Hull speed calucation

For a normal hull it is the square root of the waterline length times a factor of between 1.3 and 1.5 at best. So a 25 ft waterline will give a speed of from 6.5 to 7.5 knts. It is all to do with the Froude Number which comes from the work of a marine designer called Froude. This does not work for cats or planing hulls.
 
I have always been puzzled by this formula, if I construct a displacement hull say half a foot long then its maximum speed would be 0.707 x 1.4 = 0.9898 basically less than 1 knot, but anyone who has watched kids with model boats knows that this is not correct

My boat has a waterline length of about 28 feet, which even at a factor of 1.5 means I should not exceed 8 knots, but I know my boat actually seems to top out at 8.9 knots (often measured where no tide is running and confirmed by GPS).

Anyone able to explain?
 
The hull speed is not an absolute limit. It is just the speed at which the boat fits into the wave it is creating. To go any faster it must use more power to start climbing its own bow wave. With enough brute force higher speeds can be achieved; most of the extra power is used to make a bigger wash.
 
Exactly Dave there is really no such thing as max hull speed. What the term ios trying to describe is a point on the graph of power versus speed where the increase in power gives only a small increase in speed. (The graph line gets steeper) (The curve is less steep with narrow hulls light displacement and actually folds backwards when a full plane is acheived.) The formular gives an indication that this speed is where speed increase becomes a poor return for the power increase required. The formular is perhaps more usefull to show the comparison of probable speed with different hull lengths. yes flying spud my 21 fter has reached 9 knots and probably more but the pressure on the rig must be orrible. (spin up)
regards olewill
 
Froude was interested in tank testing of scale models for ship hull design. I guess most of us would start by thinking that if the model was say 20 times smaller it would act the same as the real ship at 20 times less speed. He found it was a square root relationship and not linear. That is for hulls of the same shape. The 1.3 1.4 1.5 figure is a sort of goodness factor which varies with hull shape - keel etc. It applies mainly to most reasonable hull shapes with length/beam of about 3 to 1. Most hull/engine power/prop fit calculations assume a factor of 1.5. If can get hold of one you can try for yourself.
As with cars the easiest way to make your boat go faster is to retune your speedo.
 
Interesting that you say it relates to a 3:1 length / beam relationship. Would that mean that a ‘thinner’ boat can go faster? (on the basis a square tank would be horribly slow one assumes), and what about the shape, does that have any effect?
It also seems to me that the actual displacement must have an effect, after all, the bow wave is, I assume, created by the water the boat displaces, the more the displacement presumably the bigger the bow wave at a given speed, so the quicker you would reach hull speed. Would that be correct?
Was Froude actually looking at yachts and their sort of shape, or was he working on designs for ships?
 
Sure. 'Thin' hulls can go faster - e.g. Helen's cataran hulls and other 'wave-piercers'. Froude was a ship man but the basic hydrodynamics about the square law remain. Even today yachts have to be about 3 to 1 for stability and just as importantly accomodation. As you say a flat fronted square boat would be most interesting but a bit slow. Shape and displacement do matter of course. That is why long distance racers worry about the weight of their toothbrushes. Two of the other things which slow a boat down are called form drag and surface drag. Some hull forms are more 'draggy' ( your box ) and the more 'wetted' hull area the more drag. These are as well as the wave factor which Dave described. Designers are always looking for speed, sea kindlyness, more people room so it is all agreat balancing act. All in all it is a very complicated subject and I am rapidly getting out of my depth.
 
It does sound like it's a useful guide as to when you have achieved a reasonable optimum in rig terms where to try to achieve an increase in performance is probably not worthwhile. So my little 16' displacement hull cruiser achieving 4.5 to 5 knots is probably sailing as near to it's best as I am likely to get which is useful to know.
 
Yes AithW that is correct especially if when sailing on the wind you can sail free er and go faster while pointing closer to the wind means slower but pointing better. Now when you get into the hull speed range then sailing freeer will give only a slight improvement in speed while pointing higher will get you to windward faster even though the speed is a little slower. That speed compromise is of course for you to discover for your boat. (the fun of racing)
regards olewill
 
Being a dinghy sailor you can't help tweaking stuff all the time even though you know you are sailing something never intended to be 'raced'. You just naturally want to be sailing efficiently. This summer will be about settling the boat into a comfortable groove and making some adjustments in the way controls are handled. It's all part of the fun of being on the water under sail. Thanks, as always, for the input.
 
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