# Displacement Speed Question

Discussion in 'Boat Design' started by Tug, Jun 14, 2009.

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### tom28571Senior Member

A simplistic way to look at the long vs short boat resistance thing is to consider two extremes. Take a boat shaped like a hockey puck with a length/beam ratio of 1. As it moves forward, a lot of water has to get out of the way very fast to allow its passage and the round stern has to suck this same water back in after the puck passes. Take the same weight and volume and make it very long with sharp ends (say length/beam of 10 or more). As it moves forward, there is much less water that has to be moved sideways to allow its passage. If the ends are stretched out to a sharp bow and stern, this smaller volume of water is accelerated sideways at a much slower rate, meaning that there is a much lower bow or stern wave. Both of these actions take far less energy than is required to push a hockey puck of the same displacement through the water.

Some people have looked at exploiting this phenomena in a monohull of normal (wide) hull with a length/beam ratio of 3 or so. A half model of one of my versions of this boat is attached. At full scale, the lower canoe hull would contain about 75% of the total displacement while the much bigger upper hull only needs to support the remaining 25%. The idea is that the canoe hull has a large length/beam ratio of 12 or so and can thus run in the high teen knots with low power like a similar catamaran hull would. The upper hull is flat in the aft sections and, with very low loading factor of weight per sq ft of area, can plane at low speed.

It's only an idea and a model and but the numbers look promising. The intention is to develop a cruising boat with very nice accommodations of about 30' LOA and beam of about 9.5' with displacement of 6 or 7K lbs that will cruise in the teens at low power and low fuel use. The initial goal would be 25hp and 1 gal per hr fuel use. Type of power would dictate the aft length of the canoe body. Ideal is a diesel inboard sunk in the canoe hull with a large slow turning prop on a horizontal shaft..

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### Ad HocNaval Architect

tom
you are basically decsribing the well known laws of Froude.

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### tom28571Senior Member

Of course, that was the intention. Much better than being in opposition to Froude. On the other hand, where is the boat that exploits Froude in the same way as this model? At this stage, I am never going to be able to build such a boat but am looking forward to some larger scale model tow testing.

I should add to the second paragraph that the aft wide hull can plane at low speed because so little dynamic lift is required. This means that the needed dynamic lift can be developed over the large area at a very low trim angle. This low trim angle is required for the canoe body to do its thing. The aft bottom would have a built in down angle of one degree or so and the whole boat can then run at zero trim angle.

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### Ad HocNaval Architect

"...On the other hand, where is the boat that exploits Froude in the same way as this model?.."

Don't understand your point?

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### tom28571Senior Member

The point is, to my knowledge, no such boat has ever been built.

Are you aware of one?

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### jehardimanSenior Member

Actually, it is strength and arrangements that limits a hull as the actual hull is not a large cost of the vessel. So much of what works in thoery by extension does not work in practice.

Also look Tug, you need to look closely at what you are comparing. You are really comparing the location of the first wavemaking hump and the power needed to get there. While a longer, narrower, vessel may use less HP/ton, it may have more tonnage for the same carring capacity and therefore go slower. Or it may be so long that forefoot emersion from pitch would be an issue in anything but calm seas. There is no one best hull form, only the best hull form for the mission requirements.

As a side point, there is no such thing as "max displacement speed". The boat will always go as fast as the power input will allow. The idea of a "hull speed" is left over from the early theoretical work by Froude and Havelock and propagated by some magazines in the inter-war years. What it best represents is the first or second discernable wavemaking "hump" for a Wiggly shaped hull. Other effects such as block, prismatic, midships coefficient and length-displacement distribution contribute much more the location of this hump which can vary between a Froude number of 0 and infinity (yes, there are some hull shapes that have no wavemaking hump....they are called infinite planks...).

However, as far as maximum speed is concerned, the "economic" speed will depend on wether the hull has enough power to climb over the first or second wavemaking humps (actually, they are the third and fourth, but the first two are negligible for most real vessels) and into the hollows between/beyond them. The first hump for most displacement hulls occurs about Fn ~0.27-0.3, the second ~ 0.47-0.5. Most displacement hulls do not see a decrease in power in the hollows, but rather a "flat spot" where very little power is required to increase speed. Some hull forms like SWATHs and powercats have such deep hollows that they become uncontrollable in speed as they clear the first hump and launch themselves across the powering curve at the next upslope.

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### UlfJunior Member

This one is very simmilar: http://www.alsphere.at/dg/index.shtml
And it seems to work.
But wouldn't the planing part be sensetive to changes in displacement for examle if the displacement increase by 12%, the planing part would have to carrie 50% more load...
And what happends in waves? I would expect the drag to increase a lot...

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### tom28571Senior Member

Yes,

The DG is similar. The lifting body is quite a bit different and I don't know how that affects the performance. They do claim some magic for the shape of the hull. Loading increase would affect the upper hull of my model in the same way it would a normal flat bottom planing boat. Changes in loading would probably have less effect on the DG. However, the bottom loading on the lifting hull starts out so low that it could increase quite a bit and still be much lower than "normal" planing boats. I still think it has promise as a low power, moderate speed cruiser.

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### WillallisonSenior Member

Ahhh - I thought this idea showed too much promise for you to just just let it slip by Tom
As you know, I've been playing around with similar hullforms myself - "fat" displacement catamaran shapes, that I've coined the 'monomaran'. In fact, I think there are a few people lurking about the place that are considering similar vessels. Once I get over my current build, I hope to experiment with a large scale model myself.
All my research suggests that this does indeed hold the potential to produce a very efficient platform at low to moderate speeds. It's not an entirely new idea, of course - there are so few of those these days! - some of the box-keel shapes share similarities and of course the displacement cat form made popular by Malcom Tennant is similar too.
The displacement glider is an interesting case - though the shape of the 'upper' body would, I expect, only be optimised to a specific speed, though their website suggests otherwise...

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### mydauphinSenior Member

You guys don't know my canoe. 18 x 2.5 feet, 25 knots on 5hp... Runs all day on 2 gallons. You can make a canoe plane, you can make a destroyer semi-displacement, But it helps for it to be narrow....

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### tom28571Senior Member

We hear of such boats but I have never actually seen one quite that good. I expect it needs a lightweight driver to get that speed and mileage. Those are probably 5 large horses too.

My old Destroyer was 396' LOA with a beam of 35' or L/B = 11.3. It could do 36 Kts with all four boilers straining for a speed length ratio of 1 .8. The Japanese had some that did 40 Kts.

Will, maybe someone will get interested enough and actually commission such a boat.

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### TugJunior Member

I built a 17'6" cedar epoxy glass canoe a few years back....was extremely fast....
I researched this build as well.....
Of course in those days research came at a much higher cost then this wonderful forum...
I found an OLD marathon canoe racer and arranged a meeting....
He spent about 1 hour talking about designs and then dragged me out to paddle and talk...for about 4hrs i tried to talk and keep up paddlin with this old fart.....it nearly killed me...
He invited me for a 4 day paddle/camping with his family to try all his own different designs....
He forgot to mention that his family were ALL marathon canoe racers...4 days of workin my *** off chasing canoes dissappearing ahead of me....
It was a great lesson...i learned every different design particulars from actually paddling them and sitting around the fire listening too him explain why he made each choice for that design....
He made a great teacher....
I even went out and cut my own white cedar tree to enjoy the whole process...
So the new funky looking magic hulls might claim to be more efficient i think i will wait till some old guy has made 8 or 9 hulls himself and has figured out all the little problems assosiated with them...
Thanks
Tug

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### mydauphinSenior Member

My canoe is doing the speed on Plane with my wife and me. About 400lbs and engine 5hp Nissan 4 stroke. Honestly any more power would be dangerous. My wife scoots in at mid canoe. I have be very careful at speed. With One person alone it really flies but becomes a handful.

I would really like know what the new destroyers do. They say only 33knots, but with all that power, they might do more.

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### mydauphinSenior Member

Is a destroyer doing 33 knots planning, semi-planning?

15. ### apex1Guest

Overforced displacement rather than semiplaning. Say 112meter LWL

square root LWL = 10.58 by 2,43 = 25,71 the so called hull speed.

Regards
Richard

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