Your hull type

Here is an example of an extremely steep QBBL. Very high displacement hulls can be drivin w a minimum amount of power w a stern like this.
Something tom think about:
If you are driving your boat at hull speed regularly it's probably not a full displacement boat.

manyboats said:

Marin,
Sounds like you did'nt realize the GB was so flat and straight aft.

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No, I've known exactly what the hull looks like ever since we pulled it out of the water in Alameda for the buyer's survey thirteen years ago. But it's always interesting to see it again when we haul out for bottom paint or insurance surveys. It's kind of like the chassis of your car--- you know what it's like but unless you have it up on a rack you never think much about it.

I will say that I did not really realize the significance of the design until you pointed out the characteristics of various hull designs and how they affect the movement of a boat through the water in various posts over the years. So I learned a lot from you in that regard.

Gulf Comanche said:

Full displacement, I assume.

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Definitely a full displacement hull. I'll bet her ass barely slaps the water.
I'll bet you can average 6 NM/Gal. On sailboats without sails up, they definitely rock and roll. The rocking on sailboats is eliminated by the the sails. The pressure from the wind heels the boat over to one side and keeps it there. Without a real keel, I would not recommend sails without having a sailmaker involved. You dont want to have the wind overpower you and knock you down. Any good sailmaker would figure out the max size of a mast and max. sq. footage of sails.
Shrimp boats in the Gulf of Mex are also full displacement hulls. When not dragging nets, the leave the outriggers all the way down and out. The overboard weight from this dampens the rolling motion to a more comfortable level.

What are paravanes.

Paravanes are the delta shaped devices which are attached to the outriggers via wire rope to stabilize the boat. Somebody please correct me if I am wrong.

Gulf Comanche said:

Paravanes are the delta shaped devices which are attached to the outriggers via wire rope to stabilize the boat. .......

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Thanks, that is what I thought they were. Just wanted to make sure.
I have also seen them in disk shapes where the disks are stackes with spaces inbetween.

Tony B

Even with the fish out of the water, the poles down help dampen the roll. Underway they are fully delpoyed 90% of the time. The fish run 15' below the surface and are about 300 sq" in size.

Here's a shot of Bucky (Krogen Manatee) when I pulled her for a bottom job and boot stripe in Daytona last year. For sure, the corners of the stern are out of the water when at rest, and indicates what Eric is saying about the buttock angle. The design makes for a wonderful experience in a following sea, and up to 7.5 knots or so, the wake is almost flat. So many of the PWC's running around here chase me down to jump my wake, and their so disappointed.

Sailboats are basically canoe shaped at the waterline. So does this mean that since I have no width, I cant expect any speed? Like, just stand still.


If you look at the shape of the hull it sort of resembles a spoon this in itself helps to pull the boat through the water.

It is called, I believe the Brounelli effect.
If you take a spoon and dangle it by the handle under a streem of water, instead of the force of the water knocking the spoon out of the way it will instead suck the spoon into the flow of water.
As the hull moves through the water it does the same thing. So water moving past the curvature of the hull aids in the movement of the boat.
this is especially true in a sailboat or full displacement hull design.
SD

Marin,
Good and thanks.
Tony B,
I question 6nmpg. My Willy only gets 1.
healthustler,
I see your Krogen Manatee is a more pure FD hull than I thought. Wider than most of course but you have tons of wonderful space on broad and I'll bet that was the primary focus of the designer. And one of the reasons I did'nt buy a bigger boat this winter was the added moorage cost. I'll stay w my short boat. For the space on board you may have the most easily driven hull on the forum. And w her generous beam a very stable and usable platform on the water. I love all the Krogens.
Dude,
I think inertia explains things better than the so called Brounelli's effect.
Sailboats are often more beamy than same size power boats. They are skinny at the ends and fat in the middle. A diamond shaped hull as viewed from above. That is the biggest difference in the hull form of sailboats and trawlers. Trawlers are full at the ends to provide space and support for refers, air conditioning, generators, big engines and other comforts of home. Sailboats need to be VERY efficient when powered only by the wind that is not very brisk most of the time. My 30' trawler needs 20hp to cruise a bit over 6 knots. A typical sailboat will do the same on 10 hp. Trawlers full at the ends gives us great carrying capacity and space per foot of length but same size sailboats are far more efficient. At one time I wanted to buy a 36' sailboat, strip it to the hull, remove half the keel and build a Nordic Tug house on the hull. Just a crazy idea. Not long after I got the idea I saw a sailboat that had been converted to a powerboat in Everett WA. There are reasons the idea is'nt 100% good though. As to the spoon in your post Dude .....yes water tends to follow a curved surface and the mass of that water moving or changing direction or position causes an equal and opposite reaction of force. That's why a FD hull squats at hull speed and above. Planing boats squat for different reasons. But when a FD boat is moving at a bit less than hull speed the water that the bow pushed aside is bouncing back under the stern so to some degree the boat is surfing and the stern wave is, to some degree pushing the boat along. The boat is sitting in it's own wave w the bow on one crest and the stern on the other w the amidships in the trough. Square stern submersed transom boats do'nt benefit much from this as their sterns are'nt shaped like the face of the stern wave.

I question 6nmpg. My Willy only gets 1.My 30' trawler needs 20hp to cruise a bit over 6 knots.

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So Eric.. When your 20hp is getting you 6 knots, you're burning 6 gallons per hour? You need to repower.

Here's a shot of our boat on the hard. She's really a sailboat!!!

Chip ...........

One's allowed to go brain dead once and awhile ...Yes????

DUHHHH sorry.

2DASEA
Totally FD w a very steep QBBL. I'm sure she's VERY easily driven.

manyboats said:

.....Tony B,
I question 6nmpg. My Willy only gets 1...................

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Taking a guess, I'll say your 30' boat has a waterline length of 25'.
That will give you a theoretical hull speed of 6.7 KTs.
Since you prolly dont have a full displacement hull, lets say you get 90% of the 1.34. That will now give you a theoretical hull speed of around 6 Kts. to be fuel efficient, you should stay a little below the theoretical hull speed because you are still pushing more water the faster you go. So, lets take 90% of the 6 kts and we have a new economical speed of 5.4 kts.
So traveling at 6 kts can be substantially more fuel consumption. But not enough to explain the 1km/gal.
Also it takes 2 or 3 hp per ton to cruise economically. Assuming your boat weighs 8 tons, it will take 2.5 HP/ton or 20 HP. The engine will make about 10 hp from each gallon of gas. Given this, your engine should be using 2 gals/ hr which would give you at least 3 km/gal.
Something is bad wrong with your fuel consumption figure or your propulsion system. Maybe your gages are off.
BTW, I got most of the calculations from various old posts on this forum.
I had a 30' Catalina Sailboat with a 25HP diesel. There was a noticable difference in rpms required to move the boat from 5.5 knts to 6 kts. I always motored my sailboat at 5.5. From habit, I still motor my current 39 footer at 5.5 kts. It weighs about 25,000 lbs loaded and uses around 3/4 gal per hr or less in diesel.

I was going to suggest you back off the boost on that turbo or the trip south was going to get mighty expensive.
Brain dead is good sometimes....trust me...I know.

Tony,
Sorry. I need to engauge brain before mouth more frequently. Diesels get about 18hp per gallon and Willy burns 1 gallon an hour. I was going to send you a PM apology but perhaps this will do. Have you ever seen a sailboat converted to power?
Oh ....... Willy is a FD hull w a WLL of 27.5'. We cruise at 6.15 knots. 2500rpm gets us 6.4 knots. Hull speed is obviously 7 knots. Not even sure WOT will attain 7 knots. Would be about 2 gph if it did.

Hi Eric

You are a wealth of knowledge on hull design. My company works with CFD programs regarding mass airflow. Similarities to water flow and airflow are remarkable. Difference between gas, fluid, and solid is usually just a matter of temperature. Hotter things get, faster molecules move, more fun it becomes!

Thumb nail is pict duplicate bottom to my Tolly... not my exact boat though... simply because I never took a pict.

She planes real nice and yet is economical at just below what calcs at hull speed, especially with only one screw turning at a time. Great to see and learn about all these bottoms.

Thanks! Art

manyboats said:

Tony,
... I need to engauge brain before mouth more frequently. .......

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That is too funny. When that happens to me, I refer to it as "having a senior moment". Lord only knows, i have plenty of them.

And yes, I have seen a few sailboats converted to power. Essentially all they did was remove the mast. They were usually on shallow draft boats.
I have never actually been on one during a run. I would suspect the keel would give the boat a quicker motion without the mast to dampen the effect of small waves. The quicker motion would be uncomfortable to most. This would not be much of a problem on rivers or lakes. Then again, it's only a guess.
I could easily see how it could be economically on a sailboat like a Morgan Out Island. They have probably the shallowest draft of any sailboat their size. They also are heavy and very beamy boats and somewhat overpowered for sailboats. Hey, you got me talking myself into it. LOL.
Older model Morgan's (Out Island series) are really inexpensive compared to most other sailboats. Lots of room inside

wow...some medieval thoughts on hydrodynamics....more homework is required

manyboats said:

Have you ever seen a sailboat converted to power?

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Here's one for sale out of Sequim WA. 36' steel hull. Nomadwilly was going to go down and take a look at it, but I haven't heard any report yet.

I really like it but I've decided to keep my Willard. I see in the picture she's perched right in the middle of the two waves just like a FD boat should and it looks good. Now that I'm going to be living back in Washington state the moorage is too high for a 30' so I' do'nt EVEN want to look at any longer boats. And I have no idea how long (or short) it will be before I decide $250. per mo is too high and get out of the game. But I really do like that boat. As I recall it was an odd layout inside though. A sailboat w shallow draft like a "drop keel" (I think that's what it's called) and also full (for a sailboat) at the ends like an Albin 27 would make the best trawler .......I think. A sailboat w/o her mast would probably pitch a lot if it had skinny ends.

Art,
Thanks for the nice words.
If your Tolly is economical at slow speeds it's probably due to her light weight but I do'nt think it's very light. Lighter than a Uniflite though. The Tolly's have a well proven medium vee hull narrower at the chine thus making them more easily driven. Tolly's are beautiful boats.

With a large-keel-for-a-powerboat vessel like that, she deserves a couple of sails. Otherwise, what's the purpose of the huge keel?

manyboats said:

I really like it but I've decided to keep my Willard.

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Great! We/I love your Willard.

We hav'nt needed sails for a long time. Keels are for boats. Keels give lots of directional stability and roll stability as well ...not to mention propeller and rudder protection. I would'nt want sails on Willy.....could'nt even get into covered moorage. You have a wonderful boat Mark but sails are an also ran in my book.

OK, Eric. Meanwhile I can enjoy less "rock and roll" and add a half or knot or so whenever I get the gumption to hoist my sails (with only a fraction of that boat's keel).

Rainha Jannota will be a full displacement with a D/L 347. The D/L (displacement / length) is the ratio that defines displacement, semi displacement or planning hulls. – hulls, not trawlers! The reason to state “hulls not trawlers”, is because G. Buehler boats, the dieselducks, are not full displacement by definition, but are definitely trawlers.
The D/L is the result of the following formula:
(Displacement in pounds/2240)/(LWL/100)3
The “experts consider that any hull with D/L above 260 is a displacement hul. Below 220 is a planning hull. What is left in the middle is a semi-displacement. A good design with nearly perfect hydrodinamics will evntualy kills this theory.

Regards

Fernando

Portuguese said:

......The “experts consider that any hull with D/L above 260 is a displacement hul. Below 220 is a planning hull. Fernando

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I have to disagree with this. A "displacement hull" is a function of shape. I'm sure I can come up with a box or a barge shape and have a D/L above 260.
The D/L has nothing to do with shape. If I take a Rectangular shape and put a motor on the narrow end, then the length of the water line is measured using the long side of the rectangle.
If on the other hand, I take the same shape and put the motor on the wide end (understandably not the normally desired configuration) then the length of the water line would be the short side of the rectangle.
The displacement had not changed since the boxes are identical. The D/L ratio will have changed dramatically which upsets the whole premise. And we are still stuck with a box or barge shape which is not a displacement hull shape.

I would think that the D/L ratio would be something used for speed calculations when comparing boats of the same designed water line length. For example, I am looking at 4 semi-displacement hull motoryachts of the same waterline lengths with the same HP. The one with the higher D/L would be the slowest. Not only is the higher numbered one heavier, requiring more HP to move it, but it will also have the greater wetted surface creating more friction in the water and having more water to push.

So, to me, the only useful purpose of the D/L ration would be to compare similar sized and shaped boats to determine which class I could put them into for comparison purposes, such as light cruiser, medium cruiser or heaver cruiser.

markpierce said:

OK, Eric. Meanwhile I can enjoy less "rock and roll" and add a half or knot or so whenever I get the gumption to hoist my sails

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If one is willing to do the work I think a sail assist on something like Mark's boat is a good idea. Fuel saved is fuel saved. If the stuff was $1.63 a gallon that would be one thing but at $4-$5 a gallon, knocking a bit off the bill--- or being able to use the same amount to go a bit further--- seems a good idea to me. If our boat could accommodate a sail rig and the boat's configuration was such that it could get some benefit from it we might be inclined to use it, particularly on longer runs when the wind was favorable.

Marin:

"If one is willing to do the work I think a sail assist....." In the open, as long as you are not changing course, there isn't much work. Wind direction usually dont change unless a front is coming through. The sails on a motor-sailor are relatively small and easy to handle. There are times when just being able to not rock are worth the trouble. In a river, especially with turns like most rivers have, it would be a lot of trouble to deal with and not worth it.

I'll stick with my little Kubota...
having that piggy screaming 'weeeeeeeeeee..waaweeeeee' in my ear is just not worth the fuel savings.

Having cruised on working boats and pleasure boats in open waters (various models, designs, sizes, and power-ratios) I found to restrict every boat’s hull design as belonging strictly to any level of a three tier category (FD, SD, or P) is simply not applicable in all instances (however it is in some). Having experienced boats categorized FD well exceed their “hull” speed without much extra power as well as boats considered SD to have broken into full plane without exorbitant power I copied the following from Wikipedia. I feel this best explains the notable per-hull-shape variables as well as the speed/efficiency potentials for hull-shapes. The hull category I have noticed that most often holds true to its definition/capabilities is P... and, in that category there are hull-bottom design details that can increase their high-speed-planing efficiency, such as “stepped” and other hull designs to further reduce wetted surface or to produce “skipping”. From what I’ve noticed each hull-design category holds a broad range of added potentials that can be developed upon by incremental design improvements. Being a person who designs and co-engineers mass airflow apparatus I appreciate the capabilities of what minor apparatus design adjustments can afford in the movement of fluids past stationary material items as well as the movement of material items through, over, or into fluids or gasses. Some material items we use in CFD tests are rigid configuration while some are flexible and/or expandable with capability to alter their shape in accordance with needs apparent to coincide with the properties of fluids or gases encountered. For water and air, many appreciable similarities exist between fluid-drag, fluid-release, fluid-bounce, fluid skipping, and fluid-deflection. High end CFD programs at times reveal unexpected fluid dynamic efficiency relationships between the design of material items and fluids or gasses. In my opinion, correlation between power ratios, water, and boat-hull-designs in fluid mechanics offer grand opportunity for fluid design research; as is consistently ongoing in the marine industry. Loving boating as I do maybe in future I will direct my efforts toward hull designs... just no time to indulge right now!

WIKIPEDIA:

Hull speed, sometimes referred to as displacement speed, is the speed of a boat at which the bow and stern waves interfere constructively, creating relatively large waves, and thus a relatively large value of wave drag. Though the term "hull speed" seems to suggest that it is some sort of "speed limit" for a boat, in fact drag for a displacement hull increases smoothly and at an increasing rate with speed as hull speed is approached and exceeded, with no noticeable inflection at hull speed. Heavy boats with hulls designed for planing generally cannot exceed hull speed without planing. Light, narrow boats with hulls not designed for planing can easily exceed hull speed without planing; indeed, the unfavorable amplification of wave height due to constructive interference diminishes as speed increases above hull speed. For example, world-class racing kayaks can exceed hull speed by more than 100%, even though they do not plane. Semi-displacement hulls are intermediate between these two extremes.

Hull speed is often called the "speed-length ratio", even though it's a ratio of speed to the square root of length. The concept of hull speed is not used in modern naval architecture, where considerations of speed-length ratio or Froude number are considered more helpful.

Froude number: Dimensionless number defined as the ratio of a characteristic velocity to a gravitational wave velocity. It may equivalently be defined as the ratio of a body's inertia to gravitational forces. In fluid mechanics the Froude number is used to determine the resistance of a partially submerged object moving through water, and permits the comparison of objects of different sizes. Named after William Froude, the Froude number is based on the speed/length ratio as defined by him.