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Old 04-10-2012, 01:45 AM   #61
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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.
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Old 04-10-2012, 02:24 AM   #62
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Marin,
Sounds like you did'nt realize the GB was so flat and straight aft.
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.
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Old 04-10-2012, 08:36 AM   #63
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Full displacement, I assume.
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.
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Old 04-10-2012, 08:58 AM   #64
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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.
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Old 04-10-2012, 09:31 AM   #65
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Paravanes are the delta shaped devices which are attached to the outriggers via wire rope to stabilize the boat. .......
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.

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Old 04-10-2012, 09:46 AM   #66
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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.
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Old 04-10-2012, 10:24 AM   #67
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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.
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Old 04-10-2012, 10:47 AM   #68
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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.
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Old 04-10-2012, 12:06 PM   #69
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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.
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Old 04-10-2012, 12:17 PM   #70
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Quote:
I question 6nmpg. My Willy only gets 1.My 30' trawler needs 20hp to cruise a bit over 6 knots.
So Eric.. When your 20hp is getting you 6 knots, you're burning 6 gallons per hour? You need to repower.
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Old 04-10-2012, 01:01 PM   #71
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Here's a shot of our boat on the hard. She's really a sailboat!!!
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Old 04-10-2012, 01:13 PM   #72
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Chip ...........

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

DUHHHH sorry.

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Totally FD w a very steep QBBL. I'm sure she's VERY easily driven.
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Old 04-10-2012, 01:20 PM   #73
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.....Tony B,
I question 6nmpg. My Willy only gets 1...................
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.
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Old 04-10-2012, 01:20 PM   #74
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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.
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Old 04-10-2012, 01:33 PM   #75
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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.
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Old 04-10-2012, 03:21 PM   #76
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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
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Old 04-10-2012, 03:30 PM   #77
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Tony,
... I need to engauge brain before mouth more frequently. .......
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
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Old 04-10-2012, 07:30 PM   #78
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wow...some medieval thoughts on hydrodynamics....more homework is required
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Old 04-10-2012, 07:55 PM   #79
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Have you ever seen a sailboat converted to power?
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.
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Old 04-10-2012, 10:35 PM   #80
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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.
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