Help With Economical Semi-Planing Designs

I think Leo has the idea.

Some of the boats resistance comes from the bow forcing the water aside.

But the speed handicap is when the stern sinks , as the boat climbs the bow wave.

With the aft section being held up by the prop wash , the boat doesn't suffer the handycap of climbing a wave , simply pushing it aside, as it runs "on top" all the time.
Displacement being carried by the submerged box keel.

The Atkins boats were reputed to be similar to the Sea Bright Skiffs in being able to haul immense weights of fishing gear, at speed thru breaking inlets.

So light weight might be required for the best NMPG , but not to operate the boat.

I would speculate the stock Atkin power requirements were to get economy of operation , with out the higher stresses of high speeds , which would be harder and more costly for a home builder.

FF

 

It doesn't seem that it would be that hard to test the basic theory. A pipe and valve extending above the waterline fitted to a through hull just in front of the prop would either jet water when you opened the valve, or suck air. I'm betting it will jet water. I'm also betting that opening the valve at high speed will spoil some of the lift and help the boat remain stable at the limit.

 

I will take the opposite bet because I think it will suck air, here's why ...

The propeller suctions water that's in front of it, then pressurizes water that's behind it. This suggests (to me anyways) that a pipe installed in front of the propeller will suck air since that portion of the water is under suction, not pressure.

 

A different way to look at it is with jet aircraft engines.

Long ago a very small diameter engine took some air and pushed it aft at tremendous speeds . noisy and not very efficient.

Same as our std boats , where a prop pushes a stream of water , the bigger geared prop does a better job , but its still the same. Just an un-ducted water pump, tip losses, shaft angle losses ...

A modern turbofan uses a 10ft diameter prop , in a nice ducted fan cage , with the engine exhaust a very minor portion of the propulsion effort.

Both are just air pumps , but the later units move more air , at a lesser speed , with far more efficiency.

I think the reverse deadrise is fed accelerated water and the propeller accelerates the entire water volume thats contained in the aft section by the hull sides.

Result is a larger water mass moving aft more efficiently that also expends some energy in preventing the transom from going down,reducing drag.The large water mass may be the stern wave of the box keel.

So we get the pluses added from more efficient propulsion , a better performing hull aft , and the lessened wave making of the submerged box keel.

Since the boat runs flat , we get good sea keeping and a sea kindly ride.

Add everything up and it would seem Atkin was on to something !

FF

 

I think this is true, but I still wonder if leaving the aft sides of the tunnel section higher (Rescue Minor) is better or worse than putting the sides down in the water (Shoals Runner).

Robb White is the one who claims such great fuel economy and he built "something like" Rescue Minor ... but was it really like the RM sketches and plans? We know he created soft chines instead of using the hard chines spec'd by Bill Atkin, but were the bottom of these chines higher or lower than those in the RM drawings?

 

The sides of the tunnel will need to be IN the water (to contain the thrust) when the boat is operating.

The static at rest condition might be more comfortable with slight immersion to stop roll.

FF

 

Hi Fred, thanks for your thoughts. I wish I were as convinced as you are. Rescue Minor's design appears to shows the sides of the tunnel out of the water or possibly just barely touching the waterline, so I cannot help but wonder if there is more to it than this.

I did a silly little boat design in another thread that you might want to take a look at, it shows the tunnel sides partly open in the forward portion of each "half tunnel", then the sides drop down further aft when the two half-tunnels become one:

http://www.boatdesign.net/forums/showthread.php?t=17562

Granted this is a short fat little boat with a very wide box keel, so maybe this is the "right thing" for this type of boat but not for other boats ...

I think it's the right way to design this particular boat because those chines are still "in the water" even though I forgot to show the waterline in the design. Basically I left some space along the widest portion of the aft box keel (where the two half-tunnels begin) for water to flow back under the hull after it is pushed aside by the front portion of the hull, then I dropped the chines down even further toward the transom.

Maybe on a longer boat this wouldn't make sense -- especially one with a long slender box keel -- because in a long slender boat the water will be able to flow under the chines and into the tunnel area with much less difficulty than in the short fat wide box keel boat.

That's my theory anyways, at least until someone else comes up with some new info to change my mind ...

 

Hi Leo, thanks for the great explanation, I think you're right on the money. Your analysis brings up more questions (for me anyways):

1- What happens if we move the prop further forward -- or aft? Shall we assume that Atkin got the prop location perfect?

2- Is there a formula for positioning the prop (such as a certain percentage distance between the box keel and the transom so we can be sure to "get it right" when we design our own modern tunnel-stern Seabright skiffs?

3- Will two smaller props, one in each hull near the aft end of the box keel, perform better than a single prop aft of the box keel?

Note that this last question might make the difference between a taller tunnel and a shorter one ... and I suspect the shorter tunnel might be more efficient since less water will need to be pumped up into the tunnel, and therefore more pressure can be used for forward thrust.

 

Rescue Minor's design appears to shows the sides of the tunnel out of the water or possibly just barely touching the waterline, so I cannot help but wonder if there is more to it than this.


MY guess would be that at RUNNING TRIM , every boat of this style will only work with the sides of the tunnel capturing the wash.

Weather the sides touch un-powered is not the question, weather they allow the design to perform IS.

FF

 

MY guess would be that at RUNNING TRIM , every boat of this style will only work with the sides of the tunnel capturing the wash.

I have read that the Dave Gerr version of the Sea Bright skiff was unsuccessful at obtaining good operating speeds as it ventilated the prop at speed.

So a model test might be a good idea.

FF

 

cOPIED,
August 1, 2002
Yes, Atkin Was a Genius
By Robb White

We launched the Rescue Minor June 20th and she ran most marvelously. There was no ceremony to it. My wife and infant redheaded granddaughter and I just wheeled her down to Lake Lamonia about twelve miles down the road and untied the jackleg lines that held her on the trailer and she rolled off into the water.

The boat sits about an inch and a half down by the stem at rest but it is actually that she is up by the bow.... the toe of the stem is right at the water. It is because the boat is so light. I knew it would be like that and hoped it wouldn't be worse. When my wife and the baby got in up in the bow, she sat right down where she belonged. I hadn't brought the engine house, so, when we fired her up, she cackled pretty loud (about like a Kubota tractor) but didn't vibrate or shake the boat at all. While she was warming up, I checked all around to see how much exhaust water the little Shurflo diaphragm pump I adapted to run off the camshaft was giving (plenty) and what the oil pressure was and all. I think I was a little scared to put the power to the wheel and see what was what.

As soon as the propeller began to revolve, the stem picked up what felt like three inches and the boat began to move much faster than I would have expected from any planing boat at dead idle. Lake Lamonia is one of those lily pad lakes we have down here and there is only a narrow trail through the bonnets out to the clear water in the middle and I had fooled around looking at the engine and let us blow off so that we were heading for the lily pads but just a hint of rudder brought the idling boat right around. I have never seen an inboard boat turn like that.

There ain't no idle zone, so I ooched up on the throttle a little bit and the boat picked up speed just like a regular boat. I gave her a little more and she gave me a little more. I ran her on up and she ran on up. There was no perceptible rise to the bow at all and the wake never changed. The little engine smoothed out so that it was hard to detect any vibration at all when I put my hand on the cylinder head to see if she was warming up or not. The boat steered so stably that I could hold the tiller and walk all around the engine to check on my doings. Which, the copper tubing wrapped exhaust manifold ran cold and the inlet pipe from the keel-cooler stayed cold. There were no oil leaks and no hint of a diesel fuel stench or exhaust but I did have a damned tiny coolant leak from the plastic overflow reservoir. The outlaw graphite ceramic well pump shaft seal never gave a drop and the belt-drive transmission ran smooth as all get out.

Which, I hope I ain't ruined my credibility too bad. She ran 18.6 knots on the gps and that with the 10" pitch propeller that I put on there to make sure I didn't lug the engine while it was breaking in. That wheel let the engine run up to where the governor backed her off at 3,600 rpm. Me and Atkin think she 12-1/2" of pitch and I have that prop standing by ready to put on there. You know, I have the jackleg push-button prop nut and can change wheels by just reaching up under there with one hand. As an aside, that's a wonderful fig. I can take the propeller off a sailboat while she is luffed up in the mouth of the river. I wish I could lay claim to the invention, but it ain't nothing but something like a quick-disconnect like on a garden hose.

The boat ran most marvelously. I would have been satisfied with 12-1/2 knots (my speed) and a slightly tender feel (about like a deep "V") but the boat was so stable that my wife and I could hardly alter the running trim by both us, and the baby, sitting on the same side. She turns about level and, even then, weight distribution doesn't seem to affect the trim. I think the dynamics of the hull that control the wake hold the boat in a tight grip. It feels like it weighs about 10,000 pounds. There was never much wake at all but, like Alex's, there was a sporty looking rooster tail erupting about 8' astern. I have a little clamshell water pickup right behind the prop to give a little supplementary exhaust water at speed, and it is mighty effective (I could probably eliminate the engine-driven pump). The beautiful exhaust-water rainbow around the rooster tail made a most charming sight.

All my fears are put to rest. She ain't tippy at rest and she don't rise up by the bow and try to skitter off on that little pirogue she carries on her belly. I couldn't make her cavitate to save my life and the boat will turn, at speed, shorter than any outboard boat I ever had. I tried to make a wake to run back across so I could see what was what with that, but the boat doesn't make enough wake for a valid test. I ran across the wake of an aluminum butt head-skiff with a nine point nine that was much bigger than the wake of the Rescue Minor. I don't need no wake in no lake to tell me how she'll do in rough water. I know a sea-boat when I see one.

It is a wonderful boat and, as Alex said, "Atkin was a genius".

 

Why select a semi-planer?

Why select a semi-planer?

Most of us are familiar with the terms "displacement vessels" and "planing craft". We know that the maximum speed of a genuine displacement hull is limited by the length of the waves it generates. And we also know that the function of the nearly prismatic shape of the aft bottom of a true planing hull is to eliminate the speed barrier by creating hydrodynamic lift as water is forced underneath the bottom, to minimze negative pressure, and to ventilate the transom.

When accelerating at intermediate speeds, however, most planing hulls experience a rather pronounced "resistance hump" combined with steep trim angles and unpleasant wake wash. The consequence is that the driver normally will try to avoid the "hump" by passing it as quickly as possible either by speeding up or slowing down.

However, in any chop, while runninng above hump speed the planing hull may experience heavy wave slamming and accelerations, and the driver may be forced to throttle back to avoid making the trip too uncomfortable for the crew. Consequently, under certain conditions it may be necessary for planing craft to proceed at close to hump speed, which means generating a lot of waves and excess fuel consumption. Nevertheless, it is precisely the difficult transition speed range that is attractive to many people, in particular when cruising long distances. Under such conditions, boats designed specifically to operate in the so-called "semi-planning" speed range would be ideal.

Since the appearance of te original Saga 7000 design more than 30 years ago, the basic concept of the Saga wide-bodied "semi-planning" craft has been continuously refined. The present family of Saga boats demonstrated the same qualitites of outstanding seaworthiness, manoeuvrability, directional stability and seakindliness as the Saga 7000.

The Saga boats are intentionally designed to operate withing the tricky transition speed range. They are by origin true "semi-planing craft" characterized by a full length keel, ample lateral area and proper balance between the forebody and afterbody. Today the word "semi-planing" is occasionally misused when characterizing boats which are more like planing craft in that they lack the power needed to get past the resistance hump. The genuine semi-planing hull, however, performes as well at dead slow as at full throttle. The Sagas are providing comfortable and safe cruising right up to the maximum speed recommended by the builder.

Eivind Amble
Naval Architect, C. Eng, Mrina, Msname

*********************************************************

Found on the Saga Boats web site. Thanks for the link goes to Raggi Thor in the Scandinavian boat design thread.

Best,

Leo

 

I could not find a single picture of the bottom of this boat on their web site which is overloaded with hundreds of other pictures of the boat. Why not any pictures of the bottom so we can see what al the hype is about?

 

I have neither see what is the installed power. All I have seen is that some are available as "semi planning" with deep keel and straight shaft, and as planning with stern drive (Saga 26).

 

Doesn't appear to be anything special. Try this on Yachtworld.

OK. Try this one on Yachtworld. Says a 230 HP Yanmar.

Then see all 16 on this search of Yachtworld.

Best,

Leo