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Old 17-12-2006, 17:55   #1
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Broaching

Does the multihull concept reduce/limit the broaching effect with a following sea?
A new hull design which I have been working on has a very narrow and deep bow and a wide stern. I have not been able to test it in big seas yet, but I am concerned that the basic design could have a severe tendency to broach.
I owned a 36 ft displacement motor cruiser which was quite a handfull in a following sea. My experience with mutihulls (29ft trimarans) I can't remember ever having a broaching experience.

What I plan to do is develop my new hull design into a trimaran configuration to limit any tendency to broach.

For some years now I have been building prototypes 15ft long using a concept developed by Alvaro Calderon (who worked on the American cup team and a famous mathematician)
In basic theory, he uses a displacement hull which has a very fine entry angle(6.5 degrees on each side ) which continues (the same angle) to the stern of the boat. A double wedge shape. The hull sides are vertical and the bottom is flat.
In pure form his concept is not very practical, but it does have some very special characteristics.
Frictional turbulence along the sides is significantly reduced (no eddys) and no bow wave is formed at high speeds.
I have built a number of 15ft prototypes the test the validity of his design at high speed (20knots).
There was NO bow wave and the hull did NOT plane. The stern wave which was significant at low speed flattened out over 10knots. The bow does NOT pound
The 15ft long prototype I built with a 8hp outboard achieved 15 knots.

Because the drag associated with a bow wave is eliminated. I calculate that doubling the speed will need four times the power. Because the boat is not planninng and in displacement mode the power need is very modest.
BUT I am concerned that the design parameters, deep bow and wide stern will lead to pivoting on the bow with a decent sternwave could be disasterous.
Which is why I am asking if I apply trimaran type floats on either side, to stop the boat pivoting on the bow of the main hull. will this help stop/limit the broaching.
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Old 17-12-2006, 20:03   #2
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ALan Lucas in his book "Cruising The NSW Coast" 1976, describes his yacht " Soleare's" as having Cods head and mackeral tail type hull.

This meant her most buoyant sections are forward of amidships and thus her stern sections are relatively slack.

He reckons this was the best shape to resist broaching, but not to good to windward.

Don't know if this help's

Dave
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Old 17-12-2006, 20:11   #3
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broaching

Thanks dave, Yes I remember that quote by Alan Lucas.

I had an aquarius 36 built in Brisbane a semi displacement with a 280 hp perkins. It was very narrow and relatively fast 18 knots but it was a real bitch downwind.
I always thought broaching was caused by the stern lifing to the wave and pivoting around the bow immersed in the wave ahead.
I would love to meet with you an look at your project.
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Old 17-12-2006, 21:31   #4
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Quote:
Originally Posted by beau
Which is why I am asking if I apply trimaran type floats on either side, to stop the boat pivoting on the bow of the main hull. will this help stop/limit the broaching.
Deploying a drogue, I believe, is the best method to prevent broaching in a following sea.

Adding out riggers could impede in the broaching if they are too small or the wrong shape by resisting the sway of the hull..........................._/)
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Old 18-12-2006, 04:05   #5
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It may be my lack of expierence, but my 40 foot, full keel ketch is a real hand full in a following sea. What are some of the procedures to help aleviate the tendency to wallow and broach. She is 40 foot, 27000lbs, with an 11.5ft beam. she also draws 5.5 feet. It is a new boat to me, I have only had her out once in any real following seas.
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Old 18-12-2006, 08:56   #6
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Whoops, I guess I am in the wrong thread for this question....Sorry, should have been in the mono-hull section.
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Old 18-12-2006, 16:38   #7
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Of course a well designed boat that likes surfing is probably the best way to avoid broaching.

But yes, a drogue work's as well.

That comment i made above isn't what I actually believe, as it seem's to me that the bum would come around to the front, but Alan Lucas has probably forgotten more than I know about boat's

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Old 18-12-2006, 18:03   #8
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yes dave, allan lucas's comment didn't make alot of sense to me either. I have not had any answer from other Multi people yet.
I agree with the drogue idea, but I would like to fix the problem in the design stage if possible. At the moment my design is of a trimaran concept anyway for stability purposes and shallow draft. essential in this part of the world. I just felt that by having three pivot points instead of one it might solve the problem.
I was coming back from Dunk island in a 29ft Sailing Trimaran when fairly large seas built up, I was flying a spinnaker and doing 20 knots. It scared the hell out of me, but I was reluctant to drop the spinnaker in the conditions and we only had to hold on for 5 miles before coming in the lee of Magnetic island. we made it ok and the boat performed flawlessly but I don't know if a monahull which was prone to broaching would have went.
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Old 18-12-2006, 18:12   #9
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Quote:
Originally Posted by beau
but I don't know if a monahull which was prone to broaching would have went.
Im not up for a knife fight this close to chrissy

Dave
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Old 18-12-2006, 18:18   #10
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I thought you were a converted multihull person, this is the multihull site?

I wouldn't dare say anything like that on a "monohull site"

I did say "prone to broaching" I have had the mast hit the water a couple of times IN A MONOHULL. (not my design though)
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Old 18-12-2006, 18:32   #11
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Just had a look uptop and right you are.

Bloody monohull's wouldn't have one, keep leaning over and spilling your drinks.

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Old 20-12-2006, 09:54   #12
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Perhaps you should consider asymmetric amas. Rudy Choy and others claim that asymmetric hulls reduce the tendency to broach in a heavy following sea.
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Old 20-12-2006, 18:38   #13
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I think the narrow beam and easily driven hull form contribute more to the tendancy not to broach than the number of hulls. A lenght to beam ratio of about 8 to 1 for a tri and 10 to 1 for a cat is about right.
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Old 20-12-2006, 19:18   #14
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Quote:
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I think the narrow beam and easily driven hull form contribute more to the tendancy not to broach than the number of hulls. A lenght to beam ratio of about 8 to 1 for a tri and 10 to 1 for a cat is about right.
So, an 80' cat would be 10' wide & a 100' tri would be 10' wide. I think the ratio is a bit off.
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Old 20-12-2006, 21:46   #15
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broaching

Here is a reply from Derek Kelsall for those of you who may be interested.

Regards,
Beau



" Date:Thu, 21 Dec 2006 13:40:57 +1300




YAHOO.Shortcuts.hasSensitiveText = true;YAHOO.Shortcuts.doUlt = false;YAHOO.Shortcuts.location = "us";YAHOO.Shortcuts.lang = "us";YAHOO.Shortcuts.annotationSet = {lw_1166676199_0: {text: 'Waihi, New Zealand',weight: 0.993819,type: ['shortcuts:/us/instance/place/nz/town'],metaData: {geoArea: "6.72802",geoCountry: "New Zealand",geoIsoCountryCode: "NZ",geoLocation: "(175.84277, -37.390549)",geoName: "Waihi",geoPlaceType: "Town",geoState: "Waikato",geoTown: "Waihi",geoZip: "2981",type: "shortcuts:/us/instance/place/nz/town"}},lw_1166676199_1: {text: 'multihull_boatbuilder-unsubscribe@yahoogroups.com',weight: 1,type: ['shortcuts:/us/instance/identifier/email_address']}};YAHOO.Shortcuts.overlaySpaceId = "97546169";YAHOO.Shortcuts.hostSpaceId = "97546168";To answer your direct questioin - deep bow and wide transom will increase tendency to
broach in any boat. Whether it is serious or not will depend on lots of other factors,
particularly distribution of weights.

I do have an interest in the concept. We have produced a few small cat hulls which were
made directly from a single foam sandwich panel - taking 2-3 hours each. This was done to
test what hull shapes could be achieved. The easiest is the one you describe. Also possibly
relevant - we have a couple of power cats operating where the water line is widest at the
transom. (increasing width from stem to stern). The bow waves appear less than is usual.

I believe the concept is worth further testing - but how to evaluate is likely to be a problem.
Cat should work, with c o g well aft. I see no advantage in using outriggers.

Two points -
The impression may have been given that plumb stem is dangerous. it depends how is it
achieved. We can start with a conventional overhang/reserve buoyancy and then pull the
wl forward or we can pull the deck line in leaving the same WL length. The former would
not lose reserve buoyancy.

The fixed keels are bad news! - I do not subscribe to the widely held view that a cat will slide
sideways in front of a breaking crest. To do so the cat would have to accelerate by more
than 20 kts in half a second. The situation which I believe exists is that the crest lifts the
windward hull, but that half second later the crest has passed under to the lee hull and the
cat is back on its feet again. This seems to fit with a number of reports from those who
have done it. I have always been able to run with the storm.

Burying bows back to the mast! Do it just once and you will ensure sufficient reserve
buoyancy and not put those decks and bridge-decks right forward - on power or sailing cat.

A bit of Kelsall Cat news - we have started a "daily picture" on <www.kelsall. com>. Any
interesting pics from clients or from our work here will feature, whenever we have
something - may not be daily.

Seasons greeting from the team at Kelsall Catamarans.

Derek.
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