Measuring hull resistance.

I am currently working on the design of a IOM using the free version of the Delftship software. When comparing hull resistance - using the provided John Winter’s (KAPER) program - I have noticed that where one places the Midships Location has a dramatic effect on the results. This begs the question: where should Midships be? Is it:
[li]1/2 the length of the boat[/li][li]midpoint of the LWL[/li][li]longitudinal centre of bouyancy[/li][li]waterplane centre of flotation[/li][li]longitudinal centre of effort[/li][li]point of maximum waterline beam[/li][/ol]
I shall be most interested in your reply.

Thanks in antisipation,


Hi James … I’m in the camp of little bit of knowledge being dangerous :blkeye: but couple of points so I can follow the discussion.

For an IOM points 1 and 2 on your list are the same reference or not?

By resistance are you talking about Hull “surface” friction and or Hull “wave” resistance ?

And finally about old terminology “Midships” is this maximum Beam @ Waterline ?

Cheers Alan

Hi Alan,

In answer to your query: (all measurements are from the stern)
Point 1 would be at 500mm and point 2 would be at 494mm - my LWL is 981mm and 3mm of the hull’s stearn is out the water.

The John Winter’s (KAPER) program gives Rf and Rr, which is friction and wave, and combined.

Regarding terminology - I have raised this question because definition of what is “Midships” (required entry under Main Particulars of Project Settings) is what I want to know. The default setting is 50% of Length, but it can be changed.

I can see arguments for any of the options I have put forward, but there must be an “accepted” definition so that different designs can be compared.

I hope this helps clarrify things.

All the best,


Hi James, I thought (wrongly) that an IOM had to have same LOA & LWL, I did not realise they allowed to have overhangs :confused:

Anyway, I looked up the definition of “midships” and says it is the midpoint of the LWL …is this old sailor slang for exactly “middle point” or only approximation ? …like put it other there some where ! …sheesh lets hear what the experts say.!!

I’m interested in what your free designer software recommends should be the best position too ?

Cheers Alan

Hi Alan,
There’s no need for an IOM to have same LOA and LWL. In fact many don’t in order to reduce wetted surface area for light airs. You may like to read the Seahorse article on IOM design… There’s a link on Lester Gilbert’s site:
The Delft software is silent on the subject.



Thanks …shouldn’t have… but did & wish I hadn’t :scared:

Makes me think to what ClaudioD is always telling me, for serious competition at high level you really need two boats, one designed for light and other for heavy conditions, reading between the lines, that article more or less confines this is what is happening at the IOM worlds, level.

Cheers Alan

Hi James,
in general the max beam is positioned between 55 and 62-63% of LWL. Much depend also on the main section form and area.
Deep draft will produce deeper wave hence more resistance.
There is also a ‘fineness coefficient’ that suggest about the fluidity of the hull when sailing in the water :
LWL area / (LWL x Beam Lwl). It should be around 0.65/0.68, lower the better. I will consult my books !!!
Since you refer to the Delftship, than the answer may be already in your hands.
If you can try to modify the hull form in width and dept and check the resistance variation.
Wider a shallow hull are offering stability and reduced wave formation. The 60% of the LWL could be a good choice.

Hi ClaudioD,
Many thanks for your reply. I have learnt something new that is also very helpful, however my actual question remains unanswered - probably because I didn’t phrase it very well.
“Midships” is a required entry under “Main Particulars” of “Project Settings” of the Delftship software. The default setting is “0.5 of *L”, but it can be changed. Whilst working, I have on occasion moved the midpoint in order to see midship sections more clearly. I have then checked the “Design Hydrostatics” and “resistance calculations” to see what effect my alteration has had. If I have omitted to move the “midships” back to the default setting the “resistance calculations” give wildly different results.
Out of curiosity I have Set the “midships” at: (measured from stern)
1.1/2 the length of the boat - 500mm
2.midpoint of the LWL - 494mm
3.longitudinal centre of bouyancy - 431mm
4.waterplane centre of flotation - 429mm
5.longitudinal centre of effort 458mm
6.point of maximum waterline beam - 400mm(est.)
7.max beam OA - 450mm
to see how much the results vary and was suprised by the extent of variation - hense my question.

My overall design thinking is based on the fact that watching boats - big and small - in different conditions, it is very clear to me that one of the hardest things is to break through the wash of another boat - hense the fact that the lead boat often breaks away from the rest of the fleet.

My design has a wetted surface area of 147mm2, beam LWL of 173mm, LWL 981mm and LWL area 114mm2. Using your “finess coefficient” I get a rather high result 0.67 - so I shall be doing some extensive thinking about the implications of the “finness coefficient”.

All thoughts and comments greatfully recieved.

Kind regards,


This may not be terribly helpful as what I know about IOM’s could be written on the back of a postage stamp (and still leave space for the complete works of shakespeare). However, from a terminology point of view, midships is generally defined as the point of maximum beam, therefore has little to do with LOA or LWL. My experience is predominantly big boats and I don’t think I’ve ever been aboard a yacht where the max beam was also LOA/2 or LWL/2. If you look at the current crop of Volvo / Open 60 type boats, I’m pretty sure some of them will carry max beam at or near the stern - definitely not midships. Jim (Astute Composites) has ‘hands on’ experience of these types so if he’s about he may chime in.

I assume your wetted surface area calcs should be in cm2 rather than mm2 - if not then your genius knows no bounds and the likes of Judel & Vrolijk, Farr and Dubois will be beating a path to your door!!



Hi James ,
I went checking some of my books and very little is said about the longitudinal beam position except in one case where the general statement was “…faster the boat more recessed the beam shall be…” . nominally the same book is saying that the beam shall stay between 0.51 and 0.56 of LWL from the bow.
From the Delft studies carried 1995, there is curve that relate the efficiency of the hull against the speed and Prismatic Coefficient and suggesting the LCB position in terms of percentage of LWL.
Now I think you should not regards to the beam, but rather to the main bulkhead surface since from there the resistance is calculated.
Please see :

The first diagram
combine two curves, the LCB percentage position as function of Prismatic Coefficient and boat speed, the other referring to the Prismatic coefficient variation against the boat speed.

Tracing an horizontal line at PC = 057 you get directly on the righ the value of -3.2%. I use the sign “-” because is after the middle section sitting at 50% of the LWL where the shadow n°5 is also positioned. It can be also observed that the LCB curve is indicating that the speed coverage may go from 0.31 to 0.44 Froude or Vr from 1.0 up to 1.4 this late being valid for planning conditions that our models never reach.

Boat speed is the driving parameter. Our model when really sailing fast, they reach a VR around 1.2 or Froude coefficient of 0.40.
You can thus notice that the LCB should stay around -3.6% of LWL, while the PC is almost at 0.59.
More reasonably for a VR of 1.0, the diagram will suggest an LCB of -3.2% of LWL and a PC of 0.57.
As mentioned by Alan, you need to decide for which prevailing weather conditions you want to design your boat.

A second diagram is simply a typical Curve of Areas where you can see the LCB found at -3.0% and the main bulkhead sitting somewhere at -0.55 of LWL counting from the bow.

There are many other considerations about the hull form as the entrance and exit lines, and draft.

Hope this may help !


Hi Row,

I’ve editted my above information to add in the location of max over all beam, which is at 450mm - and for those are interested is 201mm.

I fully appreciate your point about the real location of max beam - my early offshore racing was on “MANITOU IV” - a 42 footer (with a bowsprit!) designed and built for the 1969 Canada’s Cup, which it won 4-0.
She was the first boat specifically designed to “cheet” the water into thinking the boat was longer than it actually was by having b max way aft and a transom that was 9ft wide! It was very fast in light airs, but was the easiest boat to make broach that I have ever sailed.

Regarding areas - the program shows wetted surface area as 0.147m^2 and waterline area as 0.114m^2.

Many thanks for your input,

All the best,


Hi ClaudioD,

Thank you very much for your research and very helpful input. I have some serious thinking to do…!

I have no doubt that I shall be back with further questions after I have digested ‘your’ facts and figures.

Very best regards,


Hi ClaudioD,

Many thanks for your input. I now think I understand why I was able to get exciting ideas with radical thinking… I have allowed Delftship to run on its default midships of 0.5*L instead of using the point at which the section/bulkhead is greatest as the midships. I hope I now have this straight in my mind. It does make a rather dramatic difference!

Thanks again,


P.S. Just a thought… is there an accepted total resistance area that I should be aiming for with an IOM design? I’m currently thinking in terms of 0.0060kN at Froude Fn 0.400.

I’m not familiar with these parameter values and never worked with Delftship.
What instead I can suggest is to buy the ‘Principles of Yacht Design’ of Lars Larsson

Fair enough ClaudioD. Thanks again for all your very helpful input.

Kind regards,