Practical hydrodynamics
 

Ten years ago I designed and built a 19ft wooden skiff. I spend many days and nights off the California coast persuing my hobby of underwater photography.

The hull has a flatbottom and 3 stringers running the length of the hull separated by two 7" channels. Each stringer is 1" thick and has a full radius on each edge. The 'good skiff' travels at less than 30 mph.

My question is this: Would it lessen the drag on the hull if the stringers were beveled as opposed to having a full radius? Would the degree of bevel make a difference?

I was thinking that if they were beveled, it would pack water into the two channels, adding lift to the hull but I have no way of determining this. My hull is too precious to make a mistake with.

Any feedback that you offer will be greately appreciated.

Sincerely, Brad

Re: Practical hydrodynamics
 

My gut (which shows you just how scientific an answer you get, even here) tells me to advise you not to do anything to what you've described as a good boat (built with your own hands, which I applaud). Here's what I think:

1. Beveling the stringers would probably reduce the drag.

2. The degree of reduction would depend on how you shape it.

3. The total amount of drag caused by the stringers in the present configuration is small compared to the drag on the overall hull, based on my understanding of your description. The hull is a few feet wide by 19 feet long, that's a lot of surface are compared to the 3 stringers. I'm thinking that the stringers represent 10% of the drag, at most (probably less).

Based on that, eliminating them completely would get you 2 or 3 knots, tops.

From a risk / reward standpoint, it's not worth it to risk screwing up a perfectly good hull for a couple of knots (unless you're competing for the Americas Cup, and I'm sure you've seen some of those boats fold up like a paper bag).

As you pointed out, your hull is too precious to make a mistake with, and it works fine as is.

Alton

Thank you Mr Reich
 

Thank you for your thoughtful reply. My goal with that potential project is to extend the range of the good skiff. Cost is of no importance. I can carry a maximum of 76 gallons which will allow me to cover some 240nm. If the modification would extend the range another 10nm, then I would consider it worth the effort.

While underway, the wetted surface of the hull extends about 10 to 11ft from the transom. So there is a total of about 65ft of full radius moving thru the water. Because of the fastener placement, I could put a 60 degree chamfer along each stringer. I'm looking for the difference in wetted area between a full radius and a 60 degree chamfer X 65 feet. I believe that a radius is neutral from the perspective of lift potential, but a chamfer would pack water into the channels. Would that cause lift? Would it lessen the drag?

I can build a boat without plans, but I'm not good at math!

There is a submerged mountain top that is a hundred miles off the California coast, it is my favorite place to swim. With this project, I hope to extend the range enough to visit another island (The Island of the Blue Dolphins!) on the same trip to that magical place. I never take chances, and every mile counts.

I don't want to burden you with my trivial questions, but your reply will be greately appreciated.

Sincerely, Brad

Re: Practical hydrodynamics
 

(1). I don't have any idea about the shape of your love boat, so it is hard to give any suggestion. (2). But I used to optimize the shape of centrifugal fan by using "paper clay". The paper clay can be shaped, sand after it is completely dried overnight. By gradually change the shape, removing and adding paper clay, and through testing, I could arrive at the optimum shape, which is quite sensitive to the drag. (3). I think, you can first do some research to find out the right type of clay, oil-based maybe, stay flexible type, non-permanent, so you can easily remove it if you need to do so. (4). By trial-and error, I think, you should be able to arrive at the optimum shape. Testing is necessary, because the accurate prediction of the drag, though not impossible, is rather difficult without a high power computer. Since you have already developed some kind of intuition about the boat design, this hands-on approach would be ideal. (5). For larger area, you can also cover it with high density styloform porous material first, the with the clay over and around it. So, check out your local arts supply store, model store, or wind tunnel testing store for the right type of material. It is possible that there is already an ideal type for boat application. Remember that, sometimes, small changes in the shape can have large impact on the drag. (6). The other possibility is to build a quarter scale model and test it there. It is likely that somewhere along the line, you will discover something new. Or you can always tell a story about how you increase the drag of your love boat.

Thank you Mr Chien
 

Experimenting is good advice, but it would be difficullt to actually do as I would have to apply over 60 feet of it on an upside down surface. I wish I could!

I have decided to bevel the edges when I buy a new motor in the coming months. From what you and Mr Reich have said, it sounds like the chances for improved performance are good enough to make the effory.

Thank you for your thoughtful reply!

Sincerely, Brad