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Hull speed

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

  1. DrJudyB


    Joined Jun 25, 2004
    272 posts, 183 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

    Will Gilmore and Scott T-Bird like this.
  2. Jackdaw


    Joined Nov 8, 2010
    9,844 posts, 2,898 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


    Joined Nov 8, 2010
    9,844 posts, 2,898 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! ;^)


    Last edited: Mar 15, 2019 at 9:13 AM
  4. Scott T-Bird

    Scott T-Bird

    Joined Oct 26, 2008
    3,782 posts, 829 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


    Joined Nov 8, 2010
    9,844 posts, 2,898 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.

    Scott T-Bird likes this.
  6. Scott T-Bird

    Scott T-Bird

    Joined Oct 26, 2008
    3,782 posts, 829 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.

    Will Gilmore and jon hansen like this.
  7. Jumpstart


    Joined Jan 13, 2009
    242 posts, 50 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.

    Will Gilmore likes this.
  8. Joe


    Joined Jun 1, 2004
    6,645 posts, 409 likes
    Catalina 27
    US Mission Bay, San Diego

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