Why not use a design from 1916?

It is possible to moderately modify an old design by changing the buttocks. They were often like sailboats with a "belly" in the middle. Designs like that can do better with straighter lines.

 

Great for what?
You're making the incorrect assumption of putting the cart before the horse.

A hull form, is selected to suit the SOR, not the other way around. It is just a 3D shape that allows the Design to work, based upon the requirements.
Don't over think it.

 

Thanks for your input, it made me smile. Stephen Covey said "Begin with the end in mind" in respect to be happy and successful in life, achieving whats important for you. Your referring to the SOR made med think that it is pretty much the same thing as "Begin with the end in mind" when it comes to life struggle, but it's easier said than done, I've tried to figure out my end goals for a long time, ha ha.

I guess my assumptions are that speeds >0,45 FnL seems ineffecient speeds in general, and not at all suited for electric drive trains in particular and that my SOR is I want to go as fast as possible but be able to use en electric drive train without having to maximize battery packs for reasonable range,thus stay in reasonably effective FnL range. While at the same time not going for a 28 meter waterline, so really go slower, up to 8 knots cruising, in a boat that can host 6 people on daytripps comfortably and possibility for overnight for a couple of persons, and a little gally, basically a social platform and a boat for excursions and adventures.

Point with this post being that there are so many designs already made for any one looking for an efficient displacement hull, pick and choose from the early 1900. That is why I am curious if a 100 year old design would still be valid or if a more modern design has advantages. Are there any new things we have learned about efficiency in speeds below 0,45 FnL? In that respect the model test comparison made by JSass that he mentioned above was really interesting, comparing equal displacement, equal wetted area and equal length/beam between a 100 year old design and a brand new design. The brand new achieved better efficiency but mainly in speeds over FnL 0,45 If I didn't misunderstand. My remaining question therefor is if settling for speeds up to 8 knots or maximum 10 knots, will the new design still be a better choice or are there other pros and cons with the old vs. new? I guess from the answers so far in this forum, it seems that old designs were actually pretty good if first priority is maximum speed and range with a given and limited amount of power and energy consumption. But I would love to hear what JSass thoughts are about handling in headwind with waves or other situations between the old and the new design, still going 8 knots I might ad.

Wonderful to read all the posts with thoughts and input // Anders

 

I should ad that the advantage with electric as I see it is always arriving to a boat "filled up" and ready to go (if you have solar or shore power), silence of course, less need for detours to find gas station (not that many in Stockholm Archipelago), When in critical need for energy it will be easier to find an outlet (any house will suffice) than a gas station and a nice day with solar you could get a couple of kWh to get you ashore, and lastly cheaper moving cost (electricity). The disadvantage of course being huge; less energy carried all in all and expensive. The electic engine will basically cost the same as a diesel but the energy storage of battery will be much much more expensive than mounting a tank for diesel.

 

Why assume….and define inefficient?

It makes little difference what is being used to provide power to the prop.

That means, you need to “Design” it first…forget endless what if’s. Design the boat calculate the weight of the vessel based upon your SOR, then you will know, rather than endless guessing and a blunderbuss approach.
And therein lies the rub…your SOR must accommodate several Design points.

I refer you to my previous replies.

 

My definition of inefficient speed range

Maybe a missunderstanding, of course it has nothing to do with efficiancy, that is not what I mean, but practical reasons, like noice, price, space, reliability etc. I like the idea of electric for the reasons mentioned in my last post, but electricity does not match well with the need for energy to travel in water. So if I want to have en electric propulsion then the amount of energy I can carry will be extremely limited due to battery energy density thus the need for a super efficient hull increases, and lower speeds.

It would be fun of course to design my own boat, but timely and expensive and for me also lack of skills required. Why invent the wheel again was my humble retorical question. The drawing from 1916 seems to fit my SOR fine with enough interior space for my needs, a long waterline for higher displacement speed and a narrow hull for a light and efficient construction. And of course I am curious if a 100 year newer under water hull design with otherwise comparable metrics, could generate a knot higher speed or some more miles of range for the same amount of energy and better or at least not significantly worse seakeeping capabilities. It seams you think I am on the wrong track in my head. But doesn't the question intrigue you?

 

An important factor is the boat's weight center of gravity in relation to the waterline area center. If there is a large difference, it affects the seaworthiness very significantly. That is why we have abandoned boats that were designed a hundred years ago.
JS

 

Aaah, I see, that was a very experienced answer and really what I was wondering. I have been wondering for some time. Almost every article about new energy efficient boats in Sweden mentions CGP, apparently even mentioned in your work, but no one would explain the difference, except that new designs in general seem to want a higher speed range, and I could understand why CGP designs were not the most effective designs for semi planing hulls.

Thank you so much JS.

 

Okay, so what you're basically defining as "inefficient", is simply to avoid the increase in resistance at the prismatic hump.
That is a notable objective....assuming you wish to do so, and not go beyond the hump.

But of course if the Design ends up having a high length-displacement ratio say above 9.0 (in metric) then this becomes less of an issue. Whereas if your LD ratio is low say 4-5.0, then is it a massive issue to consider.

And this is why you will be constantly flopping from one 'objective' to another, because you'll ostensibly be reacting to one liners without understanding the reasoning behind it.
Like "inefficient" for example.

From a laymans point of view, I can see that it does.
But from a naval architects point of view, it is what the Design is supposed to do...based on the SOR.

So it is far better to pose your thoughts about, what do you want to achieve..and forget the background noise of inefficient and so on....
Just have a clearly defined objective (your SOR), and then breakdown each part that has an influence to gain an understanding fo where you are going...and why.

Since no two boats are the same, and whilst one Design may not suit you, that Design works for the SOR it is Designed to, ergo, your SOR is not the same, ergo no two boats are the same.
Don't overthink the objective.

 

Very good input for me to be able to understand. Yes, I believe my conclusion from research so far is avoiding the increase in resistance at the prismatic hump to be a key factor if going electric with a battery bank and not spend a fortune of money and kilograms on the battery bank, and I only have let's say 28 kWh of energy and want as much range (distance) as possible in the speed range of 7-8 knots.

A follow up question: It was very interesting what you stated about less of an issue with the hump with LD ratios > say 9. I intuitively can realise that there would be a big difference with pushing more weight "uphill", but I have difficulties in quantifying the difference, but of course I trust you when you say the difference is significant. I guess my assumption would be that no matter the LD ratio the speed range around the hump going "uphill" would be very relatively costly, energy wise, even with higher LD ratio, thus causing a rapidly diminishing range even with a very high LD Ratio. I do not know how to calculate these effects and differences but if you made med guess I would guess that even with a LD ratio of say 9 the mile range would be reduced by at least 50% statyin in the hump area rather than lowering the speed half or one knot or so and staying in the bottom of the longitudinal wave form. Would that be a reasonable guess? And if so I guess your point beeing that the penalty for staying on the hump would be even grater, actueally a lot greter, with a lower LD Ratio.

If above understandings are reasonable, I guess my other conclusion regarding the underlying but maybe too subtle question in the topic headline would be that given a 100 year old design of a boat 9,6 meters long and 1,7 meters wide, if it where built with a LD Ratio over say 9 and I would stay under the hump in speed (probably 7-8 knots) the under water design in itself would have lesser impact on the energy consumption and thus range (or at least unsignificant). And also that one negative aspect of that design would be the seaworthiness according to the post by HJS regarding a a bigger difference back then in a boat's weight center of gravity in relation to the waterline area center affecting the seaworthiness very significantly. If those conclusions are basically correct I must say I am a bit surprised and intrigued of the fact that the actual design of the under water body ( where you put the deepest point, the angels, the radiuses, sharp shines or no shines, round bottom etc), that these considerations seem to have less significant impact for the energy consumption. It seems to be concluded that the under water body design have a more significant impact going up in speed, to the hump area and over, and into the half planing area, say speed ranges of 10-15 knots.

Again many many thanks for your time everybody, this was fun, at least for me

 

This is a chart that Ad Hoc has posted in previous threads. Note that the "hump" becomes less severe as L/D ratio increases.


Each curve is for a different L/D ratio. It shows the resistance for a family of similar hull shapes, each designed for a different L/D ratio, with constant displacement.
The shapes of curves for a different family of hull shapes would differ in detail but the overall trend as L/D increases would be similar.
If the displacement is constant then the length increases as L/D ratio increases.
The vertical axis is resistance / velocity.
The horizontal axis is volumetric Froude number.
Volumetric Froude number = Velocity / Sqrt ((Volumetric displacement)^1/3 * g)
Length Froude number = Volumetric Froude number / Sqrt(L/D)

 

Boat hull shapes are designed for a particular displacement or range of displacement. A boat built lighter than the design displacement will float higher and be less stable then as designed. It is usually a mistake to build a boat with a hull shape designed for a significantly larger or smaller displacement.

Both the L/D ratio and the details of the hull shape will affect resistance including the shape of the resistance curves. But the general trend of of the shape of the resistance curves with as L/D ratio varies will be similar for different families of hull shapes.

If low resistance is important then weight should be minimized consistent with other requirements.

Factors other than minimum resistance may be the ultimate drivers for length of the vessel. This factors can include construction cost, size of the area available for construction, overland transport restrictions, mooring and storage restrictions and costs, regulations which depend on length. lock size, etc.

 

G-5-class motor torpedo boat - Wikipedia https://en.wikipedia.org/wiki/G-5-class_motor_torpedo_boat
I've always liked this boat. Aircraft designer .

 

Hmmm..it matters little what is driving the boat, as previously noted. Since this is true no matter the power source.

It is rather straight forward.
Weight.
As previously noted, Design your boat, which means calculating the weight of the boat in the arrangement that suits your SOR. Once you have this weight, it is a simple calculation to obtain the LD ratio.

You’re beginning to conflate two entirely separate issues.

DCockey posted a graph that explains it all.

Exactly…weight is your friend or your enemy.

 

Certainly, but with abundant energy for my drivetrain I might not care I guess. The same with electric cars, to me it appears that more people care how they drive when they don't have abundant energy, It becomes ovbvious and important the amount of range that can be saved, but as a matter of fact the energy saved by driving slower has always been true even with ICE engines but just not something worth noting having a big tank and if empty stopping for a couple of minutes and fill it up.