Prop Questions

Good Morning,

I am trying to learn more about marine propellers and what works best in different scenarios. I have read some excerpts from Gerrs book which has helped somewhat but there a couple of things that I'd like to get straight in my mind. From what I gather, greater diameter will give a more efficient prop, but that will ultimately be limited by the design draft, and size of the appature, although adding some rake will help a little bit. Then its down to pitch and RPM, so a greater pitch will drive the boat further per revolution, provided the power is available at the shaft. A deeper pitch therefore is more suitable for faster boats, with a certain RPM determined, amongst other things, by the gearbox ratio.

Assuming a 60t schooner, 60ft WLL fitted out with a 150hp engine, giving a service speed of about 8 knots.

The first question is, what is physically going to happen at the prop if full power is applied when the vessel is making little or no way through the water? I have always applied power smoothly, but now I am thinking about what is actually happening. I'm thinking that at a standing start, there is little resistance and hardly any load on the propeller with no slip. So to set full RPM and put all that SHP into the propeller when it cannot be absorbed would result in a lot of cavitation and little acceleration. As the boat speed increases, so does the resistance and the load on the propeller, and by introducing SHP slowly as this resistance increases, the propeller is always 'fully loaded' and able to absorb this extra power without creating excess low pressure. Is that right?

So when I see RIB's being used to push the bows of their parent craft when manouvering, is it fair to say that they want a course pitch to be able to put that power into the mass of the larger boat - even thought the RIB itself is not actually making way? And this will also serve to bring the RIB up to a high service speed when being used as a tender? (Yes I know now we are not talking about a displacement hull). Or to put the question another way, what is the most suitable pitch for a tug a) steaming at full speed to the next job with no tow; and b) slow steaming under a heavy load/tow?

Secondly, assuming the boat is correctly propped (i.e. the power curves meet at say, MCR) and the hull/blades are clean. The boat will do 8 kts in nominal conditions - but what is going to happen in bad weather. Let's say we are punching a head sea and gale force winds trying to get to shelter. The speed might be down to 2 or 3 knots - what is happening at the prop?

What I am wondering is, how to best prop the boat to cope with bad weather, i.e. if I theoretically want to keep her at 8kts even in inclement conditions. I say theoretically because I'm also aware that the boat might actually not be able to take the punishment of being forced through those conditions at that speed - but that aside for now.

From what I understand, the heavy weather is going to effectively add extra resistance to the hull and cause an increase in prop slip. The only way of overcoming that is to actually have HP in reserve at the engine, and a deeper pitched propeller to be abe to use the extra power. This means that it nominal conditions, unless the boat is fitted with a CPP, she will be cruising round at reduced RPM's and an underloaded propeller. So one is sacrificing efficiency for reserve power, although fuel consumption will be lower anyway. Is that right?

So some beginner type questions I guess - but I'm not a naval architecht!

Thanks,

Richard

 

No, you would want a much smaller pitch to keep the propeller from cavitating and/or the engine from stalling.

 

Gerrs book has all your answers, just keep reading it until you understand.
For discussions sake, you need to narrow down your focus.
No way to compare to an unknown tugboat or RIB.
Is the 60’ schooner the boat in question?
Hull type, weight, power, Rpm, transmission ratio, prop diameter restrictions, target speed, etc?
Google Search for Vic Prop calculator, I’ve found that very helpful.
Your most valuable resource will be a reputable prop shop, provided that you are interested, at some point, in purchasing a propeller.

 

Try this post, it explains what happens to the prop/engine when the prop is under/over loaded.

Oversized prop compromise

(edit : sorry, wrong post was linked.)

(2nd edit)

BTW, "sea margin" is 5-20% power, smaller and/or route vessels having more margin.

 

Thanks chaps,

I'll buy my own copy of Gerr then and give it a good staring at!!!

Thanks Jehardiman, the post on the other thread really helped. Really I am interested in the heavy weather scenario in a biggish sailing boat as described. If I understand correctly then, assuming the boat was propped for 8kts but was making 4kts in a heavy sea at service RPM, the slip will be high, and therefore so is the torque required. As a result the engine cannot deliver the power to the the prop to get her up to 8kts and it will start to overheat. So then, for a vessel that was okay in nominal weather but struggled in inclement conditions, the answer is not to look at the prop, but to increase the engine size to be able to match q/Q and atain service speed over a wider range of conditions?

Thanks again,

Richard

 

It depends on the blade area too. If the blade area is large enough to absorb the extra power it won't cavitate. Otherwise, a bigger engine won't make the boat attain more speed. Also, if the blade area is large enough to absorb more power than the engine can develop, it will cause a stall.

 

Propellers are very sensitive to the inflow velocity, which means vessel speed. So when moving at a slower speed more power will be required to turn the prop at a given RPM. The thrust though, will also be higher. That helps with the acceleration and also with increased resistance. An appropriate propeller design will allow some engine margin at the nominal design condition to still allow the engine to reach rated RPM when there is a greater load due to fouling, sea conditions or vessel load. Part of selecting an appropriate design point also involves knowledge of the engine power curve. Most diesels can achieve full power below rated RPM and also run at above rated RPM. This does allow some flexibility to use all the available power over a wider range of speeds.

 

Let's take a look at these two scenarios. A correctly sized prop will indeed be able to absorb all of the power that your engine can produce. As you apply full power at a standing start that power is manifested as acceleration. The correct propeller will minimize slip so that your boat accelerates more quickly, more efficiently. As the boat speed increases the speed of the water that the propeller "sees" increases, thus unloading the prop a bit until you approach hull speed or maximum displacement speed. Spend some time with Dave Gerr's Propeller Handbook and a scientific calculator. You'll never get it all sorted out on an internet forum since you'll need to solve some fairly complex equations that will be specific to your boat.

Fighting a head sea or strong current more power isn't going to help you. Assuming a full displacement hull, you're going to max out at a certain speed. Physics tells us that even huge increases in horsepower aren't going to materially change your max speed. So you want to power the boat and prop it for it's maximum displacement speed or very slightly higher.

This is why displacement boat skippers need a keen awareness of tides and river flows/currents. If you've ever watched videos of boats going through places like Haulover Inlet in Florida, Oregon Inlet in North Carolina or the Columbia River you'll see that almost all are planing boats or at least semi displacement hulls. In less than optimal conditions a full displacement boat would likely not be able to traverse the inlet as it would not be able to carry enough speed to manage the strong current. Again this is just the laws of physics playing out.

Work out the equations in Gerr's very good book and you'll get a good understanding of all this. You'll be glad you did.

Oh, one more thing. Welcome to the forums and have fun!

MIA