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

I have consider the concept of not having the outboard hanging off the back but a little further up the boat.

I notice the sneak box has a pair of trailing sternlets. How does this work in practice.

Oh
If you are interested in corporate interests in the US, check out a book called " Toxic Sludge is Good for You"..... Its a good read.

cheers

Originally Posted by soundman

I have consider the concept of not having the outboard hanging off the back but a little further up the boat.

I notice the sneak box has a pair of trailing sternlets. How does this work in practice.

Oh
If you are interested in corporate interests in the US, check out a book called " Toxic Sludge is Good for You"..... Its a good read.

cheers

Whilst looking at Bolger, you might like to consider his "Slicer" - a 28' runabout designed to "cleave the chop elegantly" on a low horsepower outboard ... think of it as a 14' tinnie with a 14' pointy bit added to the front.

Current displacement is about 4,600 pounds.

The "ears" that bracket the engine well, do affect the wake a fair amount. Because the deadrise in this portion of the hull is essentially flat, flow twisting up off the deeper V'd belly of the boat, is smashed down and splayed outwards. The "hole" between the two ears, tends to "suck" the tops off the wake crest. This knocks down the curl that appears as the stern wave resurfaces aft of the transom, smoothing out the wake. I don't think, with this minimum deadrise it has much effect on efficiency over S/L 1.5.

The general hull shape is very narrow warped bottom. This shape was selected because it lifts it's bow quickly and has a large, relatively flat planning surface. Of course it is speed limiting as the drag comes to bite and restrict the top end. On the other side of the coin, the hull rises rapidly, so the only draw back was to keep her stern from squatting too badly, which permits her bow to penetrate the surge wave and once she does, it's like a turbo kicks in. She rapidly transitions from 1.3 to 2.0, usually forcing me to panic a touch as I adjust the wing.

When you look at the problem from several angles, you come against a bunch of walls. Efficient hull shapes at displacement speed, truly suck on plane, moderate plane shaped hulls, drag their butts at displacement speed, but shed the stern wave once past 1.5, relieving the burden of this pile of water. Typical displacement hulls squat, but the "inclined" narrow hull forms, of the early parts of the last century addressed this, accepted the squat and used it to advantage.

This is what the wing does. The boat still squats, but the water is effected more then the boat's attitude. The flow under the aft portions of the boat is accelerated, making the prop think it going faster then it really is. The pressure also increases on the aft quarters, primarily because of the foil shape in the wing. Then of course, there is the lifting element of the wing itself, which is more attitude adjustment then actual displacement reduction. The net result is I'm tricking both the water and the boat into faster speeds then would normal be possible. Because the stern just kisses the LWL, displacement speeds are efficient and the same is true of plane mode, because the crisp edge of the transom is in contact, a clean release is provided.

A few designers have fooled around with these "low power" high efficiency hull shapes. Phil Bolger comes to mind. He is a particularly clever user of warped and flat bottom forms to gain plane mode efficiency. The most efficient hull to plane is the flat bottom, but like all hull forms, it has it's draw backs. The double wedge or warped bottom form is a bit of a transition shape that is essentially flat bottomed aft, but incorporates a V entry to slice through chop (easing the flat bottom's flaw) and more importantly creating an effective dynamic relationship in the "transition zone" where the V quickly turns into flat bottom.

As a test bed hull, I needed a hull shape that would show me differences quickly. Rig and appendage differences are easily found on a narrow form with limited initial stability. You can remember how far out you had to hike with a previous rig or appendage in a similar wind strength, providing instant feed back. The same is true in planning mode. weird things begin to happen to power boats as they get narrower, mostly they yaw and pitch, which signifies a chine digging or trim issues. It's also a lot cheaper to build a skinny hull too.

The boat is about as butt ugly as they get. I receive a lot of comments about how bad she looks. The bottom is smooth, with a few gallons of filler and epoxy under seven or eight layers of clear polyurethane. It looks like a hodge podge of colors and fairing filler, but it's shiny. The topsides and deck have received much less attention, with very little filler and only 4 coats of poly over the epoxy. Taped seams are clearly visible as are the ink stamped plywood marking labels, which never got sanded off. It's a test bed, not a harbor queen. She's finished bright so I can see the seams and fasteners. I will instantly notice if a fastener is working loose or a seam has opened or water is getting beneath the coatings, which paint would hide. The inside of the boat has a couple of fore and aft benches and now a single athwart just aft of midship. Other then a few coats of polyurethane, they've received no special attention, still having the planner blade marks on them. It's a test bed and uses much less then a gallon per hour at 2.7 (14.66 knots) and just over a gallon at 3.1 (almost 17 knots). I can run her up to 18 knots, but just below 17, she gets pissy about it and starts boiling her water and drinking heavily (much like her daddy some times).

Terribly nice to see you strut your stuff PAR!

Thanks a heap.

To be hitting those speeds with that power and weight is rather remarkable to say the least.

Also resolving the problems of trim going crazy (nose up) which of course is the limitation of trying to make slow boats fast or fast boats slow with efficiency.

Quite a novel approach - though I was being tempted by some people last year to try something similar with a competition kayak or surfski. Do you think there would be some advantage in situations where the powering is just enough to exceed the S/L of 1.35 - such as with human powered vessels. I imagine the difference becomes greater when the powering available to get up to 2 or 3 is available.

MIK

MIK

I use to think the performance I was getting was remarkable, but have now toned down my enthusiasm, with the understanding I've just rediscovered something, lost after power to weight ratios came down to reasonably modern standards.

At the end of the 19th and in the early 20th century, powerboats were beginning to ply the waterways. 300 pounds per HP weren't uncommon so particularly efficient hull shapes were needed. Nat Herreshoff and others started working on the problem and developed very narrow, displacement shapes that could be driven over 1.3 S/L, some actually planning.

Historically, Herreshoff has been credited with the wide stern to prevent squatting, but looking at his designs, he clearly preferred the narrow, double ended form (inclined bottom type), to the "draggy" wide transom model.

Once engines got down to 100 pounds per HP, then things got interesting. Now they could plane off, if the hull was sizable enough to support such bulk and lightly built enough to get up. In late 1876, Herreshoff got the commission to design, (and his company built them) a fast torpedo boat for the US navy. The design was the first of it's type in the new fangled material (steel) and it pulled down 17 knots at sea trials. It was about 60' long and steam powered (his preferred engine). In 1880 he did a torpedo boat for the Russian navy, which was 40' long and set the world speed record on her trial run of 21 knots. His later model of US navy torpedo boat of 1890 (the first to fire an automated torpedo) topped 23 knots. Then the Spanish/American war hit and typical of war endeavors, technology took off. With 27 to 30 knots by the turn of the century not uncommon. These were all double ended, inclined bottom hull forms, with huge engine and fuel weight burdens. I'm doing little more then using similar shapes without the massive burden of the late 19th century engine technology.

I do think the wing is possibly helpful to correct trim issues, but the cost at 1.35 could be huge if human powered. I don't think it's of any advantage below 1.5 realistically. You need the power to get the wing "working", which requires at least enough speed to get reasonable flow over it surfaces. If "fatter" foil shapes were used, you could get them "engaged" at lower speeds, but you'd flutter them at higher speeds.

This crap is so much easier when working with air, like wings on a jet. Air is about 740 times less dense, so the same foil can work from say 20 MPH to 200 MPH. Just try that under water and have a good laugh as things start breaking off and self destructing.

I honestly don't think I've "discovered" anything, just "uncovered" some old, long forgotten stuff, by folks a lot brighter then me. Okay, I added the wing, but I'm reasonably sure this isn't new either.