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  #166  
Old 06-25-2007, 04:22 PM
tom28571 tom28571 is offline
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Excalibur, those photos don't reveal much of anything about the bottom shape so they wont help much.

There is an infinite range of boat hull shapes between full displacement and full planing. Just where to place a given hull shape is often problematic. It is no surprise that the definition of a semi-displacement or semi-planing hull is vague. Nowhere is this more evident than in sailboats where great changes in performance have taken place in the last 50 years or so. 50 years ago, there were almost no large sailboats that could claim the ability to plane. With evolution in hull shape and lighter weight design, current designs can tripple the speed of older boat of similar proportions. Gradual flattening out of the aft bottom sections with greater waterline beam, lighter weight (often through the use of exotic and expensive materials) have changed forever what was the mantra of boatspeed in the 1950's.

The same thing is true (in a design sense) in powerboats. Take a displacement hull, start flattening out the aft run and lightening the hull and the performance potential starts to rise. The semi hull will fit somewhere in there and will always have some rocker. Full planing boats will usually have no rocker at all and the aft buttock lines will be parallel. Very heavy boats of good hull design will still plane fast but will require lots of power and a have wide transition zone between displacement and planing speed. WWII PT boats are like that as well as many of today's powerboats. Some other boats of good design and light weight can almost ignore the transition "hump" and run at any chosen speed the power will allow.

I see no obvious magic in the shape of the Saga boats and doubt there is any. If they are good boats, I see no problem with their hype but don't expect any miracles, because there ain't any. Because there are so many slugs on the water, a decent manufacturer must be forgiven for trying to differentiate their boats from others. Caveat Emptor ?
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  #167  
Old 06-26-2007, 09:31 AM
Excalibur Excalibur is offline
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I don't expect miracles; I'm just curious as to whether there is any hydrodynamic progress at all. Powerboats and sailboats differ in this way as well. For powerboats, most of the progress has been in the area of power. We can now give a hull all of the power that it can handle (and we can afford). The main driving force of sailboats (wind) has not changed (except to get dirtier). Thus, sailboat hulls have evolved at a great pace, and have far less compromises in terms of cubic footage and toys vs efficiency. Why? Because efficiency matters when driving force is limited.

I strongly suspect that if my 40 year old hull were rendered in modern weight saving materials and given modern power, that you would be hard pressed to find a more efficient recreational cruiser hull. I don't think my hull is that great; I just don't think that recreational cruiser hulls have become more efficient. I too, wish I had the resources to build a large seabright hull. I would love to see data on the Atkins Naiad design. 28' and 15.5 kts on 52 hp. Now THAT would look like magic

But, I have what I have. I will take data for my boat. I am taking out excess weight for better performance. I will experiment with her trim. I can't help it; it's the engineer in me. She may be a 40 year old termite condo, but I want her to be the best termite condo she can be

So now a question. The importance of a clean bottom was showcased for me just last week when Thetis came back from the yard. Cleaning the hull and adding fresh bottom paint netted me a cool 2 kts at cruise. But I noticed that there is still an old unused transducer from an ancient chart recorder faired into her hull, with a prop driven speedometer drive still attached. Do small interruptions like this have a real world measurable effect on performance? If I glass the bottom will the improved fairing give me enough gain that I will see it? Other than the obvious (get rid of more weight) what else can I do?
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  #168  
Old 06-26-2007, 10:09 AM
tom28571 tom28571 is offline
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Originally Posted by Excalibur View Post
So now a question. The importance of a clean bottom was showcased for me just last week when Thetis came back from the yard. Cleaning the hull and adding fresh bottom paint netted me a cool 2 kts at cruise. But I noticed that there is still an old unused transducer from an ancient chart recorder faired into her hull, with a prop driven speedometer drive still attached. Do small interruptions like this have a real world measurable effect on performance? If I glass the bottom will the improved fairing give me enough gain that I will see it? Other than the obvious (get rid of more weight) what else can I do?
Dunno if you could detect the difference a small thing like you extra fitting on the bottom or not. I expect you will remove it at the next haulout and then we will know.

As for the difference an even slightly dirty bottom can make on a planing hull, my experience is the same as yours. Doesn't take very much to make an easily detectable loss in top speed.
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  #169  
Old 06-27-2007, 11:30 PM
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Willallison Willallison is offline
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Originally Posted by tom28571 View Post
As for the difference an even slightly dirty bottom can make on a planing hull, my experience is the same as yours. Doesn't take very much to make an easily detectable loss in top speed.
I worked as an instructor for a time in a waterski school in Greece. A couple of guys arrived in a fair sized boat with a 23ft Beneteau powerboat in tow. I was so horrified at the growth on the boats bottom, that I offered to clean it for them (Big job.... ever tried breathing underwater through a piece of garden hose...? ) Anyway, the end result was that their 12 knot boat became a 30 knot boat!
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  #170  
Old 06-28-2007, 01:20 AM
charmc charmc is offline
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Originally Posted by Excalibur View Post
My Sea Skiff is a true semi displacement hull. You can almost make out the keel that extends 2/3 of the length of the hull from the point where the hull curves up to the props. She's 38 feet, and I've seen the displacement written as anywhere from 17,000 pounds to 22,000. I'm pretty sure it's over 20,000. 700 horsepower. It cruises (3,000 RPM) at 14.4 knots, and tops out at 24.8. What makes me characterize it as semi is the fact that bow rise is undetectable from either helm and it cruises with the forefoot still in the water. I personally like the characteristics of the hull; a very smooth ride, no "hump" and an ability to cut through chop without pounding. But she is certainly wet! Spray to the flybridge in 2 foot chop.
Excalibur,

My 30' Pacemaker Sport Fisherman was an earlier carvel sea skiff. Smaller and not as much flair in the bow, but otherwise very similar hull to your Corinthian. Pacemaker and Egg Harbor built some excellent carvel hulls with great seakeeping characteristics. From your photos and descriptions, your hull is similar: deep and narrow forefoot, rounded chines, sharp vee carried well aft, transitioning to flat stern, rocker bottom, deep keelson/skeg. Classic semi-planing hull. My home port was near the head of Barnegat Bay, long and narrow like the Chesapeake. Steep 2-3' chop whenever the wind was northerly or southerly. Coming back from Opsail 76, we were exiting NY Harbor with a long run in open waters, onshore SE wind vs ebbing tide created steep 6-8' seas. Next day several skippers of hard chine glass 40- 46 footers complained of pounding, broken glass, and sick passengers and crew. We came through that same stuff at around 17-18 knots, easy rocking horse motion, all my landlubber passengers fell asleep. Single 200 HP gas Palmer gave 6.5 - 9 gph @ 18-23 knots. She would hit 30, but wide open you could almost see the fuel level dropping in the tanks.

Your description of the easy transition to planing, forefoot in the water slicing through waves rather than lifting and pounding, and soft ride in rough waters sounds familiar. Offshore, I discovered that she tracked very well in quartering and following seas. When the odd wave from a different bearing hit, she was easy to hold on course. Heavy construction helped that good ride. Like yours, she was very wet.

By the way, I like the looks of your Corinthian. Beautiful shear, and that design did a good job of matching clean, modern looking topsides with a seaworthy hull.

I understand your desire to build. There have been several good suggestions here. The heavy construction of the traditional skiff hulls helps the ride, but reducing weight via modern construction methods will definitely help fuel consumption. I'll send you the Sea Saga photo.
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  #171  
Old 06-28-2007, 01:27 AM
charmc charmc is offline
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Originally Posted by Willallison View Post
I was so horrified at the growth on the boats bottom, that I offered to clean it for them (Big job.... ever tried breathing underwater through a piece of garden hose...? ) Anyway, the end result was that their 12 knot boat became a 30 knot boat!
When I was young and stupid, I cleaned a hull by free diving; coming up for air 40-50 times made it pretty slow going. I got it done, but every time I think back on it, it was pretty stupid. The boat did pick up noticeable speed, though.
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  #172  
Old 06-28-2007, 04:56 PM
FAST FRED FAST FRED is offline
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"Can you please explain how the water gets accelerated BEFORE it reaches the prop? "

As the water gets pushed aside , skin friction drags some of the water along.

On a 35 ft boat with smooth bottom it can be 3 or 4 inches thick.This water contains ENERGY whose capture by the ATKIN reverse V shape may add to the energy the prop produces.AS accelerated water gets further accelerated by the prop, the total energy consists of what the prop produced as well as the "free" energy from the already accelerated water.

Its ALL that pushes out the rear that makes the boat go , so reusing energy that simply heats the ocean (on other hulls) makes sense.

Depressed sterns,

"I am pretty sure that these are symptoms rather than the cause of this behavior. As a planing hull accelerates above “hull speed” lift must begin to transfer from buoyancy to dynamic. The hull will always assume the trim angle needed to produce dynamic lift for the speed it is running. Thus as speed increases, the angle required will normally decrease as lift is proportional to both trim angle and speed. More speed = less trim and less speed = more trim. Some boats are much better at this because of lower bottom loading. More bottom area means less trim angle needed to produce the required lift at any given speed above hull speed.<<<<<

Agreed , but in FL we have loads of "canal patios", 25 ft Pontoon boats with 20-24 inch aluminum pipes that power well.

They hardly rise a bit , yet speeds of 30mph are common with a modest engine.In terms of SL ratio there ar 5 or 6! with no plaining loss of hull LOA.

They have the "right" plaining angle as none seem to ever have a trim tab, although the engine tilt IS adjustable. The point is fast speeds do not require plaining on a small spot in the stern, simply power and light weight.

Efficiency is a different trade off , as the long immersed hulls give a better ride in chop compared to bouncing off the wave tops.

The ATKIN concept might work to allow a long slender , light boat to cruise at better efficiency, (lower fuel burn) at moderate hi teens speeds. The concept of 16 -18K at 5 mpg would work for me , really well.The RM 35mpg won't work in an 8000lb boat.

Leo's thought,

"But there will be no comparison possible for resale value in future time."

Might be correct if the idea doesn't work, but IF it does it will surely be copied instantly.
Not many cruisers may want a 38ft boat with 7,6ft beam to fit into a std shipping container. BUT , it only takes one individual that wants to cruise world wide , sans the bother and expense of crossing oceans to sell my dream boat after cruising.

The concept tho of a high teens cruising boat with excellent fuel economy would be grabbed world wide .(If folks can be convinced not to want floating apartment blocks.

The Austrians as well as the US "creator" of similar designs claimed great speeds with minimum wake and low fuel use.

Perhaps if they had copied ATKIN better , not just the box keel, but the reverse V also , the world would not be ignoring their boats?

FF
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  #173  
Old 06-28-2007, 05:51 PM
tom28571 tom28571 is offline
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Quote:
Originally Posted by FAST FRED View Post
"Can you please explain how the water gets accelerated BEFORE it reaches the prop? "

As the water gets pushed aside , skin friction drags some of the water along.

On a 35 ft boat with smooth bottom it can be 3 or 4 inches thick.This water contains ENERGY whose capture by the ATKIN reverse V shape may add to the energy the prop produces.AS accelerated water gets further accelerated by the prop, the total energy consists of what the prop produced as well as the "free" energy from the already accelerated water.

Its ALL that pushes out the rear that makes the boat go , so reusing energy that simply heats the ocean (on other hulls) makes sense.

Depressed sterns,

"I am pretty sure that these are symptoms rather than the cause of this behavior. As a planing hull accelerates above “hull speed” lift must begin to transfer from buoyancy to dynamic. The hull will always assume the trim angle needed to produce dynamic lift for the speed it is running. Thus as speed increases, the angle required will normally decrease as lift is proportional to both trim angle and speed. More speed = less trim and less speed = more trim. Some boats are much better at this because of lower bottom loading. More bottom area means less trim angle needed to produce the required lift at any given speed above hull speed.<<<<<

Agreed , but in FL we have loads of "canal patios", 25 ft Pontoon boats with 20-24 inch aluminum pipes that power well.

They hardly rise a bit , yet speeds of 30mph are common with a modest engine.In terms of SL ratio there ar 5 or 6! with no plaining loss of hull LOA.

They have the "right" plaining angle as none seem to ever have a trim tab, although the engine tilt IS adjustable. The point is fast speeds do not require plaining on a small spot in the stern, simply power and light weight.

Efficiency is a different trade off , as the long immersed hulls give a better ride in chop compared to bouncing off the wave tops.

The ATKIN concept might work to allow a long slender , light boat to cruise at better efficiency, (lower fuel burn) at moderate hi teens speeds. The concept of 16 -18K at 5 mpg would work for me , really well.The RM 35mpg won't work in an 8000lb boat.


FF
Fred, please,

That free energy is just not there. The accelerated boundary layer water you mention is actually moving along with the boat contributing to drag and is going in the wrong direction to be "free energy" for the prop. Its effect is negative rather than positive. If there is magic in the Atkin, that is not it.

Those pontoon boats are not planing and have L/B ratios, from your numbers, of 10 to 12. 30 MPH! I'm skeptical although a well designed non-planing catamaran of similar proportions could probably do that. To be planing, a boat hull must have some angle of incidence to the water or it cannot develop any dynamic lift. Dynamic lift is the definition of planing.
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  #174  
Old 06-28-2007, 07:58 PM
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Willallison Willallison is offline
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Sorry Fred - Tom's quite right. The boundary layer is a contributor to drag: it takes energy to move the water along with the boat. Imagine a very smooth bottom (of the nautical variety). It stands to reason that it will 'drag' only a thin layer of water along with it. Now imagine a very rough bottom....the opposite would logically be true.

As for pontoon boats, they would probably be operating as a relatively inefficient displacement catamaran. With a length:beam ratio of around 12, a well shaped displacement cat is very economical up to a SL ratio of around 2.8. Beyond that the planing cxat comes into its own, though the lines are very definitely blurred. Conversely, with that kind of length:beam ratio the hulls aren't really wide enough to provide sufficient lift for the boat to plane as the bottom loading is too high.
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  #175  
Old 06-28-2007, 11:03 PM
charmc charmc is offline
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Originally Posted by FAST FRED View Post
As the water gets pushed aside , skin friction drags some of the water along.

On a 35 ft boat with smooth bottom it can be 3 or 4 inches thick.This water contains ENERGY whose capture by the ATKIN reverse V shape may add to the energy the prop produces.AS accelerated water gets further accelerated by the prop, the total energy consists of what the prop produced as well as the "free" energy from the already accelerated water.

Its ALL that pushes out the rear that makes the boat go ,
FF
Sorry, Fred, most times I agree with your insights, and I'm not meaning to pile on here, but ... on this one you're wrong. You're describing a boundary layer, and that's a net energy loss, not gain. Energy is used to drag that water along as you describe. Total energy is what the prop produces less what is consumed in accelerating (dragging) that water.
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  #176  
Old 06-29-2007, 06:25 PM
FAST FRED FAST FRED is offline
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Location: Conn in summers , Ortona FL in winter , with big dock & room for O'nite stop .
"You're describing a boundary layer, and that's a net energy loss, not gain. Energy is used to drag that water along as you describe. Total energy is what the prop produces less what is consumed in accelerating (dragging) that water.'

My thinking is the boundary layer of water is not stagnant , and stuck to the hull, with the same water in the same place place hour after hour.

Rather I would imagine its replaced constantly as it gets dragged aft .

AS it departs aft it still will have some velocity , therefore some energy .

Sure the prop would see all the water as pure drag to be overcome , but its what happens to the energy /water velocity being dragged in the boundary layer the that is trapped by the inverted V that may be being captured.

One hulls shapes induced drag might ? be a different hulls source of energy recovery.

Since wave drag would be low , due to the majority of the displacement in the box keel , with the surface hull forward simply working as an end plate over the box keel and the skin friction (boundry layer =induced drag ) perhaps being reused , it could answer the many efficiency claims.The pressure of the prop wash holds the stern from sinking , weather its plaining becomes moot.
*
The Good old Boys with the canal patios install 45 to 150 hp engines on 20ft boats!

30K is easily done , although the fish killers with 2- 250's are way faster, but they do plane, while the patios only take up an angle of attack with bow raised and then skoot.

FF

DO pile on , I am an recovering New Yorker, and can take the hit.

Also it will be 2 years of expensive and hard work to get results.

First I'm planning on building a powered scale model, that can record the power (electric motor) used at all speeds.

I can weight the model and get realistic impressions of what weight does to the hull form/ power requirements.. Additionally it will be easy enough to borrow an underwater camera and video scale speeds to see what gives.

Full sized will be in 3 stages,build a bare hull and power it with a throw away car engine to get real power requirements.
Install a mock up interior to maximize the space (its a small 38ft).

With the knowns , known repower with "correct" size diesel, and have interior done by craftsman , rather than painted ply of mock up.

FF
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  #177  
Old 06-29-2007, 07:05 PM
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Jimboat Jimboat is offline
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Interesting discussions. The latter descriptions of boundary layer theory are getting closer to the reality. What seems the most important observation re: your discussions/questions, however, is that the lift/drag of any hull can be analyzed...so while it may be complicated through different hydrodynamic stages for each hull, it is still quite possible to calculate the lift/drag for the hull. It just takes the work to do it (but it's fun...if you like that sort of thing!)
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  #178  
Old 06-29-2007, 07:42 PM
tom28571 tom28571 is offline
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Fred,

Relative to the speed of the boat, the water at the surface of a boat underway has a velocity of zero or exactly that of the boat. This velocity reduces throughout the "boundary layer" until it is at the same speed as the boat beyond the boundary layer.

All the energy required to move this water in the boundary layer must be supplied by the propulsion system and is therefore a drag on the boat speed. We might wish it were otherwise but that is just the way it is.

Not having any experience with those pontoon boats, I can't make any further comments on them.
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  #179  
Old 06-30-2007, 09:12 AM
FAST FRED FAST FRED is offline
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"All the energy required to move this water in the boundary layer must be supplied by the propulsion system and is therefore a drag on the boat speed."

Understood , the question is weather there is any recapture caused by funneling the accelerated water under the aft lifting section.

The water IS accelerated , therefore contains energy already paid for, which the uinique shape might be recaptyring?

A skin of water 3 or 4 inches thick covering the front of the vessel should have high energy from the closest to the skin and lesser out to 4 inches where its minor to non existant.

The hull "should " be doing something with this energy , if its ducted aft its presence must have an effect.\\

The SL speed of 2.8 would be dandy for my purposes , its 16 or 17K with a plumb bow , fast as I'm willing to buy fuel to run.

FF

Interesting that we can carry this conversation on as we transit the NY State Barge canal, via Wi Fi , complements of nearby sources.

FF
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  #180  
Old 06-30-2007, 09:55 PM
tom28571 tom28571 is offline
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Originally Posted by FAST FRED View Post
"All the energy required to move this water in the boundary layer must be supplied by the propulsion system and is therefore a drag on the boat speed."



The water IS accelerated , therefore contains energy already paid for, which the uinique shape might be recaptyring?

Interesting that we can carry this conversation on as we transit the NY State Barge canal, via Wi Fi , complements of nearby sources.

FF
Fred,

I see where you are going with this. I have seen this idea expressed before and think there is merit to it. Whether the Atkin design gains some particular advantage from this accelerated water shed from the boundary layer that other boats do not is not so clear to me. There must be some data on this phenomena somewhere in the books.

Lots of Wi Fi along your route huh? Amazing that you can get the use of it while traveling.

Just watched a recording of the latest race in the AC. While I was pulling for the Kiwi's, I am not sure they deserve winning the Cup. Had the race handed to them three times and insisted on giving it back. I should talk. Sailed in a race today and am not sure whether we finished at the top of the bottom half or the bottom of the top half. At least we did not screw up as much as they did but were just not fast enough. Felt like a bucket was tied on the keel.
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