Outrigger Hydro Turning Dynamics

by, Marty Davis

The dynamics of making an outrigger turn especially well (“Snap Turn”) are complicated. I have wanted to do a Technical Article on this subject for a long time. Some may not agree or understand my findings and opinions, but they are based on a long period of testing and experience. Usually I do a several part article, but this time I think I will make it a complete Technical Article about the subject.

Important Note: I want you to visualize the way that a perfectly set up hull “snap turns”. Imagine that the turn fin is the pivot point of the hull and when set up correctly, you will see almost ZERO water come off the turn fin in the turns. The boat will PIVOT around the fin and the amount of rudder throw will be so slight, that it will amaze you. I almost gave up on the “Snap Turn Characteristics” because I found that I had to reduce the rudder throw so much. With all the previous rudder throw, the boat performed Horrible. Only after reducing the rudder throw did I see what I was looking for.

Snap Turn Defined:
A “Snap Turn” is when the boat goes through the turns and appears to speed up as it turns. In addition, the turn fin will have almost zero water shearing off of it. You will see no water high off the turn fin. The boat will be pivoting around the fin with the rudder only moving the rear of the boat around the pivot.

This Technical Article will be broken down into several main components including:

Important Outrigger Hull Design Characteristics

Sponson Physical Characteristics

Turn Fin Characteristics

Rudder Physical Characteristics

Prop Characteristics.

Important Outrigger Hull Design Characteristics:

In order for an outrigger to be able to “Snap Turn” consistently, the balance of the hull needs to be such that when it hits a wave or any rough water the boat bounces level with the water surface. No bow up bounce, no transom kicking up, etc, etc. It must be set up to be balanced.

Attack Angle of the Sponsons:
The attack angle of the sponsons are one of the most important factors that contribute to a boat being able to consistently “Snap Turn”. If you have the attack angle set even a few tenth’s of a degree off, the turn will not be consistent. Some time the boat will snap turn and some times the boat will wash out and not turn well at all. With higher attack angles, the boat will run faster down the straightaway, but that will be more than offset by bad turning. The turn fin will not be able to stay hooked up consistently.

Sponson Stance:
The width of your sponson stance (the distance between the inside edges of your sponsons) is also very very important. This wide stance makes the turn fin have a very long lever arm, so that it does not overpower the boat when you make a left correction. It also makes the boat remain stable in the turns because of the “long lever arm”. The boats remains flat and level (side to side) with a wide stance. How do you know if you are too wide? If the boat “teeters” back and forth between the right and left sponson in the straightaway, you are too wide. It really takes a lot of width to get into this condition.

Flex of the Sponson Tubes:
If your sponson tubes are super “flexi” you will have a boat that changes attack angle while running and will be difficult to get consistent. Rigid sponson tubes are a must, if you want to have a consistent running hydro.

Setting the Attack Angle of the Sponsons:
I have previously talked in detail about the methods for setting the attack angle of the sponsons using two different methods. These discussions can be found at: http://1nitrorc.com/past.htm Taking time setting attack angles accurately will reap great rewards for you in the way that your boat performs.

Wheelbase of Rigger:
The wheelbase of an outrigger (distance between the running point of the sponsons and the center of the prop) is important to keep the hull in a stable condition without rocking forward and aft. You will be more prone to have a boat that rocks forward and aft, the shorter it is. The prop will be much more critical since it generates the lift that creates most of the rocking. I prefer a longer wheelbase to keep stability. You will see a much better race boat if you have considerable wheelbase. On my newest boats, I am tending to lengthen the wheelbase a great deal.

Weight of the Outrigger:
Physics will indicate the importance of having a light weight boat. The lighter the boat the faster it will accelerate. The lighter the boat, the more pitch you will be able to pull effectively. The lighter the boat the easier it will launch a big fast prop. Weight is a super important factor that contributes to an impressive and well developed outrigger. Weight is an area that I spend a great deal of time on. I don't allow ANY excess epoxy in building, I use light weight materials, I use aluminum screws and nuts, I lighten all components by milling out unnecessary materials and I don't put a ton of paint on the boat. Any weight that you can save in the building process is going to make the boat better, assuming that it is designed and refined as indicated. NOTE: I have never seen a boat that was too light, which was refined and well designed. Not sure “too light” is possible…..

Running Friction of the Mechanical Components:
The running friction of the running components (driveline, strut, alignment, etc) must be smooth and free. Any bind here will totally destroy your efforts at refinement. The smaller the engine class, the more effect increased friction will have.

Radio Installation:
Radio installation is an often neglected component of the great outrigger. The linkage between the servo and the runner assembly should not have ANY slop. You should not be able to feel any slop in the linkage when moving the rudder assembly with your hand.

Sponson Physical Characteristics:

Sponson Width:
The running surface and dihedral of the sponson also contributes to the correct attack angle required to make the boat “Snap Turn” consistently. For example, the Crapshooters that Stu Barr makes have narrow run surfaces and take a few tenths more attack angle to make the boat run consistently well without bouncing excessively. The boats that I run and the Roadrunner have a wider running surface and take less attack angle to get this consistency. What angles you say? On the boats that I run and the Roadrunner, you can't go wrong with attack angles of 3.0 degrees on the left sponson and 3.2 degrees on the right sponson. On the Crapshooter that Stu builds you need more, like 3.6 on the left and 3.8 on the right. I can't say on any of the other hulls, as I have not worked with them. If you have stats that convince you that you have the optimum attack angle figured out, let us know. Why the difference between the left and right sponson attack angles? The right sponson has .2 more because of the turn fin drag to keep the boat running straight. The width of the sponson will be the major controlling factor in how well the boat runs in rough water. The wider the sponson, the more reaction it will have off of a wave or wake. You want your sponson width to be as narrow as possible to get the attack angle working where you find turning consistency. The balance of the boat will also have a big effect in the rough water characteristics.

Sponson Toe-in:
Having a little toe in for the front sponsons is important. I personally use 1/16” toe in on the right sponson and none on the left. What this does is make the boat track straight. There will be no pulling in of the boat under acceleration toward the center of the course. The boat should run totally neutral with a little toe in.

Sponson Dihedral:
In years past, dihedral and stepped sponsons were the rage. There is not doubt that sponsons with dihedral or steps are faster on the straightaway, but a flat sponson with no dihedral is better for heat racing. The boat stays much more stable and consistent with the flat bottom sponsons.

Sponson “Belly”:
This term will be foreign to most. If you can imagine, the running point of the sponson is the contact point with the water. If you take a straight line with an attack angle of 3 degrees forward toward the front tip of the sponson you will have a sponson without any belly. If you imagine the same line going forward about 3 or 4 inches and then having an arc to the front tip of the sponson, that is the amount of belly in the sponson. Some belly is really good for running in race water. Too much belly will adversely affect the running of the boat at launch and at slower speeds. With a lot of belly, you will have to add attack angle to get consistency. For straightaway, a sponson without belly is best as it stays cleaner than anything. Some dihedral here is beneficial for SAW.

Sponson Tops:
The tops of the sponsons are important to the boat staying in a level condition as it encounters rough race water. Unless you have some angle of attack on the top of the sponsons, they can get to the point of no return if they get up in the air from a bounce. Once the sponson bounces, it creates more attack angle and air gets under the sponson. Unless you reduce speed, the angle increases until the boat blows off the water. A little (3 degrees +/-) of attack angle on the tops of the sponsons will eliminate this problem.

Sponson Sharpness:
The edges of the front sponsons should be sharp and never rounded. The sharper the better.

Sponson Finish:
The running surface of the front sponsons is very important in it’s finish. A close friend of mine, builds boats that always run very well. They appear to not need much horsepower since they are so easy to push. I asked him one time why and the reply was very interesting. Sharpness of edges, sanding TOTALLY flat on the running surfaces and increasing to 1200 grit paper for a final finish. All this sanding and flattening is done AFTER painting and will pay BIG DIVIDENDS.

Turn Fin Characteristics:

Depth of Turn Fin:
The depth of the turn fin has a great deal to do with how the boat acts when you turn left. It is important also in keeping the boat hooked up in the turns. If you have a boat that wants to flip up on its top when you turn left, you have a fin that is too deep. The area of the turn fin can be maintained by making the fin wider rather than deeper. This will allow you to shorten the fin and reduce the bad effect when turning left.

Straightness of Turn Fin:
The relationship between the turn fin and the inside edge of the sponson is very important. The fin should be perfectly in line with the inside edge of the sponson. Take a straight edge and be sure that there is no warp in the fin and the fin is perfectly in line with the inside edge.

Turn Fin Contribution:
All of you have seen boats that turned amazingly well and actually seemed to pick up speed in the turns. I first saw this characteristic with a boat that Steve and Jack O’Donnell brought to the Indy Hydro Masters in the 80’s. That boat was beyond awesome. It had a funny flat turn fin that was really deep and angled in. Boat had a LOT of trouble turning left, but sure would go around the turns. The boat set the IMPBA Oval Record at that Masters Race. The boat was an inspiration for everyone there. We tried similar turn fins and still could not get our boats to turn anything like that boat. All of the variables which I have discussed in this technical paper was the reason that we couldn't get similar results. There was MUCH MORE to it than just a turn fin. At that point I started working toward the goal of having a boat that would turn like the O’Donnell boat. It took me many years of refinement to accomplish a good result. I had a lot of help from a lot of people. Ed Lackey and I REALLY worked hard on this over several years. Over the years, I have been fairly secretive about all of the components that went into this subject. Those who know me well, know that being secretive is difficult for me. J In this technical paper, the only thing that I haven't expanded on is the actual design of the turn fin. I can tell you that the fin is only a part of the solution and unless you have all of the other characteristics right, you will never have the impressive “Snap Turn”. Many types, styles and configurations of turn fins will work well and Snap Turn so the setup and design items are much more important than the actual turn fin design.

Turn Fin Stiffness:
Any flex in the turn fin will negatively affect all your efforts at refinement. It took me many years to finally find a light weight solution to making the turn fin rigid. I use a piece of 1/8” thick flat carbon plate, sandwiched between the turn fin and an aluminum doubler. It makes the fins TOTALLY rigid and will not bend any under load.

Center of Pressure of Turn Fin:
The area of the turn fin that is under the surface of the water while running has a point where it represents the center of pressure of the turn fin. If you look at the fin, you can imagine, fairly accurately, where the effective center of pressure of the fin is. You may have to draw a water line to help you visualize the center of pressure. It is the effective point at the center of the area of the fin. Once you put a point at the COP, you will want this point to be in front of the c/g of the boat with ½ tank of fuel. On most hulls, the COP should be between ¼” and 1” in front of the c/g with ½ tank of fuel. You will have to slot the turn fin to find the “sweet spot”. Run the boat, move the fin a little and keep trying until you find the sweet spot.

Rudder Physical Characteristics:

Rudder Length and Width:
The length of the runner is also important in keeping the boat running flat and consistent. You want the rudder to be about as short as possible and still do the job. For example, on a .21 hydro, the rudder should be about 2 1/8” below the center line of the shaft. That is also a good reference for larger boats. Just take that measurement and add ½ of the diameter difference between a 50mm prop and the prop that you will run on the larger boat. For example: a 50mm prop is 1.9” in diameter and for my 67 boat a H32 is about 2.5” in diameter. So 2.5”-1.9” = .300” ½ of .300” is .150” so lengthen your rudder that amount from the base of 2 1/8”. That will get you close. I use all Speedmaster rudders and the ratio of width to length is consistent between sizes so this work out closely. A note: for Straightaway, you use a MUCH shorter rudder in order to lessen drag. If your rudder is to long, the rear of the boat will be lifted by the rudder and the boat will never run well. Having some lag in the rudder will make it even worse (bottom tip of the rudder further back than top of rudder blade).

Prop Characteristics:

Prop Being Used:
The prop is one of the “Black Arts” of our hobby. A prop that is super fast and has a ton of lift is difficult to get consistency with. An example: Several years ago, I watched Stu Barr run his favorite class (40 Hydro). He spent an entire season working with the “V Series” Octura Props. His boats were rockets, but he could not get them tamed down and have consistency for heat racing. He had to drive wide and fast to be able to race that prop. Also, in rougher water his boat would not stay hooked up very well. Now, he is back to a more consistent prop in the “1400” and “1600” series. He can run on the buoys and snap turn with consistency. Maybe gave up 7 or 8 mph but can race much much better. I personally run large diameter props on all my race boats. The reason, I want the prop to stay “hooked up” in race water so that I have consistency and good acceleration out of the turns.

I hope that this discussion will be helpful to you in running and tuning your outrigger hydro. Many of the things discussed can be applied to other type hulls including: Scale Boats, Sport 40, Sport 20, and similar design boats in the gas classes. The same physical characteristics should pay dividends for you in these other classes. I use the items discussed on all 3 and 4 point hydros.

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