Hull speed

Discussion in 'Ask All Sailors' started by dnimigon, Mar 11, 2019. Add this thread to a FAQ

  1. DrJudyB

    DrJudyB

    Joined Jun 25, 2004
    349 posts, 259 likes
    Corsair F24 Mk1
    003 US San Francisco Bay, CA
    Here’s an excerpt from an article I wrote in1999, about hull speed.
    ~~~~~~~~~~

    The Math

    Caution! Part 1 gets technical. If you want to skip it, you can go right to Part 2 of this article.

    Heavy boats with a large displacement to length ratio (D/L>240) must push a lot of water aside and they create big waves. They are restricted to the theoretical hull speed limit that is defined by the propagation of waves.

    The above equation for a displacement hull can be rearranged, 1.34 = Hullspeed/(LWL^0.5), so that the 1.34 represents the ratio of speed to the square root of LWL.

    So, the speed/length ratio for displacement hulls is 1.34. If a boat exceeds its theoretical displacement hull speed, the S/L ratio must increase. A semi-displacement hull has a S/L ratio of 1.45 or greater. A planing hull has a S/L greater than 3.0.

    But just how fast can a monohull boat be expected to go? It all depends on displacement -- more specifically on the D/L ratio (i.e., how heavy the boat is compared to the LWL). Naval architect Dave Gerr worked out the relationship, one of the great accomplishments in modern naval engineering. (David Gerr: Nature of Boats, McGraw-Hill; Offshore, Dec. 94, pp 29-33)

    D/L ratio = D[in long tons, 2240 pounds]/(0.01 x LWL)^3.

    S/L ratio = 8.26 /(D/L ratio)^0.311

    The formulas show that lower displacements permit higher speeds without actually planing. Everyone is familiar with Anthony Deane's original formula for heavy displacment hulls, and people are slow to catch that non-planing boats go faster than Deane's formula predicts, despite our observations that boats sometimes do go faster than they're supposed to.

    That's probably because most folks divide boats into two categories - planing vs. displacement. The difference between the two is visually obvious. However, most small sailboats don't get up an obvious plane, but nevertheless, they DO go faster than a displacement hull of similar LWL. Many sailboats fit into this category -- the semi-displacement hull

    We can re-write the equations to make the math easier, and then we can solve them in a step-wise manner:

    Eq #1: D/L=(weight/2240)/(0.01 * LWL)^3

    Eq#2: S/L = 8.26/(D/L)^0.311

    Eq#3: Hullspeed = S/L * LWL^0.5

    Equation 3 looks very familiar -- it's just like the traditional hullspeed formula, but the constant is no longer 1.34. In Equation 3, the S/L ratio replaces the constant allows us to take the displacement and water-line length of the boat into account.
    ~~~~~~~
    Comments by Dave Gerr
    from the Trailer Sailor Bulletin Board, where I initially posted this information

    Posted By: Dave Gerr

    Date: 9/22/99 9:15pm

    In Response To: (deleted)

    Hi Folks:

    Glad my somewhat more sophisticated hull speed formula has been of interest. It has proven quite accurate over the years and across many kinds of boats. The old rule-of-thumb, “simply 1.34 x the square root of the WL in feet” really isn’t accurate. The multiplier “1.34” is really a function of D/L ratio. That’s what my formula is all about. Keep in mind, though, that my rule describes the top hull speed that a hull MAY achieve without planing.

    Three things here:

    1) You need lots of power (wind for sailors, obviously) or you don't go faster, regardless.

    2) Many hulls that have low D/L ratios do have faster hull speeds before planing, but if--at the same time--the hull has some planing characteristics, it may well start to plane--at least to some degree--before taking advantage, as it were, of it's higher non-planing hull speed.

    3) There are lots of small sailboats that do a little of both in heavy air (in the right conditions). In other words, they get up on plane a bit, and they also take advantage of their higher non-planing hull speed at the same time for a small but noticeable double boost.

    Hope this is helpful. We're swamped with work here, so I may not have time to answer more, but I'll try and peek in occasionally.

    Cheers, Dave
     


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  2. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    10,072 posts, 3,079 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    A J92 for sure will plane in the breeze at that angle. Zoom!
     


  3. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    10,072 posts, 3,079 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    Nice! Easily one of my favorite books. Anyone remotely interested in this level of detail has to have a copy. Small world @DrJudyB , it was your post years ago that lead me to that book! I see its now a paperback, maybe my hardbound 1st edition is worth something! ;^)

    3185C6C0-145B-4A9B-9967-DD008A95C127.jpeg
     


    Last edited: Mar 15, 2019
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  4. Scott T-Bird

    Scott T-Bird

    Joined Oct 26, 2008
    3,903 posts, 895 likes
    Starwind 27
    US Barnegat, NJ
    I don't think it is necessarily planing. See the discussion that I referenced above. The point that they are making is that the definition of planing is rather murky. It is obvious that our displacement hull sailboats can exceed hull speed. My opinion is that we reach a transition to planing, above hull speed, without truly beginning to plane. Perhaps your boat does plane at some point. I am in agreement with the discussion that says that there is no binary condition, where planing is either on or off. Rather, there is a transition. I think that many of our sailboats operate in that transition. The length of the bow wave simply stretches out beyond the stern of the sailboat because there is not enough resistance to prevent the stern from escaping the quarter wave (and perhaps the bow begins to climb over the bow wave, too). I think that if our boats truly became planing, then speed would increase substantially as the power in the wind increases (as some sailing vessels can do). Mine doesn't, so I know that it is not feasible to actually be on plane. But it does exceed hull speed, so I think it gets into that transition regime.
     


  5. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    10,072 posts, 3,079 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    At 10 knots a j/92 is planing.. The water is coming off the transom just like it is in the pogo picture. Been there seen that.
     


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  6. Scott T-Bird

    Scott T-Bird

    Joined Oct 26, 2008
    3,903 posts, 895 likes
    Starwind 27
    US Barnegat, NJ
    I'll take your word! :biggrin: I'd like to be on one, but never have. I like @DrJudyB reference as well. In essence, I think the explanation of a "semi-displacement" hull dove tails nicely with the discussion that I was reading. I think of the "transition regime" in pretty much the same light as an explanation for how a semi-displacement hull exceeds the speed of a full displacement hull. In both cases, I think the definitions can be rather murky and subjective, yet we know that what we observe doesn't fit into a binary condition of planing vs. displacement.

    I will add that in some circumstances, planing vs. non-planing can seem to be binary. I knew when my sailboard was not planing and I knew exactly when it would begin to plane. There didn't appear to be any transition. It was either on or off. Perhaps if there was a transition, it was imperceptible. My sailboat is different. I know that it will exceed hull speed, but the speed has definite limits and it is certainly not planing.
     


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  7. Jumpstart

    Jumpstart

    Joined Jan 13, 2009
    247 posts, 54 likes
    J Boat 92
    78 US Sandusky
    A J/92, not mine. Look at that boat speed. Over double hull speed. It helps to have a big rudder/ dw sa/displacement of over 50, and a flat aft section.
     


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  8. Joe

    Joe

    Joined Jun 1, 2004
    6,663 posts, 422 likes
    Catalina 27
    US Mission Bay, San Diego
  9. Green Flash

    Green Flash

    Joined Nov 16, 2018
    4 posts, 4 likes
    Seascape 24
    Streaker US Honolulu
    A lot of good material. Hull speed is the speed a displacement boat can achieve while sailing as a displacement boat. The hull speed formula, 1.34x sq. rt. of waterline length in feet gives you speed in nautical miles per hour. The formula is actually the formula that describes the relationship between the speed of waves and the length of waves from crest to crest. So when you are going fast enough to make a wave with a length longer than the water line length of your boat, you are essentially causing yourself to sail up hill.
    But that's just the beginning. When sailing down wind in seas, you can sail faster than hull speed because when you are on the wave you are going down hill. We call it surfing a wave, and in my beneteau 40.7 in the big waves in Hawaii we regularly will reach 16 to 18 knots, and hold that speed for 15 to 45 seconds, even though the boat's calculated "hull speed" is somewhere around 8.75.
    If you want to sail faster regularly, a light boat with a reasonably flat bottom helps a lot. Most important is the displacement to length ratio. If you get that down around 90 or less the boat will plane easily off the wind. A bunch of fairly early light designs prove the point clearly: The Santa Cruz 27, 50, 52 and 70 designed by Bill Lee, the Express 27 and Moore 24 and the Antrim 27. Also great, the one design 35. Narrow boats plane just as well as wider boats, but we do like a wide flat stern to provide stability. A critical design factor also is that you want lots of foward boyancy. On a really good surf, a boat can actually outrun the wave it is on and stick its nose into the back of the next wave. We have had some boats do that then pitch-pole, or turn over back over front, which usually turns out very badly. So all the designs I just mentioned have excellent forward boyancy and can surf/plane extensively down wind.
    Fineness of entry also makes a huge difference, because as with my Beneteau, the entry is so fine that the waves go more out to the side instead of building up in front of the boat. Thus, in the light boats with fine entries, the transition between displacement and planing is quite gradual. But in older heavier boats, it usually will take surfing on a wave to "break out" and usually the surf/plane will only last while the wave is giving the boat a good push. There is a lot of technique involved in helming a boat off the wind to achieve the maximum amount of surfing. It takes focus, and quick response.
    So a lot of the speeds quoted in the above comments appear to be the result of temporary surfing on waves. But heavier boats, especially those with deep v hull designs which are sea kindly for cruisers, will be limited to speeds in the hull speed range.
     


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  10. Will Gilmore

    Will Gilmore

    Joined Oct 19, 2017
    4,596 posts, 2,542 likes
    O'Day 19
    US Littleton, NH
    :wow3:
    Green, nice post. You got a few points worth a mention that usually get glossed over.
    People need to keep in mind that using hull speed as a comparison standard to sail to isn't unreasonable, but, unles those factors in wave surfing and current velocity, a GPS doesn't tell you how you compare.

    -Will (Dragonfly)
     


  11. Green Flash

    Green Flash

    Joined Nov 16, 2018
    4 posts, 4 likes
    Seascape 24
    Streaker US Honolulu
    Will thanks for the comment. You are of course right in your adds.
     


  12. FastOlson

    FastOlson

    Joined Apr 8, 2010
    1,082 posts, 151 likes
    Ericson Yachts Olson 34
    US Portland OR
    For one lonely little data point.... we picked up our boat at the yard today with its new bottom paint. Motored home, taking about 3 hours. Water speed was 6.9 to 7.0 on flat water. Cruising RPM's (about 2500).
    Our design has a fairly flat run aft and at this speed the stern wave is up against the lower part of the transom about 4 to 6 inches above the static WL.
    FWIW, this is about typical with a clean bottom. While the brochure claims that we have a 28' LW, the immersed length is likely more like 31 or so, sailing or motoring. We are certainly not... violating any laws of physics or fluid dynamics . No citations received, yet.
    :)
     


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  13. jon hansen

    jon hansen

    Joined May 25, 2012
    1,932 posts, 1,600 likes
    john alden caravelle 42
    us sturgeon bay, wis
    hull speed has a simple definition. it does not ever mean or imply 'top speed'
     


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  14. Will Gilmore

    Will Gilmore

    Joined Oct 19, 2017
    4,596 posts, 2,542 likes
    O'Day 19
    US Littleton, NH
    Perfectly stated. It always annoys me to hear "theoretical" hull speed. It's not a theory, it's a definition derived from the conditions where efficiency begins to go down as speed goes up for displacement cruising.
    I wonder if this relationship is slightly different in freshwater than saltwater. The density and viscosity are different. This must have some effect on how easy it is to create waves, which, I would think, would make the speed to frequency relationship different also.

    -Will (Dragonfly)
     


  15. jon hansen

    jon hansen

    Joined May 25, 2012
    1,932 posts, 1,600 likes
    john alden caravelle 42
    us sturgeon bay, wis
    tricks to use with the understanding of hull speed: in a race a skilled skipper can lure a larger overpassing vessel to sail close by and drive in directly behind the bigger boat and catch his hull wave and get a "tow" from the bigger boat. your stem a few feet off his taff rail and his wave under your stern. it's not for the shy sailor. drives the other boat crazy. mile after mile. win PHRF races with such tricks.
     


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  16. agprice22

    agprice22

    Joined Aug 3, 2012
    2,309 posts, 540 likes
    Performance Cruising Telstar 28
    US Watkins Glen
    At 9 knots, my boat resonates as it accelerates. At 10, it gets smooth, but it is throwing quite a tail. I have had it up to 14. My friend was asking for his pfd! The calculations on my boat seem to indicate a top hull speed around 12, but it is a tri, so all bets may be off.
     


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  17. Green Flash

    Green Flash

    Joined Nov 16, 2018
    4 posts, 4 likes
    Seascape 24
    Streaker US Honolulu
    Not very familiar with multi hulls, but I do know that there are hull speed calculations for them, but they are not the same. Will's speculation about viscosity of fresh vs. salt water was interesting so I did some more reading. Viscosity and several other factors are issues, but probably the viscosity of salt water is not enough different from fresh water to make a significant difference. Salt water is a little heavier, so a boat will float a little higher ... but the weight of the water creating drag will be the same.
     


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  18. DrJudyB

    DrJudyB

    Joined Jun 25, 2004
    349 posts, 259 likes
    Corsair F24 Mk1
    003 US San Francisco Bay, CA
    Viva la difference! Whoosh!
     




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