Lindsay Lord, Daniel Savitsky, and Ocean Boats

Discussion in 'Boat Design' started by tananaBrian, Oct 15, 2017.

  1. tom28571
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    tom28571 Senior Member

    Brian, I remembered that some years ago when I was building the prototype Bluejacket 24, I took the bottom lines off a friend's C Dory 22.
    Some pertinent numbers are:
    Transom WL beam: Deadrise = 3 degrees on 65" chine beam
    At 84" fwd of transom: Deadrise = 3.6 degrees on chine beam of 67"
    At 120" fwd of transom: Deadrise = 3.8 degrees on chine beam of 64"
    At 156" fwd of transom: Deadrise = 11.5 degrees on chine beam of 29"
    At 192" fwd of transom: Deadrise = 28 degrees on chine beam of 11.7"

    That C Dory 22 had a small fore and aft chine "fence" of about an inch height which may have been to effectively make the WL beam act as it is wider. I don't know how effective the fence might have been.

    Its clear that these C Dories are very flat bottomed boats. I might guess that the intent was to have the aft buttocks completely flat. I could not determine where the stem waterline entry was but the entry can be said to not be very sharp. The result as you and others surmise, is that the ride can be rough in chop. My Bluejacket deadrise is much greater at every point and provides a much better ride although not close to a true deep V. While the flat bottom does make the boat more efficient than a high deadrise V, it in no way compensates for the lack of bottom area and is why a C Dory needs much more power to achieve planing than any of the boats discussed here. This particular C Dory had a pair of Honda 45s on the transom. One of my mantras has been that bottom loading has a great effect on ability to get on plane quickly with less power and lower trim angle. All desirable qualities for efficiency.

    Almost everyone has a different set of qualities they want in a boat but, in my view, many dories pay a high price for cuteness.
     
  2. tananaBrian
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    tananaBrian Junior Member

    Thanks, Tom. As you know with deadrise, some of the water moving past the hull moves outward rather than straight back, which means you are imparting energy (your motor) to do work (sideways movement of water) that isn't helping you ... you just want pure lift, no waste. On a relatively smooth lake surface, a wide flat-ish bottom boat will be the most efficient. On the ocean or in a larger chop, it's tough to come up with a low-deadrise boat hull design that doesn't want to 'stop' with each smack against a wave ... V-hulls, deep Vs in particular, reduce this resistance and the net is that while a boat of this design won't be as efficient on the lake, it'll be more efficient in the ocean. Lindsay Lord found that a deep (by today's standards) V monohedron hull was best offshore, but you had to balance the aspect ratio ... the boat width v. length (he gives a drawing and what measurements to compare) was optimal according to his experiments. On the Great Alaskan, I ignore the Float-tel boat designs and followed LL's guidelines as exactly as I could, for a boat as light as the Great Alaskan. The light weight disallows a deep deadrise without also narrowing the beam too much - it would mess up the aspect radio, making the boat too narrow. I placed priority on the aspect ratio over the deadrise, but went as steep as I could while a) maintaining the best aspect ratio, b) sticking with as close to a monohedron as I could (13.1 deg aft, 14.5 deg amidships), and c) allowing the finest entry that I could afford - but not wandering too far from the tried and true entries that boats in its class utilized. The same applies to the transverse metacentric height (GMt) - it's stiffer than most commercially produced boats, but not overly so. That, combined with the 'sorta dory-like' flare in the sides, is what helps the boat bob up and over waves when the boat is moving slowly or adrift - part of what makes the boat seaworthy for fishermen (my first priority! Fishing!). Anyway, that's that. It took a lot of iterations and a lot of work to optimize everything, using RhinoMarine (now defunct) and Rhino 3D software for the 3D CAD work and boat performance modeling. I won't bring up the shape of the waterline v. the on-plane trim angle at the 'most efficient / most typical' expected speeds... that took a lot of work too. There are reasons that it's not likely that I'll design a boat in this size range again .... although, a certain naval architect friend of mine compared the Great Alaskan's hull to the Calkins Bartender, and surmised that my hull may even be better if I made a couple of small changes and made it into a double-ender. It would be cool to have a planing double-ender design out there.... But it would be a LOT of work, and like I said ... ugh! I'm working on some smaller designs now, some of them fast-boats.... we'll see.

    Brian
     
  3. DCockey
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    DCockey Senior Member

    My understanding is the C-Dory Venture boats, which Brian asked about, have different hulls with more deadrise than the older C-Dories which Tom provided information about. I'm also interested in the deadrise of the C-Dory Venture boats.
     
  4. tom28571
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    tom28571 Senior Member

    As the C Dory distributor is only about 14 miles up the road from me, I will get around to taking some lines off the Ventures. Kind of busy redoing a couple of kayaksI built in the 90's. They will go to my son who is to pick them up Thanksgiving. The fleet has been reduced by five boats this year and only a couple more to go.

    Brian, I'd like to discuss more about our boats and how they got the way they are when I get time to think about what I want to ask.

    Tom
     
  5. DCockey
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    DCockey Senior Member

    That would be most appreciated Tom.
     
  6. tananaBrian
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    tananaBrian Junior Member

    Fire away! I love all the discussion!

    Brian
     
  7. tananaBrian
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    tananaBrian Junior Member

    Tom (Lathrop) ... One thing that I wanted to point out in your remarks above about the C-Dory "needing" more horsepower than the boats being discussed here (Great Alaskan, Bluejacket etc) is one of the secrets about fiberglass boats. While fiberglass layups are not significantly heavier than the average for stitch-n-tape plywood/epoxy/glass construction, the polyester fiberglass itself isn't very structural. The way that fiberglass boat manufacturers make the stuff stronger is a) making layers thicker, and b) utilizing structure. For example, a box-girder beam is a lot stronger than a single flat piece of material, regardless of material. Fiberglass boats have hollow structure built into them to stiffen and strengthen them up. This comes at a cost though ... it requires more material, as do thicker layers/layups. The bottom line is that glass boats, while the base material is not significantly heavier than ply/glass/epoxy, are heavier ... they have to be. One of the advantages of a plywood/epoxy/glass boat is that the required structure is thinner and you don't need to build in the additional structure that glass boats need. From a user's perspective, this means two things: you get a lighter weight boat that burns less gas and needs less horsepower, and you get a lot more room inside the boat that makes the boat more useful and less crowded. Aluminum boats are a different beast. They can go even thinner than ply/epoxy/glass, but it weighs around 168 pounds per cubic foot versus the ply/epoxy/glass layup's weight of around 45 pounds per cubic foot. So even though a ply/epoxy/glass boat uses more material, it still ends up weighing less than half of what an equivalent aluminum boat weighs, and I believe the polyester glass boats weigh even more. All that from just saying bigger motors are required for a C-Dory! The bottom line is weight ..... more weight means more horsepower is needed. Personally, I do not think the C-Dory waterline beam is too narrow ... perhaps slightly, but the designers were probably countering the low deadrise a little (narrower boats don't pound as much).

    Brian
     
  8. tom28571
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    tom28571 Senior Member

    Brian,

    There are a lot of worms in that can you just opened and I only want to look at one of them right now. So far, we only have a bit of comparison of a C Dory 22 and my Bluejacket 24. In addition to weight, we have the lines of the bottoms of these boats and a bit of performance data to examine.

    Given that these boats have a decent bottom design with no outstanding negative features, it is my position that bottom loading in weight per unit of bottom area is the main factor that determines how easily they get past any hump and reach planing speed. The numbers for these two boats are: C Dory 22 static waterplane area = 75 sq ft, planing waterplane area @70% of static area = 47 sq ft, planing bottom loading = 58 lb/sq ft. Comparable data for the BJ24 are: 104 sq ft, 75 sq ft, and 33 lb/sq ft.

    While these boats weigh about the same, the BJ24 is quite a bit larger and has a much larger waterplane at all speeds. This results in a much lighter bottom loading for the BJ24 than the C Dory 22. Just using these numbers without any other differences that will cause major changes, what are the results?

    With a 50hp motor, the BJ24 gets on plane at a much lower speed and the C Dory may not even get over the hump without more power if loaded very much. On the other hand, the C Dory achieves a much faster top speed with a 90hp motor. I don't know how much power it would take to give the C Dory reliable planing but I have driven one with a pair of Honda 35hp motors that performed well with two adults aboard. The thing to carry from this is that greater bottom area generally means ability to plane at lower speed where wave making drag is dominant, while having lower top speed, where surface friction drag becomes more important than wave making. So, a small bottom or greater weight means poor low speed planing but higher top speed.

    Sailors of planing boats know the importance of weight far more than powerboats as their racing results often depend on it. This is a brief look at this issue but shows a basic comparison between well designed dory hulls and well designed monohedron hulls of similar displacement. Sometimes personal desires over rules all other factors and if the owner is happy, little else matters.
     

  9. tananaBrian
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    tananaBrian Junior Member

    Yup ... The interrelated nature of all specs on boats confounds things and makes no one simple answer easy. That said, my marine modeling software (RhinoMarine at the time, coupled with Rhino 3D CAD) was very useful as I went through my iterations on the Great Alaskan. Factors that affect all of this include the CG, CB, deadrise (aft and amidships in the region where the hull comes out of the water when on plane), waterline beam, and chine width. And keep in mind that as you iterate and optimize these factors, you also want the transverse metacentric height (GMt) to stay within an acceptable range as well ... and this is primarily affected by the waterline beam. The same applies to the relationship between CG and CB. It took quite a bit of work and time to iterate the Great Alaskan's design since you have to look at results from the dynamic modeling, tune something by redrawing and recreating the hull, then modeling again ... repeat. I also put together a very-detailed center of gravity spreadsheet that added weights and center of gravity of every component, including paint coatings, 'holes' such as windows, moveable objects such as gear/ice chests and different fuel loads, and people. If I recall (It's been what? 13 or more years since I designed the Great Alaskan?) a couple of years to optimize the design, detail it out, and produce the plans. I swore then, and I repeat now, a vow that I will not design a boat of that size and complexity again - too much work and never enough time to do it. Glad I did it though.

    Back on the ease of planing topic, I agree with you that bottom loading is a primary factor and that deadrise and the change in deadrise from aft to amidships are secondary factors. Other factors fall behind those. I did NOT want a boat that stuck its bow straight up in the air as it rose over the 'hump' and started planing, so one of my optimizations was to not exceed about a 4.x degree bow-up maximum trim as the boat went on plane. Without digging up the hydrodynamics results (RhinoMarine), I believe the Great Alaskan trims up to about 4.2 degrees, max, when going on plane. When you actually operate the boat, it just feels like an elevator rising up and you don't notice any bow-up tendencies. Once on plane, the bow-up trim reduces to around 2.1 degrees as the boat accelerates to the maximum modeled speed of 40 knots (46 mph). Only Adrian P. of Gresham, Oregon has exceeded that speed so far and his boat performed very well at those speeds. The video available at my web site is his boat going this speed.

    Brian

    PS: The RhinoMarine marine modeling plugin for Rhino 3D is no longer on the market. It has effectively been replaced by the Orca 3D plugin for Rhino 3D. If I recall my history correctly, I don't think Orca purchased RhinoMarine, but believe that instead, they hired several of the key developers. That needs verifying. But if someone is interested, I believe the Orca 3D plugin now runs upwards of $3000 and is available at the following link:

    Orca3D Naval Architecture Software | Marine Design Plug-in for Rhino https://orca3d.com/




    .
     
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