Planing speed threshold

[Simon 420]

We have a Princess 420 and thinking about some longer sea trips for the summer. Last year we went out in company on organised cruises, and just followed the group speed, but I'm trying to come up with the most economical speed for us to aim for when we're on our own.
Does anyone know where I can find out at what speed our hull will be/start planing please, and any other advice for the most economical speed to aim for
Thanks

 

[Bobc]

I would have thought that the easiest way to find the planing speed would be to take the boat out, get it planing to the point where you can wind the throttles back and hold the speed, and then look at the speed you are doing.

 

[dom]

Hi Simon and welcome!

Why not also post this question over in the diesel head section (Motor Boat Forum) as few of us here get anywhere close to planing speeds without our sails!

 

[lw395]

Can you fit a 'gallons per hour' meter?

I think your optimum speed will change as soon as the sea is not millpond flat anyway?

 

[PaulRainbow]

We have a Princess 420 and thinking about some longer sea trips for the summer. Last year we went out in company on organised cruises, and just followed the group speed, but I'm trying to come up with the most economical speed for us to aim for when we're on our own.
Does anyone know where I can find out at what speed our hull will be/start planing please, and any other advice for the most economical speed to aim for
Thanks

Easy to find out when it will start to plane, take it out and slowly increase engine RPM. There will be a point then more RPM doesn't result in any real increase in hull speed, but the aft end of the boat will start to "dig in". This will be when you are trying to exceed the boats maximum displacement speed, ease back on the throttles slowly until the boat levels back out and the turbulence stops, this is your displacement speed. Note though, it will be speed through the water that counts, not speed over ground (GPS speed). You'll find that in slack water your hull speed will be somewhere around 8 knots, but if there is, for example, 3 knots of tide against you, your GPS speed will only be 5 knots, if you turned the boat around, GPS would be 11 knots.

In flat calm water, your most economical speed will be wide open throttle RPM, minus about 400 RPM. So open the engines full throttle, let them settle down, slowly reduce speed by about 400RPM. Any more and consumption will rise very quickly. In most cases consumption also increases if you further reduce RPM my too much, as you expend a lot of energy trying to stop the boat falling off the plane.

For more accurate figures, always set off with full tanks, record you cruising RPM, distance covered, sea conditions and tidal direction/speed. Takes a few trips, but you soon have accurate data for various conditions.

 

[dom]

In flat calm water, your most economical speed will be wide open throttle RPM, minus about 400 RPM. So open the engines full throttle, let them settle down, slowly reduce speed by about 400RPM. Any more and consumption will rise very quickly. In most cases consumption also increases if you further reduce RPM my too much, as you expend a lot of energy trying to stop the boat falling off the plane.

Not sure I agree with this at all, save for a very specific engine/boat combo.

I'd be much more inclined to ask Princess for consumption data as a starting point.

 

[Spirit (of Glenans)]

Here is an interesting article on calculating "Hull Speed":
Crunching Numbers: Hull Speed & Boat Length - boats.com

 

[PaulRainbow]

Not sure I agree with this at all, save for a very specific engine/boat combo.

I'd be much more inclined to ask Princess for consumption data as a starting point.

I didn't expect that anyone would agree with me, that would be far to rare.

Even if i did say

For more accurate figures, always set off with full tanks, record your cruising RPM, distance covered, sea conditions and tidal direction/speed. Takes a few trips, but you soon have accurate data for various conditions.

 

[RJJ]

oP asked " at what speed will I start planing" in conjunction with a question about economy.

Replies so far have told him how to establish displacement speed. Correctly, but that doesn't answer the question. The whole point of planing is that you get over a "hump" of acceleration that gives you disproportionate speed for your extra revs/fuel.

Your fuel consumption is primarily guided by your revs, not your speed. You can look up fuel consumption for your engine at various revs, and then by experimenting (allowing like-for-like waves and weather) establish expected speed at various revs. Then you know your economy.

Typically, I would expect it to be something like "get her planing then reduce revs back as much as possible while still planing" per bobc (post #2). Whether that's actually more economical than displacement speed is down to the weight and hull form of your boat. Almost all boats "can" plane, but for a heavier "displacement " boat it takes (a) a heavy engine that the boat can't carry (b) a monster amount of fuel that's expensive and the boat doesn't have room for. That's why the biggest drinkers on the water are the semi-displacement brigade - just look at their wash.

I am just a dinghy and yacht sailor, but I have given this some thought regarding my outboard on our little planing rib.

For a really accurate assessment, do it empirically.

 

[RJJ]

PS once you are planing , "hull speed" has gone entirely out of the window.

 

[Beneteau381]

oP asked " at what speed will I start planing" in conjunction with a question about economy.

Replies so far have told him how to establish displacement speed. Correctly, but that doesn't answer the question. The whole point of planing is that you get over a "hump" of acceleration that gives you disproportionate speed for your extra revs/fuel.

Your fuel consumption is primarily guided by your revs, not your speed. You can look up fuel consumption for your engine at various revs, and then by experimenting (allowing like-for-like waves and weather) establish expected speed at various revs. Then you know your economy.

Typically, I would expect it to be something like "get her planing then reduce revs back as much as possible while still planing" per bobc (post #2). Whether that's actually more economical than displacement speed is down to the weight and hull form of your boat. Almost all boats "can" plane, but for a heavier "displacement " boat it takes (a) a heavy engine that the boat can't carry (b) a monster amount of fuel that's expensive and the boat doesn't have room for. That's why the biggest drinkers on the water are the semi-displacement brigade - just look at their wash.

I am just a dinghy and yacht sailor, but I have given this some thought regarding my outboard on our little planing rib.

For a really accurate assessment, do it empirically.

The fuel consumption is primarily guided by the LOAD. If full throttle is selected and If the prop is too big it wont get the revs but the consumption will go through the roof.

 

[PaulRainbow]

Your fuel consumption is primarily guided by your revs, not your speed. You can look up fuel consumption for your engine at various revs, and then by experimenting (allowing like-for-like waves and weather) establish expected speed at various revs. Then you know your economy.

Doesn't work like that, unless it's a petrol engine. If you are cruising along in flat water at 3000 RPM using x litre per hour, do you really believe that if the waves are 3 metres high you'll use the same amount of fuel per hour at 3000 RPM ? As stated in post #11, consumption is dependant on load.