General Foil Question

Some one asked me the following question here at work the other day, and I didnt have a good answer. Hoping that someone here can help me out a little bit.

Stern mounted rudders VS rudders that come completely out of the underside of a boat. . . I remember hearing that foils out of the underside of a boat are more efficient from a drag stand point . . . but why, and by how much? I know that not having to break the surface again around the foil is part of it, but is there something else I am missing? And does anyone have any realative numbers to show the difference in efficiency? Is there a model of boat out there available with a stern mounted or inboard rudder that we could compare PHRF ratings on???

Any help appreciated!?!?

Todd, there are some minor efficiency considerations but there are also gains in effective waterline length for one.
In models you don’t have the generally minor ventilation problems associated with transom rudders in some conditons on larger boats-I’ve found that out thru using them and at very high speed. On models a consideration is the demolition derby that can occur in close racing-if not physically protected the rudder could conceivably be damaged in close quarters. None of my external rudder boats have ever been damaged and I am prepared with a SS or carbon “cage” to protect it if it becomes a problem. I use it because it allows easy adjustment of the foil on a t-foil rudder.
I guess in most RC classes the assumption has been that they have to give up hull length under various rules to use one and that that would be bad–I don’t think it would have any effect other than maybe being heavier than a normal rudder.
You might consider posting this question on under “sailboats” where a number of full size designers hang out that could give you more info. The hydrodynamic plusses and minuses will translate pretty well for this idea–no scale effect here.

Doug Lord
High Technology Sailing/Racing

In classes that are limited by waterline or overall length (as are virtually all r/c racing boat classes), it makes no sense to sacrifice waterline length for a transom hung rudder.

Todd, I did a little research on this subject and found that a transom hung rudder may have to be 15-20% larger than an under the hull rudder on full size boats.I don’t think the difference needs to be as great on models.However, if the rudder is fitted with a “fence” blended into the hull (see: "Design of Sailing Yachts by Pierre Gutelle, p.186 for an excellent illustration) then there is no loss of effective area.
As best as I can tell the rudder hung on the transom is part of the whole hull’s wavemaking pattern making it basically an extension of the hull from a waterline length standpoint.
I think there are good reasons not to use a transom hung rudder on most heeling models but loss of effective waterline length probably isn’t one of them.
This kind of a rudder on a movable ballast boat could have significant advantages especially in light of work done by Paul Bieker who discovered that on upright or nearly upright boats a t-foil placed in the right position on a rudder increases effective waterline length.

Doug Lord
High Technology Sailing/Racing