Critical speeds for Semi-Planing

If you review the history and writings of William Atkin, you will see that in fact he did exactly what you're saying he should have done:

1- He designed dozens of these tunnel-stern skiffs over the course of his career.

2- He bragged about how much experience he had with these particular hull types which he invented, all of which were exceptional according to him.

3- He boasted that "This form of underbody is my contribution to the development of fine shallow draft motor boats and no other designer has attempted hulls anything like it."

If this is not "running with it" then I don't know what is ...

At the same time there is never any guarantee that when a person runs with a new concept that others will jump on it and accept it as readily as the designer. Peter Payne designed an exceptionally smooth riding high-speed hull decades ago too, but only a handful were ever built in the USA, and the boats never became popular even though they solved the problem of extreme vertical acceleration inherent in all other high speed monohulls ... and until today no one else has come up with a simpler or more effective solution to this problem.


Back to William Atkin, here's what he says on the River Belle web page:

I term the hulls of this model tunnel boats, but really there are twin
triangular sectioned tunnels leading each side the box deadwood;
and it is difficult to tell where the bottom of the boat leaves off and
the tunneled portion begins because the whole bottom is the tunnel,
the shape of the bow sections being as much a part of this as the
after sections are. This form of underbody is my contribution to the
development of fine shallow draft motor boats and no other designer
has attempted hulls anything like it.

The first boat of this model I designed for The Gordon Boat Building
Company, Brooklyn, N. Y.; the year 1922, 21 years ago. Since then I
have designed 44 of this type and all have been successful beyond
expectations. The hull is easily propelled and for equal displacement
and power is faster by several miles than the usual underwater form.

And these boats if built exactly like the plans handle perfectly, ahead,
astern, in rough water or smooth. There is no cavitation running across
the wind in a strong chop, either; which is surprising. Like all very
shallow craft the boats do not lay at anchor without a lot of walking
around; but this is to be expected because of the very shallow lateral
plane.

A great advantage these boats have is the low position of the
floor boards and the very slight angle of the propeller shaft. The former
provides better headroom without carrying the deck house too high and
at the same time provides a wide floor line. The reduction of the shaft
angle adds to the efficiency of the propeller and the motor.

Another thing worth careful consideration is the fact that boats of this
particular model will sit upright if grounded or pulled ashore and do not
require shoring to keep them from rolling on their bilges while being moved
ashore or left ashore over the winter months.

http://www.boat-links.com/Atkinco/Cruisers/RiverBelle.html

He repeated it in dozens of his designs ... but because he was such a prolific designer he also designed hundreds of other boats. Not every boat should have this type of bottom on it, so I'm not surprised to see less than 100 of these designs but many hundreds of his other designs.

Not according to Atkin himself. Instead he clearly says these hulls perform several miles an hour faster than the 'usual underwater form' -- whatever that means.

 

Thanks for posting the link to Robb White's page Rick, I usually post links along with quotes but I forgot this time.

Right. By raising the prop halfway 'out of the water' but keeping it in a tunnel pocket, Atkin eliminated the possibility of aeration / ventilation except in the very beginning when the prop is put into gear and starts rotating. As soon as the prop starts pumping water it purges the air from the tunnel pocket, replacing it with water. Now that the tunnel pocket is full of water the prop cannot ventilate -- until something lets air in there again ...

I'm not so sure about this. The most efficient catamarans are going to have very narrow aft ends, and this substantially reduces the ability of the aft hull sides to exclude air from the tunnel pocket. There is simply not enough width back there for the sides to drop down to the level of the bottom of the prop in such narrow hulls.

I drew up a bunch of catamaran hulls like these, but every time I ended up getting rid of the tunnel pocket (most of it anyways) and settling on something like this instead. This is for a Philippine banca but an even narrower hull would work for a catamaran:





There's still an inverted vee bottom with a slight hook at the transom, but these hulls do not have enough lateral width for the sides to drop down to the level of the middle of the propeller or below. As you can see here, the aft hull sides end way up high near the water line, so they will not prevent air from getting to the prop from the sides.

Of course these hulls are specifically designed to suck water into the tunnel pocket from the sides -- not from below -- and the props on these boats will probably ventilate like crazy unless the hulls are submerged substantially below the waterlines shown here. But *if* we can count on these hulls being submerged much deeper than shown, they should work quite well ... and the planing shoe shown in green will certainly protect the prop from damage.

Another possible solution for very narrow aft hulls might be to extend the sides down beyond the bottom of the chines, and get rid of the planing shoe. This will prevent water from filling the tunnel pocket from the sides, and instead this water will come from beneath the hull. Unfortunately this will likely reduce efficiency since the water cannot be drawn from the nearest place which is along the sides. This change will also get rid of the substantial propeller protection of that wide planing shoe. The end result is that the boat will have a real 'tunnel' instead of the inverted vee bottom that Atkin calls a 'tunnel stern', and the same combination of features which exist in Rescue Minor will be missing.

I don't think it's as important for catamarans to worry so much about this anyways since their hulls can be very efficient without all this complication ...

 

Thnx Redu

 

kengrome,
the reduction of the shaft angle may well be the key factor in this tunnel design boat:
If there is angle in the shaft, the virtual pitch of the propeller actually varies according to shaft turn angle. Propeller blade tips are of course moving forward as does the boat, but additionally the tips are fluctuating forward and backward according to the shaft angle. For instance on left side the virtual prop pitch is higher than prop pitch and on right side it is lower. That is a very inefficient situation in some cases. If rev/min is low and prop diameter is wide the fluctuating virtual pitch effect is fatal.

I once performed some simple math evaluations on shaft angle systems, and learned that for a high efiiciency system with high dia prop and low r/min shaft angles higher than 1..2 degrees should not be accepted.

redu

 

I drew the attached cat hull. It has the prop sized to be better than 80% efficiency at 15kts. You can see that it needs to hang below the keel. If the top of the prop was placed in a partial tunnel and the side of the hull blended into the tunnel to feed water in then the prop could be placed higher without risk of ventilating.

End result is bigger diameter prop giving better efficiency with prop no lower than the keel. This would be better than what is shown.

The hull is still narrow but you need big diameter props to get efficiency.

Rick W.

 

You might be right redu. The more horizontal the shaft the greater its efficiency and the less stress on the prop due to the variation in pitch as you described. I think there are other factors contributing to greater than average efficiency too of course, but it's anyone's guess what percentage each of these factors contributes to the overall picture.

 

How often do you see the props on propeller driven aircraft angled at 10 to 20 degrees to the flow?

I am no longer surprised by the compromises that are made with boats without any real understanding of the consequences. As energy efficiency becomes more compelling the designs will take these things into account.

Rick W.

 

This is what I was trying to describe when I said the hull sides would have to be lowered or extended below the chines.

I like the concept, but it eliminates the hard protection of the planing shoe below the prop, and it also prevents water from filling the tunnel section from the sides. The prop probably won't need the extra protection of the planing shoe anyways, but it seems like filling the tunnel with water that comes exclusively from below is going to be less efficient than filling it from water on three sides like the Atkin boats probably do. The leads me to the question:

How much efficiency will be lost when the water can only fill the tunnel from below, instead of filling it from below and from the sides?

Another thing to consider here is that it is entirely possible to use a surface piercing propeller instead of a conventional fully submerged propeller. Supposedly these props are more efficient than fully submerged propellers, and better designed to deal with aerated water too. If this is true, why not use a surface piercing prop instead?

 

Ken
You do not need to take anyone's guess- It can be worked out with reasonable precision. The Michlet data I have produced for the Atkins style hulls indicate very similar results to those reported.

Rick W.

 

There is no waterline shown on my previous attachment so you may be interpreting it incorrectly. The square transom is actually submerged by about one third of its width. So it is well under water. I do not see any difficulty in channeling water into the prop from side skirts.

The other thing is that a big prop acts on more water and accelerates it less so it does not create as much pressure variation in the stream.

Finally surface props are not very efficient compared with big props on boats at moderate speed - say 20kts. Their low efficiency is tolerable when you can eliminate a heap of appendage drag normally associated with any other form of water prop.

Again the Atkins hull has the advantage that there is no need for additional shaft strut or long, exposed prop shaft adding drag. The shaft can be short coming straight out of the hull, supported at the hull.

Rick W.

 

Rick, this is encouraging news to anyone interested in building efficient 15-20 knot boats. Have you run other hull styles thru Michlet to see if they can equal or better the efficiency of the Atkin style hulls at the same speeds?

 

Are you saying (for example) that if your transom is 12 inches wide, it is actually submerged by 4 inches of water?

If this is what you mean, then your chines are also submerged 4 inches, and in this case I agree that there will be few if any problems with water being channeled in from the sides. If the boat hobby horses and the transom comes out of the water, that's when it will suck air and ventilate. Maybe a short chop would let air get to the prop as well?

One of the things Billy Atkin seems to have learned is that the chines need to be "in the water" all the time to prevent ventilation in some sea conditions. Shoals Runner was his last tunnel-stern Seabright design and its chines are well submerged as you can see here:



... but Rescue Minor (a boat he designed many years earlier) has its chines at the waterline. Some RM owners reported ventilation problems when turning and/or when running in a chop, presumably because the chines are too high so they let air under the hull and into the tunnel pocket:



I figure Mr. Atkin learned a thing or two after his first Rescue Minors were built and tested, and this is why I also like to believe that he improved these boats by the time he got around to designing Shoals Runner. This is just my opinion of course, I have no evidence that this is true. It does 'make sense' though when you think about it ...

 

Ken
It is a combination of many factors. Most are discussed above. The features include.

1. Quite high L/B ratio. The one I did for FastFred has almost 7.
2. The hull may appear as a bluff transom but the underwater portion is a canoe. This means good streamline flow off most of the stern.
3. The monohulls gain stability from the overhanging stern compared with a canoe but I have not done full analysis on this.
4. They can swing large props without needing extra draft.
5. The prop shaft is close to horizontal. This avoids vibration and large shaft forces so no need for a strut.
6. Reduced appendage drag by removing most of exposed shaft and prop strut.
7. I have seen a video of one in operation and the trim hardly changes from rest. This simply means that Michlet is a reasonable tool for analysing them - same applies to any hull with high L/B and canoe stern.

I did an optimum version of the Atkins to Fred's dimensions- see attached. This is a 7 tonne boat that should do 15kts with around 35kW.

What a lot of people do not realise is that a monohull is fundamentally 40% more efficient than a catamaran of the same displacement if there are no constraints for length and stability. In practice the constraints do come into play. Most people would not be happy crossing oceans in scaled up rowing eights - even I can see a problem with that.

This guy might have a different opinion though - it is not me.
http://www.youtube.com/watch?v=wqZeJf2HC0Y
Two things to note. The speed is deceptively fast and not much wake to speak of.
Rick W.

 

The Atkins design does not appeal to me for the ocean for the reasons you point out. It would be OK at speed in moderate conditions but in really heavy weather where you need to back off I believe the prop would see a lot of air.

I think my idea of a withdrawable drive slotted through a well towards the middle of the boat is a good solution. With larger boats the drive unit could be hydraulically lifted for inspecting the drive or removing fouling from the leg.

The electric drive I made weighs 19kg so can be easily lifted by hand. I am running 48V so get 4.5kW continuous from the drive and it can peak to 9kW. Same unit will get 7kW continuous on 72V. Ample for a light, easily driven hull.

I had very good experience with a small sail drive in a yacht under all conditions. A drive leg near the middle of the boat works well.

Rick W.

 

Rick,
thank you much for these reference data:
4.5kW, 19kg, ...
Speed at 4.5kW? Hull lenght (wl)? Hull beam (wl)? Total load?

I have got something similar:
6.5kW, 72V, 12kn, 340kg, Tornado center hull + tiny outriggers, poor propeller, Mariner outboard rig and bulb + Perm 132 motor
But plans are for a mono hull like yours.

Well, out of thread topic! Rick, have you found a good thread especially concerned el-boat hulls and techniques?

redu