Engine Size - SailNet Community
Reply
 
LinkBack Thread Tools
post #1 of 11 Old 10-11-2014 Thread Starter
Member
 
Join Date: Sep 2011
Location: Stevens Point, WI
Posts: 78
Thanks: 10
Thanked 2 Times in 2 Posts
Rep Power: 9
 
Engine Size

Is there a formula for how much HP you need per foot of boat? Just got a 24 foot and have a 4hp outboard Mercury. I assume that will be fine.
wallster is offline  
Sponsored Links
Advertisement
 
post #2 of 11 Old 10-11-2014
Senior Member
 
Faster's Avatar
 
Join Date: Sep 2005
Location: New Westminster, BC
Posts: 19,488
Thanks: 160
Thanked 587 Times in 558 Posts
Rep Power: 17
     
Re: Engine Size

It's more about the weight and the intended use. A 4 hp outboard is going to be fine for a 24 foot sailboat, esp if the primary use is simply getting out of the marina/ramp area to where you can set the sails, and the occasional return trip if the wind dies.

For more prolonged trips against wind and tide you may wish for a bit more power. Time will tell. In the meantime you'll appreciate the lightweight 4 hp.

Ron

1984 Fast/Nicholson 345 "FastForward"

".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
Faster is offline  
post #3 of 11 Old 10-11-2014
Senior Member
 
Join Date: Oct 2009
Location: Virginia
Posts: 1,598
Thanks: 4
Thanked 37 Times in 36 Posts
Rep Power: 11
 
Re: Engine Size

Faster is right on this. In most cases, 4 is enough. If you were boating in strong currents, 6 would be better. Anything over 8 would really be overkill.

Catalina 34
chuck53 is offline  
 
post #4 of 11 Old 10-11-2014
islander bahama 24
 
newhaul's Avatar
 
Join Date: Feb 2010
Location: bremerton wa
Posts: 1,842
Thanks: 7
Thanked 61 Times in 61 Posts
Rep Power: 11
 
Re: Engine Size

The formula usually works out to be two hores per half ton on a full keel boat a trailersailer about half that will work just fine

Illegitimus Non Tatum Carborundum.

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
newhaul is offline  
post #5 of 11 Old 10-12-2014
Learning the HARD way...
 
eherlihy's Avatar
 
Join Date: Jan 2007
Location: Boston / Ft Myers Area
Posts: 7,076
Thanks: 402
Thanked 320 Times in 311 Posts
Rep Power: 14
 
Re: Engine Size

I have this note ferreted away. I found it somewhere in my online reading.

A Diesel Engine produces about 20lb of thrust per horsepower

Wind resistance for a 35' boat (my boat) 225lbs of thrust at 15 knots, and 1200lb at 30 knots. A smaller boat will require less thrust per given wind speed. (there is a chart somewhere)

Detail; the very approximate calculation goes like this:
Take the engine power in Watts. If it is given in HP, then multiply by 746 to obtain the power in Watts. Call this power BSP (brake shaft power).

Multiply BSP by transmission and propeller efficiencies. You can assume around 0.5 as a maximum total efficiency of such a small prop. Call this power PE (effective power).

Convert the speed to meters per second (m/s). To do that, multiply the speed in knots by 0.514 . Then multiply again by 0.95, to approximately take into account the eventual effect of the hull wake. Call this speed Vp.

There might be no wake effects if this is a planing hull with props clear of obstructions ahead. But still I'd keep that 0.95 factor above, because it will give a safety margin to the calculation of the thrust.

To obtain thrust given by the prop, divide PE by Vp: T = PE / Vp .

This will be the available thrust in Newtons, for that speed and engine power. I'm using all SI units, to avoid errors related to inconsistent units. To convert this thrust to lbs, multiply by 0.225 .

That's all.

If you think you'll need to do more such calculations in future, and/or have them more precise, I suggest you to buy the excellent Dave Gerr's book "The Propeller Handbook".


To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

USCG Licensed OUPV Captain, US/Sailing BKB and ASA 101/103/104/105/106/114/118 Instructor - Also ABYC certified in Marine Electrical Systems,


To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
eherlihy is online now  
post #6 of 11 Old 10-12-2014
Banned
 
Join Date: Jul 2007
Location: Truckee, CA
Posts: 8,867
Thanks: 2
Thanked 117 Times in 115 Posts
Rep Power: 0
 
Re: Engine Size

Quote:
Originally Posted by wallster View Post
Is there a formula for how much HP you need per foot of boat? Just got a 24 foot and have a 4hp outboard Mercury. I assume that will be fine.
Looks like you may be on a lake? Depends on how much headwind & chop you may encounter? The 4HP should work in all but heavy wind & chop conditions?

Just put it on & see what happens.

Paul T
dabnis is offline  
post #7 of 11 Old 10-12-2014
Barquito
 
Barquito's Avatar
 
Join Date: Dec 2007
Location: Madison, WI
Posts: 3,447
Thanks: 1
Thanked 93 Times in 91 Posts
Rep Power: 13
 
Re: Engine Size

Your Rob Legg 24 is a really light boat. I think the manufacturer said 4-10hp. I would think anything in that range that you come across for a good price would be fine. Will help you finding a good deal with your boat, in that. you don't need a long shaft OB.

Valiant 32
Barquito is offline  
post #8 of 11 Old 10-12-2014 Thread Starter
Member
 
Join Date: Sep 2011
Location: Stevens Point, WI
Posts: 78
Thanks: 10
Thanked 2 Times in 2 Posts
Rep Power: 9
 
Re: Engine Size

Thank you everyone for your responses. I figured the current engine would do, was just curious if I should be keeping my eye out for a bigger one. Ill save a few $$ then and put it towards other upgrades or basic materials.
wallster is offline  
post #9 of 11 Old 10-12-2014
islander bahama 24
 
newhaul's Avatar
 
Join Date: Feb 2010
Location: bremerton wa
Posts: 1,842
Thanks: 7
Thanked 61 Times in 61 Posts
Rep Power: 11
 
Re: Engine Size

At 1500 lbs the 4 that came with it would be plenty spend the money on sails or some other upgrades
Here is the factory site for the boat http://www.rlyachts.net/index24.asp
An excerpt from the page Designed to sleep 4, the interior is open and clear of unnecessary bulkheads to provide comfortable living. The midship toilet, galley and icebox areas are convenient and the cabin can be subdivided if required. Auxiliary power is provided by a 4 to 10 hp standard shaft outboard sited in a special well designed to prevent cavitation.

Illegitimus Non Tatum Carborundum.

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

Last edited by newhaul; 10-12-2014 at 04:20 PM.
newhaul is offline  
post #10 of 11 Old 10-12-2014
Don't call me a "senior"!
 
Puddin'_Tain's Avatar
 
Join Date: Feb 2014
Posts: 968
Thanks: 0
Thanked 32 Times in 29 Posts
Rep Power: 7
 
Re: Engine Size

My old boat was about 1500 lbs, and I used a British Seagull (about 2 hp, or was it 1 gull-power?) to push it at hull speed in flat water.

"If a man must be obsessed by something, I suppose a boat is as good as anything, perhaps a bit better than most." --- E.B. White
Puddin'_Tain is offline  
Reply

Quick Reply
Message:
Options

By choosing to post the reply above you agree to the rules you agreed to when joining Sailnet.
Click Here to view those rules.

Register Now



In order to be able to post messages on the SailNet Community forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
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:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in











Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page



Similar Threads
Thread Thread Starter Forum Replies Last Post
engine size kms Gear & Maintenance 27 02-03-2016 12:07 PM
engine size blarg21 General Discussion (sailing related) 20 10-27-2012 11:53 PM
Engine size on Pearson 34 needsanap Boat Review and Purchase Forum 8 07-14-2011 01:20 PM
Is bigger better? (Engine size) cclark Newport 2 03-06-2009 05:05 PM
Proper engine outboard size Slickerish Cheoy Lee 1 07-19-2007 01:49 PM

Posting Rules  
You may post new threads
You may 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 On
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome