Hey Rick,

 

I've had to go through this a couple of times... to try to plan my campaign.  Thank you for taking the time.  It seems a shame this yahoo group is so linear... I still haven't found a way to search on past information.  I'm sure there is a treasure trove here, and items like this should be invaluable tools for now as well as other people in the future.

 

At the moment there are so many things I want analyze, avenues I want to explore... I'm damn near in brain lock.  I am trying to fix some things so I make progress.  I've reached critical mass and have come back from the Dark Side.  Its decided... it will be a HarryProa style.

 

My major concern for this thread was merely to consider seperation of the two hulls and wave interaction.  I want to evaluate the case as the boat accelerates toward the max speed condition and flying the windward hull.  I want to make sure I don't accidentally pick a seperation that causes some major drag spike that can't be overcome even though once above it, the boat would normally have kept on accelerating up to a far higher terminal speed.  You're contention in #7 is good enough for me at the moment. 

 

Anyway at the moment, I'm crunching on your and Todd's symmetric rudders.  It would make several other problems I have simply go away.

 

Dennis

 

BTW, What do you do for your day job?

---

From: Rick Willoughby <rickwill@bigpond.net.au>
To: harryproa@yahoogroups.com.au
Sent: Tue, August 3, 2010 8:32:52 PM
Subject: Re: [harryproa] Godzilla advice...

 

Dennis

Building a reasonably precise model of the boat that will cover all points of sailing over a range of conditions takes a lot of time.  A good starting point is to look at a few conditions, working through each element to determine what needs to be modelled precisely and what are second order factors that may be modelled more simply or even neglected.

Building up a VPP from all the individual elements is not too hard that I will explain if you get that far. The first step is to go right through a single iteration to see what needs precise modelling and what can be done more simply or just factored.

The following is how I view where you are now:

1.  You have a reasonable shape for the lw hull that is long and slender.  Once you get the idea of long and slender there are diminishing returns in playing with the hull shape.   The deck profile and wave clearances will be bigger factors in real life performance.  So far you have only considered the drag from the lw hull.

2.  I expect trim changes will be secondary given the Flotilla analysis.  It seems that at the highest speed you should be able to get the hull to run near flat or maybe even slight bow up.  For the long and slender hull the trim change is not going to impact drag much.

3.  I expect you will find that having masts on the ww hull will cause significant yaw moment at higher speed when on the wind.  This means the rudders will need to be bigger and add more drag than if the drive was more in line with the drag.

4. Two large asymmetric rudders that can be trimmed independently should be the best means of controlling leeway.  This means that you do not need to consider the induced drag on the hulls - there does not need to be any.

5.  Roll is an important factor from the point of view of loading on each hull. I suggest the initial iteration to look at is close hauled at the speed and displacement distribution you would like to operate at.  

6.  The list on individual hulls will be a secondary factor that does not vary a lot given the overall beam and relatively shallow draft of each hull.  I have no way of calculating the drag on a hull with list.  I do note that the amas on the big tris have an initial list so they run in trim when the main hull lifts. 

7.  My expectation is that wave interaction between hulls will not be a significant factor at your speeds of interest when sailing.  Most likely case will be motoring when the ww hull is heavily loaded or similarly if you choose to sail directly downwind.  In any case the righting moment and weight distribution will be a more compelling driver of  separation.  Can check for these cases and determine if it needs to be considered.  If not then you treat each hull individually in the VPP.

8.  Having low drag hull means the other drag components become more significant.  Windage could be similar to the hull drag when on the wind.  I have not bothered doing precise windage analysis.  I usually look at the shape and apply a Cd for the frontal area based on similar shapes:

http://www.aerospaceweb.org/question/aerodynamics/q0231.shtml

http://www.engineer ingtoolbox. com/drag- coefficient- d_627.html

9.  If you have two sails that are separated by less than say 10 chord lengths then they cannot be treated in isolation.  Their influence on each other needs to be accounted for.   Javafoil is a useful tool for this.

10.  As far as I know there are no freely available analytical models for handling added drag from ambient wind waves - Leo is working on something but nothing yet available from him.  My own testing shows that getting wet is the limiting factor on a slender hull.  This is not just from spray.  If you keep the reserve buoyancy in the ends of the hull low then the hull easily drives through waves without decelerating.  The issues that eventually slow you down will be how much of the boat you are driving through water upwind - if it is not streamline it will slow you down.  Off the wind it is the risk of burying into the back of a wave and diving.  In conditions less extreme added resistance from waves is about 10% above normal hull drag for a slender hull.  There is some good empirical data for sailing boats in the Delft series but this is for fat hull.  See if you can get hold of the 1998 Sonnenberg Keuning paper on the Delft series that covers wave added resistance - I have the paper but cannot find a link.  Most of their stuff now is anlytical rather than empirical.

So the first step is to do one iteration through looking at the sailing conditions you would hope to achieve most often.  Start with the weight distribution with a good safety margin and determine the lowest drag hulls for this condition.  If you have already settled on the lw hull then find the ww hull underwater shape that gives lowest overall drag.  I did do some work on looking at what the lowest drag ww would look like.  It has a straight centre section but quite a lot of rocker in the ends so the waterline length increases as it is immersed.  

If you develop a reliable analytical model for the boat then you can assess the merit of various ideas quite quickly.  This overcomes endless speculation.  Real world observations are invaluable in validating the model but sometimes you find that the conclusions from observations are wrong.  Being able to model it analytically from the basic physics is powerful in improving understanding.  In my paid work I have found analytical methods to be invaluable.  Also pilot scale work is extremely useful if you want to avoid high cost failures. At pilot scale you can quickly assess how all the factors come together.  Arguably a decent size working model is not too bad for learning either but for hydrodynamics the scale factors are not simple.  

Rick 

Look at what the lowest drag hulls are for this with and without wave interationOptimise the hulls for th  

On 03/08/2010, at 11:23 PM, Dennis Cox wrote:

 

This would primarily be addressed to Rick, but if anyone else might have some ideas, I'd certainly be interested.. .

 

I started getting pretty comfortable with Michlet, but Godzilla I had far less exposure.  It seemed to barely improve on the hull that I eye-balled while drawing up in DelftShip.  However, I get the feeling that I wasn't using it to its full potential and didn't really give it the free reign it would have preferred.

 

In those studies, I was mainly shooting for the "max speed" condition with the windward hull flying and the leeward hull taking all the load, but not having to contend with waves from the windward hull.

 

Now, I'd like to do some analysis using the hulls as they are designed, but varying the, speed, roll angles and pitch angles... and mainly trying to optimize the hull seperation based on the wave interactions. 

 

Would anyone have an example that would give me a jumping off point?

 

Thanks,

Dennis

Rick Willoughby

rickwill@bigpond. net.au

03 9796 2415

0419 104 821