Kurt Hughes 38' Trimaran Trawler

A neighbour is looking at this design, basically it's a power tri designed as a kayaking tender, long range/low fuel consumption. The guy is particularly attracted to its long-range cruising capability, and is likely to use it single-handed much of the time with occasional family onboard..

Any comments? It's lovely but does not seem a good candidate for a first build, looks like a bit of a handful for a professional builder in fact.

The retractable amas seem very small; would that indicate a design not intended for heavy sea conditions? I suspect this is a purely inland waterways boat despite its size, which would suit the intended use, but it might venture onto a Great lake occasionally.

Twin 15 hp outboards are mounted in nacelles on each side of the main hull; I wondered why the outboards aren't hung on the transom, but I think that's because the height of the transom.

http://www.multihulldesigns.com/designs_stock/38tri.html

 

Mentioned in this thread as well

http://www.boatdesign.net/forums/powerboats/searching-fuel-efficient-powerboat-37835.html#post461334

Why exactly?
No different to any other composite build

They are only there for balance, like the outrigger on a canoe
they do not have to counteract sail loads

These vessels also only have very small floats

http://psipunk.com/us-navy-orders-trimaran-littoral-combat-ship/

I would prefer a small light diesel for that build
a 40hp Lombardini weighs 115kg, about the same as 2 x 15hp electric start Honda's

 

With more beam more kayaks could be accommodated in protected waters but the slip size might increase. In rough seas the kayaks should probably be hauled onto the main deck which might be challenging with the limited room. I'd rather be sailing but this boat would also handle a hybrid fuel electric motor though finding deck space for kayaks and solar cells might be hard.

 

Thanks for the information and advice. I guess for a powerboat the small amas make sense: no need to size them to fly the main hull . . .

I was concerned with the vacuum bagging, composite build and cylindrical mold construction - seems like a lot of new techniques to master all on one boat, compared with a ply-on-frame design; way lighter of course.

The diesel appeals to me, gets rid of those nacelles which could be a challenge to build. More economical. It would be nice with a stern drive, although that would increase cost and probably maintenance.

The guy interested in this boat is not a kayaker, he just likes the range and low fuel consumption of this design.

Another question: the amas look to be slightly above the waterline presumably so they can be "flown" in calm weather for minimum fuel consumption. With them retracted for docking, does that create an unstable feeling when boarding? How does one board such a design anyway? I cannot visualize it and there are no pictures I have seen of the boat in that condition.

 

Unless I misread- the figures for fuel consumption are incredible: 29mpg @ 5knots for a 38' 5000lb boat is flat good-and exciting. I wonder if that speed on that boat could be achieved with current solar/electric tech?

 

Cylinder mold construction is a very easy method. My only complaint is maintaining an accurate hull is tricky. In most cases this is not a problem as absolute accuracy is not required.
As to suitability for the owners use, well he should chat with Mr. Hughes. He is straight forward about this stuff and would give the guy a good idea if it suits his needs.

 

I think the keyword is estimated.....it would be interesting to see the real world figures versus the advertising. It is interesting that higher rev out boards are called for versus the big wheel reduced gearing work boat versions. I'm wondering about prop size, pitch and slip rates.

 

My thoughts on fuel consumption and range (apart from Huh?) are, would I want to do 2,958 nm at 5 k? I mean, that’s 3-1/2 weeks of chug-chug . . . it’s really a meaningless datum - 11 k for 3 days would be bad enough for me but I have a notoriously short attention span. Of course the least puff of wind in the wrong direction and those figures will go out of the window.

However, for the kind of mileage a vacation boat might actually get it’s probably cheaper than sailing once the cost of the rig and its maintenance are taken into account.

Interesting thought. An electric trolling motor would push this thing along at a decent rate, I suspect. The “Minimal load, one engine at low speed” speed of 5 k probably requires around 2 hp or 1.5 kw, not sure what that translates into ITO solar panel area but there’s plenty of space forward.

. . . it is partially related to a strip-built method I used recently for a constant radius bilge canoe; for that I had to make 30 perfectly matched ribs - it took forever, never again! But only one cylinder mold is needed I assume, and one just trots off hull sections until one has enough. Still time consuming I suspect, but then we are talking about a much larger boat here.

 

I have also been thinking about a "mother ship" for small boats. Much of the year we live on board a cruising boat, whether in the Bahamas, Europe or the Pacific North West. We are often anchored in a safe, beautiful anchorage and don't want to leave, yet want to go out and have fun on the water for a few hours.

I started by drawing a 36ft "podcat" style powercat. Each hull had a 16ft long "hold" or stowage area as I wanted to take, not a kayak, but a 15ft sailing trimaran. The concept worked but I didn't like the styling of the centre-cockpit layout.

So I have now drawn a more conventional powercat, although the trimaran will now have to be stowed on deck rather than hidden below.

The powering estimates given above seem to be in the right ball park, except at very low speeds, although I agree few are going to motor far on tickover, whatever the fuel consumption.

But I decided to go for bigger engines, so have drawn twin 60hp Etecs which should give cruising speeds in the mid teens and 10mpg at 8 knots. 60hp because they are not that much more money (in the grand scheme of things) and the same weight as 40hps. I don't want to use inboards because they are heavier (once you including the piping/controls/shafts etc) and because the noise is in the boat not behind it

I have motored one of my 36ft sailing catamarans at 8 knots powered by twin 9.9 Yamahas. Years ago I ran several outboards on the same 35ft Banshee sailing catamaran. A 4hp pushed it at 4 knots, twin 90hps at 22 knots when towing a waterskier. So maybe a 5hp will power this trimaran at 5 knots, only a real life trial will give the answer.

Obviously a powerboat hull will be more efficient than a hull designed for sailing and it will be even faster without the airdrag of the rig.

I know from experience that the 9.9 Yamaha does about 10mpg at cruising throttle on a 35ftish 6000lbish multihull. I also know that most boats are/become overweight and you need more power to punch into big waves or strong winds. So I am assuming a 7000lb total displacement on my 36ft cat, which really has the accommodation of a 32ft boat, but on long thin hulls.

I will be experimenting some more with my 20ft Skoota over the summer prior to building the larger boat

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Hmmm I get 14+ mpg with a Yamaha 9.9 on the Nicol running at close to 6 knots and more at lower speeds. It tops out at 7- 7 1/2 but would still hit 6 with a conventional 7 1/2 hp. At cruising speeds, say 2/3 to 3/4 throttle there is virtually no slip. What I noticed about higher gearing and fast props is the large increase in slip as soon as a headwind is encountered.
I appreciate the drag of a rig but noticed one power cat still needed something below the waterline as it was all over the place in a crosswind.
I'll stick to sailing green even if fuel prices go down, at current prices I try not to start the thing at all.
I liked the auxiliary mast options as the speed increase with motorsailing would surely increase range.

 

That's probably true for virtually all sailboats. This is compounded by the way most sailors motor upwind anyhow. It's complicated by the ruled in many marinas making it a fine to sail in the maina. Sadly, this is true oven for small craft where rowing and paddling as well.

Dan

 

Not the way we sail.....
Power tris and cats are a apple and orange comparison if you are going by waterline length, the cat will carry more the tri will get better mileage. Ignoring dock fees it makes sense to make a power tri longer for the same capacity and horsepower as a cat to get higher speeds and better mileage.

 

Richard, would you care to elaborate on this?

What changes would you consider to the hull design for a *sub-displacement speed* power cat vs. a sailing version? (I'm assuming reasonably slim hulls, somewhere around 12:1 L/B)

Mike

 

"Faster without the airdrag of the rig" is, I hope, obvious. Maybe I should have added the word "primarily" before "designed for sailing"

Most sailing boats are designed for a range of speeds whereas a powerboat can be optimised for a set speed. That in itself makes the design easier, and by implication it can be made more efficient at that speed (but not necessarily at other speeds, eg planing boats are very inefficient at low speeds, but that's OK as they have the power available, whereas a sailing boat sailing slowly is usually low on power as winds will be light)

Furthermore all powerboats tend to squat aft so need extra buoyancy when compared to a sailing hull. So if you look at the photo on my website of the Banshee towing a waterskier at 22 knots you'll see it trims more than is desirable. More buoyancy aft and a flatter aft section would help lift and thus reduce drag. But that wouldn't make a fast sailing hull (remember I also sailed that Banshee at 19 knots)

Not sure what you mean by a "sub displacement" speed

Hope that helps

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Obviously I meant without rig and (most, if not all of) the keel.

Sorry - by "sub-displacement speed" I meant sub-hull speed (although that has less meaning with skinny hulls). The example of 5 knots in a 38 footer would be fine.

I suspect optimizing the boat for a specific speed would be similar, whether done for power or sailing craft. At the example speed it would probably involve minimizing wetted surface area. Would probably favor a skinny monohull with tiny amas attached - i.e. a trimaran.

Was curious if you had any powerboat specific optimizations in mind for the low speed regime.

Mike