Effect of weight carried on boat speed

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catalac08

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In the 5 years of owning my current boat ( 8M Cat) my top boat speed under sail and power has progressively dropped, possibly due to the excesive amount of spares/equipment I have accumulated on the boat in this time. Having removed 2 car loads of spare heavy warps, wood, paint, enough tinned food for a circumnavigation, spare sails,spare chain etc I am hopefull of returning the boat to its previously more lively performance.
My question is whether there is some empirical formula relating reduction in boat speed due to load carried, taking account of total displacement as clearly a ton of payload will make a big difference to a small boat and may not be noticed in a 15 ton vessel.
 
Effect of excess weight on your cat is enormous. Anything you carry requires extra power to move it, which in turn slows you down. The amount it slows you is a product of the weight itself, plus the extra wetted surface area (friction) created. Thus a heavy workboat with no specific waterline and a greater weight per inch of immersion, will, usually, be less affected for a given increase in weight than a vessel built to a design waterline.
Nicki
 
So which would have a greater effect on speed - heavy gear or weeds clinging to the bottom ? - I would have thought weeds ???
 
Well, slightly different subject I suppose, but weed does slow ya down. My boat in the Canaries grows this horrid long stuff with air sacs on it. it has so much drag that much of it can be removed by running the engine hard in reverse whilst still attached to the pontoon. This particular weed does not adhere very strongly. The rest just comes off with a broom. Setting off with it on the bottom means no steering at all and sod all progress. Embarrassing.
Having said all that, supposing there is sufficient excess weight aboard to make the boat crank, this would have a huge effect on speed. I don't suppose it would be very easy (possible) to calculate which would have a greater effect, but I would guess that, ultimately it would be weed. Unless, of course the extra weight rolled the boat under. :-)
Nicki
 
there is PPI immersion and Moment to trim 1"

POUNDS PER INCH IMMERSION (PPI): The weight required to sink the yacht one inch. It is calculated by multiplying the LWL area by 5.333 for sea water or 5.2 for fresh. The PPI usually increases as the hull sinks into the water as the LWL area is also increasing due to the shape of the hull above water.

MOMENT TO TRIM 1 INCH (MTI): The MTI is the moment, expressed in foot-pounds, that will change the fore and aft trim of the yacht one inch. For a displacement hull, the MTI is, roughly (but close enough for all practical purposes), .35 times the square of the waterline area divided by the WL beam, or:



For example; a boat has a LWL Area of 165 sq ft and a Beam WL of 8 feet. The MTI is .35 x 165 x 165 /8 = 1191 ft-lbs., say 1200 for rough figuring. Now you hang a 100 pound dink 18 feet abaft the CB. You've added 1800 ft-lbs of aft moment so the boat will trim 1800/1200 = 1.5 inches down by the stern. However, the boat does trim about its CF and, as that is usually abaft amidships, the stern will move less than the bow. You might find that she trims 5/8" down by the stern, and 7/8" up by the bow, making a total trim change of 1.5 inches.


Obviously on a cat things are slightly different.. like 2 hulls lol...
once the new waterline area is calculated you can apply the figures to SA/D (taking into account the new displacement with the gear onboard).. this gives a reasonable view of the potential change.. mainly monohull formula though..

DISPLACEMENT/LENGTH RATIO: The D/L ratio is a non-dimensional figure derived from the displacement in tons (of 2240 lbs) divided by .01 LWL cubed, or, Dt/(.01 LWL)3. It allows us to compare the displacement of boats of widely different LWLs. Some examples of various D/L ratios follow, but are generalities only as there is often a wide range within each type.

BOAT TYPE D/L RATIO

Light racing multihull 40-50

Ultra light ocean racer 60-100

Very light ocean racer 100-150

Light cruiser/racer 150-200

Light cruising auxiliary 200-250

Average cruising auxiliary 250-300

Heavy cruising auxiliary 300-350

Very heavy cruising auxiliary 350-400

STORM, a wonderful 27' LWL sloop on which I raced with Bill Luders many years ago, had a D/L ratio of 386 so she would be considered very heavy by today's standards. However STORM was 39' LOA and when she heeled to a breeze her long ends would increase her sailing LWL, thus reducing her D/L ratio to a more reasonable figure when we were beating to windward. If she picked up 3 feet of WL her D/L ratio dropped to about 281, a significant change, and one that made her a very competitive racer in the 1960s.


SAIL AREA/DISPLACEMENT RATIO: The SA/D ratio is the sail area in sq. ft. divided by the displacement in cubic feet to the 2/3 power, or SA/D.667 .

Ratios below 14 are suited for motor sailers, from 14-17 for ocean cruisers and from 16-18 for typical coastal cruisers. Ratios over 18-20 are seen on racing dinghies, inshore racers and ocean racing yachts. The more extreme screamers can have very high SA/D ratios indeed; My 60 foot design, WILD THING, had a SA/D ratio, based on 100% foretriangle, of well over 30, depending on her displacement at the moment. Her displacement varied widely as she could carry 8,000 pounds of water ballast in tanks on the windward side
 
Thought about this too having watched all the racers remove everything from thier boats. Evetually found a site about rowing & the answer they give for adding weight is that if you increase your boat mass (weight) by 1% your boat speed falls by 1/6th of 1%.
Looked at another way at 5kt after 1 hour you would have about 20 yards difference in distance travelled!

http://www.atm.ox.ac.uk/rowing/physics/weight.html#section7 - the answer as always is at the bottom.....
 
I can say from personal experience that weight on a cat makes a phenomenal difference. A seriously overloaded cat is not just slow, it is also difficult and dangerous to sail and the loads on the rigging and hulls are significantly increased. Far worse than a weedy bottom IME.
 
When my 9metre Catalac was overloaded with family and liveaboard gear plus 90 gallons of water (two young daughters on board!), the transoms were in the water and creating a lot of drag. Once lightened, they were clear of the sea and the boat was immediately more lively and could keep going better in light winds.

Martin T
 
Speaking entirely unscientifically, experience on my DAzcat 10m is that a season's growth of weed has far more effect on speed and manoeuvrability that whether or not I have the dinghy in davits or on teh mooring. Fully loaded for a fornight's cruise with dinghy, full tanks, etc, I can expect a fall or a knot, maybe a knot and half from 9.5 to 8 knots in Force 4 to windward compared to early season light-displacement sailing. At the end of the season weed will have reduced speed by up to three knots to six knots. More importantly, I will have to back the headsail unless I bring her round very carefully as the rudders are far lesss effective.
 
Agree generally with other posts. Capnslarty's generalisations are just that as he says himself but as good as any.
What it boils down to is how much reserve bouyancy you have and that in turn relates to the increase in wetted area on your hull.
Most cats go fast because they have slender hulls, which equates to low reserve bouyancy so a proportionately smaller cargo load will have more of an effect than on a fat monohull. HOWEVER. There are cats and cats. Mine, a Snowgoose Elite, has very soft bilge lines and not very slender hulls. It is therefore not as fast as many but capable of handling a cargo load higher than most with little effect on performance. Racing cats with very slender hulls only need a few extra kilos to have a dramatic effect. The 8 metre catalac has an underwater section that is V shaped and therefore will be quite sensitive to increased weight. It's surprising how it adds up. When I built mine she was just under 6 tons lightship on launch. When lifted out at the end of season she was nearly 8 tons! I really must have an inventory of the Fray Bentos pies and beans on board.....
 
[ QUOTE ]
Speaking entirely unscientifically, experience on my DAzcat 10m is that a season's growth of weed has far more effect on speed and manoeuvrability that whether or not I have the dinghy in davits or on teh mooring. Fully loaded for a fornight's cruise with dinghy, full tanks, etc, I can expect a fall or a knot, maybe a knot and half from 9.5 to 8 knots in Force 4 to windward compared to early season light-displacement sailing. At the end of the season weed will have reduced speed by up to three knots to six knots. More importantly, I will have to back the headsail unless I bring her round very carefully as the rudders are far lesss effective.

[/ QUOTE ]

My experience with a Prout was very similar - the fine south coast weed if not scrubbed off mid season could stop the boat tacking altogether by slowing it down before it went through the wind. Mind you, filling up the water tanks used to take a good know off the max speed.
 
I have some formulae for performance of multihulls. speed in a given set of conditions is proportional to

sq root(1 / displacement)
 
Case Proved

funny_picture_fat_guy_row_boat.jpg
 
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