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 Boat Design Forums Why does multihulls have pointy bows?

#31
01-26-2017, 07:04 PM
 oldsailor7 Senior Member Join Date: May 2008 Rep: 436 Posts: 2,094 Location: Sydney Australia
I think the result of bluff bows is pretty obvious.
The bluntness shoulders the water out of the way.
The energy (F) to accelerate the water comes from the bow it's self which evinces as Drag.
Newton got it right. F=MA, and the M of water is pretty heavy. (64 lbs/Sq.Ft.)
#32
01-26-2017, 11:34 PM
 valery gaulin Junior Member Join Date: Jan 2017 Rep: 7 Posts: 83 Location: Montréal
If I understand correctly from the graph, if the velocity is above 4 m/s the blunt nose hull is were it start to have less wave drag than the more pointy type bow.

Therefore at low speed, below 4m/s the blunt bow hull as a lot more drag probably due to the increase in wetted surface area for such a hull design with high prismatic coefficient.

But as we get at higher speed, above 4 m/s, it is wave drag that predominate. From what I understand, at higher speeds a higher prismatic coefficient is better. Since the blunt bow hull shape have a higher prismatic coefficient the drag is lower compared to a pointy hull with a lower prismatic coefficient at higher speed.

Got this understanding from the discussion on the proa forum quoted under here:

https://www.proaforum.com/forum/viewtopic.php?t=27

Quote:
 Attached below (hopefully) is a chart showing the variation of both wetted surface area and maximum cross sectional area with varying prismatic coefficient (Cp). This is for a hull with semicircular cross sections throughout, which is the minimum case for wetted surface area. In practice, at high prismatic coefficients this leads to very blunt waterlines, so normally the ends would be immersed, which gives finer waterlines but exacerbates the increase in wetted surface area with increasing Cp. If we assume that wave drag is proportional to the maximimum cross sectional area, it is clear from the chart that at higher Cp, wave drag is reduced while the wetted surface drag is increased and hence, given that wave drag predominates at higher speeds, a higher CP is better for a higher speed hull (at least, for displacement hulls).
Conclusion,For a cruising catamaran needing the maximum hull volume for a given maximum lenght and if it is possible to cruise at higher than 4 m/s the blunt bow hull shape can make sense. Going in average at above 4m/s (7.77 knots) by sail alone might be difficult but if it is a possibility to motorsail at an average speed of let say 8 knots the solution of the blunt bow hull on a crusing catamaran make sense to me.

Another question: Would it be confortable to navigate at 8 knots with a blunt bow hull?

Next question: At what speed could we expect a 30foot X 4foot round blunt nose hull to go on plane with a 8500 pound displacement in a catamaran configuration?

Someone know how to calculate water plane area needed to get on plane and at what speed?
#33
01-27-2017, 12:04 AM
 DennisRB Senior Member Join Date: Sep 2004 Rep: 228 Posts: 1,224 Location: Brisbane
I cant see how you can come to that conclusion when the blunt hull was horrendously worse below hull speed. You have to be kidding yourself that its going to pass that point with so much drag. What actual NAs seem to do is set the Cp to realistic known levels depending on the targeted speed range. There are upper and lower limits to what anyone would want to use. Slightly higher for a fast boat, and slightly lower for a slow boat. Not a Cp of close to 1 like the blunt bow would be. Plus are you really going to class this as a fast boat that needs a Cp so high that its off the charts of what would usually be used?
#34
01-27-2017, 01:28 AM
 bjn Junior Member Join Date: Jul 2014 Rep: 16 Posts: 85 Location: Stockholm
Quote:
 Originally Posted by valery gaulin Therefore at low speed, below 4m/s the blunt bow hull as a lot more drag probably due to the increase in wetted surface area for such a hull design with high prismatic coefficient.
The higher drag is not due to the slightly higher higher wetted area. That only makes Rv increase slightly (by the ratio 12/11). That is neglible, less than 10%. The 500% difference in Rt at 2 knots is due to wave drag.

Quote:
 Originally Posted by oldsailor7 I think the result of bluff bows is pretty obvious. The bluntness shoulders the water out of the way. The energy (F) to accelerate the water comes from the bow it's self which evinces as Drag. Newton got it right. F=MA, and the M of water is pretty heavy. (64 lbs/Sq.Ft.)
I'm surprised the difference is so large at low speeds (2 knots) though. I had expected it to work well at low speeds, but not high speeds.
#35
01-27-2017, 01:51 AM
 Emerson White Junior Member Join Date: Aug 2012 Rep: 61 Posts: 94 Location: Nordland, WA, USA
If you have a blunt bow and are sending the water off to the sides at a higher velocity are you going to end up with more pronounced internal wave interference as the two hulls pile water onto each other?
#36
01-27-2017, 02:42 AM
 upchurchmr Senior Member Join Date: Feb 2011 Rep: 579 Posts: 2,365 Location: Ft. Worth, Tx, USA
You need to be careful taking a simulation outside of the parameters from which it was developed.

Continue ignoring waves and you will be in for a rude surprise.

Please let us know any catamarans you have sailed on.
#37
01-27-2017, 08:13 AM
 valery gaulin Junior Member Join Date: Jan 2017 Rep: 7 Posts: 83 Location: Montréal
I sailed hobbie catamaran only, never a cruising catamaran.

Guys lets calm down this is a boatdesign blog, if nobody ever tries something different nothing will ever change or evolve. You can copy and exactly what you neighbor does and you be exactly the same no better no worse. You can try something different, and the result would be different, failure or success! If you succeed you will be a hero, if you fail you just learn a life long lesson.

Now this guy tried sothing different and it works pretty good for him. Nobody tried it before him and everybody thought that he was crazy. Check this video and listen to what he is saying.

You both sailboat side by side a pointy mini and a blunt bow mini. Even at displacement speed he outpace the pointy mini! If he would have listen to all the expert he wpild of have never taken the risk to try it out!

There is a bigger wave in front of the blunt bow mini compare to the pointy mini but he still outpace the pointy mini!

Lets be carefull when we compare data in percentage. Lets say that for the 600% more drag at very low speed the blunt bow needs 10 times the horspower to over come this drag. If it needed 0.2 hp for the pounty bow, 10 times for the blunt bow means 2hp. Everybody knows that it is not a big deal to produce 2hp with sail area. Now lets say that the pointy bow needed 2 hp to overcome the drag at low speed the blunt bow would need 20hp. This is starting to be a problem to produce 20 hp with sail area only at low speed. Therefore this would be a very bad choice if this is the case.

The point I am trying to make is we need to know how much horspower difference it needs to overcome this drag compare to the pointy nose. From 0.2hp to 2hp not a big deal, but from 2hp to 20hp, starting to be an issue, from 20hp to 200hp this might not even be possible or realisitc even with en engine. This is all 10 times the power needed but the technical issue is totally different. Hope you get my point.

I still beleieve that it is worth trying a model size to compare.
#38
01-27-2017, 10:23 AM
 UpOnStands Senior Member Join Date: Nov 2015 Rep: 16 Posts: 282 Location: Sydney
check the video carefully, the mono is strongly heeled so the actual immersed hull shape is not blunt but more ellipsoidal.
Also the mono is getting a lot more stiffness as the righting force is much higher with the full underbody. None of these characteristics apply to catamarans.
If you want only a racer then by all means flatten the keel and ramp up your dagger board areas. You will however (IMHO) pound like crazy even at anchor in a chop. Under sail, your cat will be really touchy (IMHO) and holding a straight course will be very tiring. Your choice.
#39
01-27-2017, 10:31 AM
 bjn Junior Member Join Date: Jul 2014 Rep: 16 Posts: 85 Location: Stockholm
Quote:
 Originally Posted by valery gaulin The point I am trying to make is we need to know how much horspower difference it needs to overcome this drag compare to the pointy nose. From 0.2hp to 2hp not a big deal, but from 2hp to 20hp, starting to be an issue, from 20hp to 200hp this might not even be possible or realisitc even with en engine. This is all 10 times the power needed but the technical issue is totally different. Hope you get my point.
If you only need a rough indication, that is easy! =)

A very simplified sailboat, not affected by waves, heeling etc, will have a ratio between windspeed and boatspeed.

Lets say a 1:1 ratio on a reach. 10 m/s of wind - 10 m/s of boatspeed. But because of wave drag, the speed will be lower than this. Especially at the hump in wavedrag, 4-5 m/s. At higher speeds the wavedrag is becoming less and less significant, and ratio approaching 1:1.

The difference between the blunt and the better hull is (as we can read from the graphs) that before the cliff, the blunt hull has a lot more drag. So with only a few knots of wind, the blunt hull will sail really slow. While the better hull will sail silently into the sunset. =)

I think it's impressive how some boats can really move in light winds. While others (like mine) are stationary.
#40
01-27-2017, 04:50 PM
 oldsailor7 Senior Member Join Date: May 2008 Rep: 436 Posts: 2,094 Location: Sydney Australia
Un-fair play.
In that video the blunt bow is not immersed.
Clearly at the bow waterline the hull is planing.
Of course it would be fast, the only drag is coming from skin friction.
#41
01-27-2017, 07:10 PM
 valery gaulin Junior Member Join Date: Jan 2017 Rep: 7 Posts: 83 Location: Montréal
It is not unfair play! This what i am suggesting from the beginning...
#42
01-28-2017, 02:32 AM
 semelis Junior Member Join Date: Feb 2015 Rep: 18 Posts: 39 Location: Barcelona
This is not going to happen. Look at any video of a multihull in waves. Imagine the slam of the blunt bow every time it hits a wave.
Parlier tried it with his planing catamaran: it got the 24h record in perfect conditions, and lasted only 2 weeks. Races were quite bad, as perfect conditions were rather rare.
If you have 2 (or more) long and narrow hulls in the water, at least 1 of them will try to cut thru a wave when beating, and blunt means no cut.
#43
01-28-2017, 07:38 PM
 valery gaulin Junior Member Join Date: Jan 2017 Rep: 7 Posts: 83 Location: Montréal
@ oldsailor7 and uponstands:
Check this video the bow is immerse at low didplacement speed and he is still as fast if not faster than the other pointy bow?

Now what is your ecplanations and or arguments for saying thst it can't work for cruising catamaran???
#44
01-28-2017, 07:46 PM
 Doug Lord Flight Ready Join Date: May 2009 Rep: 1362 Posts: 13,612 Location: Cocoa Beach, Florida
Quote:
 Originally Posted by semelis This is not going to happen. Look at any video of a multihull in waves. Imagine the slam of the blunt bow every time it hits a wave. Parlier tried it with his planing catamaran: it got the 24h record in perfect conditions, and lasted only 2 weeks. Races were quite bad, as perfect conditions were rather rare. If you have 2 (or more) long and narrow hulls in the water, at least 1 of them will try to cut thru a wave when beating, and blunt means no cut.
---------------------------
I don't think Parlier's problem had anything to do with the bow-it wasn't blunt like the mini's.His hulls were stepped planing hulls and didn't have less drag than a "normal" cat hull until boat speed was over 20 knots-then drag dropped a lot.

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#45
01-28-2017, 07:57 PM
 upchurchmr Senior Member Join Date: Feb 2011 Rep: 579 Posts: 2,365 Location: Ft. Worth, Tx, USA
Valery,

Nothing anyone has said affects you in the least.

Please just go ahead and build you fat bowed cat and report back after a couple of years.

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