Planing Hull at Disp Speeds

i should have quoted the post this was in reply to

 

I refered to SLR of 3........should have been 1.3
My bad ....sorry.
Williallison, As I recall the GB is lighter than the 33000 lb Willard 40. The W40 requires 23 hp for 7 knots, much less than the W30 since the SLR is far less w the W40. But the issue is how fast would the GB go w 23 hp? Probably about 4 knots. The GB is even lighter so drag'in that barn door through the water has a HUGE effect on efficiency.

 

No.
Immersed transom can double the TOTAL resistance;
More so, dry-transom and immersed-transom hulls will be COMPLETELY DIFFERENT hulls; in my example trim was changed by about 5...10 degrees; as a result, hull, as "seen" by the water had completely different shape.
For SAME LWL & DISPLACEMENT it will be approx. 20HP for non-immersed transom hull and 50HP for deep transom hull, at Froude number below 0.25 (metric) or so.

 

Daiquiri - sorry, I obviously wasn't explaining myself clearly... I wasn't suggesting a chnage in displacement. I meant that the power required for a given displacement might be 20hp - the amount required to move that mass. Then the resistance due to drag for an immersed transom might be 5hp, vs the resistance due to a non-immersed one might be 2 hp. So the actual, real life difference between them would be much smaller than the 2.5 times that is suggested...

PS / Easy - sorry, I just find what you are suggesting to be hard to believe. I simply can't accept the notion that, with all other things being equal, one will use 2.5 times the power of the other. And so far, nobody's produced evidence to the contrary....

 

you can play with michlet and see what it gives you for total and transom drag. It's supposed to give good numbers for slender boats, but a full transom ratio on a canoe wouldn't be nearly as draggy as one on a fat trawler.

 

Mat,
I don't remember saying anything about the submersed stern boat being 2.5 times the FD hull drag. At a SLR of 1 to 1.5 it's probably close though. At hull speed probably only 25% more.
I think a good apples to apples comparison can be found in the GB 36/42 and the Willard 40. I know the Willard requires 23 hp for 7 knots and to use the GB one would need to use the difference between the 36 and the 42 GB. If the GB 36 required 50 and the 42 70 then we would use the number 60 to compare to the Willard. Anybody know the GB numbers?

 

It's just not going to happen.......There are too many other factors involved to isolate the transom immersion as a single difference.

Most Grand Banks boats are twin engined, so appendage drag is perhaps 2.5 times that of a single engine vessel (The Willard). How are you going to account for that? What is the weight difference, a GB 42 could be up around 40,000 pounds loaded for cruising, and a 36 might be 35,000, but I don't know......Published data for the W40 states 33,000 pounds but I don't believe that represents reality........

My data for a single engine (JD6068 225HP) GB42 shows 1.9gph at 7 knots, around 40HP used.......I have no data for a single engine GB36 (I don't know if any have been built) but a twin engine (210HP Cummins 5.9's) version burns 2.5gph at 7 knots, for approximately 52 HP used. The smaller (shorter) twin engine boat uses more fuel for the same speed. I don't have real displacement figures for either boat.

 

I want to see if I'm getting the basic take-aways from this thread.

Optimal hull form for a full displacement vessel is the so-called canoe shape, defined by two symmetrical intersecting arcs. The more this is interfered with by the addition of a flattened transom the greater the drag penalty and hence the more HP required for a given speed. So far so good?

So, a couple of questions that suggests:

Does the stern shape at the waterline matter more or is it the shape of the entire underwater stern?

Does asymmetry in the fore and aft shape matter (i.e. the hull forward of midships is shaped differently than aft, as in a teardrop shaped hull)? How significant is this? Does the so-called wineglass stern solve this problem?

 

TAD,
What I see/hear is that I could get a single engined GB 42 and repower it w a 55 hp Yanmar (w appropriate gear and wheel) and cruise at 7knots burning only twice the fuel that I burn on my W30. That's only 2.75 hp per ton!

 

Seems to me that comparing the GB 36 or 42 to a Willard 36 or 40 is comparing apples to oranges because of the hard chine vs round bilge hull shapes. Wouldn't a better comparison be something like the Rough Water 35 or 41. These boat have a round bilge with single engines, either a ford lehman or a perkins. The original advertising claims for the Rough Water 35 was that it had a 750 mile range from 170 gal fuel tank at 7.5 knots as I recall. I'll leave it to the NA's chiming here to do the numbers. Just a thought.

 

Chuck,
Fine if the RW has as much emersed transom as the GB. And if we can find the power requirements for that hull. I think the hard chine soft chine element has next to nothing to do with it.

Easy

 

I would be very reluctant to use Michlet for that sort of purpose because the thin-ship assumption of small longitudinal slope is violated at the stern.

There is an excellent report on the NPL hull series by Wellicome, Molland and Couser that has a lot of experimental results at speeds where the transom is wet, partly wet, and fully dry. Maybe they can be used to get a rough estimate.

Leo.

 

So far, so-so.
An "optimal hull form" is a slippery concept. It depends on the speed, ambient conditions, etc, etc.

I would say it is still unresolved.
Search for papers by Kevin Maki, Lawrence J. Doctors, Robert Beck and Armin Troesch. They recently conducted a long series of experiments on transom stern effects, including mathematical hulls with rectangular sterns, and other more conventional shapes.

Yes it does matter.
You can try to play around with the program "Michlet" to see how the wave resistance and skin-friction are affected by fore-aft asymmetry.

In a small nutshell...

Ignoring boundary layer thickness effects for the moment, you will find that fore-aft symmetric hulls will have the least wave resistance.

Now, if the hull pitches bow-up, then a symmetric hullform will become asymmetric as the bow rises and stern sinks down.

In that case, a "better" hull is the so-called "fish-form" which has its maximum width bowwards of centre. This asymmetric hull has larger wave resistance than the symmetric hull at level atttitude, but it becomes more symmetric when the bow rises, so it will have a lower wave resistance than the other hull.

It is important to consider these sort of shape changes for high-performance kayaks and rowing shells where fractions of a second are crucial. For pointy-ended logs in a real seaway it's not so critical.

All the best,
Leo.

 

Agreed.

Optimal for who, and what?

A "theoretical” optimal shape, is only valid in the “imaginary” world. By that I mean, this ‘optimal’ shape, that is defined, in whatever manner, may be in one instance and snap shot of time, be ‘perfect’ or ‘optimal’. However, as already noted by Leo, under different conditions, this “optimal” shape/hull is no longer valid. This is true throughout the whole design cycle, not just piecemeal transient steps in time history of dynamic behaviour.

Take said “optimal” shape A…..can this hull be made quickly, easily, and cheaply, for example?

Shape B, may not be “hydrodynamically optimal”, but is quicker, cheaper and easier to make than shape A.

Shape C, may be heaver, more draggy, expensive, but very very quick to make

Shape D may be much worse than shapes A to C but very easy to maintain, thus through life costs make shape D “optimal”

And so on…

Thus, the term “optimal” is a moving goal post. It is highly subjective to your own SOR, or design constraints. This includes not just the performance but costs etc.

I personally detest the term “optimal” when used in design/naval architecture, owing to the aforementioned reasons. No two boats and thus, not two SORs are the same. Hence, what is optimal???

Optimal is in reality just another way of say…you have achieved the SOR. You have managed to balance the endless compromises that one is required to make, to obtain the objective of designing a whole boat, not just one minutiae aspect of the design under a microscope.

 

Even with the 8 paramater shape function? I thought the point of that was to look at the effects of a submerged transom? Maybe that's why I came in second in that cardboard boat race . And all this time I thought I was simply out-paddled .