Bottom Loading

I am pretty sure I understand the concept of bottom loading on a planing vessel. Most of what I know or think I know comes from the highly informative site of Tom's about his BlueJacket line of designs.

I don't know how many of you are familiar with the Bristol Bay gillnetter boats. Restricted in length to 32 feet they have become quite extreme in design. Some of them are almost half as wide as they are long. Packing substantial loads of fish and possessing huge horsepower motors people get these babies up on plane in a race for the fish. One design feature I have always wondered about is an extension of the bottom surface plane of maybe 18" or so that resembles a fixed trim tab. It is sort of like the transom was set forward.

I believe this is a method of "cheating" the overall length restriction that would lead to a smaller bottom loading figure due to the increased surface area. On the boats that I remember it best on, it is an empirical modification not "engineered". I was hoping Tom and others more knowledgeable than myself might comment on both this design feature and perhaps on the general subject of bottom loading.

 

I think you have the reasoning correct Wally. As always, every thing in a boat hull affects everything else so there are no guarantees of a simple solution. The extension would most surely help get the boat onto plane but, once on full plane, it would most likely have little effect. I wonder how these boat handle? I would think that the rule makers would frown on such a device though.

 

Hi Wally,

The articles Tom has written about his Bluejacket design ( http://bluejacketboats.com/ ) are probably among the best I've found on this topic. Most of the more academic literature concentrates either on the displacement speed range, or on higher planing speeds- or, for very slender hulls, full-displacement mode at high Froude numbers. Comfortable, controllable running with a planing hull in the intermediate speed range seems to be very dependent on bottom loading. It's interesting to note that HamiltonJet, among other drive suppliers, uses bottom loading as one of the main criteria for evaluating a proposed planing design.

The extension you describe does sound a lot like a rule-cheating feature in this case, but is not unique to the boats you mention. Similar fixed extensions are sometimes seen on RIBs and other boats that, although originally intended for high speed, end up often being used at low planing speeds. Movable trim tabs, in their zero-degree or slightly down position, serve the same function, and there are many reports of boats that, although originally hard to control at very low planing speeds, were found to level out and run smoother at these speeds with trim tabs near flat or slightly down.

I would agree that such modifications are usually designed along the lines of "well, she's not behaving like we want, let's try bolting this thing on". As many boats have proven- Tom's Bluejacket, most modern wakeboard and ski boats, etc.- a well designed, properly balanced planing hull does not have much of a need for add-on lift enhancers.

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Edit - Tom, not sure what's going on, but it looks like a few pages of your site have been corrupted somehow?

 

Interesting replies gentlemen,

Tom,
When you say that the extensions would help lift the boat on plane but have little effect once there is this because getting up on plane there is more square footage available, lowering the bottom loading but as the boat comes up out of the water the square footage decreases thus decreasing the surface area and increasing the bottom loading?

If this is so it would seem that creating a feedback loop that allows the maximum surface area when planing would minimise bottom loading and decrease horsepower requirements. I assume the devil is in the details.

As Matt indicates a well designed proper balanced hull ...etc.etc.etc. I wonder other than the obvious answer regarding weight what aspects of design and balance have greater or lessor impact.

These boats I mention are big heavy creatures relying on brute horsepower. Looking at the hulls out of the water one can see several ideas at play to help generate lift. The already mentioned hull extensions. Big gullwing lifting strakes at the chines. Memory fails me if these boats have lifting strakes between centerline and the chines. I will consult my photo archives and see if I have any shots that might provide clues.

I mention these boats specifically because I think they are crude examples of empiritically derived design solutions that perhaps focus on this bottom loading idea that I partially understand. These boats are operating at the cruder edges of the performance envelope. Big unwieldy heavy fiberglass or aluminum hulls with large high horsepower motors. Everything about these boats is oversized but the overall length, including the additions to the hull to help get these big pigs moving.

In any event your comments and observations help me to grasp the finer details of hull dynamics which I find fascinating but somewhat baffling. I have also examined some drag boats hulls since they too have hull features that are added to help change hull dynamics in an attempt to push the boundries of the outer edges of the envelope. Sometimes seeing things that are added after the fact makes the design features easier to "see" and understand.

Thanks to both of you for increasing my understanding. I don't know why I find boats so fascinating but I do.

It is funny to see these commercial boats designs being altered to fit arbitrary limits set up by Government regulators. Much like the extremes we see in sailboat racing where designing to the rules can foster some quite radical otherwise unsatifactory designs.

When regulators set limits on boat length or license numbers for conservation's sake the results are similar. Radical extremes are created at great expense that are sometimes dangerous or worthless except for the narrow requirements of maximising a specific fishery. The laws of unintended consequences show up time and time again.

 

As soon as I saw the thread title, "Bottom Loading" I knew it'd turn up a certain couple of old salts... Well, now all 3 of us are here!

I imagine that what Tom means when he says that hull extensions will make little difference once on the plane, he is referring to higher, fully planing speeds. Indeed if you go fast enough the increased wetted surface area will probably end up slowing the boat down, rather than improvinf its performance.

It's not uncommon to find extensions even on production boats. For instance I was looking at a brand new 40 footer the other day that was originally intended for shaft-drives. Reacting to the market, the builders now offer the boat with Volvo's IPS. As a consecquence, I suspect, the drives are acting to bury the stern rather than lift it. The entire running surface was extended by adding an extension about 500mm deep. I was intrigued somewhat that they no longer deemed trim tabs as necessary - or perhaps they simply had nowhere to put them....

As far as the gillnetter's go, I seem to recall seeing a film about them once - as you say - quite bizzare boats! I'm not sure I'd describe them as crude though. Certainly in the absence of regulation, the boats would almost certainly be quite different, but like many boats, they have evolved to over time to be the most efficient solution to a particular problem.

Indeed, if you look at how most pleasurecraft are used, it's not difficult to make an argument for the kind of floating condo's that attract much derision around these parts. They suffer bottom loadings that bring a tear to the likes of Tom, Matt & I, yet they too have evolved to cater to a segment of the boating market....

 

The Bristol Bay gillnetters are very weird-looking boats, no doubt about it ( example, http://www.dockstreetbrokers.com/listing_detail.php?id=1166 )
It's interesting to note that if you google "bristol bay gillnetter", the first two things that come up are a youtube clip and a list of people trying to sell them- and the third is a personal injury lawyer specializing in Bristol Bay fishing accidents. I wonder what that says about the seaworthiness of such boats.

I agree; as the boat lifts out of the water at higher speeds, the large, flat bottom that is such an asset when climbing to plane becomes something of a liability. Especially so on very fast hulls, where aerodynamic forces become a major factor, and the balance of lift, prop thrust and hull drag can actually move the spray root aft of the LCG. Still, few people actually go that fast, as much as they love to brag about their boat being able to "do 60".

My current boat (a Bolger-designed 5 m runabout) has a bottom loading of about 1 kPa lightly loaded, a bit over 2 kPa with four crew and their gear. When running light, it will plane at nine knots. Laden down to the gunwales, it definitely struggles a bit to climb, but does OK once up.

Some references, including HamiltonJet's application guides, indicate roughly 0.22 kPa of bottom loading per 1 m of LOA as being a sort of threshold, above which the boat is probably too overloaded to plane well. (Interestingly, an awful lot of "dock queens" come in well over this figure. Since they don't have to contend with high seas and don't tend to go very far, they are ideal for their purpose- getting the most luxury per slip-space dollar.)

For efficient running at lower planing speeds, and for being able to run through the "hump" range without too much difficulty, I would suspect that the threshold would be closer to 0.16-0.18 kPa per metre.

If I recall correctly (Tom, correct me if I'm wrong), the Bluejacket has a bottom loading of something like 25 psf (1.2 kPa), or 0.15 kPa per metre of length overall. I haven't seen the actual boat yet (perhaps the Lathrops will do more Ontario cruising sometime) but from the photos Tom has made available, it is quite clear that this boat has no noticeable "hump" speed, as it is already generating significant lift by that point. I have seen other boats- notably flat-bottom cargo scows when running empty- that exhibit this same behaviour.

 

The Bayboats are seaworthy enough for their purposes. They don't travel that far but go out in almost all conditions and are surrounded by other boats if something really goes wrong. When I use the term crude I don't mean that in their own way these boats aren't fairly sophisticated. In good years when money is plentiful, some guys really push the envelope trying to get an edge. I've watched them being built and know some of the guys who build them. Some of the boats have formal input from naval architects but many copy ideas that have worked for other guys or try things they thought of over the winter. If an idea works well you will see similar things show up sporadically in the fleet over the next couple years.

Will,
If I am understanding you correctly, you are saying that at higher speeds the speed of the boat itself will generate enough lift to make the extra surface area of the extension unneccessary for lift and start to increase unwanted drag from the extra unneeded surface area being pulled through the water.

Matt,
I think I know what the spray root is. Is it the leading edge of where the hull has lifted from the water and the "spray" from the water starts to shoot out the sides of the boat? Perhaps you could expand on this and the dynamics of this point shifting for and aft of the LCG. I wonder if you could translate kPa for the metrically challenged. I imagine it is kiligrams per something

My boat is a relatively heavy cabin cruiser. I am always scheming about improving my fuel effiency. I'd like to repower with one of those cummins and really take advantage of the torque available at lower rpms with a prop of sufficient size and pitch. But that is unlikely to happen as long as the old motor runs well. Another crazed idea is to decrease the bottom loading by using some sort of hull extension like the Bayboats have. I would like to optimise performance at 12 to 15 knots. Sadly my hump of resistance is right where my targeted speed lies. Most of this is mental ************, but over the years I have tackled some small but still extensive modifications. The best so far was adding small tracking keels about 5 feet long roughly halfway from keel centerline to the chines. the parasitic drag may cost me a bit of effiency but she tracks on rails. I can lock the steering and walk away from the wheel to attend to small tasks confident that the boat will take care of herself for a bit.

Again thanks to all who help add to my personal storehouse of knowledge.

 

kPa = kilopascals, 1 kPa is 1000 newtons per square metre or 20.885 pounds(force) per square foot. SI makes an important distinction between the units of mass (kilogram) and the units of force (newton), and the two cannot be interchanged (although that doesn't stop the occasional confused engineer from trying).

The dynamics of a planing hull are hard to visualize / explain in text form. Perhaps the best way to see what's happening is to draw a profile of the boat, and draw scaled arrows representing the various forces acting on it. In a stable state, the forces will always balance out. There are some good, albeit slightly academic, texts describing the phenomenon in considerable detail, for the more mathematically inclined.

 

Hi Will, I was waiting for you to chime in. In a boat that is otherwise well designed, I consider bottom loading to be the key to planing at low speed. I am surprised that so few have discovered this. Almost no commercial builders seem to appreciate this fundamental fact.

Matt, when I looked at the gill netter in the photos, I thought at first that they had squeezed the photo. A really odd looking duck. Well, maybe not so odd if they were actual ducks

As Matt said, it is not easy to fully explain planing dynamics in a way that satisfies all situations but I will make an attempt at the one mentioned. We all agree that adding the hull extensions will help an overloaded boat over the hump. Taking the boat up to full planing speed makes it all a bit different. Assume that there is adequate power to get it there.

The boat will always assume an angle of trim to develop the required lift from the sum of both dynamic and buoyancy lift to keep the hull in equilibrium attitude at the speed she is running. Without the hull extensions, she will take trim angle A. Now add the extension. If the CG were changed to place it at the same distance from the aft end of the hull bottom, I think the boat would run just as before. Same speed and same trim angle. But in our example, the CG will not change and will tend to hold the bow more down at an angle B.

If the boat were running at, or less than,the ideal trim angle for minimum drag (from adding wave making and friction drag), most boats will slow down a bit from the lower trim angle. That is, the frictional drag will increase more than the wave making drag decreases. So the result is that top speed is reduced from adding the hull extensions. This is a thought experiment but it agrees with my imperical tests, not from adding hull extensions but from shifting the CG, which should give the same result.

In the 1950s, I had a little 12' runabout with a 10hp outboard that my friends and I learned to waterski with. One of my friends was at least 200lbs and hard to get up out of the water. Without knowing anything about this subject, I reasoned that adding width to the aft bottom would help matters. I added a shaped piece of 1x4 to each chine with screws and fiberglass, polyester resin then, of course. It worked very well and made it much easier to get the boat and skier up on plane. Getting up on the skis was not like now at all and consisted of a sequence of jerks with the boat speeding up a bit between each jerk, but it worked. There is a photo of this little boat on my website.

Thanks for the conversion Matt, I was afraid that I would have to go to some old reference book for enlightenment.

I think the website works well on IE but not so well on mozilla. Gotta do some work on that.

 

Curious as to why you want to optimize performance at such a low speed. In your current configuration, your fuel econonmy will be a better at a slightly higher speed as opposed to where you are almost falling off of a plane.

If you want the better ride that you are getting at that speed, remember that if you reduce bottom loading and get the boat higher out of the water, your ride won't be as good since the boat is now more subject to pounding...

Still, more area will reduce your power requirement for that speed range. Just as importantly, adding lift aft of the transom will get the nose down, and may decrease bottom loading even more than just the effect of the afterplane. Consequently, with an afterplane you may actually have a pretty good crusing efficiency at that speed, and certainly it will be a lot better than if you are just at the top of the hump.

 

Conventional wisdom does not always work out. I find that the ride can often be improved by either increasing or decreasing speed while on plane. There is often a sweet spot for the existing conditions. Depends on the particular wave pattern, relative direction of the boat to the waves, the particular hull and hull balance as well as the speed.

Driving the bow down on a V hull with the normal warped forward sections improves ride in chop since it forces the waves to hit the fine forward sections first. This can even make a flat bottom hull ride acceptably if it has a sharp bow. I know of one fellow who routinely runs offshore at speed for large distances off the California coast in such a skiff.
http://www.oceanskiffjournal.com/index.php/osj/1EMPS/610

The author of this piece may disrupt your normal conclusions about the form a small boat should take.

 

That's all true, but since we really know nothing of the hull that we are talking about I was simply trying to keep it general. The small chop ride may indeed actually get better, but at 10 to 15 knots I was thinking that it wouldn't be that bad anyway.

In larger stuff, even with a fine entry a boat is going to be to see larger motions when fully planing, since at lower speeds the hull that is not fully on a plane is partially supported by the displacement effect. While it does follow the motion of the waves it is transversing, it is a lower frequency and is of a more damped motion than it is when it is fully supported on the surface and is therefore subject to higher peaks and lower valleys of the surface.

 

I must admit that I am not sure exactly what is being stated here so cannot offer any opinion.

 

I will try to clarify what I was trying to say, I don't quite understand what part of the above post was so difficult. but I'll give it a try. I have made some assumptions to simplify what I a trying to convey, but will talk about them at the end of the post.

What I am saying is that at speeds below full plaining speed the hull is supported by combination of forces. It is partially supported by displacement and is partially supported and lifted some (but not all the way up to essentially the surface) by the dynamic lift generated by motion of the hull through the water.

When the hull encounters a wave at these lower speeds, since the dynamic lift available is not sufficient to bring the hull to the surface, the only additional lift that is supplied is that of additional displacement which results as the hull is pushed up by the vertical acceleration. Since the primary supplier of upward force is displacement, the response is lower frequency and the response lag allows the hull to pass thru the wave without as much vertical acceleration. Consequently, as the hull interacts with wave action at these low speeds, the initial vertical acceleration in response to wave action will be lower than is experienced when the hull is fully on a plane.

Once a hull gets up to a higher speed, there is a lot more lift available than what is required to simply lift the hull to the surface. If you are 20% above planing speed (in this case going 18 knots instead of 15) you have the ability to generate 40% more lift than you did at the lower speed. Consequently when you encounter a wave at this higher speed the higher available lift is transferred into higher vertical acceleration. Simply put, when the wave pushes upward on the boat, you will generate much more force (vertical acceleration) before it begins to not follow the rising surface. Since you have applied a higher force the hull vertical speed is greater you are now moving up faster and higher in (or in extreme cases out of) the water and, as the wave passes, you have further to fall. Once the hull comes down it is moving faster downward, and, as a result of the higher force available, the vertical accelerations are higher at the bottom of the next wave.

All of these things will be dependent on the realtive wave height, speed, direction of impact, and frequency of the waves, length of the hull, fineness of the bow, trim angle, and any one of number of other factors, but I wasn't trying to relate it to a specific case.

There are some simplifications that I am making here to describe the process. I haven't considered pitch or fore an aft accelerations. The description above assumes that the effective planing area is relatively constant. If for instance the bow is carried high out of the water at low planing speed, and the hull is not a fine entry, there will be additional dynamic lift created at the low speed conditon as the wave is encountered so acceleration can be somewhat higher, but in the same manner, at higher speeds vertical acceleration will be higher if the bow is higher and additional planing area that is out of the water is immersed as a wave is encountered. I also haven't considered the effects of a deep v hull, but at these low speed the effects aren't as substantial as at higher speeds anyway.

Bottom line is that you can generally expect higher vertical acceleration (ride roughness) in larger waves even with moderate speed increases above that required to just barely achive planing.

This was the reason for my original question, as to why TollyWally wanted to cruise at speeds of 10-15 knots. If the reason is that he likes the ride he gets in that speed range, (which is what I sort of suspected) then improving the planing performance by getting the hull further up on a plane, (with increasing planing area) was unlikely to produce the ride that it did when it was not fully planing. If the ride was fine three or four knots faster, and he had other reasons for selecting that low crusing speed, then increasing the planing area might be a good thing in terms of increased efficiency.

 

Yellowjacket,
For me 15 knots is fast enough to make time, not too noisy, and not too bad on fuel. LOL if I ever get around to putting in decent sound insulation I may want to upgrade on my speed! I have a fuel flow meter and have a pretty good grasp of my fuel burn at different speeds.

My hull came with any number of engine combinations from singles to twins. It is ironic that my desired speed is at the worst of places for hull resistance.

I am hoping you might be willing to expand on the math regarding speed and lift. Your example of 3 knots supplying 40% more lift is well worth understanding more about. Are there some basic equations etc.?