Dinghy LCB

Hello everybody

I am in the process of designing a cruising 18 ft dinghy .I am currently trying to determine my LCB. As per Principles of yacht design, for sub planing speeds, it should be around 4% of LWL aft of midships. However I see that most similar dinghies have their LCB far forward of midships. Is this a try to give forward volume in order not to allow the bow to dig in, or is there something else I am missing?

Thank you

 

Are you thinking Wayfarer or Drascombe when you say "cruising dinghy". Crew weight is high relative to boat weight so their position makes a huge difference. Most dinghies keep transom out of water when not planing. Sorry, not enough info to advise you properly, maybe post a sketch?

Richard Woods

 

LCB under what condition - an arbitrary static design waterline?

 

@Men, LCB does not mean anything without taking into account LCG.

 

Discussions of where to locate LCB for minimum drag, etc usually have an implicit assumption that the boat will be designed with the LCG located directly above the LCB.

Depending on the type of vessel the LCB/LCG location may be one of the initial characteristics specified and the arrangement of the vessel is designed so that the LCG is in the desired location; or the arrangement of the vessel may be designed based on requirements and the resulting LCG determines the LCB the hull shape needs to have; or tradeoffs are made between the desired LCB/LCG location and satisfying the requirments/desires which affect the arrangement until a satisfactory solution is obtained; or some other set of requirements and decisions may determine LCB.

Open questions include:
How important for an 18 foot cruising dinghy is the LCB location which will be calculated based on an arbitrary displacement/waterline and trim?
How applicable to an 18 foot cruising dinghy is LCB location guidance cited by the original poster and what types of boats was that guidance intended for?

 

In dinghies, the weight of the crew is used to adjust the trim. The LCB changes with the trim adjustments, which is used by the crew to modify the behavior of the boat. For example, in a strong breeze when reaching, the crew moves aft.

 

Thank you for your replies.

1) Something like a wayfarer, but bigger, with self draining cockpit, possibly ballasted hull with centerboard, not with planing in mind (no submerged transom), easy to build. I do it as a design exercise, and will incorporate as much of the above as possible.
2) As a design starting point, i do imply that the LCB and LCG will coincide at the arbitrary designed water line (I must start somewhere!). The rest attitudes, will be studied further on.

3) The data is derived from the Delft series. My dinghy is well within boundaries, so I guess the LCB positioning aft of midships is applicable.
4) For a non optimum hull (non optimum LCB position) at Fn 0.35-0.4, you might have a resistance loss in the order of 10% of the displacement. While I do realize the actual sailing condition may vary considerably, to accept this sacrifice from the beginning, what would I gain? Is it improved performance in the rest sailing attitudes, is it keeping a drier cockpit?

I was aiming at this approach. It also seems practical, based on the fact that most of the weight (crew) will be quite aft, the volume also being somewhat aft.

 

Where did you obtain the LCB information for similar dinghies? Was it based on the displacement and trim that those boat are usually sailed with? Lines drawings of the hull shapes at some arbitrary displacement and trim? Something else?

Have you investigated how LCB moves with changes in displacement and trim in those boats?

How are those boats sailed at Fn 0.35-0.4? Does the crew move aft which moves the LCB aft?

 

Are you sure you have the axis the right way 'round. Longitudinal distance is conventionally measured starting at the aft perpendicular (x=0) and measuring forward. So a reported LCB of -2% on a 16 foot waterline would be 3.8 inches aft of the midsection. Also make sure you have the APP and FPP correctly located.

 

It might be useful to explain how the positions of the APP and FPP are deduced.

 

That is a common practice for "ships".

The Principles of Yacht Design (Fourth Edition) discusses LCB location in terms of percent of LWL behind midship, with LWL being the nominal design Length WaterLine (sometimes referred to as DWL). I have seen technical references on boat design which measure LCB from the aft end of the LWL, from the forward end of the LWL, from midships of LWL with positive forward and from midships of LWL with positive aft. There is no universal standard. I always check how LCB is defined in texts which sometimes takes a bit of searching.

 

APP and FPP as defined for ships are infrequently used in the design of small boats. Much more common is the foreward end of the design waterline (frequently equivalent to FPP) and the aft end of the design waterline (aft of APP for boats with inboard rudders).

 

Maybe not in ancient times, but currently one of the first things to do, both in small boats and in huge ships, is to define the design length and the other design dimensions of the boat. And, believe it or not, it is not easy to know what is the design flotation and other characteristics of the hull in the early stages of design. I am talking, of course, about the design made by a professional who always intends to meet minimum standards.

 

TANSL, I agree entirely that "one of the first things to do, both in small boats and in huge ships, is to define the design length and the other design dimensions of the boat."

My comment which you quoted above your reply was much more specific: "APP and FPP as defined for ships are infrequently used in the design of small boats." My comment refers specifically to the terms "APP" and "FPP" as defined for ships. There are other ways to define length of a vessel such as the length of the design waterline (LWL) which is much more commonly used in small boat design than the distance between APP and FPP as defined for ships. The forward and aft ends of the design waterline are frequently used as "perpendiculars". As I noted above while the FPP (as defined for ships) and the forward end of the design waterline are frequently the same, the APP (as defined for ships) is typically at the rudder post and for boats with inboard rudders the aft end of the waterline is aft of that definition of APP.

For fishing vessels and other commercial vessels (such as the vessels in some of the images in TANSL's gallery) APP and FPP as defined for ships may be commonly used.

 

All boats, large or small, need an AP and an FP.

Defining a location, for a datum frame of reference, varies with size and often vessel type. It is not rigid in that sense.

Small boats - in general - use the AP, which often coincides with the transom - but not always.
Reason being is that depending upon the loading condition the draft will vary. On a large boat this is a minor change. On a small boat this can be very noticeable. But this means the "midships" location will vary, since the LWL is varying also. Thus rather than having a design condition which could "technically" have 2 very different midship positions, it is easier to use one frame of reference that will not move, regardless of the loading condition - this being the AP, or Transom if coincident.

Since the DWL and LWL are not the same, one needs to account for the varying load condition in the preliminary design stage and one requires a fixed datum as the metric. When doing this task of prelim-design, placing the new waterlines of light and full load onto the hull Lines of a small boat, the effect becomes obvious. Thus which midship location to use and which LWL to use? Hence a DWL is the more correct reference as this is not influenced by loading, and then if the draft changes, from load cases, the influence of the location of the midships, on its LWL, may add to more confusion.

So a general simple rule is that small boats use the AP and/or Transom, as this is a fixed datum.
But like all rules - it is not rigid and always contain caveats...