Sea Sled Performance: Qualitative Analysis

@baeckmo has challenged me to come up with some solid grounding for the remarkable claims.

I'll lay them out here:

1 They were claimed to be really fast. The Crouch constant in George Crouch's famous formula, rated them at 230, when most boats were 150 to 180. Long since passed over in favour of more accurate, sophisticated formulas. Crouch did his work empirically, and determined that a sea sled was the fastest form, by far, at the power to weight ratios achievable in his day.

2 Despite the very low deadrise forward, and barge like transom, it was claimed they didn't pound in a seaway. They were so good in rough water that the US military used them exclusively for crash rescue for decades.

3 It has been reported that they had little to no planing resistance hump. Crouch determined that a sea sled could get on the plane at 100#/hp, where the next best forms could only plane at half that. Naturally that meant amazing load capacity.

There were some downsides. The original boats moved like fish, but steered like cows. And before any waterproof glue existed, they were very expensive and heavy to build. Definitely a luxury item in the early days.

 

I'm limited to qualitative analysis, because I can't do a huge amount of direct testing. In another 6 months or so I can do tests on my own. Speed/load, wave response, etc. But that won't tell us about sea sleds in general. Just about mine.

So my analysis will be based on public domain resources, and first hand observations from qualified observers.

 

So number 1:
Fast

In 1925, 50 miles an hour was really really fast. Baby Bootlegger was winning race sfter race at 48.5 mph. Orlo III was setting records at 55 mph. So the Crouch constant of 230 compared to typical boats of 150 to 180 is a huge speed advantage... when your context is around 50mph/80km/hr. Nowadays that's nothing. But different hull forms have very different behaviors at different speeds. The flat bottom boats of Crouch's day were fast, for the 1920's with low drag. But faster than that, and the extra wetted area probably made them slower than a monohedron.

 

I referred to Savitsky's 1964 work in the last few days.

The part of interest is that a zero deadrise planing surface has much more lift, and much less drag than a high deadrise hull.
The difference is striking. Roughly 30% more lift at any given trim angle and 30% less drag associated with that lift, compared with a 20° deadrise surface. Note that this is wavemaking drag, not viscous drag. Back to that later. The resulting L/D ratio of around 1.7 times better is crushing. This however is only around the onset of planing. Faster speeds and viscous drag becomes a bigger factor, and wavemaking drag less so. But for moderate speed ( or scary fast in 1925) flat bottom takes it effortlessly.

Except. There's always an except, or it wouldn't be engineering. A flat bottom boat will break your teeth in any kind of a seaway. So planing boats for typical conditions have a V hull.

So it's perfectly reasonable for Crouch to say sea sleds were the fastest thing on water in his day, provided you don't mind pounding.

As @Mr Efficiency very perceptively pointed out the other day, a V hull keeps rising up onto less and less surface area at higher speeds. A flat bottom boat can't do this. I predict that a sea sled will not be as fast as other boats, up around autobahn speeds. And if Crouch had access to modern materials and engines, his formula would have been more sophisticated.

 

Number 2

Don't pound.
I am grateful to our esteemed member @baeckmo for this one.

He examined 1955 towing tank data, from a 1/6th scale model of a sled. The boat come off very poorly. Among other things, it pounded badly. It had huge transitional resistance. Both of these attributes are the opposite of what was reliably reported to me by several current owners. So how did the towing tank data fail to reflect reality so comprehensively?
Being the unstoppable engine of science that he is, he did his own testing. It's the foam in the tunnel! At 1/6 scale, the model did not make the aerated water tgat the full size sleds make. Peak impact force is proportional to wave velocity times speed of sound in water, times other stuff. But varies linearly with speed of sound. 1200m/s in green water. Speed of sound in water full of random bubbles (foam) can be as low as 35 m/s. So peak impact force is reduced to 1/34th. Pounding goes away.

Suddenly the flat bottom in part 1 looks very attractive again.

 

Foam under the hull through the tunnel.

It has been suggested that air lubrication does not play a large part in the claimed Sea Sled performance. I read some papers. Air lubrication or whatever you want to call it, is being avidly pursued these days. Researchers exploring ideal bubble size determined that with large, highly squishable bubbles, viscous drag can be reduced up to 40%. About half the planing surface of a sea sled is riding on highly aerated water. So viscous drag will be reduced enough to make a very measurable difference. More speed. Yay!

 

3
Carry huge loads

With 70% better L/D ratio at the onset of planing, and a butt shaped like a barge (sorry babe) enormous load capacity is to be expected. The L/D ratio advantage means a much lower trim angle, and therefore much less resistance before full planing speed is reached. I'm sure many good boats could carry tremendous loads at speed, but they can't get out of the hole. And a boat working right at the edge takes off, once it can struggle it's way up onto the plane.

 

"The plane", I love it.

Shouldn't you be working on the boat?
Just sayin...

 

I am ashamed....

Maybe I should edit this into 1 post. It's a little incoherent.

 

Methinks leave them how they are - if they were all together in one post, you would still have to separate them into different paragraphs.
They are an excellent collection of observations and thoughts, and I hope that you can conclusively prove all of them when you launch your fine sled.

 

Naah, hold on now! I have NOT described the DTMB job as crap, it is a test of a series of hulls for a specific mission, and the test is performed correctly and to a common agreed standard. It is in fact the only piece of engineering quality that I have found that presents real numbers for the Hickman original shape. What I have said, is that there are a few questions left unanswered.

The most important of them is the question of the impact of surface propeller thrust line on the hull trim and resistance and then the question about the influence of the aeration/foaming in the tunnel on the resistance.

The SP prop thrust comes with a considerable (and varying) vertical force; the standard test procedure is to apply the pull along the propeller shaft direction. In the DTMB test, the Hickman hull was tested with a slight initial nose-down attitude. That could have two reasons, either an attempt to mimic the aft lift from a SP prop, or to refer to the average buttock, in an attempt to use a "neutral" angle of attack in a hull with a warped bottom. Both ways are logical and correct for the purpose of the test.

The question about the foaming is complex, since there is no way to vary dimensional scale, Froude number, Reynolds number and Weber number (We describing the influence of surface tension in relation to mass forces) simultaneously in a model test. This means that the full scale hull may react in a different way to the bubble layers than the model.

Now back to the DTMB test and the conclusions made there. I am quite certain that both Eugene Clemens and Charles Tate had hands-on experience from the full size Hickmans of the time; they are both highly respected in marine science. Their remarks on the hull performance should be taken seriously:
"Full scale trials have shown that this design is slow in acceleration to planing speed (because of the very high resistance at intermediate speeds), that it is sluggish in answering the rudder, and that it pounds severely in rough water......" (Report no 1378, DTMB 1959.)

Note that the Hickman is not competitive with the standard series 62 mother hull until a Froude number of ~4.5 (roughly 34 knots for a 3 ton hull). At all speeds below that, the sled shape is inferior. Until real evidence (power, weight, speed, sea conditions) are available, there is no backup for too much of applause; the real engineering achievement from Mr Hickman is the general application of the surface piercing propeller, not the hull shape.

My comments on the damping effect of gas bubbles in water is valid for the case where the bottom is covered by a more or less continous layer of foam in the region where the highest slamming pressures occur. The bubble layer will reduce the instant pressure peak, but the rest of the deceleration depends on the "spring constant" of the bottom shape. If you imagine the hull position when the keel(s) just touch water and then look at the travel until its full footprint is in the water. The longer the travel, the softer the ride. Bubbly water does NOT make the Hickman "original" a soft rider in any sea of significance.

The point here is that you must not equal the Hickman shape (warped bottom with no deadrise aft) with a modern W-hull with reasonable deadrise all the way. It is just as big a difference as we (who were "afloat" at the time) saw when Ray Hunt, Sonny Levi, Jim Wynne and a few others (not to forget Dick Bertram) raced deep-vee hulls in circles around the flat-bottom gang in the late fifties, making a whole flat-foot fleet obsolete overninght. I do think that a modernized form of the inverted-V hull could have its place for specific purposes, but to get there you must cut loose from Hickman religion and go for engineering science instead of anecdotal "evidence".

So, you're welcome to cite me, but don't put words in my mouth that I didn't put there myself, please.

 

A thousand apologies. I'll rewrite the relevant section, to make @baeckmo 's considered observation clearly separate from my immoderate grumping.

 

......you're welcome, we all get carried away by enthusiasm now and then........

 

That post by baeckmo was as good a summary of sleds as we are likely to see, just about anywhere, it may well be that a departure from that warped-bottom, flat-tailed orthodoxy beloved by sled "tragics" could see significantly better results, but it seems also to make a single engine a bigger headache. I would think twin engines would be easier to accommodate, but there might be trouble there too. They would need to be well outboard, probably. But then you are getting closer to the planing cat, and why not go the full hog ? On any kind of open water boat where twin engines are mandated, the cat has to be at or near the top of the list.

 

It sticks out as super-obvious that when the sled was "born", that warped bottom, flat or near flat stern, was the order of the day, and largely probably had to be, as lightweight powerful engines were not readily available, for the mass market. It is surprising no-one who hails the virtues of sleds, seem willing to embrace the "heresy" of the deep vee, with the inverted vee format, as if it would disturb the ghost of Hickman