Rocker in straight sections

So simple.

 

A vertical "chine" isn't such a problem if there is a positive angle of attack to the panel behind it. It is where the discontinuity changes it abruptly to a negative angle, that turbulence will be created. That is going to be virtually unavoidable with the aft of a displacement hull.

 

Thank you again for your replies

I guess the question is - is the performance penalty worth the savings in build time/complexity?
I read a KSS build report saying the fairing took 4 man weeks to complete!

Performance might be in the eye of the beholder - As long as the 'perpendicular chines' didn't drag you back to monohull speeds I'd still be away laughing.

If the delft ship numbers can't be trusted how would you determine the penalty of the steps?

Cheers
David

 

NZ,

I tried writing this 3 times so you get the short version.

There is no saving in making the joint you suggest. It is actually harder than making long straight panels and bending them into shape.

Get a free copy of the Gougeons on Boat Construction. http://www.westsystem.com/ss/assets/HowTo-Publications/GougeonBook 061205.pdf
Lots of information on how to make smooth plywood boats.
There is no need to fair on a plywood boat for 4 weeks unless you are building an aircraft carrier.
Check out these sites which have boats built with long smooth plywood panels.
clcboats.com
pygmyboats.com
http://dixdesign.com/index.html (a large range of boat sizes in monohull and multihull)
http://sailingcatamarans.com/ (one of the forumites who tried to suggest issues with your proposal)

Last of all - go look at a boat yard and see if you see anything that has your suggested shape.

The experienced people on the forum are trying to save you from yourself, assuming you are not just trolling.

Every computer code has its flaws or situations it was never designed to handle. The only way to tell is talk to those who wrote it, or use it professionally, or go spend $5K+ to build a boat with your feature and test it against the prediction.

One last try, what kind of a boat are thinking about?

 

Hello upchurchmr

It appears I'm frustrating you so I just wanted to start by apologising. It was not my intention to irritate. I am genuine - not trolling!

Thank you for your replies and for taking the time. I have been listening to what is being said and in between posts I have been reading and following up on things people have suggested.

In my work we are constantly questioning what has been done before - this often leads to dead ends but sometimes to innovation. Again I wasn't intending to frustrate.

Thanks for the list of links - that's my evening sorted!
I have spent some time looking at Dixdesign and many hours on Mr Wood's excellent site.

I have only found one sail boat with a stepped hull and they were going for speed records - which is exactly what Richard and Rapscallion were saying.

I was thinking about a large cat - like Wood's Nimbus - 40ft or so. But I'm not building one just now

I'll try to explain what I thought was the advantage over the ply boats you pointed to and then I think I'll lie low!

As I understand it - over a large hull many sheets of ply must be scarfed/joined, each join sanded and then the whole thing covered in fg. This is then filled/sanded and then painted.
If an entire hull is made of 3-4 large sheets of precured and gel coated fg then the prep time, sanding time and painting time are drastically reduced. If the inner sheet of the sandwich is also gelcoated then you also have a pre-finished interior. I am floating the idea of buying the cured fg on a roll so there is no prep time at all.
(Folding the sheet the other way as you suggested makes sense to get a better shape but requires several joints the length of the hull which I thought would mean much more sanding than 2-3 joints in the other direction.)
The downside is obviously a non-ideal hull shape which you could say makes the whole idea pointless....*cringing* my question was really - how pointless? Would the average cruiser notice the difference? I understand that a step (even a rounded step?) will cause confused water behind it and therefore drag. If properly designed could the hull be reasonable or is it going to be a bike with square wheels?!
I think everyone is saying its a square wheel so far - perhaps I'll try talking to Delftship before I finally take heed....unless someone has $5k?

Richard Woods - thank you for your reply. I got a little lost with Cp and Fn but that just shows how green I am. Your website and articles are a fantastic resource - so thank you.

Rapscallion - the Harry Proa site was fantastic. I don't understand the implications of no rocker but was interesting to see how he builds them.
Sailorjim on these forums has some great photos of a folded sheet boat here - https://plus.google.com/photos/11143...CIuMxdnjhcvbUg

Mr Efficiency - I found this online after reading your reply about turbulance. It has nice pictures to explain it! http://speeddreamblog.blogspot.co.nz/2012/04/speeddreams-stepped-hull.html
The fg comes rolled up so I thought the radius might not be such and issue. Sailorjim has a nice solution for the bow. (see link above)

Sorry for the huge post!

Cheers
David

 

Thanks for replying to everyone, Davo, you could actually build some small models, with and without the "steps", and see what the resistance difference is, it should scale down pretty well. You could use something as simple as a torch-battery powered air fan to propel them over a fixed distance, and use a stop-watch to check the time difference.

 

Davo, meet Derek. And he lives in NZ

http://kelsall.com/TechnicalArticles/WhatIsKSS.pdf

 

Hi again

Mr efficiency - that's a great idea. We have a 3d printer at work so I could Cad the hulls and print them out. Should ensure there is no human error in the models. But I'd be restricted to about 150mm length - unless I glue sections together. What scale would you recommend to use to get a reasonable result?

Redreuben - thanks for the link. I actually thought I'd invented KSS about a week ago! Then I found Kelsall online and my dreams were shattered
But as I understand KSS uses slits and fingers to achieve compound form and thus requires filling sanding and fairing over large areas.

Cheers
David

 

Glue the sections together, and make something around 600mm long, at least. You should see a difference in favour of the boat without the transverse steps.

 

I'm pretty sure that David Barker used basically one "U" shaped mold to put Sundreamer and Stratosphere together. Apparently he just joined them end to end for the full length of the hulls. The mast beam and side pods look as if they came out of the same mold as well. Very clever man that DaviD Barker.

 

I don't think the boat in 13 would be all that bad. There would be drag at the joint, but in that pic, you are showing 2 joints. The straight sections between aren't optimal, or all that bad. You loose volume where you might need it, but it wouldn't be the worst.

People are saying that your idea sucks because the chines (already a compromise in most cases) are not aligned with the flow. While that is true, the chines on most hard chine boats are not aligned either, and there are often multiple chines, that the water flow takes long diagonal drags around corners on. All that assumes the boat is even going straight, which in sailing is not always the case.

The problem with your earlier drawing is the steps forward. That would be crazy, but as shown, it would be hugely hard to actually model, real steps like that required lots of 90 degree joints and risers. There have been step multihulls, one in a recent ocean racing adventure, but nothing forward, and the idea was that the boat would be driven at planing speed. On flying boats, and float planes, the steps are in line with the wings, give or take, because the support pad the boat is planing on has to be under the center of lift of the wing, more or less. Which is not an issue here.

I think the biggest issue is your motivation for making this design which seems to rest on the availability at some undisclosed price, of solid glass for some scale of boat. 6mm glass is not a good material for anything I can readily . And by the way, cutting it out, unless you have an enormous water jet, or somethings is a hugely unpleasant task that will convert your home and surrounding property into a super fund clean-up site. It is like mining asbestos. Plain old 6MM ply makes beautiful hulls up to about 30 feet, so solid glass would need to be for an unusual design situation since it will be crazy heavy and stiff.

 

People are saying this idea sucks because it is entirely unnecessary.

 

Hello ThomD and Redreuben

Thanks for your posts.
Sorry for my delayed reply - new baby means free time is at a minimum!

I thought using two smoothed joints would give a better approximation than the one sharp step and as others pointed out, the sharp step would be difficult to build.

The reason for looking at a continous roll of FG was that it 'seems' like less work than fixing and fairing ply every 2.4m. But the hull shape is 'sub-optimal'

I have started the CAD to make a 3D print of the hull. Was a bit of a mission finding CAD for an existing Cat to compare it with.

Will let you all know how I get on with my idea that sucks!

Cheers
David

 

Sorry - one more.

As a kid I watched Dad sand a 22ft glass boat he made and I remember the dust!
Pre-cured wouldn't need the finishing but you are right about the cutting. What about using a multi-tool. Looks cleaner than a grinding wheel -


Cheers
David

 

If you're really determined to build in this semi-stealth facet aircraft way, then I'd suggest joining the hull not in 3 sections but 5, meaning that you have 2 shorter flats at the junction of the major 3 ... and that will soften the abrupt connections. And then you could cheat a little by slightly rounding off those areas - which means sanding the scarfed joints.
I remember Malcolm Tennant drawing early versions of the Great Barrier Express where he had an abrupt change in the rocker a little aft of the hull's longitudinal centre, kicked up sharply from the near straight rocker line beginning at bow waterline, to run near straight to the transom. That way he got rocker, little stern drag (and fast tacking). Whereas Rob Denny has a completely straight rucker (tube) with shaped tapering foam for the two ends (to make a bow and stern - they're both the same shape). You need to check out his proa hull construction. That would be the closest to your suggested approach to building, I think. Meaning the most hydrodynamic.