Economical Coastal Cruiser as faux tri

Following the Economical Coastal Cruiser thread, I’ve looked further into the possibility to build a coastal cruiser with a trimaran type stabilised monohull (faux tri). The hull form itself seems to be well suited for a fast displacement boat with an economical cruising speed of 12 kn. The challenge is to make a useful boat of it.

My ideas are outlined in the documents I enclose. For those who don’t have Microsoft Word on their computers, it’s possible to download the completely free Open Office software from www.openoffice.org. In some ways that office suite is the equal of Microsoft Office once you get used to it, and Open Office can open all of the Microsoft documents.

I believe I have the basics of the boat worked out in the document, but not down to details. I ask for opinions, advice and suggestions. The Economical Coastal Cruiser thread gave me many insights and ideas and I’m hoping for more.

I’m not able to log onto boatdesign.net every day because of my travels so I might be late with responses. Don’t interpret that as a lack of interest.

I enclose the word document with my ideas plus the Michlet file for the hulls. The hulls have been generated from equations and I’m aware that some modifications will be necessary to make them fully developable, but that shouldn’t change anything fundamental.

Erik

 

Erik
Michlet gives initial stability in the ship.mlt file as output. Michlet assumes G to be on the waterline so the GMT value it produces of 0.62m means you will have marginal stability if your CoG is 0.55m above the waterline.

So I suggest you take a look at the stability and that you need to improve it. You will have to increase the length of the stabilisers and/or increase the overall beam.

The transom on the stabilisers add a little extra drag. It is worth making them canoe sterns similar to the main hull.

Note that the way you have the offsets in the in.mlt file they are maintaining attached flow so the drag value is slightly higher than it should be. However if you go to a canoe stern you overcome this little glitch. Otherwise you need to have the last row of offsets the same as the second last row.

Rick W

 

Rick,
I wasn't aware that Michlet also provides initial stability. As usual I havn't read the manual enough. I will take a look at initial stability and the stabilisers.

A stupid question: What are the values for acceptable and desirable initial stability? I'll check with the litterature I have, but I don't remember that I've seen actual numbers for acceptable and desireable stability. My own answer would come from the acceptable heel with one and two persons on a side deck, but maybe there are already commonly recognised numbers.

Erik

 

Erik
You really need to do a full stability analysis. Freeship, not Delfthip, can produce cross curves so you can develop the righting moment at various angles of heel from that.

You should check that you have positive righting moment under various conditions.

For example consider all crew on the cabin top to see how that will shift the CoG. Also consider heavy and light state.

One of the issues I have experienced with my very light boats is the side windage. The Cd sideways is about 1 or even a little higher. So you need to take into account this as a heeling moment. You should use the second moment of the side area above the waterline for this calculation. This may not be a factor for you because it is quite heavy but it is worth checking.

You can improve the side-on Cd by angling or rounding the transitions from sides to top.

Ideally you can make the boat self-righting. It is one of the benefits of the narrow beam.

Rick W

 

Rick,
I went back to "The nature of boats" and found that the initial stability ideally should give a roll period in seconds roughly equal to the beam in meters. This implies a metacentric height of 0.9m. I'm quite far from that right now.

I'll start with adressing the problem of initial stability and put off the full stability analyses a little while. I have avoided learning FreeShip until now and it will take me some time to learn the software and then model the boat. I intended to to learn FreeShip sooner or later. It seems it will have to be sooner.

Erik

 

Rick,
I elongated the stabilisers to 6 m and kept the beam at 0.3 m and kept the transom. After increasing total beam from 2.5 m to 2.7 m I got a metacentric height of 1 m (GMT 1.57 m) and the initial stability I wanted . The power requirement increased by 6% at 6 m/s, and this size of increase was of typical regardless of speed (up to 10 m/s). I'll check later how varying the stabiliser length while maintaining the waterplane area will affect power requirement.

I then changed to a canoe type tail for the stabilisers, kept the length at 6 m and adjusted beam for the same waterplane area. The metacentric height remained at 1 m. The result on power requriement was 0 at 6 m/s and higher speeds, but at lower speeds the power requirement went down. -3% at 5 m/s, -5% at 4 m/s and -14% at 2 m/s. Unless I find a good design reason to have a transom on the stabilisers I'll go for a canoe tail. The reduction in power requirement is small at the speeds of interest, but why waste it without a reason?

I haven't yet had time to learn FreeShip and have a question. Can FreeShip calculate the stability also for a flooded boat or will I have to do it by hand? I don't really think that stability with closed doors and maintained integrity of the boat will be a problem, but I'm sceptical regarding the stability of a flooded boat.

Erik

 

Freeship does not handle flooded state. You would have to do the cross curve at some added displacement and then consider how the water is shifting the CofG. I think the stability module for Delftship handles tankage but I do not know if it does flooding. This module is an expensive addition though that I do not have.

If you are worried about the flooded state then you need to place plenty of solid buoyancy up high.

I know of one open boat built from timber that had solid buoyancy glued in under all the decking and it was intended to float OK right side up but as far as I know was never tested. Open boats with underfloor buoyancy usually end up upside down when flooded.

I worry about flooding in open waters on any hull that does not have marine hatches with strong hinges and firm latches. If you have these and batten down as conditions deteriorate there is not much risk of flooding. If you can seal various compartments to reduce the amount of flooding then that is also helpful. The best solution will depend on what conditions you intend to operate in.

There are various stability indexes like IMO and STIX that are worthwhile spending time to review to get insight into the factors that you should be considering for the intended operation. This will be a better guide on the stability required although you cannot always plan well enough to avoid worse than intended.

Rick

 

Erik
One other consideration is the need to have any initial immersion on the stabilisers.

My view on this is that in the light state you trim stern down with the stabilisers just touching at the stern. As you load up the CoG moves forward causing the stabilisers to just touch for a greater part of their length.

In operation there will be a bit of sinkage and the bow wave/s will stream back at 19 degrees and provide something for the stabilisers to sit on.

On my pedal boats I set up with a bit of flop roll when stopped to allow for the sinkage and bow wave once in operation.

With a boat of 12m length and relatively light displacement it does not take much weight to trim fore/aft.

In waves I can set the stabilisers even higher because they ride the crests.

Rick