Power predition formula

I have read of a prediction formula mentioned on a naval architects site called the Beebe Power Factor, I am curious to know more about it.

Re-powering the Tortuga, an article on the net made a good case for Wymans formula being the best. Gerr formula 1 and Keith's formula were less accurate and Kieth's formula gave the lowest answers for required power. Don't know what coefficients he used but while they are all good useful formula get the coefficient wrong and you might as well just guess. let me explain.

Gerr-1 SL = 10.665/(dis lbs/SHP or BHP)^.333

V(kts) = LWL x C x(BHP x 1000/dis lbs)^.333

In Gerr-1 C= 10.665

From a published speed power table LWL=38.33 Dis =60,000lbs 143 max Hp

6kts SL=0.97 HP=18 33.53 33.16
7kts SL=0.13 HP=36.75 53 52.65
8knts SL=1.29 HP=67.55 78.86 78.6
9knts SL=1.45 HP= 112

When you plug in the numbers Gerr-1 gives SL= 1.313 @ 9kts. To get SL=1.45 C must be 11.777 . Plug in the numbers in Keiths formula and to get 9kts C must equal 1.181 for the boat in question. I then calculated the required HP Blue is Gerr-1 Red is Keith. Not much difference is there.

 

I wish someone could tell me what this Beebe algorythm is. I have done a lot of number crunching. I have always felt the Gerr-1 formula to be my favourite ( knew the formula long before I bought his propeller book). Frankly I didn't rate Gerr-2 as Gerr-1 is simpler. Dave Gerr recons Wyman's formula to be excellent for semi planing boats. but his Formula(Gerr-2) is a little more accurate for boats up to a maximum of SL =2. My calculations lead me to agree.

I have been on Michael Kastan's site which is where I first read of the Beebe algorythm. If a professional designer uses a formula it must be good.

Gerr-2
SL = 2.3 - ((Dis lbs/SHP)^.333)/8.11 if you plug in the numbers you get SL If the boat was 100ftLWL multiply Sl by the square root of LwL and you have how fast the boat will go on installed power at the displacement weight you entered.

 

Ok When I want to know something I tend to drive myself nuts trying to find out. I went on Amazon and have ordered "Voyaging under Power by Beebe" third edition.
I checked out the index and it seems to be a mine of information. So even if the algorythm isn't any more accurate than Gerr or Wyman and that is probably too much to hope for I don't think I have wasted money. I lived most of my early life in South Queensferry Scotland, less than a mile from the river fourth. I'm an engineer with no sailing experience nor am I ever likely to get any now. I know the river Forth is dangerous in the summer apparently Beebe did ocean crossings in boats which I would think twice before crossing the Forth in them.

 

Considering the lack of different sized diesels that can be installed in a boat , is decimal point prediction of cruise HP required useful?

I think the tables are great to advise IF a certain engine you are looking at will fill the bill , and to keep from purchasing a too small, or two large engine .

The next question will be what varying loads will the engine need to work?

Hyd? , mechanical air cond?, cruise power generator?, water maker? scuba tank fill?

All these requirements may need to be handled while underway , and their loading can not be predicted to a decimal point.

FF

 

Ok Fred I agree, but I am a retired engineer and I want to be able to predict what horsepower is required for what speed as accurately as possible. My table has horse power to two decimal points in order to show how close the two formulas I was using were. As you go nearer SL = 2 Wyman's formula is better.
By the way I think your rule of thumb 1 ton displacement requires about 3HP is the best/easiest starting point when looking for an engine.

 

I now have a copy of voyaging under power with the Beebe algorythm. I quited in an earlier post detaila of engine horse power and compared the predicted results using Gerr's formula. Using The beebe algorythm I get 18, 36.75, 67.55, and 112hp as near as makes no difference.
I then used the algorythm which is limited to 95 long tons to predict (very roughly) the required power for a Fletcher class destroyer, as about 40,000SHP. I thought this to be quite good as I know warships at least have about 33% or more extra power to cope with bad weather.
The horsepower in the previous post was for a Nordhaven 46. I wonder if someone used Beebes algorythm at 48320lbs to produce these results or were they checked against a prototype vessel then published.

 

I won't use Gerr's formulas, they are not even close to be accurate...
Look at graphs - physics-based and reliable methods show close results; some 'simplified' formulas are well outside.

 

Thanks the graphs show there is a big difference between these formula. I thought you had to make sure you used the right formula for your type of boat to get accuracy. If you don't use Wyman or Gerr's formula what do you use.:?:

 

There are plenty of boats on the planet, spend your time looking at boats similar to yours and see how they go with different engines. It will depend on your hull design also.

Problem solved.

I looked at these formulas quickly it says my boat im building will do 100 mph. I doubt it.

 

Jetter I assume you have a planing powerboat if you use the crouch formula you will get close results. But both Gerr formulas are for displacement type hulls and will give stupid answers for a powerboat.

 

What's your motivation behind wanting an "accurate" prediction?

And how accurate is accurate?

 

My boat weighs 360 kilos with me in it and it has 150 hp, it is a planing hull. Will it be fast?

 

No. The first graph is for the sample boat Gerr is using in Masthead. The second graph is an attempt to compare all range of formulas, we deal with hull parameters only and not with particular hull. I fully understand which method is for what purpose and speed range.

The outcome is that Gerrs formulas do not work. They show power close to reality only in one point, and there are no limits specified by their author so we do not know what is valid range meant for those formulas. I recommend to avoid using them. Better choice are traditional methods that are available from say FreeShip where range of systematic series is included. Always use 2 to 4 methods to validate the results.

I made a review of performance prediction methods at ibex 2011 can mail handouts if you interested.

The main problem with Gerrs formulas I think is he is trying to cover power curve by one continuous line function. This never works as resistance curve has humps and hollows and can not be described by smooth polynomial function, etc. If one looks ar systematic series, they have separate formula for every Froude number.

 

Alik My motivation is I am a retired engineer looking at the theoretical design of boats. I want to be as accurate as I can be within reason. I don't agree with everything you say about Gerr's formulas, however I have no doubt your knowledge on this subject is far greater than mine. I find the Beebe algorythm works well for boats about 95 tons,which is why Michael Kastan uses it on his site.
If you can show me a better/more accurate way, I would be most greatfull.

 

I don't know what is Beebe algorithm, this is not what naval architects use. As well as Gerr's formulas - no serious designer would rely on them.

Try to study conventional prediction methods. There is good book recently written by A.Molland on performance and propulsion, so far it is the best collection of methods that You can program into spreadsheet and use.
http://www.amazon.com/Ship-Resistan...e=UTF8&qid=1356442266&sr=1-2&keywords=molland