38' power Trimaran: Kurt Hughes Design?

Please define "seaworthy", and also "too flat-bottomed"? Explain the analysis you undertook to reach this conclusion?

One measure of a hull to determine if it's flat bottomed or not would be deadrise angle. Zero deadrise would be flat-bottomed. Below are three hulls, The PL 39 which is "too flat-bottomed", the Nelson 42 MKII by TT Boat Designs, and the Duffy 41, a 20+ knot lobster fishing hull which works in the North Atlantic in all weather.

Deadrise angles
PL 39 - transom 12 degrees, midships 20 degrees, forward 60 degrees.
TT 42 - transom 0 degrees, midships 15 degrees, forward 50 degrees.
Duffy - transom 3 degrees, midships 18 degrees, forward 60 degrees.

The PL 39 that's "too flat bottomed to be seaworthy."



The Nelson 42 by TT Boat Design



And the Duffy



And in case you think I've never thought about such things, perhaps you'll advise as to whether this boat's bottom is "too flat to be seaworthy"?

 

I think that on your design there isn't a big enough keel to give the flat bottomed design directional stability. The Nelson design has a very large keel and skeg, which at the top of a wave will stop the tendancy of a round bilge boat to broach. As the hull form has less and less deadrise, you will either need to adopt a deep vee planing hull form to act as a surogate keel, or hang a substantial skeg with long keel on the hull to help to the hull to slice through a steep wave, especially when traveling at an angle to a head sea.

 

Regarding the outrigger float design, here's advice from Farrier:

Big floats...!

 

A power-tri shouldn't need huge buoyant amas like a sailing trimaran. The power tri AMA does not have to be big to resist roll. Look at Earthrace and voyager trimarans that were well designed to break open ocean records. Their amas are very small. Whereas sailing tris need to counteract sailing forces, and the fastest designs can fly their main hulls-- so amas are designed to take 150% to 250% of displacement.
I would disagree with the AMA placement of the KH design. As designed his tri would be prone to excessive roll in a following/ quartering sea... And most likely broaching. Try to imagine what happens when the bow and stern are spanning wave crests... The amas are then a long way from the water in the trough of the wave. This means the tri would roll quite a bit before the AMA submerged and recovered. This would most likely give the boat a very disconcerting ride in these conditions. If the amas are brought further aft, closer in line with the main hull transom, this tendency would be reduced. Also, by bringing the AMA aft it is now tucked into the " slipstream" of the main hull and which could reduce drag.
My comments are based on first hand experience with a power tri that is of very similar design... But for different design goals.

 

Makes sense, or maybe even longer floats.
have you got any photo's, or specs?

 

Yes the other power tri's have small amas but that could be because their amas are always in the water. The ama will be at a higher froude number, maybe 2 to 3 times higher depending on the design. It would take some careful designing to keep the ama drag low.

I asked earlier why the KH design had the amas out of the water when every other power tri has the amas in the water and supporting a percentage of the weight.

I can see the slightly less drag reason but at what cost? The boat will never sit level, always tilting left or right even at anchor. I think the reason the amas are in the air is more a result of sailboat design experience than the correct way to build a power trimaran.

 

Kurt has just emailed me with the latest upgrades.
looks like he's addressed your point about the floats being out of the water, the new setup has them exactly on the waterline.
seems to have moved the OB's further back towards the stern.

 

So no definitions of either "seaworthy" or "too flat bottomed"? And now the keel is too small as well? So it goes...... all you can do is laugh and take pleasure in today's drawing......

 

Because your passagemaker- lite is a super displ. hull form, it has no sections that generate lift like a S/d hull design. This could cause problems at 14kts with the bow digging into the face of waves, as the hull is acting just like a standard displ. hull form....ie: ploughing through the water, rather than the upward trim angle of a S/d hull at cruise.

A flat round bilge keel design would only exacerbate these design compromises.

 

So now your inventing new hull classifications aswell, to justify the preconceptions floating around in your mind. What exactly is a "super displacement hull" " in your mind? Explain it in detail please and what distinguishes it from other hulls...

 

cruise speed higher than it's hull speed.

 

This is laughable...

So a physical shape and form of a hull is not related to its form classification in any way, but rather how fast its travelling...?

Wow, i always thought that the speed of the boat at any point in time, was the direct result of the engine horsepower and thrust generated by the propeller... i guess i should have just called my boat fast, and it would have been so?

 

'speed' boats...40kts
'fast planing' boats...25kts
'swift' trawlers....20kts
's/d hulls'....15 kts
super displ. hulls....more than 1.34
Displ. hulls.... 1.34

 

So by your definition, a navy destroyer or the titanic is a speed boat or planing hull respectively! Would you care to try again?

 

Attached is a picture of the power tri that we built a couple years ago. Note the small amas midship. These were also just touching the surface of the water. They worked very well in flat water in that the bottom surfaces of the amas were flat and started working as "skis" as the boat speed went over 16 knots.... the boat became stiff in roll from the dynamic stabiltiy. Nigel Irens described a similar effect in his design breif of the Ilan Voyager trimaran. The max speed of our boat is 22 knots without lifting hydrofoils with a 20hp outboard. Boat is 24 feet long and the main hull is a displacement shape. Although it does not have nearly the rocker in the aft section of the mainhull as the KH design-- KH's design is more typical of a sailing trimaran. I would think that the KH design would start to squat above 9-10 knots--- wheras having flatter sections aft with less rocker is more appropriate to a power trimaran main hull.

Anyway the boat we built was designed to use lifting hydrofoils and this necessitated the amas midship to mount the foils on. The drawback to this was the large amount of roll and tendency to start to broach with following/quartering seas of 4-6 feet. (this was while testing the boat without the lifting foils). So for a displacement powertri without foils I would say that having the amas further aft would help quite a bit. It does look like KH's latest drawings show the amas back a bit further which looks a lot better to me. I think that having the amas immersed more could help this a bit too... but will not completely eliminate the roll problems in following/quartering seas, and with a more immersed ama you then have the added drag and hull interference problems to deal with.

None of my comments are to diminish the power trimaran design of Kurt Hughes--- I like his designs and I will never have 1/10th of the skill/experience that he has in designing multihulls. I am just trying to pass on some of my experiences in a power trimaran that is very similar to the one in this thread.