Question for the Mech. Engineer Types...???

[Maine Sail]

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..!

[Alan Wheeler]

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.

[Chief Engineer]

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

[Maine Sail]

Originally Posted by Chief Engineer

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?

[AnchorageGuy]

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.

[Chief Engineer]

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

[Evan]

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?

[migot1]

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.

[Morgan Paul]

#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/

[Ex-Calif]

Originally Posted by Acoustic

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?

[Chief Engineer]

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.

[Maine Sail]

Originally Posted by Evan

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??

[Chief Engineer]

Originally Posted by Chief Engineer

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

[Alan Wheeler]

That sounds better chief. I couldn't make any sense of the figures from the first one.

[David M]

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.