New old guy

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Alan Waters
Posts: 2
Joined: Mon Aug 08, 2016 6:13 pm
Location: Conover, NC

New old guy

#1

Post by Alan Waters » Wed Mar 01, 2017 8:12 pm

Hello to all. New to the forum. Recently retired welder. Preparing to build first boat. This is a purpose built tour boat designed to operate in shallow water. Here on the coast of N.C. shallow means one to four feet at low tide. As a Wildlife/birdwatching boat, getting close to shore is important. Let me describe the boat and perhaps you can help me with some material advise.
Picture two, Five ft wide, Four ft high, forty ft long sponsons,[.250 plate] which come to a point at the bow with a square stern and a flat bottom. These two will be connected together, four ft. apart with trusses which will be two ft. x four ft. Truss material is 2x2x.250 wall square tubing 5086 or 5083. Trusses will be on 2 ft. centers unless advised otherwise. Hull material will be double plated the length of the truss attachment points. This will be decked with .125 sheet and about half will be enclosed and heated/cooled. Need advise about stringers and ribs. Size and placement inside the sponsons. My idea is .125 material 4 inches wide on 2 ft. centers running across the bottom and up the sides for ribs. Same size for the stringers, 10 inches off the center line, both sides of the C/L.
This will be a boat which spends most of its time in flat water with a little chop on the sounds. Never offshore. Having said that, it needs to be strong enough to handle 2 to 4ft. seas which can occur on the sounds. Just in case.
Power will likely be a pair of jet outboards 50-90 hp mounted on cantilevered transoms.

Regards to all, Alan Waters

kmorin
Donator 08','09,'10,'11,'12,'13,'14,'15,'16,'17,'18'
Posts: 1379
Joined: Mon Aug 18, 2008 1:37 am
Location: Kenai, Alaska

Re: New old guy

#2

Post by kmorin » Thu Mar 02, 2017 12:36 am

Alan, welcome to the AAB.com Forum good to hear about your project.

I'd suggest you consider the book http://www.gerrmarine.com/Articles/BoatStrengthIBEX.pdf "The Elements of Boat Strength" by Dave Gerr (like bear) not the last word on aluminum construction planning but surely more informed than most designers on their first boat?

Some remarks on the concept: If the boat will haul over 6 people? and be for hire/charter/rental/ferrying work regardless of the water it will ply- there could be some USCG standards (Sub-Chapt.T Charter Boats?) that may need to figure into your planning? If not the Coasties, I'd want to understand where your State's regulatory agencies stood and what both these agencies think about your idea?

Next, I'd note; that your catamaran makes little or no sense!! if you want shallow- nothing is as shallow for ANY given load/mass/displacement than a big ole' barge. That would be a scaled up jon boat in general terms. You don't gain one single thing being a cat hull for shoal draft- you loose plenty.

What are the tall sides for? I think I'd want 2' or 30" max sides? Are the people going to be viewing 4' off the water? Not sure on this aspect but the "extra" 2 of sides seems a little tall for a tidal swamp tour?

Catamaran hulls are for a sort of limited use - that is; the gain of the hull form is limited to a set of conditions that make those gains useful when the boats' intended use can make use of them. Fast speeds with minimum power and small (relative) loads so a few passengers that need to be somewhere in short time- like a water taxi or ferry make sense. Slow cruising tours- not so much.

In the salt, Cats' speed is limited to wave conditions about 2/3 the chine-to-center bottom ht.; otherwise they slam like a jon boat. I know all the cat adherents would will not agree; but they're pretty useless at any more than displacement speeds in really high seas.

So your idea of 2- 5' chine bottom ama's with a mere 4' spread is not really very draft effective compared to a simply 10 bottom power scow. The cat will cost 2x or more to build in this size range- the draft is deeper, turning at slow speeds much worse and the deck between two 40' hulls will truly be more work than the entire scow hull.

[FULL DISCLOSURE: I have not designed or built a catamaran. I have not built more than a couple boats 40' LOA or larger. I have no formal marine engineering or naval architectural training.]

I'd tend to think of a super shoal draft scow tour boat as more realistic for the waters/shoals you describe? I'd spend the money saved from the cat hulls 2x-3x surface area on materials and labor to have small diesel inboards with Tractor Jets, and huge generator/alternator power heads driven directly off the engines so the main propulsion could be the air conditioning power. http://www.namjet.com/

A jet outboard gives up 50% of its rated hp or delivered thrust at low speeds and you're planning to move a displacement speeds - so using inboards and pumps is much more reliable in regard handing a 40'er- even in shoal water.

No point in discussing scantlings or alloys until you go around the design cycle a few times and get a final SOR (statement of requirements) this is the initial step in marine design. Once you've got the needs finalized you can start doing the preliminary research to make design decisions about the "next level" of complexity.

If the primary purpose is to haul sight seers? then the cabin to do that - house them comfortably is the purpose of the entire boat. so: "The main purpose of this vessel its to haul an air conditioned cabin with accommodation for SomeNumber of people in 12" of water- or less if possible- so the hull can beach on soft mud beaches."

See? that puts the dynamic load as an estimated value (say- 6 people at 250lb each is 1,500 lb dynamic load). They need 6 chairs with X square feet of space all inclosed in a cooled structure. (now you can size the cabin seating with it overall size) and that will give the Air Conditioning capacity rating (volume of space to cool) - which will give the size of and HP needed to run that refrid'g pump.

That is the method used to initiate a design. Hope this helps you to consider the reality of what you're describing; I'd say welding and metal decisions are Way - Way over the horizon at this point.

Once you edit the text narrative of the boat (still design cycle or design helix initial stages) then- you'd move up to the hull design with some facts in hand. The equipment list- including the holding tanks for the fresh gray and black water, fuel, engine and reefer equipment, controls, helm and general hardware wts. This list allows you to enter the hull design (shape only) phase with known adders to the hull's mass based on area of materials and wts.

This is all needed just to get an estimated waterline!

Just my view of the project. Please let us know other details, it sounds interesting especially if you're building in welded aluminum?

Cheers,
Kevin Morin
Kenai, AK
kmorin

Alan Waters
Posts: 2
Joined: Mon Aug 08, 2016 6:13 pm
Location: Conover, NC

Re: New old guy

#3

Post by Alan Waters » Thu Mar 02, 2017 7:04 pm

Thank you Kevin for your response. I will answer your questions. I have been torn back and forth about the Cat vs one big flat bottom. You are right about the one big flat. I chose the Cat for what I supposed to be comfort for the passengers. Less roll. But, there is not much roll on flat water is there. So, your answer has swayed the jury. [Flat Boat] And, this is exactly what I was looking for when I posted. I was trying to avoid the bow slap that comes with jon boats so I'll just have to figure out how to put a pointed bow on the boat. As for the 4 ft. sides. This boat must be wheelchair friendly. Trying to get near dock height as possible. Depending on the tides. Also I must [ for my dear wife sake] insure no rouge cotton mouths or alligators attempt an unauthorized boarding. Like you said. 30 inches should be plenty.

Now a question. My idea of power would be one maybe two 23 hp diesels. Has anyone used a jet pump on low hp? Don't need to go fast, just go. But, having enough hp to keep the boat under control and moving in event of high winds is a must.
Again, thank you for the help. Regards to all, Alan

kmorin
Donator 08','09,'10,'11,'12,'13,'14,'15,'16,'17,'18'
Posts: 1379
Joined: Mon Aug 18, 2008 1:37 am
Location: Kenai, Alaska

Re: New old guy

#4

Post by kmorin » Thu Mar 02, 2017 8:21 pm

Alan, The scow will roll less than the cat since the CG of the cat is much higher off the waterline and CB than the scow in section. So the scow/barge/big jon boat rolls less than the comparable cat- pontoon for the same sea state in a beam sea.

Wheel chair access, you're going to end up with some form of passerelle- gangplank that folds up/stows into the boat. http://www.nautical-structures.com/boar ... gangplanks examples only.

Dock to deck ht: In our area docks float, they're not on piles due to the tides here we see 20-30' per day changes- twice a day. So if you're working off or against a low tide area where the docks are on pilings??? I can see that adjusting deck to dock may be a good idea -however, if the dock floats- then a low deck is fine- regardless if the deck is above or below the dock- gangplanks are easier to build than making the entire hull one or another ht regarding the dock.

A 40's scow doing 6 to 8 knots is not going to transmit the wave impact at the bow to crew/passenger un-comfort!!! the mass is just too much. A 5' wide 15' long Jon boat that weighs less than I do (couple hundred and then some!) will be slapping and you'd 'feel it' but with a 40 scow you won't feel 3' footers even at a couple knots headway. The bow may pitch (lift to the swell) but that will depend on the period. If the distance between swells is 1/2 the length ( ie. 20' between them) there may be some lift but if the waves are close- chop in an estuary- it's likely the bow will just 'stomp' the swells with little motion.

A 40's scow even lighter than its cat cousin will weigh some big tonnage! I'm not discussing the boat moving fast- as you didn't indicate you're anticipating more than a 'walking speeds'?

A scow is ramped in the bow, and can be at the stern, and that ramp allows the wave impact to be a "glancing blow" of impact- not like you're planning a vertical bow.

Propulsion, look for the low speed jets and see what the min. horses needs to be? Remember if you're carrying 'air' for the cabin (?) you'll need 1/2 the power out the stern and plenty (1/2 the hp?) inside for the cooling? I've never done tropical latitude cabin air conditioning but I have done some on-board reefers for commercial fishing and the amount of power required will astound you- at least it did me. Further, IMO with all things diesel; the engine rpm has to be 10-15% either side of peak torque (rpm) on the engine curve or you're wasting your money and time.

The design cycle I mentioned is how all this is done? Don't use that method (?) and you'll chase the project's details out of sequence 'til you're pushing up "home grown tuh-maters"!

ie. if you're a retired welder you could ask your self if you'd choose a MIG, stick or TIG weld alloy without knowing the weld spec? OR; would you just start welding the pipe butt joint if there was no end prep, line up, length cuts and or other fits planned? Or, would you just light up and weld if there was no plan/prints/payday coming because the client hadn't agreed to the work yet?

Sequence is success.

I'm driving at the "cart before the horse" approach you're asking about? Where is your hard-n-fast SOR? If it ain't there, not completed and agreed.....? you'll chase your intellectual tail for a long time before you can get a single step done that is part of the final project.

I've done two 40' scows of welded aluminum and while they were not powered, many of the other boats I've done were power boats. You may not find this believable (?) but the single hardest part of designing and building these boats was to get the owner to answer my questions and then leave the answer alone. It may not seem like that would be true, and that I'm exaggerating- I'm not; it is completely true.

Engine power (a conclusion not a guess or preference) comes way down the road from where you are now! I'm using that 'assumption' as a means to point out you're not plotting the course of your build in the same way larger projects are actually designed.

Allow me to encourage you, again, to write the narrative (SOR) then discuss it, and nit-pick it. Then find locals who know the waters and get them to find fault with our concept and concept sketches? UNLESS there is another one of these already doing the work you're planning to do?

I will mention the phrase most builders share with new or first time builders- "That first couple dozen boats are the most difficult." I learned the hard way (sometimes called "My Way" or "Kevin's Way") about the design cycle and how super critical it is.

How important is the foundation of the house? If it's #1 Not there? #2 Incomplete? #3 What if the foundation is out of kilter or leaned to one corner (!) #4 How's the rest of the project going to be done? #5 what's the rest going to look like?

I'll be happy to help; but I'll suggest that exactly like the welding trade if you don't use sequential methodology to get that weld in sequence to the overall fab process (Like weld before you cut and fit- for example) things may not work out well?

Being able to weld is helpful but you're at the design stage- not the welding stage.

Cheers,
Kevin Morin
Kenai, AK
kmorin

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