Weight versus Displacement

So in contemplating a 1:10 scale boat a few questions have come up. For a smaller lighter boat such as a Melges 32, when the displacement is listed, does that include the crew or is that the bare boat. I know displacement is the weight of the water that the boat displaces when it is floating, but how does that number relate to the weight of the boat in full race preperation with crew and all. I know as boat get smaller, esp. racing boats, the crew are an integral part of trimming the hull for maximum efficiency through the water. When scaling these boats, should the “simulated” crews weight be added to the bulb, or would a moveable ballast system inside the boat of the same weight be more beneificial. I was thinking of having a weight on track that is a half circle, it will be mounted so that the ends are just aft of midship (we will call this point “M”) and the mid point of the track will be mount at “M” plus the radius, allowing for the weight to bring the bow up slightly on downwind and add a little extra righting moment when on the wind. Now I know it can be argued that the extra weight would be better off in the bulb, but then it doesn’t allow the trimming effect.

Your post raises two very good points.

First, the direct question. A designer will usually state a displacement figure which incorporates not the just the weight of the boat, but also the expected weight of the crew and equipment that will be on board. This is factored in to the calculations. In smaller modern boats, the designers often state how much they have factored in as crew weight.

However it is a little acedemic, as whatever you do, you would want the model to end up sitting on her design waterline. Thus the end displacement of the model, and the placement of ballast, would not be finally determined until you float the boat and add weight until she sits on her lines.

Second point - I can assure you that crew weight is fundemental - even in bigger boats. On the 52 footer I used to race, we sailed with 14 crew within our rating, and up to 17 crew in non-rating events. Our standard crew weight with 14 crew was 1,200kg (2,640lbs). Even on a 52 foot (15.8m) boat, that is a lot of valuable movable ballast.

This is a potential problem when building model sailboats to scale. When designing to a model rule, say the M or the IOM, we know we have no moveable ballast and we try to design boats which perform on different points of sail and angles of heel. If we use fine lines forward, we deal with bouyancy issues downwind by incorporating higher freeboard forward, raised foredecks, and flair (look at the Stollery Marbleheads).

When a designer draws a full-sized boat he knows darn well that he can shift the crew weight. Look at a Mumm 30 carrying a spinnaker downwind in a breeze. The crew are as far aft as they can be. Look at how flat many modern boats are when sailing to windward, even in a breeze - the designer, and the sailors, know that if the boat heels excessively, the underwater hull shape gets screwed up and balance and performance are lost. But in models, we often design out boats in the full knowledge that we’ll sail to windward at 40 degrees of heel.

So your question as to whether you should add weight to the bulb to get the desired displacement, or incorporate moveable ballase, is a good one, and will depend on the design you wish to scale. In terms of righting moment, obviously the bulb will be best - but if the design is one which relies on moving crew weigh aft and to windward to perform in a breeze, you may be better to try to mimic that in the scale version, and learn to sail the boat like the real thing would be sailed.

This is also why the canters (which derive so much power from the moveable ballast) and the short-handed designs (such as the Open 60) make good subjects for scale modelling. The latter are designed in the knowledge that there is no effective ballast to be derived from crew weight - so the use another system, which we can mimic in models.

Boats without moveable ballast and comparatively low crew weights will work well if they are heavy designs, typically narrow, and intended to sail at good angles of heel to windward - viz, the 12 meters and the IACC boats. It’s no surprise that these have been popular scale modelling choices.

Just my 2 cents.

My thought is that we already have the advantage of a longer moment arm because (I think) we agreed that the draft could be altered for our 1:10 class. So the righting moment is changed without adding any additional weight which translates to additonal displacement. I figure, depending on the design, the crew weight might be in the ballpark of 10% the total weight of the boat. So if I were desiging a boat that is too displace 17 lbs, I would put 1.7lbs of moveable ballast on my servo/pendulum mechanism which now allows me to trim the boat to a certain extent. Also keeping in mind and RC boat can have a much greater perecntage of the weight in the bulb than a “big boat” if the set up is kept pretty standard (2 or 3 channels).


Andrew - another thought borne out (at least until the rules caught up) was the “M” class where the displacement of the average hull stayed the same to get the boat on it’s lines, but the keel depth became a radical exercise in establishing more righting moment without adding weight.

Because to start, I was not going to work on a canting keel in the 1/10 class, I now must make some choices, since a canting keel carries much less weight in the bulb than does a fixed keel monohull sailboat. I simply can’t add more weight in the bulb to make up for the missing canting keel assistance in righting moment, and if keel depths are limited in depth I can’t gain righting moment there with a deeper/longer keel, so I may be faced with a boat that is very “tender” when sailed with the normal keel depth and weighted to the proper displacement to bring her down to her designed lines. I am grappling with this issue now - and fear that if I can’t reduce weight from canting gear and put it into the bulb - then I may be forced to engineer a canting keel system in order to have the proper lines at the waterline. I suppose I could augment with water ballast, but that is something else to slow down the process of getting the boat on the water and sailing it.

One good thing, is if others elect to forego the caning keel, we all will (essentially) be in the same boat (pun not intended) and have to deal with the necessary weight to meet designed lines, but not enough righting moment due to a non-moving keel.

Geez - hope I didn’t lose everyone on that post? :lol:

Well what brought up my question was a conversation I had with a designer who said I will allow you to use my plans, “but it won’t work because my boat depends heavily on crew placement for righting moment and trimming the hull”. I have been thinking of this “swinging weight” idea for a bit now, never more seriously than now, but thought it might be an added bonus rather than a deeper keel.

On the canting ballast side, I always wanted to make a boat with the same amount of ballast, just mor erighting moment from the canter and therefore stupid ridiculous amounts of sail area. But now that the scale thing seems like it is coming to fruition I am going with the K.I.S.S. theory and hoping my boat proves to be fast.