Recent content by johneck

Once the boat is moving, the relative velocities of the water past the oar are much lower, so many of the variables that you are looking at become less critical. That may help explain why you can get similar results from very different blades.

It is very interesting to read all the different viewpoints on this. It is what makes propeller design such an interesting topic in my opinion. It simply depends on the problem one is trying to solve and what the constraints and optimization goals are. Sandhammaren is operating in a world...

That is an interesting way of looking at the problem, unfortunately it is an oversimplification and not actually correct. If the boat were not moving, it might be right, but efficiency is defined as useful work (thrust produced) for a given input power, so simply looking at the direction of the...

Thanks for the update, very interesting. I did read the article. I agree with your statement that the only way to really make an improvement in efficiency is to go to a larger diameter prop, which is not really an option with an outboard. Best of luck with your project.

The prop will produce out of plane forces if it is operating at an angle of attack. Angle of attack can come from tilting the drive, tilting the whole platform, or induced flows from something forward of the prop. It is probably very difficult to operate the boat without any angle of attack on...

It is fun to build, but I think that Gonzo has it right. Adding blade area only helps if the prop is really overloaded, this one is not. Adding cup is really only useful when it is not possible to provide enough blade area to avoid thrust breakdown; again, probably not the case here. Slip is...

The short answer is no, the props are not too big. Even at only 8-10 kts they are probably only using about 120 HP/shaft. The problem is likely somewhere in the engine, fuel, air, ... It also seems realistic to reach 20 kts with 530 total HP and a 20-25k lb boat. Trim tabs or interceptors...

here it is

Yeah, OK. I was not trying to say that you shouldn't ever ease the sheet, but the more you can sail without doing so, the better your chance of winning the race. The flexible mast and the flexible plank help.

The extreme flexibility helps the iceboat in a few ways which are not really needed for most sailboats. It allows the sails to be flattened nearly completely since the boats sail over a very wide speed range. They also act as one of the main shock absorbers in the system to help the sailor...

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...

Sounds interesting, but why do you think that optimizing Cl/Cd of a blade section will produce a better propeller? It seems to me that more critical factors are providing an appropriate radial circulation distribution and making sections to avoid cavitation issues at various points on the...

The rule of thumb about right, so you need approx 2500 HP/shaft to get to 50 MT; maybe a bit less with a Tractor tug. How did you arrive at 273 RPM? This is probably not very workable with that RPM/Diameter combination. The power is pretty high for that diameter, so to manage blade loading...

that came from sine(beta) and cos(beta). People usually use beta for pitch angle, but for various reasons used phi here Old guys used to try and save cpu time, so if a calculated number was used repeatedly, it was assigned to a variable

One thing I just noticed is that "skew" is projected skew in radians, not expanded skew as it says in the comments. "dtor" is simply the concersion factor. it should say skew = skp(i) * dtor 'skew is skew in radians, skp is projected skew