Hull Design

I tried, but couldn’t capture the subtle hollow entry which is (IMHO) the key to its upwind surfing ability. The USVMYG has electronically restored Houk’s original drawings; PM me for details on how to obtain a copy.



I made a hullform file which looks pretty close to “riptide”,seems we can no longer attach .hud files here though:(

Yes, I had the same concern, but as I thought of it further, I tried to visualize the shape of horizontal sections sliced incrementally below the WL. These would also assume the banana shape to some degree, and exert similar yaw moment, but now in a 3-D volumetric sense. Picture a concave shaped garden shovel being dragged sideways through the water. The actual waterline will be curved & “want” to turn in that direction, but the portions below the water will “act” in the same manner.

Your explanation also makes alot of sense, thanks. I hadn’t thought of it in that way before but it’s clear that the 2 force vectors would form a couple
causing the hull to turn. I see no reason why both explanations would be mutually exclusive. It is likely that since both factors are acting in the same direction, they would be simply additive. The “lead” of the CE fore of the CLR is needed to balance the combination of these other forces.

Bill K

Put up the offsets as text and I’ll try and find time to run the “Turner Curves” on it.



Brett, please send me a copy of the Riptide hud file when you get a chance. I think it would help me as I try to understand all this. I might be able to help with the hollow entry that Earl mentioned, too.

thx…Bill H

heres some views from hullform for discussion
waterline at 0 degrees
waterline at 25 degrees heel.

I have not visited the site in about a week and suddenly there are so many posts. Earl I’m going to take some time go through your past and other posts. It might take me a little while.

I would like to thank everyone that has posted here. This has become very interesting. I started reworking on the shape of my hull but I’ll be stoping to diggest the information that has being posted.

Should I be working on Hullform instead?


Here is RipTide again,Bill found the time to fair my first attempt up a little.
Earl,You could print this view and check a couple of the sections for your magic curve??
There seems several schools of thought on hull balance.
Turners metacentric methods.
methods using symetrical waterlines.
and I have found some references to the “welch” axis.
After searching the net for quite some time I have come to the conclusion that the early model yachtsman knew more about hull balance than us modern radio sailors…and perhaps even more than modern "big boat’ designers.
Seems there is some valuable knowledge painstakingly found by those who went before us…I would like to get in on what was known as I belive a well balanced radio yacht is much easier to race with than one that is not so well balanced.
lets find out what makes a balanced hull shall we???

Before you all ask this is my understanding of what the “welch” axis is.
I have attached 3 files which show the a heavy red line showing what I belive is the Welch axis for the 3 heeled hulls.The line or curve is plotted along the deepest part of the canoe body when heeled.
Though the term is not widespread or well used I stand to be corrected.
Another possible explanation is the half beam is plotted for each heeled section.I am not entirley sure just what it is exactly but think I am on the right track.
The references I have found suggest that this axis (whatever exactly it is)should be as straight as possible for good balance.
On a double ender for example there is no way this axis could be straight,and of course there are many examples of well balanced double enders.(esp balanced by Turners methods)
You could see that a wide tansom design such as the TS2 IOM for example could quite possibly have a straight welch axis,though it will be slewed to the side,also wide transom hulls can have symetric waterlines when heeled,again slewed to the side.
So the question of good balance remains…Do turners metacentric shelf designs have it? or the wide transom types such as the aforementioned TS2 IOM which is reported to be well mannered at all heel angles also?
Is the secret in the LCB and other centres not moving on heel?
or something else altogether?

Hi Earl

Something close… I’ve added the calculations you are looking for to my ‘Circular Arcs Hull Design’ spreadsheet. So, as long as you are happy designing with circular arcs (smile)…

It is too large to upload here in the forum, apparently, so I’ll upload it to my Web site shortly. But here is a sample metacentric plot.

Cool. (the plot, dunno how I feel about the hull :-))

Please feel free to use any or all of my post in explaining the curve.



Hi Earl

Will do, thanks!

Here’s a comparison of yours to the original: one with identical waterline beams, one with identical deck beams. Yours in read, original in black. Plus the USVMYG restored sections of the original.



Thanks for the better section view,I have modified my file to suit.I am getting very close now,will post again when finally satisfied.
What I am seeing so far is that the LCB and LCF are very nearly in the same place,both shift slightly fwd on heeling.The whole hull lifts out of the water as she heels.
Also if I am correct that the “welch axis” is at the deepest point of the hull on each section as the hull heels…then “Riptides” is still down the centreline as she heels,dead straight.

Earl,on another subject.I have read someplace that WJ Daniels also developed a method of balancing hulls(circa 1904),do you know of this method??

The rising out of the water is, I think, key to the surfing behavior. It’s really a bit of a thrill to watch a “Rip Tide” get hit by a big puff. The boat dives for a moment, and then just seems to leap out of the water and go. And dead straight, as you said. I’ve always viewed Houk’s design as the “Ranger” of the the American free-sailing M’s.

I think part of the reason for that sophistication of design has to do with the nature of free sailing. Once launched, there really isn’t anything for you to do except to intensely observe the boat’s behaviour.

On your other question, Daniels’ technique using “in wedges” and “out wedges” is basically that of determining that the heeled and upright CBs are the same. The heeled/upright CB correspondence and its relationship to hull balance was known to designers as early as 1885, when it was documented in C.P. Kunhardt’s book. I think a lot of the “lore” of hull balance circulated as esoteric knowledge amoung NAs and wasn’t published much.



I said in a post a year or so ago that scratch-building “Rip Tide” was on my wish list of future projects. Following this thread is making me want to bring it right to the top if the list :slight_smile:

I have downloaded your IOM. My comments are as follows, Great boat- good on you- but I feel the bow sections look to fine. Why I say this is that some time ago I asked on this forum why my very narrow 170mm beam IOM would not go down wind. It was thought it was bulb cant. Well it wasn’t, I did find out why. I to use Hullform. I ended up with a hull of around .053-.054 prismatic co- efficient. The bow showed as .056. I thought I had a very balanced hull, but I was wrong, while the boat went upwind great, downwind it was a dog. To rectify it I bogged up the front 1/2 of the plug - layed up a new 1/2 hull - cut the hull off around the deck and just in front of the keel stuck the new 1/2 hull on and bloody beauty. It was like night and day. Interesting I used standard bow section shapes supplied on Hullform. They are obviously to fine for an IOM type boat. Mabye okay on full size yacht. I dont know how maxsurf works but I believe you need to bulk up the forward sections. This will also stop the bow going down when heeled upwind. Just my thoughts as I’m only an amateur designer who has built plenty and burned more IOM’s than I care to remember.



Thanks you for your comments. I have reworked the hull some what. I’m attaching the file. The main things that change are: reduced the beam, reduced the draft a litle. I also made my transvelsal lines stiffer. I worked on the front 1/2, which is fuller now. I started working on that before your comment, which after I read your comment made me feel more confident on what I was doing.

Here is another thing I did. I scaled up the boat to 10 m instead of 1 m. This way I can use the other tools that come with maxsurf: hydromax. Sea… Before the boat was too small to be able to seen any changes like resistances. when it heals. Should I go back to 1m or those it matter.

I’m glad to find someone with

I know very little about these computer based design programs, but you should be aware that there are tremendous “scaling effects” whenever changing dimensions such as you have done. The 2 hulls may have the same “shape” but will have entirely different performance charactersitics. This is largely due to the fact that although the linear dimensions can remain proportional, measures such as volume (buoyancy) will be changed exponentially to the 3rd power.
Bill K

That is good to know. I’ll get back to the 1m.