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?

 

With a clean bottom, and some details addressed, current top speed is 33 knots. She easily and reliably reaches that, but won't go faster.
I've quite a few runs now, watching fuel flow (our fancy BRP tach shows exact fuel flow, through the computer controlled injectors) and speed from GPS. This curve shows mileage in km travelled per liter burned at most speeds through the range. For these tests weight is ~5200#, 2400kg, with max power ~275hp, 205kW.
I haven't been able to fill in between 19 and 31km/hr because she won't stay at those speeds. Cable controlled speed: stable at 19km and 22 liters an hour, and if I touch the throttle fuel jumps to 30 liters and 31km/hr. I filled in the 54km/hr this morning. Connect the dots from 50-60 and there it is.

So this hull has a much better resistance curve than the tow tank model that was tested in (?)1952. That one was pretty dismal at low-medium speed. I attribute that to Hickman's crappy non-tripping chines. Take a look at those lines earlier in this thread. The keels curve inward to about 50% of beam on either side. With a roughly 15° divergent flow across the bottom more than half the streamlines have an ugly hook in them. Anyone recall what effect a big hook on the bottom has on a planing boat? Not even an unfortunate curve, but an actual hard knuckle? Doesn't look good in the towing tank.

Designs improve over time, and I just copied a modern non-tripping chine.

 

Load carrying ability
I have carried 3600#, 1630kg of cargo and planed at 15 knots, but she felt sluggish. That was with a dirty bottom though. No doubt better now with the new antifouling paint.

 

One issue that I really don't know about are the differences between the old style bottoms like mine with a flat transom, and more recent examples like @baeckmo 's where the tunnel carries through to the transom.
Based on my studies in aircraft design I would expect Bodo's to have a better l/d ratio, simply because the bottom has anhedral, which reduces induced drag.

I can't speculate about ride quality etc. With (as I understand) a requirement for, or at least a use for lifting strakes they would have slightly greater complexity in design and build. Noticeably larger wetted surface drag at low speed and less at higher speeds.

If I had the money to build a matched set of test hulls we'd all know.

J

 

How about building a matched set of model hulls instead, and testing these?
You don't need a fancy towing tank with a dynamometer - when needs must, improvise.
In the 80's I was working in a little design office in England - I was the junior draughtsman / tea maker. And we had been commissioned to design a 12 metre fast (25 knots contract speed) aluminium cat (with asymmetric hulls) water taxi for a client in London.
My boss was a bit worried about if we would manage to hit the contract speed, but there was no money in the budget available for getting somebody like the Wolfson Unit to do some tank testing - so we did our own tank testing.
My boss had a small motor sailer at the time, which he kept on the river. One day when it was calm, we went on a voyage to test a very nice scale model that my boss had built.
Our equipment consisted of a windsurfer pole with a pulley lashed to the end, a length of small rope, and a simple spring balance with a hook - the type that fishermen use for hoisting fish up to see how much they weigh.
No GPS then, but I think we had a log and a Decca unit, and the two of them were in good agreement. We made sure that we carried out our towing trials at slack water, in a flat calm area - with the windsurfer mast as an outrigger we could tow the model in calm unobstructed water off to the side of the motor sailer. We calculated the Froude numbers for a range of speeds, and did a couple of runs at each speed (up and down stream).
The results were encouraging - they suggested that we could easily reach the 25 knots requirement with a pair of 250 hp diesels and water jets.
And when we did the sea trials on the full size version some months later, the results were very close to our model results. And she reached 25 knots with ease, and everybody was happy.

 

More observations:
At a Vmax of 33 knots she is distinctly "floaty" . She does not feel as directionally stable as at 22 knots. No doubt it is because she is mostly riding on the aft bit of the bottom, which is perfectly flat. At 22 knots its like she is riding on rails.
We went out for a cruise in wind ~8+ m/s and 0.8-1m waves at 4s period. Extra drag from wave crests reduced speed from 22 knots to 20 knots at the same power (38 liters/hour). The ride was fine until we hit confused seas where I was obliged to reduce speed because Anne had a prior back injury which made the landings uncomfortable.

If I build another, I will carry the tunnel through to the transom, at least a little, to provide some side force in the hull to make a couple with the steering forces from the OB.

At all planing speeds she self trims to +1°.

 

But it is not "perfectly flat" because you have chines, unlike Hickman's Sea Sleds.
Carrying the inverted "V" through to the transom also departs from the Hickman design.
In fact, are you starting to veer into Boston Whaler territory...?
Interesting.

This is a pretty accurate claim ( one degree accuracy in fact ).
Do you have a pitch inclinometer on board?

 

Why? To make it more directionally stable?
Why? Do you not like the "floaty" feeling?