Prop tip velocity

It doesn't matter that they have a limited choice of engines. It will not affect the calculations at all. Seems like you are trying to re-invent the wheel. The standard calculations have worked for a long time. Unless you are doing some exotic high performance craft, which you claim you are not, it is meaningless to try to attain such a degree of precision. Particularly if you are going to use mediocre quality propellers, it is overkill.

 

You don't even know what the full parameters are yet. They are demanding. We have to get it right. That is why I am here discussing and asking. It is not just some hobby.

 

Michael, your are correct that if "if a prop is doing 1 RPS. and we increase the diameter by one foot we increase tip velocity by 3.14 FPS." and "if the prop is doing 10 RPS the tip velocity increases by 31.4 FPS".

But you are incorrect when you said that "a 10% increase in diameter results in a 31.4% increase in tip velocity". The tip velocity only increases by 10%. If a 10 foot diameter propeller is turning 1 RPS then the tip velocity is 31.4 FPS. Increase the diameter by 1 foot (10%) and the increase in tip velocity is 3.14 FPS as you correctly said above. But that is only a 10% increase in tip velocity. 3.14 FPS / 31.4 FPS = 10%.

Not "rocket science" but a common mistake.

 

More on the common mistake when dealing with ratios and percentages, and has cropped up in two different threads recently.

If Y = C * X then if X changes by DelX then Y changes by C * DelX
-but-
the percentage change of Y is the same as the percentage change of X, and the ratio of the new Y to the old Y is the same as the ratio of the new X to the old X independent of C.

Y1 = C * X1
X2 = X1 + DelX
Y2 = C * X2 = C * X1 + C * DelX = Y1 + C * DelX

Y2 - Y1 = (C * X1 + C * DelX) - (C * X1) = C * DelX

Y2 / Y1 = (C * X1 + C * DelX) / (C * X1) = (X1 + DelX) / X1 = X2 / X1

(Y2 - Y1) / Y1 * 100 = {(C * X1 + C * DelX) - C *X1} / (C * X1) * 100 = (X2 - X1) / X1 * 100

 

David, IMO:
Those who know the math, won't bother to read that one because they already know it.
Those who are not good in math won't bother to read lines of symbolical formulae because they won't understand it.
Some will simply not be interested.
At the end, IMO, very few persons will benefit from that post.
Cheers

 

Yes, I understand this now David: I worked through it myself, Thank you If the diameter goes up by a percentage the tip velocity goes up by the same percentage. The same goes for RPM, right? I was wrong, now I know better.

Maybe tomorrow I will go through the parameters we have to satisfy. All should then become clear.

 

Yes, same for RPM. A change of X% in RPM results in a change of X% in tip velocity.

 

Hopefully a few people will benefit. The same mistake was made in the http://www.boatdesign.net/forums/boat-design/why-not-very-thick-core-hull-deck-47289.html thread which was leading to some erroneous conclusions.

 

Actually I will do it now

Objective: to transport shipping containers on natural inland canals. Ideally min of 4 x 40ft or 8 x 20ft containers. i.e. max of 130 ton payload

In places water depth is down to 1m. Customer would like to do some deliveries in 0.5 m water depth if possible

Plus: locks of 8.6 m width and 35 m length need to be negotiated.

Plus: Bridges restrict air draft to 4.2 m

Plus: In places there is solid water weed

We'v got it all solved except for tug propulsion.

We will get this too in time.

M

 

Ha! I am going back to that issue of foam sandwich sooner or later. That I do know something about. I have test data that's pretty impressive, like: double the strength of a hollow beam using polystyrene and another little trick that I won't disclose. No bonding either. I am talking beam strength: resistance to failure with a central point loading and beam supported at extreme ends.

 

4.2 m air draft+.5m maximum hull draft =4.7m if you substract the thickness of the hull and the draft of keel and propellers it gives you maybe 4.0m. That means it can only carry one layer of containers. With the length restriction on the locks only two 40' containers can be installed. That is taking into consideration an empty space in the bow and machinery space in the stern.

 

Sorry to disagree on that. It is not only power to weight ratios. It is the duty cycle ratings that the engine manufacturers use.

For example, you can have two engines of the same HP. One is a leisure yachts that goes out 4 to 5 hours on weekends, the other one goes out 8 hours everyday. The yachts will need a lower duty cycle engine, maybe 20%, the other one maybe 40%.

Or two engines, same block, same make. One is detuned, lower HP, weighs a ton with heavier than usual core designed for chug chugging in a trawler all day, 24/7. The other one is tuned, more Hp but is rated to be used only half the time, say for patrol boats. This is how engine manufacturers rate their engines, Duty ratings first, HP second.

You are however correct when you said each engine has its own application.

 

You are perfectly right, of course. I have probably failed to express the intentions of that post. It was meant to just show how installing a heavy engine, although apparently better in terms of BSFC, on a wrong boat can lead to an overall higher fuel consumption.

I should have choosen with more care an example of a 180 HP heavy duty engine. A Volvo D7A TA 177 HP would be much more rappresentative in this case: http://www.volvopenta.com/volvopent...ngines/Inboard/engine_range/Pages/d7a_ta.aspx

Cheers

 

Perhaps those folks should carefully consider how much design and analysis they do on their own.

 

Good point, deserves to be repeated every now and then........