Subject: Re: [harryproa] MLM Trim [1 Attachment]
From: Rick Willoughby
Date: 9/9/2010, 9:18 AM
To: harryproa@yahoogroups.com.au
Reply-to:
harryproa@yahoogroups.com.au

Dennis
The attached is trim with CoE at 3.5m above waterline.  Made some allowance for the windspeed gradient.  I have not included the rudder drag in the trim either but then it should be small.

Flotilla produces the actual trim moment so most of the data is in the output file.  I just scale the difference so I do not have to bother with the KMl.  The output file has a lot of extra data that I do not bother with. I do not know if Leo is using all it produces.  It is interesting that the Flotilla data does not show any reduction in drag although the hull does lift overall above 15kts.

You will be getting lift from the wave making so it does counter some of the pitch down moment from the sail drive.  It now shows in balance at 10kts. I have not gone through the iterations with the sail pitch moment applied.  I expect what I have shown will therefore be worse because the pressure at the bow should rise more as the bow is forced down by the sail.  

Once the hull is in bow down trim then planing force will be small because most of the hull bottom will be angled downward.  The lift on the small flat entry area will not be much but it has a good lever.

Your hull has rapidly diminishing KMl as the bow submerges.  The slightly flared hull I have used has more constant KMl as the bow goes down.  I take the view that if you are going to have some reserve buoyancy that will slow the hull down it may as well be used to maximise the bow up before it submerges.  I was really pleased with the way V15 worked with the limited testing I did in some waves. 

I use marks and video them in operation to confirm the Flotilla data.  I also have had some discussion with Leo on whether the reduction in water level along the hull should be taken into account in the viscous drag calculation.  Flotilla has no allowance for this but it could be done.  I do not think you will have water level around the hull stable enough to get much meaning but some marks on the bows will certainly be good reference when analysing video.  The main observations will be how close it performs to predictions.  Most often things are worse because it is easy to introduce extra drag.  Much harder to reduce drag.  

On a positive note the analysis is done at 300kg.  A lighter boat will have more reserve and much higher KMl to start with - around 50m at 200kg down to 35m at 300kg.

Rick


On 09/09/2010, at 9:38 PM, Dennis Cox wrote:

 

Rick,
 
Any trim up is a good thing in my book... I'll take whatever I can get.
 
I just double checked my COE. 
 
The area centroid is 3.21 meters above the bottom of the lee hull.  This includes the main with roach and jib.
 
I have a paper theorizing the wind speed gradient as a function of height which would drive the COE up.  But since the sail warps, it would seem that the top portion doesn't produce as much lift as it would normally... thus driving the COE down.  So are there some other fudge factors used based on the area centroid?
 
Would it be possible for you to run that curve using 3.21 meter COE?  BTW - Does Flotilla do all this processing... or do you have to do some subsequent analysis to get this curve?
 
Also, if I put depth marks on the lee hull, do you think they'd be visible and worthwhile if I video'd them at speed?  It would be good to correlate your numbers with testing.  We might be able to come up with some empirical method to describe the dynamic lift contribution of planning.
 
Dennis
From: Rick Willoughby <rickwill@bigpond.net.au>
To: harryproa@yahoogroups.com.au
Sent: Thu, September 9, 2010 5:44:55 AM
Subject: [harryproa] MLM Trim [1 Attachment]

Dennis
Attached shows the trim you can expect from MLM with respect to speed.  

The curve is based on CoE at 4m above waterline (maybe low) and all weight carried on lw hull - so a limit condition but not necessarily the worst state particularly if you articulate the ww hull and it has its aft end bouncing along.  This will push the lw hull even more bow down.

If you ever see the top end of the speeds shown then nosedive will be a possibility.  In the range 7 to 12kts MLM should run with almost level trim.  

The trim will be a limitation for the height of rig you can use effectively.  So this brings up efficient low aspect rigs

It is common to see airfoil fencing in some applications like these:
But fencing or winglets are not so common on yacht  sails although there are a few keels with them. 

Rick Willoughby




Rick Willoughby
rickwill@bigpond.net.au
03 9796 2415
0419 104 821