Speed vs Power

This is lifted from the boat design net,,Boat Design Net - the Boat Design and Boat Building Site forum.

Fun contemplation ,

a startling discovery on required power.
I have long been obsessed with accuracy in maths. Applying maths to power prediction and you come up with Wymans formula, Gerr's formula and Kieth's formula. It is the latter i want to talk about. If you search the net for "repowering Tortuga" you will find a very interesting article on these three formulas, how and why the owner chose his engine. His maths more or less proves that the power required for a speed on a flat calm times 1.36 gives the power for average conditions, and times 2.5 give the requirement for rough water. Now I know rough water for the Bismarck would keep Tortuga in harbour but: I decided Kieth's formula seemed to predict flat water conditions so I applied it to boats from "Voyaging under power". I then multiplied the result by 1.36 and got results very close to the installed power. If you apply the formula to WW2 destroyers and multiply by 2.5 you get very close to the installed power.

Little Sinbad 9.2kts 150hp predicted 151 hp
Nordhaven 46 8.75kts 101hp predicted 107.6hp
Diesel Duck 38 9kts 80hp predicted 85hp
Neville 39 9.1kts 105hp predicted 101hp
Neville 48 10kts 200hp predicted 193.7hp
Seaton 256 9.8kts 195hp predicted 180.7hp
Willard 40 10.5kts 130hp predicted 140hp

It looks like these 6 designers used Kieth's formula for power and added 36% then fitted an available power unit as close to that number as they could fit in their engine space.

, Mik the stick
Senior Member
Join Date: Dec 2012
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Location: Devon

The designers of those boats all wanted an engine that could cruise easily at displacement speed and not tax the engine too much, but have enough power in reserve to make head way in rough seas.

So six good designers came up with the same solution. No surprise there.

David

This interesting and correct thread (thanks Fred) brings me to a rule of thumb that I’ve previously stated, pay close attention to, and basically adhere to (most of the time that is - lol).

I.E.

For owning any boat coming from really good marine designers and really good boat manufacturer companies listen closely to and basically follow their design-standards/guide-lines per boat model that is built-out.

Top brand, high quality boat builders spend uncountable hours and much money on having the best marine-design mathematicians figure out what is really needed (regarding all sections of their individual boat models).

This includes power ratios, engine size/weight/brand recommendations, general boat-area-weight placements, hull designs/hull-add-ons, superstructure designs, fluid tankage locations, ballast if needed, above water line front and rear periphery extensions, interior and exterior layouts… etc…

I say this because I’ve seen too many boat owners who got hankerings to modify a perfectly good boat-design with results that were less than desired… sometimes a complete failure. Then again, there have been successful modification results.

My main theme in pleasure boat owning, for having the simplest capability for ongoing “Pleasurable Results”: Be careful not to mess up what is already nearly perfect results as per marine-design experts.

Happy Boat-Design Daze! - Art

It is nice to see that "Repowering Tortuga - http://www.todddunnmicroyachts.com/tortuga/repower.html " got some positive comments. However, it is disappointing that the writer of the Boat Design Forum comment appears to have totally missed the point of the exercise written about in "Repowering Tortuga". That point being that simple formulae like Wyman, Keith, Gerr, etc. are only good for a crude first approximation and that a much more comprehensive approach is required to get a good result for a specific design.

I make the comment above as the author of "Repowering Tortuga". Incidentally, the rms error between the predicted speed as a function of engine rpms and measured speeds was 0.3 knots from idle speed to wide open throttle (diagram below). The calculations included not only the resistance of the hull to movement, but also the details of propeller efficiency at each speed. The calculations were complex, but didn't involve any serious mathematics. Finally, my "article" didn't prove anything since I was simply applying some of the more fundamental models out there. What I did was to decide which models to utilize and then wrote software to do the calculations. My intent in writing the piece was simply to document my approach. My conclusion was that you can get a pretty good result if you have the hull lines, do a few simple calculations and make some reasonable assumptions. Finally the driver behind the entire approach was simply that I am cheap and didn't want to pay someone to do something that I felt I could do.

Disclaimer - I did take some naval architecture in college and worked for a couple of years in the field (but with a capital "N"). I am a physical scientist (retired) and have taught fluid dynamics at the graduate level.

Art said:

This interesting and correct thread (thanks Fred) brings me to a rule of thumb that I’ve previously stated, pay close attention to, and basically adhere to (most of the time that is - lol).

I.E.

For owning any boat coming from really good marine designers and really good boat manufacturer companies listen closely to and basically follow their design-standards/guide-lines per boat model that is built-out.

Top brand, high quality boat builders spend uncountable hours and much money on having the best marine-design mathematicians figure out what is really needed (regarding all sections of their individual boat models).

Click to expand...

Yes, yes, yes.

If you don't like their design then find another boat, but don't choose a design and then start modifying thinking you can do better than they did. Same with maintenance on engines, with fluids, with most everything involved. The builders and manufacturers had access to a lot of information far beyond that which you have.

To me the saddest is when the new owner takes the sledge hammer to an interior he has never even spent a night aboard.

Many if not most boats were designed by a pro, or an on groth of an existing series.

Live with it for 6 months , and THEN decide whats wrong .

FF said:

To me the saddest is when the new owner takes the sledge hammer to an interior he has never even spent a night aboard.

Many if not most boats were designed by a pro, or an on groth of an existing series.

Live with it for 6 months , and THEN decide whats wrong .

Click to expand...

If it's just redecorating from primarily a cosmetic point of view, I have no problem with that. It's no different than buying a house and painting and replacing the carpets before moving in.

Quote:
Originally Posted by FF
To me the saddest is when the new owner takes the sledge hammer to an interior he has never even spent a night aboard.

Many if not most boats were designed by a pro, or an on groth of an existing series.

Live with it for 6 months , and THEN decide whats wrong .

BandB said:

If it's just redecorating from primarily a cosmetic point of view, I have no problem with that. It's no different than buying a house and painting and replacing the carpets before moving in.

Click to expand...

Gee BB - I don't think Fred was intimating cosmetics... at all. And, I too have seen abortions happen on some boats where the "used boat's" brand new owner hacked, hammered, sawed, and built-new - till that boat's originally OK design layout had become a nightmare.

Art said:

Quote:
Originally Posted by FF
To me the saddest is when the new owner takes the sledge hammer to an interior he has never even spent a night aboard.

Many if not most boats were designed by a pro, or an on groth of an existing series.

Live with it for 6 months , and THEN decide whats wrong .




Gee BB - I don't think Fred was intimating cosmetics... at all. And, I too have seen abortions happen on some boats where the "used boat's" brand new owner hacked, hammered, sawed, and built-new - till that boat's originally OK design layout had become a nightmare.

Click to expand...

Well, even then if that's what the purchaser envisioned when buying. Knowing the changes they could make might be the only way they purchased it. Guess I just feel more along the lines of "they bought it, they can do what they want." The opposite of that is not buying because they can't picture what can be done to improve it. Sometimes one likes the structure, but doesn't like the layout. They only buy with the understanding of the changes they can make. Just doesn't bother me.

One example I've seen done was a three cabin with forward master purchased for a couple. They disliked the forward master and the small cabins. So they immediately converted the two small cabins into one large master. If they hadn't known that could easily be done, they wouldn't have bought it.

Curious about whether it bothers you as much if they do that to a house? When we bought our current house, we moved a few walls around before moving in. We had a contractor look at it before finalizing our offer. Just like the boat owner above, we wanted a larger master (bedroom, bath and closet all larger) with the trade off of fewer bedrooms.

Another just like a house move. A boat purchaser doesn't like the closed feeling the wall between the salon and galley gives. Just like the home buyer who loves the house except wanted an open concept. Out goes a wall which requires a lot of changes to the kitchen.

What bothers me is changing things related to the naval architecture or engineering of a boat, especially if done without a naval architect involved. That's where I've seen nightmares. Even things seeming benign like relocating the batteries or adding or moving ballast. Anything that changes the balance of the boat. Or changing engine type or size, anything that changes the performance.

BB - Nothing "bothers" me regarding what's done to other's boat, house, car etc. Long as what they do does not infringe on my items/life. I'm just agreeing with Fred that some people go hog-wild with boat "improvements" before living with it first for some time to then be able to really make informed decisions.

I guess I am guilty then. I completely gutted the interior of Tortuga after I bought the boat. When I rebuilt it I made it closer to the original layout.

As far as an owner redoing an interior, it depends on the owner. I made sure everything was ergonomic and built to yacht standards.

Art said:

BB - Nothing "bothers" me regarding what's done to other's boat, house, car etc. Long as what they do does not infringe on my items/life. I'm just agreeing with Fred that some people go hog-wild with boat "improvements" before living with it first for some time to then be able to really make informed decisions.

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There's certainly an argument for that. I'd also say a strong argument against forcing oneself to live without changes that were an important part of their decision to buy. It can sour the entire experience. Sometimes it doesn't feel like your boat until you make those changes.

We're all different. I have an acquaintance who will never buy a boat without planning to make changes to it. If it was nearly new he'd want to start changing things so he looks for good hulls and structures, good mechanicals and equipment, and bad interiors that reduce the price and he can just go wild on. Every boat he buys will spend it's first months at a boat yard being turned into his vision.

It's like homebuyers where one wants a project and one would never buy a project. We'd never buy a project we had to do, but would buy one and immediately contract for the work to be completed in 6 weeks or less. Our first home together was a great lot on the lake but an odd little small house. 3 Bedrooms in 1200 sq ft on an upper level with an unfinished full basement. It quickly became two bedrooms upstairs with kitchen removed and 1200 sq ft family room, kitchen and dining on the lower level. So a 2400 sq ft two bedroom home. The people who bought it from us immediately turned it back into three bedrooms. Our lot was like the hull or structure on a boat. Now, my cousin would have done all the work himself and taken 3 years to do it.
We loved it and we loved the vision we saw for the little house which had been just a vacation home for the previous owners. We also knew it would never sell in NC with just 2 bedrooms so made sure it could easily go back to three.

TDunn said:

Disclaimer - I did take some naval architecture in college and worked for a couple of years in the field (but with a capital "N"). I am a physical scientist (retired) and have taught fluid dynamics at the graduate level.

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Which means that you are light years ahead of most of the rest of us in your understanding of the problem and the skills to solve it.

Thanks for sharing your experience. It is interesting.

As to modifications.....

There is a huge and lucrative industry devoted to just this desire on the part of some to modify something that was well designed, for cars. I have spent some time around car enthusiasts and low level racing. Automobile designers spend huge amounts of money and time to match engine, transmission, and suspension to the car. Car enthusiasts then turn around and spend huge amounts of money and time changing it because they think they know better than the professionals. Their argument is that the designers were building a car for the masses and not for them individually and their own compromises don't match the ones made by the designers. Most of the time however, imo, they screw it up.

I have seen much less of this in boating. Maybe because the average age, experience, and intelligence of a large boat owner is vastly different than the average car enthusiast?

The interior of a boat is pretty simple. If a person has modest carpentry skills, building a boat interior is actually pretty easy. Fussy, but easy. Similarly, the mechanicals and other systems on most pleasure boats are also pretty simple. In my experience, the hardest part is deciding which components to use. The installation is generally pretty straight forward. But then I am one of those people who doesn't like to use something if I don't know how it works.

TDunn said:

However, it is disappointing that the writer of the Boat Design Forum comment appears to have totally missed the point of the exercise written about in "Repowering Tortuga". That point being that simple formulae like Wyman, Keith, Gerr, etc. are only good for a crude first approximation and that a much more comprehensive approach is required to get a good result for a specific design.

Click to expand...

Todd, your work on constructing power-required curves for Tortuga is terrific and I want to do something similar for our boat when I have the time someday. Meanwhile I find that the simple Wyman formula gives pretty good rough estimates.

One thing that is not clear from your write-up (and the way the original authors write their formulas doesn't help, either) is that the Keith, simple Wyman, and Gerr-1 formulas all have this form:

required_horsepower = some_constant * displacement * (S/sqrt(L))^3

and they just differ in what the some_constant is. If displacement is in long tons, speed S is in knots, length L is in feet, then the values of some_constant that they are using are approximately:

Keith: 1.01 to 1.55, depending on choice of "Keith coefficient" from 1.5 to 1.3

Gerr-1: 1.85

Wyman: 2.24

So, if you are using these formulas, Keith will always show the least power required, Gerr-1 will show 19% to 83% more than Keith depending on what you choose for the "Keith coefficient", and Wyman will show 21% more than Gerr-1. But they all have the same shape, with power required proportional to displacement and proportional to speed cubed, which seems to be getting at a fundamental relationship that holds pretty well for most boats at less than hull speed.

QB, yes the simple formulas will give you first approximation results. However, they only include waterline length, displacement and speed. If you want to get an accurate result you need to take a lot of additional hull parameters into account. That is necessary, particularly if you are also matching a prop to your drive train. A large part of my approach was to pick the correct prop parameters (diameter, pitch) to match my engine and transmission choice. The curve I showed above indicates that I did pretty well on the prop. You can't really do that good a job of picking prop parameters without more information than displacement, waterline length and speed.

TDunn said:

QB, yes the simple formulas will give you first approximation results. However, they only include waterline length, displacement and speed. If you want to get an accurate result you need to take a lot of additional hull parameters into account.

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Your graph superimposing Wyman on the three L&E curves seems to show that Wyman is really a very good approximation in the S/L regimes where most of us operate. It would be interesting (to me, anyway) to see a Wyman-based prediction superimposed on this graph: