design an hull

okay guys
i am designing boats now for 10 years. and i just wondering if anybody has any input into how deep the rocker pannel should be. and where should the deepest be part. should it be at 70 % or close to the middle at 50%?, i have been putting the deepest at 50% and i have found the hull does not plane well. but it loves heavy winds and heavy seas? the seawind looks like the deepest part is at 80% and the boat is very quick in lite airs. but does not realy like the heavy stuff. what do you guys think?
i design IOMs. but this could also affect us 1 meters, marbleheads
long live the cup and cris dickson

All the best boats seem to be @ 60%. The Seawind is @ 60%. The problem with the Seawind is the freebord and not anything else. I have made a Seawind with a 3 1/2 " nose instead of 1 1/2" and it is great in heavy air and would also make a good IOM as the water line will hit better at 8#. The depth of the rocker depends a lot on the weight of the boat and the width. My US-1M are close to 1 3/4" for a 7" wide 5 1/2 to 6# boat. 2" deep for a 6 1/2" wide boat. 2 1/4" deep for a 6" wide boat. I have only US-1M’s to compete against so I have never made the Seawind into a IOM. Cut the Venom down to a 1 1/2" nose or any boat for that matter and see how it sails.

I’ve thought for a while now that changing the hull shape (within reason) will only change the preferences of the boat. If a change improves the light wind performance then probably the heavy wind performance will suffer.If the upwind improves the downwind will suffer.Its lke trying to buy a vehicle that will do everything you need-it just can’t be done. If you only have one boat it’s going to be a compromise. The best you can do is try to build a boat that suits the most common conditions at your pond. I think the way the wind changes here on the coast that I need say, 15 boats[:D][:D], at least till I have 15 then maybe I’ll reconsider[:D][:D].

Vancouver Island

this is one of the reasons. why i started this post. a ts 2 is very wide and looks like it is fast in light conditions?the rocker panel on that boats look well aft. now look at ikon. this boat has a deep rocker and in high winds. it goes well
don you were in vancouver for the worlds? what did you noticed? does anybody think that the hull needs a deep rocker pannel , but well aft?
long live the cup and cris dickson

There is another practical limitation in that the rocker shape, and maximum depth, impacts on the hull’s centre of boyancy. This in turn influences where you choose to position the keel fin, which in turn influences the position of the mast.

In an IOM, where the jib boom on the #1 rig is 400mm long, and the job boom balance weight cannot extend beyond the bow, you can’t position the mast too far forward anyway (even if you wanted too - with some interesting offwind behaviour issues). So working backwards, you are inclined to design the hull with the CoB somewhere behind 520mm, try and get the CLR not too far away, and then position your rig according to your thoughts on balance. This tends to result in the maximum rocker depth at 500mm or further aft. My current project has maximum rocker at 535mm, which is further forward than I would generally favor, but I’m trying a wee experiment.

As for depth of rocker, my preference would depend on the conditions the boat will sail most in. For a light weather biased IOM, I’d be favouring a nicely curved and deep rocker (the depth tends to be a result of the narrower hull anyway), but there’s always that 60mm draft limit to be aware of. In a fresh airs boat, I’d favour a straighter rocker profile, particularly aft, to try to get the boat “up” off the wind. Just my own thoughts Isn’t compormise wonderful?


Hi, there!
sorry for my bad English: what is a “rocker pannel”? I never heard it before.

regards from Germany

Hi Joachim, Wie geht es Ihnen?

What we are talking about is the underwater profile of the boat when looking at it from the side (when the boat is upright). The measurements refered to in my post above are distances back from the bow of the boat.

So when Cougar says in his opening post that the “deepest part is at 80%”, what he means is that the line of the keel (not the keel fin, but the deepest part of the hull athwartships) comes down from the bow toward the stern until its deepest part is 80% of the way back from the bow.

I hope this helps.



Cougar was referring to “KEEL rocker”.

“Rocker Panel” is the metal on a car or truck under the doors that curves down and under the body - replacing running boards. (at least referred to here in the US)

the rocker pannel that i am refering to is the underwater profile of the keel. start a the bow and the rocker will be 0 then go to half way the deep part would be say 60 mm. then at the stern it would be 0.
this would make a bow . even , now if you move is aft then the line going from the bow to the deepest part becomes a straighter line. does that make things alittle clearer?
long live the cup and cris dickson

what LCB position and Cp have you found to be most sucessful in your IOM designs Cougar?


thanks for the explanation!
I understood the discussion but I never heard this expression before. So I was not sure to get all details
Thanks once again.

regards from Germany

i have found that the deepest part should be around 70 %. but the problem then become nose diving. as most of the hull is in the aft section. my first fast IOM had the deepest part at 50% right in the middle and right where the keel was. my thinking behind this was to limit the wetted surface at the ends thus making the boat turn better. but he draw back to this was in lite condition the boat was a dog. it did not like going up wind. and i could keep going. my next design was moving the rocker panel around . the next boat the boat did not turn as fast , but it would go upwind better, but it also nosedived alot.
this is what i am trying to figure out. what else has people tried?
long live the cup and cris dickson

Did you change the entry angle( half angle ) of the boat when you changed the rocker position.

Yes Cougar,
I understand what you are saying about the rocker…
My question was what LCB and Cp have you found best?

I’ll echo Brett’s question - also curious Cougar, about the LCB and Cp. Further, to what extent has the LCB moved as you’ve moved the deepest point of the rocker aft. (This of course impacts on Cp too).

My guess is that Cp in an IOM will pretty much fall within a narrow range across most designs. Don’t know whether it does in reality - you experienced designers will be able to comment.

Your comments on your experience with different windard performance as you’ve move the deepest part of rocker - any comments on how the position of the rig may have contributed to this? Have you also moved the keel and rig aft? (Of course that links back to Brett’s LCB question again)

This is a great thread. Thanks for kicking off this discussion Cougar. I don’t think we are “off topic” with these questions, as all of these design factors are of course interelated.


this is a good topic. i am not sure what you meen by (cp) but the LCB was moved aft. this allowed a more gental slope. the keel i have always put at 50% i did make 2 models once with a differnt keel pos. and that initself can start a whole new thread. the reason i come to , when i say the future forward you move the rocker panel the more hull stay in contact with the water. this stops what i call slipage. there is probably a more techicnal term out there. but i still draw my boat with pencil and paper. with the rocker panel at 50% my black boat “cutlass” i have noticed that the boat like heavy winds, heavy seas, basicly anything heavy. the beam is also 7 inch
i first looked at bantock rtyhmn. and noticed that this boat like lite breeze and the deppest part was around 50%. but it was also 10 inch baem. this boat did slide sideways. in lite air. i look at my new boat bee gee. and i have moved my rocker panel to 75% and the beam is also 7 inch but the & inch is at 75%. "cutlass " is at 50% deep and wide. bee gee is at 75% deep and wide. both have the fins at 50%. what have i found out. bee gee is very quick to acclerate and in lite airs slides sideways. but for some reason that i dont know. it does not nose dive. but it likes high wind, just not waves
long live the cup and cris dickson

Cp is Prismatic coefficant.
Still no LCB position Cougar?
I have designed IOMs with Cp from 0.53 to 0.57.
LCB from 52% to 57%
Have not done enough definitive work to conclude what is best.
LCB and Cp are 2 parameters that I take great care in choosing before I start a new design

ps…anyone interested in a set of Lines for a 5.5lb 6.3 inch wide US1 meter…I drew one up today…never drawn a US1 before so thought I would see what I could come up with. I can e mail a CAD file with just the stations if anyone is interested.

My current project has a Cp of 55 - so right in the middle of the range Brett has used.

I always work out LCB for the boat when upright, and again at 30 degrees of heel. Depending on the heeled shape of the hull, LCB generally moves aft quite a bit when heeled - about 20mm (3/4 inch) on my current IOM project. I’ve positioned the keel fin, and CoG of the bulb, under the heeled LCB, on the basis that I want the boat to sit correctly when sailing upwind. The theory is that when the boat is upright, and sailing downwind, CoG will be slightly aft of the LCB, which may help pop the bow up a little. Don’t know yet - she’s still on the building board. Of course placement of the battery pack and servos impacts on this too.

Brett - are you going to build your US1M? That’d be heresy in NZ. I put my toe in the water a while ago to suggest there may be room for another class in NZ - but was soundly (and probably very sensibly) disuaded. Is it just a fun project, or do you have the export market in mind?


Brett will know from a conversation we had some years ago that I’m not an enthusiatic follower of Cp values (smile). It is certainly extremely important to know how the volume of a hull is distributed, but Cp is a little too rough for me.

As someone on a Web page said somewhere, if you design almost any normal looking displacement boat, you will almost always end up with a prismatic coefficient between .5 and .6. And, for an IOM, when volume and length are kept constant, all that the Cp tells you is the amount of your maximum cross sectional area. Interesting, of course, in the same way that the value of the maximum waterline beam is interesting, but…

Lester Gilbert

No Muzza,
the US1m was drawn just for fun.
Does the mind good to have a crack at all the different rules out there.

Lester is right, Cp is not everything…but usefull none the less to compare hull forms.
The full size texts will tell you there are speed penaltys for non optimum LCB and Cp…who am I to argue?