Sea Sled madness. It’s in my brain.

So, do the test.

 

Well okay then.

 

There cannot be any dramatic change in air fraction due to bubbles rising to the surface over the short setback length. The dominating influence is rather the propeller inflow, which tries to accelerate the approaching fluid into the propeller disc. Thus, a towing experiment can't be expected to add much knowledge regarding the actual operating conditions for the propeller.

Most of the setback's positive function comes from getting the prop lower in the water, as I see it. In order to get an idea about the cooling water inlet position, a simple scoop test with a pitot tube and a transparent hose would show at which depth "good water" is available under self-powering conditions. The runs with the SeeSnake have already given reasonable info on the shape and depth of the bubble cloud; there is only one way to get a better view, and that would be an underwater footage from below. But except for satisfying my curiosity, I don't think it is necessary right now.

 

The big question for me isn't about setback, which I agree seems unlikely to alone solve a thing, but more about attitude. If the bow comes up, how does that affect the flow..

And then would interceptors or trim tabs suffice?

 

My seesnake test runs revealed that even under current conditions the lower horizon on the bubble field covers at least half the prop disk, and the cooling intake. Probably more than half of the prop disk, considering the hydraulic sink would be pulling the lower density fluid down, which I couldn't measure without taking some risks.

So I think with such limited prop effectiveness Serenity is doing very well, and won't need anything else to fly, other than some hard water. I can certainly build the needed pod, but I may be better off paying for the leg extension. We are very very close to the limit financially out here, with the massive increase in mortgage payments, and cost of gasoline, but the fix for that is me working for other folks, not taking time off to save a few dollars.

 

Sound reasoning, fully supported! Decisionmaking is about focusing on the essentials and wash off the possible-but-not-essential. Really wish you good luck; we all feel the increased pressure now (Ukraine is not far from Scandinavia.....).

 

Thanks Bodo. I thought COVID was going to be the singularity, but Vladolph Putler has changed the world at least as much.

 

The shape of the sled concentrates air entrained water. Above the stagnation line, the forward moving water mixes with air and the shape of the sled focuses the mixture into the center of the boat. On a mono hull the air would rise and move
chine ward? and offer the prop green water.

The air entrainment does not begin at the transom but partway/forward up the hull. So the air would/ MIGHT have several feet of distance/time to move upwards and PERHAPS reduce prop depth water to be more dense ( dense being the mixture combination)

If the engine was 3 feet back, not really practical, PERHAPS the prop might be in green water.

I agree with the pod concept.

But if the air is getting entrained from the centerline out to the chines, I would install 4 strakes, (the dimensions are for discussion purposes only) longitudinally, and say 3 inches deep, 1 foot off the centerline, and 2 feet off the centerline
Any air caught in the area from 1 foot off the keel up to the chine should then be caught by the strakes and directed to the transom outside of the prop width. Ie the upper chine side corner between the top of the strake and the hull will provide a
port for the air entrained water to exit the transom without it being channeled to the prop.
By itself this might reduce the amount of the air to increase thrust. I would consider this first.

Then in a perfect world, the only air entrained that will get to the prop would be the 2 foot wide strip of air/ water mixture created from ahead of the stagnation line.
Then I am guessing that a pod positioned from the transom, and maybe forward 6 or more feet might restrict air entrainment. The pod would not be flat but rather 2 feet wide, like a mono hull, ie v shaped designed to take this amount of
air entrained water and provide 2 channels to move the air to the outside edge of the pod. Hard to describe without a picture, but looking from the back, the sled is an inverted V, the pod would be a very small upright V that provides
two channels/ports/throughways to push the air to the side.

I agree with others that the short set back may not completely get rid of all the air but with the increase of bow trim angle, (Fallguy) the prop will be lower in the water and DC said that it appeared that only half the prop disc
was interacting with the entrained water

Baeckmo, So could this current air/water mixture (lack of green water, terrible terminology) be accelerated, more efficiently, with a high pitched surface piercing propeller? Or even just a higher pitched prop albeit with slow speed performance impacted?

 

My helper got on the roof without me noticing.

 

How fast were you going in the picture of the prop wash?

You've got to admit, all the froth is coming from the prop itself. The water is pure blue on the port side.

 

Going about 6 knots. No, the air is coming out from under the tunnel.

 

Driver side slider window complete.

 

99610 views. In a day or two, 100,000 views. And in a couple weeks, three years since our first post.

 

Now that is impressive, everywhichway you look at it.

And the thread will soon be up to 2,500 posts.

 

Sure looks like bubbles are more to starboard, but sometimes cameras can trick the feeble minded..