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Old 07-27-2013, 01:55 AM   #161
Al
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Greetings- In an earlier forum regarding rolling an introduced question regarded ballast and material. As our 27' Marben was a 'Quick' roller that would about cast one off the boat if an unexpected movement was experienced. To the point that the purchase of the boat was under consideration. There were many qualified and interesting suggestions, stabilizers, rolling chocks, steadying sales and of course ballast.
We decided to go with ballast as stabilizers, rolling chocks and such didn't make too much sense for such a small craft. The end result was the installation of lead ingots 2" thick, 6" wide and approxamely 14 " long each weighing approximately 55#. these ingots totaling 825# were dispersed directly over the keel under the floorboards of the bilge area. What a total difference, speaking to the beam sea, where the boat use to 'snap' is now has a very gentile roll and the vessel raises up and down with the wave, not snap rolling. Head seas are now not a bow rearing rise but a soft slow rise reflecting the weight forward of amidships. Following seas have ceased being a wild turn to turn of the wheel.
So for our 27' overall 25' on the water with a 9.75 foot waterline beam, this addition of ballast has been the perfect solution.
Good discussion and there are lessons to be learned on the subject.
A.M.Johnson-Ketchikan (Bridge to Nowhere) Alaska
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Old 07-27-2013, 02:17 AM   #162
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Al - Yes, a good ballast certainly helps.
I have a powdered lead ballast in a sealed bulkhead down low in the center of the bow. Being sealed, it also provides extra safety if the hull is damaged hitting a reef.
I haven't seen powdered lead used in any other boats but it makes sense, as it fills the entire void and can be accessed via a sealed screw type inspection cover. There is no concern over the weight shifting in severe seas.
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Old 07-27-2013, 06:37 AM   #163
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Quote:
Originally Posted by brian eiland View Post
So you agree with me that most any hull can be operated efficiently at displacement speeds. I was NOT asking about water skiing.

As far as the Shamrock hull shape is concerned I was suggesting that this hull shape might be expanded up to the ~40 foot size, and still be a reasonable hull configuration for a displacement operating trawler? Again I was not asking for a fast boat shape that your people are not wanting to wait for.
The Shamrock hull is basically just a slab sided Downeast (lobster boat) design. Nothing fancy or techie about it.

My point is that semi-displacement or the newer thinking semi-planning hulls are designed as a compromise....reasonably efficient at speeds up to near 20 knots and reasonably efficient at slower speeds...but really not efficient at either. If you never plan on going anything over hull speed then design it that way...not with a compromise hull shape.
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Old 07-27-2013, 10:19 AM   #164
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I suspect the knotched chines as shown on post #152 is just to make a wide boat not so wide so it suffers less (efficiency wise) from it's wide beam.

Much can be done to control outwash and spray w similar chine shapes. A curved surface transverse to the flow of a fluid reduces it's velocity a lot.
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Old 07-28-2013, 09:34 AM   #165
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Aku fishing vessel & Defever trawler

Quote:
Originally Posted by Marin View Post
Well designed displacement hulls are easily driven which means you don't need gobs of power which you can't use anyway. Small-ish engines burn less fuel which means the typical single-engine Nordhavn has a great whacking range to it which is what you want if you're going to cross a big chunk of ocean.

And I don't believe there is a rule that says displacement hulls have to have round bottoms and chines. I've posted these shots before, which I did not take, but it shows one of the sampans or aku boats that were built locally in Hawaii in the later 1940s and early 50s for the tuna (aku) fishery there. These relatively narrow boats incorporated flat albeit curved bottom sections, hard chines, and even gunwale " hull bulges" (my term) to provide roll stability in the often very rough waters these boats fished in, like the infamous Molokai Channel. The "hull bulge" is obvious in the second shot and was quite effective as these boats were often rolled to their gunwales in the windy swells and waves around the islands.

But displacement boats they were, most of them powered by a single GMC 6-71. Watching them knife through the swells and waves like destroyers was a beautiful thing indeed. I filmed on a few of these boats in the 1970s and while they certainly rolled around-- hell, even sperm whales roll around in the Molokai Channel-- it was amazingly easy to keep one's balance and footing while on board. I have no idea who designed the hull-- all the aku boats had the same basic lines above and below the waterline-- but whoever it was, he understood the nature of the open ocean the boats were going to work in and how to design a hull to effectively meet the challenge.

The boats were almost exclusively crewed by Japanese Americans and I was told the word "sampan" is the Japanese term for carvel or smooth-sided planking, as opposed to lapstrake.

The bottom line being that a displacement hull does not automatically have to mean super-rolly.
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I found these two hull shapes very interesting, and rather similar in form. The DeFever has the added feature of bilge keels.

Is this Defever hull shape unique to this particular size vessel in their line?
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Old 07-28-2013, 11:00 AM   #166
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Is the boat in the pics really a DeFever?

Isn't it Ben's Eagle?

Whatever it is it's not unusual in any way that I can see other than the bilge keels.

It's a beautiful FD hull w a steep run aft that rises to the WL at the stern. All FD hulls have this feature or the equivalent such as a double end flat bottomed dory.

The hard chines increase stability very slightly and dampen roll very slightly and increase wetted surface very slightly so really play almost no part in the design overall.

The boat Marin posted is significantly unique in that she is very "pointy" in the bow and as a result carries very little of her displacement fwd. If she were a car you could say she had a short wheelbase. Also the stem is straight and extends all the way to her keel that is (I believe) also straight.
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Old 07-28-2013, 11:34 AM   #167
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Quote:
Originally Posted by brian eiland View Post
I found these two hull shapes very interesting, and rather similar in form. The DeFever has the added feature of bilge keels.

Is this Defever hull shape unique to this particular size vessel in their line?
See my answer here: Trawler Forum - View Single Post - Hull Shapes----Show us your girl's bottom

Yes, in some ways the hull form of Island Eagle is similar to a sampan, although a sampan is much more stretched out.

If you are interested in hull forms and their effect on boats, I would highly recommend Gerr's The Nature Of Boats. You should be able to find it in most public libraries, or here: http://www.amazon.ca/The-Nature-Boat.../dp/007024233X

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Old 07-28-2013, 02:12 PM   #168
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Quote:
Originally Posted by Marin View Post
And I don't believe there is a rule that says displacement hulls have to have round bottoms and chines. I've posted these shots before, which I did not take, but it shows one of the sampans or aku boats that were built locally in Hawaii in the later 1940s and early 50s for the tuna (aku) fishery there. These relatively narrow boats incorporated flat albeit curved bottom sections, hard chines, and even gunwale " hull bulges" (my term) to provide roll stability in the often very rough waters these boats fished in, like the infamous Molokai Channel. The "hull bulge" is obvious in the second shot and was quite effective as these boats were often rolled to their gunwales in the windy swells and waves around the islands.

Attachment 12999
Hi Marin.
Can you define the 'hull bulge' portion of the vessel that you speak of?

Is it up near the gunwales, or that somewhat bulgy flat hard chine portion below the waterline?
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Old 07-28-2013, 04:19 PM   #169
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Marin is retired, he only lurks here.
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Old 08-11-2013, 07:56 AM   #170
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Roll damping on two New England trawlers: an experimental study

This reference was just brought up on another forum by a fellow from Australia
Quote:
Originally Posted by watto99
In my own search for this information I tracked down a report and bought it on line for $40 from the US naval archiect association.
It's well worth the money and answers the questions about benefits and optimum design of bilge keels etc


Roll damping on two New England trawlers: an experimental study C. A. Goudey, M. Venugopal 1989, April.

An experimental study on three types of roll dumping devices bilge keels, passive bilge fins, and paravanes is described. These tests were conducted in the MIT Ship Model Towing Tank using scale models of a 76-ft single-chine trawler and a 119-ft double-chine trawler. The models were fitted with each damping service and excited in roll by a hull-mounted moment generator. Roll motions were measured at zero speed and at trawling and steaming speeds.

Nondimensional damping ratios have been calculated and the effects on roll damping of each device are compared.

The relationship of bilge keel and bilge fin area as aspect ratio to damping ratio are studied and some design considerations are presented. Practical aspects of each device type are also discussed.
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Old 08-27-2013, 08:59 AM   #171
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Art, I am looking into getting a 1985-1988 44' Tollycraft w/ twin diesels and I will be living aboard in St. Maarten full time with cruising the Carribean as a plan. I will be retired and I was wondering about your thoughts on a Tolly in this boating environment. Also, do you have any fuel burn or mileage estimates for the Cat, Volvo, or Detroit motors?
Thanks
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