HULL of a Que$tion

I am contemplating a cruising boat that could operate in good weather to "hi" speeds , and would hope to be able to carry on when the going gets rough , by slowing down.

The Question is at what point does hull construction get REALLY EXPENSIVE , for that last Knot?

On displacement boats hull speed SL 1,34 is not a WALL, just the place where it gets really expensive to power faster.

While I doubt I would install enough power for 40K or 50K (16K and 24K are hoped for designed cruise speeds) I am wondering how the cost difference in a 30K hull construction would compare with a 50K hull?

As I like foam core , would it simply be a bit thicker core (say from 3/4 to 1 inch) and a few more layers of glass inside & out?

While full engineering shops can do huge and complex structural grids inside , as was described in PBB for a north Euro lifesaving service , my preference would be to just make the hull more robust .

Not too elegant , but suitable for a small shop , with out the engineers.

Skinney Boat 38.6 LOA 7.7 BWL and hopefully under 4-5 tons trailer weight.

FF

 

Very rougth assumption that need full checking.

Bottom pressure (MPY and iso 12215) id in the form :

P = A (1 + ncg)

ncg = B V²

So all parameters identical, a 50 kts boat should resist bottom pressure 4 times higher than a 25 kts boat.

( I do not speak of other minima as Pi Pb in MPY or Pmin in ISO)

Now, SM is linear with P.

So you need a SM * 4, so likely double the core thickness, and double the skin thickness.

So roughly said, going from 25 to 50 kts is likely to double the hull cost.

 

Fred:
Judging from some of your other posts, I would reckon that you are a pretty smart guy. Why, then, would you even think of a 38 foot boat with 50MPH potential ? FCFC has given some general information about hull skins. But that is only the tip of the iceberg. The rest of the structure must be engineered to absorb the huge horsepower that you'd need. Add the cost of the big power and shafts, props, tankage, fuel costs and........you will have an exponential increase in total cost that exceeds the bounds of reason for most of us. I've driven a few very high speed boats that gave me some thrills. In all cases the thrills were uncomfortable to say the least. In a case or two, several days recovery time for my bruised body was the price to pay for thrills.

Consider that you will need something on the order of one shaft horsepower for every ten pounds of boat weight for the purpose of approaching 50MPH. With a 5 ton boat, you are flirting with the thousand horsepower range. You can go 25Kn with 30 pounds per horsepower. That is a no brainer if first cost and/or operating cost is a factor. If you gotta go really fast then I hope you have as much money as Bob Nardelli (the Home Depot corporate rapist) or some other person with money to burn

Excuse the mini tirade please. A screen name like FastFred should have shushed me in the first place.

 

I think that about 25 mph is about the most efficient speed for most planing boats. At higher speeds than that the resistance due to skin friction increases in an exponential fashion requiring those scary horsepower numbers mentioned above.

 

What I'm after is a speedy cruiser semi displacement (to fit into a shipping container) , 24K being about as fast as could be ever afforded , in reasonably smooth water, 16K being the "reasonable" cruise speed a more modern Atkin Sea Bright hull design could efficently do. And of course a speed of 6-10 would probably work best for longest range.

I was wondering if more power would be useful at times , since the hull cost is only a small percentage of the Total boat cost.
IF if the about 200hp 5.9 Cummins were replaced with a 475hp Yannmar , I wondered what the expenses would be .

Not worth it to me , but I "had" to ask.

MY handle FF was given to me decades ago, As a Navy pilot 2 fellows in a squadron can not have the same name and operate . A fellow named Charles Fred XXX,(already there) hated "Charley" so was "Fred" for radio com..

When I showed up in the squadron with a Corvair 145HP stuffed into a Porsche Speedster ,FF it was.

FF

 

Fred;
Look into the Crouch formula for generalities about powering your boat. See Gerrs' Propeller Handbook among the more familiar sources. A lot of guidance in that book including Crouch.

I'll bet we could get some spirited collective arguments started about foam construction. I think that foam core is good for ice boxes and very small boats. For larger boats there may be some question. It is not a cheap way to build either. Klegecell and Airex costs a plenty. A messy, sticky, stinky way to build too. To its' credit, teredos are not much attracted to that construction.

I have a test panel of half inch Klegecell in front of me. It has FRP inner and outer skins of sufficient thickness to make it pretty rigid but it has not been over done. Vacuum bagged, it is not resin rich. It weighs 1.44 pounds per square foot. That is about the same weight of a similarly dimensioned piece of wood. Obviously there is no advantage in terms of bouyancy for the individual part. The stuff will not hold a fastener so that means that you'll have to do a lot of preplanning to locate wood inserts in the structure. Planning, like where you may put a cleat or some other piece of hardware. Of course you could always scab on a wood or metal plate to accept the hardware. If so, the boat may not be as tidy as first dreamed about. Then there are really critical structural elements like engine bed stringers. I think the foam core system can be used for a larger boat, and has been used, successfuly, but I suspect there are many problems to solve.

Hmmm ! Porsche/Corvair. Neat. I have a customer who is busily involved in stuffing a Subaru into his puny 914. That'll be neat too, and the 914 will no longer be a red haired stepsister.

Where is Ortona ? Between Orlando and Daytona maybe (something like Deltona) ? On the Caloosahatchee near Ortona Locks west out of Okeechobee ?

 

???
http://www.bcompsoc.org.uk/events/proceedings/060627/07 inspec and repair marine.pdf
http://www.barracudatec.com.br/pdf/RE_Paper.pdf

If both UK and Sweden rescue boats are built cored, perhaps it is worth something.

http://web.usna.navy.mil/~phmiller/Impact_study.pdf
http://www.compositesworld.com/hpc/issues/2004/January/331

On that one, I agree. A sheet of Corecell A500 will make a mahogany marine plywood sheet look inexpensive. I did not even speak of occume or meranti.

 

"Airex costs a plenty" .

But it makes a superb cruiser ,IF the weight of a single skin is used , simply split.

I have built such boats to USCG Sub T , for passenger service' and was very pleased with the boats cruising ability.
Not that messy , after the first dozen drums of resin.

Insulated , both for noise and heat .

The layup was 3 layers of 3/4 oz mat with 24 oz woven roving between , both inside & out. The inner layer of mat was not needed , but makes a fine easily painted and maintained interior.

As this boat will be subject to man handling going in & out of "the box" a robust hull is required. Also the ability to take the ground (the box keel) will require similar strong construction.

At 3lbs per sq ft of hull , I think a trailer weight of 8,000 to 10,000 with a 1,500lb power plant is reasonable.

I would entertain either wooden (Cold molded or strip plank heavily covered) or aluminum construction, but the added expense of HAVING it built might be a problem. Budget of $100K US.

My goal is a really simple boat , no rare unobtanium woods or fancy (and heavy ) joiner work.KISS , from end to end.

Probably will be far to Spartan for the "Eye Candy" set , but should travel , and wear well.

FF

 

No question that foam core is a viable method of construction. I defer to the Swedish and UK rescue boats. To be sure they will fulfil their purpose. Here's hoping that the Swedes and the Brits do not have a beaurocratic system as inept and irresponsible as our US dimwit agencies. Who else but The US would pay 500 dollars for toilet seats ? The sneaky inference here is that the method in question is not the most economical and not neccesarily more durable.

Just for fun consider the following true tale of boat building determination. There are a bunch of fun loving guys at the Bradenton Florida chapter of the Traditional Small Craft Association (TSCA) chapter. One of them was Dumpster diving, one day,when he found some huge styrofoam slabs. Boat guy that he is, he was compelled to play like Michaelangelo. Those blocks of foam had a boat inside he reckoned. He hollowed the inside a bit and pointed the ends a bit and rounded the chines some. Yup, looks like a boat. He had also found a bunch of unused latex house paint. He applied a lot of the paint and let it harden for a couple weeks. Then he applied glass and polyester resin over the whole thing. He explained that the latex kept the polyester from melting the foam. More house paint on top of the glass and he had a dugout canoe or more nearly a sit on top kayak. It worked rather well but it was as ugly as sin. That was the epitome of a minimalist foam core boat. I am confident that Fred's boat will be far more sophisticated.

I still think that construction would be faster, more simple, and cheaper with a cold molded plywood system as used by Reul Parker and others. One wood advantage I can envision is; you hole the boat on a reef in Tonga or traversing a Mongolian canal or some other remote place. You can find some wood for patching but may have a hard time finding some more Airex.

 

You guys are forgetting your history...

What about Lulworth?

40 ft long

8 ft beam

275 horsepower

8 tons(Is that right? I'm too lazy to look it up)

40 knots.

I've always wanted to see a modern version built. Tell us how it turns out, Fred.

Lance

 

I would not be so affirmative. I agree that plywood construction for boats under 28 ft, with thickness 3/8 or 1/2 is attractive if you accept hard chine boats.

Now for boats over 40 ft, where you need to scarf over 6 sheets in length, when you need a multichine design because 4ft wide sheets are not wide enough for the planking (say bottom planking at the middle, or top planking near the stem), when you need to stack sheets to build the thickness, or allow bending of 1" or 3/4", plywood become more complex to use than other building methods.

Between the two, your mileage may vary.

And also ask a professional welder if he would prefer build a composite cored or a plywood boat.

Beware of remote location. I have lived in such a remote location 20 years ago. It would have been difficult and expensive to find anything but coconuts at that time. But I can tell you that now, it has broadband Internet connection, police have speed radars, it is complex to find a parking place at the supermarket, and there are mooring fees. And of course, DHL, Fed Ex or any other similar can deliver anything there within 48 hours.

 

Reul Parker's method uses longitudinal planking of regular wood (recommended tongue and groove) sandwiched with two layers of rather lighter plywood strips in a double diagonal fashion with a glass epoxy covering. These layers are all glued with epoxy.
I think this could turn out to be a very good choice for the boat in question.

 

I do not understand the building process

is it from inside to outside:

glass
longitudinal planking
layer 1 diagonal plywood
layer 2 cross diagonal plywood
glass

but no sandwich there. It is a variant of strip planking + cold molded veneer, the veneer being replaced by plywood.



or


glass
layer 1 diagonal plywood
layer 2 cross diagonal plywood
longitudinal planking
layer 3 diagonal plywood
layer 4 cross diagonal plywood
glass

but what do you build first ? If it is the longitudinal planking, how do you build the diagonal layer inside?



or

glass
layer 1 diagonal plywood
longitudinal planking
layer 2 cross diagonal plywood
glass

same question as above.



All theses methods seem heavy and time consumming. Why not strip planking only : the longitudinal planking only over a mold + biaxial glass inside outside. No need for 2 or 4 diagonal layers of plywood. or stich and glue : one developpable layer of plywood sheets on a very ligth mold or bulkheads, and glass inside outside. No longitudinal planking, no diagonal strips to cut and glue.


Or maybe it is another way I did not understand.

 

A boat with a really narrow beam is going to need some really fine bow construction to flair the deck enough to get even a tiny work surface.

It IS necessary to be able to stand way forward to pick up a mooring , etc.

Strip plank might be easier than twisting thin ply.

FF