Surface drive with large diameter propeller

Thanks for your reply!


Hull speed - Wikipedia https://en.m.wikipedia.org/wiki/Hull_speed

Just wondering if the propeller design has to be tweaked for better efficiency when traveling above Hull speed compared to below..

My guess is that an oil tanker that is full of cargo will coast further than the same empty one, when power is cut, but I don't know if that effect would be a consideration in propeller efficiency..

 

It's complicated - but the momentum doesn't care one whit. It's all buried in how you achieve it with mechanicals.

cross posted with baeckmo - I was off line for a bit.

 

At first look, it made sense as a good guess due to extra mass when loaded but

"When fully loaded, the ship sits about two-thirds underwater or 75 ft below the surface." pirated from the American Petroleum Institutes site

So the amount of water that the ship would have to move out of the way would be much higher loaded and the ship unloaded would not be able to get the extra efficiency gain due to the bulbous bow not being immersed.

 

Hi Barry, thanks for your response! I think the he bulbous Bow would be immersed when the tanker is fully loaded which means it would get the extra efficiency.

A search turned up this link which addresses the question about loaded versus empty tanker stopping distance about halfway down, and I couldn't find anything else that would contradict it,

Stopping Distance, Turning Circle, Ships Manoeuvring – Knowledge Of Sea https://knowledgeofsea.com/stopping-distance-turning-circle-ships-manoeuvring/

But the information you wrote makes a lot of sense, so I don't really know what the answer is, ha! The question might not be as simple as it seems to be, lots of references had complex looking calculations which I did not have time to explore.

 

In this forum, it occurs so frequently that answers are much more complicated as what would appear on the surface.
Especially the neophyte OP's, "I want to build a boat, what should I build" entry remark .
And some of the very patient frequent responders start the query process to find out what the OP wants.

 

It has none. They are unrelated except in that a prop is optimized for one condition and is less efficient the farther it is operating from that condition. Coasting is unrelated. Of course a vessel with the same waterplane area, and therefore similar wave making drag coasts farther than it would empty. But unless dolphins pushed it up to that speed there isn't any free energy there. It got to that speed with its very own prop and fuel.

Naturally a drive train set up for slow and heavy is different from one set up for light and fast. Hopefully that's not a revelation.

But none of this was @Barry 's actual question.

He asked about large surface drives at low speed. Or if Kt/Kq is superior with a large surface drive compared to a smaller submerged prop. The drag from all the stuff underwater: petticoats, garters etc, is absent but at low speed that matters less than at high speed. The obvious superiority at high speed has less of a clear case. No one who hasn't tested exactly this knows, although I'm prepared to bet actual folding money that a phalanx of fellows imagine their opinion is better than hard data to 2 significant digits.

 

Thanks DC !

My guess is that it takes more energy (wasted prop slip) during the process of quickly getting up to cruising speed (that is above hull speed) than it takes to maintain the cruising speed, (possible application of Newton's Laws about Bodies In Motion)? So I thought maybe the propeller design for best efficiency would be different while cruising at or below hull speed as compared to accelerating? If the same thing be true with a surface piercing propeller, what kind of length and pitch might be involved or is it better to use gears (or adjust depth of piercing) and stay with a fixed length and pitch?

 

OMG, gears!
DC's favourite topic!

 

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There are so many things going on in that question I can't make a generic answer. And since no one else has answered, I guess that's generally the case. At low speed, below planing, bigger is generally better with a prop. But as the diameter increases there are diminishing returns. The reason is that thrust is a momentum transfer, over simplified as mv: mass of water accelerated backwards times how much it was accelerated. But accelerating that water is a kinetic energy function. Over simplified as ½mv². So for good efficiency, accelerate more water, but less fast. Big prop.
But turning that prop isn't free. @jehardiman pointed it out quite well. Turning a prop in the water without making any thrust still takes work. And that work goes up with prop diameter. The sweet spot changes with speed of the boat. Slow boat: big prop. Fast boat: small prop. At the ultimate end you have top fuel drag boats reaching close to 300 knots. They have 10,000 hp, and put that through 11"/28cm surface piercing props. They use 6 speed gear boxes.

 

Can someone weigh in about a surface piercing props being impacted by the aerated water that Serenity produces at the transom? The reduced mass per cubic inch/foot/ meter due to air entrainment ahead if the prop must have an effect.
Running jet boats with mixed flow or axial flow impellers in air entrained-white water reduces thrust immensely.

 

Sleds were typically built with twin engines. The aerated water is only about the middle 20% of the transom. The only OG sea sled I know of with a single prop is Miss Lakeside who has a 26" SP prop on 200hp. Catch that much water, aeration apparently doesn't matter.

 

The only direct comparison between jets and props that I have seen first hand was a comparison made by a boating magazine.

They used a single boat. Some vanilla common FG runabout, around 25' and ran it with twin mercs. Something like 150's. First a pair of their standard prop models, then the mercs with the same power but jet drive legs. After much tweaking of props, trim, depth etc to get the best performance out of each, (proper science lads) they determined the jets gave 80% the speed and mileage of the props. I don't know how a more apples to apples comparison could be done.

Assuming merc has competent engineers, I would imagine the jets were limited by the above relationship: mv vs ½mv². They accelerate a relatively small m of water to high v. Reducing that further would be punishing in the extreme.

 

No science, but an amusing story.
Several years ago I was approached to design a 15m/50' Harryproa for a French solo Transat. They didn't think it would win, but they thought it would be good publicity for the race and sponsor. One of the rules was a motor that did not need deploying, and that would push the boat at 5 knots in flat water. A prop under the hull was draggy, wouldn't work in both directions, and almost certain to hit something, so I looked at a 2 bladed prop mounted on the beam. Think it was 6m diameter with the hub mounted on the beam 2m above the water ie 1m of blade in the water. When not in use it was horizontal next to the beam. I asked one of the NA's who helped me design the windmill boat blades whether it would work, and how much power would be required. The answer was (I think, it was a long time ago) around 8 hp, which would have been mounted with a gearbox in a box behind the beam.
The blades were triangular section, flat side in the rotation plane, apex as the leading edge. Thickness dependant on shaft diameter. The french guys got quite excited (the boat was 2 tonnes, compared to 6 for the tris at the time), to the point that proas were banned for the race.

 

Great thread Barry, thanks.

I can't help but wonder if a Thai longtail style surface drive
off the back of Serenity wouldn't move it very well indeed.
Rope-and-tiller style controls could allow for remote helm control but
given the visibility from within Serenity's cabin, why bother.
It would suit John's penchant for V-8's and
the low aft deck may lend itself to mounting.
The fuel tank may not even need to be moved,
although it would be of benefit closer to the CoG.
The aft door would become useless,
and the doorway may need to be widened.
There would be no limit to prop sizing.
He could even incorporate gearing, manual or automatic!