Efficiencies for Surface drives.

[gonzo]

If you are looking at a whole package, talk to Sonny Miller at WWW. CAL500.COM He manufactures the drives, and will calculate propeller, HP, etc. Where in the UK are you? We will be in the UK for the team meeting of the Cowes-MonteCarlo race in mid March.

[fpjeepy05]

I guess what I was getting at is in a conventional inboard straight shaft an overloaded boat just goes slower. And an over loaded SP boat can critically over load an engine, but if one can adjust immersion (as in Arneson) then load on the engine can also be controlled as to not cause damage, and prop efficiency can be relatively constant (as shown by Ferrando)
Right?

[gonzo]

Not at all. The dynamic behaviour of the boat and different loads has to be calculated to the design the propeller. Off the shelf components bolted together will give mediocre performance at best. At worst, they will produce an unsafe to operate vessel.

[fpjeepy05]

Never suggested "off the shelf components bolted together" or an improperly designed prop.
So you are saying that if a boat is overloaded and the prop immersion is reduced the engine will still be overloaded? Or the boat will not go as fast? Because if I were to add significant weight to anyone's boat, I doubt they would be surprised if the boat went slower.

[gonzo]

If the boat is overloaded there are several scenarios. It can be trimmed down by the bow, in which case the propeller will be at the right level, but the boat will oversteer and broach. It can be down on her lines but level. Then, if you trim up the drive, the thrust will not go through the CG and the boat will porpoise. There is a different change if the propeller goes in and out of the water than if it is completely submerged all the time like in a straight inboard. The type and diameter of a propeller are more critical in the surface drive.

[fpjeepy05]

Attitude and prop immersion do not have to be mutually exclusive. Things that can be used to change attitude include CG changes, trim tabs, and rocker plates. Also changing prop immersion does not mean that you have to change thrust angle; an example of such would be the SCS drive (Sage #8).

[mydauphin]

I love those Cal500 drives; interesting the rudder in front of props, I guess that is ok on a race boat. How would it work at lower speeds?

[gonzo]

I agree that a boat with more load and same power will go slower. However, the whole behavior can also change, depending on the design characteristics.

[jmiele3]

Well, I tend to agree with Gonzo and Fpjeepy... Higher load at same power will reduce the speed. Changes in drive trim, use of trim tabs, or ballast can compensate to a great extent, depending on the hull design. When we are designing a system for a vessel, the most common problem in addition to heavier weight is the shifting of the LCG. We tend to try and design the propeller for a slip of between 6% and 11%, the higher number at full load. This is definitely where trim tabs and trimmable drives become more important. Effectively, the main advantage to a surface drive is providing the lift necessary to achieve plane. A heavier vessel becomes more difficult to get over the "hump"... However, though there will be some reduction in speed, the impact is minimized at higher speeds.

One other factor to consider... Vessel efficiency deteriorates over time, particularly regarding maintenance or lack thereof. Very seldom is a propulsion package as efficient 4 years later as it was on the day of commissioning.

I hope this helps answer your question... I won't compare to what others do, but I can tell you that on virtually every project, we are providing calculations on different sea states, and vessel loads so that customers can accurately determine the actual performance and efficiency of the drives under different conditions.