H.W,

I was not trying to brag about my skills or show my superiority. I could barely understand the graph myself when I had a look at it yesterday, because I had nothing to go on except the very cryptic labeling and my slowly recovering memory. Took me a while to remember that the numbers in the end was the displacement. I do happen to be pretty talented with numbers, which comes with a lack of skills in some other areas.

Anyway, what I thought was so interesting with that graph. was how the "humps" completely disappear on light boats. I've been sailing keelboats, and it is so obvious on those boats that they will reach a certain speed, and then not go faster. You may be able to push it 0.2 knots faster when the wind picks up. But I was once on a pretty long and light keelboat under spinnaker in a breeze, and it starting to go faster! it was very exiting. That boat had about 10m waterline length, and a displacement of 4 tonnes (so just like the purple curve), and we reached 12 knots or so on a broad reach with a 90 m^2 spinnaker and 34m^2 mainsail. So it is possible, under the right conditions, to climb the hump and reach a higher speed. But I have also watched the M32 sail in almost no wind, but they are still moving in a decent speed. And then it just takes a puff to lift the WW hull and reach 12 knots instantly.

If we look at the numbers of the Harryproas, it looks like they will have a almost as good performance as to the M32, but as cruisers!

So that is one of the things which excites me about these boats! Even though they don't tack or gybe. But from watching the M32, and Sailing the H14, I can say that it is very hard to make a good tack. A lot of the times the boat stops completely, and with me on the rudder, even sail backwards. So I believe Rob when he says that shunting is not that much slower then tacking. And then of course, with the H14, I have experienced the "zone of death" while trying to gybe in a strong wind, and capsized it completely forwards. Even just starting to sail the boat from a stop has capsized it because the sails hit the shrouds and can not be eased further. So I understand a lot about how unstayed masts must make life so much easier aboard a big boat..

But still, there is something very rewarding to making a good tack/gybe and maintain the speed. But I guess when sailing on the open ocean, you do those maneuvers less than once per hour, so not much of a reward or time difference at the destination... =)

On Wed, May 30, 2018 at 7:52 AM, Doug Haines doha720@yahoo.co.uk [harryproa] <harryproa@yahoogroups.com.au> wrote:

 

The length of the lw hull is not necessarily the measure of the size of the boat.

If it is a problem for mooring or trailering, then design a folding bow like Ian Aitken did. This seems like the best solution to get a decent length and still fit a trailer.

My opinion was that lw hull should be even longer than as shown in some of the HP designs.

Bucket/Sol is a good benchmark: 15m lw hull and only minimalist ww hull.

For the constraints on cost and weight etc it is easy to add as much length onto hulls, especially lw hull without increasing cost.

So for better speed and safety, make hulls even longer.

One big advantage of HP's. Why not make the most of it?

Doug

On ‎Wednesday‎, ‎30‎ ‎May‎ ‎2018‎ ‎11‎:‎37‎:‎08‎ ‎AM‎ ‎AWST, StoneTool owly@ttc-cmc.net [harryproa] <harryproa@yahoogroups.com.au> wrote:

 

Björn:

    An impressive graph with all it's colored lines, humps and squiggles, but entirely over my head.   I have no idea what the units represented on the X and Y axis are, nor can I decipher what the chart on the right that represents the lines on the graph means.   The purple curve as you say shows the drag of a 10M  4 ton hull.  I'm not sure how I'm supposed to get that out of Rt / (D*Fr^2) 4.00.   In any case, it's obvious that I'm not in the same league with you and the rest of the list members here, who I'm quite sure took one look at that graph and instantly understood it in it's entirety.   I appreciate your taking the time to run it off, but the effort was wasted on me.

    In any case I do understand the principles you are driving at in a general way.

    In the real world, a 37 foot trimaran would meet my criteria as far as load capacity (1140kg) and performance, but would require a much taller mast and larger rig to drive it to it's potential, and have more upkeep.   A 31 footer would be sub par on payload at 680KG, but would drive easily on a reasonably sized rig

    A 28 to 30 foot cat can meet my payload criteria, and 1200 kg (for the 28 footer). If lengthened to 30' with the same design, the payload increases in theory by about 86kg, hardly enough to justify the extra work and material, but there are other benefits.  If built of foam sandwich, this could increase by as much as 500 kg to 1786 kg simply by reducing empty weight.   We are talking a weight envelop I can very easily live within.  

    There is no doubt in my mind at all that the 37' trimaran would "kick ass" in performance, but there are costs I don't want to bear.    I drive a Subaru, and a 6 cylinder Chevy 1500 instead of a Corvette and a Dodge Diesel 3/4 ton 4x4 for the same reasons.    I'm a conservative man in the original sense of the word.  

    The 30 foot stretched cat can be driven by a rig of a decent size to a level of performance that is sufficient for my purposes, and is small and maneuverable it offers a bridge deck cabin where I can keep watch in dirty weather that is on a level with the cockpit, unlike the trimaran, where one has to climb up and down the companionway every time you go into the cockpit or cabin, and that is HUGE when one is single handing.  Fatigue is the enemy.

    The Harry Proas that fall into my 30' length range are simply not  going to carry the payload for voyaging........... That's pretty well established as far as I'm concerned.   There is little I would enjoy more than sailing into some remote port in a Harry Proa, and dropping anchor among monohulls and cats, but I have no intention whatsoever of building a 40 footer of any kind.  


                                                                                                                                            H.W.

On 05/29/2018 08:48 AM, Björn bjornmail@gmail.com [harryproa] wrote:

 

I originally started to reply to H.W in another thread, but it was far off topic. And then when I started to discuss Bucketlist, which was even more off topic, I decided this needed its own topics, so here we go!

There are some questions for Rob below!

H.W, regarding your posts about building a 30' catamaran because a trimaran of the same displacement needs to be 40', I sense that you need to learn about the length to displacement ratio. Which displacement is your target on the 30' feet boat? I hope it is not too heavy, because then you will most likely not sail faster than "hullspeed" with your catamaran unless it is a very windy day. It might not be faster than a monohull of the same length in most conditions.

I made a series of simulations in Michlet to investigate into this last winter. The following graph shows the ratio between drag and displacement, so lower is better. But that is not all to it. Because since drag goes up with the square of speed, I have divided the drag with square of speed, to "normalize" the numbers. So that is what the graph below shows.

If the only source of drag is the friction from wetted surface area, the curves in the graph below should be horizontal. This seems also to be the case with the very light boats - almost horizontal, so wave production has low impact. But the heavier boats shows a large increase in drag at different speeds at or below "hull speed", which you can see in the graph. This is most likely because a heavy boat will produce a large wave which I guess the boat needs to "climb" to be able to reach a higher speed. And that would require quite a large sail or a strong breeze.

https://pasteboard.co/Hnqe6bn.png

The purple curve for example, shows the drag of a 10m 4 tonne hull. It shows quite a pronounced hump in the curve.

On a 10m hull (33'), Freude number is more or less 1/10 of speed in m/s, or 1/20 of speed in knots. At Fn=0.4, where the hump starts, is roughly 0.4*20 = 8 knots with this waterline length.

We can also see that on very light boats the friction from the wetted surface area dominates. So it might not be optimal to have a too light boat. It depends a bit on the sail area it can carry I suppose, especially in light winds.

The optimal seems to be about around 1 tonne for a 10m hull (yellow), from the looks of these graphs. (Given that it has enough sail area to use all the righting moment the weight provides.)

This gives us a simple formula: D = L^3, where length is in meters, and displacement is in kg.

(Two hulls complicates things, but I guess that if we are pushing the boat and it starts to "heel", it will sail mostly on the lee hull, so to me the results for the single hull are the dominant for sailing in some kind of breeze. And the catamaran result are dominant when running on motor or in light winds.)

And it seems like "my theory" aligns with the real world regarding this as well. I don't know of any 32' boat faster than the Marstrom M32, which has a lightship displacement of 550kg, and should have a racing displacement of about 900 kg with a crew of 4 and some gear. 9.6^3 = 880 kg, so pretty much spot on my formula.

We can also look at the A-class catamarans. 5.5^3 = 166kg, which is more or less the weight of the boat with crew (80+80kg).

This also takes us to why the proa seems to be a good concept compared to a catamaran. When heeling, weight is transfered to the lee hull, so the lee hull should be longer to not start producing a larger wave and drag. But I think that in light winds, the trimaran is most likely the best, with only the wetted area of one hull in water. (On my Hobie 14, I sometimes use the trapeze on the lee side of the boat, to lift the WW hull out of the water and decrease wetted surface. Its a bit hard to sail it properly from that position though, so I'm not sure if it has been faster.)

The original Bucketlist would have been an extremely light boat. My formula says 12^3 = 1700kg. According to Robs measurements, Bucketlist was on target to have around 500kg lightship, 700kg with crew. So less than half of my formula. Which means between the red and blue curves above. So it looks like it would have been completely without a "hump" at hull speed, it would just go linearly faster with windspeed until it runs out of righting moment. But would it have been too light? Too much wetted surface for the sail it could carry? I guess we will never know.

Rob, I think it was very sad to see Bucketlist shopped into pieces! Now we can't see how it would have performed! It would also have been interesting to see if it would plane on the lee hull! I think Bucketlist would have been the lightest boat in the world for its length, so I think there exists no real world data with regards to planing on that kind of narrow, flat bottom, very light hull. If it would plane, it would get rid of some of that extra wetted surface area, and sail even faster. Was it because of all new foiling boats getting popular, that you stopped development?

Will the new one (triscarph) be as fast? I don't really understand the Triscarph configuration. Will the waves cancel out in some way? Or will it be as draggy as two very short hulls with bad length to displacement ratio until it foils?

And I should add that I have never sailed on a larger catamaran, only small beachcats. And I more or less only have the theoretical knowledge of boats from reading ship theory on the internet and running Michlet simulations during the snowy winters in Sweden.

/Björn