Thread: The blunt or not so blunt end???

All of the above.
Except that the stern generally seems to dig down when the boat speed significantly exceeds the natural wave spacing for its length. The stern wave is left behind & the stern drops ingto the trough between bow & stern waves. At this point the boat is trying to climb up over its bow wave, and to go faster, you need enough power to drive the boat up-hill & out in front of its own bow wave...

follow links from http://www.unold.dk/paddling/article...tatistics.html for further elucidation.

I should add that generally accepted wisdom is that an efficient hull at displacement speeds is not so efficient on the plane. Likewise, an efficient planing hull is not so efficient in displacement mode. Particularly if one speaks of out-board powered craft, where the stern has to be bulky to support weight of motor. It also has to be strong to do so, & the bottom made stronger to resist pounding, and unplanned detachment of the strengthened stern... Which adds weight & makes the boat less pleasant to row or sail...
Conventional wisdom is that a planing hull works best with a long straight run aft for the boat to sit on whilst planing. A displacement hull tries to allow the displaced water to refill the hole it made cleanly & with minimum turbulence.
Exceptions to every rule of course, for example, modern sailing skiffs have the long flattish run aft to plane on, but when crew weight is moved forward, the boat is trimmed for displacement efficiency. Of course, you'd need to keep a motor on one of these things very small (weight) & very water-proof whilst sailing... ;-) You might also consider something like Bolger's "Light Schooner". http://www.ace.net.au/schooner/INDEX.HTM
cheers

I wouldn't pay much attention to the vast majority (there will always be exceptions) of aluminium or fibreglass boats - they don't have to be efficient because the wally buyers will fit the largest motor they possibly can anyway and so the boats are designed to be produced cheaply as opposed to working efficiently.

Richard

Following from Daddles comment about motor size, lots of boats have a broad square cut bum end to help carry the weight of the big motor, fuel tank and in some cases operator with tiller control engines.

Does not help a lot in making them seaworthy as such as there not a lot of reserve bouyancy to help avoid getting 'pooped' by waves approaching from behind, either when the boat is stationary or travelling slowly. Anything can be made to go like a cut cat with enough power, and shouldn't get pooped, but they have to slow down at some stage.

On the other hand there are the double enders, with virtually as much reserve bouyancy at the back as the front, and able to cope with heavy seas coming either way. But not really capable of planing operation.

Operating parameters such as means of propulsion, speeds, intended water conditions, loads, etc should dictate the configuration, but often style, looks, etc overtake these considerations.

Sterns ... one of my favourite subjects.

Pointy sterns can go like rockets on a light enough boat. Sailing Canoes are a good example of that, but in general MalB, you are right - you won't get a high top speed out of most canoe sterned boats.

However you do tend to get exceptionally high cruising speeds with properly designed ones as the excess wetted surface of a wide transom is reduced. So the boat finds it a lot easier to get up quite close to hullspeed.

For example many years ago David Wilson - the foreman at Duck Flat built a canoe sterned boat - Ethel - 13ft long.



It was around 160lbs for the hull and around 112sq ft of sail. Design is from George Holmes around 1890

I designed and built my first boat - Beth. Hull weight was 70lbs - sail 82 sq ft and dead flat on the bottom.

Here shown reefed at Clayton. I was heading out to give my friend on the Sailboard a good drubbing - which I could do if I was sneaky enough to tease him onto a broad reach - Beth is particularly potent on a broad reach - which is where sailboards think that they will have the advantage hehehehe.



In open waters Beth was so much faster - she probably can hit around 15 to 18 knots on a broad reach. Effel (as I call her to D Wilson's face) bogs down pretty badly at around 5 1/2 knots.

But in more variable conditions such as further up on the Murray River something interesting happens. Beth will get a gust and blast off at a million miles an hour and end up ahead by a couple of hendred metres. Then the wind goes light and I push her along as well as I can and Ethel just keeps getting closer doing maybe 4 1/2 knots in very light wind compared to my 3 1/2 in the light stuff.

Then as Ethel gets closer there will be another gust and I will get a couple of hundred metres ahead again.

As Kurt Vonnegut would write "And so it goes".

In general you are right Soundman. For a boat to be efficient at low and moderate speeds you need the transom completely above the water by a bit including when the boat heels a few degrees (though a little vee just touching the water in bigger boats won't impede progress excessively).

For good handling you need to balance the stern with the bow. If the bow is fine you need to pull some volume out of the stern a bit or you end up with something quite nasty to sail (the way of fast Nthern Hemisphere boats as the moment - who haven't yet noticed how Oz boats get the speed by reducing the volume for handling in sneaky ways. From our skiff classes and the Moths.

If you go and observe it is mostly paddle boats and rowboats that have completely pointy sterns. There is the occasional dinghy or yacht as well, but they are vastly outnumbered with properly designed transom sterns. The transom sterns do give more power to prevent the boat from heeling as much.

Note too that the fast canoe stern boats have trapezes or leaning planks so they don't have to rely on the hull for stability.

Convention can be a great guide even to the unconventional!

(Howzat - a story with illustrations!!!)

MIK

MIK

Originally Posted by Boatmik

Note too that the fast canoe stern boats have trapezes or leaning planks so they don't have to rely on the hull for stability.

Except for your "Beff"... ;-)

What ever happened to David's 'Ethel' ?

Hmmm very much As I expected then.

Hadnt considered the bow wake, stern wake concepts, but I can see what is going on there.... I'll think on that.

I would have to agree that lots of commercial boats are not particularly efficient.
Just look at some of the bigger stuff on sale....you need that 225 HP just to get it out of its own hole.

I read a review of a US style pontoon boat recently... the reviewer sait it was rather spritely rode a heavy chop much better than his bass boat.....Of course it rode chop well it weighs over a tonne, and yeh it would get along with 350HP up the tail .

we are seeing lots of fellas moaning about fuel prices......yeh they all got big heavy boats with 100HP pluss motors.

people who fish the inland impoundments are having to think a bit...many of them have internal combustion engine bans of HP restrictions.
so efficient hulls and sail is liiking good to me.

If I can get a fishing boat to do 10 knots under sail I would be happy man.

well see....I think a rrrrude conoe will be first.

cheers

Howdy Sound ...

And remember that part of the reason the boat needs 225hp is because it has the weight of 225hp on the back.

Part of the problem is that production 'glass boats have every edge trimmed in terms of saving material cost and labour cost.

Solid glass with stiffening of plywood - usually the cheapest available grade of ply too. And everyone knows now that ply in polyester/glass structures will get wet and will rot out. Might take a decade or two, so of no concern to the first or second owner.

So despite the dazzleflash of the deck mouldings they are very crude structures with almost no development of the actual hull shapes. Many of the hullshapes are the same as they were in the '60s though deck mouldings are changed regularly so the hulls can be presented as "new boats". The Holden/Ford trick.

So they are heavy, crude objects - and have to be because of our small market where they are all trying to survive.

This is what the Europeans are up to


We will see some shaking up in the next few years as power boat builders attempt to address the fuel cost issue. There will be a shift to new materials in an attempt to lighten the boats and later, some development of hullshapes that are more efficient and designed a whole heap lighter.

This will be complicated by people's expectation to go fast. It is possible to go just as fast on something like half the power currently being used by better hull design (incorporating a few "flats" in the hull shape for goodness sake) and much lighter construction.

The 225hp outboard is about to be a dinosaur except on large boats that can generate income to support that brontosaurus in the style (fuel quantity) that it is accustomed to - you won't be seeing them on a back of a 20 footer anymore soon.

In the very distant future there might even be more wood appearing in the structure, which though its price is rising (because of scarcity of the resource because of mismanagement in every country on earth) it is not specifically linked to fuel prices and is properly recyclable unlike plastics of all descriptions.

ie Plastic recycling doesn't bring the material back to its original state for reuse - but you can always grow more trees which recycle the carbon out of the environment again.

Best wishes
MIK

Fat butted pleasure craft are driven more by marketing team desires and current industry niches, then an efficient hull form. Wide sterns mean an aft cabin with a double berth in lengths unheard several decades ago.

Working craft typically are governed by load capacity and working deck space, then practicality.

If you pay attention to history, these trends follow oil price adjustments, which seem to make dramatic swings every 20 - 30 years or so, until a new "norm" is established. In the USA, there was a fuel shortage in the early and mid 1970's which adjusted then 20 to 30 year old averages (per barrel) up to a new standard. This was done by decreasing output, forcing a willingness to pay more, then a gradual easing of production limitations. Prices then stabilized (with a slight yearly advance) on the new price.

This has happened several times since the turn of the 19th to 20th century and we're experiencing yet another "adjustment", which will likely settle around $120 per barrel. This occurred in the pre WWII years, again in the 50's, again in the 70's again in the 90's's and now we're seeing yet another adjustment.

In short, oil prices, which seem high, actually aren't, particularly in direct comparison with other mechanical forms of propulsion. This will change, if the general belief is that oil will continue to double it's barrel price every couple of years. Most of those in a position to understand these trends, know full well, the market will not tolerate much further acceleration in price and has to stabilize. If not, alternative propulsion options will suddenly become competitively priced, likely to receive more research funding and the additional sales and research will lower the costs for these systems, further sweating the oil invested. Alternative fuels and energy sources received a similar boost in interest in the 1970's after the oil shortages of that era.

Unfortunately, our oil related infrastructure is well entrenched and dislodging it will require more then just a new energy source. The oil related industries have to be able to get "on board" or world wide economic collapse could result. Imagine if you will, a brand new energy source, yet announced. One that doesn't pollute, is essentially free, produces 10 times more HP per dollar then diesel and costs 90% less per gallon. Great news right? Nope, the world would fall into a huge depression with the loss of oil revenue and the related industries, like automobiles, trucks, aircraft and boats. You just can't slam the door on that many industries and expect things to smell pretty in the morning. As much as half of the major industries in western society are heavily related to oil in some way. Enough of a connection that the removal of oil from the mix, in spite of having a better choice will crush these industries. Fleets of aircraft that have to have new engines and fuel delivery systems, ditto cars, trucks and boats. Even the small little label printer on the corner will shut their doors, because the contract they had with Shell oil to print 93 octane labels for gas pumps is no longer needed.

Back to boat shapes. Because fuel prices have been reasonably stable in recent years, tossing a few hundred HP at a boat was just a part of ownership, in fact a source of pride among some (mine is bigger then yours . . .)

I've been playing with a displacement hull that is particularly efficient at S/L ratios up to 1.4, where of course she runs smack dab into the wall of her own wave train. She's a narrow hull, elliptical stern, which is near double ended below the LWL. Just over 1.3 S/L she begins to squat and power requirements rise dramatically as efficiency drops off. I've placed a wing, under her stern, forward of the prop, which comes to bare as she reaches S/L 1.3, preventing her stern from falling into her self created "hole", which keeps her head down and knifing through the bow wave which is moving aft. This permits her to develop a dynamic relationship with the aft moving bow wave and she lifts up bodily, decreasing wetted area.

Currently I can get this hull to S/L 2.8 with reasonable efficiency and do so through the full range, from slow displacement speeds to moderate plane speeds. Nope, it doesn't have a full size double berth in an aft cabin, nor could it support one, but it sips fuel like a bird.

There are options, but much of these are like being a preacher and keeping all the parishioners awake during the sermon, without using fire and brim stone, let alone being taken seriously.

Originally Posted by PAR

Currently I can get this hull to S/L 2.8 with reasonable efficiency and do so through the full range, from slow displacement speeds to moderate plane speeds. Nope, it doesn't have a full size double berth in an aft cabin, nor could it support one, but it sips fuel like a bird.

This parishoner is awake... Please, speak more of this wondrous vessel !!
Sounds like an improvement over even Bolger's planing power sharpies (Sneakeasy & Idaho, etc)
Pics ?
Funnily enough, both MIk & I had the same thought regarding the little wings I've tacked onto my Teal - Maybe they could turn it into a planing hull. I don't think so - placement is wrong & I'm too fat.
It sounds like you are further down this path in practise than our dreamings.
cheers
AJ

I quite agree with most of what you say PAR (as per usual).

However the transition is already underway with the big petroleum companies making serious investment in alternative technologies.

They are discovering that they are in the "ENERGY" business and not just the "OIL" business that they thought a comparatively short time ago. British Petroleum (BP) was the biggest manufacturer of solar cells in the world a few years ago, but now is in about 3rd place. Big companies are pressuring governments to make changes rather than it just be a smattering of environmental groups.

A lot is going to change. Including social pressure on people with 225hp outboards.

You can sort of be discreet that your car develops 225hp and consumes fuel to match but it is rather hard to hide it when it is written all over the outboard cowling.

4.5 times the fuel consumption of a 50hp.

I'll be betting that we will start to see Outboards sold on the basis of cubic capacity real soon - so they can hide the direct link there is between horsepower and fuel consumption.

Keep up the good work with the hullshapes PAR! I'll keep working on it too. Plus reminding consumers that there are other ways.

I once sailed from Adelaide to Sydney is a relatively slim and light cruising yacht. We ended up motoring most of the way. Took us around 10 days. Cost $100 in diesel split 3 ways and we had a nice time.

When talking about slim and light boats I always seem to linger on an example of sitting in one of the Bolger Tennessees that was launched in this neck of the woods. We were able to motor at around 8knots at 75% revs of the Yamaha 4 stroke. Passengers sitting in the cockpit, playing cards and reading newspapers. Thought is that you can take turns driving over a 4 hour period and get around 36 miles up the river.

The 225hp demon would do it in a bit under an hour. But what state will the crew of the faster boat be in?

I know it depends on what you are trying to achieve - if it is just to "get there" a car is much, much better. But if the purpose is to travel by water ...

MIK

I tell you what is a thing to behold..... one of the brisbane "city cat ferries".

I had a close look at a couple of these things underway and starting & stopping at the pontoons a few weeks ago......I now see what all the fuss is about.

these are a big boat and they have a pair of pretty solid diesels pushing them, but man they get along and accelerate very respectibly.

When you consider they are a boat about the size of a decent middle class house, they are more amazing.

they sit on two very narrow very pointy hulls with an almost vertical bow.

interestingly the sterns ( there are two) are (what I can see) square.

I realy must look further... I'd realy like to see what they are like below the water.

the thing that is realy striking is how little wake they leave, mant they must be slippery.


Id have say on the subject of lousy hulls.....I have seen a workshop dustpan that looks like a respectable hull shape in comparison to some....( yeh we dod have a waterskier at one time that had a stunt with a shovel)

on the subject of mor efficeint pleasuer boats I recon something like the "Fish Cat" is a posible alternative to our floating roof tiles.
www.common-sense-boats/fish_cat.htm

cheers

The tobacco companies in this country have "divested" their interests into other, less politically sensitive areas. Most only carry 25% of their annual net in tobacco now. I suspect the oil and related industries are beginning to believe their own 20 year projections and are spreading out their money to absorb the oil interests when they make the inevitable turn. In the USA, we're pumping the last of the easy to get stuff from the ground now, meaning it'll cost much more for the same number of daily barrels in less then 10 years. From a business stand point it's just good sense to divest and move into new growth industries, possibly capitalizing on the infancy of some and forming another energy monopoly (which is what the oil companies have currently) on western society.

About my experiments in extending displacement hulls into moderate plane mode. The current hull would be a fine sailing hull of relatively narrow form. 33' LOD, 29' 6" LWL, 6' 3" WL beam, V bottom, but uniquely shaped for lots of bearing aft and a very fine entry. Her deadrise aft is just a few degrees. The beam is brought aft a little more then usual, but not enough to make a triangle shaped LWL.

The outboard is mounted is mounted in a covered well, with just enough clearance to swing clear of the transom in the up position. A small cutout is provided for the lower leg and prop wash. The wing which is about 4' 6" in span and has a 12" cord is mounted on two foil shaped legs, exactly the maximum depth of the hull underbody. This places the wing below the prop a few inches.

The wing's pitch is adjustable with two worm screws and a ring gears mounted on the wing (trailer jacks incased in the legs) The worms work in unison (which took some trial and error to remove twist from the wing) with a set of linked cranks.

In displacement mode the wing is left neutral or permitted to follow the flow by releasing the worms and letting the wing "run free". In this mode the wing moves up and down gently with the general motion of the boat and surrounding sea state.

As I approach 1.35 S/L I introduce positive inclination, usually progressively, of just a few degrees. This is enough to keep the stern from squatting and not bleed off any speed from the additional drag. The flow seems to be pressurized in this area of the hull and it's trying to reattach to the bottom and 2 - 3 degrees helps align the flow. The stern begins to "fly" at this point and with greater applications of power. Initially I need to crank in a fair amount of positive , but once the bow "pops through" I back off to about 3 degrees and the bow wave is at about 1/3 the LWL aft of the stem. She's in full plane mode, but the boat is actually riding on her V'd forward half (the dynamic portion of the formula) and the wing, with the chines well clear at 12 knots. Because the wing has no dihedral, she tracks very well and stays level, even in turns, which is a bit weird at first. A few degrees of dihedral would permit the lean we all know from powerboat turns, but I'd loose a little efficiency on the wing.

Foil shapes and locations, I ain't talking, amount of wing area to wetted surface, ditto, but the concept is pretty simple and not a new thing by any stretch. I am able to get a fairly large boat on plane with very modest power and fuel savings are incredible. Of course I have to live with a narrow and lightly built boat, but if you knew how aircraft were made, you probably not want to fly again either.

Howdy PAR,

Is this conceptual or launched? If still conceptual, I am really looking forward to launch reports. having a proper stern foil makes an awful lot of sense from a trim perspective - a decent size one will make a lot more difference than those mostly toy ones they fit to outboards (it is a good concept but hasn't really been stretched yet).

And to know the differences in speed with and without the foil deployed would be great - and a selling point. Trim tabs try to do the same thing but are very draggy for the amount of lift compared to a foil.

Good stuff.

Michael

It's splashed Michael and has been a test bed for several ideas I've been working on. Without the wing, she's captured within the speed limitations governed by her LWL. With the wing left free, she has the same issue, though her wake appears "flatter", likely a slight flow redirection around the "floating" wing. With 1 degree of positive pitch, she squats about half as much as she would without her up to S/L's of 1.5, where she then begins to squat pretty good. With 2 degrees she doesn't squat and holds her stern up nicely, but has difficulty piercing her bow through unless you have a follow sea and can time a wave to initiate surfing the forward sections. At 3 degrees she'll stick her bow out like a buck toothed kid in a candy factory and she accelerates rapidly up to 2.1 where I have to back off the wing pitch to gain more speed.

It seems she likes a process, wanting a little pitch initially to keep her butt up, then more to get the bow through, then slightly less to max out the 30 HP I have hung on her. I've managed her over 3.3 in very flat, no wind or current water, but she's laboring above 3.0 and much less efficient over 2.8. Prop selection could increase top end, but I'd lose displacement efficiency. Basically I can't have my cake and efficiency with a fixed pitch prop. I suspect she do much better if I used a variable pitch prop, maybe a hydraulic setup, but the budget is maxed for speculation and experiments on this boat.

I've learned a lot off her, including a short stint under hybrid electric power. She's also worn a new idea I've had for moderate aspect ratio rigs, a few different rudder and board ideas and now I'm thinking about a forward wing, with dihedral to mimic the deadrise so it can retract nearly flush with the hull. This much lift will require a real jack plate instead of my home made job and the "one who must be obeyed" has determined I've dabbled (her words) enough on this hull and wishes for "something pretty" (her words again, which seemingly have no end). She's going to be when she finds out Pete and I are working on a new racer to beat a standing record next spring. It isn't pretty, but it'll be God awful fast, if we can keep it upright. Maybe, I put a new coat of house paint on the test bed and officially name it after her mother, just before Pete announces the new boat (he never could keep his mouth shut for very long).