and pitch you should have for that motor and boat.
__________________
4900bruce
4900bruce is offline   Reply With Quote
Old 17-05-2008, 00:03   #6
Moderator Emeritus
 
Ex-Calif's Avatar

Cruisers Forum Supporter

Join Date: May 2007
Location: Singapore
Boat: Maxi 77 - Relax Lah!
Posts: 11,514
Images: 4
Send photos...

I was given a prop and had no idea what is was other than it was a feathering prop. We had the original Volvo saildrive folding prop.

After I posted a few photos the folks here identified the prop within seconds (maxprop duo). I downloaded the manual from the internet which explained how to set the prop up. I expected a bit of "trial and error" but we seem to have nailed the adjustment on the first try.

With the same 10hp engine we now get 6.5 kts vs. 3.5 before.

I would guess if the blades were damaged you would see evidence on the blades of striking something - mud might be a different story - but also I would expect you to get vibration at any significant engine speeds.

photos would be nice...
__________________
Relax Lah! is For Sale <--- Click
Click--> Custom CF Google Search or CF Rules
You're gonna need a bigger boat... - Martin Brody
Ex-Calif is offline   Reply With Quote
Old 17-05-2008, 07:29   #7
Registered User
 
svHyLyte's Avatar

Join Date: Mar 2006
Location: Tampa Bay area, USA
Boat: Beneteau First 42
Posts: 3,434
Images: 25
The flares at the tips serves two functions. Firstly they provide a little drag to ensure that the blades fold when not powered. Secondly, they redirect the tip-vorticies that form as the blade spins, which would otherwise swirl under the back sides of the blades killing some of the lift provided by the blades. A good example of this is the up-turned tips at the outboard ends of some of the higher performance jet powered private aircraft.

s/v HyLyte
__________________
"It is not so much for its beauty that the Sea makes a claim upon men's hearts, as for that subtle something, that quality of air, that emanation from the waves, that so wonderfully renews a weary spirit."
svHyLyte is offline   Reply With Quote
Old 17-05-2008, 09:53   #8
GreatKetch
Guest

Posts: n/a
Quote:
Originally Posted by Alan Wheeler View Post
A common cause of bending prop blades is to ground in mud. The effort exerted on the blades will bend them like Bananas instantly. It is possible the boat has been aground and the prop has been in something harder to move than water.
That certainly might be true if the prop is buried in mud and turned, but given the bottom configurations of most sailboats that seems a unlikely scenario.

If you are lucky enough to live in an area that has a propeller specialist you might bring some photos to their shop.
__________________
  Reply With Quote
Old 26-06-2008, 22:06   #9
Registered User

Join Date: May 2008
Location: Port Townsend, WA
Boat: 37' Samson, Ferro
Posts: 69
Images: 9
reply

Quote:
Originally Posted by 4900bruce View Post
just remembered ,there are hull speed calculators and prop sizing calculators on the internet which will tell what speed you should be travelling at and what size prop and pitch you should have for that motor and boat.
hey Bruce, send me an email, I'll send you pics of my C-Shell / Bird.
__________________
Borealis is offline   Reply With Quote
Old 26-06-2008, 22:07   #10
Registered User

Join Date: May 2008
Location: Port Townsend, WA
Boat: 37' Samson, Ferro
Posts: 69
Images: 9
reply

Quote:
Originally Posted by 4900bruce View Post
just remembered ,there are hull speed calculators and prop sizing calculators on the internet which will tell what speed you should be travelling at and what size prop and pitch you should have for that motor and boat.
hey Bruce, send me an email, I'll send you pics of my C-Shell / Bird.
usnaquaman@hotmail.com
__________________
Borealis is offline   Reply With Quote
Old 26-06-2008, 22:13   #11
Registered User

Join Date: May 2008
Location: Port Townsend, WA
Boat: 37' Samson, Ferro
Posts: 69
Images: 9
photo, good idea, i may even have one from my last haulout...thanks.
__________________

__________________
Borealis 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
Bent anchor shank sluissa Anchoring & Mooring 41 15-04-2008 19:19
Bent Steel Davit Claims Benjamaphone Construction, Maintenance & Refit 24 31-03-2008 01:25
Bent Boom clausont Deck hardware: Rigging, Sails & Hoisting 10 29-11-2007 19:34
Straighten Bent Stanchions Spirit28 Deck hardware: Rigging, Sails & Hoisting 3 30-07-2007 11:40
Philosophical bent... bcguy General Sailing Forum 10 30-12-2006 11:16



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 06:53.


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.