How to design an outboard on an Atkin Seabright Skiff???

Discussion in 'Boat Design' started by SalmonMan, Mar 30, 2007.

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kengrome Senior Member

It's very funny and coincidental that I've been beating up on these same questions, trying to come up with an idea that will result in high mileage, reasonable speed, very shallow draft, and good load capacity boat design ...for Interior Alaska's rivers.
Click to expand...

I think it is also coincidental that Renn Tolman lives in Alaska and has designed his new Tolman Seabright Skiff for precisely the purpose you've described here! This is what he wrote about his new boat design in his building plans:

There's a lot of thin water in Alaska -- tide flats and rivers, and a skiff that draws only 6-7 inches has tremendous attraction for a hunter and fisherman like me. And as a lot of boaters know, there's a lot of other places on earth with shallow water as well. So maybe the Seabright skiff's time has come -- again.
Click to expand...

Kenneth Grome

kengrome, May 7, 2007

#46

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sal's Dad Atkin/Bolger fan

Some photos... I'll be off-line for most of the summer, though

Sal's Dad

sal's Dad, Jul 9, 2007

#47

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artemis Steamboater

Hope you read this before you go offline for the summer.

Read your comments on the Atkins forum. Couple of thoughts:
1. Without people and much dunnage she should float as drawn in the Atkin plans - at the waterline. She obviously doesn't which makes me think that location of the outboard motor needs to be offset with some weight forward of same. The motor Atkin recommended was around 360# as I remember. To float properly to her lines she should weigh about 850# so balllast her according.
2. That will probably take care of the problem you mentioned on that site of the bow digging in and very sensitive handling at speeds over 12 mph.

Ron

artemis, Jul 9, 2007

#48

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sal's Dad Atkin/Bolger fan

First, sorry for the cryptic postings, but there are separate discussions going on here, at the woodenboat forum
( http://www.woodenboatvb.com/vbulletin/upload/showthread.php?t=52985&goto=newpost )
and AtkinBoats group ( http://groups.yahoo.com/group/AtkinBoats/ ). There is a full album of construction photos on the Atkin group.

Yes, clearly she is squatting quite a lot - it is even more pronounced with the bigger motor. I added some weight in bulkheads, tanks, and lockers, but thought it was roughly distributed fore and aft. Shifting weight forward only increases the extent to which the bow digs in at speed.

Bare hull is about 550 lbs, so figure 1000 with motor (150) skipper (200) and gear/lookout.

I'll be working on it . . .

sal's Dad, Jul 10, 2007

#49

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brian eiland Senior Member

Let's face it. One Atkin design has been launched, a handful of Gerrs, and a Carlson. Robb White's light-weight round-bottom variation supposedly has few (if any) negatives, All the rest have some drawbacks.

Until we get them on the water for sea trials, nobody really knows how the original designs (let alone the variations!!) will perform. All this speculation is a bit premature.

Sal's Dad
Click to expand...

tom28571 said: ↑

Well Dad, that is why it is called speculation If it weren't premature, would it be called speculation? At least 75% of what we write here is probably speculation, hopefully based on other than pure guesswork.

I find this whole realm of the box keel very interesting. I wish someone would build some of the versions with high displacement in a narrow canoe body keel and a low deadrise upper hull. That is, with the keel L/B ratio over 10:1 and 75% of the displacement married to a planing hull bottom that supported only 25% of the displacement.

Would these two entities fight for a divorce or compliment each other to give a boat that ran quietly and efficiently up to, say 18 to 20 kts. Up to this speed, the keel would not be constrained by any "hull speed" restrictions and the upper body would have such low value of bottom loading that the trim angle required to generate adequate dynamic lift for planing would be very low.

I'm not looking at this as a rough water boat but as a cruising vessel that could run in this speed range with small power and fuel use. The advantage over planing boats like my own would be the ability to carry all the room and weight that live aboard trawler cruisers like.

I don't have the time or wherewithall to get into it other than speculation .
Click to expand...

So I'm just reading thru this subject thread and noted the date 2007. What has been learned about these hull shapes since then?? Are there bigger examples?

...a sailboater trying to learn about powerboat shapes

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brian eiland, Aug 20, 2013

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brian eiland Senior Member

FAST FRED said: ↑

"I find this whole realm of the box keel very interesting. I wish someone would build some of the versions with high displacement in a narrow canoe body keel and a low deadrise upper hull. That is, with the keel L/B ratio over 10:1 and 75% of the displacement married to a planing hull bottom that supported only 25% of the displacement.

It might be possible to allow the box keel to have the plaining angle of attack , so the hull above would still be fairly level.

The trim might be changed by venting the aft negative deadrise section with an OTS trim tab set.

While a 10-1 L-B boat would be interesting , the Old Camper and Nicholson pre-war 50 ft launches ran 20K on 100HP, so its a proven concept.

Adding the light weight of todays materials and engines should allow the LB ratio to fatten and still be cheap to push.

We are planning on a 38.6 loa with about a 5.5 ft BWL, 7.6BOA (all to fit into a container), so it wont be as fine as 10-1.

FF
Click to expand...

Did you ever get around to this experiment?
Brian

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brian eiland, Aug 20, 2013

#51

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brian eiland Senior Member

kengrome said: ↑

Here's a little one I've been designing in the evenings while building my Tolman Seabright Skiff during the days. It's 12 feet long, 4 feet wide, 24 inches high at the bow, 18 inches high at the transom, 15.5 inches high at the lowest point along the gunwales, and should weigh 250 pounds or less including the engine.

I plan to build it using mostly 1/4 inch plywood (maybe 3/8 inch on the box keel bottom panel) and power it with a cheap 6.5 HP industrial gas engine. The transmission will be a simple v-belt system with idler pulley for forward and neutral, and no reverse except via oars or pole or paddle of course.

My concept here is a "Personal Seabright Skiff" ideally suited for one person to use as a recreational / fishing / hunting / coastal explorer, a boat with great fuel efficiency and enough seakeeping ability that the owner won't feel threatened if he gets caught out in the rough stuff. I also want this boat to be able to sneak into just about any shallow backwater areas a person can find, and to run at top speed in less than a foot of water, and to beach it anywhere there is a beach available -- all without the need for a jet drive and with virtually no risk of damage to the prop or drive train or rudder.

At 6 inches of draft it has 9.5 inches of freeboard and its displacement is 500 pounds, 250 for the boat and 250 for a big person and some fuel and gear. A nice feature of this little boat is that it still has 6.5 inches of freeboard at a total displacement of 980 pounds -- which means it can actually haul 3-4 full-sized adults around safely when the seas are calm enough.

Note that I colored the sides light teal, the garboard/tunnel sections orange, and the flat box keel bottom white. I also used white on some of the pix to show the submerged portion of the boat. The profile shows 500 pounds displacement (typical running load with one person) and the underbody pix show both 250 and 500 pounds displacement to illustrate the waterline difference between an empty boat and "running mode".
Click to expand...

Did you build this??

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brian eiland, Aug 20, 2013

#52

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brian eiland Senior Member

kengrome said: ↑

Hi Dick,

I'm no fan of fiberglass either, and before I built the hull of this Tolman Seabright Skiff:

http://www.bagacayboatworks.com/images/seabright02/

... I was thinking that a fiberglass bottom would make everything a whole lot easier. But once I built this bottom in plywood I realized that it is only the first build that would create problems. The second and third and all future builds will be much easier, especially when a custom jig is used that makes the panel-twisting task easier. Now that I rebuilt my jig specifically to make twisting this panel easier, I don't even think about using a glass bottom.
Click to expand...

Interesting, then a metal hull might not be so difficult,...a frameless type construction with an external jig to hold the plating?
http://www.trawlerforum.com/forums/showpost.php?p=172896&postcount=18

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brian eiland, Aug 20, 2013

#53
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brian eiland Senior Member

...from another subject thread

Ad Hoc said: ↑

mydauphin

It is more than just looking at hull shape and saying...hmm..flat bottom, with a Vee, must be a planning boat...or wow, boat is fast so why is it not called a planning boat.

Any "body" moving through the water, ie at the air-sea interface, will create pressure variations around the 'body'...these pressures variations manifest themselves as waves. These waves are a measure of energy and hence drag.

The shape of a hull can affect this pressure distribution considerably. In a nut shell to cut a long story short, the stern experiences suction pressure fields. Speed then also begins to play a part....ie trim/squat, the faster one goes.

For a "normal" boat, the faster one tries to go, the more trim and the greater the power required for little gain. The back is sucked down and dragging creating a lot of wash/waves. Too much energy is being used in making waves. The reason is the hull shape and its length displacement ratio. This is seen in the resistance curve by humps, and the main prismatic hump. A hull must over come this main hump to go faster, ie make "lesser waves" or better still no waves. Long thin hulls, hydrodynamically, behave differently to short fatter ones.

However, just making the aft section flat with/out a vee, doesn't mean that is all that is required just to go fast, or get over the "hump" in the resistance curve. That is just hull shape.

Making the hull longer and thinner, reduces the 'near vertical' curve in resistance of a 'normal' hull form...ie its length displacement ratio. Not only does the resistance curve become less step, but also the main hump is much less pronounced. The longer and thinner one makes it (that is light for its total length), the curve slowly approaches a smooth curve and almost no discernable pristamtic 'hump'. This is why fast ferries, for example, go fast, have a high froude number, but are not planning. They are long and thin.

The length displacement ratio (ie long and thin) allows the hull to be driven faster than would normally be the situation. The wave making reisatnce gets less and less, the longer and thinner the hull becomes.

The down side is, one of these long thin hulls is generally far too unstable to be used effectively. Solution, put two side by side, ...a catamaran. Utilising the benefits of the long thin hydrancimc effects, but providing a stable platform for use in almost any application safely.
Click to expand...

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brian eiland, Aug 20, 2013

#54

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How to design an outboard on an Atkin Seabright Skiff???

kengrome Senior Member

sal's Dad Atkin/Bolger fan

artemis Steamboater

sal's Dad Atkin/Bolger fan

brian eiland Senior Member

brian eiland Senior Member

brian eiland Senior Member

brian eiland Senior Member

brian eiland Senior Member

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