Full Keel Vs. Fin or Modified Full Keel - Page 6 - SailNet Community

   Search Sailnet:

 forums  store  


Quick Menu
Forums           
Articles          
Galleries        
Boat Reviews  
Classifieds     
Search SailNet 
Boat Search (new)

Shop the
SailNet Store
Anchor Locker
Boatbuilding & Repair
Charts
Clothing
Electrical
Electronics
Engine
Hatches and Portlights
Interior And Galley
Maintenance
Marine Electronics
Navigation
Other Items
Plumbing and Pumps
Rigging
Safety
Sailing Hardware
Trailer & Watersports
Clearance Items

Advertise Here






Go Back   SailNet Community > On Board > Boat Review and Purchase Forum > Sailboat Design and Construction
 Not a Member? 


Like Tree1Likes
Reply
 
LinkBack Thread Tools
  #51  
Old 11-15-2013
Jeff_H's Avatar
Moderator
 
Join Date: Feb 2000
Location: Annapolis, Md
Posts: 6,564
Thanks: 5
Thanked 93 Times in 70 Posts
Rep Power: 10
Jeff_H has a spectacular aura about Jeff_H has a spectacular aura about Jeff_H has a spectacular aura about
Re: Full Keel Vs. Fin or Modified Full Keel

I am not sure whether you are talking about rolling in a short chop taken abeam or larger breaking waves. In either case, it depends on the specific design of the boat, its loading, and the shape of the wave in question. In a chop abeam, there will be situations were each type would be superior but a long keel with cut away forefoot would generally be worse in all cases. Iin a large, breaking wave taken abeam, except a long keel with cut away forefoot, the reality is that each keel type has benefits and liabilities.

The case is often made that a full keel is safer in a roll situation since it is typically shallower than most other keel types and so experiences less wave surface sheer forces (the force that results from the difference in speed between the water on the surface of the wave and in the center of the wave). But full keels generally have a lot of side area and does not stall so that area tends to generate a lot more rolling force even if the wave sheer speed difference is smaller. Typically full keel boats have smaller form stability so they have less force trying to initially roll them into conformance with the wave face (good) but that same lack of form stability means that there is less force trying to right them in the trough (bad). Traditional full keel boats tend to have large roll moments of intertia which resists rolling the boat at the start of the incident (good), but which may tend to cause them to continue rolling at the trough (bad) and since the roll moment of interia often comes from heavy spars, that tendency gets amplified (bad).

Probably the best choice in a large breaking wave taken abeam would be a keel-centerboarder with the board up. It would have a shallow keel and so would experience the least wave surface sheer, and has a small side area so the combined side forces would be smaller as well.

The deeper fin typically has a lower vertical center of gravity and a smaller area working in its favor. But being deeper the fin keel is exposed to a greater speed difference between the wave face and bottom of the keel due to sheering of the wave face. Mitigating against that higher speed is the tendency of deep fins to stall at large angles of attack which by definition a wave abeam would be. Then there is role of roll moment of inertia. Fin keels especially with a bulb when combined with the typically taller rig on a modern design produces proportionately high roll moments of inertia which would initially minimize the induced roll angle.

Probably the worst option is a long keel with cut away forefoot and an attached rudder is the worst option being deeper than a full keel but unable to stall as easy as a deep fin, and with a higher vertical center of gravity. All bad...
mitiempo likes this.
__________________

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay and part-time purveyor of marine supplies

Last edited by Jeff_H; 11-15-2013 at 08:22 AM.
Reply With Quote Share with Facebook
  #52  
Old 11-16-2013
Junior Member
 
Join Date: Nov 2013
Posts: 3
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 0
vtoz7053 is on a distinguished road
Re: Full Keel Vs. Fin or Modified Full Keel

wow, um im an old sailor, too much detail, thank you, but i am confused. um i have a ericson26, with a short mast, keel i think is a fin and im going to sail her in the pacific. mostly with the wind (downwind), im a liveaboard. I have sailed a homemade square jig from maine to FL and back on a 23' wodden sail. not too big on education. ill figure it out as i go. or die, so im tring to figure out how bad i am going to rock n roll out there. i know she wanders a lot sitting at anchor in the bay, at ruff seas, and the wakes from other lovely boaters are mild. i only travel downwind (i like it slow) .
so what am i looking at .. strap everything down n hold on, or kiss my but goodbye or my coffee cup might fall over???
Reply With Quote Share with Facebook
Reply


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

 
Posting Rules
You may post new threads
You may post replies
You may post attachments
You may edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Fixed or swing keel, which is better? TimG2 General Discussion (sailing related) 17 4 Weeks Ago 09:23 AM
My First Boat...Boat Term Question... Kacper General Discussion (sailing related) 38 09-23-2008 06:52 PM
Wing, fin, or bulb...what are the trade offs? langousta Gear & Maintenance 17 09-17-2008 04:54 PM
Keel Anxiety! Spang Boat Review and Purchase Forum 8 06-19-2006 03:09 PM
Short Keel vs Normal SEAJM General Discussion (sailing related) 1 03-12-2003 02:13 AM


All times are GMT -4. The time now is 03:11 AM.

Add to My Yahoo!         
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
SEO by vBSEO 3.6.1
(c) Marine.com LLC 2000-2012

The SailNet.com store is owned and operated by a company independent of the SailNet.com forum. You are now leaving the SailNet forum. Click OK to continue or Cancel to return to the SailNet forum.