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Bkay 11-13-2018 11:16 AM

Aluminum Semi-Displacement Catamaran Build
Good morning. I have communicated with a few of you over PM while building my specifications thank you to those who provided good food for thought! At the suggestion of one of our members, Ive decided to try documenting the build of a new boat.

The vessel is a 38 x 14 x ~2 semi-displacement, aluminum catamaran being designed in Canada by Scott Jutson and being built by US Workboats in Hubert, NC. This is my second new boat build and my first aluminum boat. After many years of having both wood and fiberglass boats mostly yacht finish boats Ive decided to experience the pleasure and pain of aluminum for a change of pace. However, Im getting too old to continue painting and varnishing (and was frankly never very good at either) so Im keeping the exterior of this straight up workboat finish and leaving her unpainted above the waterline. Interior will be a little more yachty, but not too much.

US Workboats (who spun off of Armstrong Marine in Washington State) do primarily commercial vessels, almost all of them aluminum and many are cats. Ive seen and been on several of the boats the companys owner has built over the years and I was fortunate enough to get a sea trial on one of their 42 x 13 semi-displacement cats that Id call a close cousin to my boat. While the PNW is loaded with aluminum boat builders, they are almost unheard of out here on the East Coast. I hope they do a great job with the build and I hope the boat shows well for the company (and for me, of course). I think we need more quality aluminum boat builders on the East Coast and the only way to get that is if there is demand.

She will be powered by two M-1 rated John Deere 4045s. They will produce 160 hp at WOT of 2300 RPM and should hit my target speed of 14 knots without too much effort. She is in design stage, but we anticipate a fully loaded displacement in the 25,000 lb range (hopefully a little lighter). Ill do my best to post the type of info that I find interesting when looking at other peoples build threads. Ive learned from a number of people over the years and figured Id pay the favor forward if anyone else is considering an aluminum or cat build.

Oh, as an aside, I dont usually post on forums and I have a pretty bad sense of humor so Ill apologize in advance if inadvertently offend anyone. My wife usually proofs my emails to make sure Im behaving in public but shes not participating in this thread. 😊

Now - to see if I can upload a couple perspectives.

Bkay 11-13-2018 11:20 AM

2 Attachment(s)
Apparently I can't upload perspectives...let me try again.

These are working models - but cutting files are being built and cutting starts in short order.

Bkay 11-13-2018 12:35 PM

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Alright, trying again.

Bkay 11-13-2018 12:37 PM

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There, I think I broke the code!

djmarchand 11-13-2018 01:51 PM

Very nice project!

Sorry I didn't comment while you were in the planning stage, but let me offer a comparison with the Island Packet Cat 35' that I owned for several years. That boat had a beam of 15', a center hull for breaking waves under the bridge and also for lowering the cabin sole to also lower the cabin roof so you could easily see over it from the center helm position near the transom (it was a sailboat after all).

You shouldn't have that problem with your pilot house helm position and your cabin height is much taller than mine.

Your weight target seems entirely achievable, maybe a bit conservative. My boat weighed about 18,000 lb light ship (measured on a travellift). Yours is longer but narrower and doesn't have the center hull or mast so you should easily hit the weight of 22,000 lbs.

My boat was really a full displacement hull, even though the hulls were narrow so the 1.34*sqrt(lwl) rule doesn't really hold it butwas very inefficient. It took full power from its twin 27 hp Yanmars to hit that value. It should be able to almost do that on one engine if it were an efficient full displacement hull.

Two things caused that I believe: the drag of the center hull and the relatively heavy weight for its length. So keep the weight down as best you can.

Post some interior renderings when you have them.


Bkay 11-13-2018 02:17 PM

Thanks for your comments. We've changed the interior layout so all I have is the original concept (which didn't fit well). I should have the updated layout shortly.

I don't have what I guess you are describing as the center nacelle - but I have a fair tunnel height. I was fortunate enough to get some time on several of Armstrong's cats on a recent boat hunting trip out to Washington State. This tunnel design has received some positive comments from guys who fish commercially in rougher waters than I'm seeking out.

Yep, weight always seems to be the killer with cats.


Bkay 11-13-2018 02:36 PM

I looked up Packet Cats. I have never been on one but I see what you are talking about. It's not an actual nacelle. I like their name for it - Deltapod has a nice, space age ring to it.


Bkay 11-16-2018 10:25 AM

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Let's see if I'm getting better at uploading images?

This is a second round general arrangement just to see how things fit. We'll be deleting the Dickenson on the starboard side so we can extend the settee and swaping out the sink for something more in scale. Everything else is just tweaks and adjustments to use space better.

Dealing with a builder that uses fixed price contracts and change orders gave me some concern. I just got to experience our first change order discussion and I liked the way they handled it. It's all been fair and above board so far.

The designer has also been good to work with. He has a good reputation but with some NA's, bedside manners are not in keeping with their design skills. Jutson has been good to work with and appreciative of my questions and inputs. Granted, I'm not telling him how to design the boat - just what my uses and preferences are.

Hope this is of interest to folks.

djmarchand 11-16-2018 11:34 AM

Interesting. That centerline queen forward with steps down from the main cabin on either side. Who gets the easy access to the head without having to go up, around and down?


ssobol 11-17-2018 01:52 AM

Doesn't look like there is much elbow room at the business end of the head.

sean9c 11-17-2018 12:58 PM

I'll admit to not knowing much about much and less about cats but the interior sure looks small considering it's 38'X14' and I always thought that weight was a big issue on multihulls. 25k# sure sounds heavy for something with not much in it, a whole lot of aluminum I guess.

Genecop 11-17-2018 06:58 PM

I really love the looks of your future Cat...As far as the toilet placement, as long as you fit your rear end in, your good to go...

Bkay 11-19-2018 12:10 PM


Originally Posted by djmarchand (Post 715225)
Interesting....Who gets the easy access to the head without having to go up, around and down?


Ha, the short answer is "no one". There is a lightweight bulkhead on the inboard side of the berth. So both have to walk around to the head and one person has to crawl over the other. In answer to "who has to crawl over to get out" the answer is my wife (unless she wants to be responsible for checking on things that go bump in the night).

Bkay 11-19-2018 12:13 PM

To those commenting on the head - yep, that's a valid issue. We are currently in negotiations on how far aft the toilet can move to balance elbow room at the toilet (although it's not really my elbows that take up a lot of space) and keeping as much space as possible aft of the head.

It's always a compromise.

Bkay 11-19-2018 12:23 PM


Originally Posted by sean9c (Post 715532)
...the interior sure looks small considering it's 38'X14' and I always thought that weight was a big issue on multihulls.

This boat follows my tradition of cramming a 35' interior into a 38' boat. The reality is side decks take up space, a large cockpit with engines takes up space, a boarding platform takes up space, etc, etc. However, these were all requirements that I felt were worth giving up cabin space for.

The original boat I looked in when deciding on this plan was 42' x 13' and was built out in PNW. I put a lot of thought into layout and how small a package I could fit my requirements into and this is the result. I'm satisfied with where we stand now and I have held firm at 38' - there is always pressure to go another foot or two and I'm glad my requirements fit without making that concession.

Certainly not a boat for everyone, but it's a close as I can make it to a proper boat for my needs.

Insequent 11-19-2018 03:29 PM

Good luck with the build. Sounds like you have researched and planned thoroughly.

Having a fixed price plus change order relationship with the builder is perfect. You will always know exactly what the cost is going to be, not often the case with boat builds. Of course that relies on the design at the time the fixed price was set being effectively complete, and being so close to previous boats they have built it should all go well.

I would have the 4045's rated at M3. They will not use any more fuel, and there is potentially a little more top speed if you want it. Rating at M1 is really for running 24/7 all the time, which is very unlikely. You can still run M3 '24/7' if you need to occasionally, provide that you stay 400 rpm below WOT. More likely you would in practice be at an even lower rpm, so you wont be stressing the engines much at all.

djmarchand 11-19-2018 04:46 PM

Perhaps this is a good time to share my contracting experience, both with fixed price plus change order contracts and time and material or cost plus contracts. This experience was not in boat building but from forty years of building heavy industrial projects.

My company's prime contracts were probably 70/30% cost plus/fixed price. Our subcontracts were about 90% fixed price.

Cost plus contracts rarely had contentious financial problems. Yes when we sometimes overran the budget the client wasn't happy. In most cases there were legitimate reasons but not all and in a few rare instances when it was obviously our fault we made a price concession.

Probably 80% of our fixed price contracts had contentious financial problems. In virtually all of these it was due to the client contracting with a loose scope. When the inevitable change orders came up the client didn't want to pay, almost always because he told his management that the fixed price was a fixed price. In our case we recovered about half of our proposed change order amount and in about half of those we lost money on the contract as a whole. One thing that was quite different from your situation is that we hoped to do more work for these major companies and sometimes would just suck it up for good will. That is not likely to happen in a boat building contract.

Our subcontracts faired similarly but in these cases we had a much more solid scope because we knew the dangers of a loose scope and these were pure construction contracts and our engineering was pretty tight.

So our biggest problem and yours too was a loose scope that begs for lots of change orders.

But even with lots of change orders there were subcontractors who bid low and tried to make it up with unreasonable change orders. We negotiated hard and in almost all cases, well all actually as I had no litigation over change orders in my career, we came to a decent (I won't say reasonable) price accommodation.

You no doubt hope your scope is tight and that your builder will be reasonable as the inevitable change orders come up. I wish you luck.


Bkay 11-20-2018 12:12 PM

Doug Hylan (a great American and darn fine boatbuilder) wrote a great article in Wooden Boat magazine a number of years ago discussing Cost plus Fixed Fee contracts.

I have come to agree with his position that these provide the best alignment of interest between a builder and buyer. Both Cost Plus and Firm Fixed Price contracts provide incentive to the builder to use change orders as a profit center. [Unfortunately, I've been tangentially involved in a fair amount of contract negotiation in my military life]

To date, I've seen no indication that my builder is doing that and I did business with them because I have faith that they won't. But I guess I won't know for sure until we are done with the build. If they do a good job, I'll certainly report on that.

Bkay 11-20-2018 12:24 PM


Originally Posted by Insequent (Post 716005)
Good luck with the build.

I would have the 4045's rated at M3. They will not use any more fuel, and there is potentially a little more top speed if you want it.

Thank you. We are in the final stages of layout and cutting files should be built within a few days.

Regarding the rating - of course you are right that it wouldn't cost me anything in terms of fuel to rate at M3 and I'd have some additional speed. But my logic is I could rate at M3 and have 200 hp at 2500 RPM - but I don't really need that speed.

Alternatively, I could rate at M1 and only be able to turn up to 2300 RPM and hit my target speed. But at the same time, we will also increase propeller pitch so the engine was fully loaded at 2300 (I honestly don't know if they go for 100% load at WOT or something slightly less; but it will be more pitch than if I were in an M3 engine that was fully loaded at 2500 RPM).

So at any RPM up to 2300, I will be more fully loaded and will travel faster on essentially the same fuel burn as an M3 engine. Your right, I can't go OVER 2300 RPM with my engine - but not going over 2300 because I don't want to or not going over because I can't is immaterial.

In reality, if the John Deere 150 hp engine was still available I would have chosen that. Overall you are probably right in that I might have been better off with M3 or M4 rating - but I'm not one to pine over speed I don't have and we optimized the hull shape for this speed then found the best engine that would produce the required HP.

djmarchand 11-20-2018 12:29 PM

Well, in general I agree. The fixed fee covers the builder's overhead and profit and it is truly fixed, well unless you add 5' to the boat. He gets paid all of his labor and material costs on a reimbursable cost basis, so change orders become a way to track the budget, not a profit center for the builder.

Sounds like you have some experience in the contracting business.


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