| Subject: Re: [harryproa] My Little Mule (sketch) [1 Attachment] |
| From: Rick Willoughby |
| Date: 8/22/2010, 6:00 PM |
| To: harryproa@yahoogroups.com.au |
Reply-to: harryproa@yahoogroups.com.au |
Rick,Updating VPP - I've uploaded the fdm files to the files section. Thanks Rick.What a drag - Jeeze! That bad???? 15%! I pretty much figured it was bad and will eventually fair it... just not that bad. This sketch was a 3 AM throw together and really meant to lay out the structural geometry... I'm at the stage of the lee hull build that I needed to find the pickup points for the beams and mast step structure. Unfortunately, wedge section or rotating the beam 45 degrees greatly reduces the bending stiffness and strength and would have to be compensated with lots more material. It will be far easier structurally, lighter and fabrication wise, and far superior aerodynamically to add foam, faire and put a light 4oz shell on it. Besides, drawing a 3D blended curve using Google SketchUp is a bitch.BUT! For 15% I may have to re-think those positions very quickly. I can see myself getting it out on the water before our season ends here in the Northern Hemisphere if I launch as the sketch shows. Then add fairing during the winter. Which brings back the old question... of Instrmentation. Any ideas on how to document a before and after? I would really like to quantify and document that kind of gain.
Aside - Although sent to Rick, please, ANYONE chime in. John, Skip and Doug's contribution about the UHMW or Toilet Dividers is just the kind of back yard engineering that can do the job. I know when I read other people's messages, I don't feel like I'm qualified to answer. Maybe... I need to respond more myself.Dennis
From: Rick Willoughby <rickwill@bigpond.net.au>
To: harryproa@yahoogroups.com.au
Sent: Sun, August 22, 2010 7:43:42 AM
Subject: Re: [harryproa] My Little Mule (sketch)
Dennis
If you are decided on the hull shapes I can update the VPP with those shapes using your fbm files if you want.The windage on your beams and diagonals as drawn will cost you 15+% of the sail resolved thrust when going to windward, which is more often than not when you are sailing near wind speed considering the apparent wind. If those flat surfaces clip waves they will really slow you down. You should look at faired shapes like back-to-back wedge beams. Just rotating the main beam 45 degrees will reduce Cd to a fraction of the flat face.Rick
On 22/08/2010, at 6:43 PM, Dennis Cox wrote:
This is my current thinking... although, I want to re-check my numbers based on Rick's last message about using an aspect ratio of 7 to 8 and see if how much I can reduce the size of the rudders. Since I have no way of doing a non destructive rudder strike, I will be evaluating that some. Its configured to articulate the hulls and I have some ideas to restrict that motion, but they're not in the drawing. Windward hulls been analyzed with Godzilla is about 10% more drag than a semi circle cross section... again... hopefully, can get it planing early and overcome that deficiency. Can always sit on back of windward hull (with the articulation) raise the nose to encourage planing. With the big flat bottom, expect it won't be too difficult. Although it will be a bumpy ride!