Hickman Sea Sled Information

Cool, thanks.
The Moderator may be willing to move some of your relevant posts from this thread.
I look forward to your new thread soon, I have more to say...

 

It has rarely been that easy Steve in my experience, having been capsized a couple of times and lost gear.

 

The original sea sleds built by Hickman had the tunnel fading away to zero at the transom. That is what I built with Serenity. It's what I knew.
They also had twin drives, generally, because all the air , and aerated water from port and starboard bow waves collected on the centerline and were run over, reappearing as highly aerated water at the transom. This aerated water is not a suitable environment for an outboard engine.
We had cooling problems, traction problems, and frequently both. Two years and thousands of dollars later, it's not really cured. To get the ob low enough to get suitable cooling water we added a leg extension. That was an ordeal.
Cooling solved, we tried higher power runs, but were plagued by a lack of traction. Not cavitation, because the disk loading wasn't high enough, and not really ventilation, because the prop was extremely deep. Just mushy squishy water that was too aerated. So we tried a larger prop. That helped some, but got us up to speeds where dragging too much leg through the water blasted fountains into the air, threatening to drown the engine. And still the prop frequently lost grip.
So we bought a large SS prop, and had it cupped. That almost entirely solved the traction problem, but the increased speed made the fountain problem serious.
So we added a fin on the centerline to part the stream, and protect the upper parts of the leg from high stagnation pressure. That also worked.
But. The prop is now too big. She's overpropped. Can't turn above 5000 rpm. Flat transom sleds seem to need twins.

Modernized sleds, like those built to @baeckmo 's design have the tunnel carried through to the transom. They seem to do fine with a single.
I have no opinion about relative strengths and weaknesses otherwise, but that's a big one.
We will soon re-engine Serenity accordingly.

 

Another prop would be way less money and work than an engine(s).

If you buy new, many prop makers will swap them out until you find the one that works correctly. If she were mine, I would take the extension off, get a manual jack plate, rig up a davit or engine lift and start trying out props and height settings. Double cupped would be better than cupped. The catamarans have the same issue with a single and all of them solve it with double cup, height adjustments and a splitter (fin).

 

I could do that, but I'm a couple years in. More experimentation will happen when I retire. Now I want reliable operation. I've been using her for 5-10 km jobs. Now it's going to be 190 Nmile trips through notoriously dangerous waters. Seems to call for twins.

 

The 250 Etec has an exceptionally flat power curve. It reaches its max power already at 4600 rpms and from there to top it stays completely flat. This means that Serenity's engine is now producing its nominal power (provided it is getting the correct amount of fuel at the correct moment aso). Ergo: the present propeller is reasonably correct for the light (?) vessel. To get a better margin for higher loads, it should be "downpitched" an inch. I did recommend a rather heavy cup (combined with a reduced pitch) early on in this process, based on my experience with both catamarans and inverted V's.

I'd say the aeration under the center of the "classic" sea sled is a thinner layer, spread over a wider surface, compared with my inverted V hulls. The gripping problem is not unique for Serenity, but I think most people, including DC, don't realize the difference a cambered blade profile makes in aerated water (or ventilated/cavitating for that matter). It is a fundamental shift, based om first principles of physics.

At present, Serenity's top speed is ~25 knots (??) at full throttle, but we have actually no idea about her weight today. Consequently, we can not say much about the real performance numbers or even come up with a decent comparison. With the challenges during the coming job season I understand the wish for twins, but the hull is what it is and new engines still have to be protected against barnacle larvae and other trolls invading the cooling system, particularly during zero-activity periods. I hope DC will take "Saint Albert H" less seriously and listen to todays experience a little more; Good Luck John!!

 

Here the power characteristics of the Etec

 

Indeed I will Bodo. I always use the best information available to me, whether I like it or not. And thank you again for your good wishes.

Current prop is a 16"×17" prop, which is the largest that can fit. It has heavy cup per your recommendation.

Top speed in the current configuration is indeed 25 knots. We had previously reached 30 knots under propped. And 20 knots thus loaded at 11usg per hour. Currently 25 at 26usg/hr. The big change is the small fin. Perhaps it makes terrible drag?