Trailer Cruiser summary

Read all 20 or so pages of an older thread , where a gent was searching for 12K at 1gph with a boat to be trailered.

There seem to be a number of hulls plaining and disp. that seem to come really close.

Besides light weight , propulsion efficiency seems to be the only area where large gains can be made.

The secret seems to be (as always) an efficient propeller.

Rick suggests a deep reduction gear and VERY! large diameter 2 bladed prop.

Interestingly the prop seems to be more important than the vessels speed (within limits) but an 18 or 20K cruise would seem to be available at modest fuel burn.

So the question is HOW do we get a cruising hull that can take a big prop , and not destroy itself in the process.

The only two concepts I come up with is the transom mounted engine and exposed shaft as seen on many far eastern boats.

How this could be made acceptable for a modern boat , where folks don't want to spend their time at the engine , adjusting the prop immersion is a question. AS is the danger from the exposed child chopper


The small Atkin design by, http://www.stormportboatworks.com/ is small and light , but claims 1/2 gph at 18K!

From my figuring with more displacement taken by the box keel , and even more reverse deadrise a quite large prop could still be protected from grounding .

IF the best guess for more efficiency is a really BIG prop, any other hull ideas?

FF

 

Fred

You may have made a mistake in interpreting the claims of this boat. They do say 18kts as well as 1/2 gal/hr but they do not say these numbers are at the same time. Maybe yes, maybe not.

I have been playing with a hull that takes part of Atkin concept further. that is the canoe body glider or monomaran or whatever you call it. It combines the high L/B ratio of the load bearing canoe lower body with a very lightly loaded upper hull which is nearly flat to the water flow. The canoe body has very low drag at speeds in the teens and the upper body is so lightly loaded that it offers little frictional resistance. Wave making should be minimal. The depth of the canoe body allows for a larger than normal propeller. I don't see the prop getting to Rick's 30" but 20" should be easy and with a small tunnel, 30 might be approached while the prop is still well protected.

I see power as a 25-30hp diesel in the canoe with a horizontal shaft. Displacement could be as high as 6 to 7K# or more to make a full appointed cruiser out of it. LOA would be about 28 to 30 feet which is plenty large for two people to cruise comfortably. If the holy grail of 12kts at 12nmpg is attainable with a full sized power cruiser, this just might be the one to do it.

 

It seems to me that the reason large propellers are so efficient is the very high aspect ratio of the blades and the low disk area loading. Look at the modern windmill. If it's not practical to hang a larger dia prop on a boat perhaps several smaller props w high AR blades and huge disk area would do the trick. Penta screws anyone?

Easy

 

Hi Easy,
What I have learned from various threads, it seems twin blades is not a bad idee. Any change to give a reply to the e-mail to your Yahoo e-mail box?
Bert

 

Tom, There was an article , Southern Boating? ,where I first found the info on this boat.

His claim there was the boat burned the same 1/2 gph at speeds between 5K and 18K.

Thought perhaps he fell on a combination where the engine efficiency rose with the speed.

Most diesels stink at low loading so 5K would be pretty inefficient and as the rpm rose so did the loading and efficiency? Although it was only a 10 in prop.

I hope you will share your updated concepts of a trailerable boat as you finish up the design.

I found that model testing is very difficult as the model will not work when simply dragged .
Due to the hull shape it Must be powered , and measuring meaningful results , is far beyond my abilities.

FF

 

I think this may be the same boat that won a high prize at the Perdido WB show recently.

You are correct that the tunnel concept would not seem to prove much unless a prop was powering the tunnel.

I talked recently with Mark Bayne of Sea Island Boatworks in Charleston who built a Rescue Minor. He reported similar experiences to those found by Robb White as to stability, wake, etc.

 

The Atkins were, without a doubt, skillful practitioners of their art. I would not dare accuse them of prevarication, but I might suggest that they were infected with expanded optimism. Somewhere in my digs, is a yellowed copy of: How To Build 20 Boats. (old timers will remember that series of magazines). The Atkins had done a box keel Seabright Skiff style boat of about 19 feet. It was powered by a Baby Palmer. It was to make 17 knots with the puny little engine. Wow!

The Oyster Cracker, from the referral, planes at 5 knots? Pardon my raised eyebrows. That must be one heck of light weight boat. Maybe that canopy over the helmsmans station works like a parasail.

I am not being sarcastic here. I am merely having some reservations about claimed performance. I hope the claims are true because, if so, I want one. A wise old bird once told me that......girlfriends, race cars, and boats never quite live up to our hopes.

 

A very wise old bird indeed.

I passed right over the 5kt planing claim as just another wild one. I wonder how one would even determine that in a 20' boat? Hey, the boat is gliding along almost level, it must be planing! Bet your boat does it too

 

There are all sorts of claims for almost magical performance due to hull shapes that simply make no sense, such as box keels and inverted V hulls...

There are also very clear examples of boats that do achieve substantially better performance that the typical powerboat.

The human powered and solar powered vessels that actually work tend to exhibit the following common characteristics:

1) They are light. To move a boat, work must be done to move the water out of the way. The more water you move to the side, the more work to move the boat forward. One simply cannot get away from this fundamental fact of physics.

2) The are long and thin. Work is force times distance. By pushing the water to the side (and down) less, that means less work happens for a given displacement. The result of putting less energy into moving the water to the side is less waves. Again, you simply can't get away from the underlying physics.

3) The have very low wetted surface. The best performers have round bottoms. Think rowing shells: there are zero at any level of competition that do not have essentially round cross sections.

The efficiency of rowing shells is quite remarkable, especially considering they must perform at a range of speeds: they accelerate with every stroke, and then rapidly decelerate. An 8 man boat changes speed from about 10 to 14 knots throughout the race, and must remain very low drag across these speeds. These speeds are from about 1 to about 1.4 times hull speed for a 56 foot 8 man rowing shell. An olympic time is about 5 minutes 20 seconds for 2 kilometers, or about 22.5 km/hr or a bit over 12 knots average speed, or 1.2 times hull speed.

An 8 man shell weighs about 200 lbs, each scull weighs 7 lbs, each of the 8 person crew weighs about 200 lbs, the coxswain about 120 lbs, so a total displacement of including the coxwain is 200+8*(7+200)+120=1976 lbs, say 2000 lbs for a nice round number.

Various studies and analyses suggest that olympic level athletes can average between 250 to even 400-500 watts for the duration of a race. That's 2KW to 4KW (2.6 to 5.2 HP) to sustain 1.2 times hull speed!

At 1 gallon per hour for 10 HP in a typical gasoline outboard, that means between 23 and 46 nautical miles per gallon at 12 knots.

Note that the extreme displacement length ratio of 5, and beam length ratio of 36 makes the performance quite a lot better than the normal displacement boat models suggest: the Delft series suggests only 12.5 nmpg for such a boat. Other long thin light boats similarly greatly outperform the common models: iLan Voyager is about double the mainstream models.

The Gerr model that respects narrow hulls does match the iLan performance very well. Using his model, an 8 man rowing shell should be able to cruise at 35 knots at over 8 nmpg!

No special weird propellors required.

So I continue to think that a long narrow and light boat can easily exceed the target of 12nmpg at 12 knots. A typical boat sure won't.

 

David,

I don't have any doubt that long lean and light can make for the most efficient hull shape in fuel in vs speed over water.

The problem is that all such vessels ignore the very practical demands put on a boat intended for trailer cruising by a family. These are: accommodation, amenities, capacity for carrying all the stuff cruising people want with them, and the need to trailer the whole thing behind typical vehicles like motorhomes without exceeding legal limits. Working within such limitations plus high dockage fees for long boats, designers are driven to more normal L/B ratios of about 3:1 and greater weight.

Such long lean and light vessels just don't offer much to solve the trailer cruiser equation.

 

Such long lean and light vessels just don't offer much to solve the trailer cruiser equation.

However if the boat is seen as two parts , a long lean submerged hull supporting the weight ,LB 12+ or so,with no wave making resistance ,

and a top "plaining" section where the loading will only be 25% or less of the boats total weight you are neat the Austrian concept of a "Displacement Glider" , posted years ago to have very low wake making.

Only the drive setup and batteries live in the canoe boat , the above part looks and lives like any boat that size.

Combine that with the biggest most efficient propeller and it gets downright interesting.

"The Atkins were, without a doubt, skillful practitioners of their art. I would not dare accuse them of prevarication, but I might suggest that they were infected with expanded optimism."

We all love our work , no doubt the Atkins loved theirs.

But times have changed using their "lightest" construction the FRAMING then would weigh more than the entire boat would today , and that's using inexpensive expensive construction , no NASA needed!

FF

 

We can make the discussion a little more complex by contemplating engine output characteristics. A for real measure of efficiency is called brake specific fuel consumption (BSFC). That is a measure of: pounds of fuel consumed per horsepower per hour. Note...Pounds not gallons. Gasoline weighs something on the order of 6 pounds per gallon. Typical BSFC numbers for an engine in good mechanical condition is 0.50. That figure is only average because the engine will exhibit different consumption rates at diferent RPMs. A practical minimum for piston engines is 0.45 and may go up to 0.55 at some RPM range. (For 10 HP we can calculate.....0.50 x 10 x 1 = 5 pounds or about 0.83 gallons. ) From this you can see that an engine may have a sweet spot where efficiency is maximized. This is a good generality for any piston engine such as the one in your car, your boat, or your gas powered welding machine, etc.

Those figures are reliable enough but that does not settle the matter. We can produce Z amount of HP but how much of it can be translated into forward motion for our boat. That question brings us to prop efficiency and boat efficency. Boat design must influence the flow pattern at the prop with turbulences, vortexes, cavitations, and all that mysterious stuff. Our boat has a speed range which is optimimal in terms of power requirement compared with the speed that satisfies us. Call it sweet speed. All we have to do is match the engine sweet spot to the boats sweet speed and we have a winner. That aint gonna be easy. If our boat needs 23 Hp to run within its most economical speed regime, we need to find an engine that is most happy while producing 23 HP. Such an engine may have a rated max output of 100 HP or it might be rated at 36 HP max.. Our quest is to determine which is the right one. Some, but not all engine manufacturers, will provide BSFC information. Bless those who do.

P.S. The stated BSFC figures do not apply to Diesels, but diesels have their own numbers. Also we are talking about normally aspirated engines, not blown, nitroused, or turboed ones.

Fred and Tom, I have built several models with the canoe shape underbody with "wings" as mentioned here and on another thread. My models are relatively large, about 36" LOA. I have used them with RC electric power and also towed them. So far the models have been disappointing. After several iterations, I have abandoned the idea until I learn more. My failure does not mean that the idea is flawed. It only means that more work is needed until we discover how to do it.

 

"Some, but not all engine manufacturers, will provide BSFC information. Bless those who do.

P.S. The stated BSFC figures do not apply to Diesels, but diesels have their own numbers."

The easiest concept to use is called a Fuel Map.

HP up one side , RPM along the bottom.

The map looks like a set of clouds or circles one in the center (with the lowest fuel consumption ) then the next a bit larger , and with a bit poorer fuel consumption.And so on for 4 or 5 different fuel burns.

The few I have ever seen were very strange shaped , not round .

I HATE the fact that these are harder to get from engine mfg/marinizers than plans for a T-88 nuclear device .

Without them any choice of engine operating HP/RPM is simply a guess, use the torque peak to cruise is the rule of thumb , bit its still could be 200% off from a really efficient workup.

To use the mfg simpleton method of full rated rpm at WOT is to assure short poor fuel consumption at very modest RPM where most boats cruise .Here the CPP could be a good investment.

OR should the engine be chosen with 80% power at 90% rpm for cruisiing , there will be nothing left for high winds or large sea state progress.

I notice there are research being released from the Euro (Dutch) towing tanks which suggests modifications to the hull shape in front of the propeller (the area is no longer simetrical) will increase prop efficiency a few percent.

Of course when a ship is burning tons of fuel per hour , even a small percentage is of big interest.

Even Nordhaven has chosen to copy this research in its newest hull offering.

FF

 

There's a Rescue Minor being built around my parts too... I haven't had a chance to check it out yet, but look forward to doing so.

I would concur with Tom's comments about the practicality aspect of going long and lean. Having just completed such a design - a variation on my earlier monomaran hullform - I can attest to the difficulty of providing adequate accomodation within such a restrictive envelope. I hope to build a smaller version in the not too distant future, so we'll see how closely its consumption figures mirror those produced by the relevant performance prediction programs.
There's nothing terribly revolutionary here, however - long, lean and light isn't exactly a new mantra that we should all espouse. The trick is to whether one can manitain the efficiencies of the three L's and wrap them in a more palatable package.

 

The peak BSFC precisely coincides with peak torque.

I agree, Tom, that long and light often does not correspond to the typical packaging dimensions (L:B=3:1). And I agree that most people think they want so much junk on their boat that there is no hope for high efficiency, regardless of design.

However, I was raised in a family of 7, with a "4 place" airplane. Back then, there were not actual restrictions on the number of humans, but weight and balance was (and still is) inescapable. So we all learned how to pack very, very light.

Later, when I was in boy scouts, my pack would weigh 8 or 9 lbs and everyone else's packs would weigh 30 or 40 lbs. Guess who had fun on those hikes, and who thought they sucked?

Similarly, I've actually spent a lot of time on boats: over 80,000 miles at sea. Sounds like a lot, but I'm 52, and learned to sail years before attending school, so that's only a few thousand miles per year. While many boats have had TVs, in all these years, I've watched exactly one movie and zero TV shows on board. Many have had air conditioners, and yet I've turned those systems on exactly zero times. A few have had water makers, and these have worked properly for a complete cruise exactly zero times.

I spend a lot more effort taking stuff off of my boats than I do putting stuff on.

My wife and I are almost empty nesters: our third and last child moves out this summer. We will sell the house and build a boat to live aboard and cruise. We both have lived on boats before. We've been experimenting for awhile to see what we really need on board, versus what we have at home. Turns out, as expected, that boat stuff and house stuff are very, very different indeed.

I'm sure we will fit into a 5000 lbs 53 footer with less than 7 feet of waterline beam. 8.5 feet of overall beam. That's an easy to trailer load, no permits throughout the USA.

Trailerable liveaboard? Yup, because a lot of our adventures will be on lakes and rivers, and you've got to get from one to the other.

We'll keep a house somewhere for all the house stuff.