Fuel economy discussion...more important now than ever!?

On the other hand, the result of a modified planing hull is likely to be substantially inferior to a newly built displacement boat, and in fact there is no way of knowing how much fuel might be saved after such modifications anyways. Not only that, but what if the modified boat handles more poorly or even unsafely after the modifications?

Plus it makes no economic sense for an owner of recreational boats which are used a few tens of hours a year to spend literally thousands of dollars for reconstruction plans and labor and materials simply to save on fuel. He would have been far better off if he had simply used the original planing boat only in displacement mode, assuming he was disciplined enough to do so.

In my opinion, modifying a hull in such a dramatic manner *may* make financial sense for commercial vessels, but it almost never makes sense for pleasure boats -- not when simply "going slower" is so easy and takes no additional money whatsoever.

Of course this all assumes that the costs and risks of such redesign / rebuild efforts are a concern to those who might be paying for them ... and they will be, right?

 

There probably aren't any in production in the USA, but in some places outside the USA boats like these have never gone out of production. Mostly I'm thinking about Europe for hulls shaped the way Americans would like them, but the countries of Southeast Asia have thousands of examples of displacement hulls you could buy and have shipped to the USA if you like their hull styles.

Displacement power boats are relatively easy to design anyways. I took a Bolger Diablo and modified it for full displacement service. I chose this particular boat because I like the original shape, and it has long been known as a heavy load carrier. So, given the fact that I want to carry lots of people at slow speeds, this seemed like a great hull to start with.

Here is what I came up with:

 

These 38 footers get around 4 mpg with OB's, more with small diesels.

The photo gallery shows some side views out of the water that might interest you.


http://www.supercat.co.za/

I am trying to desgn a 24' mono that will get over 6 mpg.

 

Ok Ken - so what's with the little chocolate soldiers - and how come two of them have their bum's poking thru the side of the boat

Bullshipper - 6nmpg is very easily doable - hell we manage that with a 225 df1 outboard pushing a heavy deep-vee planing hull at about 4 - 5 knots. But for good economy, the rules are pretty simple - long, narrow and light. Beyond those three simple rules the gains are marginal.

 

Oh, - - Will, - - - - -
They ARE very light cats that also go up to 20+knots from Saffa Land or somewhere in that region.... Check out the posted site - - Quite neat but lacking the range I am looking for (2 to 3000 Nm...) at 12 to 15 knots.... which means 2 to 3000 litres of fuel.....

Close but not quite there yet.

 

This deep vee measures 24' x 7'7", weighs 2900 with a 150 hp motor, passengers, gear and fuel. I hope to attain 6mpg at a 30 kt cruise, not 4-5knots. This economy is over 2x what the 23 Mosely Seacraft gets, so that is setting the bar high enough for a rookie like me.

 

Do you have any pictures of this to post? I'd love to see it.

 

I think those two ate too much ...

The truth is, I never moved them toward the centerline because I couldn't be bothered. I only wanted to see how many people could sit side-by-side comfortably so I could determine the boat's true human carrying capacity. To me it looks pretty good for a 15 footer. There's still 17+ inches of freeboard at 2500 pounds displacement which is something like ten 200 pound people plus a 500 pound boat.

When it comes to open boats I really like side bench seats too. They make people walk down the center of the boat, they allow infinitely adjustable weight distribution ... and when they are built as boxes they behave like very stiff longitudinal stringers, they can contain lots of 'stuff' that can be locked up securely, they add plenty of excess flotation capacity, and they substantially reduce the boat's interior volume which ends up being a 'good thing' if a wave tries to fill the hull.

 

I'm finding this hard to understand because when the boat is not planing it is plowing, which is very inefficient. I would be inclined to think that getting this boat on plane quick with a 4/5 blade prop, dole fin, and maybe even trim tabs and maintaining the lowest possible planing speed (10-15 kts) would be most efficient.

 

Of course if you already own a good boat and outboard it makes perfect sense to just slow down.

I was speaking in general terms of ways to improve economy and handling of a hull designed around a huge heavy outboard (or two) hanging on the transom.

There are some basic principals of naval architecture that differentiate planing and full displacement hulls. Look at the curve of areas for either hull, note that the aft end of the curve for a planing hull is lopped off square. The curve for a good full displacement form is double-ended or nearly so.

If you substitute a smaller (say 40 HP, 200lbs) for twin 150-200 Hp motors (1000-1200 lbs) the boat will certainly float bow down. At low speed this produces some less desirable handling characteristics. To reduce the trim by the bow you reduce volume aft, by changing the bottom. This will lower the prismatic and reduce hull drag at low speed.

I certainly agree that any modification must be viewed with cost/benefit in mind. It makes no sense to modify a boat that will only be used a few hours a year. But I know many avid sport fisherman use their boats for 100's of hours a year, or they would like to. If you are looking at buying a new engine and you'd like to spend a bunch less and go slower, a hull mod might make sense.

 

See http://www.tadroberts.ca/about/pdf/power-boat-design-form-and-function.pdf

 

The Diablo is a planing hull, less than ideal for displacement speed. Below is a 16' displacement hull designed around a 10-15HP inboard engine. I would not advocate 10 passengers though.

 

Canivour...

If you click on masa's name you get a link to his gallery where pictures live.

Cat's are great until you try building a good one to trailer, which was part of the original request. A cat 9-10' wide ends up with a fairly high VCG, due to the clearance needs for the bridgedeck. Also narrower cats can have problems with increased resistance due to the hulls being so close together.

 

"I have experienced how governments have responded to water shortage in Australia."

Happily from what I saw on a Science TV show the Reverse Osmosis power requirement has been reduced by 60%.

Test plant construction is well underway.

"In my opinion, modifying a hull in such a dramatic manner *may* make financial sense for commercial vessels, but it almost never makes sense for pleasure boats -- not when simply "going slower" is so easy and takes no additional money whatsoever."

Probably the best savings come from a re-power , so the engine (esp a diesel) can be run at 80 to 90% of rated load.

A big engine pulled way back frequently becomes Very fuleish as well as wet stacks and wears out at a rapid pace.Just pulling a large engine back may not save a dime.

The biggest problem with small diesels is the high speed, turbo, and electronic controls required for high efficiency.

And then its only a car engine , so those tales of 10,000 service hours will need to be reduced to perhaps 2000.

On a TOTAL cost per 2000 hours , might be a gas car engine would be better than any diesel.

FF

 

Rick;
Cadence is something of a departure from the initial premise of the thread. Nonetheless, I want one of those. Now the prospect of importing a Cadence, or similar, from Oz is not in my budget. I respect your intellectual property so if I ask too many questions you can tell me to "buzz off" without offending.

I am interested in the innards of the boat. Obviously some kind of gear box is involved to translate rotary motion athwartships to rotary motion along the centerline. Possibly a set of cables and pulleys might change directions as well. You appear to be using a simple system of connections to the rudder from the hand grips. At 24 inch beam the boat will be tender and demand some degree of vigilance. When using a skinny kayak, the foot braces are very important to the stability and maneuvering of the boat. When you pedal, your feet are moving and I wonder if that has an effect on the attitude of the boat.

Gene

Excuse me fellows. I did not mean to hijack the thread. Rick, perhaps you might PM me for a more lengthy discussion.