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| Frequently Asked Questions... and Answers |
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For high performance vee hulls, tunnel boat design and performance. |
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 mph 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 degradation 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.
My boat has a chine walk problem. What causes chinewalk and how do I fix it?
Answer: chine walk is pretty common on performance vee-pad hulls. As the hull accelerates, lift increases and the wetted running surfaces that are required to support the hull are reduced (more Speed = more Lift = less Surface). As the speed increases throughout the velocity range, the hull often gets to a point where the lifting surfaces become very much reduced and the hull is now "balancing" on a small area of the vee-portion or the "vee-pad" of the hull. When that surface becomes sufficiently small, it becomes very tricky to "balance" the hull on its vee or pad. The result is a rocking of the hull from side-to-side. This rocking can tend to get a little more extreme with each motion, and so the "balancing" must then be provided by additional driver (steering/throttle/trim) input in order to maintain the hull in a balanced state.
I wrote a full article on 'chine walking' that details the secrets of Chine Walk in performance powerboats- why it happens & how to fix it!
Will my performance vee hull go faster with a 'pad'?
Answer: Here is an article on vee hull and vee pad design that details the speed secrets of vee pad design, vee hull design and performance powerboat design
How much does my lower unit / outdrive contribute to the drag of my performance hull setup?
Answer: The lower unit profile and configuration and the height adjustment of your outdrive can make a huge difference to the performance of your hull. I wrote a full article on 'How Trim Angle and engine height affects performance'.
My boat seems to do allot of porpoising. How can I fix this?
Porpoising is pretty common...any vee hull (or tunnel hull) can be susceptible to porpoising, depending on design and setup. Flatter bottom vees are more prone to porpoising than steeper deadrise vee hulls, but there are several contributors to the occurrence and any vee hull can find the problem caused by dynamic instability.
The "bouncing" or porpoising comes from a rapid change in the location of the center of Lift as the boat accelerates. The relocation of static weights is one way of dampening the rate of change of the CofL...so it's not always obvious whether to move weight fore or aft in order to cause the "dampening". The solution can be calculated, but we use boat performance software (TBDP©/VBDP©)for that. It's not too difficult for you to find out through testing, whether moving weight fore or aft will help your particular problem.
The resolution to a porpoising problem with a hull design is most always addressed by causing the boat to run with less trim. There are many different ways of achieving this. Here is a full article that I have written on 'Why your boat is porpoising, and how to fix it". Also, here are some notes on S&F chat forum about 'hump zone' and 'porpoising'.
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