To plane or not to plane...

Greg2

Greg2

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Much has been said recently about the relative merits of semi-displacement boats with the possible loss of the derogation on red diesel i.e the ability to travel slower and use less fuel.

It has also often been said that a planing boat needs to plane in a seaway. Is this true?

Having had a semi-displacement boat (Broom Ocean 37) in which we cruised at 10-12 knots I know that it could roll a lot and was prone to being wet in a head sea. I have never had a planing boat but in a recent sea trial in a 38 foot flybridge Princess in lumpy conditions I thought it felt more comfortable than our Broom and handled perfectly well at less than planing speed. It didn't roll as much and didn't take any water over the bow (nor did it slam).

So, with the more economical performance of a planing hull at speed are semi-displacement hulls really any better for 8-12 knot cruising than a planing hull?
 
I have often come down to 8 knots or less in really bad seas, the boat stays quite comforable, bit amazing as once the window screen was covered in green sea for quite a few minutes. Think the only trouble is with head seas. A plaining boat is designed to go over the top, which makes the wave twice as big as going through it. On the other side. In a following sea, a modern planing boat is much, much better.
 
totally agree with everything hlb has said (scary or what?)

would add that the last sentence you have posted is not the case however - although this will be dependent on the mode the boat is traveling at so it's down to speed v waterline length. At 8 knots a 45ft boat will be in displacement mode regardless of hull design so economy would be similar but the finer entry on the semi's hull would probably give a smoother ride. But at say 12 knots and 40ft the planning hull will be chewing up a lot more fuel for most hull designs being right on it's resistance hump than a true semi displacement. Put simply within +/- 10% of the boats hull speed hull shape could significantly affect the relative fuel consumptions - and for that reason most fully planning craft are not operated at those speeds except when adverse conditions are experienced and they can be the most comfortable (on everything but the wallet!)
 
If it's bad, and folk need to get down from fly bridge. I turn the boat with sea behind, slow down, or can even stop. The boat dont move, gentle up and down maybe, but no lurching.

8 Knots is enough speed to keep the boat stable. Biggest problems are with short steap head seas where the bows are lifted more than one would like and you run the chance of going airbourn. So I'd say, maybe the semi or displasement is better if sea is right on nose. But if it can be kept a few degrees off. A GOOD planing boat will beat it hands down!!
 
The following seas bit is, I thnk, standard thinking. I am puzzeld by it all though.

The D hull boat would be expected to behave much better at displacment speeds than either an SD or planing. I just cannot see how a planing hull, with the shape it is behaving anywhere near as good at displacement speeds than a D or SD hull.

In a following sea, I just made a trip in a f7/f8 in a deep V planing hull boat at cruising speeds of 26 knots - sometimes 32 knots with surfing assistance - and that was with no slamming. So there is little doubt that the planing hull does perform well in a following sea.

The same trip saw the same boat going into a current and wind with a full head sea and we had to slow down to 17 knots in a f6, we would have had to be down to less than 10 knots in a f7/8. At these speeds the planing boat does not handle as well as the D or SD boats.

I am waiting to see how an SD hull with stabilisers and the option of doing 25 knots will perform in a following sea..... it will be interesting.
 
But would the planing hull be chewing up more fuel than than SD at around the point it gets over the hump?

In our SD boat fuel consumption doubled between 10 and 12 knots (from 4.5 to 9 gph). Had we pushed it further to cruise at say 15 (we never did due to age of engines) I wouldn't mind betting that we would have got a similar increase in consumption because the boat never truly planed.

I think SD hulls may handle a head sea better but I am inclined to think that they use as much fuel to cruise at say 10% below planing speed in a planing hull.
 
I have calculated on my new SD boat that up to 18 knost it uses less fule than my current planing boat but over 18 knots it uses more fuel. Crusing at say 14 knots shows a considerable saving and my planing boat 14 knots is on the hump and not efficient at all.
 
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The D hull boat would be expected to behave much better at displacment speeds than either an SD or planing. I just cannot see how a planing hull, with the shape it is behaving anywhere near as good at displacement speeds than a D or SD hull.

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I tend to agree but is there a diference with the type of planing boat?

Planing cruisers with outdrives can sometimes be hard work at low speed but a slightly larger planing boat with shafts seems to handle better at low speeds.
 
Interesting. My boating is in the 37/38 foot range so perhaps larger planing boats are less efficient than smaller ones?

I have nothing to support this but I wonder if the power/weight/economy stats show a steady increase or whether there is a point at which there is a marked decraese in economy.
 
Even big ships are down to six knots with strong head seas. I'm not saying planing boat is beter, all I know is that my boat, only 35 ft is perfectly happy with at least F 7 behind it, maybe more. It dont like it on the nose though!! Anywhere else it seems like the bigger the sea the better. It dont like short sharp chops, or maybe I dont!!
 
Planing boats achieve their high speed by lifting out of the water and reducing the hull drag. When they are just over the hump they have the cost of much of the lift without the benefit of too much of the reduction in drag. Fuel consumption tends to be about the sane once over the hump until almost at top speed then increases sharply with the last few knots to get top speed.

Seems to me there is little sense in going slow in most planing boats - if over the hump take them up to full crusie becaise you will get the same fuel consumption.

Outdrives.. they may be a bit different.
 
simple answer and huge generalisation is yes - almost all the consumption curves have a steep rise approaching the hump speed then linear for planning hulls and a lesser 'steep rise' then linear or possibly a slight upward curve for the SD.
Absolutely agree it's still a pretty steep increase in that area for SD hulls as well - they can't avoid the resistance in that area and you only have to look at their wakes to see they are 'wasting' energy too.
 
I agree on the hlb, you never get two seas the same.

All designs are a compromise the main reason I have gone over to Sd is because of its smooth ride in all normal seas and the comfort it offers my passengers.
 
Hi Greg I posted recently about handling in head seas (cant remember the thread) and stated that I found my current planning boat in head seas down to about 8-9 knots is, if anything, more comfortable in that it rolls less than the SD Crown I had. (same lengths overall)

I also stated that the amount of roll can be lessened by good helming i.e. use of throttles and angle to waves etc, and it is possible that its just my helming is better than when I had the Crown.

The SD was however naturally wetter in lesser head seas.
 
Yes but what would the crown have been like with stablisers on?

I think by their wave cutting nature SD boats must on average be wetter than planing boats.

Akk three types of hull and I guess that they are not distinct but shade into each other, are a compromise. In an Sd boat in those heads seas you slow down to you could probably get twice the speed.
 
Planing boats develop hydrodynamic lift in order to plane and it is this lift that resists rolling when above hump speed which is why a planing boat rolls less than a D or SD boat at speed. Conversely at D speeds you would expect the planing boat to roll more principally because it usually has'nt got a large keel, as fitted to D and SD boats, to resist rolling. Yes, planing boats need to keep planing to resist rolling
As regards head sea performance at D speeds, I think its too much of a generalisation to say SD boats are always more comfortable. Some SD boats have hard chine hulls which look much like planing hulls to me and therefore you would'nt expect a significantly more comfortable ride than with a planing boat into head seas at D speeds. Other SD hulls have much more rounded hull shapes specifically designed to plough into a head sea but because of their rounded hull shape, they can roll a lot in beam seas.
What makes a difference with most SD hulls IMHO is weight. Because they are not designed for speed, there is less emphasis on weight saving so, length for length, SD boats tend to be heavier than planing boats. Weight means a more comfortable ride into a head sea
 
I agree with most of that.

The trouble is there is no clear distinction - some SD hulls are much nearer to a planing hull than they are to a D hull.

Weight does matter - and in general, I agree, length for length, SD and D boats are heavier.

I find that planing boats do roll a lot more at slow speeds and are uncomfortable for that reason.
 
Yes you are right. What I would call a traditional SD hull is something like the relatively narrow beamed round bilged Nelson/Aquastar hull which only flattens well aft to give some lift. Your Trader hull is beamier and has much squarer chines which look to me as if they are designed to provide more lift than the round bilged hull. In fact you might call the Trader hull semi planing rather than semi displacement. The squarer hull should resist rolling better than the round bilged hull even without stabilizers
 
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