Aluminum Semi-Displacement Catamaran Build

[Bkay]

So they are in the process of taking big pieces of metal and cutting them into small pieces of metal then welding them back into big pieces of metal.

Here's a CNC router if you've never seen one.

 

[Bkay]

This is one of 2 watertight bulkheads. This one is the forward end of the engine room(s). A well known drawback of cats is chopped up engine rooms - but they have done a good job designing ER space within the constraints.

I'll have well over 5+ feet overhead clearance under the deck and large hatches to give unlimited overhead room in the cockpit plus we pushed the ER bulkhead forward to give plenty of room forward of the engine.

I'm getting old and crusty and don't really want to contort to do maintenance.

 

[Bkay]

The frames and skin are all 5086 and the design is IAW Lloyd's Special Service Craft Rules.

 

[Bkay]

So what do you do when you are Type A, it’s a rainy holiday, you don’t have a boat in the water, and your builders are moving like pond water? You restore a dinghy – that’ll burn up a couple weekends. I got this old girl from a fine gentleman in Annapolis a few weeks ago – I have not rowed her yet – but she looks like she’ll row just fine!

I had an interesting exchange via email on this subject and figured I’d share it with anyone who cares. First, all the common boat building materials have their strengths and weaknesses and if you play to the strengths and mitigate the weaknesses you can produce a great boat from any of the common materials. With aluminum, one of the major weaknesses is avoiding contact with dissimilar metals. In the case of the dinghy, it had a number of old, bronze fittings and an ablative bottom paint that surely had copper in it. To keep water from washing out small particles of bronze materials or copper bottom paint (both are more noble that aluminum), I removed all of that and replaced most of the screws and garboard drain plugs with stainless steel and I will repaint the bottom with a paint appropriate for aluminum hulls. I'm thinking red bottom paint.

If you have any questions, ask. I probably won't know the answer, but ask anyway.

 

[Bkay]

Turning rope into dockline is another good way to kill a few weekends. Cutting, splicing and whipping took about 1 beer per line. For some reason I can only do one or two lines at a time...

 

[Bkay]

I got a couple update photos. Looks like transom, both watertight bulkheads, and all 11 frames are in place with a few stringers tacked into place. These aren't great pictures, I'll see how they upload and delete them if it's too hard to tell what they are. I'll try to get some better pics my next trip down there.

The first shot is the transom with the propeller pockets visible. The second shot is the pointy end.

I've only had a couple questions and comments via PM. Maybe I should ask if anyone has an opinion on what anchor to use? JK - I'm using a 10lb mushroom anchor with 35' of 1/4" nylon.

 

[Bkay]

I got an email asking for an update so I figured I'd do it here.

We anticipate hull flip will be around 1 Feb. That's a little bit behind schedule - but the biggest hold up seems to be that one of my two John Deere engines is being held hostage in Mexico. More accurately, they are having issues getting it through customs. I don't know whether that's tied to the government shutdown or simply because my engine's asylum claim is being reviewed.

I'll try to get some more photos posted. BTW - if anyone knows how to get an engine from Torreon, Mexico to North Carolina, I'm all ears!

 

[Bkay]

I'm not sure who is reading this besides my mom, but on the off chance anyone is watching her come together, here's another update.

Framing is complete in the first shot. Those pretty bits of aluminum running fore and aft are the engine stringers.

Since the engine is delayed, they are constructing out of sequence a little and starting on tacking the house together in the next two shots. I'm still hoping the hull flip will be in a few weeks. They will flip the hull once the majority of the hull skin plates are installed. I understand the last couple skin plates get put in place after she is right side up.

If anyone is still awake, let me know. (actually, I just wanted to practice putting in an emoji; I don't have kids to show me how these things work.)

 

[sean9c]

Hey, I'm reading them. Thanks for posting and keeping us updated

 

[Bkay]

Hey, I'm reading them. Thanks for posting and keeping us updated

Thanks for the comment. I was starting to think about the old adage "When you are chatting in a bar, you're amiable. When you look around and your alone, you're a drunk."

 

[cardude01]

Cool project! Keep the pics coming!

 

[Symphony]

Yes, thanks for the pics too

 

[leeman]

Thanks for posting your build, I am very keenly following.
I don't know if you stated this already, but what are the fuel tanks made of? Capacity?

 

[leeman]

A question for the moderators. I normally open the site to "general discussions" and, after browsing those, look to the right side of the page to the "Trawler Discussions" column. I assumed any new discussions would show up there. I've not seen this thread show up there so I'm curious if I have my thinking wrong. I, only now, stumbled upon it. What goes into the Trawler Discussions column?

 

[AdmiralS]

A question for the moderators. I normally open the site to "general discussions" and, after browsing those, look to the right side of the page to the "Trawler Discussions" column. I assumed any new discussions would show up there. I've not seen this thread show up there so I'm curious if I have my thinking wrong. I, only now, stumbled upon it. What goes into the Trawler Discussions column?

Click the "New Posts" link in the top red bar. It will show you unread posts.

 

[Bkay]

I don't know if you stated this already, but what are the fuel tanks made of? Capacity?

Leeman, thanks for the question. The fuel tanks are aluminum. They are separate tanks (NOT integral to the hull) and are 150 gal each (two total). I considered all sorts of increasingly complicated fuel plumbing arrangements with different polishing systems, and ultimately settled on the most simple answer: two tanks, each with single draw and return and no separate polishing system.

The builder pushed for smaller tanks to keep weight under control, I wanted larger tanks just to brag about my range. Ultimately I settled on tankage that would give me a good combination of range at slower speeds but reasonable fuel turnover at higher cruising speeds. Since my cruising area is east coast, primarily Chesapeake Bay, I'm not as concerned about how much range I can get. If I were building a Bahama boat or I was in the PNW - I'd reconsider that.

 

[leeman]

Click the "New Posts" link in the top red bar. It will show you unread posts.

I started a new thread as I still don't comprehend the Trawler discussion sidebar.
Why does this discussion not shown up there?
The OP was wondering if anyone was following and I wasn't for the above reason.

 

[leeman]

Leeman, thanks for the question. The fuel tanks are aluminum. They are separate tanks (NOT integral to the hull) and are 150 gal each (two total). I considered all sorts of increasingly complicated fuel plumbing arrangements with different polishing systems, and ultimately settled on the most simple answer: two tanks, each with single draw and return and no separate polishing system.

The builder pushed for smaller tanks to keep weight under control, I wanted larger tanks just to brag about my range. Ultimately I settled on tankage that would give me a good combination of range at slower speeds but reasonable fuel turnover at higher cruising speeds. Since my cruising area is east coast, primarily Chesapeake Bay, I'm not as concerned about how much range I can get. If I were building a Bahama boat or I was in the PNW - I'd reconsider that.

Thanks, you answered all my questions, although what would you feel the maximum tankage might be?

 

[Genecop]

Looks great...THanks for the photos.

 

[Bkay]

Hey Gene, hope all is going well! Have you decided to pull the trigger on an aluminum cat yet? You might want to hold off and see if mine floats upright. Delivery is expected in May/June time frame. If you are still thinking about it you'd be welcome to come down and take a look.

Good luck in your quest!
Dave

 

[Bkay]

Thanks, you answered all my questions, although what would you feel the maximum tankage might be?

Leeman, I look at changes to aluminum boats as 3 level:
Minor changes that don't impact the cutting files and are taken care of in house or on the shop floor;
Material changes that require tweaks in the cutting files but don't impact the hull or major movements of weights, these are also done with in house engineers or outsourced at minimal cost.
Major revisions that require you to build essentially new cutting files.

Having said that, I think you could easily go to 175 - 200 gallons with only minor or maybe a few material changes and stay within my 38' footprint.

The first boat of this type I inspected had 2 x 200 gallon tanks and there was physical room for more, but it was a 42 foot hull. He had similar range at WOT as I will have because he had larger engines. You could probably fit even larger tanks in if you make them integral to the hull - then weight will likely be the limiting factor, not the space.

If you do a major revision you just need to set the volume of fuel and they can design the boat around that. But if you don't set some reasonable constraints, you'd obviously risk driving around in a freakishly large floating fuel tank.

As an aside, I set my water tankage and fuel tankage to give me a target number of autonomous days. If you increase your fuel tankage you would need to consider increasing your water tankage or add a watermaker. I suspect either of those would be easy fixes.

 

[Aquabelle]

I'm certainly reading....and enjoying....your posts. Please keep them up!

 

[Aquabelle]

Aluminium isn't regarded as a perfect choice for fuel tanks....did you consider going with plastic or some other material ? Or is there that being in an aluminum fabrication shop that material becomes the default for everything ?

 

[Bkay]

I'm not fully convinced you could get 3 boat builders together and come to a consensus on the perfect choice for fuel tanks. I would certainly be open to a proper plastic tank, but in this case, the ability to weld the precise shape to fit the available space and to weld the tank into place is a real benefit. Aluminum certainly isn't the default on everything - my water tank is plastic.

I have an aluminum tank on my current (wooden) boat and I've been happy with it despite the challenges of proper installation. But I think an aluminum tank makes a lot more sense on an aluminum boat. We have not gotten that far in the actual build, but if they propose a change to proper plastic tanks, I'd accept that in lieu of aluminum. I wouldn't accept any other material on this build even though there are well justified opinions of other materials being best.

Having said all that, all 3 of my prior boats had aluminum tanks. So maybe I am missing something.

 

[Bkay]

While I’m twiddling my thumbs waiting for my engines to be granted asylum in the US (see post 38) I thought I’d philosophize. When I started looking at building a boat about 10 or 15 years ago, I couldn’t get my brain around why they cost so much and frankly, I assumed everyone was wrong when they said you really can't get a good cruising boat cheap. Now I’m older, but not really any wiser, I figured I’d share my experience in pricing out new build boats.

I’m technically not under a non-disclosure agreement with my current build, but since I adhere to the philosophy that gentlemen don’t discuss women or the price of their boat, I’ll give you some real world numbers from research I did a couple years ago that never resulted in a contract.

First, I used to look at boats on a cost per foot basis. But anyone bothering to read this probably already knows that cost per foot is a poor metric because it ignores total volume. Cost per pound of dry displacement is more accurate because it accounts for all the "stuff" that went into the boat. It is of less use when comparing two different types or materials (aluminum costs more AND it is lighter - so you'd expect the cost per pound of an aluminum boat hull to be different than steel or fiberglass).

In the summer of 2017 I had a well-known NA develop study plans for an aluminum monohull new build. Here’s a sketch I did – it looked a little better when the NA was done with it, but I’m not really at liberty to show his product here. That boat was 38' x 12' and we estimated the weight at 19,500 lbs with a half load. It was to be powered by a single 160 hp John Deere. We estimated 10 to 12 knots cruising speed.

I reached out to 5 well known aluminum boatbuilders and all the estimates came in around the same range (give or take $25K). Including the cost of design fees, NC cutting files and additional engineering costs (which totaled about $28,000 for a 1st time build), the best value came to $539,553 total or $27.67 per pound (that included all design costs). That bid was from an east coast builder that produced good, honest boats without a “yacht-level” finish.

One of the bidders on that job also had produced several semi-displacement catamarans and I looked at the specifications for a catamaran with a comparable level of finish that was under contract at the same time. That catamaran was 40' x 13' and had an estimated displacement of 25,000 pounds. Since the cutting plans and engineering for that boat were already in existence (this company had built several similar cats before) the total cost came in at $563,860 or $22.55 per pound. While this was almost $25,000 more than the monohull with similar length, it was a much better price per pound. And this boat had 2 x 225HP John Deere engines.

As an aside, at that time I was told these engines ran around $30,000 each with reverse gear – so at 1300 pounds an engine costs about $23 per pound; similar to the overall boat cost per pound average. Frankly, I suspect these 1300 pound JD engines costs less than 1300 pounds of high end teak and mahogany joinery.

So in the case of the cat, the old adage of catamaran hulls costing double because there are two hulls didn’t hold true. I think the reality is builders often hedge their bets on a new design – so if you can build a boat that has been built before, you lower your risk of performance shortfalls and you lower the builder’s risk of underestimating construction costs.

One more point of comparison: at the same time I was helping a friend develop specifications for a weekender lobster boat built on a Duffy 35 hull with a 2’ extension in the cockpit. This boat had a shelter top and basic accommodations for two below. The final dimensions were going to be 37’ x 12' - almost exactly the same as my initial aluminum monohull. We estimated the finished weight would be 15,000 and the cost to build this boat at a nice, but not luxurious, level of finish by a well-respected builder in Maryland would be around $450,000 including design costs. So that’s significantly cheaper than either of the two aluminum options on a dollar per foot basis, but the cost per pound was close to $30.00. So in the end, it was a cheaper boat but a worse value (from my perspective – everyone has their own tastes and requirements). For reference, that boat had a single 300 hp diesel if I recall correctly.

I used to read every source on boat construction costs that I could find and I appreciate the work of those folks who educated me. Hopefully this is useful information to someone. The most useful thing I got from reading other’s input is that boats really aren’t cheap and just because it doesn’t make sense that a simple boat costs more than the median house price doesn’t mean it’s not true.

 

[Bkay]

A few more update photos - The first two pictures give a hint of how the propeller pockets are shaped. They also exhibit that universal truth, there's precious few things you can't fix with either a hammer or WD-40.

The small pieces of plating inside the tunnel and on each hull are done first. This is the most time consuming part because the smaller the plates, the more linear feet of welding required.

I understand they will cover a lot more ground doing the fairly large pieces of hull plating on the exterior of each hull. Obviously, the more you can plan in the use of large, uninterrupted pieces of plating, the faster the welding. A disadvantage of aluminum is the skill of the welder must be much higher (than for steel) and I understand you have to have greater control of the environment. A benefit is that the welding tends to go faster than with steel.

The 3rd pic is obviously the house with overhead frames tacked in.

These photos were from last week. They are targeting a hull-flip date of Friday; but I'm trying to exhibit a little combat patience.

 

[mcarthur]

Really interesting to watch!

Just for your information, our boat is aluminium too but uses the Strongall(TM) method (translated). Ours is 12mm hull and 6mm superstructure, minimal framing. I presume even more careful welding and skills are needed!

 

[Bkay]

I've heard of Strongall method, but have not inspected one. 12mm? Geez that's heavy!

 

[mcarthur]

I'm not an expert, but it seems that while the individual panels are heavier (possibly much heavier!), there's few/no ribs and few joints which saves a lot of weight. In theory strongall should be stronger due to the thicker panels and fewer joints, and the various expedition-level boats appear to bear this out although I doubt anyone has run the same style boat into a reef or an iceberg to test the theory!

 

[Bkay]

I'm not an expert, but it seems that while the individual panels are heavier (possibly much heavier!), there's few/no ribs and few joints which saves a lot of weight. In theory strongall should be stronger due to the thicker panels and fewer joints, and the various expedition-level boats appear to bear this out although I doubt anyone has run the same style boat into a reef or an iceberg to test the theory!

I recall reading a comparison about the two methods but I don't remember the results or the source (I'm sure it's hard to get an truly unbiased comparison). I'd guess it would be pretty hard to get a room of experts to agree on "best" or "strongest" - but it would be amusing to watch them argue!