Hull Speed: Reality vs. Theory

[BillyBudd]

Checked out our Bolger Chebacco's speed with GPS this summer. Fastest on sail = 5.3 knots. Fastest with a 4.5 or 5 hp. British Seagull (Silver Century) for power = 5.3 to 5.4 knots. Usual formula for max. hull speed (displacement hull)as a function of length (19' - 8") = 5.9 knots. We're fully pleased with our boat at 5.3 when sailing. I thought the outboard would power the boat to hull speed but it did not--this is an old Seagull (longshaft) with the 5 blade prop.

It can be some ride when sailing with bottom of the topside chine digging in, boat heeling and surging ahead.

I'd like to know if others reach theoretical hull speed or not.

Pleased with our boat, but curious.

[Raka025]

Are you using Overall Length or Waterline Length? The formula is for LWL. I'm sure you knew that but just checking?

[Ian McColgin]

I reach and exceed regularly on all boats.

"Hull speed" is not like the sound barrior where once you break through you regain some efficiency. Rather, it's a phenomenon of diminishing returns where you can always make a hull go faster but as you pass "hull speed" it takes more and more power to gain only small increments of speed. Eventually you get to the point where the power - motor, sail or human - available will not give you any more speed. But more power will. This is why motor boats feel like they have a firmer hull speed - only so many ponys in the mill - but sail boats may well get vastly more horsepower in a gale, if they can stand up to it.

Anything moving through water makes waves in porportion to its speed. As your local 100' ferry pulls out and moves up to a harbor speed of say four or five knots, you'll note perhaps four wave formations coming from the bow, stern and two evenly spaced in between. As she moved up to her running speed of say 14 knots (good hull design) there will only be the bow wave and the stern wave perhaps a bit abaft the stern.

"Hull speed" is the place where the boat is moving fast enough to make only a bow wave and stern wave with the stern wave so far abaft the stern and the bow wave so large that the boat is basicly squatting in a hole in the water and trying to sail uphill.

There's a practical or economical "hull speed" where the stern wave is still at or near the stern and getting to it has required a nearly liniar addition of power. After that, to go faster will require what feels like a geometric application of power. Row a dory or some other boat designed to efficiently carry a load but not designed for flat speed. Leeward, my Chamberlain gunning dory, was easy to move along at I could maybe hit 5 briefly but it was huge work, and even with a second oarsman we were not getting past 5-1/2k.

Hull speed to length depends on design. A really good boat can have a hull speed in knots equal to 1.5 times the square root of it's waterline length in feet. A real needle, like Granuaile - 55' Marco Polo - has almost no real hull speed as she makes so little wave anyway. A boat like that has her speed limited by the relativly small sail area she can carry and by the typically small engines most voyagers preferr. Even so, Granuaile has made a 60 mile perfect high wind but pretty flat water beam reach in 4 hours 20 minutes.

A fat cat like Marmalade, in contrast, begins to squat at 5 knots, though on Friday's triple reefed run to Vineyard Haven we were keeping a steady 6-1/2 k. through the water with over eight knots sliding off the face of some stupendous (for Nantucket Sound, i.e. 7-1/2 feet high and 50' crest to crest) chop.

In contrast to my "Achey Arms" thread about the same sun a couple of years ago, I'm in better condition and had a total gas, and we did well through the rain in the Moffett, thank you, and had a glorious sail home in light weather but felicitious current and wind and sunshine except for a five minute shower off Craigville, but I digress . . .

Which partly explains why LFH used to snort that there was no such thing as "hull speed", just a maximum speed for that boat as laden and powered.

[johngsandusky]

Also, even hull speed formulae contain another variable. It is common to use 1.25 or 1.33 x the suare root of wl length, but some boats will make 1.5. Others blow it away without planing (Soling one designs, iirc). Still others are just a bit slow. My 20' Pennant sloop was pretty fast, my 26' Friendship not so much.

[gimmellsmom]

I belive our 'theoretical' is somewhere around 7 (mod. H28), correct me if I'm wrong please. We regularly clear 6.5 in good wind, proper trim and our new genoa. We could push her more, but we generally sail with little people who are not too fond of heeling & going all that fast!
Last week we were over 7 at a point... that was fun!

[dingbat]

I reach and exceed regularly on all boats.





I have heard that, due to the reduced drag, sailing with your shirt off will alow you to push a boat past theoretical hull speed......

http://i70.photobucket.com/albums/i99/neclassic/NantucketAugust2007070.jpg

[Jay Greer]

I belive our 'theoretical' is somewhere around 7 (mod. H28), correct me if I'm wrong please. We regularly clear 6.5 in good wind, proper trim and our new genoa. We could push her more, but we generally sail with little people who are not too fond of heeling & going all that fast!
Last week we were over 7 at a point... that was fun!
Just off the top of my head, I believe, the theroetical hull speed of the H28 is 6.5Kts. However, our boat, "Bright Star" easily reaches 8kts on a reach. We have clocked the boat, under working sails at over ten miles per hr. by GPS. Check out the vidio "Bright Star at 8.5 knots".
Jay

[Ian McColgin]

Wow dingbat, what a fabulous pic. Thank you.

[dredbob]

I have heard that, due to the reduced drag, sailing with your shirt off will alow you to push a boat past theoretical hull speed......

Yes, but only if you shave off your chest hair :)

[Ian McColgin]

Heh. I don't have much chest hair. When I was on swim team in college, I had the least shaving to do. (Those of a certain age will get this.)

[P.L.Lenihan]

what a fabulous pic.

It most certainly is! Mr.Ledger should get a copy of it too as inspiration and encouragement for his boat!
You're a lucky man Ian !!

Cheers!



Peter

[john welsford]

There are a couple of points here, one is that the 5 hp Seagull outboard prop is from memory only pitched for about 5 knots max, after that the engine runs out of revs so even with a very light load you wont go any quicker.
That said, the Seagull racing guys tweak them and get a lot more revs and speed. ( and noise!)

The other is that people tend to forget that slender hull theory also applies to very shallow hulls. It should by rights be called displacement angle theory or some such. Essentially the less momentum the water acquires as it is pushed aside, the lower the waves, so the less effect the waves have upon the drag curve as the Speed Length ratio goes up. To reiterate. This applies to shallow as well as narrow.
Chebacco is shallow and not that wide on the waterline so under favourable conditions should easily exceed the 1.34 S/L figure

John Welsford



Checked out our Bolger Chebacco's speed with GPS this summer. Fastest on sail = 5.3 knots. Fastest with a 4.5 or 5 hp. British Seagull (Silver Century) for power = 5.3 to 5.4 knots. Usual formula for max. hull speed (displacement hull)as a function of length (19' - 8") = 5.9 knots. We're fully pleased with our boat at 5.3 when sailing. I thought the outboard would power the boat to hull speed but it did not--this is an old Seagull (longshaft) with the 5 blade prop.

It can be some ride when sailing with bottom of the topside chine digging in, boat heeling and surging ahead.

I'd like to know if others reach theoretical hull speed or not.

Pleased with our boat, but curious.

[RFNK]

That said, the Seagull racing guys tweak them and get a lot more revs and speed.
:eek:

Please tell me there is really no such thing as `Seagull racing guys'! What sort of boats do they use?

Update: I looked it up: http://homepages.ihug.co.nz/~lealand/seagull.htm
Good grief! The rest of the world frequently comments `Only in America' but, happily, I guess, we in Oz have our good neighbour to entertain us instead!


Rick

[BillyBudd]

There are a couple of points here, one is that the 5 hp Seagull outboard prop is from memory only pitched for about 5 knots max, after that the engine runs out of revs so even with a very light load you wont go any quicker.
That said, the Seagull racing guys tweak them and get a lot more revs and speed. ( and noise!)

The other is that people tend to forget that slender hull theory also applies to very shallow hulls. It should by rights be called displacement angle theory or some such. Essentially the less momentum the water acquires as it is pushed aside, the lower the waves, so the less effect the waves have upon the drag curve as the Speed Length ratio goes up. To reiterate. This applies to shallow as well as narrow.
Chebacco is shallow and not that wide on the waterline so under favourable conditions should easily exceed the 1.34 S/L figure

John Welsford

John, Many thanks for your observations. I'll take it that Ms. Seagull has done her stuff at 5.3 knots, and I'd never want her to talk any louder than she does. Why tweak a Seagull, huh? But--and more critically to be sure--I'll also take it that the Chebacco (indeed shallow and certainly not all that wide at the waterline although when heeled there's more in the water) can be eased beyond 5.9 kts., if indeed we are so inclined and able. That little event may well be a product of increased sailing skills + winds different from flukey lake stuff.

Thanks for others for their observations/suggestions. Ian, that's one pretty boat. I'm impressed by the care you've obviously bestowed on her, and on your sailing adventures too.

[alkorn]

Which partly explains why LFH used to snort that there was no such thing as "hull speed", just a maximum speed for that boat as laden and powered.

I second (or is it third?) Ian McC and LFH. Hull speed is an arbitrary number. For a displacement hull, power goes up more and more quickly as speed/sqrt(length) gets higher. "Hull speed" is simply the point where you decide that putting in more power is not worth while.

[LaCroix]

Is there an approximation formula for speed in a design?

Something like "sailarea at L/B/D ratio times factor equals theoretical speed at X knots wind"?

There is such stuff for engine usage, but I couldn't find it for wind power

[P.I. Stazzer-Newt]

Checked out our Bolger Chebacco's speed with GPS this summer. Fastest on sail = 5.3 knots. Fastest with a 4.5 or 5 hp. British Seagull (Silver Century) for power = 5.3 to 5.4 knots. Usual formula for max. hull speed (displacement hull)as a function of length (19' - 8") = 5.9 knots. We're fully pleased with our boat at 5.3 when sailing.....

I'd like to know if others reach theoretical hull speed or not.

Pleased with our boat, but curious.

I've seen figures on the GPS way over theoretical hull speed - with a helpful tide under me - memorably 13 knots while being flushed out of Langstone Harbour.

[andrewe]

Yes, and one of the crew of a Caladonian Yawl showed me his GPS in Port Navalo (Brittany) with over 15kts recorded. But the streams are pretty fast.
I have seen 8kts downwind in my Whilly Tern with no current tho.
A

[Dr.Spoke]

I regularly sail over hull-speed, in relatively moderate winds. Max speed ( GPS, no current) close-hauled was 7.4kts. The boat is 6.3m long, wind-speed 29kts, and about less than half the sail up.
On the run, Dobra Kusa can achieve over double hull-speed... But she's a real handful to control. I maybe push her a bit hard:D

[Bob Cleek]

Ian gave an excellent explanation. Andrewe alluded to a possible explanation for the tenth of a knot apparent discrepancy you noted. A GPS isn't a particularly accurate mechanism for determining hull speed. GPS will, at best and limited by it's own accuracy tolerances, only measure speed OVER THE BOTTOM. It does not take into account current factors which will add to or reduce speed over the bottom. One would have to devise an accurate method of correcting for the movement of the water itself before being able to rely on GPS as an accurate indication of hull speed THROUGH the water.

[Ian McColgin]

The other problem with GPS arises if the time between positions used by the instrument is too short. Depending on the GPS, the area you may be off by can be up to about a football field. If you have a short period between readings used for the speed calculation (3 seconds is not uncommon) and one or both reads are off in different directions, you can easily get a knot or so error and you may think you speeded up or slowed down for some odd reason.

[john welsford]

The other problem with GPS arises if the time between positions used by the instrument is too short. Depending on the GPS, the area you may be off by can be up to about a football field. If you have a short period between readings used for the speed calculation (3 seconds is not uncommon) and one or both reads are off in different directions, you can easily get a knot or so error and you may think you speeded up or slowed down for some odd reason.

If GPS is not doing it for you, there will soon be a much better system,
http://en.wikipedia.org/wiki/Galileo_(satellite_navigation)

John Welsford

[mcdenny]

I've been doing experiments with a trolling motor using a GPS to measure speed. Its (a brand new Garmin) instant speed was useless for trying to resolve 0.1 mph - it jumps all over the place. The fix was to use the trip computer function. Reset it and then check the speed after a certain period. I used 30 seconds, the speed fluctuated in the first 5 or 10 secnds but smoothed out quickly.