The Displacement Glider

Displacement Glider

When I first saw the picture in PB #86, I thought for sure it was a Phil Bolger design. I am surprised to be the first to bring it up. We have a lot interest in boats that operate in the area between displacement and planning. If you don’t get PB, here is a link. Displacement Glider They seem to have refined the box keel, but unique concept its not. As shore erosion becomes more of an issue, here is something that could help.


Gary

 

PB always takes a while to work its way south, so I have only recenty received #85 - but I look forward to reading about this boat.
It looks to be a variation on the Jersey Sea Skiff - but the most interesting thing is the shape of the chine....curving down from the bow in the normal way before going up and then back down again towards the transom.
There are faint lines drawings in the background on their web pages - it would certainly be interesting be able to study them in detail....

 

I seem to remember seeing Japanese boats that looked identical in concept. From 60 or 70 yearsa go.
Nothing is new....

 

Will,

The web site tells more than the small piece in PB. I also squinted at the lines in the background.

Gary

 

I also saw this hull type on Japanese fishing boats over fifty years ago but I think their basis for the design was very different. The Japanese boats were very stable from no load to full load and even small ones were found well out to sea. The Jersey Skiffs used the central hull with its flat bottom to allow launching from the beach in addition to keeping the engine weight low and the prop & shaft completely protected behind the keel.

The Glider has that strange upper hull shape that Will mentioned. This is the only thing I see that should be patentable. Although they show a smooth speed/power curve, this convoluted hull should have a particular speed at which it's best and it should be less efficient at all othe speeds above or below this optimum range. It's very interesting but I don't yet see how it works. Presumably the variable cross section between the keel and the hull creates wave interference much like the bow bulbs on merchant ships and they only work at a particular speed.

The pictures of the small wake are certainly impressive and indicate that they have made a real contribution to displacement hull design.

Tom Lathrop

 

Here we have another case of something old being called something new. A funny humpy bottom is going to reduce required power by 30%? I don't think so, but I've been wrong before.

On this coast it's called a pod-bottom, the Camano Troll is a popular production version. They also claim excellent economy at semi-displacement speeds. Mr. Bolger calls it a box-keel, in the 40's the Atkin's called it a Seabright. Chuck Paine calls it a "Steady Sailor"!! The Japanese have been calling it a Sampan for,.... probably thousands of years.

But nobody mentions what weights they are working with, and what assumptions are being made in their comparison? A nice graph of power vs. speed, but what is the other "gliding hull"? Whose deign is she, are the weights identical, is it just a computer calculated comparison against a mythical hull?

They claim three quarters of hull displacement is in the keel, okay, it would be easy to push that. But there is a problem, no room in the hull. I found this out looking at the configuration a while back for one of my own boats. No accommodation space below deck.

The humpy bottom amidships reminds me of assertions by a few folk that reducing the midbody to smooth out the curve of areas by compensating for keel volume would reduce total resistance. Phil Bolger has done work in this area, as have others.

All the best, Tad

 

Tad,

I agree that the claims are a bit suspicious, but then so are moat press release claims.

What interested me was the apparently small wakes generated by a 14M boat. As you said, the weight certainly would have a big effect on that. What is your take on the wake pattern? Notwithstanding the novelty, or lack of it, does this idea have any merit?

All of Bolger's box keels that I've seen on powerboats almost disappear into a nearly flat plaing surface near the stern, so they are quite different from the 3/4 displacement keel of the glider.

Tom Lathrop

 

The wake reduction is also what interests me. One question is how would over or under loading influence the performance?

Gary

 

Hi Tom,

Yes, it is certainly a viable idea, (IMO) but like all these nifty tricks, it's a question of degree.

The wake appears very flat, this is good, but what is the wake from? I did notice mention of a hull length, but nothing more? It would be interesting to compare this vessel's wake with that of a catamaran, or a wave piercer, or a swath hull. But only if those were of similar weight and load carrying ability.

Commercial use implies carrying some loads, this hull, by implication, would have a small waterplane. This would not bode well for load carrying. Once the upper part of the hull is down in the water, you are pushing just what everyone else is pushing.

Bolger has done a fair number of variations on this theme, from, as you say, flat bottom @ transom, to something much closer to this "DG" hull. He does mention in one of his comments on the type that he believes the flat transom handles better. This is another area for potential trouble, what will this tall narrow canoe hull handle like?

All the best, Tad

 

There's ample room for improvement in the semi-displacement monohull state-of-the-art (in my opinion). It surprises me that more progress hasn't been made before now. I understand recent U.S. Navy research at David Taylor / Carderock has resulted in signifigant improvements. At the other end of the high-tech research scale, I suspect Phil Bolger may have some things right, as may the designers of the boat above.

Here's my shot at it:

 

Stephen,

How about some section lines?

Tom Lathrop

 

Looking at the views you posted, it seems that you also ahve a combination of a central displacement hull and a flat (planing?) surface. The flat surface also has a positive angle of attack to promote lift and/or prevent stern squat? I have also used this feature in chines which get wider aft to maintain a level attitude through the transition from displacement to planing. Seems to work for me.

What is the projected performance?

Tom Lathrop

 

I confess I haven't worked the numbers much, but she's a 25' launch with an 8'-6" beam, and the goal is low wash and drag at a cruise speed of about 12 knots. I've had in mind a jet drive, but I'm not satisfied I can achieve decent efficiency at at my target speed. I'm now leaning toward a Van Cappellen Pegasus Nozzle Rudder with fitted propeller. If I can keep the displacement in the 5000 pound range I'm confident I can get to 12 knots cruise on under 100 HP. But how good is it? I have no way to know at the moment. The inspiration is the work on stern flaps at David Taylor / Carderock, David Giles "Fastship" container ship, Dave Gerr's "Gerr Vee" hull, Phil Polger's designs, and those commuter type and lobster boat hulls I feel work well. The intent is to combine a flat planing surface aft with a narrow displacemrnt portion forward, resulting in wake cancellation at the target speed and a pressure bubble in the region of the prop or jet intake.

 

Stephen,

How did you arrive at the angle of attack of the aft surfaces?

Did you perhaps assume a level running trim and make it the optimum 4 to 5 degrees?

I like 12 kts as a cruising speed but what do you think might be the upper practical limit of this configuration?

I would think that with a 90hp outboard on the stern, that the displacement could be kept well below 5000 lbs. In any case, your design looks promising to me. Interesting to consider the performance characteristics of a 36 foot cruising boat using this hull with the accomodations of a typical displacement cruiser of similar size. The average trawler owner would go nuts to get 10 to 12kts with 6kt fuel usage.

Tom Lathrop

 

I don't have the AutoCAD file at my current location, but if memory serves the angle is shown at 2.5 degrees to the static waterline. My intent is a running trim of 1.5 degrees for a 4 deg angle of incidence, so you pretty much have it right. My intent is to have the CG forward relative to a planing design, hence the engine shown under the center console. My real impetus for this design is to have the perfect boat available when fuel cells become widely available. Meanwhile I envision power being diesel, probably Yanmar or Cummins (whichever matches the RPM suited to the final drive choice). BTW 5000 pounds is intended to include some fuel & passenger weight. I'm hoping for a dry (lightship) displacement under 4000 pounds.

Unfortunately the forward CG probably makes running above 15 knots unadvisable.

Note that I've edited my previous post to include other influences.