Prop lift, blueprinting, and rear shoe depth

[nicholashansen]

Hello,

I had a few questions that I was hoping to have some more experienced racers to hopefully shed some light on, as this topic has been a little bewildering when I attempted to do some research on it. Id like to note that this is more in regards to modern rear winged, turbine hulls, basically in the last 25 years or so.

From all the galleries and pictures of scale builds ive tried to accumulate, the more important ones ive sought out are of the bottom sides of hulls, as I enjoy seeing the variations between different designers hulls. However after doing my recent modifications to the sponsons and rear shoes on my U-88, ive come to wonder if I had the correct approach....I notice alot of guys tend to add depth to the rear shoes on their hulls, with 1/4" being a common addition on alot of glass hulls ive seen. Now I know that in the straights, the prop is supposed to be providing the lift and ideally, the rear shoes should not be in the water. With this in mind, I imagine that in theory, adding this depth to the rear shoes would in turn allow the rear of the boat to float a bit higher so that it can more easily support better straightaway performance, correct? But if this is so, it confuses me in the sense that all sponson modifications that I see usually consist of the backend of the ride surfaces being deepened as well, and thus a higher angle of attack. Wouldnt this then obviously raise the front of the boat more and make it more difficult for the rear shoes to stay out of the water? or is it basically just as simple as having the rear shoes set at a higher "ratio" of depth relative to the sponsons?

Also, for the more modern hulls, specifically since the T6 and on, the rear shoes are offset in how far they extend from the transom. With my U-88, I noticed this is especially true. For my recent modifications, to generalize, I added the same amount of depth to each sponson and rear shoe, and ive blueprinted the bottom so that when the hull is set on a perfectly level surface, each shoe and sponson is equally resting on the floor. Is this how it should be? I assume that all 4 points coming to rest equally on a level surface is more dependent on a side-to-side skew, rather than front to back, so if I were to add equal depth to both rear shoes now, the hull will remain equally blueprinted right?

 

[Hydro Junkie]

First off, all boats are different and, therefore, act differently on the water. I know of one boat that had the deeper shoes on it that carried the left sponson down the straights and had a very hard time turning. After adding a lot of weight, resetting the prop depth multiple times, changing props multiple times, trying different rudders and angles thereof, etc, it was discovered that the depth of the shoes was the culprit. Upon raising the shoe bottoms up, most of the weight was removed, strut reset to it's original position and the original rudder was reinstalled. The result was that the boat turned into a screamer with no bad habits.

With all that said, the full sized boats have the shoes fairly close to the bottom of the air traps. I know for a fact that the former Oberto(now Homestreet Bank) has runners on the shoes that are only 1" below the trap but a full 4" below the hull bottom. The sponson runners are only around 3 or 4 inches below the traps IIRC.

You might consider building your sponsons with a fairly low AOA to begin with. If they don't work well, you can always increase the AOA if need be later by building them up with wood or dense foam covered with either ply or FG.

 

[nicholashansen]

well yes, of course all boats are different. However, I was referring to a specific type of boats sharing the same design, so basically 1-2 kinds of boats at most. It seems like it wasnt until the 90s that most boats on the circuit had rear shoes, and then until the 2000s that those rear shoes extended from the transom at different lengths.

This boat and apparent observation you speak of that had a difficult time turning, do you have any idea how reducing the depth of the rear shoes helped the boat corner better? I mean I would imagine that when tuning any boat, it would be best to only change 1 thing at a time, in order to better pinpoint what exactly is effecting the hull and to what manner and extent. But thats another topic somewhat.... im more concerned with a stable, fast ride in the straights, and how certain design implementations (such as the ones I mentioned) are relevant towards achieving this.

its not that im curious about what exactly a good AOA is or what exactly to do with my sponsons and shoes, but rather trying to understand the bigger picture that can fuel a train of thought throughout the configuration of ones hull. Its important to me that I can learn to understand how something works fundamentally, in order to then be constructive and improve thereafter, rather than just having someone tell me what to do. There shouldn''t be any contradictions or confusion if this is followed, just experimenting and executing. Its already been made clear to me that following the full size counterparts to a 100% tee is not an ideal setup for performance.

 

[Hydro Junkie]

This boat and apparent observation you speak of that had a difficult time turning, do you have any idea how reducing the depth of the rear shoes helped the boat corner better? I mean I would imagine that when tuning any boat, it would be best to only change 1 thing at a time, in order to better pinpoint what exactly is effecting the hull and to what manner and extent. But thats another topic somewhat.... im more concerned with a stable, fast ride in the straights, and how certain design implementations (such as the ones I mentioned) are relevant towards achieving this.

Best guess is that the shoes were catching the water and acting like rear turn fins, countering rudder inputs. By raising them, this was, for the most part, removed as a factor in handling. As far as to why the boat was carrying the left sponson, that one eludes me as well.

 

[lohring]

I wrote a series of articles on props for the NAMBA Propwash. They were designed to give an introduction to propeller theory, geometry, and the practical methods used to finish a rough casting. However, there's no substitute for attending races, talking to the experienced racers, and experimenting with props on your own boat.

The Articles:

Props the Theory part 1

Props part 2 - Geometry

Props part 3

Lohring Miller