Hull speed

[rbcoomer]

This might be a stupid question, but here goes... I am new to boating and am in the process of purchasing a falcon 27, she is a sports cruiser with a planing hull. Based in the water line length of roughly 24ft , if I chugg along at 6.5 knots will it be more economical than getting her up on the plane and cruising at 20 knots?

Clearly I would get from a to b quicker but I guess I want to find out which method will give maximum range.

Apologies to o/p for drift, but I think this depends on your hull and your own displacement speed. If your quoted 6.5kts is below your displacement then I'd agree with Carlton's unequivocal 'Yes' The closer you get to planing speed however, the more fuel you will burn and there will come a point where planing is more efficient than pushing that wall of water. Mine probably isn't a typical example being a small and lightweight vessel, but I've been surprised with my own findings. I've found that a day pottering on the river at 5kts consistently uses 0.9L per mile whereas a day on the sea - typically at 15-20kts (max is about 23kts) uses 0.7L per mile. There's probably a bit of tide and river current affecting both, but figures are consistent over a number of trips. My boat however starts to plane at around 6-7kts so I could perhaps use even less fuel at 3-4kts? For info, the boat is a 14' Fletcher ArrowFlyte power with a 30 hp Evinrude 2-Stroke.

You may also find displacement speed more expensive/less range when running against the tide...

I think you need you measure your own fuel burn at different speeds in order to find an optimum speed - or more importantly for planing hulls, perhaps the range to avoid in between?

 

[dj43]

Reference to regard engine ventilation and adequate supply of air for combustion,is it better to travel with fans on bringing in air, when underway,or off and allowing the normal engine room ventilation vents to supply air,for cooling and combustion,as reading this thread has got me thinking more air in is better,what do other,s do when underway.

 

[MapisM]

You may also find displacement speed more expensive/less range when running against the tide...

Nope, that's actually just a common misconception.
Actually very common, but still nothing else than an old wives' tale, sorry.
For any boat, no matter how fast it goes, going against a 2kts tide means that the MPG is reduced by 2 miles, it's as simple as that.
And the other way round when running with tide, of course.

PS: of course, if you meant that those 2 kts traslate in 20% less range when cruising at 10 kts, and only 4% less when cruising at 50 kts, that is true of course.
But I thought to specify because many boaters think that tide has little or no effect when on the plane, while it's actually the same in absolute terms.

 

[MapisM]

Reference to regard engine ventilation and adequate supply of air for combustion,is it better to travel with fans on bringing in air, when underway,or off and allowing the normal engine room ventilation vents to supply air,for cooling and combustion,as reading this thread has got me thinking more air in is better,what do other,s do when underway.

It pretty much depends on your boat.
If your e/r stays within 10° C or so above the external temperature also without fans, you can avoid to run them.
It can also depend on how you're using the boat: in most planing boats, when cruising at slow speed, the e/r temp actually gets higher, because the engines breath less air and therefore fans are necessary to keep an adequate air exchange.

 

[fireball]

Nope, that's actually just a common misconception.
Actually very common, but still nothing else than an old wives' tale, sorry.
For any boat, no matter how fast it goes, going against a 2kts tide means that the MPG is reduced by 2 miles, it's as simple as that.
And the other way round when running with tide, of course.

PS: of course, if you meant that those 2 kts traslate in 20% less range when cruising at 10 kts, and only 4% less when cruising at 50 kts, that is true of course.
But I thought to specify because many boaters think that tide has little or no effect when on the plane, while it's actually the same in absolute terms.

Technically correct - although you do have to consider the time factor that you're operating in the adverse current at the different speeds ...
If you get somewhere that has currents at 4+ knots then there is little point in sitting in that at displacement speed as you're effectively standing still - so if you can open up and get the hell out of that adverse current it will help - even if you do still end up burning more fuel overall ...

 

[Latestarter1]

It pretty much depends on your boat.
If your e/r stays within 10° C or so above the external temperature also without fans, you can avoid to run them.
It can also depend on how you're using the boat: in most planing boats, when cruising at slow speed, the e/r temp actually gets higher, because the engines breath less air and therefore fans are necessary to keep an adequate air exchange.

If you can achieve a delta T of 10° C is good most manufacturers accept a max of 17° C before rise in T.I.T impacts on performance. Repowers generally fail in this aspect.

Whenever possible I like extractor fans rather rather than forced induction.

 

[Elessar]

Nope, that's actually just a common misconception.
Actually very common, but still nothing else than an old wives' tale, sorry.
For any boat, no matter how fast it goes, going against a 2kts tide means that the MPG is reduced by 2 miles, it's as simple as that.
And the other way round when running with tide, of course.

PS: of course, if you meant that those 2 kts traslate in 20% less range when cruising at 10 kts, and only 4% less when cruising at 50 kts, that is true of course.
But I thought to specify because many boaters think that tide has little or no effect when on the plane, while it's actually the same in absolute terms.

Nope he's right!

For example, journey 20 nm.
D speed 5 knots.
D consumption 2 litres/mile
P speed 20 knots
p consumption 3.5 litres/mile

slack water,
journey time at D speed = 4 hours, 40 litres, 2 L/mile over ground, 2.25 MPG
journey time at P speed = 1 hour, 70 litres, 3.5 L/mile over ground, 1.28 MPG

adverse tide 3 knots
journey time at D speed = 10 hours, 100 litres, 5 L/mile over ground, 0.9MPG
journey time at P speed = 1hr 10 mins 81 litres, 4 L/mile over ground, 1.13 MPG

 

[Deleted User YDKXO]

Nope, that's actually just a common misconception.
Actually very common, but still nothing else than an old wives' tale, sorry.
For any boat, no matter how fast it goes, going against a 2kts tide means that the MPG is reduced by 2 miles, it's as simple as that.
And the other way round when running with tide, of course.

PS: of course, if you meant that those 2 kts traslate in 20% less range when cruising at 10 kts, and only 4% less when cruising at 50 kts, that is true of course.
But I thought to specify because many boaters think that tide has little or no effect when on the plane, while it's actually the same in absolute terms.

Yup agreed +1

 

[rbcoomer]

Oops! Now I've gone and done it! I'll get my coat....

 

[dj43]

It pretty much depends on your boat.
If your e/r stays within 10° C or so above the external temperature also without fans, you can avoid to run them.
It can also depend on how you're using the boat: in most planing boats, when cruising at slow speed, the e/r temp actually gets higher, because the engines breath less air and therefore fans are necessary to keep an adequate air exchange.

Mapism & Latestarter1 thanks for the explanation, sorry for high jacking your thread roy,as we have our 1st major trip planned for this weekend,with this boat,we shall be finding out,whats the best set up,trip is 102nm speed restrictions at start and finish,so going to run last leg with fans on.

 

[Elessar]

Yup agreed +1

-1!

You're right mpg over water but wrong mpg over ground which is what matters. See above!