forward bouyancy vs displacement

<font color=“blue”></font id=“blue”><font size=“2”></font id=“size2”>[:-graduate]
this is a post to get a discussion going over a hard to answer question
what is the difference or importance of bouyancy vs hull displacement. A member is design his first boat and i look at the blue prints and tell him that the bow looks too narrow. “NOT ENOUGH FORWARD BOUYANCY” , so his comeback was. why dont we fill the bow up with foam. like the do with rc subs?. i even have a hard time. getting my head around it
so maybe we can discuss it here. and find out what is more important to a fast yatch?


Do the subs let water inside the bow?


This is not a serious question right?
Correct me if I am wrong…your asking what difference foam inside the boat will make to the hulls performance?..


mate…i think you should re read your post…well as i am no specialist…putting foam into the boat will:

  1. make it heavier
  2. in case of sinking [:-batman]…well maybe it can slow it down or avoid it!

but then again I am no specialist in hydrodynamic[:-banghead]


if it isn’t broken, don’t fix it!


Not sure exactly what the question is here???
There is really no difference between displacement and buoyancy. Displacement is the weight of the yacht and is equal to the weight of the water its hull displaces?
Longitudinal center of buoyancy (LCB) is the geometric center of the underwater volume of the hull and will lie directly under the center of gravity when the vessel is in a static condition.
Position of the LCB moves as a hull heels and pitches and is a function of the shape of the hull. Sticking foam inside the hull will do nothing (because it doesn?t change the shape) but building up a bow (ie making it fuller) with foam will move the LCB forward whenever the foam is in the water and hence reduce nose diving.

Does this help?


ok i guess i did not make myself too clear
1st to dan yes the sub allow water almost everywhere except the part that has the electronics
foam is put into place to counteract the wieght of the battery and other devices. so the foam provides floation
2 nd i am not questioning putting foam in the bow. for performance. we have a new designer and he wanted to make to bow realy narrow. i told him the forward bouyancy is a must. relay improntant on the downwind legs. his response was putting foam in the bow. for added bouyancy. he is not too concern about wieght yet
when i tried to explian to him that bouyancy and displacement wher 2 different things. he got confused. displacement is the amout of water a object ( displaces) when put in water. bouyancy is the amount of hull (in htis case) that keep the boat afloat.
i am trying to find a way to explain you need a certian amout of forward bouyancy to stop the bow from digging in. and stopping. he answer was that the bow would cause less resistance to water flow , and thus get a faster hull
i hope this clears it up
i always have too much forward bouyancy. must be a wind thing.( if you see the movie you understand)
thanks for the input.

I think I see what your getting at…
You can have a narrow bow with lots of bouyancy, you just make it deeper below the waterline and/or higher above it, that way as the bow goes down you have more bouyancy to push it back up. Alot of the modern fulsize cats use that trick.
Putting foam in the bow will not help keep the bows up, its just unwanted weight in the wrong place. It would only keep the bow up when the rest of the boat is flooded!

Luff 'em & leave 'em.

The way to find out whether your friend’s design is in or out of the ballpark is to compare it with other successful boats. In particular, what you want to plot is the “curve of areas,” which shows how the bouyancy is distributed fore and aft. This requires calculating the underwater area at each station, and then plotting those values on a graph. If the graph of your friend’s boat is radically different from those of a couple of successful designs then he may be taking a risk.

If you go to and follow the links to the “Design” section you’ll find a description of how to perform this and other basic calculations in designing a sailboat.



Greg is right (again)!

Given the height of the rig on most RC boats, you get a lot of nosediving tendancy. You need to have volume (displacement) forward to counteract that. Yes, you want to have a fairly fine entry to cut down on wave makeing resistance, but if you go too fine, you will have problems with nosediving. What you really want to look at is the displacement distribution from bow to stern. You want it to be a fairly smooth distribution. If you have any section where you have a sharp increase or decrease in the displacement, you will generate waves from this section. Wave making resistance is a large source of hull drag. Of course you need to balance this with wetted surface area as well. And of course you need to evaluate all of this for the heeled hull.

In fact Yacht designers have another parameter which measures the amount of displacement change versus heel which they call the prismatic coefficient. I am not sure exactly how it is measured, but it measures the change in righting moment that comes from displacement as the boat heels. This can lead to harder of softer chines and also lead to a shift aft or forward in the buttocks.

All in all, hull design is a very complex craft. Kudos to your freind for taking a swing at it. but it sounds like he is thinking about things in far to simplistic terms. There are a lot of good references and software packages out there that he should consider investing in if he wants to produce a winner. If he is just in it for the fun of it and does not mind some trial and error (and some slow boats) then he might not want to bother with such sophistication.

Good Luck,

  • Will

Will Gorgen

“IMPULSE” - Main Hull: [ M01sm.jpg]( Lemke/2004213102228_M01sm.jpg)

Cougar -
if you look at line drawings - whether they are your NZ lines or my trimaran design (above) the cross sections “tell the tale” - so to speak.

In these posted lines, the right side of the drawing is looking at the bow of the main hull towards the rear to about the middle of the boat. The lines on the left, are the rear of the boat looking forward to about the middle. The lines represent the crosss sections at each “station” along the length of the hull. They are simply split vertically down the middle to take up less room on the drawing. **

If you look at the right side, you can compare the height and cross section width of each section and can visualize the lines of the hull when built.

You will notice that the bow of the main hull is extremely tall (deep) compared to the middle of the boat. The hull was designed to have buoyancy forward to slow down pitchpole, but at the same time it is designed to be narrow to help cut through waves. Further toward the rear, the hull gets fatter across, but isn’t as tall since it doesn’t need to be thin - just buoyant.

As noted by others, the #2 cross section has almost the same volume as section #8 - so buoyancy would still be similar - it is just that the bow is thinner and higher to allow wave piercing.

Now whether I stuff foam into section 1 to 2, or section 7 to 8 ---- it isn’t going to change bow buoyancy - it WILL add weight and the hull will actually sit lower in the water. Lower in the water is more displacement. More displacement is more wetted surface. So whether the boat is full of foam, or if it is empty, it isn’t the foam that will prevent/reduce pitchpoles.

Not sure if it will help you explain it to friend - but as noted, unless you make the cross-sections bigger, foam inside the hull won’t help. If you stuff 20 PFD’s inside the bow of a sailboat, they won’t prevent pitchpole either. Hopefully will help in his thinking.

** if interested in making a full size pattern for building for those who want to try for the first time - just fold a piece of paper and place the vertical line along the folded edge. Trace the line of half the hull and cut out. Unfold and you will have a full cross section. (Kind of like cutting out Valentine hearts in grade school) Once you have all the sections cut out, you can glue/attach to a board so the water line (or reference line) is parallel and horizontal to the board surface and can proceed to build. For more detial - see the US 1 Meter Construction Guide available under the Class Pages on the AMYA web site.

A couple of followups to the great points already made. As I learned to design hulls,I changed my perspective of looking at a hull going through the water to one of the water going past the hull. I saw a lot of hulls with a large “hydraulic” wave in the middle of the hull. You know, the wave on the side of the boat when it is heeled over that almost lets you see the keel blade as it enters the trunk. If you measure the surface length of the wave from the bow to the stern,it is signicantly longer than the boat = slow boat. On a good design, the amplitude of this wave is short, its surface length is short, which means the water is going past the hull in a shorter period of time = faster hull. Last summer, I waited each day for each selection of picts from the IOM Worlds and looked for hull designs that had lower, shorter hydraulics at a variety of heel angles. I found that most of the designs at the top of the fleet filled this spec. And most of those did not pitch bow down when heeled.
I have used a couple of programs that allow me to enter the heel angle and predict the pitch angle. I modify the design to add bouyancy in the bow or take it from stern sections. Thus, when the hull is heeled, the waterline length is maintained. As I designed lighter boats with narrow bows and beams, I also learned I needed to move the LCB, CF,and LCG aft, keep them as close as possible. Using conical sections (more circular than parabolic) helps keep the pitching to a minimum and WL as long as possible. When I measured and analyzed some IOM’s, some lost over 4" of WL when heeled 30 degrees or less.
Folks who want to start dabbling in hull design should try the free version of Hullform. Just play with the hull shape, do not worry about the keel designer, not suitable for high aspect models.
Search the internet for Hullform 8S or later. It does a good job, if you spend the time to learn it. I use this program once in a while to validate my other programs. Now, I use some design programs licensed to my company for hemodynamic tests, the math (the pre-computer version of computational physics?)is the same, I just like the chromatic displays.

<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by svandre

Last summer, I waited each day for each selection of picts from the IOM Worlds and looked for hull designs that had lower, shorter hydraulics at a variety of heel angles. I found that most of the designs at the top of the fleet filled this spec. And most of those did not pitch bow down when heeled.
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Steve - if photos are still available, is it possible to post an example of each?

Label one “faster” and one “slower” so we can see the difference. If you have a photo program, perhaps crop to remove sail number if possible, so we don’t “point fingers” at anyone. I think I understand what you are saying, but having two example to compare might be helpful if you have them. Based on overall length of the hull at one meter, it should be easy to scale the hull wave to estimate total length.

Greg: If you look at the S/W it does not have a sharp front end but a low 1-1/2" compared to about 3-1/2" for most 1-M. It’s angle at the water line is about average for a fast boat. 10 degree --1/2 angle.

<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by Matthew Lingley

You can have a narrow bow with lots of bouyancy, you just make it deeper below the waterline and/or higher above it, that way as the bow goes down you have more bouyancy to push it back up. Alot of the modern fulsize cats use that trick.

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If you have the choice between a deep narrow bow and a wide shallow bow (same displacement)what would the differences be? I can see more or less lateral resistance. What about drag?

Vancouver Island

With a deep narrow bow the CLR will move forward a bit. I think the general rule is that narrow bows are fastest in chop/waves, but chop to a model is anything over 1/2", it doesen’t take much wind to get that. You’ll have to wait for the experts to tell you about drag in a scientific way!

Luff 'em & leave 'em.

Greg - just a hypothetical question.

What if you were to take a US1M and build it to resemble the shape of a 110 Class boat - but instead of flat decks, make a very high “crown” to them? Thus you would wind up with a very thin (narrow) hull with a deck, that if it does submerge, would tend to shed water quickly.

You wind up with a planning type of hull, and canoe bow/stern, and very fine entry and exit.

Does/has anyone emulated the 110 design that you are aware of in r/c sailing? I was always fascinated by the ease in which the boat was driven through the “chop” and have wondered if the design would make sense. Isn’t the “ZIPPER” a pretty narrow, fine entry boat?

And then there are the Tucker “Ducks” …

H.B. Tucker, the great UK designer, designed an M boat around 1939, but didn’t build it until 1945. To get the lines he took a successful 10-rater and chopped off the overhangs on both ends, and rounded off the bow. The boat was named “Donald Duck” and was very successful in the heavy air conditions of the UK. Here’s a (fairly lousy) picture of one. Kind of like a skinny Inland Scow.
Tucker made and sold them, but never published the plans, so data about them is fairly hard to come by. Funny nobody tried the same idea with an IOM.



Download Attachment: [ duck.jpg]( Boebert/2004214182225_duck.jpg)

Very narrow with fine entry hulls can work in rc sailing if the boat is designed properly-but not necessarily well in all classes. Multihulls have beam to length ratio’s from 9/1 or so for a main hull to 18/1 for some ama’s. On a tri the boat can be designed so that the boat CG is behind the ama center of buoyancy creating a designed in anti-pitchpole moment when the main hull flys.
Monos can be designed with a flared high freeboard hull forward to provide RESERVE BUOYANCY as Bantock has done on the F100( a fairly fine entry).
In classes like the F100 and some multihull classes movable ballast or hydrofoils(including but not limited to rudder t-foils) can be used to increase resistance to pitch.

Doug Lord
–High Technology Sailing/Racing

“Bows Down” in multihulls, and I suspect perhaps similar advantages in monohulls, are two-fold…

First: the bows down raises the stern to keep the boat from “squatting” in the water, which in turn adds to potential speed - especially in light air.

Second: Bows down provides a forward point of lateral resistance, which (in multihulls) tends to aid the pointing ability of the boat, allowing it to sail closer to wind on weather legs.

A result which is also gained, is that by heeling the boat to leeward in light airs, keeps camber in sails, which might normally be hanging vertically with no camber at all.

Doing the “Wild Thang” on a beach cat in light air, definitely improves upwind speed. Basically one sits on the leeward side near the front beam - causing the cat to heel to leeward with the lee bow pressed down and both sterns higher off the water. You are really good when you can sit “down below” and fly the weather hull, thereby further reducing the surface drag on the weather hull, board and rudder. Takes practice confidence and a quick had on the sheets in case of a small puff! [:D]


I crewed on Hobie 18s for a little while in the mid 80s. We raced one regatta on Lake Mendota in Madison WI in 1985 that had very light winds. The normal position for the crew in that type of wind is to be all the way forward on the leeward bow. I would lay down prone on the bow with my hand on the forstay bridle for stability. My legs would straddle the bow. Getting out there required some careful shimmy-ing and it took a while to get back onto the tramp for a tack. The skipper sat just to leeward of the mast on the forward cross beam. That is as far forward as the tiller extension would let him get.

  • Will

Will Gorgen