Planing hull

[robertt]

No doubt a silly question, but if you have a boat with a planing hull, can you simply cut back the revs so that the boat doesn't rise onto the plane and use the boat in "displacement mode" in order to reduce fuel consumption and increase range, albeit at much lower speeds?
I imagine the stability of a deep vee hull might be less compared to a displacement hull and the wind might blow you off more easily but otherwise is this a feasible option?

 

[Renegade_Master]

yes indeed, lots of big boat owners having the boat taken to a dfurther destination by crew, get them to go at displacement speed to save fuel. Make sure it is dispacement i.e.
about 9 knots, dont go half way between at 11-12 knots cos this will burn more as the boat is trying to "climb the hill" as it were.

 

[robertt]

Brilliant, thanks for that, it helps me to decide whether to have a planing or displacement type hull, as I would like occas to go fast but would like the option of cruising longer distances without running out of fuel

 

[mm1]

Hi this is what i find so great about plaining hulls they can do all three modes very well,mine (a fairline targa) is great @6/7knts displacement,12/14knts semi disp & fantastic @ 37knts flying,not many boats do all three,regards mm1.

 

[robertt]

Thanks mm1, that sounds just what I wanted to hear. I saw a Fairline Targa 27 advertised locally but with twin petrols, is that what you have?

 

[Renegade_Master]

like I said though it needs to be in displacement or planing any in between i.e. 12 knots not quite on the plan, will negate the benefits of 3 knots slower.

 

[robertt]

So it sounds like really it has to be one or the other, and not in between

 

[Ripster]

So it sounds like really it has to be one or the other, and not in between

Yes it does. We have a planing hull and it will bimble along at 7-8kn at 1300rpm and do lots MPG (well more anyway!)- no superchargers or turbos in. If we try to do 12-13kns - we need about 2400-2500rpm to hold that and burn nearly as much fuel as would at 18-19 @2800-2900 sat nice nicely up on top. Take the revs up to about 3300 and we get about 23kns.

 

[jhr]

All speeds are possible - but some are more expensive than others.

As an example, my 22 foot deep vee mobo will do about 5 miles per gallon at 5 knots and 2.5 - 3 miles per gallon at 25 knots. At 10 - 12 knots, climbing on to the plane, I'd be lucky to get 1 mpg.

That's with a 5 litre V8; other engine types may return different figures.......

 

[robertt]

It's interesting to learn that, in the "in between" range the fuel consumption can be worse than when flat out on the plane

 

[mm1]

no sir mine is a larger targa with twin diesels & my response was all about the advantages of a plaining hull & nothing to do with economy!,however @ 12/14 knts is not the most economicle but in rough conditions it will get you home quite comfortably, for economy i would plump for a slightly larger single engine craft that can plain but would be very economicle @ lower speeds,something like a nimbus purely for economy you understand, i love my olesinsky(not sure of correct spelling)hull they are one of the best around,regards mm1.

 

[Chris_d]

It's interesting to learn that, in the "in between" range the fuel consumption can be worse than when flat out on the plane

Remember displacemnt speed will vary with the length of the boat, the equation is 1.1xsq root of the waterline length, so your Targa 27 will have displacement speed of about 5.5knts, it will probably do about 5mpg even with twin petrols. However at 7 knts that will be halved to 2.5mpg so you have to go at the right speed, in practice a small planing cruiser can do 0-7knts or 17-30+ knots but nowhere in between.

 

[jfm]

No doubt a silly question, but if you have a boat with a planing hull, can you simply cut back the revs so that the boat doesn't rise onto the plane and use the boat in "displacement mode" in order to reduce fuel consumption and increase range, albeit at much lower speeds?
I imagine the stability of a deep vee hull might be less compared to a displacement hull and the wind might blow you off more easily but otherwise is this a feasible option?

Yes, as others have said it's perfectly feasible

At the "awkward" speeds in between true displacement and true planing, remember it is not correct to say for example "my boat behaves like such and such at 13kts and does x miles per gallon". The behaviour at these "middle ground" speeds will be different depending on whether you have risen thru 9-10-11-1kts to get to(say) 13knots, or whether you have come down from 16-15-14 to 13kts. Invariably the latter will be better. So if you want to run at an awkward speed, go faster than it, then throttle back. Indeed you often find that if you climb to 13 knots the boat isn't planing and is burning loads of fuel struggling up the hump, whereas if you go to 20kts then come back down it will plane at 11 knots (or whatever precise speeds apply to your boat)

 

[MapisM]

it helps me to decide whether to have a planing or displacement type hull, as I would like occas to go fast but would like the option of cruising longer distances without running out of fuel

Well, if that 's what you want, there's not a lot to decide really.
I mean, we can argue for ages about whether it makes sense to buy a planing boat and use it at slow speed 99% of the time (and actually, we already did! ) Frinstance, imho it doesn't make a lot of sense, for various reasons, but each to their own.
At the end of the day, if you really need the occasional spin at 30+ kts (but do you, really?), you MUST go for P hulls, period.
It would take a nuclear power destroyer to go that fast at displacement...

 

[Elza_Skip]

The in between semi displacement speed can sometimes be useful in poor conditions. We found with our 25 footer that in moderate sea she would slam too much at planing speed but pitched and rolled at displacement. At semi displacement (c12 knots) the boat rode much more comfortably and got you there quicker allbeit burning loads of fuel

 

[Renegade_Master]

yup cos its trying to climb the bow wave

 

[carlton]

...the equation is 1.1xsq root of the waterline length...

Could be wrong, but I believe the formula is approx. 1.5 x sq. root waterline length, so simple examples would be...

20' loa (say 16' w.l.) = 1.5 x 4 = 6 knots max.

30' loa (say 25' w.l.) = 1.5 x 5 = 7.5 knots max.

42' loa (say 36' w.l.) = 1.5 x 6 = 9 knots max.

My 33' planing boat is quite happy to chug along at 7 knots and is economical at that speed.