Hard Chine vs Radius Chine/Round Bilge

I know, I know, I know... another one of these questions, but despite reading other threads I'm still unsure about the advantages of radius chine and round bilge over hard chine. I'm particularly curious about the differences in drag and stability between the different types, and the difference in upwind performance (I read somewhere that no hard chine boat will ever point as high as a round bilge, can someone please confirm this?) Is it really worth the effort spending extra time and money (provided that looks aren't important) to build a radius chine or round bilge hull, in terms of small boats? Any input will be greatly appreciated. Thanks.

 

you can read all about it in Dave Gerr's "the Nature of Boats". A very enjoyable book.

 

In general, a hard chine creates turbulence as water move across it and this can increase drag.

The bow wave system is a culprit of where this water comes from, for as a boat moves through the water some will be pushed to the side and some will be pushed down under the boat. That shoved to the side is the principal bow wave. That which is displaced under the boat, or at least some of it, will subsequently be pushed to a side and reemerge as a secondary bow wave.

Here is a link to a discussion of the bow wave where the writer employed remote control models, photography and some gadgetry to help visualize the different flows: http://www.ivorbittle.co.uk/Article...he bulbous bow for my site This one again.htm .

 

If you are designing a boat exclusively for upwind performance, probably the first thing to do is to limit the operation parameters. That is, for example, wind speed range, sea conditions, ballasted or not, etc. The size of the boat will greatly determine how those parameters affect the design. Whether it is worth the effort and money depends on what the final result will accomplish or gain.

 

Welcome to the forum.

Given the the limited hull parameters, the choice of bilge type will have little to do with the other choices, you need to make decisions about. Simply put, the notion a hard chine can't point as well as a round bilge isn't possible to access, unless both hulls are the same in every other regard, which isn't likely. As a rule both hard chine and soft chine (radius) are employed so you can use sheet goods (plywood, metal, etc.) building methods. These two hull types are generally easier to build than round bilge. This is their only real consideration (for the most part). Some round bilge hulls will still employ a chine in key location(s) to promote some hydrodynamic attribute, but without a fully defined SOR, making grand statements about the values of each is meaningless. Refine your questions a bit, so better replies can be made. For example, hull form choices have a lot more to do with initial stability, than bilge type. A slack bilge, hard chine can be very weak in initial stability, compaired to a firm sectioned round bilge, but this doesn't mean the hard chine is poorer in this aspect, just the sectional choices made, comparatively.

 

Not many yachts could beat John Spencer's hard chine plywood Infidel in her day. Most modern higher performance racing dinghies are hard chine, although in many cases they are now built of composite materials that would easily accommodate round bilge design. Hard chine profiles are often used to promote early planing. Further to what PAR explains, also keep in mind that pointing and velocity must always be considered in unison. For any given design and conditions there will be an optimum pointing angle tha gives best VMG. Pointing as high as you can is only occasionally the fastest way to windward all else considered.
What type of sailboat are you thinking of?

 

Either form can be optimized for the design point, and the performance will have very little to do with whether it has a hard chine or soft chine. Measurement rules may favor one over the other though. If two boats, optimized for the same conditions, measure differently and rate differently, that will likely be what determines the answer.

You can spend a lot of time studying the stability implications of the choice. A reasonable way to start is to just look at midship cross sections. For similar looking boats, but round vs chined, comparing the transverse stability of the entire boats will be pretty similar to comparing the stability of the mid sections. By similar boats, I mean the same displacement curves at the design point.

As a general rule, chined hulls are a bit more sensitive to changes in the draft at the bow. Added weights should trim the stern down and not the bow. Stern counters are therefore more common in chined hulls where the load is variable. Chined double-enders are hard to do in a sailboat unless you know the weight quite accurately.

An issue that is usually conflated with chines is whether or not the plates are developable. They don't have to be. There are many chined hulls with compound curved bottoms. A hull made of all developed surfaces is a bit harder to design than a round bottom hull. Usually, it cannot be quite as versatile, but at one design point, it can do very nearly as well.

Chined hulls are often a bit heavier than the equivalent round hull. Even at the same level of build quality and design expertise.

 

Thanks for all the answers, I'm considering designing and building a small planing sailing dinghy, in the hopes that I end up with something that performs ok and I learn a few things along the way. I've never built a boat before, so a hard chine hull makes sense to me. My main requirement is that the boat performs well downwind and reaching, and is exciting to sail. Upwind performance isn't as important, but still needs to be decent. Based on these requirements, at the moment I'm considering a v bottom scow with very little dead rise, about 13 feet LOA and 4 foot beam. For the rig, I'm probably going to buy a standard laser rig.

 

The first thing to ask yourself is, what is your target speed? Since the higher the speed the more prone a round bilge is to loss of stability especially in high speed turns. But this is relative, since I am talking about high Froude numbers, over 1.0.
Round bilge are generally softer rides, but hard chines better at higher speeds.
At low speed, the chines add about 5 % or more drag and if loaded incorrectly can chine walk. Etc etc...

Thus as noted above it is horses for courses and each have their good and bad points. Just select which you want and then justify why.

 

NE, A 13 foot scow gives away some waterline length because when viewed in profile the nose is curved upward to or near deck level. See profiles of Inland lakes scow types. If by the use of the term scow, you are thinking in terms of a pram or garvey, then there is a forward transom. That works well for a slow speed boat but the fore transom can be problematic when at speed in a chop.

A vee bottom is also fine for a pram but not advantageous for a scow that is expected to deliver brisk performance. As a generality, the flatter the bottom the quicker to plane. The angle of the run with respect to the waterline is another factor that promotes or discourages early planning performance. The smaller that angle the better for planning initiation. One has to be careful with that feature because if not enough aft rise in the bottom the corners of the transom will drag when heeled. That gets us back to the plan view and how wide or narrow the transom is.

A real scow will go to windward very well when heeled at somewhere about 14 degrees. The underwater sections are then vee shaped, the boat holds on because of the heel angle and the underwater shapes. Upright when on a reach or run, the bottom is flat or nearly so and lifts the boat more efficiently than a vee bottom is likely to do.

It would take some extensive fiddling with the scow plan form to get it right. If optimized the section centroids will fall into a near straight line when the boat is heeled to a certain angle, as before, usually about 13 or 14 degrees. In that case the wetted surface will be dramatically reduced and the boat with go to weather like a bandit or in this case with so short a boat, like a shoplifter.

The sides need no flare. Plumb sided boats are a little easier to layout and build. Give the sides some flare to gain extra deck width and easier hiking at the expense of boat weight and a bit of building complexity as well as the use of a little bit more material useage. By all means be mindful of the weight of the boat. Light weight boats displace less water and are more lively.

Post some preliminary sketches and you can get some comments or suggestions from other forum members.

Go for it.

 

Messabout, thanks for your input, when I said v bottom I was thinking along the lines of a y flyer type hull. However, after reading your post, a scow seems like it could be fairly difficult to get right. Since I've never designed a boat before, I'm thinking that I might go towards a more conventional hull shape which is more likely to 'work'. I'm not giving up on the scow idea, but I'll look into other hull shapes as well. Regarding your comments on flare, I was thinking that I might make the build a little bit more complicated and have a fair amount of flare to keep the waterline narrow while keeping it relatively easy to stay upright. I'll definitely work on some sketches or a model sometime to get my ideas across more effectively. Thanks again.

 

Considering your level of understanding in regard to the basics of yacht design, you'd be best advised to simply buy a set of plans. There are many (hundreds) of low cost, easily built, performance sailers available, including a few scows. The cost of these plans will be less, than the cost of a modest paint job for your proposed build.

 

PAR, I appreciate the input, but the idea of sailing a boat that I designed myself really appeals to me. It will be a while before I actually start the build, and until then I'll learn as much as I can and apply that knowledge to a boat that I think might work. I understand that it might be a complete flop, and I could just end up with a box with a mast on it, but even then it would have been a huge learning experience for me. I know I would end up with a better boat if I just bought plans, but it's not just the end result that I'm interested in.

 

You could do a fairly simple scale up of a NZ Moth, easy to build and a great reaching boat in flat water. Laser rigs belong on.... Lasers. Keep your eyes open, you could find something much better for not alot.

There is a site with many of the 70s scow moth lines plans, maybe even construction details. Let me know if you want it, its saved on an old PC

 

JRD, thanks for posting, I'll look into the moth idea, and I'd really appreciate it if you sent me those moth lines.