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  #106  
Old 05-23-2007, 12:50 PM
Excalibur Excalibur is offline
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Can I get back a page or two and return to seakeeping? The question was asked if real seakeeping abilities are needed. I'm not certain if the need to brest 25 foot waves and live is there, but there can be consideration given to how the boat will react when it's just a little snotty. I'll go fish the Chesapeake in 2-3 feet (real feet, not tall tale feet) if the boat I'm in has a comfortable motion in a chop, but it will stay at the dock if it snap rolls me right out of my seat (been there, done that). Is either the Devlin or the Atkins Seabright known for it's easy rolling motion while trolling in a chop? Would either boat be able to maintain cruise speed in 2-3 foot waves?
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  #107  
Old 05-23-2007, 01:52 PM
SAQuestor SAQuestor is offline
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Quote:
Originally Posted by Excalibur View Post
Can I get back a page or two and return to seakeeping?
Please!

Quote:
Originally Posted by Excalibur View Post
The question was asked if real seakeeping abilities are needed. I'm not certain if the need to brest 25 foot waves and live is there, but there can be consideration given to how the boat will react when it's just a little snotty. I'll go fish the Chesapeake in 2-3 feet (real feet, not tall tale feet) if the boat I'm in has a comfortable motion in a chop, but it will stay at the dock if it snap rolls me right out of my seat (been there, done that). Is either the Devlin or the Atkins Seabright known for it's easy rolling motion while trolling in a chop? Would either boat be able to maintain cruise speed in 2-3 foot waves?
A bit of clarification if you please. A 3'er is about waist high on me. (Chest high on my buddy ) Depending on the period of the wave train, waves that high could be a nuisance or more serious. So in the areas you frequent, would you say the wave period is very short, i.e., close together from crest to crest or what exactly?

I've never been aboard a boat on the Chesapeake, but I must assume that there are no swells underlying these 3'ers. Right?

Next, what's your definition of reasonable cruise speed in the conditions you frequent for a boat that's in the mid-20 foot range like some Devlin and Atkin designs? 15 knots? 12? 10? 8?

Thanks for clarifying the conditions you're asking about.

Leo
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  #108  
Old 05-23-2007, 08:16 PM
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u4ea32 u4ea32 is offline
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Sorry, got distracted (dinner was ready) and forgot the links and the total data.

The conclusion: the semi planing boat is about as good in low planing speeds, and much worse the rest of the time.

The two boats:
Beneteau Swift Trawler 42, 33000 lbs displacement
Beneteau Antares 13.80, 33960 lbs displacement

8kts:
8kts: ST42 burns 5.3 gph
8kts: AT13 burns 4.2 gph -- 25% better mileage

10.3kts: ST42 burns 12.7 gph = 0.81 nmpg -- 6% better mileage
11.0 kts: AT13 burns 14.3 gph = 0.77 nmpg

15.5 kts: ST42 burns 21.5 gph = 0.72 nmpg
14.3 kts: AT13 burns 19.3 gph = 0.74 nmpg -- 3% better mileage

23.3 kts: ST42 burns 37 gph = 0.63 nmpg
24.0 kts: AT13 burns 31.7 gph = 0.75 nmpg -- 20% better mileage

28.5 kts: AT13 burns 39.1 gph = 0.73 nmpg

Full reports:

http://www.beneteauusa.com/wps/wcm/r...tares%2013.pdf

http://www.beneteauusa.com/wps/wcm/r...%20ST%2042.pdf
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  #109  
Old 05-23-2007, 08:26 PM
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Willallison Willallison is offline
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1st conclusion - both are pretty thirsty! (But typical)
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  #110  
Old 05-23-2007, 08:36 PM
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u4ea32 u4ea32 is offline
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Hi Tom,

I've read your posts many times over the years. This is a very good argument:

Quote:
Originally Posted by tom28571 View Post
If a boat is on motion on the water, it must be supported by either buoyancy, hydrodynamic lift, aero forces or a combination of these.

Waves do not disappear at high speed. Once a boat is planing and almost all support is from hydrodynamic force, they do get smaller the faster the boat goes but the energy in them is nearly constant relative to time. That is, the work done by the water in supporting the boat must be constant, disregarding aero forces. Work done has a time element so the waves are spread out over a greater area of water due to the speed of the boat, thus appearing to be of lesser importance. Unless we can somehow get rid of Newton, this must be true.

Dynamic lift and momentum imparted to the water are exactly equal and opposite vectors. Doesn't matter whether it's a regular planing monohull or a stepped hull. Momentary imbalance of these forces do occur but the average must be equal to the weight of the boat or the boat will either sink or fly. There are other minor considerations but the above covers the basics.
The thing about waves is that the longer the wave, the more energy they consume.

Read this... Dang! I hate losing my references, I'll have to find it. The paper I am looking for is by Fox Associates in Seattle, Washington on the All American built Condor Express, launched in February 2002. They studied the wave energy of the ferry as it passed through a narrow strait (Rich Passage) at various speeds. The interesting thing, to me, was that as the boat hit a certain speed, their wave train wave length was halved (I can't remember why, had something to do with bottom configuration) and the wave energy at that point plunged: a big discontinuity. The reason: the volume of a wave is the square of the wave length, as both the length and height drop. The volume is where the energy goes, so it is proportional to the wave drag.

Now this ferry was using a foil for hydrodynamic lift, but a lifting bottom is exactly a supercavitating foil. In fact, the ferry drag dropped when the foil started supercavitating.

So don't let the foil stuff confuse the issue.

A stepped hull is so much more efficient than a non-stepped planing hull for exactly this reason: two little waves instead of one big wave.

Also, each planing surface becomes higher aspect, and on a lifting surface (like the supercavitating foil that is the bottom of a planing boat) increasing the aspect ratio increases lift to drag. For the same reason, by the way: a shorter wave.

So don't think that all waves that provide the same amount of hydrodynamic lift are the same. They are not.
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  #111  
Old 05-23-2007, 08:37 PM
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u4ea32 u4ea32 is offline
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Originally Posted by Willallison View Post
1st conclusion - both are pretty thirsty! (But typical)
Right! They are both HEAVY!!!

Gotta be light to be efficient.
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  #112  
Old 05-23-2007, 08:40 PM
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u4ea32 u4ea32 is offline
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Seakeeping: Long and thin makes slower accelerations.

Slow speed through the water helps too.

A semi-planing hull is smoother than a typical equivalent planing hull because more of its length stays in the water, and because it goes slower.

You can make a planing hull act like a semi-planing hull simply by having sufficiently large trim tabs: it will be smooth, slow, and burn lotsa fuel.
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  #113  
Old 05-23-2007, 08:45 PM
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u4ea32 u4ea32 is offline
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Originally Posted by SAQuestor View Post
Please!

Next, what's your definition of reasonable cruise speed in the conditions you frequent for a boat that's in the mid-20 foot range like some Devlin and Atkin designs? 15 knots? 12? 10? 8?

Thanks for clarifying the conditions you're asking about.

Leo
Out here near Los Angeles, the typical weekend has a lot of boat wakes, and the wind kicks up in the afternoon from about 8 knots at noon to a peak of about 15 knots at sunset, pretty much every day (thermals from cold ocean to warm land). The chop build up to about 3 feet trough to peak. The swells are long, and so don't come into play under 15 knots.

I find that a very fine entry boat from 20 to 40 feet long can easily make a very smooth 12-15 knots. No real improvement going slower.
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  #114  
Old 05-23-2007, 09:49 PM
tom28571 tom28571 is offline
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Quote:
Originally Posted by u4ea32 View Post
A stepped hull is so much more efficient than a non-stepped planing hull for exactly this reason: two little waves instead of one big wave.

Also, each planing surface becomes higher aspect, and on a lifting surface (like the supercavitating foil that is the bottom of a planing boat) increasing the aspect ratio increases lift to drag. For the same reason, by the way: a shorter wave.

So don't think that all waves that provide the same amount of hydrodynamic lift are the same. They are not.
Yes, there are a lot of factors that can change the effect of wave interaction with the boat. The bulbous bow comes to mind. Newton is not contradicted, just fooled a bit. Wave collector chines and the Hickman sled are two more. Some power trimarans also use waves created by the central hull to add lift to the amas and thus reduce wave size away from the boat.

A stepped hull has a huge amount of longitudinal stability which allows both planing surfaces (single step, of course) to have two good features. They are a constant and optimum angle of incidence and, as you point out, a high beam length ratio of the planing surface. I know the latter runs counter so the beliefs of many, but a high aspect ratio is just as important to a planing boat as it is in a boat sail or a sailplane wing. And yes, the negative effects of wake from two smaller waves is less than that of a single wave but the total energy to support the boat must remain the same.
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  #115  
Old 05-24-2007, 08:36 AM
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u4ea32 u4ea32 is offline
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And yes, the negative effects of wake from two smaller waves is less than that of a single wave but the total energy to support the boat must remain the same.
Exactly.
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  #116  
Old 05-24-2007, 08:51 AM
Excalibur Excalibur is offline
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Originally Posted by SAQuestor View Post
Please!



A bit of clarification if you please. A 3'er is about waist high on me. (Chest high on my buddy ) Depending on the period of the wave train, waves that high could be a nuisance or more serious. So in the areas you frequent, would you say the wave period is very short, i.e., close together from crest to crest or what exactly?

I've never been aboard a boat on the Chesapeake, but I must assume that there are no swells underlying these 3'ers. Right?

Next, what's your definition of reasonable cruise speed in the conditions you frequent for a boat that's in the mid-20 foot range like some Devlin and Atkin designs? 15 knots? 12? 10? 8?

Thanks for clarifying the conditions you're asking about.

Leo

The Chesapeake is broad and shallow. It's 4 miles wide at the top, 30 miles wide at the bottom, and about 200 miles long. The average depth is only about 20 to 30 feet. With 20 knots of wind from the (worst case) NW, it builds a very vertical chop that measures about 2-3 feet from trough to crest with a short period. I don't think I've ever been foolish enough to be in the Bay with any conditions that would build swells. The chop though, gets up very fast, and because it is very vertical, can cause pounding easier than you might think. My (current) 38' x 12' 10.5 ton semi planing hull bridges that chop very nicely, and my speed is not regulated by it. I expect 32 feet would do so as well. Cruise on my boat is 12.8 kts @ about .9 gph. (should read .9nmpg edit). I'm satisfied with that cruise speed (although like most, more would be better), but I hate feeding the twin 454 monsters that drive it. My dreamboat for the bay has minimal accomodations (porta potti, canvas bunk, bait prep sink, a place to keep dry), has a slow even roll in the above conditions, is diesel inboard, and can carry enough gear to make 4 recreational fishermen happy. It cruises at 12.5 knts or better, and gets at least 3nmpg at that speed. The classic work boat for the Bay (the Chesapeake Deadrise) is about 35 to 45 feet long with a beam of nine to twelve feet. It has a fine entry and a shallow V at the transom. It is a fine boat, but with a gross tonnage of 10 to 15 tons it's not much more efficient that what I have. I've been looking at some of the largest Atkins designs in the 28-29 foot range. The Devlin I've been admiring is 32' 10". Of the two, the Devlin would certainly be easier to build, but it has a very conventional semi displacement hull and I am wary of Devlins claims of 18 knts and 4.5 nmpg at that rate.
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  #117  
Old 05-24-2007, 09:57 AM
tom28571 tom28571 is offline
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Cruise on my boat is 12.8 kts @ about .9 gph. I'm satisfied with that cruise speed (although like most, more would be better), but I hate feeding the twin 454 monsters that drive it. My dreamboat for the bay has minimal accomodations (porta potti, canvas bunk, bait prep sink, a place to keep dry), has a slow even roll in the above conditions, is diesel inboard, and can carry enough gear to make 4 recreational fishermen happy. It cruises at 12.5 knts or better, and gets at least 3nmpg at that speed. The classic work boat for the Bay (the Chesapeake Deadrise) is about 35 to 45 feet long with a beam of nine to twelve feet. It has a fine entry and a shallow V at the transom. It is a fine boat, but with a gross tonnage of 10 to 15 tons it's not much more efficient that what I have. I've been looking at some of the largest Atkins designs in the 28-29 foot range. The Devlin I've been admiring is 32' 10". Of the two, the Devlin would certainly be easier to build, but it has a very conventional semi displacement hull and I am wary of Devlins claims of 18 knts and 4.5 nmpg at that rate.
Surely that is a typo and you mean 9gph not .9gph.

I also am fond of Devlin's Topknot. It would be serious build project though and those numbers do sound pretty generous. Maybe he got them from a client.
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  #118  
Old 05-24-2007, 10:20 AM
moTthediesel moTthediesel is offline
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Surely that is a typo and you mean 9gph not .9gph.

Aw shucks, I was gona ask him about his special carburetors that give that kind of economy from a pair of BBC's.

Perhaps the new "Loaves and Fishes" model that I've heard Holley is working on

moT
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  #119  
Old 05-24-2007, 11:17 AM
Excalibur Excalibur is offline
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Yup, brain f**t. It should read .9 nmpg. I don't think you want my carbs
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  #120  
Old 05-24-2007, 12:36 PM
kengrome kengrome is offline
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Some power trimarans also use waves created by the central hull to add lift to the amas and thus reduce wave size away from the boat.
I think this is what Atkin tunnel-stern Seabright skiffs do, don't they?
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