Semi-Displacement?

No,

it complicated the drama.

The simple rule was:

do´nt ask, it functions!

Now we have a bunch of contradictive arguments to encounter.

Regards
Richard

 

There is some good info on this thread. Thanks. and thank-you, Richard, for adding such beauty as your boats to the world.

 

Let me introduce one more complication to the thread.
(copy/pasting here a part of my reply on a thread about planing double-enders)

This is a "gozzo", a very popular traditional boat of the mediterranean area (our friend peter radclyffe is building one) :



It is a pure displacement boat, as we can see.

But, with these simple add-on fixed plates at the stern, slightly above the water line:



it becomes a semi-displacement, or even a planing craft - depending on the engine power and boat weight. The one in the right pic has some spray rails too, which also help.
I've talked with fishermen in Liguria (north Italy) and they have assured me that with 20-25 HP O/B and that transom add-on, their 5-6 m (16-18 ft) boats can make 10-12 knots or more. The weight is an important discriminant here. Without the plates, they won't go beyond the "hull speed" - will just dig the stern in the water as the power increases.
A gozzo modified in this way will not be as efficient in SD mode as a specifically designed hull would be, but the point here is - a purely displacement hull has been fairly easily transformed into a semi-displacement one. And it still remains very efficient in displacement mode, when the stern plates are out of water.

This case shows the importance of both the stern area shape and of weight reduction, in order to create an SD boat.

 

The gozzo boat is not only appealing to look at but they also seem to have achieved an elusive goal. That is having it both ways. FD- SD and planing modes are usually thought to be mutually exclusive capabilities.

Skeptic that I am, I must wonder what the down side of such an arrangement might be. I have been persuaded by a long history of personal failures that one can not have it all. Would the horizontal appendages make the boat squirrely in a following sea, etc...?

Never mind the skepticism, the modification is to be admired. After all the Italians are famous for having created elegant and inspired stuff...Lambrogini and Ducati and Riva and all... not to mention Sophia Loren.

 

I bet those add-on planers make great swim-steps as well.

I'm nuts for any dual purpose features.

 

Could get pooped more easily w a following sea. The big flaw in the type as I see it is the apparent fact that nothing would be lost by building the sides and the transom up from the tacked on afterplane. I see very nice FD hulls made into a silly morphadite like thing. It looks like folks are too lazy to take on the admittedly difficult job of designing a good semi-disp boat. And I think it's obvious these ******* boats going at FD speeds would be better off w/o the afterplane and going faster would be better off as a proper semi-disp boat. This proper SD boat could be like a wineglass stern Whitehall type w the transom enlarged and built lower. I believe a really good design could be as beautiful as the centuries old double enders. The classic look of of the old double enders seems to be the draw that has created these boats and I think good design should be the draw.
There is (I believe) a manufacturer of afterplaned double enders in Spain.

Easy Rider

 

You are right to be skeptic, there are downsides. As I said, it is not an efficient SD boat. It just delivers what it is asked to deliver.

A gozzo is a boat mostly used by fishermen. What they need is a boat able to cope with a choppy sea, and to move efficiently under oars while deploying / hauling nets. A closed deck and a double-end form derives from these requirements and also because gozzos were originally rigged with latin sails. The mid-section is pretty full, because it often had to be pulled on the beach (so it stayed upright), and also to give it more storage volume.

With the advent of modern times and modern engines with a good power/weight ratio, those people felt a need for higher speeds while still preserving the traditional shape of the boat. So the aft trim plates are probably the simplest possible modification which delivers the required speeds, while definitely not harming the grace of the gozzo shape.

But under the plates there is still a stern-type hull, which means a lots of rocker in the aft part. And we know that a big rocker at high speeds means suction force, with a resulting higher drag and a bow-up trim. So, not the most-efficient way to go semi-displacement.

Yet, these downside characteristics are still ok for those folks, because they need the speed just for the final rush to the harbor with the freshly katched fish. For the rest of the time they will navigate at displacement speeds.

And have their traditional beauties not spoiled with transoms and vertical hull walls.

 

In this case traditional heritage is more important than hydrodynamic efficiency. I can understand their choice.

However, there are manufacturers which did the modification you have described. Aprea Mare is one of them, for example: http://www.apreamare.it
I must say that I definitely prefer the traditional shape.

Yes, Menorquin: http://www.menorquin.com
In my view, Aprea Mare and Menorquin boats are the real "********", much more than the old gozzos equipped with stern plates.


I have just recalled that a year ago there was a greek guy who has asked about the way to make his traditional boat go faster than hull speed.

So, after a quick search I've found the discussion:
http://www.boatdesign.net/forums/boat-design/displacement-speed-question-27879-5.html#post286735 (post #70)
This is his boat:
http://www.boatdesign.net/forums/boat-design/displacement-speed-question-27879-5.html#post287111 (post #73)
And this is how the other fishermen in Greece have resolved the problem:
http://www.boatdesign.net/forums/boat-design/displacement-speed-question-27879-6.html#post287869 (post #83)
A same problem has led to nearly the same modification, in two very different areas of the Southern Europe. Interesting, isn't it? Evidently, it is a simple solution which works well for what is required.

 

Daiquiri,
I wonder how fast I can make my Willard Go w the afterplanes?
37hp and 8 tons may have something to say about it.

Easy

 

Like this one? http://pacificmotorboat.com/willardboats/w30searcher

I would say - something like 1.25-1.35 times sq.root (Lwl)...

I am affraid you won't obtain any appreciable improvement with plates. As I said before, the weight is an important issue. I've been told by the fishermen who have applied the plates that it works for lighter boats only, and theirs were mostly below 2 t, with engines in 15-30 HP range.

Your boat is way out of that scale...

Cheers

 

Yes, only w more of a sedan house.
Oh dang ..I'll just have to stand down my shop and tools.
I'd sure like to extend the stern and clean up (straighten out) the buttock lines aft and then I'd probably cruise at my present top speed burning less fuel. See how the QBBL curls up at the stern
Too much work and it looks good and performs well as it is if one doesn't push it or dream of speed too much.

Easy

 

Easy, just to give you an idea about how far-off your boat is...

You have named the spanish Menorquin. Well, they have a model MY100, which has the following main dimensions:
Loa = 34.2 ft
Boa = 12.1 ft
Displ. = around 9 tonnes
Power = 2x200 HP
http://www.mondialbroker.com/detail.taf?B=213974&K=BPS

Well, with 400 HP they claim 14 kts cruise, and 18 kts max speed. And it does have a big hull extension behind, flat aft buttocks and everything. But it also has 9 tonnes to drag around, which is not much far from your 8 tonnes. It tells you that there is no hope that any afterplate could help you to increase your max. speed, with the engine power you have.
By the way, I like your boat.

Cheers

 

My understanding of a semi displacement hull is that the stern wave breaks free of the transom, but the bow does not climb over the bow wave. My understanding is also that lighter displacement boats do this better than heavier displcement ones. A sharp bow, a slightly immersed transom, and a long straight run from the deepest section to the transom are what I would expect to see in such a boat.

A lot of boats designed for planing are semi displacement in performance but not fuel efficiency. I see them all the time with their bows pointed up in an extreme angle and their sterns dragging a tsunami like wave behind them. This is probably because they are too heavy to actualy climb their own bow waves.

A semi displacement boat is less weight sensitive than a pure planer, has better low speed controllability and efficiency, and is fast enough to get somewhere in a reason amount of time. Especially in larger sizes.

 

Sharpii2,
Of all the definitions of SD that is possibly the best
"My understanding of a semi displacement hull is that the stern wave breaks free of the transom, but the bow does not climb over the bow wave."
That's as specific as possible and still a bullet prof definition. Great.
Thanks Sharpii2
And thanks Daiquiri for the boat compliment.
Here's a better picture of Willy.

Easy

 

For sufficiently high Froude number.
Below F=0.35, vessels can have a bow-down, stern-up trim.
Of course, the actual trim is small at those Froude numbers.

Cheers,
Pedantic *******.