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Marin wrote:
I think semi-displacement is a totally bogus term thought up by marketing people who wanted to associate their boats with the supposed ruggedness, seaworthiness, etc. that "displacement" conjures up. Same marketing mentality that applied the term "trawler" what in reality is a cabin cruiser.

Displacement is like dead. You either are or you aren't. You can't be semi-dead, nor can a hull be semi-displacement.
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*OK, OK.* Using Bruce's definition, the Coot is a "medium displacement" boat.* Wish we had started with common definitions.
 

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" Boy I can't wait to go 18knots."

GE makes a great turbine that would provide the required power.

Your fuel mileage may vary!
 
We've got a full displacement hull on our 58' Hatteras LRC. Hull speed of about 9.5 knots, but we've only seen that on a couple of occasions with a strong helping tide. :)
 
We have a full displacement Willard. Maybe a little "fuller" than most Willards because ours is the trawler on a 8 ton cutter hull. ;-)

Keith Olive
La Conner
1977 WIllard 30/4
 
kolive wrote:
We have a full displacement ..... because ours is the trawler on a 8 ton cutter hull. ;-)

Keith Olive
La Conner
1977 WIllard 30/4
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*Should be very fuel efficient. I'll bet a 12 year old can push that 8 tons around with no problem.
 
kolive wrote:
We have a full displacement Willard. Maybe a little "fuller" than most Willards because ours is the trawler on a 8 ton cutter hull. ;-)
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*I'm sure Eric would like to chime-in here and be counted among the full-displacement group, but I sense that his conservative views with regard to HP and hull design may be only to mask Willy's true capabilities.
 

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healhustler wrote:kolive wrote:
We have a full displacement Willard. Maybe a little "fuller" than most Willards because ours is the trawler on a 8 ton cutter hull. ;-)
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*I'm sure Eric would like to chime-in here and be counted among the full-displacement group, but I sense that his conservative views with regard to HP and hull design may be only to mask Willy's true capabilities.

Wow! And to think Willy can do that with only 36hp!
 
As I think has been said in this thread, semi-displacement is kind of a misnomer. All boats have displacement. The equation for volumetric displacement, which is the symbol for Delta in an inverted display, is the function of Delta, the weird s shaped line, times Delta. The function is substituted with the mass volume for salt or fresh water. Salt is 35 ft cubed over Long Tonnes, and fresh is 36. This sum can then be used to find all of the coefficients-Block, prismatic and waterplane. You can also find the midship section coefficient. Full displacement hulls are flat on the bottom with a radius at the bilge strake. They generally have vertical side shells with a slight flare at the sheer strake. Semi-displacement hulls have chines that help the boat "lift" itself out of the water as it speeds up, thereby having less hull in the water which leads to less volumetric displacement in the water. This is the time when a boat is semi-displacing it's own structural volume. I hope this helps.
 
FF said:
Simply look at the stern.

The width of the transom at the WL in feet will be close to the speed expected in K.on a 40-50 ft boat.
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The waterline of my sailboat, as with most full displacement sailboat hulls, comes to a point at the stern at the waterline. 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?
 
Tony B said:
The waterline of my sailboat, as with most full displacement sailboat hulls, comes to a point at the stern at the waterline. 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?
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Actually, those lines at the stern are made to reduce the amount of energy from making waves, thereby reducing the amount of energy needed to propel the boat through the water.
 
Makes me wonder what the effective horsepower is generated from sails on the typical 35-to-40-foot sailboat produce with winds of 15 knots or so.
 
Anthonyd said:
Actually, those lines at the stern are made to reduce the amount of energy from making waves, thereby reducing the amount of energy needed to propel the boat through the water.
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I don't really know what lines you are referring to.

The stern coming to a point at the waterline reduces drag from eddy currents produced by a flat stern. Very similar to how a hatch back car produces eddy currents on the rear window most noticably by the dirt and soot it collects from a vacuum like force much more so than a sleek, sloped and tapered window.
A full displacement hull is designed to slip through the water with the least amount of effort. The downside to this is a very limited speed. A planing hull is designed to lift itself out of the water and go like a bat out of hell with the limits of speed being the engine size. A semi-displacement hull design is somewhere between the 2.
 
markpierce said:
Makes me wonder what the effective horsepower is generated from sails on the typical 35-to-40-foot sailboat produce with winds of 15 knots or so.
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Good question Mark. I have a 37HP Westerbeke in my boat. It is 39' and weighs over 10 tons empty. At about 16 to 1800 RPM I will go about 6 kts and at that sustained speed I will burn about 1 gal/per hour. At 5.5 kts I will burn about .75/GPH. I can easily do these speeds at 10-12 kts of wind.
At 15 kts of wind, I can do about 7.5 kts. with 90 gals of fuel and 150 gals of water.
This is done with worn 20 year old sails.
Maybe someone here can do the math with what info I provided.
Keep in mind that I can sail faster than I motor.

I think the simplest way to come up with the answer to your question is for me to find the total sq footage of the 3 sails using the 15kts of wind as the other constant. I'm too lazy to look this all up right now. I just finished working for the day.

The interesting thing would be the answer. I would suspect that the sails generate more HP than most would expect and the sailboat reuires much less HP than most would suspect.
 
Tony B said:
I don't really know what lines you are referring to.

The stern coming to a point at the waterline reduces drag from eddy currents produced by a flat stern. Very similar to how a hatch back car produces eddy currents on the rear window most noticably by the dirt and soot it collects from a vacuum like force much more so than a sleek, sloped and tapered window.
A full displacement hull is designed to slip through the water with the least amount of effort. The downside to this is a very limited speed. A planing hull is designed to lift itself out of the water and go like a bat out of hell with the limits of speed being the engine size. A semi-displacement hull design is somewhere between the 2.
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Sorry Tony, I wasn't very specific. I was referring to the shape of the hull and the way it tapers from aft of mid-ships to the stern. I say lines as a reference to the design plan. Full displacement hulls are also designed to achieve a balance between moving through the water and stability. Semi-displacement and planing hulls are made more towarde the speed factor.
 
It takes roughly 17K of breeze to create 1 lb of force per SQ ft of sail.

So going down wind 750 Sq ft would create about 750 lbs of push (more because of the hull and rigging) so lets sat 1000lbs.

A cheap prop and not fancy drive setup will create about 20 lbs of push for each HP delivered to the prop.
Bigger bucks and better engineering will get close to 25lbs per hp.

1000 divided by 20 is about 50 hp worth of push.

17k will easily push most sail boats to "hull speed" , and 50 hp will be 2.5 to 3 GPH with most marinized engines.

Most folks would happily give up a K and cut the fuel burn in half. Give up 2K and even better range is possible from the same tankage.

These are all rather crude rules of thumb , but are fine for noodeling.

The first edition of Passagemaker has better tables if you wish to plan a fuel burn for a voyage with a known boat.

FF
 
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FF said:
It takes roughly 17K of breeze to create 1 lb of force per SQ ft of sail.

So going down wind 750 Sq ft..................
A cheap prop and not fancy drive setup will create about 20 lbs of push
..............1000 divided by 20 is about 50 hp worth of push.

17k will easily push most sail boats to "hull speed" , and 50 hp will be 2.5 to 3 GPH with most marinized engines.................These are all rather crude rules of thumb , but are fine for noodeling............
FF
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Those rules of thumb you quoted are not quite as crude as you might think.
My sailboat is 39'. Factory supplied sails are 700 Sq. FT
My boat is smaller than the one in your example and my engine is 37HP.
Most sailboats my size and just a few feet longer do fall into the 50HP range.

The only reason I dont travel near hull speed is because the engine noise is deafening. At 5.5 kts, you dont hardly hear my engine outside but it is noisy inside the boat. At 6 kts it sounds like 2 skeletons **************on a tin roof.
My theoretical Hull Speed is around under 7.5 kts Most sailboats will reach their theoretical hull speed at 12 to 15 kts of wind.
17 Kts of wind speed is usually way too much for most sail boats to travel without having to reduce (reef) their sails.
There are also other factors that come into play with sailboats but are not all that relevant to this topic.

All in all, your calculations are very close.
Thanks for the info.
 
ah wrong

koliver said:
The ultimate semi-planing hull is the Canadian Navy Frigate, Halifax Class. It pokes along under diesel power, to 16 knots, displacement speed. When the gas turbine engine is used, it planes at 40 knots.
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Actually the canadian patrol frigate has a cross connect gearbox and the ability to run on either a single cruise diesel to a maximum of 18 knots. At this speed it is using 1.7 cubic meters of fuel an hour. When the gearbox switches over and the gas turbine is used the single engine produces 26 knots and a fuel burn of 5.55 m3. At this pt the second gas tubine can be brought online and the combined speed of the two is 31.8 knots which is the rated speed of the canadian patrol frigate. Of course this is also a 4800 ton ship. The power is there to drive it faster but it is subjected to the gearbox. As for displacement speed well it is four hundred feet so that comes into the ratio.
 
Don,s east bay has a bit of displacement character because of it,s deep deadrise. If you had 45 Degrees of deadrise the boat would run even more like a displacement hull but would still be a full planing boat. The quarter beam buttock line angle tells the story. If 100 percent of the transom is out of the water at rest it,s a FD hull. If giv,in enough power a planing hull will run 40knots. Semi-Planing hulls can,t qualify as either full planing or full displacement. Almost all Trawlers are semi-displacement. Most or all DeFevers are semi disp. Many trawler owners think their boats are Full disp. A full disp.boat will have almost all of her transom out of the water at rest. Just trying out my i-pad. I,ll take it on the boat and pop up now and then.
 
The hulls speed for a 300 ft boat is 17 to to 25 knots. That is why large ferry’s ships can go so fast. They can be pushed faster with a lot more HP. It does not take that much HP to push a boat through the water at full displacement speed. We/I calculated based on the fuel burn that the Eagle, 51 ft on the water and 40+ ton uses about 75 hp at 1500 rpm, 3 to 5 gph. The DD 671 total Hp is 165. The Eagle hull is tapered toward the back so as the speed increases the bow tends to raise out of the water creating a larger bow/hull wake with very little increase speed. To bad we lost our hiastory as there where some good discussion about hp required. :ermm:


I read this morning that the Washington State ferry’s are going to convert to Natural Gas as the price of diesel keeps going up and the price of Natural Gas has gone down. The tanks are going to be installed on the roof which sort of surprised me. Be interesting how they convert the diesel engines or if they replace them. Probable have to replace them.


I did talk to a local rigger about adding sails to the Eagle when I heard it had been done on a sister boat. He figured about 1,000+ square ft of sail would be needed at a cost around 30+ grand, so the discussion was very short. I don’t understand people think that over all cost of a sail boat is cheaper and when out sailing its free. I mean sails and rigging are not cheap. :confused:
 
I did talk to a local rigger about adding sails to the Eagle when I heard it had been done on a sister boat. He figured about 1,000+ square ft of sail would be needed at a cost around 30+ grand, so the discussion was very short. I don’t understand people think that over all cost of a sail boat is cheaper and when out sailing its free. I mean sails and rigging are not cheap.
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And, sails and rigging wire and line must be replaced as they wear out. A full time cruising sailboat probably needs new sails after 5 years.
 
May I recommend an older 70’ to 80’ wooden schooner that has top sail and flying jibs with tall wood masts and wood boom and of course with oodles of varnished bright work all over her superstructure... so there is plenty of relaxation due to reduced engine upkeep and savings due to truncated fuel usage... as compared to trawlers, that is. Not!

No offence meant to sail boats or their owners. Just fact about boat styles regarding efforts and $$$ spent... portrayed in sardonic tongue-in-cheek manner.

PS: With sail boat one must also remember to plan way ahead on any IWW journey to be sure and call-ahead or completely miss bridges.
 
The Coot is Full displacement. It,s not unlike some Seaton designs. A very small amount of submersed transom but a quarter beam buttock line that is quite steep. The QBBL is an imaginary line half way between the keel and the Chine. The angle of this line reveals the hull type. There is a specific angle in degrees that is considered a norm that separates the three types of hulls. I. Don,t know the actual numbers. But it would,nt be easy for you to measure the angle on your boat anyway. The TT,s are, as far as I can recall all semi-disp. Ditto the CHB 34's, the DeFevers, the GB,s and many others. Willard,s and Krogens are Full disp except the Krogen Express. Mainships and Camano,s are planing hulls. There are some that are hard to classify like the GB. I,d say they are semi- disp because of how they perform but a case for planing hull could be made by their fairly straight QBBL and very submerged transom. Show me your stern when hauled out and I,ll give my opinion on your hull. I think I have a good understanding of this but of course it will just be an opinion. That,s what forums are for ....right. Opinions. Or I better get the fluck outa here.:ermm:
 
manyboats said:
There are some that are hard to classify like the GB. I,d say they are semi- disp because of how they perform but a case for planing hull could be made by their fairly straight QBBL and very submerged transom.
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The GB hull is semi-planing. Add enough power and you can get the thing to mush along on something of a plane a good five to seven knots above hull speed. But with the weight and the big keel I don't think the boat is large enough to hold an engine or engines powerful enough to fully plane the boat.

American Marine initially chose the semi-planing hull even back when they didn't put enough power in them to take advantage of the speed possibilities because of the stable nature of the ride. Or so they said in all their literature and brochures at the time. No mention was ever made in the 60s and 70s of being able to cruise the boat fast. With one or two FL120s in them, "fast" was not really an option. But AM made much of the hull's stabiity and efficiency. As I've mentioned before, AM never used the term "trawler" in describing and marketing their Grand Banks line of boats. To them, a trawler pulled a trawl net. The Grand Banks slogan was "Dependable Diesel Cruisers" and their literature spoke only of comfort, stability, and efficiency.

Even with one or two FL120s in them, the GBs of the 60s and 70s normally cruised at a wee bit above hull speed. Some of the photos of Spray, the 1963 prototype for the first GB model, the GB36, show her moving along at a pretty impressive clip. But speed was never anything the company marketed until the early 80s based on all the GB literature I've read over the years.

As customers began wanting more speed out of their GBs starting in the 1980s, AM and then Grand Banks, LLC began installing larger and larger engines to take advantage of the GB hull's ability to semi-plane. So you ended up with GB42s with a couple of 400+ hp engines in them. They can do 15 knots or so, but burn about 25 gph to do it.

We just had our boat hauled this morning for a long-overdue bottom paint job (every time we scheduled one Boeing sent me off to some other part of the world). Looking at the boat out of the water, that hard-chined, flat afterbody that Eric speaks of as being an essential component of a planing hull is very evident. But that massively deep forefoot and large, full-length keel makes it pretty obvious that the only way our hull is every going to achieve anything resembling a full plane is for us to bolt a GE-115 turbofan (115,000 pounds of thrust) to the top of it :)
 
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Full displacement, I assume.
 

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GC - That's a darn pretty hull. Great entry edge! With correct engine bet she just sips fuel at hull speed or just below. Looks like displacement to me... even with a just touch of flatter bottom in water at rear.
 
Thanks Art, uses about 1 gallon/hour at 1600 rpm. But, that round bottom is damn uncomfortable in 2-3 foot beam waves. Hence, going back to the paravanes.
 

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manyboats said:
The Coot is Full displacement. It,s not unlike some Seaton designs. A very small amount of submersed transom but a quarter beam buttock line that is quite steep. The QBBL is an imaginary line half way between the keel and the Chine. .... Show me your stern when hauled out and I,ll give my opinion on your hull.
Click to expand...

img_82157_0_677222ace3f1b05ba863ca3d8d4a4476.jpg
 
Marin,
Sounds like you did'nt realize the GB was so flat and straight aft.
Gulf Comanche,
Full Disp to be sure .....but a fast FD hull as it looks like there is a bit of submerged transom and that lowers the QBBL angle and makes it faster. You've got a FD hull but not quite "pure". You should be able to cruise faster than a full Disp hull of the same length. No doubt it could use the stabilizers. So could my Willy.
KJ,
A planing hull w a big keel very similar to the GB but w a lower QBBL. Your boat should be faster than the GB but less efficient at slower speeds.
Mark,
Your QBBL is a bit flatter than I remembered and there is a bit more submerged transom. Too close to a FD hull to call it SD so I stand by my original assessment of a FD hull. I'll bet the designer would agree.
The 34 Mainship actually has a negative QBBL angle and is a planing hull. The bottom is referred to as a warped bottom. The bottom keeps getting flatter as it goes aft all the way to the transom. The opposite of a constant deadrise hull like a "deep vee". A problem w this type of hull is that most of the stability is realized from the very aft end of the boat. They are level riding and quite efficient as well. The helmsman needs to do a lot of work in following seas but this hull type is very level and smooth riding with a minimum of pitching. Next?
 
manyboats said:
Mark,
Your QBBL is a bit flatter than I remembered and there is a bit more submerged transom. Too close to a FD hull to call it SD so I stand by my original assessment of a FD hull. I'll bet the designer would agree.
Click to expand...

Thank you, Doctor Eric.
 
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