Displacement speed for planing hull

[Tranona]

Everyone seems to have forgot about tides. Working hull speed out is all well and good, but the actual SOG will depend on tidal effects. If hull speed is 6 knots and you're in an area where the tides run at up to 2 knots (just for instance), SOG will vary from 4 to 8 knots.

Real World trumps paper figures here, so increase speed until you start to create excess wash/dig a hole in the water behind the boat and then slow down slightly.

Note that poster is based in the largely tideless Med where SOG is essentially the same as speed through the water. It is common to cruise long distances at displacement speeds (8-9 knots) in planing boats like his. Otherwise going for example from the Spanish mainland to the island costs a fortune as well as being uncomfortable. other passages for example to places like Malta Corsica and Sadiniaa are not possible at planing speeds because of range restrictions. It is therefore a question of collecting data on fuel burn against speed to find the optimum.

 

[PaulRainbow]

Note that poster is based in the largely tideless Med where SOG is essentially the same as speed through the water. It is common to cruise long distances at displacement speeds (8-9 knots) in planing boats like his. Otherwise going for example from the Spanish mainland to the island costs a fortune as well as being uncomfortable. other passages for example to places like Malta Corsica and Sadiniaa are not possible at planing speeds because of range restrictions. It is therefore a question of collecting data on fuel burn against speed to find the optimum.

Doesn't matter where he is, if he cruises at a speed just short of creating excess wash/digging a hole, he'll be at max displacement speed, no matter what tides may or may not be doing.

I know all too well the difference between running a large mobo at displacement speed or planing. I would strongly dispute that running at planing speed is uncomfortable though. We can comfortably run at 20 knots or so in 25knots of wind over tide.

 

[Tranona]

I was responding to your point about tides. As for comfort I am only reflecting what owners have told me about the long passages across the Med where comfort is one of the factors affecting the choice to run at displacement speeds and my own observations of rarely seeing such boats running at planing speeds well offshore. This is not disputing the bility of the boat to handle such conditions as you describe, just the choice that many owners make.

 

[PaulRainbow]

I was responding to your point about tides.

I'm aware of that, but as i said, it doesn't really matter. Real World observations of what the boat is doing is a much better guide.

As for comfort I am only reflecting what owners have told me about the long passages across the Med where comfort is one of the factors affecting the choice to run at displacement speeds and my own observations of rarely seeing such boats running at planing speeds well offshore. This is not disputing the bility of the boat to handle such conditions as you describe, just the choice that many owners make.

Pretty sure the main governing factor for deciding on displacement speed or planing is cost. We can run at displacement speed using 25 litres per hour or full cruising speed at something like 250 litres an hour. Although 25-28 knots is perfectly comfortable, we do most passages at displacement speed for that reason.

 

[Chiara’s slave]

We obviously have the long slim shallow draught hull. There is no noticeable transition from displacement to planing, nor any ‘hole’ at what would be forced speed on another boat. We simply have a small displacement wave form at 7knots and a planing wave form at 8. This also is very small compared to your big fat MOBOs. Your wash is very useful to us if we’re going roughly the same way, we can surf that and hitch a ride, very fast sometimes.

 

[dancrane]

Isn't the question of whether diesel engines suffer from running for prolonged periods at much lower revs than their rated output, an issue? I'm glad, if it's not.

I'd had the thoughts described in this thread, in mind for ages. There are petrol outboard boats that I've always fancied, except for their notorious thirst at planing or semi-planing speeds...

...but if, as a matter of habit (and until I occasionally need or want to go faster), a biggish outboard is happy trickling the boat along at a peaceful idle without jaw-dropping lightening of the fuel tanks, I could easily be persuaded to ditch sail for a few years.
.

 

[Chiara’s slave]

Perhaps a lightweight small motor cat, Dan. You don’t need such big outboards, and 4 strokes are generally fine on small throttle openings. You might find a 1980s V6 2 stroke rather more trouble.

 

[PaulRainbow]

Isn't the question of whether diesel engines suffer from running for prolonged periods at much lower revs than their rated output, an issue? I'm glad, if it's not.

I'd had the thoughts described in this thread, in mind for ages. There are petrol outboard boats that I've always fancied, except for their notorious thirst at planing or semi-planing speeds...

...but if, as a matter of habit (and until I occasionally need or want to go faster), a biggish outboard is happy trickling the boat along at a peaceful idle without jaw-dropping lightening of the fuel tanks, I could easily be persuaded to ditch sail for a few years.
.

You're thinking of bore glazing Dan. Can happen running diesels for prolonged periods without being under load. Running at lower RPM at displacement speed, the engines are still under load. If i want to go particularly slow, say 6 knots in a river, i just use one engine, pushing 20 tones of boat along at 6 knots, it's got to be under some load.

 

[B27]

Isn't the question of whether diesel engines suffer from running for prolonged periods at much lower revs than their rated output, an issue? I'm glad, if it's not.

I'd had the thoughts described in this thread, in mind for ages. There are petrol outboard boats that I've always fancied, except for their notorious thirst at planing or semi-planing speeds...

...but if, as a matter of habit (and until I occasionally need or want to go faster), a biggish outboard is happy trickling the boat along at a peaceful idle without jaw-dropping lightening of the fuel tanks, I could easily be persuaded to ditch sail for a few years.
.

My dinghy club runs 4 stroke outboards, often at low rpm, for many hours per year, and our fuel bill is not huge.
But, we have relatively small light ribs, fuel consumption is closely related to displacement,
Mate of mine had a river cruiser for a while, proper displacement hull, something like an old cortina engine hardly used any fuel at the 5 mph speed limit or whatever it was.

OTOH, I've known knowledgeable people say that some (power) boats are poor at low speed, there's sweet spot where the transom is lifted a bit reducing drag.
Like with a dinghy, if you both sit at the back in light air, it's dog slow.

The reality seems to be, most power cruiser owners don't do big numbers of engine hours per year, so the other costs tend to eclipse the fuel.
The trick is to keep it somewhere where there's a good choice of nice places to go which are not too far away.

 

[dancrane]

Brilliant answers, thanks all. Glad to have clarification about diesel wear and loading.

I was actually thinking of the outboard version of the old 1980s Hardy 18, which is probably about as heavy and hydrodynamically inefficient as any boat its size...

...I had long enjoyed the idea of the motorsailer version, whose top speed is necessarily about five knots, being a displacement hull. With the recommended maximum 20hp four stroke turning a big prop, I'm sure it can reach top speed at fairly low revs, and in moderate conditions, probably only sips fuel...

...but, if the motorsailer (with its barn-door aerodynamics) can be driven frugally by that rather bigger outboard than is usual on an 18ft hull with sailing pretentions, I'm assuming that the planing, motor-only version of the same hull (said to be 660kg, compared with the motorsailer's 1,200kg) powered by a 50hp four stroke, could similarly idle along inexpensively at five knots, while reserving the option to go 20 knots if it matters.

The flexibility of such a motorboat is very appealing, considering the poor sailing ability its motorsailing sister, and the relative insignificance of fuel among so many other costs.

I guess my point is that as a sailor I'm accustomed to going very slowly...so routinely staying at hull speed in a planing motorboat wouldn't be an intolerable frustration.

 

[Chiara’s slave]

Fuel consumption of 4 stroke outboards is more closely related to power being used than top power outputs. Of course that doesn’t mean a 200hp V6 uses the same, at idle, as a 4hp at full throttle, but it won’t be far off. A 3 or 4 cyl. 40 or 50 is quite abstemious at low revs.

 

[B27]

Fuel consumption of 4 stroke outboards is more closely related to power being used than top power outputs. Of course that doesn’t mean a 200hp V6 uses the same, at idle, as a 4hp at full throttle, but it won’t be far off. A 3 or 4 cyl. 40 or 50 is quite abstemious at low revs.

Indeed.
And a boat like that Hardy, if memory serves, doesn't have a stupid amount of transom dragging in the water a displacement speeds.
Yamaha have been selling fuel injection 40 HP outboards for some years now, I think they are much more economical for pottering about than the old carb versions. There are probably a lot of carb models still out there though, in fact there's still plenty of two strokes working.

 

[Chiara’s slave]

Indeed.
And a boat like that Hardy, if memory serves, doesn't have a stupid amount of transom dragging in the water a displacement speeds.
Yamaha have been selling fuel injection 40 HP outboards for some years now, I think they are much more economical for pottering about than the old carb versions. There are probably a lot of carb models still out there though, in fact there's still plenty of two strokes working.

You can buy a lot of fuel for the difference in cost of an old model outboard and a spanking new one. My old RIB had twin 150 Yam 2 strokes, and was a total bargain to buy. Salt tears at tank filling time of course, but the thousands off the purchase price compared to one with modern engines was more than the extra fuel. Then there's the gut wrenching toque, unburstable power, and the noise, and the lovely smell

 

[dancrane]

The noise is something I'm keen to reduce if possible. I too have sat aboard dinghy-club safety RIBs with 50hp four strokes, burbling along peacefully for long inactive periods.

I doubt anyone who mainly wants to go fast, buys a boat as unnecessarily sturdy as a Hardy. I think I've read that the Hardy's lines and great weight prevent it planing efficiently, so I hope I would reserve about 40 of the 50 horses for sudden traffic avoidance or the option to shorten the long journey home (plus occasional mindless showing off).

By aiming mostly for fuel efficiency, I'm hoping I'd always enjoy the equivalent of best straight-line sailing speeds without any of the unpredictable delays, discomforts and other discouragements that stopped me getting value from a sailing yacht.

Although of course, I still want a sailing yacht, too.

 

[Chiara’s slave]

You could get a decent dinghy and a Hardy. Whats not to like? Solo? Something like that has a decent splash of adrenaline without being too demanding either afloat or ashore.

 

[dancrane]

Exactly my thinking. I mentioned it here to a motorsailer-fan about ten years ago. It occurred to me that one of those "mooring whips" popular in the States, would keep a derigged sailing dinghy secure at a short distance astern under tow, at least in moderate conditions.

When the wind's right and with mademoiselle helming the accompanying Hardy, nowhere would be far out of dinghy sailing range. Not that having such support is in keeping with the authentic dinghy cruising ethos, but I was never much persuaded by that comfortless airbed and boom-tent purism.

Sorry about the drift.