Godzilla

Displacement mode performance is certainly a factor for all hulls, but more often than not is completely ignored in the case of boats that will normally be on plane. Ever notice how an 18' bowrider, no matter who built it, will rarely track in a straight line at 3 knots? A hull meant to go fast will necessarily compromise most of the characteristics that make a hull pleasant at slower speeds.

Sailing catamaran hulls tend to be slender and operate mainly in displacement mode, so- if you give enough thought to how you set up the problem- it's possible to get reasonably useful comparative results out of Michlet for such a boat. However, issues such as pitch stability may well prove to be more important than absolute minimum resistance for such a boat.

Keep in mind that any of the following will contradict the assumptions made in deriving the equations Michlet is based on, thus yielding questionable results:
- Hulls that generate significant dynamic lift (ie, anything that planes); this changes trim and immersed volume in ways Michlet cannot predict.
- Any shape that would tend to induce flow separation (ie, waterlines/buttocks that curve sharply inward near the stern, tugboat-style); Michlet has to assume the flow will follow the hull.
- Any shape that is wide relative to its length (although I've heard of hulls as fat as L/B = 4 giving OK results in Michlet, results for anything fatter than L/B of 6 or 7 should be taken with a good-size chunk of salt until confirmed by some other means).

As you can probably guess, then, Michlet tends to work fairly well for streamlined, slender hulls that don't tend to change trim very much as they speed up. Given proper formatting of the input file, it can also handle a clean breakaway of water from a transom.

Hope this helps.

Thank you very much Matt Marsh for your help. Based on your reply I'll keep playing and considering michlet to evaluate my boat for future development, present one is almost ready to go into the water.
Regards.
Andrea.

Hello.
Dihull have the same Maximum displacement Speed as Single-Hull ?
Single hull 4m long, if I' m right, should have a max speed 2.49 m/s.
The diagram Rt-V I get from Godzilla does not show any change in slope at that speed for a 4m cathamaran case.
Andrea.

dallolio said:

Hello.
Dihull have the same Maximum displacement Speed as Single-Hull ?
Single hull 4m long, if I' m right, should have a max speed 2.49 m/s.
The diagram Rt-V I get from Godzilla does not show any change in slope at that speed for a 4m cathamaran case.
Andrea.

Click to expand...

What will limit the maximum speed to 2.49m/s?

Rick W

Rick,
Andrea is thinking in terms of 'hull speed', ie. V in knots = 1.34 * sqrt (LWL in ft). For a 4 m hull this gives 2.49 m/s.

Andrea,
'Hull speed' is not a hard speed limit, not in the same sense as Mach 1 or c are speed limits. 'Hull speed' is just a rough indicator of the speed at which you run into the law of diminishing returns when adding more power to a displacement hull of 'typical' proportions. It is of little relevance to hulls that are very slender or very light for their length, ie. most multihulls.

It is quite common for relatively light, slim hulls to show a decrease in wave drag with increasing speed that is roughly comparable to the increase in frictional drag with speed; thus, the resistance curve levels off and the indicated power curve begins to approach a linear increase with speed.

Keep in mind that Michlet does not calculate dynamic changes in trim or sinkage, so caution must be exercised when interpreting its results at higher velocities.

If you have concerns about a particular hull, feel free to attach the Michlet input file and one of us can take a look at it.

Rick Willoughby, Matt Marsh,
Thanks for your emails.
The question I have is what speed I have to optimize my hull for, particularly because Godzilla output looks to change strongly between 2 and 3 m/s. Below 2 m/s the hull is curved bottom (side view), 3 m/s and above it’s straight. Following your indication 'Hull speed' is not an issue, so I focused on sail drive force (http://www.wb-sails.fi/news/SailPowerCalc/SailPowerCalc.htm), Venice Italy has an average wind speed 2 m/s. It looks like I have to stay with the curved bottom, actually that make sense considering Hobie Cat hulls. I’d have rather preferred to stay with the straight bottom for construction reasons. Attached file is optimized for V=3m/s, please if you have comments let me know.
Andrea.

Andrea
With a sailing cat the only time the hulls are loaded equally is when sailing dead down wind or you shift weight around accordingly.

To determine the most suitable hull shape for a sailing cat you need to consider more than one point of sailing. Reaching and upwind performance will more likely dictate the best shape. Hence you need to consider the load on each hull.

In the limit, when you are flying a hull, the boat acts as a monohull. In this case you have a single hull displacing 230kg rather than two with 115kg.

Also when you are sailing you need to consider the apparent wind. If the true wind is 2m/s then the apparent wind could be considerably more or less depending on the point of sailing.

The actual speed you will achieve in given conditions will also depend on the sail area.

It is best to look at a range of loading on the hulls and a range of design speeds so you get some feel for how the hull changes under different conditions. You then apply judgement on what will give the overall best performance.

Rick W

Thanks Rick Willoughby,
I think I was a bit to easy on the true, apparent wind !
Talk you later..
Andrea.

Rick Willoughby, Matt Marsh,
I enter the attached spreadsheet with a guess boat velocity and true wind speed and direction, it gives me apparent velocity and direction and from http://www.wb-ails.fi/news/SailPowerCalc/SailPowerCalc.htm I get the sail drive, if the Godzilla output file Rt x 2 at the guess velocity equals the sail drive it means guess velocity is the actual boat velocity with that true wind and direction. At 90 deg and 2 m/s true wind I have a boat speed of2.35 m/s , such speed is right above the value where straight bottom hull are better than curved one. With true angles smaller boat speed increases, with true angles bigger 90 deg and true wind at 2 m/s curved hull are better. Conclusion : at 2m/s straight hulls start being competitive at some true wind directions, 2 m/s true wind is not much, with stronger winds straight hulls are better, I’m thinking to go straight hull. Does that make sense ?

Your link to the sail calculator does not work for me so I cannot see what it is producing.

I do not know why you are using Rt x 2. Your input file was for a catamaran so the result will be the total boat drag.

You have an implied assumption that you can use body weight to completely balance the sail forces so the hulls are equally loaded and in level trim.

Hulls with little rocker do not turn as easily as hulls with more rocker. Hence tacking upwind and turning at buoys will be slower. So you have to also consider if turning ability is going to have a significant bearing on your overall performance.

Rick W

http://www.wb-sails.fi/news/98_11_PerfectShape/Main.htm#
then click sailpowercalc
x2 I mean hull 1 + hull 2.
Yes for 2 m/s wind I assume I can not fly on a single hull
I use one center board at each hull.
Andrea

The sailcalc seems to give an accurate result. It may be a useful link for others.

Typically what you find with these complex interactions is that there will be something you did not consider and it has some significance to the outcome. When you make the next design you can make allowance for it.

You are using reasonable tools that give a sound basis for a design. At some point you need to get your hands dirty and build it. This is the quickest way of finding out if your calculations are good.

Once built and tested you should see opportunities to improve and these are things that can be accounted for with some adjustment or possibly a new design. Tools like sailcalc and Godzilla can take guesswork out of the optimising process. However you need to know what constraints exist and their significance to use these tools effectively.

For example your design basis of 2m/s windspeed may be meaningless. Reminds of sailing on the Barrier Reef off the Australian east coast. At this time of the year the temperature is balmy with an average windspeed of 12kts. However from 7pm to 7am there is hardly any wind but from 10am to 5pm it blows at 25 to 30kts. A boat suited to 12kts is really not well suited to the conditions it will encounter here.

These are things you will only find out through a lot of observation that is best done on a boat in the conditions. You learn very fast by doing.

Rick W

Thanks Rick W,
I had thought something like you wrote about the average wind meaning but reading also from others sounds much more reasonable. I'm ordering the material to follow your recommendations about getting hands dirty.
Andrea.