IOM Design Question

Hi Guy’s,
I realise that wetted surface is king in IOM design. But what I do have a problem with is righting moment. Can anyone describe what numbers I should be looking for - or is it even worth worrying about? Program I use is Hullform.

Current design is;

Beam .210mm
W/L Beam .170mm
Wetted surface .149 sqm
Waterplane area 0.112sqm
Co-efficient .0565
Forward co-efficient .0510 ( I have problems with getting a balanced co-effecient with narrow waterlines)
Stern co-efficient .0610

Looking forward to all input


Your max beam 210 and w/l beam of 170 are very middle of the road for current IOM. The main way to increase stability would be to go to a much wider hull, like the TS2 at 290 beam, but at the expense of wetted area. The best way to increase stability is to make your A rig as low as possible to lower its COE.

One thing to be careful about. Hullform tends to produce a hull that is very fine forward (sections 1,2,3 come to a V). This may lead to early nose diving compared to a more rounded, fuller forward shape.


Thanks for input John,
I use the free version of Hullform and you’re correct about the “V” shapes in it’s standard format. Moving away from Righting Moment for a “moment” LOL, with only 6 lines to play with “U” shaped bows and flattish sections between water lines and the gunwale can be a little difficult to achieve so after getting everything I need I sometimes have to use a bit of hand drawing to finish it off. Also spot on about being fairly standard stats wise. The class has,I believe, over the years worked itself into a corner. I feel the design gains have now become minuscule.
Going back to TS2 type of hull is in my opinion is just that going back. A great boat in it’s day but there are now large numbers of designs out there that are quicker, in my view.

Hi guys,

I’ve been using Hullform for several years now, and I find that by using the lateral and vertical control lines I can achieve almost any shape that I want. The attached pic is an example.

Have fun…Bill

Hi Bill,

I believe you are using the pro version which is a lot more sophisticated than the free version. For example with only six lines available it’s very difficult to obtain a rolled gunwale. Also once the shapes start to get complicated things start to go pear shaped very quickly. After looking at what you’ve produced maybe I should spend some money.


I’m a cheap guy, Tony…I use the Student version of Hullform and the free version of FreeShip from before Delft got it.

Play around…the key is to use the control points as well as the offset points. The offsets make the line, and the control points change the shape between the lines. You can manipulate the control points either when editing sections or when editiing lines.

Take one of your designs that came out too V shaped. Start by editing a master section…leave the offsets alone and use the control points to change the V to a U. Do the same to your other master sections to get them to the shape you want at key points along the hull. Then switch to editing the lines to smooth everything out. It will take several iterations before you are happy with the result, but persevere!

It takes some practice to learn how to get the effects you want, but it’s worth the time invested. The sample I posted uses five lines, and represents a lot of hours of work…but most hulls are a lot easier.


Tony, I made an IOM version of the boat I posted earlier. I’d be happy to send it to you if you PM me your email. You could use it to play around to learn the effect of control points. This forum doesn’t permit uploading Hullform files.