tomcat's power vs. speed calculator
 

I wrote a program that I use a lot for determining power vs. speed and prop selection. What I would like to know is whether you would like to see it put on the OSO website. There are other calculators out there already but this one is different and hopefully you will think it is better.

Bam's calculator is based on Mercury equations that work well for a given type of hull when you know the weight. I use a formula that asks you to enter 1/2 power speed, to calculate a hull efficiency factor for a particular hull, and you don't have to know the weight. Either way works as well as can be expected, since there are so many things that can affect the answer, not the least of which is X dimension.

The neat thing about the program I wrote is you don't just plug in a max HP figure and get a speed. You enter the entire engine dyno curve, which is HP vs. RPM. You can then enter a prop pitch and the program converts this into a engine HP vs. speed curve. Then it plots that curve on top of the hull HP requirement vs. speed curve. Maximum speed occurs where the two curves cross. You can change engines, hulls and props to your heart's content and see the effect of more power, better hull and the best prop, without having to go back and forth several times between a speed calculator and a prop calculator.

This is a powerful little technique and it really gives you a feeling for how to prop the boat. For example, the excess of engine HP over hull HP at any given point is your potential for acceleration. You can see what happens to this excess power as you prop up for top speed. You can also see the difference between supercharged and normally aspirated power and why they should be propped differently.

The program can handle two sets of data at once so you can see the curves of both the existing engine and the proposed engine on the same hull curve. That's how I use it most of the time. Sometimes you have a great custom engine that you will keep when you sell your boat and you want to know how it will perform in a new hull. So you would plot one engine on two hull curves.

I've been thinking about substituting Mercury's formula into my program and making the program available on this website for members to use. Since not everybody has Desktop dyno or dyno curves for their engines, I would have to include a menu of engines, Mercury and common high performance combinations, to enter into the program. Members with dyno data or a dyno program could enter their own data.

I have posted the idea as a separate topic to see what the members think. If you guys like the idea, I will talk to Jeffery.

Here are some posts from another thread where this subject got started:

Katdrvr
Member
Member # 950
posted 01-17-2002 04:17 PM
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Tomcat,
Sounds like a cool program. I'll bet everyone would love to be able to run some demo's.


mike

[ 01-17-2002: Message edited by: Katdrvr ]


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Posts: 853 | From: Gretna, Ne | Registered: Feb 2001 | IP: Logged

traviss
Member
Member # 284
posted 01-17-2002 07:34 PM
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Tomcat, what is the name of this program? Would be able to get it somewhere? It probably is very expensive being boat related
thanks


Travis

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"Loose is fast and on edge your outta control"


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Posts: 4286 | From: in nothing anymore!! | Registered: Oct 2000 | IP: Logged

tomcat
Member
Member # 995
posted 01-17-2002 08:11 PM
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traviss - I didn't buy the program, I wrote it. It's a bunch of formulas in a spreadsheet, that produces a beautified chart for the output.
I proposed putting this program on the website to Jeffery a while ago, but I don't know anything about website design and I would need some help converting the spreadsheet into an application that can run on the website. I would also need to input more data on different engines and hulls to make the program user-friendly. Since I would have to pay a computer person some money to make this work, I didn't pursue the idea any further.

I think if Jeffery was convinced of the interest of the members, this program could be provided on the OSO site free of charge. Any computer whizzes out there who can help?


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Posts: 414 | From: Lake Simcoe, Ontario, Canada | Registered: Feb 2001 | IP: Logged

traviss
Member
Member # 284
posted 01-17-2002 08:23 PM
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Thanks for the info Tomcat..
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"Loose is fast and on edge your outta control"


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[ 01-17-2002: Message edited by: tomcat ]

It sounds like a good idea to me!

Tomcat, I think it's a great idea. However, I'd like to see it available in it's current state. There are a lot of us gearheads on this site that would love to plug all the torque curve numbers in....

Bob

It would probably make sense to have two versions, a quick and easy one with menus to choose from, and and expert version that allows you to enter all your own data. What I have running on my computer now requires all data to be entered, and I can change the formulas if I want. The expert version could also allow you to choose from the Mercury equations or the other equations which can be "calibrated" by test data from your own boat.

Something I do manually now is adjust total prop HP until the calculated speed matches the GPS results from a test. For example, a member gave me GPS results of 60 mph with a pair of 260s. I used the typical hull efficiency factor for deep vees and adjusted the total prop HP until the program gave me 60 mph. The final prop HP was 475, not far off for a pair of 800 hour small blocks rated at 260 crank HP. Once the baseline is established this way with actual data, you can throw the bigger engine into the program and see what happens.

I don't claim that any program can be perfectly accurate, but playing with the program is an education in how performance boats work.

Tomcat/

I think it would be super if you could get your program on OSO. Sounds like you have developed an interesting approach to analyzing performance and I would think the members would not only learn alot but find the program benefical in their own applications once they got their boat's performance and the program showing comparable numbers! Good luck!

Fantastic idea! Nice of you to offer it for OSO members!

Tomcat, Great work. You and I share the same enthusiasm for boats, power, speed and numbers. I am calculating all the time to figure things out. Your programs sound great. I had thought about plots of hull efficency and hp but couldn't figure how to put it together. Can you download your programs to CD? Maybe that would be initally easier. It would be the best to have it on OSO. Maybe OSO needs a 'Reference' area which would have topics of great interest like yours. Yopu're exactally right about the when the hp curve crosses the hull curve is the max speed-I love this stuff. Keep it up. Marty.

It sounds like a great and valuable tool. Yes I would like to see the program on the site, or what was mentioned before on disc. You are very generous to offer this :)

Tomcat, I am willing to help you develop it into either a desktop application, or a web based program. Email me and we'll talk.

You got my vote :D

Tomcat, your plotting of the speed/hp is a good improvement on the calculator presently on my site. I wish there was an accurate (and inexpensive) way to measure the hull power requirements.

My calculator (Speed calculator) will do the HP calculation if you enter the weight, constant and speed, in fact it will solve for any one of the numbers if you supply the other 3. The way I find it most usefull is to find the constant for a given hull/setup combo. If you have good numbers for the weight, speed and power the resulting constant is very accurate. If you then make changes to one of the other variables I have found the resulting calculations are pretty close.

The hardest part is coming up with accurate numbers. It is good to use the uncorrected HP as this most likley represents the power you actually have while running.

[ 01-18-2002: Message edited by: Mbam ]

Mbam - Thanks. Marc. I like the simplicity of the Mercury formulas, and I like being able to work backwards to get the constant for your own hull. For example, my old Excaliburcat has a constant of 264, halfway in between a fast vee and a cat.

You and I think alike, because the ultimate answer is an on board dyno that can measure propshaft HP and therefore the hull HP required at any speed. I think it could be done. How easily and how expensive I don't know. This is a whole other subject so I started a new topic on it.

Tomcat
This sounds very intereasting. Are you coming up with a hull number based on performance, or your dyno numbers ? The only question I would have is are you able to correct for the inaccuracies of most desk top dyno type programs ?
Doug

The hull constants come from Mercury if you use their formula, or from your own test results if you use the other formula. Desktop dyno numbers can be very accurate if you use the cylinder head flow data on the chevyhiperformance.com website and cam card data from cranecams.com.

Is this spreadsheet file to large to email or download from a web site?

I would like to download it and play with it.

One question, if I do not know the engine's dyno sheet, only the engines max HP and operating RPM, will I be able to use this program? i.e. Mercury 500 EFI operates at 5300 rpm.

Tomcat
I agree that some of the better dyno simulators are very good, but to use them you almost have to build the engine to have the right data to imput. I find that most people do not get very good numbers from the Desk Top dyno versions due to the data they imput. I deal with this on a daily bassis. My main beef with them is you can enter number that are not possible of working, and still get data.

Rik - The program is not too big to download, but it's a one man program right now; too many things you have to know and do manually to make it work.

What I would like to do is get a streamlined version up and running on the web as soon as possible. That means going through the logic of every cell in the spreadsheet, with a computer person at my elbow telling me how it should be designed to run automatically on the web.

After that, adding an advanced version would be icing on the cake. If I do it this way it will minimize everyone's learning curve, and maximize enjoyment.

You do need a dyno curve with data for RPM range 2500 up to a max of 7000, at 500 RPM intervals. A single point will not generate a HP vs. speed curve, therefore no intersection of curves and no Max speed.

What I do is simulate the engine on Desktop Dyno using the known parts and adjusting exhaust and/or compression to get the known HP @ RPM. This gives you a very realistic curve. In the future I would like to go directly to Mercury for their dyno curves, or use dyno data from aftermarket shops that have dynoed the engine.

lets do it! sounds like a great software tool.
Mark :cool:

Where do you get this "desktop dyno"?

You can buy Desktop Dyno from most car speed shops. There is supposed to be a CD with cam data available too.

I have sent a couple messages to Jeffery about putting the speed calculator on the site somehow. Stay tuned!

Tomcat. First off, I must say you sure are the Rokit Sientist of the OSO with out a doubt. But on a techinal note (or better yet question). The other night on the SV Dragboat competitions they were mentioning water temp and the density of water affecting the speed due to prop lock-up. What are your thoughts on that with respect to the calculations? Would the prop slippage be less (if you would) with colder water which I guess would be more dense? Ie. Would the boat speed in 70degree be better than in 85degree water?

Bob

It sounds like what i need.

Hi Tom,I was waiting for someone to drop the water density curve ball into your equations :D All in all,I think it is a great idea to make it run web based.Come on Jeffery,let him have at it.The TOMOSOCALCULATOR sounds like a great addition to the site. ;)

Actually, I figured the density question would come up with respect to sea water/fresh water. But here goes; first let me put on my propellerhead hat.

I checked my books and a drop from 86 to 68 degrees F would only increase water density by 0.35%. Is this small increase going to make a noticeable difference? I don't know. Considering all the sources of variability in a drag boat race, I wonder if it is possible to prove that a speed increase was due to reduced prop slippage due to such a small increase in water density. Since air and water temperature tend to go down at the same time, a corresponding reduction in air temperature is going to have a much bigger effect on engine horsepower, which seems a more likely explanation for extra speed.

However, the density of sea water is 1.025 g/cm3 at 15 degrees C, which is an increase in density of 2.6% at that temperature. This is a much bigger effect, and it is common to hear people claiming slightly higher speeds in salt water. Is this due to less prop slippage or increased buoyancy/reduced wetted surface? I don't know, but I can believe that this effect is noticeable and repeatable.

I don't plan to try and allow for either of these effects in the computer program, because there are other sources of greater variability. But it was a good excuse to put on the hat.

I think the saltwater thing is not valid for a planing hull. Once on plane I don't think buoyancy is much of an issue. On a displacement hull it's another story, I think the difference would be significant. I personally have never seen a difference in speed.

Sounds like salt water is another pointless incremental improvement! One thing that reinforces the "salt water is faster" theory is when you buy a "true" 80 mph boat in Florida, and it only goes 75 mph when you get it home on the lake. :rolleyes:

cobra marty - Check your email.