Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 12-09-2008, 09:56   #1
Marine Service Provider
 
Maine Sail's Avatar

Join Date: Jan 2007
Location: Maine
Boat: CS-36T - Cupecoy
Posts: 3,060
Question for the Mech. Engineer Types...???

Hi All,

I have a couple of questions.

#1 I am looking for a prop thrust calculator that would show me how much thrust my boat can develop. Can anyone point me to a link that might help me determine this how much thrust my boat has?

#2 Does thrust equal pull? By this I mean, if I tied my boat to a dock and put a device in the line, then put her in gear and let her rip does 1000 pounds of thrust turn into 1000 pounds of "pull" on the line???


If anyone can answer these two questions I'd really appreciate it..!
__________________

__________________
Marine How To Articles
Maine Sail is online now   Reply With Quote
Old 12-09-2008, 14:13   #2
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
Answering your last question first, yes the "thrust" is equal to pull. In fact the propeller is pulling the boat through the water more than propelling. Perhaps we should call the propeller a pullpeller instead. About 60% of the forward movement is generated by lift. A prop is a Rotary wing. A water jet produces thrust. Whic now brings me to the first question. Becuase a prop is a rotary wing, it's ability to do what it is doing is dependant on the design. So a means of calculating thrust is not so easy. So hence why most vessels are tested using a static bollard pull. A set of scales measures the pull.
I imagine individual prop makers would have specs they could give you if you asked.
__________________

__________________
Wheels

For God so loved the world..........He didn't send a committee.
Alan Wheeler is offline   Reply With Quote
Old 12-09-2008, 14:44   #3
Eternal Member
 
Chief Engineer's Avatar

Join Date: Aug 2006
Location: North of Baltimore
Boat: Ericson 27 & 18' Herrmann Catboat
Posts: 3,798
Try this formula

Thrust (in Pounds) = 62.72 X (Shaft Horsepower X Diameter of Prop).067

Rule of Thumb for tugs is 2,240 lbs per every 100 Brake Horse Power.

I am sure this will spark some friendly debate
__________________
Chief Engineer is offline   Reply With Quote
Old 12-09-2008, 14:49   #4
Marine Service Provider
 
Maine Sail's Avatar

Join Date: Jan 2007
Location: Maine
Boat: CS-36T - Cupecoy
Posts: 3,060
Quote:
Originally Posted by Chief Engineer View Post
Thrust (in Pounds) = 62.72 X (Shaft Horsepower X Diameter of Prop).067

Rule of Thumb for tugs is 2,240 lbs per every 100 Brake Horse Power.

I am sure this will spark some friendly debate
Tugs are geared differently and have overly large wheels for the given HP so does that change that equation at all when compared to a sailboat spinning a small wheel with different gearing?
__________________
Marine How To Articles
Maine Sail is online now   Reply With Quote
Old 12-09-2008, 17:40   #5
Marine Service Provider
 
AnchorageGuy's Avatar

Join Date: Mar 2003
Location: Wherever the boat is!
Boat: Marine Trader 34DC
Posts: 4,618
Most prop calculators for pleasure boats will give you prop size and pitch for given boats and I don't recall any that calculate thrust.
__________________
Chesapeake Bay, ICW Hampton Roads To Key West, The Gulf Coast, The Bahamas

The Trawler Beach House
Voyages Of Sea Trek
AnchorageGuy is offline   Reply With Quote
Old 12-09-2008, 19:15   #6
Eternal Member
 
Chief Engineer's Avatar

Join Date: Aug 2006
Location: North of Baltimore
Boat: Ericson 27 & 18' Herrmann Catboat
Posts: 3,798
The formula I gave you is from "The Propeller Handbook"

There is a different (slightly) formula for tugs it has to do with size of wheel in feet.

BTW you should be using inches for your prop diameter.

You can get really carried away with slippage, lift, drag, profile of props, cross sections. Which you really need to know about if you are running Miss Geico or any racing power boat
__________________
Chief Engineer is offline   Reply With Quote
Old 13-09-2008, 01:40   #7
Registered User

Join Date: Nov 2006
Location: Vancouver, Can.
Boat: Woods 40' catamaran
Posts: 277
It's a pretty poor modern tug that can't get > 25 lb/HP.

But Tugs are generally propped for maximum thrust at zero speed, whereas most sailboats are propped for max thrust at ~hull speed.

For a sailboat prop, you would be lucky to get 20 lb/HP.

Does this help?
__________________
Evan is offline   Reply With Quote
Old 13-09-2008, 02:21   #8
Registered User

Join Date: Jun 2007
Location: Israel
Boat: Southerly UK 37ft
Posts: 104
At Volvo they have a computries plan to optimies PROP versus BOAT. try to contact them. Giving the boat model and weight they can get the best suggetion for a new, right size prop.

if you are thinking of a Folding Prop....that is much more difficult.
__________________
migot1 is offline   Reply With Quote
Old 13-09-2008, 06:09   #9
Registered User

Join Date: Feb 2007
Boat: 1973 Morgan 36T
Posts: 808
Images: 17
Quote:
#1 I am looking for a prop thrust calculator that would show me how much thrust my boat can develop. Can anyone point me to a link that might help me determine this how much thrust my boat has?
This might help

Thrust

and

http://www.alberg30.org/maintenance/...r/propcalc.xls
found here
http://www.alberg30.org/maintenance/...ion/Propeller/
__________________
Morgan Paul is offline   Reply With Quote
Old 13-09-2008, 09:55   #10
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
Quote:
Originally Posted by Acoustic View Post
Hi All,

#1 I am looking for a prop thrust calculator that would show me how much thrust my boat can develop. Can anyone point me to a link that might help me determine this how much thrust my boat has?
I am curious why you want to know this?

Is this something we should know about our boats?
__________________
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 13-09-2008, 13:28   #11
Eternal Member
 
Chief Engineer's Avatar

Join Date: Aug 2006
Location: North of Baltimore
Boat: Ericson 27 & 18' Herrmann Catboat
Posts: 3,798
I have a buddy who has a big hanging scale....just for fits and giggles I am going to try a test...I'll post the results when I do it.
__________________
Chief Engineer is offline   Reply With Quote
Old 14-09-2008, 17:03   #12
Marine Service Provider
 
Maine Sail's Avatar

Join Date: Jan 2007
Location: Maine
Boat: CS-36T - Cupecoy
Posts: 3,060
Evan...

Quote:
Originally Posted by Evan View Post
It's a pretty poor modern tug that can't get > 25 lb/HP.

But Tugs are generally propped for maximum thrust at zero speed, whereas most sailboats are propped for max thrust at ~hull speed.

For a sailboat prop, you would be lucky to get 20 lb/HP.

Does this help?
This is the rough type of estimation I'm looking for can you point me to a source for the 20lbs. of thrust per HP??
__________________
Marine How To Articles
Maine Sail is online now   Reply With Quote
Old 14-09-2008, 20:29   #13
Eternal Member
 
Chief Engineer's Avatar

Join Date: Aug 2006
Location: North of Baltimore
Boat: Ericson 27 & 18' Herrmann Catboat
Posts: 3,798
Correction to formula

Quote:
Originally Posted by Chief Engineer View Post
Thrust (in Pounds) = 62.72 X (Shaft Horsepower X D/12).067

D=Diameter of prop in inches

Rule of Thumb for tugs is 2,240 lbs per every 100 Brake Horse Power.

I am sure this will spark some friendly debate
Let's see how that works
__________________
Chief Engineer is offline   Reply With Quote
Old 15-09-2008, 13:29   #14
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
That sounds better chief. I couldn't make any sense of the figures from the first one.
__________________
Wheels

For God so loved the world..........He didn't send a committee.
Alan Wheeler is offline   Reply With Quote
Old 15-09-2008, 17:13   #15
Moderator Emeritus
 
David M's Avatar

Cruisers Forum Supporter

Join Date: Sep 2007
Location: San Francisco Bay
Boat: research vessel
Posts: 10,150
The problem with getting an accurate figure for maximum thrust for a properly pitched yacht is that the boat must be moving through the water with the engine at maximum horsepower to get this figure. For most yachts, you will never get this figure with the boat tied to the dock because the propeller will cavitate or lose its laminar flow before you get to maximum engine horsepower....or you may not be able to reach maximum horsepower at all because you are not moving through the water.

Tugs are paid according to their bollard pull (amongst other things) which is measured by how hard they can pull while tied to a pier. But tugs have very low pitched props, controllable pitch props, Kort nozzles or Voith-Schneider drives....yachts don't have any of those. The Marine Exchange in San Francisco does the bollard pull certification here.
__________________

__________________
David

Life begins where land ends.
David M is offline   Reply With Quote
Reply

Tags
engineer

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
Unlimited Engineer looking for some answers dansmith1982 Training, Licensing & Certification 15 15-11-2008 11:38
First Time as Engineer sloopz The Sailor's Confessional 6 24-02-2008 01:18
Electrical Engineer wanted with.... Yachts66 Electrical: Batteries, Generators & Solar 0 25-09-2006 19:47



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 07:32.


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.