Old design, modern power?

[graywolf]

Well, some interest, thanks, guys.

Only thing is I did a bit of web searching and found stuff that was not up the last time I looked. Apparently that tunnel hull thing was Atkins take off. Photos of old Seabright Skiffs show basically a round bottom rowing skiff with a box keel. When they motorized them they put in a horizontal shaft drive with the propeller sticking out of the end of the box skeg.

Here is an inside view:

Photo of an old boat: http://www.stripersonline.com/content/type/61/id/1634384/width/1000/height/1000
Sorry, the photo showed in edit window but not in post. Note how the prop just sticks out the back.

A rear view drawing of Rescue Minor from the Atkins site:

Note how the prop is halfway out of the water.

 

[graywolf]

I was actually thinking of this Atkins design: Atkin & Co. - Little Water http://www.atkinboatplans.com/Utilities/LittleWater.html

Says it was designed for a Graymarine Six-226 which was rated at 93hp and weighed 600#. There are a lot of 600-pound motors these days that easily produce 200-250hp.

Another thing that interests me is that there has been a lot of improvements in tunnel drives the last few years. Surface piercing props, controllable pitch props, vented tunnels, etc.

 

[Mr Efficiency]

No mention was made of the speed, you'd have to think 15-16 knots would be a good maximum cruise for that. There is just too dramatic a change in shape aft to be getting much more. I could see that Little Water being well suited to a diesel, and lends itself well to be made from GRP sandwich on a male mould, with the complex shape of it. I don't like the plumb, flat transom. Looks horrible !

 

[Mr Efficiency]

Bow rise not too much here:

 

[Mr Efficiency]

Somewhat similar lines of the two boats, "Everhope", and the larger "Little Water". Both would be a bit of a build nightmare in the original conception, I'd say. I'd like an expert's view of which had the most speed potential.

 

[Rurudyne]

I was actually thinking of this Atkins design: Atkin & Co. - Little Water http://www.atkinboatplans.com/Utilities/LittleWater.html

Says it was designed for a Graymarine Six-226 which was rated at 93hp and weighed 600#. There are a lot of 600-pound motors these days that easily produce 200-250hp.

Another thing that interests me is that there has been a lot of improvements in tunnel drives the last few years. Surface piercing props, controllable pitch props, vented tunnels, etc.

With a modern 93 HP engine that frees up a lot more displacement for beer ... er, "cargo".

 

[PAR]

The majority of the hull volume aft is supported by the box section of the keel and the slightly immersed chines, which do pick up a modest amount of additional volume, as the boats squats underway. The hook has to be designed for a specific speed range, which is it achilles heel. Below these speed ranges, it does very well, presenting a clean flow to the prop, providing a prop shaft angle at more prefered angles and having a clean release from the chines. The result is the boat can motor above normal displacement speeds easily, but only up to a point. At higher speeds the hook can't offer enough upward lift to keep the stern from squatting more than desirable, the shaft angle goes up, the chines dig in more and the more you push it, the worse this gets, until you're dragging so much wake, with a bow up trim, she's locked in. At sea, the prop can uncover, but this is less a problem than you might think, unless in a really steep sea.

This is a more common example of the box keel approuch and essentially a skiff like hull with a deep forefoot, box keel aft and conventional bearing area aft too. This does better with more power, but the buttock angles limit how fast she can go.

This is much closer to the original SeaBrights and you can see the box does a lot more work. Billy started the development and John continued, when he joined the firm. John discovered he need to reduce wetted area, so the bottom of the box was made much narrower, like the first image shown.

This is the one I own and the last commission John took for a yacht. It's obviously much larger and double ended, but the principles still apply. You'll notice the development in the aft section on mine, with a longer and flatter hook, because she's designed to operate at higher speeds and even the chine has a subtle hook.

BTW, all of these designs shown are carvel, not lapstrake.

 

[Mr Efficiency]

One might assume the later designs were the most evolved, perhaps one should look for a time stamp somewhere ! I just think they seem attractive for diesel propulsion, and a speed band that most boats do not handle well. I guess your double ender might represent the final summation of what they learned from all those different models, but it is certainly not as radical as some of the earlier, smaller versions.

 

[graywolf]

Tunnel drives are strange. Let's consider a common one, the Penn Yans.

If you browse the web you will read folks saying they throw a massive stern wave and only can do about 15-mph, and you see other owners saying that does not happen with their boat and they have no problem doing 30-mph.

The government used a lot of those Penn Yan tunnel drive boats. If they did not work well, why would they have kept buying them? I have been reading a lot about them (Hey, you can pick up a 21-footer for $1500-2000). It seems there are two things you have to look out for. A dirty rough tunnel will kill the performance. The wrong prop will kill the performance. I can very easily imagine a previous owner needing to replace the prop and buying the cheapest thing they could find, the 4-blade one it came with was probably expensive.

I do not know about those Atkin designs because everything I have read seems to be by folks who are enamored by the low power efficiency. On the other hand, Jersey Speed Skiffs are fast as hell. They do not have a tunnel drive, however.

 

[PAR]

A box keel and a tunnel hull are entirely different animals and not a far comparison. Atkins does provide dates of the their designs and it was an evolution of the type.

The Jersey Speed Skiff is fairly fast, but very unstable at high speed and does have longitudinal instability issues once pushed hard. Just watch any race and you'll see. These are typically lapstrake, but nothing like the Sea Bright Skiff, as you can see below.

 

[graywolf]

Yes, I am aware the speed skiffs are an entirely different design, actually flat bottomed with a rounded lapstrake topsides.

Let's drop the Atkin's designs for the time being and just look at what needs to be changed in a design to handle a much more powerful engine. That was actually my main question, not the particular boat, sorry if I misled people. But my questions about engine stringers, hull planking, etc. should have made that clear.

In other words, my inquiry was intended to be far more general than it seems to have been taken.

 

[Mr Efficiency]

Atkins does provide dates of the their designs and it was an evolution of the type.

Where to access that information ?

 

[PAR]

Generalities are usually not as practical as you might think. Most of it depends on "targets" and without a specifics to aim at, a fruitless effort. I think most of the problems in this thread are attempts to focus on widely different hull forms, inverted V aft sections, box keels, Jersey Sea Skiffs, etc. Your original questions were about the Sea Brights. Which would you like to focus on now?

Many of the drawings have dates, though you can also call Pat (John's widow).

 

[Mr Efficiency]

Graywolf may have lost interest in "little water" after hearing it is a semi-displacement boat suited to lowish power and quite speed limited. He'd probably have lost interest anyway, when he realised building it is not a small undertaking. It should be noted these boat's plans all require lofting, which is somewhat of an inconvenience, to say the least ! Still, that boat or similar does seem to have potential for a trailable craft with diesel propulsion that would be very economical to run, if you can bear to have people scooting past you in faster planing hulls. Mid teens mph is very respectable progress unless you have long distances to travel. The material that would work in that boat today is GRP, imo, which would ideally be female moulded, but that is a massive undertaking. It could be done with one-off GRP sandwich, by someone with a little experience, but in the absence of the right kind of plans, not that easily. I should add the tumblehome would be an issue with moulding, and might need to be gotten rid of. A boat with similar underwater shape, but above WL changes, could be designed reasonably readily.

 

[graywolf]

Which would you like to focus on now?

Well, it seems that the Atkin's tunnel hulls fascinate folk on BD, and I can see why, a few of them fascinate me. I now think the Design forum was maybe the wrong place for my question, as I was more interested in what needs to be done when repowering older boat designs with more modern engines. The answer may be right there in what Lymon & Chris Craft did when going from the old flat head sixes to the newer ohv V8's around 1960. I was thinking the change was just marketing, but maybe not.

And some of the problem is that I have been fixated on lapstrake sea skiffs since I was a boy. Back in the 1960's I read a lot about boat design, even designed a few, but none of them got built. Back then I was more interested in sail because I thought it was cheaper. Ha! Power you pay a little bit at a time, with sail you have to pay a lot at fairly long intervals, but the cost per mile kind of works out to be about the same overall.

If I was to actually build a boat it would be a 20'-24' open sea skiff with a 350 block inboard using glued lapstrake and laminated rib construction. I also guess I would mostly just copy those older boats using the modern construction methods to make it stronger and more durable rather than lighter. I kind of suspect it would cost about the same as restoring an older one would.

Is that about right?