Best way to design sprayrails...?

[Boatbuilder NL]

Looking for some help on how to design sprayrails, any inputs are highly appreciated!

Thanks in advance!

(Tried to attach two renderings, but doesn´t work... )

[jiggerpro]

Do you mean how to draw them with 3D software or the actual sizes placements etc ??

[GuestR01312011]

Look at the spray root and stick one in line with it, then if deck still getting wet put more above, towing a model will be very helpful.

[Eric Sponberg]

There is very little information on spray rails, either for powerboats or for sailboats. It is every bit an art, and people usually go at by trial and error.

I have from time to time added spray rails on my sailboat designs, Project Amazon being the most notable, and they definitely worked. What I had to guess at (estimate?) was the overall shape both in section and fore and aft. Also, how high up on the hull to put them. To my knowledge, there has never been any academic study directed solely at the design and placement of spray rails. I have long been of the opinion that this would be an excellent thesis for someone in college working for either a bachelor's degree or other advanced degree.

One of my absolute favorite sailboat designs with spray rails is a 35'er named QII, owned by Mary Falk in England, and designed by Michael Pocock. An excellent design with unique spray rails and hull shape. I had the good luck to meet Mary at the end of the Transatlantic Race in Newport, I believe in 1998, where she not only one her class, but was second in the 40' class ahead of her. I believe she has also won the Round Britain and Ireland race with QII. A great design exploiting spray rails to the best advantage.

http://www.icr.ac.uk/support_us/supp...lk/index.shtml

Eric

[ted655]

My skiff is SO sensitive to load & balance! It REALLY matters how the weight is loaded in the boat in relation to speed. If the weight is forward, the spray comes in over the sides in the 1st. quarter of the length (20' total). when the weight is moved aft, the spray waits & comes aboard in the rear quarter of the boat.
I don't think a empty boat can be towed & get a fair idea of spray patterns.
I notice the drier boats have big flared bows that kill the spray right @ the bow. At high speed, a spray rail JUST moves the spot at which the spray comes over the side.

[Eric Sponberg]

Ted,

Your observations are certainly correct. In the towing tank under controlled circumstances, the model is allowed to heave (move vertically up and down) and pitch as the hydrodynamic forces dictate. So in a scientific setting with money enough for a few different models, it would be possible, I think, to quantify a science for the design and placement of spray rails. As I said earlier, to my knowledge, this has never been done before.

For those who are interested in this topic, one company that I have had good experience with is Smart Rail. They make and add-on spray rail that can be used on existing boats as well as new boats. Their website is:

http://www.thesmartrail.com/

Eric

[GuestR01312011]

Ive seen tank testing data/images on some private powerboats that were getting green water on deck, and went to the Wolfson Unit to find a solution. and the procedure was to video the model towed at various speeds and then place rails at the appropriate position and retry until all spray deflected. The dynamometer would only need to be active if you were concerned with the added resistance / supposed ammount of lift gained from re-directing the spray.
Im not sure with spray rails on a yacht? Maybe the newer semi-planing hulls, but most of these are racing machines and it doesn't matter if the crew get wet, maybe the drag from the spray and rigging etc. But at low speeds the rails are submersed and developed huge drag no?

[Sean Herron]

Hello...

I know I may not be your most favourite person but do you know of anyone who uses 'circulation tanks' where the model is stationary and the water flows past it via pumps in a big doughnut like tank...

Much like a wind tunnel - same process...

Sorry to hi-jack the thread - if the original poster wishes - I can post a video of how I lay strakes and knockers on my hulls using Rhino 3D - if that helps at all...

Thanks in advance...

Also I would like to ask again about how the USA is handling the whole issue of licensing N.Arch vs. 'small craft designer' - what makes what distinct...

SH.

[Eric Sponberg]

Hi Sean,

I know that the Institute for Marine Dynamics at the Memorial University of Newfoundland in St. John's has a Flume tank which does exactly as you say--you put the model in place and the water circulates past the model. It's a pretty big one. You can view the model both above and below the waterline in the glass-walled tank. The booking agent and technical advisor for testing there is Oceanic Consulting Corp. Their website is www.oceaniccorp.com.

As for PE licensing in the US, not much has changed. SNAME continues to maintain the tests on behalf of the NCEES (the authority that actually gives the tests) and also offer a naval architecture/marine engineering brush-up course over the internet that starts every January and runs until the PE exam in April.

As for small craft designers, to my knowledge, the states are pretty much leaving them alone. I know here in Florida, which is particularly strict on the issue, the PE Board has taken a rather laid back stance and is letting the small craft designers practice without hindrance, just so long as you say you are not doing "engineering". It is OK to be doing "naval architecture" or "small craft design" because those terms are not protected terms (i.e. they are not written into the state law as being protected). It is also OK for me to say that I am a PE in Connecticut, where I hold my license, as long as I say "PE (CT)" after my signature. I did have to take the word "engineering" out of my website, so I replaced all the "e"-words with "naval architecture", which is OK as far as the state is concerned.

So, you can all go about your business as usual, and if you want the PE, it is there to apply for.

Eric

[Sean Herron]

Hello...

Thanks very much - very much appreciated...

We have a tow tank associated with the University of BC - but it is a drag tank...

You cannot put colouring into the water in a drag tank...

Again thanks - but St. Johns is the bloody other coast - looks like I may have to set something up myself...

Do you know of - or use any good CFD programs...

I appreciate your response...

SH.

[Sean Herron]

Hello...

Find the root entry - lay diagonals in station or body to same - project to hull surface - might curl in plan - in plan - straighten same - project to Cplane then back to hull surface - do a one rail sweep horizontal and vertical and trim each surface to same...

This is how I have had success doing lifting strakes - with Rhino 3D and other - anyway...

SH.

[Eric Sponberg]

I do not use CFD programs because it takes a special type of person to run them, and in my end of the business, I cannot afford to keep such a person on staff. My clients have little need for it. That said, I know people in the business, and if I had occasion to need a CFD analysis, I would subcontract out to those people and charge the fee to the client. Personally, knowing the state of the CFD science, I would prefer to go to a towing tank.

Eric

[BMcF]

I have been designing surface effect ships with their assymmetric catamaran hulls for over 20 years...and the number and location of the spray rails has always been - and still remains - a 'wait and see' design feature; yet is also a very critical feature for the SES to reduce drag and enhance pitch stability against high-speed plow-in or tip-over events. We show up at the model tank with the bare hull model and a 'bunch' of spray rail material. After a couple calm water runs, we return the model to the 'beach' and work off the photos and our direct observations and start afixing rails. More runs and some tweaking of the rail locations/lengths and number of rails and we call it good when it looks good.

-Bill McFann

[Sean Herron]

Hello...

I like tanks too - it is an immediate answer to intuitive questions...

But I would prefer a 'Flume' tank - (thank you) - where dies could be introduced to the flow...

See http://youtube.com/watch?v=3X8Ul8Lvx1M ...

My - that tank at Oceanicorp is just crazy - fantastic...

Thanks again...

SH.

[GuestR01312011]

You cant put dyes into a towing tank alright but you can put the dye onto the model, it used quite alot. Paint streaking, or oil mix and the flow will distort the oil/paint along the streamlines. I think towing tanks are more accurate, cause the flow in a water tunnel is affected by alot of things, boundry layer growth, pump/fan interaction, flow straighteners... lots to comtaminate the steady flow. Even in towing tanks after a test you wait 10 or so mins to let the currents settle and there's still some risiduary left... Although if your no taking readings I suppose just to look at the fllow they're good?