Sailing Dinghy Design

Here is a link to the SDN homepage. I wish it would run on Windows 2000/XP so I could see what it is like but that's another topic.

 

To run on Windows XP Pro, select the application (or a shortcut), click right button, goto the 'compatability' tab, and choose 'Run with compatability for... Windows 95'.It will then run under XP Pro. Whether something similar will work on XP Home and 2000/ME I don't know, but that's how I did it.

Cheers,

Tim B.

 

I noticed that on the previous page Phil S gave some comments about the 16 foot moth derivative that is now sailing. Has anyone got any photos? I'm still working on my own take on the idea (14ft, 2.5m beam for towing and righting moment, 10m2 of pocket luff sail...) and will post some renderings shortly. I certainly wonder if wide wings, masses of sail and a trapeze may be a step too far for control of a narrow waterline hull, and that sitting out in a lighter, more efficient boat may be a better solution. Refinement of the rig package would be my priority here. Any suggestions?

Incidentally, Phil S, were you just going to stick your moth hardware on a longer hull? I would guess that the boat would almost certainly be slower, as for the same power available you would have a heavier boat with more wetted surface area. It may be less slow than one might think though, especially as you go up the speed range. Bearing in mind that beam increases as length to the power of 0.7, the longer boat would have less wave making drag and so would be faster in heavier conditions. Add a bigger rig to couteract the increase in wetted surface and displacement, and wider wings to balance the bigger rig, and I would guess that the bigger boat will end up substantially faster than the smaller one in all but the very lightest of conditions. Phil S's construction method may just be the ideal way of building a prototype for this 'new' class...

Andy

 

No photos of Bruces big moth. Yes I think he went too far and consequently has control and stability problems.

My thoughts were to chnage only the hull and use all moth hardware so as to minimise cost and be able simply try the idea. No construction yet maybe next year, the family are enjoying the absense of boatbuilding at home.

I also think that 14 ft would be a better size. Easier to handle in and out of the water. 10 sq m would also be better to help the big people. But I am wary of increasing mast length too much. I tried a moth mast 500mm longer once and it made a huge difference to stability even in moderate weather.

The wetted surface penalty might not be as bad *** you might think. Without calculating I suspect that it would float higher enough to reduce WS.

Andy, I spent a year in Scotland once and I think you must be very hardy to go out and get wet in that climate.

 

If you look back a few pages to some graphs I posted from Michlet, they suggest the longer boat would be faster even with the same size rig in light to medium winds where wavemaking drag dominates.

If the width of the boat is the same, making it longer increases the wet surface area. The WSA numbers i got for moths of various lengths are as follows:

9ft Moth - 1.28m2
11ft moth - 1.57m2
14ft moth - 1.87m2
16ft moth - 1.91m2

Wet surface drag increases as speed squared, so the advantage is lost at higher speeds - but you have the advantage of being able to drive the boat harder without risking pitchpoling so much.

That said, I'm convinced the longer moth will be a faster boat most of the time, but may not have the same top end speed as the current moths.

 

Nice to hear some more thoughts. Phil, your comment about the increased rig height is interesting - perhaps beefing up an existing mast and adding a jib on a carbon boom (self tacking and more efficient when reaching) might add the extra power without raising the centre of gravity or centre of effort too much. On the other hand, the longer boat would have more static stability due to a waterline beam increase, and more righting moment when sailing due to larger overall beam.

About the wetted surface - using Barkla's suggested scaling method we can see that if we assume a scale factor of L between two similarly proportioned but different sized hulls, then wetted surface area increases as L to the power of 1.7. I'll play with my Rhino model in the next couple of days and try some different sizes using the corresponding displacement scaling factor to see if this is right, and I'll post the wetted surface and so on.

Mad Engineer - Phil S noted that the displacements of the boats you compared were all the same. have you run the tests with more realistic displacements? I would then look at the way the wetted surface drag and the wavemaking drag change with speed. The total drag I would guess would be higher no matter what, but would the wave making drag be comparatively less than for the shorter, comparatively fatter hull moth? I think a dimensionless ratio of total drag to length plotted at different speeds would be a good comparative tool here. That would give us an idea as to the comparative efficiency of two designs, and give a clue as to the powering requirements as well. Anyone got any ideas? Incidentally, there are a few nuggets worth pondering here http://www.kelsall.com/Kelsall Formulae.htm
and here http://www.steamradio.com/JSYD/Articles.html

Also, the more I think about it the more I wonder if a bigger moth might not be faster in all conditions except a drfting match (where wetted surface and overall boat drag are dominant). I don't think anyone has ever taken a Tornado anywhere near 45 knots (and lived to tell the tale), and yet that is the top end speed Gilles Ollier is talking about for Bruno Peyron's new mega catamaran. Bigger boats seem to generally be more efficient, even if they are mostly less refined. Look also at Alfa Romeo - 35 knots on a good day! The cat hulls operate more like a moth hull tho.

Finally, Phil S - yep Scotland is b*/*/*d cold and wet at times. If anyone knows of a good source of space suits then I'd definitely be interested...in the meantime I'll keep sailing in my thermals. That said, if you get the weather right its still in my opinion the most beautiful place in the world to sail, and the beer is cheap!

Andy

 

I14

Ok, the I14 is back on route again, after a short stop due to the lack of both a fast computer and good CAD Package. Unfortunately, I've not got it right with Version 4, so it's back to the CAD package for Version 5. The following rendering shows why version 4 is not going to work (not well anyway)... I should have a version 5 by the end of next week (or this week coming if you're really lucky.)

Cheers,

Tim B.


Version 4 is now floating on it's revised lines at a displacement of 160Kg, which I still consider to be too light, but it should be somewhere near.

 

Ok, a (very) rudimentary I14v5 half-hull, has anyone got any comments, if so, I'll adjust things now rather than later. This was done in Rhino 3 incidentally, surfaces are not too bad on this hull, but are not yet smoothed.

Cheers,

Tim B.

 

Displacement wise I reckon you need to be closer to 230Kg - allowing 75Kg for each crew member, minimum hull weight of 74.25Kg plus all ropes, mast boom, sails etc.

Any chance of posting some sections (similar to Jim Walsh's Woof sections back on page 3 or so) and some water line plots?

The bow looks a bit odd on v4 but that may just be the angle of the image...

 

Yeah, you can see the reason for my dislike of v4. I've got to a half-reasonable hull-shape now in v5. I'll post some sections for you to ponder. I've had to tighten up the aft fillet (read get rid of it) to get a decent surface. Looking at the rules, the min weight excludes mast, boom rudder, centre-board etc. but includes the spinnaker pole and associated gear. I was allowing about 60kg per person (9.5 stone), checking it in the hydrostats claims only an inch extra draught to 230Kg, the transom will be deeper than that anyway. I don't really want to get into playing with the keel now, I quite like it. I am considering hanging the rudder under the boat, rather than off the transom. What do you think?

 

Sorry Tim but your drawing gives me the impression of a 505, which was designed in 1954. Its the fullish bow, smallish stern and round bilge that does it to me.
You seem to have excess V in the keel for a modern planing boat. You need to present a flatter area to the water for quick planing and also to lower the volume and float it higher. This is especially important if you are designing in a rule with chine measurements like the I14, NS14, Int Cherub etc. You need to lift the wide chines as high out of the water as possible by adding volume down low.

By the way we raced 6 Moths in a mixed fleet at Lake Macquarie on the weekend with some dozen or so top 505s, a few less Skates (14ft twin sliding seat asymetric for the non Aussies) and a some Cherubs.
2 races in 20-15kts on 505 course: triangle, return, triangle. Places were a bit like 505, 505, moth, 505, 505, skate, moth, then a mixture of 505s, skates with Moths interspersed, and Cherubs later. On the reaching legs the Moths covered enough ground during the other's spin hoist and lowering process to minimise losses, but were passing the big boats upwind. The top Moth easilly won on yardstick of 104.

 

Phil,

That's a pretty good indication of the Moth strengths...

By the way, in those sort of winds are the Moths fully powered up on the reaches?

Tim - I agree with Phil's comments on your sections, I would also worry about the hull curving in to the stern causing the boat to steer itself when heeled, which at I14 speeds could be fairly dramatic...

Does anyone out there know hoe to get sections out of Hullform in a presentable format?

 

I 14 design

None of the sketches I've seen in this thread look anything like a modern I14, or any skiff.

Look here (http://www.i14.org/ovi/) to see the latest from Paul Bieker, the B4. Scroll down and pay particular attention to the first photo, the one of the stations without any skin. That is what you should be looking at for your design.

The desire for keeping the boat as narrow as possible to reduse drag while maintaining the required rise of floor results in a very slab sided shape.

You folks need to get back to square one if you are serious about this project.

 

Paul,
So tell us - who pissed in your cereal this morning?
;-)

 

Paul B,

I hate to seem somewhat cutting, but,

1) From what I can see on the I14 site you quoted, and certainly from your rendering, the waterline beam is certainly greater on the Bieker design. The problem with that is the wave drag at low speed. Slender Bwl, lower wave drag.

2) I would disagree with your slab-sided theory. Geometrically, one wants the section rule to intersect (lie in) the hull surface. That green point on my I14 (amidships) is just marginally inside the hull, to cover measurement in-accuracy.

3) If you are suggesting we get back to square one, then please read the rules well and tell us where we should start, with reference to well-known theory, or appropriately derived equations.

Anyway,

Mad and Phil,

I know the bow is quite full at present, hence that render is preliminary. Any heeled boat (except a barge) will have an assymetric waterline. This is what gives rise to the moments causing the effects you mentioned. I'll carry on with that in my next post.

Cheers,

Tim B.