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

[Murv]

Apologies for the stupid question, but I really just don't understand!

So, theoretical boat, it's 20' long weighs two tons, has a 150hp engine and is capable of 30 knots at WOT which is 4000rpm.
As I understand it, the hull speed is restricted by the length of the boat, the boat is travelling in it's own wave with the start of the wave at the stern, ending at the bow. In this theoretical case, hull speed is 6 knots. At 2000rpm it does 6 knots, at 2,500rpm it also does 6 knots but the bow is high and at 3000rpm it also does 6 knots but pointing at the sky.
What speed will it plane at?
If it can only do 6 knots at 3000rpm, but 30 at 4000rpm, once it "climbs the hump" does the boat speed increase disproportionately? So, for example, 6 knots at 3000 but 15 knots at 3100rpm?
What would minimum planing speed be, does it mean that the boat cannot travel at, say, 10 knots? can it only travel at minimum planing speed or maximum hull speed with nothing in between?
One on the plane, if it's throttled back slightly, will it then drop back to displacement speed?

I know what I'm trying to ask, just not how to really explain it!!

Yours,
Confused from Swale

 

[fisherman]

We used to get just over the hump at 2200 revs, 14kts then could throttle back to 2000, less if there was a ruffle of wind on the water, until someone sneezed or we made a sharp turn.

 

[piratos]

Apologies for the stupid question, but I really just don't understand!

So, theoretical boat, it's 20' long weighs two tons, has a 150hp engine and is capable of 30 knots at WOT which is 4000rpm.
As I understand it, the hull speed is restricted by the length of the boat, the boat is travelling in it's own wave with the start of the wave at the stern, ending at the bow. In this theoretical case, hull speed is 6 knots. At 2000rpm it does 6 knots, at 2,500rpm it also does 6 knots but the bow is high and at 3000rpm it also does 6 knots but pointing at the sky.
What speed will it plane at?
If it can only do 6 knots at 3000rpm, but 30 at 4000rpm, once it "climbs the hump" does the boat speed increase disproportionately? So, for example, 6 knots at 3000 but 15 knots at 3100rpm?
What would minimum planing speed be, does it mean that the boat cannot travel at, say, 10 knots? can it only travel at minimum planing speed or maximum hull speed with nothing in between?
One on the plane, if it's throttled back slightly, will it then drop back to displacement speed?

I know what I'm trying to ask, just not how to really explain it!!

Yours,
Confused from Swale

First of all the boat must be able to plane. There are boats that are able to plane "good" and others "not so good"

Once a boat starts moving through the water it starts making waves. Has the boat reached the theoretical hullspeed she will have one bow wave and one stern wave.
Having reached the hullspeed and power is increased so is the speed. The boat will move up on the bow wave, and the more it moves on its bow wave the more effective will the power used be transferred in speed.
It requires a lot more power to move up on the bow wave than to stay at speed - therefore you can reduce the power once you have passed your own bow wave. In your examble there is simply not enough power to pass the "hump" and therefore the feeling is more like looking into the skye. Reason for the lack of power can be plenty - set up engine/propeller could be changed to easier climb the hump, but might give less top speed (Maybe turbo kicks in too late)

 

[rhino_mac]

I'm sure their is a science behind this but it's beyond me.

I can use my boat at any speed. 6 knots, 10 knots, 14 knots (when it starts coming up on the plane) or above. However, if I bury the throttle with 8 people on board at 4 knots, it buries its bum in the water and won't speed up. If I slowly increase revs, it increases speed until it eventually pops up on the plane, then it increases speed without increasing revs as resistance in the water decreases.

So you should be able to do 10 knots (for instance in Poole Harbour so you're within the speed limit, but still going as fast as you can) but, you will be burning loads of fuel because you're pushing the boat against an ever increasing wave. Once you get over that wave, the boat planes and life gets more efficient.

If you bury the swim platform in the water and stare at the sky, all the energy is in the wrong direction (up rather than forward), hence you don't speed up (you just lift a bit more of your bum into the air upwards).

So 6 knots is easy tickover speed, for 10 knots slowly increase throttles until you eventually hit 10 knots, for anything above, throttle as fast much as possible without burying your bum in the water.

That's what happens on my boat anyway (25' diesel sports cruiser)

 

[Murv]

That's fantastic, thanks very much all
That's the bit I really didn't get... the theoretical boat will cruise at higher speed when on the plane at lower revs than it takes to get up there. I still don't understand why, but then I don't need to, I just need to know that it does!
Although, Rkino_mac has of course explained it perfectly, as the resistance drops, the speed increases for the same rpm. I can sleep soundly now

So, if in frustration, I dumped the theoretical boat engine in the skip and replaced it with a twin turbo charged V8 developing 1000bhp, is there a danger that it could just point vertically if throttled forward too hard? is it a case of gently does it for all boats? Incidentally, theoretical boat is a planing hull, sorry, should have pointed that out!

 

[rbmatthews]

As a rough approximation the maximum displacement speed in knots is between 1.34 and 1.51 x the square root of the length at the water-line in feet. As you have calculated, for a 20ft boat, this gives you a max displacement speed of between 6.0 to 6.75 knots. If you go any faster than this, you are pushing up against the bow wave until you get over it when you start planing.

A formula for approximate planing speed V (in knots) is V = 7.2 * LCG / sqrt(B), where LCG is the longitudinal center of gravity from the transom (in m) and B is the planing beam (in m) (ignoring the mix of units!). Assuming that LCG is half the boat length (which it may not be), then for a 20ft boat with a 2.5 m beam this would give a planing speed of 13.9 knots. There are other equations for calculating planing speed based on displacement wt and power which are probably better and which I am sure other forumites can provide, but this one gives you an idea at least. It gives plausible figures for our boat.

So, to answer your question, you can do any speed in the range between max displacement speed and full planing (i.e. 6-13.9 knots in the example above), but it means that the boat is struggling away to get over the bow wave, and you will be using a lot more fuel per distance travelled than you would be either side of that range.

Hope this helps
R.

 

[lisilou]

That's fantastic, thanks very much all
That's the bit I really didn't get... the theoretical boat will cruise at higher speed when on the plane at lower revs than it takes to get up there. I still don't understand why, but then I don't need to, I just need to know that it does!
Although, Rkino_mac has of course explained it perfectly, as the resistance drops, the speed increases for the same rpm. I can sleep soundly now

So, if in frustration, I dumped the theoretical boat engine in the skip and replaced it with a twin turbo charged V8 developing 1000bhp, is there a danger that it could just point vertically if throttled forward too hard? is it a case of gently does it for all boats? Incidentally, theoretical boat is a planing hull, sorry, should have pointed that out!

I can only speak for myself but I take 'Lisilou' straight and quickly up to around 3300 revs then, once she's over the hump, drop her back to just under 3ooo. If I do it too slowly/gradually with her, she points skyward and labours hard to get over that wave. She doesn't like it and neither do I.
L

 

[Murv]

As a rough approximation the maximum displacement speed in knots is between 1.34 and 1.51 x the square root of the length at the water-line in feet. As you have calculated, for a 20ft boat, this gives you a max displacement speed of between 6.0 to 6.75 knots. If you go any faster than this, you are pushing up against the bow wave until you get over it when you start planing.

A formula for approximate planing speed V (in knots) is V = 7.2 * LCG / sqrt(B), where LCG is the longitudinal center of gravity from the transom (in m) and B is the planing beam (in m) (ignoring the mix of units!). Assuming that LCG is half the boat length (which it may not be), then for a 20ft boat with a 2.5 m beam this would give a planing speed of 13.9 knots. There are other equations for calculating planing speed based on displacement wt and power which are probably better and which I am sure other forumites can provide, but this one gives you an idea at least. It gives plausible figures for our boat.

So, to answer your question, you can do any speed in the range between max displacement speed and full planing (i.e. 6-13.9 knots in the example above), but it means that the boat is struggling away to get over the bow wave, and you will be using a lot more fuel than you would be either side of that range.

Hope this helps
R.

That is helpful, thank you Maths is not my greatest strength, I have to be honest, but it's clear that if theoretical boat has an incredibly heavy engine, that will have a substantial effect on planing speed due to altered LCG.

I can only speak for myself but I take 'Lisilou' straight and quickly up to around 3300 revs then, once she's over the hump, drop her back to just under 3ooo. If I do it too slowly/gradually with her, she points skyward and labours hard to get over that wave. She doesn't like it and neither do I.
L

So, the complete opposite to Rhino_mac!
Looks as if "suck it and see" will be the only way to determine the best way to get theoretical boat on the plane


Many thanks all, I now understand how it all works

 

[omega2]

just put the thing back together, and the go out, and see if you can pull the revs at wot, without any black smoke, also watch the boost pressure gauge, it may rise a bit but then again it may not because it is shot.

 

[Murv]

just put the thing back together, and the go out, and see if you can pull the revs at wot, without any black smoke, also watch the boost pressure gauge, it may rise a bit but then again it may not because it is shot.

Oh, don't get me wrong, that's very much the plan!
I just wanted to try and understand how it all worked. As a land lubber, if something is doing 6mph at 75% throttle, it can't possibly do 30mph at 100%! that's the bit I wanted to try and understand.
I'm not sure if you lost track on my other thread, but the CAC is now clean, resembled, and I'm just waiting on the prop to be returned to see how it goes

 

[rhino_mac]

I should have been a bit clearer - reading back I thought it wasn't quite right.

If my boat only has 2 passengers and a light fuel load, I can bury the throttle and it will plane quickly. If I have 8 people and full tanks (normal!), I tend to find using too much throttle will bury the bum and I end up going nowhere so I have to ease the throttles forward slowly.

Regardless though, the trim of the engine will always affect thrust. If you're staring at the sky, the prop will be pushing you upwards, not forwards. So keeping the boat at its optimum angle will surely propel you forwards with the greatest efficiency.

And regardless of all of this, trim tabs will affect how the boat planes and the angle of the bow. It's odd though, when everything is trimmed correctly everything just seems to be "right". You can feel when the boat is happy in the water and when it's stuggling.

 

[Murv]

I should have been a bit clearer - reading back I thought it wasn't quite right.

If my boat only has 2 passengers and a light fuel load, I can bury the throttle and it will plane quickly. If I have 8 people and full tanks (normal!), I tend to find using too much throttle will bury the bum and I end up going nowhere so I have to ease the throttles forward slowly.

Regardless though, the trim of the engine will always affect thrust. If you're staring at the sky, the prop will be pushing you upwards, not forwards. So keeping the boat at its optimum angle will surely propel you forwards with the greatest efficiency.

And regardless of all of this, trim tabs will affect how the boat planes and the angle of the bow. It's odd though, when everything is trimmed correctly everything just seems to be "right". You can feel when the boat is happy in the water and when it's stuggling.

Thanks for the clarification, that makes perfect sense

Even with the trim tabs fully down, theoretical boat was still staring skywards because it lacked that extra bit of speed to help them take effect.
I was wondering how to fully utilise the trim tabs when on plane, if it's something you can feel then I'll be fine

 

[rhino_mac]

I can feel when mine are badly trimmed without any issues. You end up leaning over to one side. You can actually tip my boat badly if you are trimmed wrong, then trim it even more wrong especially as you only have one hand on the steering wheel while pressing the tab button.

I have to apply a little left hand tab to keep the boat upright (single engine). Trimming fully down to get the boat on the plane is needed when fully loaded, but when up on the plane, I find tabs full down leads to poor handling when turning. The stern seems to "slip" or at least feel like it will. So once on the plane, I lift the tabs almost fully up, leaving just a hint of tab on the left to stop it heeling left.

It took me a while to get all this sorted in my head. I fitted Bennett trim tab indicators which are a wise investment. The rams move slower than I thought. It made trimming perfectly much much easier.

 

[Murv]

I can feel when mine are badly trimmed without any issues. You end up leaning over to one side. You can actually tip my boat badly if you are trimmed wrong, then trim it even more wrong especially as you only have one hand on the steering wheel while pressing the tab button.

I have to apply a little left hand tab to keep the boat upright (single engine). Trimming fully down to get the boat on the plane is needed when fully loaded, but when up on the plane, I find tabs full down leads to poor handling when turning. The stern seems to "slip" or at least feel like it will. So once on the plane, I lift the tabs almost fully up, leaving just a hint of tab on the left to stop it heeling left.

It took me a while to get all this sorted in my head. I fitted Bennett trim tab indicators which are a wise investment. The rams move slower than I thought. It made trimming perfectly much much easier.

Thanks for that, it gives me a starting point. Trimming when on the plane is some way off for me yet, but it seems a bit of a dark art to use the things properly.
I shall have a look at those indicators in the future, there is absolutely no way of knowing, currently, what they're up to. Except, of course, the view of the sky changes as they raise and lower!

 

[rbmatthews]

At the bottom of a very steep learning curve, gazing upwards, feeling confused...

So will you be changing your signature line now ...?

R.

 

[rosssavage]

And if you can stretch to it, bennett auto trim is a godsend when you're busy. Get the boat trimmed nicely and save that as a reference point. From then on, whenever you start to plane, hit auto and the tabs will sort themselves out (in pitch and roll) to get you back to your optimum trim automatically. It will compensate for different loading and wind direction and strength all hands off

Tbh it's quite expensive and I wouldn't have bothered had I not seen how good they are. Current boat came with them fitted, and if the next boat doesn't I would seriously think of installing them.

 

[Murv]

So will you be changing your signature line now ...?

R.

I just tried to! seems my lack of ability extends to editing my signature!

And if you can stretch to it, bennett auto trim is a godsend when you're busy. Get the boat trimmed nicely and save that as a reference point. From then on, whenever you start to plane, hit auto and the tabs will sort themselves out (in pitch and roll) to get you back to your optimum trim automatically. It will compensate for different loading and wind direction and strength all hands off

Tbh it's quite expensive and I wouldn't have bothered had I not seen how good they are. Current boat came with them fitted, and if the next boat doesn't I would seriously think of installing them.

I've just been googling them funnily enough...
Not too badly priced from the states, although shipping is a little eye watering. They do look very nice though.
As I say, it's all a long way off for me at the moment, but my tabs are in pretty poor condition so will probably need replacing at some point in the near future.

 

[Spi D]

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:

 

[rbcoomer]

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! )

 

[Spi D]

150 hp will make 2 tons plane if hull is almost flat and weight distribution is good.

When calculating weight ALL gear and crew needs to be taken into account. The easiest (and free) way to improve performance is to remove weight. As mentioned shifting weight has a double effect since you both remove it from one and add it to the other end/side.