Box Keel

I think the interesting thing about the RM type hull is the potential to use the prop stream to exaggerate/simulate the stern transverse wave system, and recover that energy with the aft hull, so I would agree towing couldn't model it well....

Perhaps a hull truncated at the prop, with a really big trim tab, could broaden the range of such a system. Or as was already said, just take out the hook and add off the shelf tabs. It may be too much like chasing a conventional OB/trim tab solution, but I think there is potential in a more integrated system....

Which brings another question, anyone know how the tolman skiff with seabright bottom performs? I have seen only one picture on the web, and the transom appears to be cut for OB. Very curious how (if?) it will work with OB trim.....

 

AT 12 kts, the next crest is (12/1.34)² ft behind your bow = 80 ft. I do not see how any boat smaller than 40 ft can regain any energy from the wave system. But I may be wrong.

 

MOT, FF;
Building models for experimental purposes, is one of my numerous character flaws. I have an attic full of them. Some of them actually worked as hoped. Some of them did not. In either case they were/are fun.

FF I suggest that you build a model to a much larger scale than 1" to the foot. Here's why. Say that your full sized boat is to be 25 ft. LOA. The all up weight will be 3000 pounds. (These are only arbitrary numbers for illustration) Then your 1' scale model will be constrained to a maximum weight of 1.74 pounds. That is hard to do. What with servos, motor, reciever, and batteries the boat will be overweight and similitude will be lost. With a 1/8 scale model the prospects are better. The 3000 pound simulation can weigh 5.86 pounds. That is more realistic for the working model. 2" scale is even better. and model weight can be 13.8 pounds which gives you some latitude for shifting weight around in the model boat to simulate the real thing. What is more, the larger model is far easier to build to acceptable scale tolerances. Also, knarled fingers work more easily with larger parts. By the time you get to a model of 50" length, you can use door skins or 3 mm Okoume for many of the parts including planking. The larger the model the more nearly it can deal with the wrinkled water on the pond.

As a matter of fact I wish this whole damned subject had not attracted my attention. Now I will be forced to think seriously about building an RM model.

 

A 1/4 or 1/3 scale model can make a nifty bookshelf, too! Just cut every other station out of a nice 1/2" ply. But it is hard to get those underbody curves, even with wetted luaun doorskins.

(see photos of rougher models in the Yahoo AtkinBoats group files).

 

2 in to the ft would be even easier to use to see if the hoped for interior and design features would fit into.

With such an unusual hull shape would the std computer performance stuff work?
I'm now just using a simple nomograph found in Lord's book , and a modern similar one from Pro Boat Builder to estimate weight vs speed .

Although I plan on building a full sized junk mockup ( cardboard and damaged ply) to work on the first 12 ft of hull.

With a very narrow beam forward and my personal requirement to NOT have to climb out of bed over the pillow (prefer narrow side isle on one side) the drawings sez it will fit , but....

"You don't BUY beer , its only rented"

FF

 

FF;
The Propeller Handbook by Dave Gerr has some good stuff about weight, power, speed. You'll want to select a prop sooner or later. Very good stuff about that too. Get the book if you do not already have it.

Sorry Brands01, we seemed to have gone riding off in several directions. We did not intentionally hijack your thread.

An old 1945 edition of "Boats you can build" has an Atkin SB that is both powered and rigged with a sizable sail. The text claims that it sails well including windward work. Sure enough the keel turns down a bit aft of the prop. In this iteration of the SB the bottom of the transom is rounded up toward the chines. Presumably in deference to heel under sail.

 

I'm wondering if a hair brained idea might help speed.

As the box keel could as easily be built of metal, the surface could work as area for a keel cooling setup.

IF the cooling water was run thru piping fixed to conduct very well would the "waste heat" warm the water in contact with the appendage enough to lower the waters viscosity?

I'm looking for "free" speed from the engine waste heat?

FF

 

Hi Messabout - no worries about hijacking the thread - Its far more interesting now than when it began!

 

Yes, that's the plan! I wouldn't expect a very modest change in temperature to have much effect on efficiency, though.

 

Yes, that's the plan! I wouldn't expect a very modest change in temperature to have much effect on efficiency, though.

The big boys (PT Boats)found a prop that is fine in 70 deg water may need 2 inches of diameter taken off for freezing water ops.

Of course 40deg F is a huge difference , but an engine running 3gph will still be able to dump 100,000/hr btu into the keel surface which might allow a speed improvement?

FF

 

The folks at Shannon ,,

http://www.shannonyachts.com/38faq.html

have a hull structure that mimics the Atkins to a large extent although the keel is better faired , and the area aft of the prop does not perform lifting the stern as much.

Same claims tho for excellent sea keeping , and low fuel burn at speed.

FF

 

fcfc wrote:
"AT 12 kts, the next crest is (12/1.34)² ft behind your bow = 80 ft. I do not see how any boat smaller than 40 ft can regain any energy from the wave system. But I may be wrong."

Well, my simple thinking is:
The crest of the following transverse wave is there because a volume of water has been rising since it left the transom (or before...).
Propelling the boat gives the water the energy to form the wake. If part of the wake is a wave, each part of the wave is as much a part of the wave as the rest. I can't see how this would exclude water under the boat, although it may take longer equations to describe.
Although I am unschooled, maybe have the wrong idea.... I am a naif in hydrodynamics.

This is a neat design because it challenges rules of thumb, so I have not tried to think of rules of thumb in understanding it. I see no free lunch in the designs, just unusually sophisticated optimization for a small boat.

My trim tab idea is quite crude, just an idea for cheap variable geometry, perhaps allowing a boat to retain the shoal performance and some other qualities of Atkins skiffs, but act like a typical planing form at higher speeds, at some cost in efficiency, obvious questions with LCG, etc. Probably PARs idea is better -simpler and off the shelf, recyclable.
My apologies if the trim tab idea seems an affront to the charm of the Atkins hull shape, which appeals because it is just that, and needs no fancy hydraulics or materials.

 

After looking briefly at the page for the "SRD 38"
I will venture to add,
vortices ain't all bad.

 

On most plaining boats the trim tabs lever the hull Down to a more efficient running angle.

With the Sea Bright design the tabs could work very differently. The aft end of the boat curves down , forcing the back to lift at speed. The Motor Boating Article described a problem with too much lift from doubling the engine power.

Atkin's cure was a simple set of wide chines forward that lifted the bow to stock running trim.

It is possible that if the aft curve WAS extended trim tabs , going faster would only require there retraction , to lessen the aft lift , and regain efficient trim.

It might be possible to use the trim tabs (getting thinner ) to lower the bow to the cheapest/fastest running angle at any speed.

FF

 

Sounds like an excellent idea Fred. However, the worst things about the bow down attitude of too much power on the Atkin hull are handling problems and safety like bow steering and foundering in turns or waves at speed. Wetness is a nuisance and we should remember that these boats were designed to give best performance at low planing speed and are not ideal for high power and high speed no matter what modifications are made.

Another thought is that adding lifting strakes on the bow also requires some energy to counteract the aft lifting force of the tunnel downturn. Both of these reduce the efficiency of the boat below that of a boat designed for the higher speed in the first place. Your tab mod might give better overall satisfaction throughout the full speed range though since it could be trimmed for each speed.