38' power Trimaran: Kurt Hughes Design?

What do you think this Kurt Hughes design Trimaran?
.....and the hull form, would it be only suitable for sheltered waters?
Are the fuel consumption figures realistic?
Advantages/Disadvantages of Tri's?

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

Length overall: 38'-0" (11.6 m)
Length at waterline: 37'-6" (11.5 m)
Beam: 16'-11" (5.16 m)
Draft: 1'-6" (0.46m)
Weight: 4,125 lb
Displacement: 5,278 lb (2.394 kg)

Half load, both engines at mid speed (4,000rpm).
Fuel consumption is 0.62 gph. per engine
Speed is 11 knots.
100 gallons fuel gives 887 nautical miles


Half load, one engine at low mid speed (3,000rpm).
Fuel consumption is 0.407 gph. per engine
Speed is 8 knots.
100 gallons fuel gives 1,965 nautical miles

 

Tris are relatively seaworthy. The argument against all multi-hulls is the same, if filliped over not self righting. Multi-hulls of all types have traveled the world for years now. For coastal and intercontinental use they are well proofed. I have no motor tri experience other than powering with my 26 ft sail tri. I would place that tri well above the average 36 ft motor boat for sea keeping and way above for fuel economy. The down side trade offs are less interior room-high sensitivity to weight (not for pack rats)-and docking with outriggers (swing units solve that problem). Also a good motor tri is not an off the shelf item and must be built one off or modified from a sailing tri.

 

I have heard that the motion of a Tri is different to a Cat, is this true? My biggest concern is that the ama's will get buried in a steep wave, does this happen often?
The cross beams seem to be close to the waterline, but persumably they would present very little resistence to a wave.

The aft section is a round bilge design, do you think this is why the cruise speed is limited 12 kts, due to the flat shape, to avoid slamming?

 

In general trimarans haven't been popular as cruising motorboats because of the docking and boarding problems that eyschulman referred to. How DO you get on the boat shown if it is at a dock? Even getting to the bows once onboard looks challenging

Also the outriggers are not just "training wheels", they go in the water and then are running at very high Fn, so have high drag. A stepped hull might make sense, as on Ilan Voyager (IIRC)

I'm not sure that there are enough advantages of the type over a powercat to make it worth building

You don't say how big the engines are, but I assume the boat in the photo is motoring around, so the performance figures are true ones

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Hi Richard,
How is your trip in the NW of Canada going? is the Skoota skooting along well?
My Cat build is progessing slowly, just got delivery of Aalco composite panels for the roof, but the weather is holding up the bonding....
trampolines on the crossbeams would allow acess from the pontoon........
A forward hatch could allow you go forward inside the hull, then stick your head out!
The quoted power for the design is twin 15 hp.
The advantages over a Cat would I persume be more payload, given that the main hull has more volume than a Cat hull. Also no low bridgedeck compromise.

The hull is speced with 100 gal fuel, way above what a cat could carry?

 

My cat is 35ft... will carry 150gal in internal tanks, room for much more in storage...
Fuel consumption would be similar if speced with the same engines...

One advantage might be in the build complexity - this tri looks quite a simple boat to build and the build time would be quite fast.

A big disadvantage is the amount of accommodation space available.

 

other advantages of the Tri design I see are these:

Will fit on a truck/lowloader with the outriggers disconnected (2.59mtrs); a 35' Cat would have a beam of 17'.
the volume in the main hull would allow quite good accomodation all on one level; basically you have a tube 8' by 38' long because the OB's are mounted outside the hull. seating could run along the hull sides, sliding doors would make sleeping accomodation area's forward and aft.
If the OB pods were made detachable, the hull would just squeeze inside a 40' shipping container.

 

I am actually in the UK right now

Looking at the photo and 3D drawing I know my wife couldn't get on board, even with a trampoline and no kayak. Mooring alongside also looks tricky with a very short, low hull on the dock/stone wall

I do find the nearly 20mpg at 8 knots hard to believe

My Skoota 28 is truckable without a permit. The boat shown has far less accommodation than Gropers or my Skoota 36

Richard Woods

 

Maybe they're using the Suzuki leanburn!!!

Question: Looking at the hull form, the aft section ls a a round bilge design.
My guess is that at more than the design speed of 12 kts, the aft section at 4' wide would be too flat for short chop conditions and have a hard ride?

 

You might find this (forgotten) thread useful

http://www.boatdesign.net/forums/multihulls/kurt-hughes-38-trimaran-trawler-38006.html

Richard Woods

 

I thought the kayak on the cross arms was a very inelegant idea too.

It is at risk of damage, and as a tender - very awkward for carrying gear back and forth from a shore.

That needs a lot more thought.

 

I did seach the forum, missed that link.....thanks

 

A Tri seems to offer lots of advantages over a Cat, I'm not going to regurgitate all the old well worn clichés about bridgedeck clearence, leeway, pitchpolling, uneasy twisting sea-motion, uncontrollable acceleration down the back of large waves, deep-vee hull carving 'oversteer' leading to broaching, rapid deceleration in waves due to light inertia etc etc.........

err, besides those things i was'nt going to mention about Cats, the Tri seems to overcome most of those short coming???

 

Lee way? - this is a powered mulithull we are discussing here, not a sailing vessel!!!

Pitchpoling - again, see above!

Uncontrollable acceleration - applies equally to any fast vessel and the same methods are used to control all of them

Deep-vee carving/oversteer - i THINK i know what yoiur refering to, often described as "bow steering", again applies equally to all vessels and needs sufficient rudder or skeg to deal with it.

Deceleration in waves - again, applies equally to all vessels and is considered in their design SOR and hull volume distribution.

The uneasy twisting motion, applies equally to tris and cats - which is a function of a multihulls high static stability, large metacentric height, and the wave frequency and amplitude which causes higher accelerations and higher motion frequencies but at lower amplitudes compared with a typical monohull.

The only issue which is relevant in your list, is the bridgedeck clearance. Insufficient clearance in head seas of critical wave height, amplitude and freqency, in combination with critical vessel speed and natural pitch freqency can lead to bridgdeck slamming - which limits the vessels speed in these conditions. A tri with a flat floor bridgedeck would suffer the same considerations, as opposed to a tri with beams only. Same goes for a cat tho! 2 hulls and 2 beams with only a trampoline floor.

Compare apples to apples, it all comes down to the floor plan and the arrangement of what is required from the boat. A tri has its limitations, as do cats, as do monohulls, it all depends on the SOR as to which suits a particular purpose more fitting the need.

 

Of course leeway on a power Multihull with it's high superstructure caused by windage would effect course made good over the ground, forcing the boat to crab sideways in steep waves with the threat of capsize. A following ' breaking' sea could push the stern up and the bow down into the wave in front, cause pitch polling; The bouyancy in the bow sections of powercats is often a very slim volume making it easy to drive, but also easy to bury in a wave.

if you've ever towed a Cat, it becomes very apparent how the hull steers with one hull first digging into the water, until a turn is completed, then the weight get transfered onto the other hull and it steers in the opposite direction: It's weight tranafer which causes the side to side carving action.

Comparing the bow section of a Tri to a Cat, the Tri has double the bouyancy. So no worries there. But if you watch videos of the ama floats they seem to get buried into the front of steep waves, causing a severe twisting motion to the boat.

I can't see that bare cross beam poles would cause more or less drag than side bridge decks, as the waves meet the poles head on.

So is the Tri design a fast power displacement hull, a sailing hull with outboards; or a slow semi-displ. hull. The design cruise speed of 11 kts would suggest a bit of both......

Would the broad 4' round bilge aft section give a hard ride above 11kts?