Help With Economical Semi-Planing Designs

[ALowell]

For some reason I've never quite believed that Seabrights are neccessarily better than other designs. I'm sure they're just as good or better than many boats at low speed efficiency, but there are other ways of accomplishing the same thing with less complexity if one isn't looking for shoal draft.

I should probably take a harder look at what Gerr has to say before I say something regretable....

~ ALowell

[kengrome]

For some reason I've never quite believed that Seabrights are neccessarily better than other designs.

I suppose this depends upon what you mean by "better" ...

I'm sure they're just as good or better than many boats at low speed efficiency, but there are other ways of accomplishing the same thing with less complexity if one isn't looking for shoal draft.

They aren't that complicated to tell you the truth. I've designed a dozen or more of them, and if you get away from the split scarph joint in the middle of the boats like Rescue Minor and Tolman Seabright the forward portions of the tunnel panels can transition into integrated spray rails and lifting strakes, then the entire panels will have very smooth and easy bends.

[kengrome]

I should probably take a harder look at what Gerr has to say before I say something regretable ...

"Perhaps the finest thing about Seabright Skiffs is that they are really good at everything. This claim is made for many hulls, but unfortunately is seldom so.

"Seabright Skiffs, however, are easily driven at low, moderate, and even at semi-planing and low-end planing speeds. They carry large loads very well.

"The Seabright Skiffs' shoal draft and beachability make them ideal gunkholers, yet their stability, buoyancy, and easy motion make them excellent rough weather boats."

-- a quote by Dave Gerr in "The Nature of Boats"

[Excalibur]

Originally Posted by ALowell

Over the years of reading about and observing boats I have come to a few conclusions about semi-planing boats, one of which is:

A hull that has a hull-speed type run aft (up-swept) will squat when pushed harder than hull-speed as everyone knows. However, a boat with the same overall shape but significantly narrower beam will squat less (though still more than might be optimal) and go reasonably well above hull-speed, up to about 18 kts when performance deteriorates.

Is the relationship between narrow beam and the ability to exceed hull-speed something that has been written about?

~ ALowell

Not certain that you are asking the right question there. There is some data that says that while wave propagation speed is a constant, "hull speed" is not. The much used waterline-length-square-root-times 1.34 seems not to apply to very narrow hulls like catamaran hulls. 1.5 or even 2.0 as a multiplier seems to apply to them. OTOH, maybe they are just better at overrunning their bow wave.

While "hull speed" may or may not be a constant, it is certain that narrow boats with lower displacement need less power to drive them than fat heavy boats. You just can't get something for nothing. And hulls that are designed to make smaller stern and bow waves will need less power than those that are not.

Boats with rocker aft tend to allow the water to follow the hull at displacement speeds and as a result will not develop a vortex at the stern. But as speed grows, that same flow turns into a large stern wave that sucks a lot of energy. And where there is a crest, there must be a trough. That trough forms right under the stern, and the hull falls into it. Thats squat. A narrow full displacement hull will make a smaller crest, and so the trough will be smaller as well. Hulls like my semi displacement Chris have a lot less rocker, and the water separates from the hull near the top of the displacement speed range. While this does allow an energy sucking vortex to form, it also delays formation of that big wake. Thus, my hull is better at speeds just over displacement. You pays your money and you takes your chances.

Those are my opinions in any case. If anyone with real degrees and/or experience would like to correct me, then I'm always willing to learn!

[FAST FRED]

The Atkin seems to be able to "fill in" the aft hole at up to SL 2.8 or 3 .

So they seem to be super efficent at that speed range and slower.

Faster and its pure plaining (less the L/B ratio is 12 -1 like in a cat).

Hard to live or cruise in a 12-1 boat , but 5 or 6-1 with a SL of 3 would seem like a nice economical cruiser. Esp if it fits on a trailer to extend the range at low cost.

FF

[Excalibur]

Atkins designed a lot of boats, and most of them were not Seabrights. The largest Seabright currently offered on the Atkins site is the 28' Naiad. Has anyone seen any plans for one larger, say about 46'?

[kengrome]

Atkins designed a lot of boats, and most of them were not Seabrights. The largest Seabright currently offered on the Atkins site is the 28' Naiad. Has anyone seen any plans for one larger, say about 46'?

I don't think Atkin developed anything that large. Dave Gerr has Summer Kyle, a 42-foot tunnel-drive motorcruiser, beachable on unique underbody with just 23 in. draft:




... and Peregrine, a 47-foot tunnel-drive motorcruiser, beachable on unique underbody with just 28 in. draft:




... and Kamarea, a 50-foot tunnel-drive motorcruiser, beachable on unique underbody with just 26 in. draft:




More details on these boats here:

http://www.gerrmarine.com/power_50.html

However, none of these boats reach the same speeds of the Atkin boats, probably because of their weights coombined with their relatively small engines, and possibly because of their hull shapes which may not be optimized for 15-20 mph like the Atkin boats.

I suspect the reason Dave Gerr used the tunnel-sterns on these cruisers is more for their shallow draft, upright beachability and protected propellers and rudders than anything else.

Note that these are Dave's "tunnel-stern" Seabright designs. He also uses the non-tunnel Seabright hullform on some of his other boats on the page linked above.

[eponodyne]

I just had the thought that perhaps the Sea Bright sailing craft was never fully developed when internal-combustion engines were adopted as a motive force. Were they a fully mature technology when sails were abandoned?

[ALowell]

Originally Posted by kengrome

"Perhaps the finest thing about Seabright Skiffs is that they are really good at everything. This claim is made for many hulls, but unfortunately is seldom so.

"Seabright Skiffs, however, are easily driven at low, moderate, and even at semi-planing and low-end planing speeds. They carry large loads very well.

"The Seabright Skiffs' shoal draft and beachability make them ideal gunkholers, yet their stability, buoyancy, and easy motion make them excellent rough weather boats."

-- a quote by Dave Gerr in "The Nature of Boats"

The reason I question the utility of seabrights isn't because all of the above isn't true but rather because all of the above (with the exception of draft) can be said of my present boat, an 18' lobster hull. My Eastern 18 doesn't have any tunnels, spray rails, lift strakes, etc. and yet it performs at all speeds from 10 to 21 mph with no discernable decrease in average efficiency (no "hump" speed). The hull shape of the Eastern and other similar hard-chine versions is inherently easier to build than the tunneled seabrights, whatever people say to the contrary.

~ ALowell

[kengrome]

ALowell, I don't see how you can question the utility of the tunnel-stern Seabright design when it offers more features than your current boat -- the first of which is shallow draft -- a feature that perhaps others find more useful or appealing than you do.

Another very practical feature is the boat's ability to be beached almost anywhere there is a beach.

Another feature is its ability to sit upright on its flat bottom box keel when on the beach or on a tidal flat, or even on a cheap flatbed trailer.

Another is its excellent fuel efficiency which is very hard to match in almost all other planing power boats because of the way the tunnel-stern Seabright design re-uses the energy trapped in the accelerated boundary layer water. There are other threads that explore this unique phenomenon if you are interested.

Your lobster boat may be a great boat, and it may actually be a better boat for you than a tunnel-stern Seabright skiffs. But shallow draft is very important to many people, and so is fuel efficiency. For these two reasons alone I think many people will prefer the tunnel-stern Seabrights.

Unlike you, I never worry about how difficult the boat might be to build either. Instead I prefer to focus on the features I want from a boat, then do whatever it takes to build that boat, even if it requires more effort or takes more time during construction. Then I will end up with the boat I really want, rather than something that only comes close ...

[FAST FRED]

"The hull shape of the Eastern and other similar hard-chine versions is inherently easier to build than the tunneled seabrights, whatever people say to the contrary."

Most boats are built on a simple mold on a strong back, hardly any extra effort to go with the SB,amd NO extra cost.

The lower engine hight and ability to run aground AT SPEED! with no problems are worth far more than an extra afternoon building the mold.

FF

[ALowell]

I guess I shouldn't be arguing with 'true believers". What I meant to say isn't that there is anything wrong with Seabrights, just that it isn't a free lunch. Having a tunnel stern reduces planing efficiency -- ask Dave Gerr if you don't believe me. This is a trade off that one has to consider against the benefits -- beachability, shallow draft, etc.

~ ALowell

[longliner45]

jc boat,,,longliner

[FAST FRED]

The largest "Sea Bright " (reverse deadrise) hull design I have seen from Atkin was about 150 to 200ft for the military during WWII.

Never got built , the war ended , and it wasn't "our" idea from the USN.

But big ones do exist. A River Belle is about 35 ft , but the low freeboard and huge glass area condem her to be a river runner (as designed).

FF