Using multiple bulbous bows as blocks?

Hey all!
I know alot of you folks have been playing with many of the software packages out there and I need a little advice. I would like to buy a middle class (Vacanti 98 or similar) software package. Here is the question. Could you develop a hull and fit different bulbous bow shapes as blocks? Then I could try the shapes and refine and weed out the shapes and profiles that did not provide the best efficency. The final shape of the hull could then be machined and tank tested. does anybody have a favorite software that could do this? I realize that the wave and friction drag figures would only be close and a real surface program would be needed to really get it right. I just have no love for those trawler builders out there (we will not name names) that take the easy way out and scab a piece of pipe on an otherwise good hull. On the other hand only a fool would spend 400 man hours cutting, splicing, rolling, bending and cussing steel into a work of art that does not work!
Thanks in advance 8Kts

 

Probably most programs would let you do what you describe. In Prosurf, you could create separate shape files of each bulb, and then also a file for the master hull. Open the master hull and then import any of the bulbs. You can trim off intersecting surfaces where the hull and bulb meet. Then you can save each complete hullform as a separate file.

Really, the only way to test the efficiency of a bulbous bow design is through model testing, and the bigger the model the better. Have a look at my website for the Moloka'i Strait 65. www.sponbergyachtdesign.com. We have a nabla bulbous bow which we model tested at the Institute for Marine Dynamics in St. John's, Newfoundland. It is one of the most sophisticated model tanks in North America, and quite modestly priced.

We tested only one bulb design for the MS 65, and we made "educated decisions" regarding the bulb design on the MS 72, which is currently in detailed structural design. For the upcoming MS 85, I would like to go back to the tank for further testing of multiple bulb designs, and do pretty much what you propose regarding different bulb designs.

There is very little technical literature available for bulbous bow design, and the best of it is written in a series of German technical papers from the late 1970s. Only one of the papers has been translated into English, and it is available through the Society of Naval Architects and Marine Engineers. The remainder of the papers are not easily available, unfortunately.

Basically, bulbous bows are very sensitive to the primary shape parameters: Length, breadth, depth, cross-sectional areas transversely and down the centerline, and volume. If you change any one parameter just a little bit, you can change the optimum drag reduction (bulbs reduce wave drag only) very significantly. You can also inadvertently increase the drag, which is a bad thing. All of this is highly dependent, too, on the master hullform that the bulb is attached to as well, so a bulb design that works on one hull may not work at all on another hull. This is why model testing is so important for any given hull design--you just don't know where you will end up with any given bulb.

Eric Sponberg

 

Thanks for the input

Eric, Thanks for the insight. I have seen your boats in PMM. Great work! I did see one thing that troubles me though, Those port lights right down to the waterline bother me. I think a possible (educated) boat buyer would ask alot of questions. Please understand that I am a self proclamed doodler NOT qualified to evaluate your work at all just an observation by an admirer of fine boats. 8Kts

 

The windows near the waterline on the MS 65 (and MS 72) are 1/2" thick laminated glass installed in a welded-in fixed port. The glass is recessed in from the hull 4". They cannot be opened. The windows are made by Freeman Marine in Washington state, one of the best suppliers of windows. We have addressed the issue of safety, and in concept, these are no different than those found on many other vessels.

Eric