IOM vs a Hybrid IOM CBTF

Here’s a hypothetical question to compare a standard IOM with an IOM CBTF, that?s fits within the measurements of the IOM rules, ie
LOA 1000 mm
Hull draft < 60 mm
Keel draft < 420 mm
Keel weight < 2500 grams
OA weight > 4000 grams
Sail area A, B & C rigs

Hull Measurements

(If someone can throw these figures into a hull design program to do the number crunching for wetted surface area etc would be great and even a vpp for each boat)

--------------------------------------IOM,----------IOM CBTF,
LOA------------------------------10000 mm,-----------1000 mm,
Beam max---------------------------240 mm,------------220 mm,
Beam WL----------------------------200 mm,------------180 mm,
Hull draft--------------------------56 mm,-------------40 mm,

(Pity that funnybones - has not got the information on the various IOM designs on his web page)

Righting Moments

There have been some assumptions in working out the following.

  1. The centre of buoyancy has been taken at 50% 0f the hull draft.
  2. There has been no allowance for the increasing RM as the hull heels due to the shift in CoB.
  3. Any thing else that I have forgotten!!!

----------------------------------------IOM,----------IOM CBTF,

Keel weight----------------------2500 grams,---------900 grams,
Keel draft
(centre of bulb to CoB---------------377 mm,------------370 mm,
Cant angle------------------------------n/a,------------50 deg,
Heel angle---------------------------20 deg,------------20 deg,
RM-----------------------------32,275 gf/cm,------31,292 gf/cm,

Weight Break down

----------------------------------------IOM,----------IOM CBTF,
Foil & bulb----------------------2500 grams,---------900 grams,
A rig-----------------------------275 grams,---------275 grams,
Hull weight
(includes rudder & servo,
sail winch
receiver & battery)--------------1000 grams,--------1000 grams,
Hull makeup weights---------------125 grams,-----------0 grams,
Front rudder & servo----------------0 grams,---------125 grams,
Keel drive unit---------------------0 grams,---------400 grams,
Total----------------------------4000 grams,--------2700 grams,

The IOM CBTF is about 1/3rd lighter.

The Race.

Since it is all hypothetical, the wind would of a consistent strength and direction so we can bang the corners on the wind. My guestimation is that the IOM CBTF would be faster as it carries the same sail area, same lwl, less wetted surface area, less displacement (1/3).

Ok guys go for it.
But no personal attacks please, just have great time.

whats your theoretical wind strength for this match race?

Is this really necessary? Do you miss the days of DL?

John, you’ve built a canting keel boat, go sail it and please report back on the results. I, for one, would love to find out how all of the things you are trying to accomplish have worked out.

Lets not go down the road of “my theoritical boat in hypothetical wind can beat your generic boat …” Those kind of discussions only create bad feelings. And really, the only answers that can be given are opinions with no real world evidence to support either side.

In light air i would expect the CBTF boat to be just as fast or faster than the conventional IOM.
However, when the wind starts picking up, as you said, the CBTF has less displacement, less wetted surface, but the same sail area. That would mean that on the wind it will most likely start to suffer severe leeway in the higher winds than the conventional IOM. I know a whole bunch of you guys are saying, but the CBTF boat has got the twin foils, so it shouldnt suffer leeway. Obviously one would thiink that, but on the R/C CBTF boats the foils cant be as precision controlled as on a fullsize boat. Also, the less wetted surface would mean that its easier for the wind to push the boat. That is a good thing, except hard on the wind. Beam reaching and close reaching, you can just have the boat point high so the leeway factor negates itself, but on the wind you dont have that option. Also, if this was an actual match race with a smart skipper driving the conventional IOM, than probably it would be able to outsail the CBTF boat to weather, if they engage in a tacking duel. The tacking duel would slow the CBTF boat down, because of the foils and keel swinging to the correct side. I would be incorrect on this upwind theory if someone has figured out how to completely swing their keel in less than a second. I race Soling 1 meters now, and in winds over 6 knots, we can tack the boats in 2 seconds or less. Another thing the tacking duel would do is wear out the CBTF’s batteries. So basically my conclusion to this would be to not sail the CBTF upwind in a breeze, and if i had to, i would try everything to avoid a tacking duel, because i know i could easily make the distance up on an offwind leg.


In your post you are contradicting yourself, at the top, you say the canter will be greater than 4000g, and then further into your post you say the all up weight would be 2700g.

I would hope a boat they weighs 2700g would beat a 4000g boat (with all other things being equal) if not, there is something wrong with the design or the skipper.

A more realistic test would be to keep them at 4000g, and see if the extra righting moment helps the canter at the top of the different rigs. I.e. they can carry them into higher winds.


Have to say I’m starting to agree with Roy on this one (OMG!), numbers are all fine but there really is only one way to prove it.

Luff 'em & leave 'em.

“--------------------------------------IOM,----------IOM CBTF,
LOA------------------------------10000 mm,-----------1000 mm,
Beam max---------------------------240 mm,------------220 mm,
Beam WL----------------------------200 mm,------------180 mm,
Hull draft--------------------------56 mm,-------------40 mm,”

In that case I go for the IOM [;)]


_/ if it isn’t broken, don’t fix it! _



Well my hull for me CBTF F100 is a one metre hull and I have a female mould which I have made and have started to build a fixed keel option just to see what the differences are so when I get everything finished I will let you know.

USA2 the leeway from a cnating keel boat is not as great as people think and in a breeze the forward foil has a lot of flow over it and tends to work very well and negate a lot of leeway.

Any way time will tell guys.

Cheers Gappy

The leeway on a canting keel boat isnt great-ur right about that- but thats on full size ones. I said that people would think that the CBTF boat wouldnt be expected to suffer leeway, but an RC one is likely to because you cant fine tune everything and the force on the sails will quickly become more than the foils can pruoduce enough lift more.


It all depends on how flat the boats is & how deep the canard of the canter is, if the canard is small & both boats are way overpowered, then the IOM will take the canter because it will go sideways, hardly any forwards movement, until you let the sails right off, well, thats what happens on mine, which has a small canard. have a deeper canard & it will probably stop it.

I see said the blind man to the crippled nudist who put his hands in his pockets & promptly walked away.


yeah it appears that once you are under way that the boats have plenty of power and height and not a lot of leeway but if you let them tip over in a gust before you have any speed they do go side ways quite quickly it is bvery important sail trim and keel trim to get going but once they are moving they can be very nice to sail it does take a while to get used to but it is great fun.

i am progressing with boat two and hope to have my mast made this week and then fit it out and then start painting everything so will see how long before i hit the water .

Cheers gappy

what you makin your mast outta? am still working on designs for using an assy kite on mine.

I see said the blind man to the crippled nudist who put his hands in his pockets & promptly walked away.


yeah i am doing my own carbon mast with a built in track in the back so will see how it goes I am doing this so that I can use the same mast and change mains easily with a bolt rope.

Cheers gappy