Why chines work on IOM boats

Taken from the R/C Forum on why chines boats work thanks to Craig Richards SA 


There is a discussion on a boat design forum about chines. I have quoted an iteresting take below: 

I mentioned earlier that I thought the Britpop had a "light" bow .. this explanation would suggest that it allows it to point better upwind.

http://www.boatdesign.net/forums/hyd...m-41874-2.html.

“I read through the newsletter section about chined hullforms, and a few things are apparent to me from the discussion and the photographs. The picture of the IOM chined boat on page 14, compared to the pictures of other boats on later pages, shows the hull with comparatively full sections forward, a lot of rocker, and very slight sections aft. The other boats, such as the SKAs, have finer sections forward, less rocker, and fuller sections aft.

The new IOM chined hull seems to have better pointing ability because as the boat heels, the volume distribution and particularly the waterplane shape at heel promote lifting the bow. As the French designer in the article indicates, the chine is just a consequence of how he wanted to shape the heeled waterplane--he wanted slack sections aft, not full sections, so that as the boat heeled, the narrower waterplane aft allows the stern to sink a touch more and, correspondingly, the bow to rise, pointing a little more to weather, which is the direction you want to go anyway. And as the boat heels this way, it also increases the angle of attack on the keel and rudder, which increases lift, and with more lift, there is more power pulling the boat to windward. the boat balances better with less increase in weather helm.

This effect has been known for well over a hundred years, and as I have stated before elsewhere in this forum, most latter-twentieth-century designer types, collectively, seemed to have unlearned this lesson. This has more to do with heeled waterplane shape and controlling the center of that shape, the longitudinal center of flotation (LCF) than it does with the chine. Therefore, I agree with the French designer.

Capt. Nat Herreshoff, the winningest designer of the latter 19th and early 20th centuries, promoted this feature in his hull shapes--slack stern sections compared to fuller bow sections to control the LCF. As the boat heels, LCF should either stay in the same fore/aft position, or move forward slightly. As the boat heels, the bow lifts, the stern sinks, angle of attack is increased, the boat sails to weather better.

In modern times, over the last few decades particularly, we have seen hulls getting wider and wider and with fuller sections aft. Of course, you can get more stuff into such hulls, but at the same time, the builders and designers of such craft have been touting "powerful stern sections" as an indicator of superior performance. Nothing can be further from the truth, and in my opinion, "power stern sections" is a hoax. Such boats do not sail well, they balance poorly to more bow-down attitudes and pick up weather helm as a result. They're cranky. You'd see this in their heeled waterlines, they are fuller aft; the LCF moves aft as the boat heelsl. So, heeling over, they pick up more buoyancy aft, which raises the stern, depresses the bow casting it to leeward, and reducing the angle of attack on the keel and rudder--all just the opposite of what you are seeing on your new chined IOM models.

So, the better performance does not come strictly from the chine--rather, it is all about area distribution of the waterplane which is shaped by consequence from the chine to produce the desired bow-lifting effect. You can do the same thing without a chine, but the chine, in this case, seems to accentuate the effect.”


In the Photo page there are 4 photos of new chine boats