View Full Version : flat vs v
08-18-2003, 11:04 PM
I am new to boat building and have a question for the gurus out there. I want to build a wood(ply) center console skiff/dory/salty fishing boat.It will be used mostly in Southern California. 75% of the time it will be used for inshore fishing(Not Flats)But I want to be able to run to Catalina and maybe even Clemente if the weather is favorable. Also it will be used in Baja where beaching is a plus.
I have looked at just about all the designs on the net, GlenL,Tolman,O'brien,.... but still can't come to a decision between the v vs. the flat hull type.
It seems to me that many commercial's use the flat bottom style but most are not working offshore;
Dories are stable because of the flat hull and easier to build but just how much speed is sacrificed? Say a semi v can go 20kts. in a 4 ft.swell with our typical afternoon winds,how much is the flat hull going to have to slow down? I want to hear from everybody that has an opinion one way or the other.
Hi Hal. Hopefully the gurus will respond soon but in the mean time... :D I don't know of any advantage to the flat bottom, including beaching. They aren't even any easier to build. I have built two small Glen-L boats, one a v-bottom sloop and the other a power skiff that is v'ed in the fore but flat and wide at the transom in the typical small outboard design.
08-19-2003, 12:13 AM
I'm sure as **** not a guru, but a flat bottom planes earlier, is more efficient and can have shallower draft. Theoretically it also has better roll damping, but in practice the difference is probably negligable.
A V bottom trades efficiency for a softer ride.
For anything other than enclosed waters, forget about flat bottoms for planing boats. Not only will they probably tear their engines out and break up if pushed too hard, but they'll do freaky things to your body. The loads on the joints (particularly the knees and spine) can be really horrible in any kind of planing boat in rough water, but a flat bottom is like whacking the soles of your feet with a boat.
Imagine you're pointing a garden hose at a flat bit of ply - the most force is transmitted to the ply when the hose is perpendicular to the surface. When the stream is at an angle to the surface, the water (and hence force) is reflected to one side, transferring some of the perpendicular force component to a lateral one.
For boats used in open water, 20 degrees seems to be a common compomise between efficency and ride, and a bit less if the boat is narrow or primarily intended for semi protected water. Lighter boats will need more V due to less inertia. I'm not a big fan of power boats, so I'm not really up to speed on the finer points. I'll let someone else wax lyrical over the benefits of constant vs tapered deadrise. In flat water a planing boat can essentially be viewed as static if the reference point is taken as being in the boat. A boat at sea is extremely dynamic, and they do all kinds of funky **** like launching into the air off waves, crashing back into the water, and then trying to bury their bow in the next wave.
Planing boats have a lot in common with aircraft, and are actually newer technology. The first successful planing boat was launched several years after the first plane flew. If you imagine that beam is the counterpart of wingspan, and the aft section of the hull in contact with the water is chord (and deadrise as dihedral) then you'll start to get the picture. Probably the most important development in the history of offshore powerboats is the use of spray deflector strips along the bottom, which detach water flow from the bottom, decreasing wetted surface when planing, hence increasing efficiency and therefore speed.
And just to finish off on a belligerent, opinionated note - outboards suck arse. Whoever thought of putting the engine weight up high and hanging it off the back of the boat should have their scrotum stapled to their forehead smile.gif
Outboards are the fast food franchise of the recreational marine industry. (where's my 'tongue-in-cheek' icon?)
[ 08-19-2003, 12:45 AM: Message edited by: Aramas ]
08-19-2003, 11:30 AM
That V imposed on a flat bottom in the current WB is an interesting go. Just be sure you have a plan for any tendency towards rot, difficulties of inspection for damage, etc.
The power dories used off the beach in Oregon and California are pretty nifty. They will get up efficiently, even with a load, and they are plenty seaworthy. You're looking mostly at pretty long waves and much of the running about is parallel to the seas, more or less, anyway so the premium that an east coast boater might put on the ability to punch into a steep sea is not so vital to you.
If you want to blast through the Gulf Stream at 60 kt, a deep V is the only way to come out with your kidneys still over your a$$.
But if you want to tool around your waters at an efficient 20 kt, mostly nice weather but in a boat you can get home in if caught in a hard chance, I think the power surf dory might be hard to beat.
08-19-2003, 02:08 PM
Originally posted by Hal Forsen:
It seems to me that many commercial's use the flat bottom style but most are not working offshore;
QB]You answered your own question.
08-19-2003, 04:26 PM
I always feel a bit nervy to disagree with Tom Lathrop because I admire his work and respect his opinions but a couple of things:
Along shore, say within your normal fuel capacity of the coast, such as you describe as your intended use is quite different from actual off-shore, which is beyond the range of a normal power boat in the range you describe.
A sharp bow is indeed nice - more than nice! - for punching into a head sea but the bit with the bottom flat or not is not all that relevant for a boat such as you plan. You're not setting off-shore race records. You're not dashing out into a Force 8 rage through a breaking inlet.
There are a lot of ways to find suitable seaworthiness for your purposes that may include a V or flat or round bottom.
The smaller Bartender is nice as well though thought by some to be a bit antiquated.
Or a kind of combination, like the Pete Culler style file bottoms that are pretty sharp and V'd at the bow and flat astern - narrower in general than other power boats. Very efficient. Good turn of speed for modest horse power and fuel consumption. Bring you back alive. And with proper work boat finishing, a head turner anywhere.
Personally were I looking for a boat to use as you plan, I'd go with the Culler type first choise since in the end I still like really sharp entry, but the classic beach power dory is a good choise for out there. Unlike the Culler, it's not widely favored on this coast though there is a unit called something like the Carolina dory skiff that's quite a bit like the west coast power dory, perhaps a bit finer in the bow.
The dory type I've mentioned have a fair bit of rocker which somewhat diminishes their flat water speed and efficiency. The Culler model has less rocker and has a bit wider range of efficiency.
08-20-2003, 05:54 AM
Must be a yank thing. I can't think of a single flat bottom powerboat type used anywhere in Oz - except the ubiquitous 10'-12' 'tinny' punts used for fishing in rivers and small lakes, and they're considered to be the bottom of the food chain.
But then for most of us, 'bar running' is not optional - it's mandatory. Perhaps the Tasman and Southern Ocean are a little lumpier than in your part of the world, but from what I've heard, the conditions on the Oregon coast are quite similar to the East coast here. Southern CA might be a little more user friendly, but with a trailer boat you can go anywhere - you may as well make sure the boat won't restrict your options.
Doing 20 knots in any kind of chop will deliver unhealthy shock loadings to the body, even in deep vees and shark cats. Flat bottoms make it a *lot* worse. At anything over displacement speed they'll pound like a president on an intern.
08-20-2003, 08:19 AM
No worries Ian,
I am not the last word on powerboat hull design or anything like it. Just a worker and observer of such. I do get to observe lots of small powerboats in action on coastal waters and talk to their owners though. This is the home of the "Carolina Dory", the "Carolina Skiff" and the "Simmons SeaSkiff". The Simmons is a dory hull of sorts and has a reputation for good performance in near coastal water. It has a fairly narrow, very low angle deadrise bottom with little or no rocker aft and flared sides. The boats known by most as Carolina Dories are similar to the Simmons, usually with more rocker which limits their top speed. The Tolman Skiff is also similar to the Simmons. The larger C Dory cruiser is also of this type.
The Carolina Skiff or Skimmer are true flat bottomed boats. Latest versions have made several changes to make them less injurious to human crews. They have added some longitudinal strakes and/or non-trip deadrise chines. They still ride like unsprung wagons in the rough stuff though. With their great stability, they find much use as crab skiffs in our calmer creek waters but tend to stay off the river and sound when it gets up.
It is each individual's choice as to which style they prefer for whatever reason or bias. It is not rocket science though to visualize the flow of water over the hull bottom. All of the various choices have their advantages and disadvantages. All of the ones we normally work with will pound your teeth fillings out if driven too far beyond their "comfort zone" in rough water.
For me, the best compromise is a bow fine enough to enter waves with minimal shock, changing to an aft deadrise angle consistent to the speed and use the boat is to be put to. Flatter for more speed in calmer water and deeper for more challenging offshore conditions. The latter is necessarily heavier and less efficient, requiring more power for the same speed. For some conditions, I would choose a flat bottom like the Jon boat but not for trips to Catalina, which, if I remember the song correctly, is 26 miles across the sea. Even in Southern California, weather can get lousy in a day's time.
Other style hulls such as the multihulls or submerged bouyancy types are a whole 'nother game.
[ 08-20-2003, 08:25 AM: Message edited by: Tom Lathrop ]
08-20-2003, 08:40 AM
Off topic regarding a homebuilt boat, but my daughter just returned from a year in Australia (East Coast) and Tasmania. The powerboats she was most impressed by (especially for their rough water performance) were the powercats. This squares with the observations by John Kiley, a yacht designer friend on Cape Cod, who's designed a series of effective sportfishing powercats. (Search for 'benchmark' on yachtworld.com if you're interested in seeing a 36' and 38' sportfishing cat.)
Back on topic, I agree with Ian... but then I almost always do! :D
08-20-2003, 02:01 PM
Yeah, the power cats over here are generically known as 'Shark Cats' after a popular brand from the old days. They were developed to operate from east coast river entrances that are notorious for their bars. They're basically like two very narrow deep V planing hulls with a very low bridgedeck. Initially they were widely used by the volunteer rescue organisations, and now most police boats are also of that type, along with a lot of recreational and commercial fishing boats (for those that can afford them). Their advantages are mostly due to their 'like on rails' directional stability, which allows them to run bars and confused seas, when most monos would be inclined to broach. The bridgedeck clearance is quite low, so they pound and throw spray around like a toddler in the bath.
In the 80's my stepfather had a 14 foot powercat called a "Boston Whaler", which I always assumed was a yank design, but that's only because of the name. On the nasty, short, steep Port Phillip Bay chop she was a real knee-breaker, and wet as a half-tide rock, to boot. She sure did like to go straight though.
08-20-2003, 04:49 PM
If your stepfather's 'Boston Whaler' was made in the US, and sported the red Whaler and curved harpoon logo on the sides aft, it was a Yank design... by Ray Hunt.
Bouncy, indeed, but great fun for running over sand bars, as I did in mine (and John Kiley did in his!) in the early 60's. I once broke the middle thwart when wave jumping, so they weren't smooth riders. :cool: :D
08-20-2003, 05:50 PM
While I can't say the Tolman skiff is the ultimate design for your use, I would venture to say that they have seen more use in mixed to severe waters then any of the other designs out there, and I've yet to see any gripes from the owners.
Here is an example of extreme use of the hull, but also proof of it's seeworthiness http://www.nas.com/boat/tales.html
Everything in design is a tradeoff, and while ideal on one type of condition, less then ideal in another sort. To me, the Tolman has a good balance of design tradeoffs, and a solid track record from builders and users.
08-20-2003, 06:06 PM
perhaps you could look a bit further afield and see what else is about.
I think we have incorporated all the good attributes of the CAT/Boston Whaler/Hickman Sea Sled, the cutting and tracking ablility through waves, stability,no bow wave & reduced stern wave,less pounding of the new IVB - Inverted V Hull coupled with the fact of requiring up to 20% less HP to achieve similar speeds in hull lengths.
Though we still get the same responses as did Hickman in his day after 4 years of using this hull my reply is untill you have experienced the hull dont judge it!!!
08-20-2003, 06:49 PM
How 'bout the 19 ft. semi-dory in Gardner's The Dory Book ? I've alway wanted to hear how one of those works out. smile.gif Looks a lot like the Oregon surf dories Ian mentions, but perhaps a little more elegant. Partially decked it would be about as seaworthy as anything out there.
08-21-2003, 09:15 PM
Hal here's a professionally built 24 foot Culler launch .
Here's the 30 foot Culler . This could be you crossing to Catalina .
[ 08-21-2003, 09:18 PM: Message edited by: Bill Perkins ]
08-22-2003, 10:09 PM
Pick a reasonability narrow "V" bottom with flattish stern hull form. You'll get offshore with your fillings intact, enjoy the softer ride inshore and use little extra power to move her to speed. Not the radical "V" that has taken the center console boat designs by storm, but the modified "V" hull that we've all seen a thousand times before. Punching thru a chop or trying to blast into waters not intended for a flats boat will teach you quickly about the merits of "V" bottom boats. I wouldn't venture very much farther from shore than I'd care swim back in a boat with no deadrise forward.
I learned this in a 27' dory I built 25 years ago and sailed from VA to Jersey. I was young, and had nice tight, fresh fillings then . . .
Originally posted by SailBoatDude:
[QB]Punching thru a chop or trying to blast into waters not intended for a flats boat will teach you quickly about the merits of "V" bottom boats. I wouldn't venture very much farther from shore than I'd care swim back in a boat with no deadrise forward.
QB]There was an article in WB mag about truly flat bottomed surf fishing dories. They are apparently built that way to accomodate beach launching whereby the tow vehicle is backed out onto the beach as close as possible to water's edge without getting stuck and the boats are then dumped off their trailers onto the sand or maybe a couple inches of water where they wait for a big wave or a rising tide to float them free. The flat bottom is necessary as any deadrise might easily sink into the sand. Aside from launching purposes I don't recall the article having anything worthy to say at all about the merits of the design once past the breakers.
John E Hardiman
08-24-2003, 11:57 AM
Originally posted by JimD:
There was an article in WB mag about truly flat bottomed surf fishing dories. They are apparently built that way to accomodate beach launching .....Surf dories (both east and west coast types)are very specific boats for a very specific purpose. They are designed to operate off gently shelving beaches with rolling rather than plunging surf. While they operate in swell, they do not operate well in chop.
One of the problems in trying to address the "best boat type" question is that each geographic region has spawned boats suited to that area. The west coast is generally an area of swell with an un-landable coast and an onshore breeze; the east coast is all chopy with lots of hidey-holes and an offshore breeze (i.e going "down" east while traveling north to Maine). These conditions call for very different boats. Inland lakes and rivers are altogether different. IMHO I would feel far safer 100 NM offshore in a 24' open boat on the west coast than on the east (i.e. there is a real reason the early circumnavigators named it "Pacific"). I feel safer in 8', 12 second swell than in a 4', 6 second sea. The nature of the seaway tends to drive the hull shape and to answer Hal's question; the Bight of California is exposed to storm swell from the Southern Ocean (why the surfing is good) and somewhat protected from the coastal storm waves to the north by Point Conception (which is why the beaches are being built along the bight). While it can get rough between the islands in the outer channel, on mostly any day that would be worth going out in (i.e. a recreational vice working underway) any open boat hullform (provided it had enough freeboard for open water ~24"+) that could get you back in 4-8 hours would be useable. However, you would still have to read the weather and run inshore is it started to blow. The further south you go from Point Conception, the more exposed the coast becomes and the shorter and steeper the average seaway becomes until just south of the border the coast becomes ironbound again.
Remember, when dealing with the sea, there is always a "small but real" risk that you may encounter a "statistically significant" seaway event. :eek:
08-24-2003, 12:41 PM
That's good John, thanks for that.
So the moral is to seek out a local type, though it would be interesting to look at the lines of both West and East coast semi-dories/power dories and compare. I'll bet they are much more similar than different, but I don't know. I mean, there's only so much wiggle room.
Great hullform for almost any water. I'll have to look at Gardner's 19 footer again. With a forty horse four-stroke it would make a good boat for what the thread is aimed at, I think.
Surf dories (both east and west coast types)are very specific boats for a very specific purpose. They are designed to operate off gently shelving beaches with rolling rather than plunging surf. While they operate in swell, they do not operate well in chop.
That sounds about right, John. The flat bottom won't pound in a swell, but it will in a chop.
There is a wealth of information (and photos) of Long Island surf dories, and their use in ocean haul-seining of Striped Bass, in Peter Matthiessen's Men's Lives.
The boats were simple. 16' in length (at the flat bottom, with 2 rowing stations. They were crewed by 3 men, 2 rowers and the sternman who set the net. After the whole crew hauled the dory out of a pickup, or off a tractor-drawn trailer, they'd place it into the surf. The rowers would board and take up their oars. The shore crew and the sternman would shove the boat into the surf until they were chest deep, at which point the sternman would scramble into the boat, and take up an oar to help keep the boat bow to the sea.
Check this out on pacific dory from the 30's.on dngoodchild.com website, old books on boats.
#5028--BALBOA: A 17 1/2-Foot Pacific Dory
by Hi Sibley
This staunch dory is a good model for the amateur to build because with the flat bottom and straight sides no problems are presented in construction that demand special experience. This "Balboa Dory" makes no claims for beauty of line, but is about as inexpensive as one can build in a roomy and satisfactory craft. The original is now in its third season (as of 1938). It has never shipped any appreciable amount of water in its three trips to Catalina Island, 28 miles offshore from its home port. Like all dories with their peculiarly narrow bottoms, it rolls easily in a calm sea when passengers move about, but by virtue of this very design a rolling sea does not affect it as much as other types.
08-30-2003, 11:20 AM
You guys have pretty much covered this subject, but I want to point out one additional characteristic.
Most people, when talking about stability, are thinking about resistance to heel when a vessel is at rest, i.e. static stability For equivalent boats, I agree with the comparisons of static stability already presented. However when underway, the comparisons do not always follow the same general rules.
The deadrise on a v bottom boat usually allows them to have superior dynamic roll stability at planning speed. This is due to change of pressure on the bottom. As the boat begins to heel, one side rapidly picks up higher pressure as the opposing side looses pressure. This pressure difference grows with speed. At low speeds this is not significant. But since the pressure varies with speed squared, the static stability characteristics of a fast moving boat are quickly overcome with this dynamic principle.
To summarize, if planning, your stability underway will probably be better with a v-bottom.
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