Efficiencies for Surface drives.

There seems to be a dark art in the finding of the efficiencies of surface drives. Does anyone have advice or actual figures on the efficiencies of a lets say ASD 10 unit. All the factory will do is size them , I would like to know what percentage of engine power gets through to the prop. I know i need 530 KW to reach my topspeed for a given hull resistance. This is effective power , the installed power needs to overcome the gearbox, shaft losses and equal the the effective power. Therefore the effective power gets divided by the efficiencies to give me a KW rating that i need to find in the engines. I hope that is clear enough. Many thanks Matt

 

Err, don't you think you missed out propeller efficiency? Compared to the losses at the prop your gearbox, shaft bearings and the like are almost negligible...

 

The props are also in there, mustve just ommitted them, still Im looking for the overall efficiencies, seeing as the suppliers also supply their props with them, if any one has info regarding all the efficiencies for me that would be ideal.
Many thanks Matt
oh and how to go about sizing super cavitating props.
Thanks

 

Bringing up an old thread

Does anyone have any information on the prop efficiencies charging on a surface drive as the prop become more or less in the water?
How much of an effect does this have? For instance if a boat with say Arnesons was loaded a little heavier than usual (5% heavier) and the props were trimmed so that rather than 50% of the prop being in the water only 45% of the prop diameter was in the water (43.6% of the prop area.) To prevent the engines from over loading. Would there be any major problems with this? At what point would problems start to occur? 25% of the prop in the water? I see that thrust is going to be decreasing, and soon the slightest upset in the water is going to cause RPMs to become erratic.
How about the other way? If the tank is empty, boat is empty except me ... can the props be lowered to say 60% in the water to get a little more thrust? How far can one go in this direction before props stop ventilating and start acting like normal props?

 

There are a number of litterature references in another thread under this main title. Look for papers by M.Ferrando and A. Scamardella, something like "Performance of a family of SP propellers....".

 

Calculating shaft HP is really straightforward. You only need to subtract gear and friction losses. Propeller efficiency is determined by its design, RPM, and load. There are formulas for surface piercing propellers. Also, propellers in surface drives are calculated to be submerged only on the lower half. When the boat takes off, it usually need to ventilate the propellers or the engine won't have enough power and bog down. The most common way to do it is with the exhaust gasses.

 

Can one over-ventilate a surface prop??

 

Do you mean on take-off?

 

Or in general ?

 

On take-off, the engine will bog down without induced ventilation. As soon as the boat starts lifting, the upper blades come out of the water and ventilate. Ideally the hub is barely touching the water. With diesels there is more low RPM toque than gas engines so there can be less ventilation. I am not sure there is a general rule. It depends on hull resistance at displacement speed, HP to weight ratio and other variables.

 

I thought hub would be half underwater...

 

Check out "maritimo" Bill Barry-Cotter is heavily involved in international powerboat racing and his boats have held several world championships - sirt of like automobile Formula 1 where they race at special venues around the World doing 200kmh and more in open ocean racing...

Barry-Cotter used to run diesels but was winning ALL the races so they banned diesels The boats are about 40ft LOA weigh about 4,500kg (Have very specific measurements to which they must comply), Make a hell of a lot of noise and turn on around 4g in lateral force...

The 6 attached images may give some idea... The ventilation is a piece of plumbing plastic pipe with a portion of hose attached... The drive is Non steering but adjustable for height by the engineer/throttle-man whilst racing... The engines for local races push out about 750hp (de-rated to enhance the spectacle of close racing LOCALLY, and because 4 classes race simultaneously and traffic is comparatively busy...

 

Oops I forgot, the propellers, fitted - RAZOR sharp - Surface piercing only come into their own for High Speed water craft... If you want a very fast boat - he may sell one of his previous seasons boats....

 

I guess that shows that efficiency is relatively even for different immersion ratios. Thrust will be reduced with reduced immersion, with the exceptions of a few out of applicable situations (i.e. low pitch to diameter ratio with high advance coefficient.)

So going back to my question. In the case of the heavily loaded boat, reducing immersion is going to reduce thrust, causing the boat to slow down, then a slower boat with have more wetted surface area more drag more load on the engines, so reduce immersion more? Is there a solution to an heavily loaded boat?

 

Without having checked Baeckmo's reference yet I learned SD's should be trimmed up with bow down having the prop half and horizontal in the water at speed, transporting weight a different prop might be better. ZF gives seminars on SD's here, probably there too, arneson also reply's their e-mail. Now let me see about these papers..

edit: did so and thanks but lack of data is mentioned there too.. Paul Kamen wrote an article on SD's beeing plausible for displacement propulsion as well