Z keels

Hi Guy’s,
I have a keel question. Is there any place for a “Z” keel in the IOM class, or for that matter any rc class? Here’s a link to what I’m on about; http://rcd.typepad.com/personal/2004...sayers_mi.html

What for , probably as a weeds keeper ?

It seems to be a slow page, or not working.

thr Z shape could allow more surface area with less length? What material would you use for construction? IMO, carbon fibre would be the required material.

Site is working but the link seems to be incomplete and leads to a white page …

This is what I found about “Z” keel :


Of course it can be wide and short but the ballast will be also closer to the waterline !
In order to keep the righting moment constant, the ballast weight shall be increased.
To avoid sinking due to ballast higher weight, it is necessary to increase the Displacement at Hull level !
Larger Displacement means also larger mid shadow surface and therefore larger drag !

Of course a 38ft yacht with 4.5mt deep keel may have some mooring difficulties in some harbours…expecially with tides …


Another link to the thread


this link works - thanks.
For my opinion the “Z” is necessary to balance the hull and keeping simultaneously the bulb in place. The daggerboards are moving the center of effort forward …

I’ skeptic about, for me there are too many independents surfaces each one producing their own turbulences at the trailing edges of rudder, “Z” keel, and dagger board.
I also suspect possible gyration problems developed around a buoy…!
Wet area is increased !
I do suspect also oscillatory vibrations due to the bending arm distance between CG of bulb and Z-fin attachment to the hull ?

As far as the IOM class is concerned the forward dagger board is not permissible. These foils were around in the early 90’s in the IOM class as were scow shaped hulls. both have since disappeared, maybe for an obvious reason.

I’ve just built an IOM hull and only have the keel/mast box to go so am wondering if there is any advantage to such a keel?


I do confirm, daggerboard not allowed on IOM.
As said before IMO there are no advantages, but probably the contrary. The experiment shown was made in 2000 and since the “Z” Keel was not seen anymore. Unfortunately a whale put an end to the ‘experiment’, (it would had happen with a strait vertical Fin too) : http://www.maritimenz.govt.nz/Publications-and-forms/Lookout/Issue-14-12.asp

Hi Claudio,

I think I understand your thumbnail attachment. Is it correct to assume that a “Z” keel would generate more drag going through a tack compared to a vertical (standard keel) configuration? What little I’ve been able to learn is that a "Z"keel will ventilate less and be more efficient at slow speed than a vertical keel. Plus the extra length is a benefit for a given maximum depth as allowed in the IOM class.


With my imagination about the giration effect when the boat is turning, I could search a certain analogy with the 3 rear axels of a lorry. When the lorry turns at a close radius the rear 3 wheels are sliding and rubbing the floor.
While the cantilever effect exist with the bulb attachment to the Fin and this late anchored to the hull with a long arm between fin top base and bulb CG. See Torsional effet.
For our models is also a good method to retains the weeds !
One possible advantage is that the Rig can be positiond toward the transom and possibly reducing the nose down !!! who knows !!
May be a wrong assumpion ?