Flatbottom skiffs

[Bradley]

Ten years ago, i designed and built a 19ft wooden flatbottom skiff powered with a (100hp outboard) tiller. After all of those years, i can't imagine running the seas in a 'V' bottom.

Here in Southern California you see very few flatbottom boats and i know of no commercial manufacturers who offer them for use offshore. I know that a flatbottom with proper hull configuration will work exceptionally well on the open sea. I was wondering if anyone here has a flatbottom skiff and what they did to overcome the common problems that tend to make most flatbottoms run rough on the seas.

[imported_Sean]

My first boat was a 15 ft "Uncle Gabe" flattie sailing skiff with leeboards. Though it weighed about 400 lbs, it would almost plane downwind. It was stable, tough and sailed very well.

Ever since I have had a great fondness for flat bottom skiffs.

[Sailing-Randy]

Okay, my interest is piqued. I have a flat bottom skiff and it seems all the opinions I've seen see them as less than wonderful on the open water. I would love to read more on how a flat bottom makes for a good open water boat.
Thanks!
Randy

[Bradley]

Olddboat, so glad to hear your affirmation of this much maligned hull configuration!

I refer to my boat as the 'good skiff' and she has been very good to me over the years. I have worked in aerospace most of my life, so when i built the good skiff, i used the design principles of airframe structure rather than traditional boat structure. I needed a boat that was tough enough to run the seas particularily in rough weather. I looked at many boat plans before deciding to wing it.

My problem with traditional boat designs is they use too many structural parts. The more parts, the more seams, the more seams, the more potential for failure. The primary difference between the aircraft structure and boat structure is that aircraft use as few structural components as possible, each part being as big as it can possibly be. The stringers in modern commercial aircraft wings are one piece of aluminum angle as long as the wing, in some cases over 90ft (no splices!). So instead of many small thwarts, i used 3 major bulkheads, tied into 3 stringers.

One important design feature is center of bouyancy. I wanted the skiff to accumulate water in the belly for 'worst case scenario'. In order to do that i needed to build a slight rocker into the hull. The datum plane touches the bottom 2/3 fwd of the transom. It rises 3" to the transom and 6" at the bow stem. The single most significant and common design flaw in most small boats is that they take on water at the transom. I call that a design flaw because that particular characteristic is responsible most sinkings of small vessels.

Most of the flatbottoms i have seen make the fatal flaw of drawing to bow into a 'V' that tapers to flat as it goes aft. I assume that many designers think that the bow will split the water and make for a better ride. I did the opposite. 2/3 forward of the transom is the widest part of the hull. That is what i call the impact zone. That is where i put the fuel (weight). That is the place where the water gets crushed and pushed aside rather than split. The widest part of the skiff is 59" (impact zone) and the transom is 51".

It is critical that the bow has a flat entry because you must induce exagerated transom lift. With common 'V' hulls, the same amount of lift would cause 'bow steering'. With a flat entry, there is no such effect. I use a fixed SST tab that runs the width of the transom with a cutout for the engine. The tab extends out 11" and is bent down on the ends. The degree of lift from the tab is 'tuned' to augment the lift capabilities of the trim on the outboard. The tab creates lift at the transom which presses the bow against the surface and causes it to resist lifting. If the bow doesn't rise, it doesn't slap back down. With the skiff trimmed properly, the bow only rises once, it does not flutter.

I hope to further this discussion sometime soon, but i'm off to bed now. I will be heading out in the morning for a couple of nights on the water!

Brad

[Jim Goodine]

Interesting. I was watching the fishermen in their flatbottomed skiffs (16' to 20') on the west coast of Puerto Rico. The water there is quite rough most of the time but it doesn't stop them from running at full throttle. My first thought was that these boats must really slam in the chop but they don't seem to. The only trick I saw was that they turn the boat a little just as they are about to slam down off a particularly big wave and the boat lands slightly on the chine instead of totally flat on the bottom.
These boats have the same frames from the transom forward about 2/3 the length of the boat. Then the forward 1/3 curves in to a slightly curved stempost. The sides are the same height over their entire length.

[Don Maurer]

Bolger has designed quite a few flat bottom boats for offshore use. All have qhite a bit of rocker. The biggest criticism I have seen of them is they tend to pound in a chop. Sharpies are by and large flat bottomed and were certainly capable of open water passages.

[Bradley]

(we have a little wind offshore this morning so i'm waiting for the lull before i make the run)

I didn't specify that the rocker in was not a radius. It is a straight line from 2/3 forward of the transom. The 1/3 forward of the rocker is a radius, but that portion of the hull is normally out of the water. That's important because while under pressure (underway) you want the hull to plane on a flat, straight surface. If it were a radius all the way to the transom, the hull would never achieve (theoretical) planing stability. The boat would always pitch on that radius. The continuous radius does give some relief from over stiffness, but i chose another way to accomplish that. What i did instead was to taper the width of the bottom (from 2/3 fwd of the transom) to the transom. The skiff is 8" narrower at the transom than the widest (impact point).

That configuration has an inherent conflict that actually has an advantage. The straight taper to the transom actually decreases pressure at the transom and the fixed tab reapplies that pressure in a controlled and 'tunable' manor. The net effect is that you have lift at the forward end (wide impact point) and lift at the transom. There is a penalty in performance for the conflicted double lift configuration, but it is neglegable. The double lift actually keeps the hull stuck to the surface and that makes for a much better ride in seas.

Brad

[G. Schollmeier]

Brad,
It seems to me you have a lot of horsepower for a 19' skiff. Is this required for your design?
Gary

[JimD]

I thought sharpies were named for the fact that the bow is designed to be long and pointy - sharp - where it enters the water, reducing the pounding

[Bradley]

Hello G Schollmeier

The skiff is fairly heavy, 1/2" sides and 3/4" bottom with lots of mohogany, etc. With the 100hp, i cruise at 22-23kts @ 4,300 RPM fully laden. Here is a link to a picture that was taken during construction.

http://www.ufix-it.com/personal/bradskif.jpg

I happened to find 2 pieces of marine ply that were 5' X 9'. I scarf spliced them (8 to 1) end to end to make the entire hull one single piece. That determined the length of the bottom. I use 3 3/4" stringers on the inside of the hull with 3 corresponding 1" stringers on the bottom of the hull. The 2 outboard stringers are 5" wide and the center (two) are 10" wide. Where the stringers are, the hull is 2.5" thick. The weight of those stringers helps to further lower the c/g.

The splash rail is made of 2 17' x 3/4" mohogany planks that were cut to strips 1.5" wide. They are laminated 4 deep and thru bolted at every every frame and BHD. It is a great way to add strength to the hull without taking any room inside the hull. I have since added wood to draw the stringers fwd to the bow. In severe following seas, the hull required more lift at the bow.

About 5 years ago i installed a 6" diameter window right in the cockpit floor. I can't tell you how absolutely incredible it is to be able to see whats below the boat. I have seen many incredible sights through the years, from whales to tuna!

(That dern wind knocked me off my trip this weekend)

JimD, i'm not familliar with the sharpie, i don't know how they work.

Brad

[JimD]

Bradley, a sharpie is a sail boat that is designed about like your motor boat (nice boat, by the way), kind of a dory hull. I only know what I read about these designs, but they ride rough because they don't slice through the water except at low speed in calm water because the point of the bow never leaves the surface. As soon as the bow leaves the surface they start to pound (the marine equivalent of getting shin splints from slappin your toes on the ground as you run). The sharpie design creates a sort of psuedo v-bow to prevent this. Its probably not as bad with a flat bottomed sailboat because the sail boat heels to one side as it sails and the chine becomes a v-bottom that does the slicing. On a powerboat I would imagine those chines grab the water when you'd rather they didn't. I've never heard of a solution for a power boat except to not go too fast, and not to let the boat get too light at the front end, assuming I understand your situation. Does this make sence or have I had too much beer already tonight?

[Bradley]

Hello Jim, have another one on me and i'll do the same!

I don't really know anything about other flatbottms except the few that i have actually seen. I've heard a lot of the negetive stuff about them pounding in seas though.

I have traveled over 70,000 miles of open water in two skiffs. My first one was a 16 footer that someone else built. It didn't ride well at all when i first got it, so that is where i experimented with the modifications that i later incorporated into the Good Skiff.

For me, building the skiff was incidental to actually being on the water. I do underwater photography as a hobby (passion) and i need a boat that will get me to all of those incredible places off our coast. I routeinly run 75-100nm straight offshore and swim with my camera.

I have been on a number of small 'V' bottom boats over the years and i would never be able to get to those places in such a hull. I always go for two nights at a time, so i have to be able to sleep in the skiff, most often exposed to the wind and seas. Typical V hulled boats will sync with the swells and wind chop and rock all night. The good skiff simply doesn't rock side to side so i'm able to sleep well even in significant wind and sea conditions.

I do run on the chine if i need to. I use 4 12 gal. tanks and one 6 gal. tank that nest as far forward as they can be. I use portable tanks so i can redistribute the weight as the trip progresses. In some sea conditions i will give the skiff a bias by moving an empty tank to one side. The tiller allows me to really exploit that weight bias if i choose. Running on the chine is a great option, works well in the right situation.

Brad

[JimD]

Brad, since i really haven't any advice to offer you I've decided to take yours and have another one. What sort of photography do you do? Hope that's a fair question. One of my hobbies is painting in oils - I'm hopeless with a camera...

Jim

[Bradley]

Here is a link to a gallery on the web

http://www.scisland.org/Photo-Gallery.dir/Photo-Gallery.php?page=99&path=/Photo-Gallery.dir/Wildlife.dir/New-Underwater-Photos-by-Brad!.dir

That site is run by the US Navy. They provide the schedule for the training at one of our local islands here in So Cal. The site gives us that closure schedule. Since the big hoopti-do at Viacas(sp) more of the training has to be done there.

This site is a huge asset in planning trips, and those boys are dern serious about their training. Last year i was swimming on a submerged mountain top a hundred miles off the coast called the Cortes Bank. I came up for a breath and there was a Navy hilo hovering directly above, guy in the open door signaling for me to LEAVE! Silly me, i'm just looking for a little solitude and i gets kicked off the bank!

I respect the fact that they have important work to do, so i got back in the skiff, stowed the gear and went back to the island.

I don't photograph for money, so it was a privlege to allow them to use the pics for their site. I've spent many a night at San Clemente Island, so it is only right that i give them something in return.

Hope you like the pictures. There is a lot of beauty out there, i just want to share it...

Brad