Buttocks

The OP is saying these boats ride well, I have trouble believing it, if cathedral hulls by and large rode poorly, I can see little reason why the sled would be any different, they both basically have to "eat" waves entering the "tunnel", some tri-hulls that did not end in a flat or near flat stern, rode OK. But to revert to his original concerns, about negative angle of attack for part of the running surfaces, it clearly wasn't a problem. You could build a similar boat and eliminate it, maybe use a more pronounced chamfer if worried about tripping.

 

I dont think the OP is saying that. He's just seeking what the AH hull is the way it is, as he too finds it odd.

The issue of 'tripping' was merely down to lack of research at that time in to the mechanisms of why 'tripping' occurred and also that the hull used by AH are high sided, i.e they would invariably have a high KG and thus a low GM....ergo one of the principle mechanisms for the insatiability he cites!

Thus without hard verifiable data, it is all subjective and everyone will have an opinion one way or another - yet it does not solve the OPs dilemma, to him I assume, of should he use the hull. Since that is a subjective reply by all.

 

Well, he said this...."Actually they were, and are, famously easy riding".......If he meant they planed easily, well yes, I imagine that would be right, but easy riding implies seakindliness to my way of thinking. I doubt they delivered that, though I recall the late forum stalwart PAR thought they rode well enough. I just suspect at least part of the reason they have not taken off, is hard riding under at least some circumstances.

 

No, they really have the reputation, and have maintained the reputation for over a hundred years, of riding easier than any other planing form, under all conditions. I spoke to a fellow a few weeks back who made over a dozen with his own 2 hands, and he kept coming back to that point. They also went faster under less power than anything else, and carried enormously greater loads than anything else, at a given power to weight ratio. At least that is what is reported, without exception, from the folks actually using them, again, for over a hundred years.

As to your very reasonable question, why aren't they taking off? I think human nature. Folks assume that since everyone else is using something, it must be right. Although historically, high speed tunnel hull designers considered their work to be an offshoot of Hickman's sea sleds. And consider your own dismissive attitude. You've never seen one, or ridden in one, but you're casually willing to dismiss a century of description and reporting, based on a casual glance, where it doesn't look like how you've come to expect a boat to look. I don't mean that disrespectfully, or mean to suggest that your assessment is superficial, but rather that you are a reasonably intelligent, better than average informed observer, and you wouldn't build or buy one. Why would anyone else?

Ad Hoc, you mentioned you thought it unlikely that trapped air in the tunnel could meaningfully contribute to lift. Consider the Wing In Ground Effect craft, and the extensive work done there. A wing loading of 24#/ft^2 is not at all high in aircraft, but is the loading of a heavily laden planform. And that aircraft can fly at speeds only slightly higher than the speeds of a sled. Doing so without the WIGeffect boost to lift. It doesn't take much of a contribution to get a significant edge in performance. We all know how much money is spent on America's Cup yachts to go 3% faster. That much edge is crushing.

Anyway gentlemen, I'm extremely grateful that you've taken the time you have to discuss my conundrum. I'm hoping more folks will wade in, because I'm going to apply saw to wood very soon.

 

Oh, it occurs to me that the hydrodynamics and aerodynamics forum is under visited. Should I repost in Power Boats or Boat Design? What do you think?

 

That depends upon what you are seeking?

If you want a general chat/gossip and debate on many things technical or otherwise every man and his dog has an opinion, then perhaps yes. But don't expect the holy grail answer from anyone.
Otherwise, what do you think you can gain that has not already been researched and known about, just presented differently?

 

You are dancing all around the proposition that these things slam, saying they plane easily, carry big loads etc etc, none of which I am contesting, but until I see accelerometer readings for them, I'm not convinced they don't dish out a hammering ride.

 

Yup. There is plenty of subjective hearsay in this marine community. The classic fisherman's tail.
Unless there is independent quantitative evidence of such, it will only ever be what it is...just nice sounding words that make the eyes go all misty and rosy.
Engineering is not based on emotions or feelings or sound bites, but facts.

Yes there has.
But it is completely incorrect to equate the wing in ground effect to that of a small mono/cat with partially trapped air between hulls as providing similar effects. They are so far removed to being similar in nature that it will just get you into more of a technical pickle if you decide to continue down this path. The fact that there is no circulation and a shedding vortex in an air trapped setup is the key give away compared to a WIG. Thus it will only muddle your thinking further.

There is far too much subjectivity on AH's hull form that the only thing one can say with confidence is that it has been built. That is about all. Every thing else related to its claim is purely circumstantial.

 

Such air trapped without enough gap at the rear end can be a concern in itself (beyond the slamming occurrence), analogy with what is called « Cobble stone effect » faced by Surface effect ship at high speed on waves (even small ones) : the air volume varies at high frequency due to the passing waves, and so do the air pressure, leading to an uncomfortable ride. Can be mitigate by either a good design of the flexible multilobes stern seal (geometry and very slight overpressure, the venturi effect helping the seal deployment close to the water surface) allowing the flexible canvas to fly over the water surface while maintaining a good air gap and avoiding peaks of air pressure, and if not sufficient, by using a RCS (Ride Control System), consisting of opening /closing air vents at high frequency (but edged and costly technology).

 

I heartily agree. I've never been in one either. I'm about to build one, and I'm wondering about issues as concrete as Ad Hoc could hope for, since it's impossible to cut wood using metaphor and anecdote as measurements. When it's finished, would you be interested in taking the accelerometer readings? I'm certainly not set up to do that.

Ad Hoc, I certainly can't disagree with your comments about circulation.

Dolfiman, you are correct as well. I've wondered about exactly that phenomenon. No one reports it, but they might feel it under their feet, and not realize that's what it was. Oddly, I find most folks incurious, and not given to reflection.

Gentlemen, getting back to my original question. What do you feel would be the likely effect of making those bottom chines parallel, rather than curving in towards each other? Other than the geometrically obvious effect of making the aft surface of the non-tripping chine narrower in proportion to the overall beam, and eliminating the unfortunate effect shown in the first drawings I posted, of the surface of said non-tripping chine rising from station 5 all the way to the transom?

J

 

DogC,

I've experienced my share of "chine-hopping" ( or I guess "chine-walking" as it appears to be more widely known ) on RIB's.
Trim down engines 2 degrees, and it's gone. That's pretty much it for my knowledge there, sorry.

I admire your tenacity.
Sadly, you won't know the answer to your design question until you've sea-trialed it.

BB

 

Thanks BB. Ad Hoc and Mr Efficiency are correct, in that there's a lack of concrete data on this hull form, which is baffling, considering sea sleds were the first high speed rough water boats on Earth. Period. New paragraph.
They were built in the thousands, maybe tens of thousands. Then they just slip out of the record. I actually can find more data on the Victorian plank-on-edge yachts, and they were built in just one country, for a tiny period of time, and in relatively minescule numbers. Strange.
Well, there's a lot of impressive knowledge in this conversation. Maybe when I put mine in the water, we can add to the body of knowledge.

J

 

This tripping thing I am assuming is only a problem when pushing into a hard turn, I don't think you need your chamfers converging, maybe just use a bigger chamfer ! That would be a complication for your build, too. Another issue is the prop working in the aerated water these boats send under the transom, you might get away with stainless five-bladed props, on an outboard, I know that has worked pretty well with cats with centrally mounted single engines, but that is not quite the same thing. The only sled-like vessel I have seen in the flesh, was built of alloy, looked to be in very good condition, but was going very cheap in a boat-yard. I took from that, that it was an experiment that did not please, for whatever reasons. Come to think of it, it may have had no chamfer at all, at the bottom of slab sides. Possibly it had put the frighteners into the driver through that omission, but we shall never know.

 

Now this is getting to the heart of the matter - Design.

You need an SOR....what is your design/requirement intended to do?

Since depending upon the SOR, what you plan to do may or may not have an influence. Without knowing the basics of your design, it is impossible to say one way or another.

 

Ah. I'm learning the appropriate vocabulary. This will a 25' by 10' work/commuter. On the Sunshine coast around where I live, there are countless small islands where folks with far too much money need houses built. Tragically there's no accommodation there for workers, and minimal ferry service. I'm going to drive it like a tradesman's truck, not a donut burning yob. But the distances can be long, so it needs to move. It needs to carry heavy loads while doing so. And it needs to be something I can rough it in for a few days at a time. In my youth I was a combat engineer, so my idea of roughing it can be fairly rough.
Mr E, that was the direction I was thinking. Lose some aggressive turning ability, gain some load capacity. Fair trade.

Ad Hoc, I must confess I didn't read the first link you provided in sufficient detail. Around paragraph 35 or so (he did ramble on! Must have loved the sound of his own voice) in Hickman's patent application he discussed that issue of lost lift from the tunnel to the chine surface. He proposed a series of steps so that each area of chine was always descending, making lift, and adding a fence to reduce lateral flow out of the tunnel to the non-tripping chine surface. And that's a lot of extra building complication. He was selling boats on the basis of being the fastest thing on water on the east coast. I just want to get from the lumber yard to the job site fast enough to put in a profitable days work for my crew and I.

But I was serious when I suggested accelerometer work to Mr Efficient. There's missing lore here. We should know for sure exactly what the numerical values are. I'm going to give up about a thousand hours of highly profitable time. I hope to have something worthwhile at the end.