Critical speeds for Semi-Planing

This is a really interesting development, and I'm willing to put up the last 600$, as I'm no expert on these designs, I think Ken should decide which hulls to build...

Regards

Alan

 

Reported where? By whom? How? In relation to what?

This is the kind of speculation that can be substantiated or disproved with a decent precise and rigorous testing program.

 

Alan, your $600 offer is huge and generous, much more than I would have guessed any one individual might be willing to contribute to such testing. Thank you!

I think the decision of which hulls to build should not be a unilateral decision made by the tester. Instead we should go with the wishes of the majority contributors since it is their money that may make this possible. If others see enough value in this approach we can build more hulls later. Tad was right when he said that ideally several hulls would be available for comparison, but for now two hulls is a fine start.

I'm sure I can build the boats and run any number of side-by-side tests to see which one performs better in a variety of different settings. One reason I'm suggesting direct comparison testing is to get a definitive answer to this question at the lowest possible cost:

Are Atkin tunnel-stern Seabright skiffs more or less efficiently
driven at 0-15 knots than other reportedly efficient hulls?

I think this is the same question most of us want to see answered. Speaking for myself only, I would really like to "know" which hull is the most efficiently driven at these speeds, rather than guessing at it!

I can measure the fuel used in each boat while running side-by-side tests. This will tell us which boat is more fuel efficient, and by what percentage. To insure against differences in propulsion system performance we can swap the drive systems and run the same tests again.

I can run a bunch of these tests at different speeds and in different sea states, then report my results to the sponsors. If we learn that one hull consistently outperforms the other in terms of fuel efficiency, we may finally have the answer we seek!

 

OK - 1st can I suggest that we move this discussion to somewhere else so that Leo can have what's left of his thread back!

I have always been a skeptic of the supposed advantages that the SB skiff holds in terms of efficiency. I would quite happily be proved wrong and if performed correctly then model testing would at least go some way to giving some answers.
I think Kengrome makes a valid point about the importance of the propulsion system in analysing the performance of the SB skiff. However, this also complicates its comparison against other vessels - particularly in the testing regime that he is proposing. By limiting the propulsion system to a simple shaft drive arrangement - which is inherent in the SB skiff's design - you are instantly restricting its "competition's" ability to compete. I would expect that a small 4-stroke outboard powered, lightweight vessel of normal form would outperform its shaft-driven counterpart every time, in the kind of model size that you are talking about.
Now, if you want to know which hullform is the most efficient with a given powerplant and drive system, then the experiment could be at least be classed as valid, though not definitive.

 

Good points, Will.
I would argue, though, that trying to find the most efficient hullform for a given powerplant and drive system is a rather odd way of going about it. Except in a few specific cases- like when a jet pump is mandatory because of draft constraints (ie, river-running)- I think it would be more useful to identify the most efficient hullform, period, for a given set of constraints (L, B, displacement, speed, etc) and then optimize the propulsion to suit it.

I too am at least a little skeptical of the SB skiff's efficiency claims; I know many of them are very good but I think at least part of that is due simply to their horizontal shaft angle and the nature of the craft they tend to be compared against.

 

Yes - sorry - that's exactly the point I was trying to make. By building hulls around a given powerplant you are limiting the test to less than optimum packages, thus defeating the point of the tests in the 1st place

 

I agree with both of you, Willallison and marshmat. I'm as skeptical as the next guy when it comes to the efficiency claims I've read about these boats. Then again, I suspect there is something to them or they wouldn't be repeated so much.

If this claimed efficiency results from little more than the horizontal shaft angle, it still makes the shape a good one for better efficiency at the design speeds, don't you agree? The hull has other appealing features too, but efficiency seems to be at the top of everyone's list these days with the current high fuel costs.

Maybe so, but I don't want to compare propulsion systems, I want to compare hull efficiency at 0-15 knots. I should be able to do this by keeping everything else the same except for the hull underbodies, right?

This is exactly what I had in mind. Just use the same engines and the same size boats in a head-to-head comparison and see which one consistently has fuel left over when the other runs out ...

 

If all you want to achieve is to compare the efficiency of two hullshapes for a shaft-driven boat, then I have no argument with your proposal.
BUT... if you want know whether the SBS is more efficient as a hullshape than something else, then you won't silence the disbelievers with your tests. They (we ) will - quite rightly - simply point out that the vessel that you compared it to isn't as efficient as it could have been.

 

At the risk of repeating myself, no scratch that, I am repeating myself.

I believe there is a good chance that a large part of the efficiency reported time and time again by Atkin was due to his successful optimization of that hull form.

He stated several things over and over. He modified the keel shape for smoother water flow.

He was quite specific about the propeller and shaft speed.

He almost always mentioned the benefits of a straight shaft angle.

He often splayed the shaft a few degrees across the centerline of the keel.

He placed the propeller farther behind the keel than normal and would warn against placing it in a more conventional position.

The tunnel setup was used when draft was constrained, later boats did not show a continuation of the tunnel thinking if shallow draft was not needed. The tunnel boats were usually mentioned as a solution to people cruising in shallow water not as a more effiecient form of the seabright skiff. The tunnel boats do not show any amazing degree of speed or effiency compared to the non tunnel Seabright boats. Robb White picked a tunnel boat for his experiment because his dock was in a Florida mudflat.

I am excited by the idea of some actual testing being done. However if superior optimization is part of the reason for these hulls working so well it is unlikely that an arbitrarily chosen 6.5 horse diesel and propeller are going to necessarily reproduce the historical results.

Ken has a completed tunnel boat hull that is Renn Tolman's interpretation of a Seabright Skiff. Wouldn't that make a worthy testbed? Atkin would probably have put a motor of 25 or 30 hp turning at 1500 to 1800 rpm with a 1:1 gear into a boat that size.

Will's last comments are right on the money but could be applied in either direction. We need some way to quantify the results. I am not a naval architect, I wish I was because there is much I don't understand. But I do know a little bit about boats. They operate within a performance envelope. What we are trying to determine is whether or not these hulls are operating outside the normal performance envelope. The parameters and metrics used to determine the normal envelope are the key to determining what if anything lies outside and beyond normal.

 

Yes, this is all I care about -- comparing the relative efficiency of two different hulls pushed at the same speed in the same conditions. One will likely use less power and therefore burn less fuel than the other, and that will be the more efficient hull. Simple!

Neither boat will be "as efficient as it could have been, so I don't see the big deal with using cheap inboard engines in these tests. Will, you seem to be focused on maximizing efficiency, but I don't care about maximizing efficency. Mostly what I care about is knowing which boat hull is more efficiently propelled at 0-15 knots, and it shouldn't matter what engines are used for this type of comparison as long as they are both the same.

I have no reason to think that simply by putting more efficient engines on the same hulls, that this will make either of those hulls move further under the same power than they would have with the less efficient engines. They will burn less fuel, with more efficient engines, but they will still require the same amount of power to move at the same speeds ... and they will still burn the same percentages of fuel -- relative to each other -- to cover the same distances.

The problem inherent in using an outboard in a Seabright (compared with using an inboard in the other boat) is that it will require an additional expense of probably $1500 or so. First I would have to purchase two new identical outboards since I have none to test with. Then I would have to modify the Seabright hull so the propeller is located where Atkin says it needs to be. This means the engines won't be swappable and therefore we cannot discount the possibility of engine performance differences in the final results.

Personally I think it doesn't matter one bit whether we use fuel efficient outboards or inefficient inboards in these tests, as long as we use two identical engines and run double tests, the second with engines swapped so our numbers never get skewed by always having a more efficient engine in one boat all the time.

If I am missing something here please tell me what it is, thanks.

 

Yep - I think you are missing something. A number of posts have mentioned the gains made by incorporating a horizontal shaft angle (there can be losses too, but we won't go into that here...). The SBS will enjoy this benefit.
Your identically powered 'normal' hull will have to have an inclined shaft, so will be compromised relative to the SBS right from the start.
I think it's fantastic that you are prepared to take on the task of construction and subsequent testing that you're talking about - like others, I just want to see you -and us! - get the maximum benefit from your efforts

 

I understand your point and I thought of this, but I don't know a way around it. Obviously the boat with an inclined shaft will likely be outboard powered in real life, and outboards are typically set at a downward angle -- so this might be a blessing in disguise when trying to compare two 'real life' boats.

Thanks Will, but all I want out of this is to confirm whether or not the Seabright hull is more or less efficient than a simpler hull. I suspect that it will be since it is optimized for 0-15 knots whereas others are typically optimized for higher planing speeds, but we really won't know until we do some tests.

In all honesty I would be happier to learn that the other hull is more efficiently driven. These tunnel-stern Seabrights are not so easy to build with all the twist in those garboard/tunnel panels. I like them for shallow draft, upright beachability, and protection of props and rudders, but inboard installations are a relative hassle compared with outboards too.

 

"Are Atkin tunnel-stern Seabright skiffs more or less efficiently
driven at 0-15 knots than other reportedly efficient hulls?"

The Atkin hulls under discussion were NOT Seabright Skifs.

The Box keel reverse deadrise setup is similar . The Seabrights were created to be beachable , and efficency was not part of the issue.

Hauling huge loads at speed , avoiding the USCG , and getting into shallower water than the coasties was key to a profitable smuggeling operation.

I too am building a model 1/6 and received a very good recomendation on an electric motor . The Voltage and amperage of an electric motor is easy to log and monitor.

I too believe dragging a model will show a hugely inefficent hull, and only a powered test would be worth the effort.

I'll spring $200 towards the experiment , IF its a power on test.

FF

 

I have battled with myself about how to do a comparison test with an Atkin or Seabright and more "normal" type. I just don't see how it can be done objectively.

What parameters will be held constant between the two models and which will be varied? LOA, LWL, beam, draft, displacement, wetted surface, prismatic and block coefficients? All these will have some effect on operating performance, especially in the speed range specified. No matter how the tests are done, unless the differences are dramatic, there will be considerable doubt of their validity.

I think Will brought up a most valid point. If one or the other hull is restricted in using a propulsion system that is not most effective for that particular hull, the results are no good. I think Ken disagrees with that view and it is only important to know the relative efficiency of the chosen hulls with the same power system. One problem is, it's been pointed out that it is impossible to use the same power system with both hulls since the shaft of a "normal" hull will be angled down.

I go along with the argument that each hull should be powered with the system that is most efficient for it. Otherwise, it looks like the tests are biased toward one hull or the other and most likely toward the tunnel hull. I also think it would take many models of the tunnel hull tested over a long period of time just to optimize its efficiency. Otherwise, how will you know that the tested model is the best one?

The measured parameter of efficiency would be the fuel input (same fuels) at the same measured speeds at the same total displacement with as many other hull parameters as nearly alike as possible. This allows the "normal" to be powered with an outboard. That still leaves room for questions though.

I'll be glad to find a few bucks to add to the pot if we can see that the
tests are going to yield some useful answers rather than add to the list of unanswered questions. I have done enough lab and field work in a previous life to know that the most likely outcome of any tests are more questions rather than the answers you are looking for.

The more I think about it, the more I think it needs a government grant over several years at MIT to get some decent data. Damn, that is getting pessimistic.

 

Hi Fred. I agree with you, Heron, Nibble, Rescue Minor or Shoals Runner are most definitely not the same design line as the original Seabright skiffs. I think the originals are better sea boats, and these are better shallow draft vessels. Nevertheless I have been referring to them by the names/descriptions found on the atkinboatplans.com website:

HERON - 17' Tunnel-Stern V-Bottom Seabright Skiff
NIBBLE - 17' 10" Tunnel-Stern V-Bottom Seabright Skiff
RESCUE MINOR - 19' Tunnel-Stern V-Bottom Seabright Skiff
SHOALS RUNNER - 22' 1" Tunnel-Stern V-Bottom Seabright Skiff

I really wish there were a shorter descriptive name for these particular boats. Maybe we can coin our own name for them?

And wow Fred, thanks for the $200 pledge!

Since we are already "over the top" with your $200 and Alan's $600 pledges, I think it may be time to start a new thread, which I have done here:

http://www.boatdesign.net/forums/showthread.php?p=202955

Tom, sorry but I already created this new thread before I saw your post so I will respond over there ...