Comparison testing of an Atkin tunnel-stern v-bottom Seabright skiff and another boat

Fred, I think you must have missed the critically important part of my testing scenario where I said I would run one test -- then swap engines, shafts and props and run each test again -- in an effort to eliminate the possibility of one propulsion system always being more efficient in the same boat and therefore screwing up the results.

 

Ken:

This is my first post on this forum, but have been reading it for several months now.. and I'm particularly interested in what you are proposing to do here. I was considering doing the same, except with 4ft. long -1/8 scale models, and using R/C components. It may only be one data point, but it will be a very interesting data point! And your results may eliminate my need to build the "standard" model.

One quick and potentially easy to make the test more more "scientific" would be to swap as much of the propulsion systems as you can, and run the series of tests a second time... And in speed racing, ie - Bonneville, they run the "tests" in both directions, back to back, to minimize wind effects. I would suggest considering doing the same.

Best of luck,
Jim

Edit - Ken posted his reply above, while I was typing.

 

Ken,
Comments by posters here having reservations regarding certain aspects of your proposed two hull fuel burn tests have been neither disrespectful or denigrating in my opinion. The question of optimization vs inherent effiency is a fundamental issue here that deserves consideration.

It is not unreasonable to imagine that one hull might need a propeller of a different size and pitch turning at a different speed from another hull to operate at peak effiency. Conclusions derived from from testing identical components in differing hulls are neither simple or clearcut. This is all people are trying to say.

Everyone here is interested in the same thing. You are to be applauded for your willingness to attempt this but should not take offense at the valid concerns of others who share your interest. It will be enlightening to see the results of Tolman's experiment after he gets her dialed in.

 

Ken, dont take it personally, nobody is wanting to harm your project.

Maybe shop it out to some other forums like WBF or ABB. This venue might be a little too analytical and seeking out some seriously complicated tests to get exact numbers.

E

 

Sorry folks, I apologize for that "disrespectful or denigrating" comment I made earlier. I'm getting feedback about this project from several sources now that I've mentioned it elsewhere, and I inadvertently referred to a comment someone made in another forum.

I think you're making the mistake of thinking that I want to compare the relative efficiencies of the "hull plus propulsion system" combination rather than comparing only the hulls ...

In my opinion it doesn't matter which propulsion system we use, nor does it matter whether they are the most efficient, somewhat efficient, somewhat inefficient, or ridiculously inefficient -- because as long as they are the same in both boats, then both boats will be pushed by the same amount of energy from what is essentially the same source.

Thw whole idea here is to avoid using two different propulsion systems, not to optimize each one for a specific hull. This is true is precisely because I'm trying to learn which hull is most easily propelled, while keeping the propulsion systems "out of the equation".

On the other hand, if I wanted to know which "hull and propulsion system" combination results in the best efficiency, then of course I would optimize one hull with one propulsion system, and then the other hull with another propulsion system. But that's not what I'm trying to do here.

I clearly understand that the inboard engine with the small fast turning prop is not going to be the most efficient propulsion system for the second boat ... but hey, it will not be the most efficient one for the Atkin boat either. If we use a larger, slower moving prop we can increase propulsion efficiency on both boats. But what purpose does this serve when the goal is only to equalize the propulsion systems, not to make them more efficient?

 

I am a bit puzzled. If I am understanding you correctly it seems like Tad's original proposal of towing tests would be a better first step.

"I'm trying to learn which hull is most easily propelled, while keeping the propulsion systems "out of the equation"

I understand that perhaps you have some concerns about the prop filling the tunnel etc. but doesn't that hinge on proper propeller choice?

 

You're not understanding me correctly ...

By this I meant equalizing the propulsion systems, not that the tests should eliminate the propulsion systems completely!

When you make a factor on the left side of an equation equal to one on the right you can eliminate those factors from both sides of the equation because they will affect both sides equally and therefore cancel each other out. That's all I was getting at here. I'm proposing to use the same propulsion system in one boat as in the other, so they will 'cancel each other out' leaving only the differences in hull design to affect the results.

 

I understand what you are trying to accomplish but remain puzzled as to what you will prove. I think it might be hard to gain clear insight with an arbitrary choice of powertrain. But no matter, you don't have to explain it again.

What size, pitch, and and shaftspeed will your proposed system entail?

I am interested in the particulars of the chinese diesels you have mentioned. Are they watercooled?

What sort of gear do they have?

Electric start?

Generating capacity?

 

Probably 6 inch diameter, whatever pitch the stock propellers come in, and shaft speed same as the engine -- direct drive.

I never mentioned 'diesel' when I talked about engines.

I plan to use standard off-the shelf China made gas engines -- clones of the Honda 200GX engine. They only cost $100 (last time I checked) and they start and run as good as the original Hondas from what I can tell. They are rated 6.5 HP (max) and should easily push the proposed boats at 15 knots ... or the full-sized equivalent thereof.

 

Prop tunnel reference

http://www.zeroemissionboats.com/batenbilden11.html

This is a 120 deg. prop tunnel.There is no keel box as in Atkin hulls. The boat was developed and built in Sweden for efficient zero emission elboat. Skinny centerhull and tiny outriggers. Prop axle is nearly horizontal. Gap from prop tip to channel wall is small.
They succeeded. This boat is really effective: 3.7kW, 540kg, 10kn. Atkin boats never could show corresponding power/speed figures with 540kg total load? Why? As Atkin hulls have more wetted area and more wave drag.

What is interesting is that this 1/3-tunnel propulsion is very effective. One would think that a propeller in all free water, far from any other objects, but under a hull should be the best possible solution?

redu

 

My guess is that this hull may have more wetted surface and much narrow hulls and the wave drag reduction more than offsets the added surface drag (if any).

 

I can see why that 120 degree tunnel would make the boat exceptionally efficient. How many boats have a propeller that's virtually the same diameter as the hull's waterline width? Not many I suspect.

Basically the tunnel traps and channels the water so it can only be pushed aft rather than sideways ... and by using the hull instead of a separate nozzle to contain the water there is no added drag.

Good design for efficiency!

 

Ken, and all, the image posted by redu is not very dis-similar to the design I am playing with for my cat... I had the prop - in some versions with a bigger diameter than hull beam but with an almost flat "tunnel", and a fairly fine return to encase the shaft. The hull form was relatively efficient and with a 700mm dia screw and slow, about 900 rpm I thought I was going well but the early model was sucking air, overweight 11.5 kg when it should have been 7kg for a 1:10 of 1.2m loa....

So use mine as the basis and change the tunnel sections - rest of the hull unchanged so it could be a simple matter of adding a tunnel or a flat tail area?

Have a look at the cad stuff I sent, is it do-able that way?

 

propeller channel

Masalai,
would you think, that a flat bottom width same as propeller diameter would not be a good enough "cavitation plate"? Is this 120deg "tunnel" really a must?

Would it be that outboard motors are out, if a large diameter propeller is used for effective propulsion?

redu

 

The "flat" bottom has been used on several boats, the C10 by Robin Chamberlin (photographs in my gallery) which worked well as did "foriegn Affair" also by Robin, Kanga Bartles power cat designs (Australian builder) as well as - - Oh brain access failing - that New Zealander who died recently also.

Have a look here and all the following pages for test/performance results http://www.icecat.com.au/objectives.htm and is still voyaging through New Caledonia and beyond...

Second question... According to Ric W, our expert on that stuff, the bigger and the more like, (sort of) an aircraft propeller, the more potentially efficient (lots of caveats, but you get the drift?)

More efficient to have a straight & horizontal shaft with no need for struts to hold the shaft.