Sea Sled madness. It’s in my brain.

John, I'm not content with the drawbacks with the pods in your case. I think it should be possible to attach a vane/scoop to the OB leg instead, see pics below. The protruding nose needs support down to the torpedo of course, but the rear end could be simply bolted with distances to the AV plate. Material alu ~6 mm or some of the sticky stuff you used for the hull. The scoop will eventually run across the cooling water inlet, but it does create an increased pressure on the upper (aerated) side that should force pure water into the opening.

I'll see if I can come across an E-rude rig for measurements (maybe a scan), my 3D model shows a Yamaha 150 hp; no big difference in shape, but for the good sake....

 

That looks a lot more readily built than the pod. It will exert a net down force, I think. But significantly less drag than the pod. 6mm Al definitely won't be overdoing it. Imagine the forces on that thing at 80km/hr

 

Yeah, the downforce is the inescapable cost for the vertical acceleration of green water; to be payed wherever the scoop is applied. Only ways to avoid it is to lower the prop disc into green water or increase the disc size to compensate for the low density and low speed of sound in the foam (that requires a fan rather than a propeller, but the faneller is not invented yet....).

Don't think you have to design for 43+ knots though.....

 

I do not profess answers; just questions.

Is the bubble trail concentrated in the ships center? Or is each demi creating froth that goes athwart? At one point you mentioned twin engines.....

If the froth is rising, the engine attachmentis a separator? So, cutting the bubbles to the top and sending a stream of turbulent water that is less full of air..but how do you know it is sufficient? Or is this a test?

And finally, would it be better to get a hydraulic jackplate, so that he has the ability to lower the leg or in combination with a 5" extension? Or is that step two and three?

More generally, when the speeds increase; do the bubbles increase as well? Or, conversely, does the boat get up on plane and the froth stay up higher? And the reason I ask is simple. If he runs a hydraulic jackplate to stsrt in good water; he'd at least in my brain be able to come up and out if my guessing is at all right.

But I can see where the boat may need hydraulic jackplate even with the water scoop.

All the best. It is very interesting stuff. John is super lucky to have such good help. I am also getting some good support from Ad Hoc with an issue on my boat.

 

No, I suppose not. That was the expected speed 3 years ago when the design had about half the wind resistance.

 

Fallguy, right now we are exploring possible alternatives, since there is no ready solution to exactly this situation. As far as Johns observations go, the foam source is the point up front, where the two spray lines (~stagnation lines) from each stem meet. The bubble cloud seems to spread slightly sideways, having a width at the transom of about one meter and a depth of ~0.2 to 0.3 m. This follows the general trend from my inverted-v hulls, though there the cloud was more concentrated to the center line due to the v-shape continuing all the way to the transom.

My guess is that the cloud is flattening slightly as speed is increasing, but I don't particularly like the idea of a movable engine jackplate. I'd prefer either a fixed bracket that lowered the engine about 150 mm, or a corresponding leg extension. With the engine further aft (as with a bracket), it would reach into more solid water when the hull is trimming nose-up. This arrangement would require a simple vertical fin with an endplate to "shape" the flow past the ob leg; that comes with very low additional drag if correctly designed.

 

Thanks for the clarity. Like I said, it is a curious matter.

I have one more follow up.

Is this a picture of the sneeze zone? Is the tunnel simply starting too soon? If the frothy business starts further back would it change? Like, more precisely, is this supposed to be back 3 feet? Just as a trim issue?

 

Finishing the front and back sliding doors, in the new shop.

 

The point where the tunnel disappears into the foam is about 8' back. Sea sleds always ingest and ride on their own wake. There are a lot of advantages, and some disadvantages. And yes, we've crossed short steep waves at a right angle, where the wave caught a large volume of air behind it. Then there was dramatic sneezing.

 

I see a market opportunity! Time to invent the fanellor.

 

Front slider. 100% found material, scraps and offcuts

 

Progress.....a closed front calls for increased speeds hmm!? So I've looked into various solutions for evading the aeration dilemma.

First some criteria (out of my head and admittedly not discussed with John so far):

• The prime limitation is cooling water inlet position. If air volume fraction (avf) is roughly linear from max at hull bottom to ~zero at -300 mm, then the water inlet should be lowered about 80 mm to submerge it into reasonably bubble-free water.
• The experience from my inverted-v shows that a propeller with a medium-to-heavy cup (or better interceptors) could operate with the AV plate about some 120 to 140 mm below the tunnel top.
• New hull penetrations should be avoided, particularly in the bottom.
• Movable devices should be avoided in the search for robustness.
• Finicky precision machined gadgets to be avoided for the same reason.

This leaves us with two alternatives:

• First, of course, the obvious quick fix: the 5" leg extension. Provided the shape of the new piece follows the shape of the upper part of the gear housing, an additional pod along the bottom may not be required.
• Second is mounting the engine on a set-back bracket; keeping the standard engine configuration. A set-back distance of 200 to 250 mm seems to allow the steering ram to clear the back of the present transom, while lowering the engine to a position with its lower edge of the attachment console flush with the hull bottom. If required, a flow shaping pod can be bolted to the bottom panel of the bracket. This solution brings the engine weight further aft, which is good for the hull trim and the prop submergence.

First alternative is tasty if Serenity is required on immediate "earning duty", but the investment will not have a resale value.

Since I grew up under the "no money-fix-it-yourself-from-what's-available" star, and I am a "metal man", I would go for the second alternative and simply weld a bracket from two alu U-profiles, 100x200x100x12, finished off with a flat plate flush with hull bottom. It would bolt through the existing transom holes (yes, you need to cut access holes in the aft deck, but that is required right now for torque checking anyway!).

A decent carpenter (guess you know one....) would have no problems in creating a wooden bracket with a 2" new transom piece, perhaps with some of the sticky stuff on top, likewise using the existing holes for long bolts to hold it to the hull transom.

 

baeckmo
You mention a set back. One might wonder if this would be a cure. If the prop is back far enough, the air may disperse to atmosphere prior to it reaching the prop
This is a paragraph from one set back builder
"Setback brackets re-position the motor into cleaner water to increase propeller efficiency and minimize prop slippage. Drag is reduced and the boat exhibits more bow lift, which results in a drier ride. The increased distance between the propeller and the hull also improves boat operation in reverse."
There was an article that I had read quite some time ago that the water after the transom is coming up to existing water level and the motor can actually be raised somewhat above the hull trailing edge. Not sure of the amount,
something like 1/8th of an inch for every inch of set back.
The only way that DC could check the air entrainment at a distance back from the transom is to have someone tow his boat up to planing speed with his engine up, and see if the bubbles have disperse. Mind you it would take strong cleats and a pretty hefty horsepower pull

 

It is an interesting experiment, imo.

 

In my opinion too. 28hp gets us ~7 knots. How much for 14 knots?



I don't think these do it justice. But we just about stuffed the bow. And in the cabin, it was nothing.