Help With Economical Semi-Planing Designs

Hmm - some real numbers... thanks Tad
Incidentally, how is the Transport Efficiency calculated?

It brings us back to a comment made by one of our very wise contributors quite early in the piece...

 

Hi Tad,

I wish I knew more about these Et's and FNv's and how they are calculated so I could join in the technical part of this discussion in an intelligent manner -- but I don't (at least not yet) so I will stay away from trying to interpret those numbers in any more than a simplistic manner.

Here's what I think in layman's terms:

Most of us who want efficiency are focused on running at certain speeds that will satisfy us. As long as we can run at these speeds, our next goal is to find ways to use as little fuel as possible at our favored speeds -- given all the OTHER requirements we have for our individual boats.

Leo tried to focus this discussion a bit in his first post when he said:

I think you may have done this with your Slicer example:

... but then again I don't know how Et's and FNv's convert to knots and NMPG's so I could be way off on this one. One thing I do know is that I ran my father-in-law's full displacement 22 foot Philippine banca boat on an 8 HP B&S engine at speeds which I believe were probably at least 15+ mph ... and I wasn't running WOT either, so I can easily believe that Slicer can run at 18+ mph on almost twice the HP.

But Slicer is a very simple and lightweight open boat, nothing like Whio. So don't we have to define the boats in this discussion in a more focused way, perhaps by specifying some parameters that make our comparisons more "comparable"?

My first thought in this regard is to specify not only a speed range like Leo did, but also a displacement range, and maybe a couple of other parameters as well. For example, here's a question we might ask which would focus our attention on a more specific set of parameters:

What hull design and engine combination will provide the greatest propulsion efficiency under these conditions:

1- Continuous cruising speed of 15-17 mph
2- Cruising displacement of 2000-2500 pounds
3- Propeller/rudder draft no more than 12 inches
4- Easy trailerable -- no more than 30 feet long

I'm not trying to focus the discussion on these arbitrary parameters, I'm only asking if we might end up with a clearer understanding of the issues involved here if we did?

I noted this in Leo's first post too:

It seems like Slicer does this even though it is a dramatically different boat than Whio, so I don't think Leo was thinking of a boat like Slicer when he posed his original question. Slicer could very well be "full planing" boat too, which should actually leave it completely out of this "semi-planing" discussion -- unless of course we get rid of the focus on"semi-planing" and replace it with a specified speed range -- not a bad thing to do in my opinion.

As people are beginning to learn, I have a particular interest in the tunnel-stern Seabright skiffs modeled after those of the Atkin's. But this interest is not solely based on my desire for fuel efficiency or that these boats might run best in the semi-planing speed range. I actually believe they have several more important features to offer ... extreme shallow draft, upright beachability, protected props and rudders, etc. to name a few.

Do they provide the fuel efficiency Leo is seeking? I'm not sure and I have my doubts, but I certainly hope so.

 

" extreme shallow draft, upright beachability, protected props and rudders, etc. to name a few."


For my concept of a cruiser the ability to be installed in the box with simple pipe rollers , or to slide on a simple flat trailer for land movement is also a plus.

A bigger plus is the ability to RUN aground with no hassles, damage.

Cruising in new places , a grounding is simply part of the game.

But BEST!! of all is the ability in many places to avoide the overcrowded moorings and packs of boats.

If the boat draws 2 or 3 ft , and is happy as a clam taking the ground , many near shore places become suitable anchorages.

So far the claimed "efficency" of the Atkin boats comes from 3 items.

1.The box keel supports the boat creating LESS wave making drag.
2.The reverse deadrise keeps the stern from droping at higher speeds.
3. There may be some added efficency from capturing the skin drag.

One other thing that Atkin chose was is 1-1 tranny no reduction gears.
This gives a smaller diameter , higher pitch prop , perhaps suited to the already accelerated water aft of the box keel.

The other is weather the bottom of the Box Keel serves as a plaining surface , providing some lift.

Comments gentlemen?

FF

Tom , read your T?S adventure , will be somewhat different in out 50 ft Navy Utility launch!!

FF

 

I have used something similar to the Et that Tad mentions but have no idea of how that one is calculated. Would be interested in how each of those numbers is arrived at. None of my books mention an Et although Gerr uses something similar.

Fred, since your boat is a Launch, what kind of cabin structure do you have? Any photos?

 

Quick paste from an old piece of writing.

The comparison of MPG without regard to either load (displacement/weight), or time (speed), seems an incomplete discussion. There is a very useful comparison of speed/power/weight called transport efficiency (Et ). With this coefficient, plotted against a dimensionless speed, we can compare the efficiency of displacement, semi-displacement, and planning hulls.

Transport Efficiency takes into account not just the power required (in effect MPG), but also the weight (displacement in the case of a boat) being moved, and the time required to move that weight (speed).

Et = W * v / Pd * 550

W is operating displacement, in pounds.
v is speed in feet per second, (knots * 1.6889).
Pd is the horsepower required for the above v.

For Rambler I guessed at a cruising weight of 1600 pounds.
6 miles per hour is 5.2 knots is 8.798 feet per second.
I took the power as stated 3.2 HP.

Et = 1600 * 8.798 / 3.2 * 550
Et = 7.998

Plot this against Volume Froude Number

FNv = v / (g * disp.^.666)^.5

v is speed in feet per second.
g is acceleration due to gravity, (32.2)
disp. Is displacement, this time in cubic feet.


Tad

 

Well, if we are going to discuss live-aboard cruisers like the one Fred describes, these seem to be desirable features:

upright beachability
extreme shallow draft
protected props and rudders
run aground frequently without damage
bottom designed for easy flatbed trailering

These features are all offered in the Atkin style tunnel-stern Seabright skiffs assuming the bottoms are built thick and/or strong enough. These are all features the average everyday boater will benefit from on smaller boats as well, which is why I like these boats so much regardless of whether or not they are any more efficient than similarly sized non-tunnel stern boats.

... and most definitely suited to a tunnel that is shorter in height!

As I mentioned previously the prop does not have to work so hard lifting water up into a shallower tunnel, and therefore more energy is available to push the boat forward. Small diameter props are also cheaper, and so are 1:1 transmissions, and the boat will have more useful volume inside with a shallow tunnel. These are all "good things" in terms of economy and features.

I kind of doubt this when talking about the Atkin-style bottoms which are all perfectly flat. The Atkin boats are designed to keep the forefoot in the water, which suggests to me that they are really designed to perform more like displacement boats than planing boats, so I don't think much planing comes from the flat bottoms.

In some of my designs (including the twin tunnel on the previous page although you can barely see it in those pictures) I use a very shallow vee on the FORWARD bottom of the box keel only. I think this will soften any pounding that might occur when running in a chop. It is not there to induce planing, but when running at speed it might lift the front of the boat a little since this very shallow vee actually warps into a fine entry.

I do not think the planing effect here will be noticeable though, and I certainly do not want the bottom to ride on top of the waves -- but the recurved aft tunnel should lift the stern and push the bow down to prevent this anyways, just like it does on the Atkin boats.

This shallow vee on the forward bottom will provide a benefit not available on the Atkin boats. It will concentrate wear and tear on bottom of the vee since it is lower than the rest of the bottom in this section. If damage occurs here more than in other places on the forward bottom it will be easier to find and fix. The bottom can be strengthened along this vee as well to deal with the higher stresses that focus here.

 

The box keel may well be operating primarily in the displacement mode, but given that the boat speed is well byond the displacement speed of the boat as a whole - and provided the bottom is offering a positive angle of attack - then it would definitely produce a degree of lift.
The down-angle of the stern , however, may counteract any natural tendency for the bottom to assume a natural trim angle.

Thanks for the info too Tad.

 

Hi Tad,

Thanks for the info!

Using Et's is clearly a more 'universal' or 'theoretical' way to discuss efficiency since it dispenses with the need to define both displacement and speed. But I always think in terms of numbers I can relate to -- like 2000 pounds and 17 miles per hour for example. These Et's and FNv's are so new to me that it's hard for me to get a grip on them ...

I think this is why I suggested that we define the displacement and running speed of a particular "discussion boat". Then we can talk about real numbers like MPG's ... and pick out some real boats that match our "discussion boat" and compare MPG's very easily.

Sorry, I'm just a simple guy I guess ...

 

I agree PROVIDED the bottom is actually offering a positive angle of attack -- but I question whether or not this positive angle of attack actually exists?

Robb White said the faster he runs his boat the closer the forefoot gets to the water. So if there's a positive angle of attack at slow speeds, it appears to diminish when the boat speeds up -- effectively reducing or eliminating the positive angle of attack needed to make the boat plane.

So how can we resolve or explain the apparent inconsistency here?

A lowering angle of attack suggests less and less planing as speed increased -- yet the boat is moving INTO (not out of) the planing realm while this is happening.

Seems like a paradox to me.

 

Actually, the faster a boat goes (once on the plane) , the lower the required angle of attack - or trim - in order to supply the required amount of lift. So, in fact you could expect this to happen regardless of anything else that is going on.
Having said that, I agree that as far as the SB skiff goes, significant lift is going to be created by the aft sections. Indeed, if you go fast enough, the bottom of the keel would be presenting a negative angle of attack and may produce a corresponding negative lift! A sort of amplified porpoising action, if you will. Let me observe from a nearby boat when this starts to happen...

 

I have a Toyota engine and transmission that I may end up installing in my Tolman Seabright if I don't sell it to someone else first. That engine must be at least 40-50 HP or so, way more than the 20-25 this boat should be able to handle, so I think it will provide plenty of power to run the boat too fast and experiment with this effect.

Why don't you join me on the boat? Then we can both experience this "super-porpoising" together! It will be fun to talk about for a very long time, assuming we live through it ...

 

I'll bring my wetsuit!

 

Graph of Et against Volume Froude Number from Donald L Blount and Associates.

 

Tad, Will, Fred, Ken,

In a fit of (probably false) bravery, I will throw in a couple numbers of my own.

2500#(2 adult crew), 20.5kts, 50hp - (Et = 3.14), (FNv = 1.78)

Same boat 3175#(5 adult crew), 19.1kts, 50hp - (Et = 3.72), (FNv = 1.53)

I wish I could give some numbers at lower speed but haven't the faintest idea of what the power factor would be.

I attribute the improvement at higher load to having the 2 largest crew at the stern. This is a trade of higher wave making for lower wetted surface at higher trim angle. This was an experiment. Trim of the motor has very little effect on hull trim angle. My usual cruising speed is between 10kts and 15kts.

In defense of Schulz, I think his boat is probably designed to operate at higher efficiency in the "transition" speed range as is mine. Like mine (and Atkin), his stern shape will depress the bow resulting in a lower top speed than otherwise might be expected.

Have at it.

Without taking into account the actual use that the boat is put to, these numbers don't mean very much. If we are talking about a boat that can transport people in some degree of comfort for a period of time, space and other desirable characteristics will negatively impact these numbers. Efficiency is a good comparison tool only for boats that are intended for the same purpose. Many of these design objective have been listed above. My goals are little different in that they are not so dominated by the shallow draft requirement but are also pretty shallow.

 

Our current boat is not bad on fuel use, about 3gph at 8K.

If the box boat could double the speed at the same fuel consumption I would be delighted.

16K to 18K on 3 gph.

LUCY is 50ft loa 15ft bwl, about 30,000lbs with simple structure and all cruising gear.

FF