What is 'cruising speed'

When I was gathering data to purchase our Monk, I ran across a old magazine article for a 42 Grand Banks with 375 hp Cat 3208TA's. It stated "She will cruise at 18 knots at 10 GPH". NOT!!!
Be very skeptical of any such claims, even by "experts".

For a displacement boat, hull speed (most efficient) is computed at 1.34 x square root of the water line length. On my Gulfstar, a water line length of 33 feet gives me a theoretical hull speed of 7.69 knots. When I first got the boat, someone on this forum suggested to increase rpm 100 at a time and watch the bow. When it starts to rise, back off because you're wasting money trying to climb over your bow wave. I found a sweet spot at 1750 rpm on my two 85 horse Perkins, which will give me 7 to 7.5 depending on wind and waves. I plan on 3 gallons per hour usage, and usually do better. Canal cruising at 6 knots or less does even better, hovering above 1 gph. Slow and steady wins the day (and the wallet).

For a displacement boat, hull speed (most efficient) is computed at 1.34 x square root of the water line length.

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Nope. That is the speed beyond which inefficiency goes up exponentially, which is what you describe later in your post.

Cruising speed for most FD boats will be well below hull speed. Cruising speed for a FD boat is only a very narrow range usually 1/2 to one knot below hull speed.

On this boat, a knot below hull-speed is at least twice as efficient as far as gallons per hour goes (less than two compared to nearly four gallons an hour at 6.3 versus 7.3 knots).

markpierce said:

On this boat, a knot below hull-speed is at least twice as efficient as far as gallons per hour goes (less than two compared to nearly four gallons an hour at 6.3 versus 7.3 knots).

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Fuel burn and speeds almost exactly like mine and mine is supposedly semi-displacement...but it leans heavily towards the line between the two.

With our mainship 400 (twin 240 hp yanmars) which I think of as sort of a hybrid trawler/cruiser I often get this same question. "What's your cruising speed?"

I always say that we have two: Slow cruise, which is most efficient, is at 1850 rpm, about 8.5 kts. Fast cruise, which we use when we want to get somewhere or when it is rough is at 2850 rpm, about 15 kts.

So for me there are two cruising speeds. Depends on the day.

Anything over 7 knots is unsafe, sucks the wind out of one's lungs. When we do see 7 knots we consider breaking out the skis!

Al

I looked on the can of ablative bottom paint I used this spring. It said NOT to exceed 7 knots or the bottom paint would slough off too fast. Who am I to argue? Or was it 7 coats? Maybe 7 beers between coats. I forget.

The NA of your boat knows what the cruising speed or speeds is.

But conversationally cruising speed is just the speed you run at most of the time.

Mine is 6.15 knots.

manyboats said:

But conversationally cruising speed is just the speed you run at most of the time.

Mine is 6.15 knots.

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At that speed, I'll get there five minutes before you.

markpierce said:

On this boat, a knot below hull-speed is at least twice as efficient as far as gallons per hour goes (less than two compared to nearly four gallons an hour at 6.3 versus 7.3 knots).

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At 700 rpm my boat cruises at 3.8 knots and uses 1.5 gph. I can save even more fuel if I don't exceed 2.0 knots. The biggest savings in fuel, however, is if I don't go anywhere at all!

Codger2 said:

At 700 rpm my boat cruises at 3.8 knots and uses 1.5 gph. I can save even more fuel if I don't exceed 2.0 knots. The biggest savings in fuel, however, is if I don't go anywhere at all!

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Don't worry about it. You knew what you were doing when you bought your boat. Surely, your fuel bill is doable. ... Am anxiously awaiting hearing about your next/current cruise!

I started sailing as a kid and began competitive racing in college. I continued racing on and off through several boats till about 2000. For that whole time 'cruising speed' was just how fast the boat happened to be going when not racing.

The next few years were spent battling cancer which I beat, but left me much less agile than before. I decided solo sailing was too difficult, and life was too short so I bought a Donzi to feel more alive again, yes, a mid-life crisis for sure. 'Cruising speed' for me then was a relaxed 50-55 kts with top speed well over 80.

I came to my senses a few years later and on a whim, I bought my present Fales 30. 'Cruising speed' is now 6.75 kts at 70% power. Top speed is 7 kts. and the fuel burn is negligible. This has proven to be too slow for the way I now find myself boating, mostly day cruising, where i am very limited in same-day destinations. I am searching for something small, economical and easily handled solo but, air conditioned with accomodations for the occasional overnight or weekend.

I am investingating the Mainship 30 Rumrunner Sedan. Looks like with the large Yammy, an easy 'cruising speed' of 17-18 kts and top of 26-27 kts is available at a relatively modest purchase price and fuel burn. This will almost triple my effective day-cruising range and greatly expand my choice of destinations. Also, shouldn't be too hard on the wallet, unlike the damage done by that scream-and-fly Donzi . . . which I do really miss sometimes.

So I guess my personal definition of 'cruising speed' really depends mostly on when you ask me, and what is waiting for me back in my slip.

Ok, just for the hell of it I'm gonna be controversial, by saying cruising speed is somewhat just below hull speed in a displacement vessel, and at the lowest revs that keeps a planing boat comfortably on the plane. There is no such thing as a semi-planing vessel..!

Not really…is there..? Sure the hull shape might sort of look like a planer, but there is no way that boat is going to truly plane. They just have enough power to almost plane, meanwhile pushing a larger and larger wave, or rather wall, ahead to it and dragging similar behind it, and consuming head swimming amounts of fuel. It also means they ride along with a weird bow up look and trim, because their boat is always trying, (but failing) to climb that ruddy great bow wave. Sure it probably feels great - nice and smooth - no hint to occupants of the devastation their wake can leave behind them. I'll add in there, if they are not careful, because some actually are. But many are not.

So...as my crew and I grab for anything potentially launchable/breakable from the huge rolling experience thereby imposed upon us slower (displacement) dudes, I swear just a little, but comfort myself by thinking - ok enjoy your ride my boating friend, (note I call him friend, 'cause I'm nice really), but you (my friend) are using as much fuel today as will keep me going for a year…so there..!

Interesting thread

30ft 4.5 ton 120hp Perkins (max speed 10knots )1100rpm 6.8knots 2gph engine is smooth little vibration

Same boat 165 turbo Volvo (max speed 15 knots just on the plane) 1600rpm 7knots a touch under 3 gph engine feels to be under load


Suppose you need to take into consideration gear ratio and prop pitch

CRUISING SPEED is the speed that suits better and comfortably both your pocket and your boat

Larry,
Seems you were a good person to ask about speed.

Peter,
Any boat that exceeds hull speed is clearly not a FD boat .. unless a severly over powered one. And enters into a hydraulic and dynamic state of velocity that is partly planing (semi). Identifying at what point it emerged out of the semi-planing/not FD and entered the world of "planing" ...... that would be much harder to identify.

Many that know much about this identify that amount of speed as a function of the speed/length ratio.

Your center paragraph clearly defines a typical trawler ..fully intended I assume.

manyboats said:

... fuel consumption has nothing to do w what speed the hull was designed for.

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????

Picture one sitting a dock

Picture two at 7.5 rpm =980

Picture Three at 10 rpm= 1355

Picture four at 11.5 rpm=2125