Boat thrust calculator? - Page 2 - SailNet Community
Old 10-03-2018
Senior Member

Join Date: Dec 2013
Location: Tampa FL
Posts: 476
Thanks: 7
Thanked 39 Times in 39 Posts
Rep Power: 7

Dock
Re: Boat thrust calculator?

Here's a decent prop calculator that can help describe a suitable prop.

https://www.vicprop.com/displacement_size.php
PhilCarlson is offline

Old 10-04-2018
Senior Member

Join Date: Jun 2008
Location: Tampa, FL
Posts: 1,385
Thanks: 1
Thanked 49 Times in 48 Posts
Rep Power: 12

Re: Boat thrust calculator?

Quote:
Originally Posted by Captain Canuck View Post
Thanks denerd0n. Does this equation assume a 100% efficient prop, or is the prop inefficiency factored in?
As I recall it does NOT assume 100% prop efficiency, as that is impossible. Rather it assumes a propeller of proper specifications. Another way to say that would be that it assumes you selected a prop that is perfectly sized for your boat, but then accepts that there is a small loss even with a perfectly sized prop.

Quote:
Originally Posted by GeorgeB View Post
DenverdOn, what is the "POWER" variable in your equation?
POWER is a math function within Excel, raising the value to the power provided. Has nothing to do with engine "power" if that's what you were thinking.
denverd0n is online now
Old 10-05-2018
Senior Member

Join Date: Sep 2006
Location: Scotland
Posts: 2,458
Thanks: 0
Thanked 13 Times in 13 Posts
Rep Power: 14

Re: Boat thrust calculator?

Be careful with the concept of "hull speed".
Your boat often will go much faster than the calculated "hull speed" figure.
My own 11 ton ship has a waterline length of about 33 ft, and the calculated hull speed for that waterline is close to 7.7 kt.
In flat water (no surfing), in the right wind, I have seen her reach 10 kt and hold it for minutes on end.
The 35 hp motor will move her at 8.5 kt, flat out, in flat water, no wind. I don't hold that for long on the old engine though.
If we add more propulsive force to any hull shape and any displacement, it will always go a little bit faster. It has to to obey the action:reaction law of that fella Newton.
So treat hull speed with caution.... it is a somewhat nebulous concept.
Rockter is offline

Old 10-05-2018
Senior Member

Join Date: May 2007
Location: Glen Cove, NY
Posts: 4,551
Thanks: 5
Thanked 205 Times in 197 Posts
Rep Power: 13

Re: Boat thrust calculator?

Not sure why you think you need thrust. Note that the propeller thrust will equal the hull drag, and as others have said, this is very complex and is not something you can calculate by just plugging into a simple formula. And the BIG variable is the prop. Sailboat propeller efficiencies are pretty low, many in the vicinity of about 50%.

You are better off using one of the rules of thumb to estimate horsepower and then get help selecting a prop that matches your installation. I'm guessing a fixed 3-blade will be the most efficient, but will hurt your sailing performance.

If you really want a thrust estimate, the following from high school physics (well maybe freshman college statics) could be used:

Power = (Thrust x Speed)/33,000

Power would be the engine horsepower at that operating point times propeller efficiency and speed would be in feet/minute. The thrust will be in lbs.

S/V First Tracks
1985 Cal 33-2
JimsCAL is online now
Senior Member

Join Date: Oct 2014
Location: Deale, MD
Posts: 375
Thanks: 0
Thanked 2 Times in 2 Posts
Rep Power: 6

Re: Boat thrust calculator?

Better propeller efficiency is my primary goal. Looks like "bigger and slower" are the key to better efficiency. I saw a guy selling electric outboards at the Annapolis show last weekend that were *considerably* more efficient than their gas counterparts. His max prop speed was 600 rpm. He was using a fairly big, relatively skinny prop with a high pitch, and it worked beautifully. It allowed for a tremendous amount of thrust from a far smaller package than I could have believed possible.

I need to know the thrust so I can reverse the calculation and work out the pitch and size of prop required to push my boat at cruising speed at 600 rpm for a given horsepower. This might lead to a prop diameter bigger than I can safely put on my boat, but that's OK. There are always compromises. I'm aiming for a best-effort rather than sacrifice-all-for-efficiency dynamic.
Captain Canuck is offline
Old 01-28-2020
Junior Member

Join Date: Jan 2020
Posts: 1
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 0

Re: Boat thrust calculator?

Captain Canuck did you ever get your answer? I'm also looking to re-design the propulsion system for a catamaran and it seems the answers aren't very straight forward. I'll be changing the motor and prop and I have an infinite window of options so I need to start the same as you, with thrust/torque required to push the boat at hull speed and work back from there to determine RPM requirements and HP at those RPM's. I'm even working in cad to design and machine my own prop based on those specs so I'm not even limited on prop design or specs.
Bob Rubano is offline

Message:
Options

By choosing to post the reply above you agree to the rules you agreed to when joining Sailnet.

## Register Now

In order to be able to post messages on the SailNet Community forums, you must first register.
Please note: After entering 3 characters a list of Usernames already in use will appear and the list will disappear once a valid Username is entered.

User Name:
OR

## Log-in

 Currently Active Users Viewing This Thread: 4 (0 members and 4 guests)