Semi-displacement boats don't generate lift?

interesting, thanks

 

We purposely design surface effect ships with the LCG aft of that required for the optimum running trim and then "correct it" with transom trim tabs or intercepters. That design detail results in a reduction of the total resistance by a very significant amount..in excess of 5% in calmer conditions at speeds in the 40-50 knot range (and higher).

 

( Haven't read the thread. )

Don't forget about "squat" ( anti-lift ).

 

Elaborate on what you mean.
JS

 

The drop of pressure below the ship's bottom causes a vertical sinking of the ship's hull in the water, at the same time and depending on the vessel's block coefficient Cb she will trim forward or aft or will sink deeper into the water on even keel. The sum of all vertical sinking and trim is called “SQUAT”. A down force, the opposite of lift.

 

That's consistent with what Paul Bieker found adding foils to I-14 rudders to suppress the stern wave. The sailors could move their weight farther aft; the overall effect was they won the world championship.

It would have to be a long SES for 40+ knots to be semi-displacement speed by my definition, though.

 

Nigel Irens' launches: Hull Design for Propulsion in Small Craft - Clara Boat Company by Nigel Irens https://claraboat.co.uk/hull-design-for-propulsion-in-small-craft/

 

Jurgen (HJS),
What would you think of plotting the resistance curve for your new boat on the curves above, from Nigel Irens' website? Can you find a way to compensate for the different displacements? I've often seen resistance curves expressed in terms of HP/Displacement on the Y axis.

Is this correct? (from How Much Horsepower Do I Need for My Boat? | Formula Boats https://www.formulaboats.com/blog/much-horsepower-need-boat/)

You also need to know a well maintained four-stroke gasoline engine is estimated to burn about 0.5 pounds of fuel per horsepower per hour, while a similar diesel engine is estimated to burn 0.4 pounds of fuel per hour. The equation is GPH = (specific fuel consumption x HP)/fuel specific weight.
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I see somewhat different values elsewhere, and don't have source material I trust handy at the moment.

I've proposed a group project in this thread.

 

There are too many variables that have to be matched against each other to get a satisfactory result when converting from one measurement method to another.
I have checked the chart against other similar studies and found it to be OK.
Right now my concept is being converted to electric operation. It is very easy to measure kW with electricity.
I look forward to receiving satisfactory data after new tests in the near future.

JS

 

Thanks. When you're ready to say more or publish a graph perhaps we can ask Nigel for data using your preferred units & parameters. I'd be interested in seeing how your semi-planing (your preferred term) hulls compare to your planing hulls, in the style of the Nigel Irens graph. Also how you're using a transom immersion criteria - is that informing hull shape, LCG placement, or running trim?

 

Numerically speaking, the best information I know of lies in tank design. Post WW2, most tanks had gasoline engines, but were converted to diesel engines in the 60's and 70's. As a result, tens of thousands of vehicles had their gasoline engines replaced with comparable diesels, allowing the elusive apples to apples comparison. There was typically a 30% improvement in range, despite the fact that the diesels were considerably heavier for the same power. Later developments in diesel power brought about engines that had better power to weight performance than gasoline engines, but that wasn't the question here.

 

Sorry, what was the question?

I found this article interesting given GodCalvary's last (that's a typo I'm not going to bother correcting!)
A little dated at 2018 but relevant.
I disagree with their view of little safety gain. I've attended too many boat explosions at the fuel dock ( and it's never diesel ).
I would add a pro that the article leaves out for diesel and that is ease of certification in Canadian waters for commercial vessels.
https://www.boats.com/on-the-water/boat-engines-choosing-gas-or-diesel/#:~:text=At max speed the Cummins,about 240 gallons of fuel.

 

Certification is why I'd even consider it.

 

If you mean my question, it was about the units of the Y axis of the graph. If you understood it as about fuel, though, I'm with Jurgen: let's go to electrons!

 

Very impressive wake difference 4 1/2 minutes and 12 minutes into this video: