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

There are more photos of my Rescue Minor on the AtkinBoats Yahoo group. The hull is as close to the plans as I could get in aluminum, but the sheer is about 3" higher in the bow, and the transom has more curve ; overall length is 20' 1/2". The interior is substantially different from the plans.



Mrs. Atkin (the daughter-in-law of the designer) declined to sell me the plans, as she disapproved of my intent to build a rough workboat finish, so I found Motor Boating's Ideal Series #41 "The Plywood Boatbuilder" http://www.boat-links.com/Ideal/ on ebay, and built from that.

I still intend to use inboard diesel power, but have nagging questions about the details - the offset prop shaft, performance with various prop combinations, etc. and figured the outboard is a quick and dirty way to try things out at minimal cost (and to get it in the water ASAP). But there are likely to be three (at least!) problems with this: introduction of exhaust air into the tunnel, sealing the tunnel roof, and getting cooling water into the engine at idle and in reverse.

Sal's Dad

(by the way - would this be considered an inboard, or outboard, by the government types?)

 

Why not have the input shaft from an outboard power head cut down and re splined thus eliminating the leg and positioning the engine at or just above deck level? Atkins specified a 91 cubic inch motor @340 lbs with 10" dia.x 12" pitch prop @ 2000 rpm. Im not sure how much hosepower an old inboard "91" made , but im sure there is similar output power head. A custom made flanged adaptor could be fabricated. Is the prop turning at 2000 rpm critical? That would have to be determined and suitable reduction installed. By the way does anyone know where to see the Tolman sea bright skiff?

 

To answer my own question there are pictures of the Tolman s.b.s. at fishyfish.com . The cutout tells me theyre hanging an outboard off the transom, score another one for Renn...

 

I see the Tolman "Alaskan" skiff on fishyfish, but not a "seabright" version -- what am I missing?

As to the cut down outboard: Maybe. But at that point why not go with a diesel inboard? One of the big advantages of the outboard setup is that I have one, about the right size, with a variety of props; just need to lift it off my Bollger Diablo and caulk up the anti-ventilation plate. Or at least that's what I keep telling myself.

I think I asked, facetiously, whether an inboard-mounted outboard (NOT in a well) would be counted by the USCG and other government types as an inboard. But it really could become a problem, meeting "code" on electrical, fuel, and exhaust installations.

Sal's Dad

 

www.fishyfish.com/renn_tolman/index.html

 

It will be interesting to see how this design performs. With an outboard aft of the tunnel, the prop will likely need to be fully immersed for it to work well. Atkin's tunnels are designed to have the prop shaft at about the static water level. This is quite different from Renn's design. I'm not seeing just how this boat is going to work other than protecting the prop with the keel. Will just have to wait and see.

 

I also will be very interested in how the Tolman tunnel hull will perform. Not only will the prop be outside of the tunnel, ( at least it looks that way ) the surface that forms the tunnel seems different than the lines on the RM. It could be just the way the picture was taken. Here is a sketch of the RM bottom fairly true to the RM lines.

Gary

 

THat's not quite how mine looks!

 

Sorry this model was faired and rounded but the lines are still close. What I’m looking at is where the tunnel surface and the box meet. I have been following your progress also.

Gary

 

I have been drawing these to long I went back to the original and see that I am the one who is deviating. LOL!!!
Gary

 

Removing the prop from its designed location will surely only have negative effects
The concave (or inverted v) sections are somewhat similar to a prop tunnel in that they reduce the amount of lift available on the afterboady. This is normally offset by the thrust from the prop on the aft end of the tunnel which features that rather drastic downturn. Without it the boat is reduced to being similar to a semi-displacement hull - with buttocks that sweep up towards the transom - and a massive trim tab on the back in some sort of attempt to offset the suction ( Tom)
The obvious question here, is why bother with the complexity of the SB shape?

 

In a word THE RIDE,at speed in rough water!

From my research it would seem that the SB hulls are capable of creating a far smoother ride (at the lower SL speeds of 2-4) than any other aft section.

On a launch their ability to press on in an offshore seaway , with out beating the stuffing out of the operators seems to be a difference between these and other hull shapes.

Of course the folks that need more than 20K will go for the deep V and $7000 each seats to keep their spines intact.

For us folks that cant afford 1/4 nmpg and don't need 60-100K for cruising, the ride , and the other attributes , such as beachability would make the hull shape worth the extra time to build.

Weather the keel should float 25% of the boats weight or as much as 75% is a good question, here Efficiency" should include a very small wake , so that 18-20K can be had even in built up areas.

The other question is if the box keel can be more efficient with a more "modern & hydrodynamic" rendition.

The huge load carrying ability would suit a light weight cruiser, as the difference between light ship for coastal cruising , and loaded with full fuel and water for a couple of months in a more remote area , is a good percentage increase of the vessels displacement.

Im not sure any of the above would apply to a boat with an outboard, mounted aft..

FF

 

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

 

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 .

Oh yes Will, I caught the reference.

 

Point taken. But for questions like "How will the Tolman outboard perform" I have my doubts, but so little is known about this shape, we will just have to wait and see.

As to rough water ride, Noble Cab reportedly has a problem with ventilation in a cross sea - air gets under the chine, into the tunnel.

It's just hard for me to understand going very far from the Atkin designs, without knowing in detail how the originals perform.

Sal's Dad