Could someone please explain speed/revs/planing to me?

[Murv]

If the boat hull design allows planing it will occur at the speed where the hydrodynamic lift exceeds the weight of the boat, hence making it plane upon the water rather than going thorugh it.

Weight is a big factor, weight distribution even so. Planing hulls typically have best mpg at idle or at the lowest speed where it maintains a stable plane - also if helped by trim(flaps).
Any speed where power (=fuel) is used for other purposes than forward movement (like keeping the bow high) is considered a waste. So, full flat out until on plane and then back off.

Some designs have no planing threshold, they just glide onto the plane. A good sign of a clean and stable plane is the wash behind the boat. If you leave it behind you, you are on plane.

Notice the change at 0'37:

Fantastic, thank you I've been searching for videos of what actually happens when planing speed is reached, that's a superb visualisation

A couple of extra bits that may/may not be useful...

First, if budget doesn't stretch to Bennett, consider smart tabs. Similar principle, but not as sophisticated and thus considerably cheaper!

The type of planing hull can effect how quickly or otherwise you can plane - a very shallow V is more likely to plane easily as it naturally wants to skim on the water (think of days at the seaside skimming stones! ) - great when flat... The downside of this is stability and what happens when you hit a wave (or wake of another boat) - a deeper V will cut through the wave better whereas the flatter one will try to go airborne! Thus the deep-V not only helps stability, but is also less inclined to bounce over the waves and therefore makes a more comfortable ride.

150HP seems ok for 30kts to me on a 20' hull providing it's not too heavy.

If you are struggling to plane and 'pointing skywards' then you could also try adding weight to the bow as there comes a point when the stern is too heavy for the hull to work effectively. A simple way to do this is to shift as much weight forward as possible or even secure a container of water in the bow (if it doesn't work you can empty over the side!) When the stern sinks because the boat is too heavy (as per rhino_mac's example) this is because of prop slip and is akin to a front-wheel drive car pulling a heavy trailer - no traction. Thus planting the throttle gets nowhere - the marine equivalent of wheel spin except instead of burning rubber, you're creating bubbles in the water around the prop (known as cavitation - i.e. prop has nothing to push against)! A lower pitch prop will help, but has other implications like the possibility of over-revving the engine or not reaching top speed when lighter (e.g. only 2 aboard) (Using the car camparison, driving on the motorway in 2nd gear! )

All very useful stuff, thank you
Hopefully I won't have any problems getting on plane, the theoretical boat was to try and give me an understanding of what actually happens with revs/planing/cruising etc.
I've had long running issues with my engine which are hopefully now resolved. she should, hopefully, ship across the water like a frisbee when I get the prop back from rebuild!

 

[gordmac]

Basically between maximum hull speed and planing you are trying to push the boat up a hill, going on the plane is a bit like getting to the flat again after climbing the hill. When you have the boat heavily loaded you may have enough power to hold the boat on the hill but not enough to push it up.

 

[Billy Blue]

Basically between maximum hull speed and planing you are trying to push the boat up a hill, going on the plane is a bit like getting to the flat again after climbing the hill. When you have the boat heavily loaded you may have enough power to hold the boat on the hill but not enough to push it up.

So, where does a semi displacement hull fit into this?
Is the hull climbing/pushing that bow wave continually but designed to do it more efficiently that a planing hull hence the deeper keel?

 

[gordmac]

That is a question I would be interested in knowing the answer to.
With long narrow hulls the bow wave produced is small and can be easily pushed through (cat for example) but that doesn't really apply to semi displacement hulls.