Bottom Trim Tabs / Rudder T-foils

Bottom Trim tabs as used by "Bols"probably have very limited application in rc models because most models have little pitch stability and tend to go bow down easily. The trim tabs on “Bols”, which apparently work very well, are there to push the bow down. The only models that could benefit from this (possibly) now are spinnaker boats utilizing lifting spinnakers. The point of the original post was to expand the horizons of model sailor/builder/designers with a technology that while not necessarily applicable now does illustrate the “out of the box” innovative thinking being done by some full size designers.
On models ESPECIALY canting keel models, trim may be able to be controlled with a rudder t-foil as long as the angle of heel of the boat is lmited to a max of 15-20 degrees.
On International 14’s the rudder t-foil is used to allow the crew to position themselves aft upwind using the foil to lift the transom.Off wind the crew stays in the same place but the angle of incidence of the foil is changed to pull down. This system could conceivably be designed to work on ck boats that have a greater pitchpole tendency deaddownwind that do fixed keel boats. It, of course could only work in conditions where the speed is fast enough for the foil to work; in lighter conditions the foil would be removed or at least set to zero degrees.
At higher angles of heel than above the T-foil would tend to have a cross coupling effect with the rudder and would’t be very effective so its use(for monohulls) is pretty much limited-at least upwind-to CK boats.

Doug Lord
–High Technology Sailing/Racing

Thinking T foils for my CKT; How much area and what shape? Would a micro servo on the stock be enough to control it? A fixed one would screw up the trim too much upwind…

Luff 'em & leave 'em.

What I know works: 98sq. in. SA per sq. in of foil area. What I think will work: 164 SA. per foil area.(Divide this figure into your sail area to get a figure for the area of the t- foil) Each half of the foil should have a 3/1 to 4/1 aspect ratio though higher works on foilers. But on the CK I’d stay around that.
I’m putting a t-foil on one of my 30’s and nothing on the other-that should teach us something…Make sure you go with anti-diving planes-both my boats have them-they definitely work to prevent pitchpole.
My feeling is that if you set the angle of incidence at zero(parallel with the wl) when the boat is just sitting there that that will work upwind and downwind because there is going to be a pitchdown moment in both cases at least on my skinnies. You might try that but build it with a 25% flap so you could hook a micro servo up later(build in the pushrod tube as I am doing on both).
You might want to consider building another exact copy of the boat you have though you said you would design a new one.Either build another of this one or two new ones; that way, with a little help, you can get some good results…

Additioal info: According to Peter Birch some Australian multihuls use 131 sq. in. of sail area per sq. in of foil area for the t-foil on a mini 40.

edt:4/7/04 add info
Doug Lord
–High Technology Sailing/Racing

doug -
in my 8 months or so of experience with the T-foil on the 14, 15-20 degrees is too much and causes major stall issues (as well as other funky stuff that without NA degree cannot be described by me). 10-12 is about max on heel…i’m sure a boat with less of a hard chine would respond differently (the chine really helps increase augering when heeled that far). also, crew weight is farther aft on the uphill (as opposed to position sans T foil), but not like downhill. we definitely move back from the uphill position (over 18 kts and im standing at/on the back of the rack)

an unmentioned benefit of the T-foil is reduction of stern wake (done by positioning the foil at the right elevation under water) and the reuction of hobby horsing, a very nice aded benefit with 25’ tall rigs. not positive, but these two benefits might have a cross over effect to r/c boats


Kurt,thanks for the info! How far under the boat is the t-foil on the 14? And what is its span?
My experience with t-foils is so far just on one monohull and two foilers over a two year period. But that mono never heeed more than 10 degrees or so. These boats may heel more and you may be right that 15-20 degrees is too much heel with a t-foil-I’m not so convinced on these boats; I’m about to find out.

Doug Lord
–High Technology Sailing/Racing

doug - good on ya for giving it a go. there is a whole new world out there in foil development alone.

since you asked, i will respond?the foil is approximately 14? beneath the hull. the T-foil that i am currently running is a Bieker (Type 1) design. the length of the horizontal sections are about 14? long (30? total span, approx.), and about 5 ?? across the chord. it is an assymetrical foil section (don?t remember which). the development currently going on with foils is incredible. out of 14 boats at the NOOD last month, 6 different foil designs were present. please keep in mind the comment in my previous post regarding flattening of the stern wake, our rudders are pod hung about 12? behind the transom. i believe without a stern hung rudder, this benefit will <u>not</u> be seen

to bring this back to an r/c compatible topic, an to somewhat respond to matt l., the loading on the rudder is quite severe (watch out for those micro servos). for variable pitch T-foils to transition to r/c applications, it is my belief that you cannot produce an internal foil adjustment system, but rather you need a gantry that is pinned at the top an adjusted fore and aft at the bottom. this ?rake? on your board adjusts not only your angle of attack, but also helm balance

hope this answers the questions an still keeps us on the r/c track


Kurt, thanks for sharing the 14 foil info; provides much food for thought!
On my two 30" CK Trainer/F100 proto’s the two rudders are transom mounted with a rudder made with an integral pushrod tube. One of the boats will initially be setup with a t-foil using a flap(foil set at zero degrees relative to the static waterline). This has worked on the 56" F3 with 1668 sq. in. of SA. and I’m going to play with the area starting at the same(relatively-see below) as the F3 and working down. The F3 flap is operated by a microservo that can deflect the flap with the boat on foils at very high speed.Interestingly, the flap is NOT necessary; it can be left at zero but I found that out after I had outfitted the first test boats with a servo actuated flap…
I’m not convinced the CK foil needs to be a variable angle of incidence(or flapped foil) because of the
characteristics of the boat-but I really just don’t know yet.

Doug Lord
–High Technology Sailing/Racing