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gcarter
06-06-2006, 07:50 PM
I have a book called "Naval Architecture of Planing Hulls" by Lindsey Lord, D.Sc in N.A. Dr Lord was the GODFATHER of deep vee hulls. Since the '30's, Dr. Lord was instrumental in the development of P.T. boats, patrol boats, and fast freight boats. He first published this book in 1946 and the edition I have was published in 1962. The end flap says I paid $6.00 for it.
All through the book he preaches the doctrine of a hull form he called "monohedron" which had a constant section bottom of about 20* deadrise. You would recognise this hull, although C. Raymond Hunt introduced lifting strakes, round keels, and an even deeper VEE in the late '50's.
The point of this exercise was to eliminate what Dr. Lord called "Warped Bottom Hulls". If your wondering what that is, just look at any wooden runabout, most ski boats, and modern sportfisherman, motoryacht hulls designed to run at about 20-30 MPH. A warped bottom hull will plane easier with less HP but has a very distinct upper handling limit which most of these boats are incapable of reaching. In fact he says the "Warp" of the hull generates a substantial vacuum under the rear part of the hull which pulls the transom downwards, raises the bow and carries along a substantial amount of water, slowing the boats forward progress. As far as forward progress part of it goes, just lower your tabs and the same thing happens, the boat slows down.
I know there have been a few boats with hooks designed into them. Matty has shown us several examples. When they're designed into the hull, the fact seems to be obvious. They run parallel to the transom and, infact, look like partially extended tabs. They obviously play an important part in the handling of these boats.
Now look at the pictures of the hooks that, at one time, resided in my Minx;
http://www.donzi.net/forums/showthread.php?t=43778&highlight=hook
They don't look the same. They look like long, thin inverted bowls. They also encompass the lifting strakes. I also found that the lifting strake on the port side not only was warped upward, but also inward, i.e. warped in two planes simultaniously. Hooks like these are products of, maybe bad bunking, but more than likely, are the result of the molding process. I know that every Minx I know of personally has them, also many of the 22's of the same era.
What's the down side of them being there? They generate a vaccuum and carry along some of the water you're running in, thereby slowing the boat that has them.
Since I re-jelled the bottom and removed the hooks in the Minx, I've only had three opportunities to run the boat. The weekend of the Mt. Dora event, I would just as soon forget. Then Elaine and I went out in it two weeks ago, But finally last Friday, Phil Reed came by and we loaded up the GPS and did some testing.
My boat has a 1.5:1 Alpha drive with a 15 X 22 Hydro Q4 prop. The power is a Chevy small block w/ Vortec heads and SM manifolds. But we're not talking here about WOT runs because I've been less than satisfied with my carb package.
So what we did was to run at 50 MPH and check the RPMs against that.
Before the bottom job, it required 4000 RPM to maintain 50 MPH, or about 11% slip (check me on this). Last Friday, we were able to maintain 50 MPH @ 3800 RPM, that's about 5% slip, i.e., there was less drag in the hull and the boat operated more efficiently.
I know there's all kinds of factors beyond our control, for instance on Friday it required 50 more RPM to maintain 50 MPH into the light breeze than down wind. Also my hull before the re-do was really nasty, so a small part of the difference can be attributed to the bottom condition.
I'm really pleased with the results. I'm actually pleased with the way the hull looks now. Before I really didn't want anyone to see the boat on the trailer.
Also, the cruise speed has improved, making a more enjoyable boat.

gcarter
06-06-2006, 07:57 PM
I forgot to mention the handling of the Minx has improved. It's dead steady. I trimmed it all the way up on a WOT run and the trim indicator was almost to the top before the prop blew out. It was a significant improvement. Phil called the handling completely benign.

rustnrot
06-06-2006, 08:04 PM
George, wasn't Lindsey Lord the one who said there is so much vacuum near the transom of a warped hull you can drill a hole in the hull and no water comes in?

Not true, I have also read, the water WILL come in.

Ed Donnelly
06-06-2006, 08:10 PM
George; I know you just hate taking holidays because you just have to tinker with a boat. Take 2 weeks this year, come up to Toronto, and I will let you do the bottom of the Criterion. Its the least I can do for a fellow Registry member.:boggled: ...Ed

gcarter
06-06-2006, 08:27 PM
George, wasn't Lindsey Lord the one who said there is so much vacuum near the transom of a warped hull you can drill a hole in the hull and no water comes in?
Not true, I have also read, the water WILL come in.
It is true.
I previously owned a Pacemaker FBSF and installed a speed/temp pickup that incorporated a through hull fitting. It was located just off the C/L in front of the props. I pulled the sensor several times while under way. No water came in.
If you read his book, you'll find he built a large number of towing models that were highly instrumented including pressure transducers in the bottom. The guy knew what he was talking about.

Morgan's Cloud
06-07-2006, 07:08 AM
Thanks for that info George.
I am very glad that this worked out well. After that workout it would have been a bit disappointing if nothing good had happened huh ?
As I mentioned a while back, I also had two hook like deformations forming in my bottom over the last few seasons before I undertook it's restoration and I definitiely felt as though I was loosing trim and trim response. I look forward to seeing what the improvements will be one day , eventually ...
I have a strong feeling that there are many boats (even here on the board) that could benefit from what you have done.
Once you're back up to full RPMs you must give us a wfo report :biggrin:
Steve

rustnrot
06-08-2006, 10:24 AM
George, nothing like real life testing! Would make a good Mythbusters segment, methinks.

gcarter
06-08-2006, 03:33 PM
I guess one of the important things to consider is removing the hook has a substantial impact on the cruise speed where we spend most of oour time running. Not just WOT.