The Aluminum Unicorn

Greetings boat design gurus.

I have dreamed up the idea for the perfect boat to add to my collection. The problem is, from what I can tell, it does not exist! This of course only means one thing, time to build it…but first, I want to see if anyone on this forum has any knowledge of a design like this.

Let me walk you through what I’m after.

I am located in the Puget Sound, and I love sailing and cruising on the trawler. The issue is that I am simply mortal and must work to afford such great luxuries. This means that I am typically stuck with choosing between only a handful of destinations for a weekend sail/cruise. This boat is my solution to that. Fast, functional, sleep on-able, durable.

I would like to keep this boat relatively small - 24ft max so that I can trailer it. I am planning on building it out of aluminum. I like the low maintenance aspect of aluminum and I plan to use this boat as an adventure rig, so durability is a must. It must be able to plane, preferably with a top speed in excess of 20 knots. I know that speed seems low, but that is because I want it to be an inboard diesel. I am thinking v-drive might be best because I am pursuing a classic tug design that is going to put the bulk of the super structure towards the bow. Direct drive would be ideal however.

I have worked on fishing boats in Alaska for years, and being regulated at 32’ in Bristol bay, designers have gotten creative and started building wiiiiiide boats. 32 feet long x 18 feet wide in some cases.

I would like to do something similar with this boat. I am thinking 10-10.5 feet wide will make up some nice extra space on such a short boat. I also want the extra width to add stability, because I intend to put a mast and crane on the boat to allow me to load my DR650 motorcycle on the back deck - no more being limited to areas local to the marina/anchorage. Finally I plan to have a low cabin behind the wheelhouse, just enough room for a mattress and to be able to sit up on it, and the head in the bow. In my mind, this is the ultimate adventure rig for 2.

So, now for the gurus. I would love to design the whole boat myself, but I am not so proud to think that my design would be better than one that’s already been proven. So, I am looking for a hull design that would meet my criteria. I fully intend to design the superstructure myself.

approx 24ft in length
approx 10ft wide
Planing
Aluminum
Classic styling, vertical bow shape, not sloped forward
Open to stern drive, but would prefer inboard because I can only imagine the cost of diesel stern drive parts…

is anyone familiar with a hull design like this? The closest that I can find are seine skiff designs, but the styling is wrong.

Also, please chime in on the practicality of this design. I know it’s going to be very unique, but it really fits my needs perfectly. I am thinking 5.9 Cummins for a power plant.

 

Probably not what you're after, but one might argue that this kind of loading ramp makes for a rather vertical bow And perhaps it'd come in handy for unloading the bike.

foil assist catamarans — Jutson Marine Design https://www.powercatamaran.ca/foil-assist-catamarans (or other's) foil-assist approach could probably help a lot with comfort in a chop for such a small vessel, and also to help it to transition / semi-plane at speeds low enough not to beat you silly.

Folks tend to associate foils with speed, but they work just as well at 20knots.
You can simply weld in a bigger one for early lift, stabilisation and efficiency.

It might allow to downsize the engines so much that you could have the redundancy of twins despite the small package, less noise and vibration, and also reasonably small props leading to better chances to stick with shaftlines.
Or opt for outboards (midsize commodity ones, no luxury stuff needed for that size i think), offering kick-up safety, lightness and more flexibility with the interiors.
Playing the weight/power spiral downwards puts beachability in reach, perhaps eliminating the dinghy (hard to fit a decent one on a 24footer), and will also make the boat easier to trailer.

Styling and living arrangements should be quite easy to adapt to your tastes on such a wide and stable platform.

 

I think all of your points are reasonable, however, I am pretty set on a more classic design. I understand the appeal of the cat for practicality sake, but i don’t think it I can bring myself to do it

 

There were some older designs floating around, seems like in the 90s that rough size was popular paired with some type of turbo 4. Guys off the road system that didnt trailer as much had a few built. Can think of one in kodiak that was closer to 28 but 10.5 or 10 wide witb a divorced v drive thats been around since the 90s. Doesn't trailer easy with the big wheel and the keel, and the deck is not clear.

Ive been kicking around the same idea as insurance and the cost of operating and amortization on my modern bay boat is high.

The 5.9 becomes a Chinese finger trap, if you put it aft for weight balance like the sb marine style with a divorced zf 220v... it either sticks up through the deck or makes you deal with a deep keel thats harder to trailer. Do an angle transmission and you end up sitting on it in the cabin.

Downside of going to the 4 cylinders is cost. The 4045 is 38.7k in a high output version and the new 4cta is 45k, add a gear and some driveline and it makes 300 hp outboards look like a positive bargain.

Remeber a 24 foot that was made somewhere in south central that had a marinized power stroke on a v drive to a small wheel. Rode on it when a fellow on alitak had it. Super efficient, relatively slow, and had to wear ear plugs in the cabin.

Seine skiff designs are a no go... those are purpose built little turds... expensive ones at that.

 

In the intended location is it permitted to tow such a wide boat without special permits?

 

I would only be towing a mile to store for the winter, and I might just leave it in the marina anyway. At that width, it’s just a quick phone call, so not a big deal to tow for me.

and what I would really like is to put the engine under the house, then have a few steps down into the aft cabin which is completely behind the engine. Thats why a direct drive would be preferred. I am also planning on putting ballast tanks at the stern for balancing the boat when the motorcycle is on board. (Planning on loading it on the port side for various reasons) makes me wonder if the engine weight is too far forward if adding a little ballast in the back would help balance it. I realize adding more weight is rarely the solution in a planing hull, but if 200 lbs makes a difference, that’s not a ton of weight. As for the ear plugs, I am well aware that I am going to have to put some serious work into sound proofing if it’s under the house

 

I can't really help you with a design, but I can give you affirmation that what you are describing can be a very useful, pleasurable boat to cruise and fish from. I have a 26 ft. solid glass deep vee design from New Zealand that I use for fishing, diving, and picnics here in the tropics. The hull is only 8 foot beam but carries the beam to the waterline with no taper (commercial crayfishing boat so designed to haul weight). I put a Cummins 6BTA 280, velvet drive trans, and Konrad 520 outdrive into the boat and so far performance has been flawless. The pilothouse had an Alaska bulkhead but I took it out since ambient temps are in the 90's here.
I think if you build out of aluminum you will have to locate the Cummins in the center of gravity of the hull, due to the lesser weight of the hull. This opens up the opportunity of either straight shaft, or vee drive and tunnel. I would not go for an outdrive in the log-infested waters of the PNW. Good luck with your journey, I have no doubt it will be very rewarding.

 

This is exactly what I was looking for. Good to know I’m not crazy. Why don’t small boats with diesels exist?? Boat fires are a fear of mine after some incidents I’ve witnessed, and I don’t sleep on any boat that has a gasoline engine on it now.

 

Because small diesel engines have been made so expensive by emissions that the added weight and system complexity for an inboard over an outboard makes it difficult to compete. 15 to 20 years ago we didnt have as many options for reliable big hp outboards and many makes of solid, reliable small diesels. For the relatively small market share, its difficult for the broader market to rationalize diesel power.

For me diesel is easier to get in remote areas and id like a 22-26 foot diesel thats about 10' wide and planes efficiently because Id like to run some small quota and have hydros. Reality is a honda running a pump is likely cheaper and easier to make and will have a long enough lifespan.

Your familiar with the bay, designs optimized to length restrictions.

The laconners of the late 80s and early 90s were fast relative to the horsepower inputs. Deeper than the competition but much more efficient. Would scale down to 24 @ 10 foot wide. The rozema, curry, and kvichaks had deeper tunnels and took a lot more power to have similar speeds (low 20s). Actually many of those had a hard time hitting 20s with rsw. Would think a laconner hull shape would be relatively easy to scale down to 20 some odd feet for a 5.9 in the low 300s to push right along. Would end up relatively deep with the drop keel design, but they were good sea boats.

 

No you're not crazy. Having seen a man go up in flames in a gasoline fire on a dock, I don't ever want to see it again. The good thing about aluminum construction is one-off design and build is not so labor and cost intensive as fiberglass. I would lean heavily toward prop-in-tunnel design so as to have a reasonable draft for beaching and trailering. One of the nicest advantages of a diesel inboard is you can have a bus-type heater at the helm station for when the engine is running and a diesel furnace for your fore and aft cabins while at anchor.

 

I like the idea of going with the laconner design. Those are nice boats. Do you know of any hull plans like that? I’m not opposed to designing my own hull, but a tried and true design would be preferred. I’m not too worried about draft as I have access to a steep boat launch, and I don’t plan beach often if ever. Riding motorcycles on beaches is typically frowned upon

 

You mean Unicorn ?

 

A wide-beam, semi-planing aluminum workboat hull adapted with modern underwater lines should be quite practical in Puget Sound, especially for weekend range and shoulder-season use. Expect the design to live more comfortably in the 18–22 knot semi-planing range rather than true high-speed planing, particularly with an inboard diesel like the Cummins 5.9 and crane loads. Starting from a proven PNW seine skiff or utility hull and redesigning only the topsides and weight distribution is likely your safest and most cost-effective path.

 

I am just wondering why you 'don't think you can bring yourself to do it' re considering a cat - what do you not like about them?

From what you have mentioned so far in your Statement of Requirements, a 24' x 8' (or 10') ally cat would have a lot going for it.
And if petrol is an absolute no-no, then you could have a diesel outboard perhaps (or two even, depending on what your budget is).
There are some neat little ally cats here -
Our Work — Chris Tucker Marine Design https://www.ctmd.com.au/our-work

This 24' ally cat from Specmar might be suitable, with a bit of adaptation?
24.5 FT CATAMARAN (2770) | Aluminum Boat Plans & Designs by Specmar https://www.specmar.com/aluminum-boat-plans/aluminum-catamarans/245-ft-catamaran-2770/

 

Unfortunately the cats are out of the question. Partially aesthetics, partially outboard cost. Can’t afford gas outboards, and definitely not diesel. I also can’t stand the look of an outboard. If I’m going to put all this effort into building a boat, I better love every part of it. And diesel is a must.

it’s one thing if it’s not feasible to accomplish everything in one boat, but I’m really thinking it is, so why change the design?