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Old 02-02-2011, 13:52   #1
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Effect of Draft on Seaworthiness

What effect does draft have on a passage maker's seaworthyness?

Is there such a thing as a "too flat bottomed boat"? If so, what is the risk?

What are the risks of boats with shallow draft in making passages?

Are there any general guidelines?

Thank you!
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Old 02-02-2011, 14:18   #2
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Originally Posted by ViribusUnitis View Post
What are the risks of boats with shallow draft in making passages?

Are there any general guidelines?
The plain fact is that many people have gone to the extreme ends of the earth in flat bottomed french centerboard boats. And they have not gotten into trouble any more frequently than the keel boats.

The ultimate stability index of these boats is usually quite poor and you would think in theory they would have a capsize problems but they do not seem to. No-one has really explained why not, but Steve Dashew's theory is that they don't trip over their keels (as keel boats do) but rather slide sideways.

The problems with flat bottom boats are #1 the centerboards can be unreliable, and #2 the boats often don't go upwind very well (but they can motor sail upwind ).
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Old 02-02-2011, 14:21   #3
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Depending on the boat, they can pound you to death when beating to windward.
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Old 02-02-2011, 14:26   #4
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Oh, please note this was posted in the power boat section, with the question about "passagemakers".

How relevent are flat bottom french centerboard boats to trawler type? I'm not saying they're not, just I don't understand the reference.

And I REALLY don't understand "beating to the windward" on a powerboat!
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Old 02-02-2011, 14:45   #5
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If the bottom of a boat is too flat (even with enough draft), it will experience some slamming at speed in a seaway. Sometimes, it is even possible to have a flat and shallow stern slamming at anchor.

The general guideline is: big, deep, heavy, Vee-hulled slow boats are generally more comfortable. The idea is to keep the bottom underwater at all times. For a more detailed discussion, google "axe bow".

Kunst - Amels Sea Axe Yacht report

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Old 02-02-2011, 15:14   #6
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My dad spent a year on a boat one week coming across the Gulf of Mexico on a 135' flat bottomed sea-going tug. He said it rolled 50 degrees in clear sunny skies. One day, of the 30+ man crew, he and one other person cooked lunch and 3 people ate.

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Old 02-02-2011, 15:48   #7
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How relevent are flat bottom french centerboard boats to trawler type? I'm not saying they're not, just I don't understand the reference.
Sorry, I just was looking at new posts and I did not notice it was powerboat section.

But, the french boats are an indicator that deep draft 'stability' is not essential for offshore 'safety'
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Old 02-02-2011, 21:01   #8
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what I have noticed with my current planing hull, which is a modified 'V' hull, compared to the displacement trawler I had in the past is that the planing hull is less efficient and uses more fuel for comparable speeds (of course the trawler really only had two speeds, slow and slower) but does not roll anywhere near as much as the round bilge boat did both underway and at anchor. my boat is not a "passage maker" by most definitions, but I suspect that this is would apply in similar comparisons. I do not have a sense that this hull type is any less seaworthy than any other despite popular opinions to the contrary.
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Old 02-02-2011, 21:13   #9
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I worked on a 135' LCU 1600 in Alaska. On the inside passage it was a dream. Across the Gulf in fair seas it was an interesting ride. The slamming of the bow against the swells sent a shudder down the hull but she handled it all very well.

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Old 02-02-2011, 21:22   #10
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what I have noticed with my current planing hull, which is a modified 'V' hull, compared to the displacement trawler I had in the past is that the planing hull is less efficient and uses more fuel for comparable speeds (of course the trawler really only had two speeds, slow and slower) but does not roll anywhere near as much as the round bilge boat did both underway and at anchor. my boat is not a "passage maker" by most definitions, but I suspect that this is would apply in similar comparisons. I do not have a sense that this hull type is any less seaworthy than any other despite popular opinions to the contrary.
most planing hulls are not designed to be driven under water.. what a lot of people forget, is that all hulls are displacement hulls until they reach plane... if your boat is designed to plane, but you are not reaching plane, it will be extremely inefficient. so "comparable speeds" is not a good comparison for these different hulls... in other words... You need to give it a little gas bro
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Old 02-02-2011, 21:22   #11
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There is a difference between seaworthy, stability, and sea kindlyness. Just because a boat has a v or a flat bottom doesnt make it any more seaworthy than the other, its all about keeping the ocean out. Typically a flat bottom boat has a higher metacentric height and therefore a higher innitial stability than a v bottom, the ultimate stability will depend on deck and superstructure, cg's and the like. So in my opinion considering each type keeps the seawater out, and has self righting ability, the real deciding factor is seakindlyness, after a day of pounding and rattled insides I would have wished for the v bottom over the flat bottom.
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Old 02-02-2011, 22:46   #12
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You need to remember "SPRAY" of Joshua Slocum fame was essentially a Chesapeake bay oyster boat , basically flat bottomed but wide.

My boat is flat bottomed but all the flat is below waterline.
With a very fine bow entry, no slamming, cuts through waves like a hot knife through butter.
Draft of 1 ft for a 43ft boat.
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Old 03-02-2011, 10:10   #13
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most planing hulls are not designed to be driven under water.. what a lot of people forget, is that all hulls are displacement hulls until they reach plane... if your boat is designed to plane, but you are not reaching plane, it will be extremely inefficient. so "comparable speeds" is not a good comparison for these different hulls... in other words... You need to give it a little gas bro
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Crazy, I understand what you mean Bro, however since my boat is diesel not gas, I am sure you mean 'throttle'. Most hulls I believe are designed to go through the water not under it with the exception of submarines of course. As Beau points out, at displacement speeds (which is the comparable speed I was referring to) the planing portion of the hull is beneath the surface not on top as when on plane. When at displacement speeds however, the fine entry with a flared bow does not slam into waves and the hard chines and flatter running surface actually act similar to a 'flopper stopper, or stabilizer plane that is running below the surface would also, but with a much larger moment and wetted area yielding a relatively 'flat' ride.
In terms of "extremely inefficient" my displacement trawler cruised at just a hair shy of 6 knots on about 1.5 gph of diesel and this boat cruises at 6.6 knots at 3 gph for the comparison I was making, which is not an extreme difference to me although you could argue the point I guess since instead of 4nm per gallon, I get only 2.2 nmpg. I usually cruise at 8.3 knots and 5 gph, if I throttle up to say 1,500 rpm then I am making 10.1 knots at 8 gph, still not too bad for my needs.
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Old 03-02-2011, 10:27   #14
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The general opinion seems to be that there is no particular reason that the boat might not be "safe", just not comfortable. Expecaly not when going into the waves.

I think I can live with that.
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Old 03-02-2011, 11:20   #15
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The hydro dynamics of hull forms is very complex. Depending on the type of boat you are building the benefits of different degrees of draft will vary. How a given power boat will handle in rough water will depend on the overall balance, propulsion, as well as the draft. Non planning trawler type hulls have a tendency to have a sharp entry that carries about midway aft and very little deadrise at the transom. A deeper draft on this type of boat will produce less lateral motion and slower recovery times in rough water. A rule of thumb on most larger power boats is shallow draft equals a poor ride quality. That is why many manufacturers revert to prop pockets to achieve less draft while still retaining a good ride quality. The answer to your question if it is an important factor for you in which type of boat you will buy should be answered by a qualified designer.
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