Propeller calculations - HELP!

Mike, The prop power v rpm curve is thus:

RPM (new) = 3rd(root) of [(RPM(original))^3 x Power(new)/Power(original)]

Thus if you had an engine say 1000kW @ 2000RPM

At 90% MCR (900kW) the rpm is 1931.

But at 90% rpm the power is 729kW, big difference!

 

JSL: I'm not sure I understand. Also, the sketch isn't complete, it's very basic, so it may well be confusing. I attached it to show the new hull shape, together with the third photo. The "outside keel" visible in the second photo is original, it was added to reduce drifting in a strong side wind. I'll keep it as it is. The 2 parallel lines running from this keel to the bottom end of the rudder shaft are supposed to show a strong stainless steel tube which is meant to protect the propeller and also support the lower rudder shaft bearing. It will be supported by 3 V-shaped braces going up to the hull, which are not shown on the drawing.

With this information, could you please explain your comment? I'm guessing the skeg is what I called "outside keel", but I don't understand "shoe". Thanks for your help!

Gerold

 

Ad Hoc: Here's the power curve I used. It's not great, but it's the best I could find.

Gerold

 

When you are done and the boat is operational,

by running to full throttle , noting the RPM and pulling back 10% (usually 300RPM or so ) overloading is avoided.

 

Fast Fred: Yes, that's exactly how I'm planning to do it.

Thanks, Gerold

 

Ok.
Happy new year!

 

Thanks! I'll need some time to study this...

Gerold

 

The 'shoe' is your SS tube running from the keel to the support the rudder. Common term used in our area but may not be technically correct.
Your keel-shoe are not a straight line so if you haul the boat on a 'ways' or trailer make sure the chocks ar set up so the boat rests on the keel and not the shoe.
Note that SS does not rust but it will corrode. You should be using 316L or 317L. If type 304, you will probably be relacing it within 10 years...... maybe sooner if your are moored in a 'hot' area or have used dissimilar, more noble metals (bronze, monel, etc) on the underwater gear.
As to protecting the prop. Should be okay for small stuff like ropes, etc. but not too effective with rocks, logs, etc. Mother Nature usually wins.

 

JSL: Thanks, got it. That's a good point, it hadn't occurred to me. We'll give it some thought. Considering the boat will be used in fresh water in a cool climate (inland waterways in France, Belgium, the Netherlands, Germany), do you think using SS is a problem? And yes, I'm aware the prop protection is relative. I feel it's better than nothing, even if it isn't perfect.

Gerold

 

You should? be okay in FW. As to your skeg and shoe, local fishboats have a similar arrangement but many also have a bar running from the aft end of the shoe to the transom to guard the rudder.
Also, make sure you have enough clearance between the prop and the rudder. It's nice to be able to pull the prop without removing the rudder or disconnecting the steering system.

 

We always plotted the power curves on log paper, much easier to work with straight lines

For the sake of completeness and as you know but others may not:

It’s wise to add 20% to the prop power curve if required to operate into a head sea. Small craft are much more adversely affected by sea state. Towing also produces a heavy running prop. Design should match the engines continuous power curve to a heavy running prop curve. But designers try to wring the last drop of performance from a briefly intermittent rated engine which is what sells a luxury yacht. And all too often the prop is matched for smooth water performance on a perfect day with a clean smooth hull.

The operator is often not very savvy about diesel engines and without the benefit of a ships engineer to moderate the demands on his engine, the throttle is cranked up to the max stop and the fuel pump delivers everything it's got but the RPM stays low if there's no power reserve because the engine can’t spin the prop.

Then a small 'testosterone' based diesel at the limit of it's heat capacity will heat damage it's valves and even melt it's piston crown because it's the RPM that pumps air/gas through and it's the air/gas that cools those components.

 

Actually I'd use steel, paint it and maybe protect it with a small magnesium anode. Also I'd watch the vibration of such a long strut in the vicinity of stern gear.

Stainless is not a great choice in that situation. Also why 316L ? people mistakenly think that it's somehow better than 316 but take th ecarbon out and its significantly weaker and it's no more corrosion resistant than 316.

 

Thanks for the comment. Yes, mild steel would be my choice but some people are absolutley sold on SS and mentioning anything else is heresy.
Some years ago we did an installation [very similar to the one shown on Sailor000 sketch]on a couple of 38' (12 knot) fishboats. We used a 4" x 3" mild steel tube and it was fine after 10 years.

 

Thanks everybody! Your advice really is very helpful!

About corrosion: I was aware stainless steel passivates when exposed to oxygen, so it will indeed corrode under water. I didn't know mild steel was better. Here's yet another question: This boat will sit on the hard for several months each year. We're not going to use it in winter, anyway. And when we'll spend time at home, rather than pay expensive berthing fees and worry about the potential for an unnoticed leak, we'll probably haul her. So there'll be plenty of oxygen at least once a year, probably more often. On the other hand, won't mild steel rust when exposed to the air? Considering this information, what's your advice, what do you recommend?

Thanks, Gerold

 

To be honest, I have never seen an SS shoe on a boat (be it steel, aluminum, wood, or FRP). Probably because it is more expensive and in some ways less predictable. The biggest problem may be using a tube but with mild steel this could be solved by sealing or filling with mineral oil as is done on hollow steel rudders. The exterior can use coatings like Ameron etc. which have proven quite adequate. Our own boat has had a steel bar shoe for 25 years, zincs and annual maintenance have been sufficient.
The shoe you illustrated might protect you from some small debris and rope. I doubt that it will do anything during a grounding other than significantly increase the repair bill & labour. Plus, the short keel- long shoe arrangement could make hauling with a Travelift or slings a real challenge. Sometimes a clean installation (keel, shaft on strut, spade rudder) and careful navigation is a better solution.
I am sure other members can provide some good input