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Design, Building and Setup Secrets For Recreational tunnels, Offshore
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| Frequently Asked Questions... and Answers |
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For high performance vee hulls, tunnel boat design and performance. |
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Answer: Hi Peter: No
problem. Due to something in your computer setup, one file appears to have
not been installed. There is a simple fix for this. Just copy the
file 'drvMerc25efi.gif' from your TBDP CD to your TBDP program directory...
- put the TBDP install CD in your CD drive: (this is
often your D: drive, but not always)
- highlight & copy the file: 'drvMerc25efi.gif' or 'drvMerc25efi.jpg'
(or both) from your TBDP
installation CD to your TBDP directory....
in Explorer...
- highlight 'drvMerc25efi.gif' then click on menu.>Edit>Copy
- go to directory C:\Programs\Tunnel Boat Design Program
- highlight the directory 'C:\Programs\Tunnel Boat Design Program' then click on
menu..>Edit>Paste
- close explorer
- run TBDP
Answer: Increasing pad width increases the aspect ratio of the hydrodynamic lifting surface, making it more efficient. Going from 12 inches to 14 inches could change the AR from 1.0 to 1.3; which will generate 5% more sponson lift. This translates directly into better acceleration (maybe 1 second improvement accelerating from 10 mph to 70 mph). You should recognize, however, that handling could be negatively impacted, especially in heavier water conditions. It all depends on the specifics of your hull design and setup, of course.
Answer: I'm glad that you've enjoyed the STBD book. To answer your 1st question, the calculation for water lift coefficient CLw for varying deadrise (BDR) is very complex, which is why I didn't put it in the book. The TBDP software (Version 7) allows for the input of specific sponson deadrise and also a center-pod deadrise (for modified vee designs), and completes the analysis of performance and stability based on these and many other inputs. In your 2nd question, you mentioned the question of induced drag due to edge vortices. This affects both water drag and aero drag. I think you were referring to aero drag. There are edge vortices associated with the airflow over the deck surface. The tunnel walls (under the tunnel) reduce overall vortex shedding significantly, but there remain some affects of air spilling off the deck into lower pressure areas in the chine and sheer areas.
Answer: A tunnel boat is like an airplane wing operating close to the ground - or, in what's called "ground effect". Close "ground" proximity increases lift coefficients, making more efficient lift/drag ratios for the craft. For example, reducing the depth of a typical tunnel from 10 inches (at the transom) to 8 inches could improve the aerodynamic lift coefficient by 6% - which means something like 12% more aerodynamic lift at higher speeds. This means less lift required by the sponsons, and an ultimately faster top speed (in your case 2-3 mph in the 85 to 90 mph range). Remember that with the tunnel roof closer to the water, there's also more risk of water interference, and intermittent splashing and increased drag in heavier water.
Answer: Every pound of weight means additional horsepower needed to lift it. This is the easiest way to improve the performance of your boat. If your boat is a high performance 21 foot modified tunnel configuration that weighs a total 2400 pounds, then the 100 pound weight reduction will mean you will save 3-4% horsepower. This is now available for better acceleration, and better top speed (in your case as much as 3-4 mph in the 85 to 90 mh range).
Answer: The drag of the cockpit area can be a very complex area to analyze, but also a real source of aerodynamic drag. You're better off than many if your cockpit and motor fairing are a well streamlined design already. The difference in aerodynamic drag between an open cockpit and canopy style cover is significant. You can reduce the appendage drag coefficient by 50%, and drag by 75-100 lbs (at top speed) by using a streamlined canopy (like a F-1X) series jet fighter canopy). This will translate into as much as 5 mph at top speed (120 mph range).
Answer: Whether you do increase or not is up to you, but I can tell you what the performance result of the change will be. A higher deadrise angle will give better performance in rough waters, but a lower deadrise angle is more hydrodynamically efficient and thus can generate better acceleration and a faster top speed. The effect is complex, since the more efficient hydrodynamic lift also means less wetted length, changing sponson aspect ratio. The difference in going from a 15 degree deadrise to a 10 degree deadrise angle on conventional sponsons is a hydrodynamic (sponson) lift coefficient increase of 65%. This can translate into big performance improvements, even as much as +10 mph in the "over 100 mph" range. You've got to be prepared to accept the stability and handling degredation that will come with the changes, however. The TBDP, Version 7 does a great job of analyzing the combined impact of changes like these.
Answer: To accelerate from a given velocity to a higher velocity requires reserve power. By calculating the power required for a specific design and setup to go 30 mph, you can then calculate the time increment required to achieve an incremental increase (say to 31 mph) based on using all the reserve hp your engine has to give you. This process can be used iteratively to derive an acceleration map or curve all the way to 70 mph. The TBDP, Version 7 has a feature that does this analysis for you.
Answer: The amount of aerodynamic lift generated by the hull (as a % of total lift) depends greatly on the cockpit configuration and the tunnel configuration. The AO3100 has ALLOT of POWER, and so it goes really fast. The tunnel is 16" deep (for heavy water), and the cockpit is very open for passengers, which interrupts airflow over the deck surface. Never-the-less, the AO3100 generates 225 lbs (3% of total lift) at mid velocity (75 mph) and 550 lbs (7% of total lift) of aero lift at maximum velocity (>100mph). It's a very well designed hull - and super fast!
The % of LA on pleasure boats is always lower than it is on higher performance or race-type boats, as you suggest. As an example, the (AR® Report) performance analysis of the STV Euro 19' is more a performance boat. This boat generates 18% LA at mid velocity, and 29% (425 lbs) LA at maximum velocity. It has a much smaller Height/Chord ratio (more efficient lift) and a fuller, more aerodynamic deck surface (generates higher L/D ratio). Another example would be a full race boat, like a Seebold F1 boat, that generates 65% LA at top speed. This is with a very small Height/Chord Ratio and a fully canopied cockpit with very aerodynamic deck surfaces. You can see how these design features contribute to the ultimate performance of different tunnel boat design concepts. Keep in mind, that the selection of each design feature is always somewhat of a compromise between top speed, acceleration capability, stability, comfort, seaworthiness and reliability. The designer has to match the design features to the performance expectations of the boat in all operating conditions.
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