The technology discussions area hasn’t had many posts recently so i thought i would kick one off and having scoured just about every thread on this site I’ve seen very little mentioned about transom designs/shapes.
There are two aspects that i can see being important (but I may well be missing some others!)
first of all from a performance point of view what are peoples thoughts on the differences between a more standard solid transom…
to the more ‘skiff’ like designs i.e something along the lines of
Secondly when i raced full sized performance dinghys we would spend hours with car body filler and sand paper trying to get the edges of the transom as sharp as possible in the belief that this would aid the water ‘detaching’ from the hull and not curling up a rounded edge. is this just a myth or is a sharp edge better?
I’m asking because a lot of the IOM transoms i see seem to be very rounded and I was wondering if this was done for performance, just for the looks or whether its just easier to laminate that way?
I think after all this rambling I can narrow this question down to - overall what’s the optimum transom design for a IOM? (can you tell I’ve just had some plans for a Bantock Vektor delivered and need to make a desicion about the transom? lol :))
Hi Tim,
My thoughts have always been along the same lines as yours, sharp edges. It is not really any more difficult to layup, you just have use gel or thickened resin in the corner, with the cloth following the more gentle radius of the fillet.
I have just about finished the molds for a Vektor, and have pulled one hull and deck from them. I went with the skiff design because I like the looks better
It may drain easier, but it is probably a bit heavier. I’m working on a fin, of which I will make a mold of, then the mast box/fin box.
I’m a big believer of a sharp edged transom to break the flow and reduce turbulence.
I used to have a little electric runabout with a 6v motor and battery. It would chug along with the transom in the water and a bubble of turbulence right at the transom. I took a piece of thin plastic and taped it to the transom as a trim tab. The boat now zoomed about on a full plane. There was a significant increase in speed while power remained constant. The difference was the reduction in turbulent flow and drag around the transom.
I wonder if anyone has ever done this on a sailboat. Somthing like making an IOM 1/2 shorter than allowed then add 1/2 of plastic or just make proper length then inset the transom in 1/2"
I’m also a believer in the “suction breaker” or “sugar scoop” transom. Move the vertical transom forward and have an extension of the hull shell go aft “for a bit” (1 inch on my 36 inch Restricted boats, 1/2 inch on an RG65). You can definitely see the difference on the stern wave. A steeply angled transom will probably have a similar effect. The idea is to have the airflow over the hull “reconnect” with the water without a sharp pressure drop at the end of the hull.
What about this on an ISIS IOM by Barry Chisam from about six or seven years ago. Does this count as a ‘Mini Sugar Scoop’. I have yet to see water tumble into the short transom in any wind speed.
The boat design is called FH (Flying Haggis). The designer/builder (originally from Scotland, obviously) is a local sailor on Salt Spring Island. And yes, it is very beamy (280mm). Should be good in stronger winds but may suffer from too much wetted surface in light winds. We should find out in a couple of weeks. It is almost finished. Just the fin and balast to install. First regatta outing should be the Western Canadian IOM Championship in early June.
My thoughts on the wide hull are that it should be stiff and resist heeling from the wide hull. Also the heeled sailing length will be a bit longer than a ‘normal’ IOM.
I am experiementing with the extremes of the class. I am also sailing a borrowed MIOMI and that’s one of the narrowest IOM design around (165mm). Being narrow, it has minimum wetted surface for light airs and penetrates waves well.
This weekend is Round 5 of the All Island IOM series and I’ll be sailing the Miomi at Maple Bay Yacht Club (near Duncan, BC).:zbeer:
Fascinating to hear everyone’s take on transom design. Seems like sharp is the way to go and I think I will incorporate a ‘sugar scoop’ style. Thanks for everyones input so far.
Flying Haggis ( I have been at the wrong end of one!!) looks like a take-off of an Australian TS2. Next thing we will need elastomer bumpers on the transom corners to prevent damage to other boats in a wrongly timed slam dunk.
Dinghy boats, if they are planing types, will profit from a sharp edged bottom. If the dinghy is strictly a displacement type then the transom should not be in touch with the water surface. In that case the sharp edge, or lack thereof, is of little consequence. In the case of an IOM or other RC hot rod; while they may not plane, the exit will best be made somewhat sharp edged. The lower edge of the transom will, at some time, be engaged with the water.
Angled transoms and sugar scoops are more nearly an effort to dimish the weight of the ends of the boat than to accomodate some hydrodynamic principal. Light ends are to be sought because that tends to reduce the pitching moment of the hull. Hobby horsing is bad news for a boat of any sort because it diminishes speed big time. Particularly true of a boat that uses a fin and bulb.
