Planing & Displacement

Nope was'nt you! However I thought that to get properly on the plane the boat had to climb the bow wave and get in front of it so that it was going downhill, a lot of the bioats I see are smotoring along still climbing the bow wave. I presume that if they applied a bit of power bunged a bit of weight forward then they could climb the wave and make things a lots easier and cheaper for themselves?

<hr width=100% size=1>.. when's that again, but ..
 
No silly. It's supposed to have the bow up a bit, then go up next wave and not down, belly flop, nose down, under waves and sink!!

<hr width=100% size=1> <font color=blue>No one can force me to come here.<font color=red> I'm a volunteer!!.<font color=blue>

Haydn
 
Not if done just right. As you come alongside raggie at full tilt, you trim the legs right out. This causes the boat to come down to semi displacement mode though at high speed, causing tsunami like wash.....and the really clever bit is the rooster tail which drenches them from above at the same time.

Awesome effect. However requires amazing skill and experience to pull off

<hr width=100% size=1> I asked an economist for her phone number....and she gave me an estimate
 
Sorry Jimi, but you can't fail to know when boats on plane, maybe didn't have it trimmed properly, but at 20+ knots would definately be planing.
Other explanation is he was a forumite and recognising Glen Rosa was just trying to make you feel more a home by giving you some big waves to play with/forums/images/icons/smile.gif

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When the 10mph speed limit is introduced on Windermere all the planing craft will be restricted to semi-diplacement mode with the susequent wash as you so poetically described.

This will upset the raggies more than ever and rather seems to defeat the object of the exercise, unless the idea is to get rid of all motor boats from the lake - surely not.

Ah well, that life

Martyn


<hr width=100% size=1>Do you think a Fleming 55 would look out of place on Windermere?
 
Throttling back?

ooh, you are a laugh, jimi.

To make it go faster, you push the throttles forward, and to make it go slower, you pull them back. Yes, there's a sort-of point where it's just not quite planing, and needs a bit more to get the hul out and with the boat *just* out of the water is cheaper/faster than *just* sitting right down. But on a short run in the solent, in won't make much difference. At faster speeds the boat sits higher and higher so i9t's only when you reduce speed from say 25 to 12 knots (it varies) that you really notice a dropping off the plane. But it's not like aeroplanes which reach crusing altitude and then can noticeabley reduce power (in any case i think it might be the yottie pilots who do that whereas thrashers in easyjet wham along) although you can trim a bit once planing, but not much.

Good fun is yotties having a go on a powerboat, and then after a long thrash they try and call over or in a marina they go over from the big powerboat to other yotties and explain that "actually, you know, I'm a yottie" and the other yotties think they are really really daft. Tee hee.

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Re: Throttling back?

So all this stuff about overcoming one's bowwave to get prperly on the plane is just a load of shite then?

I'm not having a moan about wash .. does'nt bother me .. just some powerboats when powered up and planing look a lot faster ,unstrained and create a lot less wash than those going just a bit slower and revving their guts out.
Are'nt those who are exceeding their hull max theoretical speed but sitting on the back of their bow wave having to work a lot harder but are going slower than those who have "broken through" and are sitting at the front of it?

Or is this just some sort of nautical myth?

<hr width=100% size=1>.. when's that again, but ..
 
Re: Throttling back?

Boat trimmed right or wrong might make a difference of say two knots. Less of course theres twenty fat folk sat on the back. Depends what size of boat your talking about. Smaller, say 22ft boats can get stuck on the hump with nose high in the air and making loads of wash.

<hr width=100% size=1> <font color=blue>No one can force me to come here.<font color=red> I'm a volunteer!!.<font color=blue>

Haydn
 
I have only planed a couple of times, the 18 knots was a special occasion, I was doing 9 knots on a broad reach (trying to keep up with the Whitbread boats) and I had the engine ticking over. I had a big wave get under her skirts, and opened the throttle wide, whoosh up to at least 18 knots (surprised the guy in a mobo who was trying to pass me more than somewhat!) /forums/images/icons/smile.gif. Dont normally press her that hard cause I am not in that much of a rush, but everything gets incredibly smooth, and she feels like she is running on rails

Her theoretical hull speed is abt 6 knots

Under engine I get abt 7.5 kts. (27 hp)

Directly into 40 kts of wind, I get 5 kts.

The wartime minelayers well exceeded their theoretical hull speed because of the power of their engines, but they were not planing.

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Re: Throttling back?

Um, yes, i think it is (in fact, i don't know of the myth anyway)

I agree that going faster they do look less "stressed" but this is an illusion. For example, on a boat that will do (say) 30 knots flat out, max cruise (hour after hour)might be (say) 25-27 knots. But the boat will use twice the fuel or more than if it was doing say 20 knots, so there no real "free" part of the graph where one would always want to be - going slower is alweays cheaper than going faster.

For maximum range, a planing boat shouldn't plane at all and i know several planing boats that have bimbled long distances at 9ish knots.

However, as you suggest, there is a fairly narrow gap of throttle setting between "just planing" and "just not planing" and of course it more mpg "just planing than "just not". This isn't a big issue for short distances, though the boat is easieir and smoother to handle at planing rather than non-planing speeds.

As hlb says, when planing, trim makes a difference of only twoish knots, i think on ours i can bog around with flaps to get 1 extra knot around 20 knots if i can be bothered, or maybe two at nearer 30 knots (if the bottom of the boat and props are clean)






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Hump resistance

so "hump resistance" purely relates to women and is not applicable to boats?

<hr width=100% size=1>.. when's that again, but ..
 
Re: Throttling back?

After having dig around on web, not so much as overcome but to get on top of bow wave, with present boat takes about 2500 rpm ( approx 13 knots) to get boat on plane, but with clean bottom 3000rpm gives about 30knots.
Normally adjust legs and trim tabs to give max speed, or more comfortable ride depending on sea state. Once on plane you can throttle back slightly if you trim boat properly, but not by much

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Re: Throttling back?

Starts to make sense .. you need a lot of power to get on top of the wave and keep it there but not a lot more to increase speed considerably? So a graph of power v. speed would show a steep gradient initially followed by a plateau then another steep gradient when air resistance kicks in .. presumably?

<hr width=100% size=1>.. when's that again, but ..
 
Re: air resistance

Probably would, but don't know as boat not fast enough for air resistance to be a problem

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Dave, I know you welsh can be a bit thick, but the resistance from a pontoon is completely different to the aatmosphere. One important fact often neglected by boaters is that wooden resistance kicks in at just above 0knots whereas air resistance builds up a bit more slowly. Hope that helps

<hr width=100% size=1>.. when's that again, but ..
 
Re: air resistance

no, jimi is quite right, i think - air resistance is proportional to speed, but somewhere around 100mph i think the air resistance become proprtional to the square of the speed. Probably wouldn't trouble yer average solent boater tho...

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