Rounded vs Hard Bilge

What effect would it have on performance, stability and other behaviour if a flat-bottomed dinghy with a hard chine were modified to give it a rounded bilge, say about 3" (76 mm) radius? I imagine the effect would be less pronounced for a multichine boat.

 

terry, I thinl would make her tippy, have you sailed a laser? very tender

 

A round bilge creates a more benign motion transversely, hence the roll is 'softer'. But being so small 3", i doubt it would make a significant difference in this case. It also, depends upon the speed you're running, ie Fn, as it can also reduce resistance. Depending where the LWL is, can stop the "wave slapping" on the chines.
Down side is that it can affect dynamic stability at high Fn's..

It all depends what changes you expect or want and hence is it worth doing. What you gain/lose in one, you gain/lose in the other...

 

I wonder how much effect a 3" dia bilge would have on a typical dinghy, around 48" wide. I imagine if I gave the bottom some deadrise at the same time it would make it a bit more tippy. I was hoping slightly rounding the bilges would increase non-planing speed by reducing turbulence without sacrificing too much stability.

I haven't sailed a laser; a sunfish is about the only small sailboat experience I have had, although I will be completiing a small sailing flattie soon. It is making me wonder what changes I could make on the next one: building a boat has that effect to me.

 

On Dace Terry, you wouldn't gain anything. If you put the big rig on her, the "rolled" chines would actually slow her a bit, because the flow wouldn't be able to "release" cleanly up on plane. With the two smaller rigs, she doen't have the HP to really offer the speeds, where the chine shape would matter much.

A Laser isn't especially tender as dinghies go. It's bilges are very firm and it's roll fairly slow compared to other hull forms.

 

well I sailed the farr designed cherub, in NZ chines, I raced lasers and they are tender, chalk and cheese, other than that terry cant help sorry
the cherub was 12 foor(think) 2 man, trapese, it was not tender , mayne beam as you described , maybe google it, although this was 1962) oops no I think the cherub was a Spencer(infidel--ragtime) Buccanner

 

So other than reducing performance the only effect would be appearance. And she looks fine anyway. I was trying to understand why boat builders would go to all that trouble instead of having a simple flat bottom: must be a reason.

 

I think a boat as to be taken as a whole, and the radius chine against the hard chine is only one of the parameters.
Size, propulsion, utilisation, feasability, material, all this enter in the design.
As transforming an allready design hull, I will not do it.
Could you be kind to elaborate about your project?
Cheers
Daniel

 

I am finishing a small sailboat, see http://www.boatdesign.net/forums/sailboats/small-sailboat-design-advice-requested-26169.html and in a moment of idleness I started thinking about the next one. I am not planning to change the existing design, this was for a future boat that I am thinking about, to try out some ideas.

 

It's somewhat a choice of materials and construction methods.. Flat is hardest form to make without having plates

 

Chines can be good in dinghies

As stated before in this thread chines help the water release more cleanly when on the plane. High performance dinghies usually have chines - skiffs like the 49er and 18, 16, 14, 12 foot skiff all have chines. The water cuts cleanly away from the bottom on the plane.

It doesn't do this as well on something like a Laser but they still plane fine - just probably not as fast as a chined boat. Chines have an awful lot to do with dinghy design and are still used even when they are not needed for ease of construction. The Cherub (correctly identified as designed initially by John Spenser - but a development class so it has many designers - fastest at moment are Matthews hulls) has a strange chine shape to fit in the rules - they are almost straight to the mid chine measurement point come to a point and then straight again to the transom - strange but fast.

An interesting experiment was done in the eighties when an enthusiast built a round bilge replica of a 60's 16ft skiff. Ken Beashel had swept all before him in the 60s with his round bilge Elvina Bay. In the early 80s the replica (Gunnamatta Bay) had modern sails, foils etc but couldn't keep up with the chined boats. No one else repeated the idea since.

Probably the most interesting experiment happened in Moths. My good friend's dad - Greg Marshall wanted to build his own Moth in the late 60s- almost all Moths were homebuilt then - but he didn't want to go to the trouble of cold moulding for the round bilges. He got his girlfriend to sit in his old cold moulded boat and drew the waterlines on it as it heeled. He drew and built a Moth that had chines that corresponded to the waterlines. He then won the next Australian titles - the cold moulded Moth died then and there.

Yet I loved sailing the 420 and Laser. Both are fab to roll tack and work well in light airs. Manouvrability and light air performance is better in round bilge boats (no chine to stick in the water and more volume per unit surface area). So it depends on the boat's use.

cheers

Phil

 

Yes I remember. You said:

I find the hull very sweet, very well done.
I hope you will find a proper design and do not "idle" too long.
Yes round chine is sweet, and rewarding at the end of the day. I am not sure, but perhaps it can make some differences by very light air.
In general you get a slight gain in wetted area in hard chine, and less in round chine, but also depend the position of the chine. Flat panel are not very efficient on light brezze ( I talk only light brezze, because when the wind pipe up, everything fly!).
But even the Flying Dutchman with the long flat aft end, we managed to go well by light brezze, by heeling some degree.
I supose it depend you dedication and patience.
Remember the Fireball? pretty fast, but mostly due to her very slander hull. What a nice racing boat. I think it shaw that hard chine can be interresting. The Contender, flat panel with rouneded bilge, I raced against it in Medenblick in 66 if I remember well the date, for the choice of a new single crew dinghy.
Our design went very badly, it was a great desapointement. We learn from our mistake.
But I am perhaps of topic of your interrest. I apologize for that.
Cheers
Daniel

 

This has been an interesting and informative thread; my thanks to all the contributers. My earlier boats were canoes and I started out with flat-bottomed ones for ease of construction and a misguided assumption that they would be more stable. I was misled by the apparent similarity between canoes and pirogues. Different animals. As I progressed with my canoe designs I realized how important the underwater shape was to stability and how wrong that assumption was, for canoes at least.

I realised that, while beam at the waterline was important, I also had to keep my weight low with a narrower bottom, leading me to my more-or-less standard 5-plank approach to small, single-seat, double-paddle canoes. They are far more stable than my first, flat-bottomed boats, and faster too as a result of reduced wetted surface.

Sailboats have different lessons to teach me, so my education continues! As a long-retired techie, I don't remember engineering for nuclear, aerospace and telecommunication being as difficult to learn as boat design. Nor as much fun as boatbuilding. I think I have finally found my true vocation!