Trimaran motorboat / stabilized monohull

Sorry Manie

Sorry Manie, I just want to share informations on "faux tri".
Sorry if this kind of boat do not please you !
 
erik818 said:
I've read all posts on "faux tri" power boats with great interest. What I have in mind is a 10 m long, 3 m wide boat with a displacement of typically 2.5 tons (minimum 2 tons, maximum 3 tons). Max speed would be 15 kn and typical cruising speed 12 kn.

How should the results obtained or predicted with faux tri hulls of similar length be scaled to get a first estimate of the power requirements for this hull at these speeds?

Erik
Click to expand...

Erik
With 2.5t displacement on 10m length and cruising speed of 12kts you would get lower drag going to a catamaran.

The stabilised monohull idea works well where the length approaches the length that gives the lowest drag. For a 2.5t boat, designed for 12kts, the length for the lowest drag hull is 23m. Obviously this is way beyond the length you have prescribed. However if you could build the boat this long it would only require 4.6kW on the hull to do 12kts.

You need to think in terms of low weight construction to get the full benefit. For example an average layup of say 3 to 4kg per sq.m of hull. Once you consider it as a total package there are features of the concept that help reduce weight. One vital factor is the low drag reduces the power required. The easily driven hull also improves propulsion efficiency so there is a weight saving in fuel for a given range.

The hulls do not experience the high loads of a planing surface and can be made to wave pierce so impact loads are reduced considerably.

A reasonable cruiser for a family could be built with a displacement under 1 tonne. It would have a length of 12m and beam of 2.5m so trailerable. A design speed of 10kts could be achieved with about 2kW. This power is more or less scaleable from the 130W I need to do 6kts in a 6m hull displacing 100kg as it is close to the lowest drag hull for those conditions:
http://www.boatdesign.net/gallery/data/500/Hobie_view_of_V14.AVI

Rick W
 
Rick,
Thank you for answering my question. Excuse me for being late with my reply.

I have followed one of the links to Leo Lazauskas work on hull restistance for long slender displacement hulls at high speed. It seems that the water resistance at speeds exceeding hull speed very much resembles the drag for a supersonic projectile, provided the shapes are well designed for these speeds. I think I have figured out how to estimate power when scaling up and when adjusting speed.

Correct me if I'm wrong. When scaling up a factor 2 the following happens:
- Displacement increases 8 times
- Required power increases 8 times for the same Froude number
- Hull speed increses with sqrt(2)
- For a long slender hull at approximately double hull speed (Froude number =0.8), water resistance is approximately proportional to speed, so the power requirement is then approximately proportional to the speed squared.
- Consequently, when scaling up a factor 2, the power requirement will increase 4 times for the same speed.

My problem now is that I don't know how to scale for increased displacement when keeping the length constant since this will effectively change the submerged hull shape by altering the water line. Maybe this cannot be scaled, but has to be calculated or tested with models?

Minimum water resistance isn't my only target for the boat I have in mind. Fuel efficiency is high on the priority list, but "good enough" is good enough. I need the boat to carry a payload (myself, passengers and various goods) that could vary a ton from minimum to maximum. I see an advantage with the stabilized monohull regarding internal height while keeping the external height low and could accept some penalty regarding fuel efficiency for that. I think 2 tons displacement at minimum load is realistic, but maybe it's possible to go lighter as you suggest. I'm not the best sailor in the world and tend to bump into things when it's tight and wind and waves are unhelpful, so I prefer not to make the boat too fragile.

Also with the maximum load and 3 tons displacement in a 10 m boat, it should be possible to keept the length/width ratio for the central hull above 10.

Another more illogical reason reason for preferring a long slender stabilised monohull is that I think they are elegant.

Still another question: What is the strategy to handle load variations in a stabilised monohull? Adjust the amas?

Erik
 
Erik
The best way to approach this is to set your constraints and determine the lowest drag hull based on those unique conditions. You need to nominate target displacement (the most likely operating displacement), target speed (the most likely cruising speed) and the absolute maximum length.

Remember that the concept works best for low displacement on a long waterline at moderate speed. So give this some thought and then post the three numbers you are interested to work though. It only takes a few minutes to arrive at the lowest drag shape. Once you see the first set you might want to compare with another set. You soon start to see how things interact. Sometimes there are surprises.

There are a few ways to handle variation in load.
1. You can make the central hull inherently stable. This is possible if you have a lot of weight like batteries set low in the hull.
2. You can have planing style outboard hulls that slope aft when light but level out when fully loaded. Under light conditions the boat is more bow up than when loaded.
3. Use "V" shaped outriggers having some rocker to cover the range of loading. Their slenderness is almost constant with loading but the waterline length increases as the boat is loaded.
4. Live with the flop roll in dead calm conditions when the boat is lightly loaded. Typically this is rare because as soon as there are a few waves (maybe even ripples) the outboard hulls will be hitting the crests.

I have tried different shape outriggers on the pedal boat. The best set up depends on the operating conditions. My current set up is good for calm water operation but they are too close to the longitudinal centre of the boat and too short for operation in short waves above 0.5m high. The boat tends to flop roll when driving over waves this size because it is suspended by the bow and stern with the outriggers a few inches above the water. It is possible to keep balance but there is a tendency to flop roll onto one of the outriggers.

So there is a bit of thinking to do in the design and positioning of the outriggers to get the best result.

Rick W
 
Erik,

Regarding your 'scale up by x2' comment: The first three points are about right; the last two are debatable but probably close enough for a first guess. This is assuming, of course, that the geometry is scaled equally in all dimensions. Testing with models is, essentially, an extreme case of scaling.

As Rick pointed out earlier, a "stabilized mono" ie. a tri with very tiny amas, works best when it is long and slender, and if you can't make it long, a cat is sometimes a more efficient configuration. There are, of course, other reasons to go for a trimaran. For example- if you are swimming, diving, working, etc. off the side of the boat, a tri with a lot of reserve buoyancy in the amas might be a better choice than a slim monohull with similar speed and power.

Ultimately, we recreational types don't choose boats based purely on logic and practicality- that is for the shipping industry. What we think is cool, beautiful, elegant, etc. plays a big role in choosing, or designing, our own boats.
 
erik818

Sorry to contradict, but your assumptions are not totally correct.

1) Correct, since 2 ^3 = 8, ie your scale factor, f, is f^3.
2) Incorrect. There are many formulae that are involved. However they can be summarised as EHP(old) = EHP(new)x f^3.5
3) Then you need to establish same speeds V = V(old)xsqrt(f)
4) Don't understand what you mean by "water resistance", in what context?
5) Not strictly correctly, see above. Since the EHP is different at the same Fn.

As for:
"..My problem now is that I don't know how to scale for increased displacement.."
Assuming that all is changing, is the displacement, it is pro-rata, thus:
EHP(new) = EHP(old) x Displacement(new)/displacement(old)

for small-ish increases in displacement.

I can't comment on Rick's "fog" worded explanation!

PS.
A delightful comment
"..Your personal attacks are getting creepy.."
Well, if you had a backbone to debate this publicly to explain yourself, would be useful. But that just exposes your own ignorance and prejudices...oh well. I don't think you would know what a personal attack is, compared to a factual verifiable justification if it hit you on the head. Still..ignorance is bliss.... :)

That's 3 neg hits today....oh of course, it's hallowev'en, silly me. What else have you guys got to do....
 
Class Response

Eric DEBORDE said:
Sorry Manie, I just want to share informations on "faux tri".
Sorry if this kind of boat do not please you !
Click to expand...

Eric,
Best collection of boat images I have ever seen. I can see how you are building such an awesome watercraft. It is "vision" that inspires creation.:idea:
Keep it up.

Dumb Boat Mechanic
I want the boat from the movie "Waterworld".
 
Another one. Its a big one.
Rick W
 

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Hi Eric I think that you did a great job with the design and construction of the proto its very swan like in aperance. I noticed in the photos that you made some modifictions to the out board edges of the tunnel entrances. What was the prolbem that you were trying to solve? If you were to build the same size boat again what changes would you make? Here are a couple of things I was considering Give the stem some forward rake in order to create more reserve bouancy forward even thow I like the looks of the plumb stem beter. sharpen the stem of the main hull at the water line to reduce water spray. widen the fore deck to make mooring the boat less of a tight rope walk and easier. Add a main bulkhead and eliminate the coach roof bracing. create crash bulkheads in the forward sectons of all three hulls and fill with foam. Re shape the forward water line sections of the out board hulls into more of a V shape that transition into the dead rise.
If you think that there is meret to any of these ideas please let me know. I need to build a boat this winter for comuting to work on the Islands off the maine coast. The boat that I use now cruises at 25 Kts but burns 8 GPH and I put over 200 huors on it last summer.
Would you have a prolbem if I used your drawings and information to design and build my own vertion of your boat.I look forward to hearing from you.
Thanks Dick
 
I seem to recall reading further back that the ideal location for the ama's is determined by the Fnv. Can someone tell me how you go about doing this? (The ama's not calculated Fnv)
 
Thanks Dick,

The out board edges of the tunnel entrances where just to flat and made lots of sprays, now the sprays are captured in tunnels.

I'm nearly full satisfied with the prototype except that there is to much rolling in ¾ back seas, has Rick explain, when the central hull is in bi twin waves, the amas are not so much in the water and the boat tend to roll downwind.
The new drawings shows a wider boat : 3,6m vs 2,5m for the prototype.
Amas are longer on new drawing.
An other modification is to flatten tunnels at the rear of the boat to have more hydrostatic stability.
The turns are to flat so I increase the bottom angle of amas.

All your comments make sense and I would be happy to share and to help you on your project.

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bottom+rear+fat+12.jpg


bottom+front.jpg


bottom+1+front.jpg
 
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