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Old 01-08-2006, 06:56 PM   #1
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question

just been thinking.....
what effect does the shape of the front of a boat have and the postion of the spray rails,for example the front of my boat is quite blunt and rounded and the spray rails are low down,the ones ive seen on an early phantom 18 seem to almost go right up to the nose and there are more of them
see pic below
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Old 01-08-2006, 08:11 PM   #2
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I would leave them off. I you keep them than same place but smaller to each other. Not so width as now.
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Old 01-08-2006, 08:14 PM   #3
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Quote:
Originally posted by bigboy
I would leave them off. I you keep them than same place but smaller to each other. Not so width as now.
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Old 01-08-2006, 08:15 PM   #4
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Quote:
Originally posted by michaelcaine
any ideas jf ?
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Old 01-08-2006, 08:40 PM   #5
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Quote:
Originally posted by bigboy
I would leave them off. I you keep them than same place but smaller to each other. Not so width as now.
bigboy i think michealcaine is talking about the spray rails, not the stainless ones on the deck.........
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Old 01-08-2006, 10:11 PM   #6
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The deeper and sharper the vee of the boat, the better it'll cut through the rough, my picton hull is designed to produce a large flat wake for skiing behind so slams in the rough.
However, i think a sharper vee is more likely to hook as it cuts into the water as opposed to skimming the surface.
Pretty sure the spray rails are designed to keep the boat on a straight course so the twist from the prop doesn't make it "crab" accross the water, not sure what other effect they have,
hope this has been of some help, anyone please correct me if i'm wrong...
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Old 01-08-2006, 10:30 PM   #7
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Some people just love a real damp ride
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Old 02-08-2006, 12:04 AM   #8
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AFAIK the spray rails are there to reduce the wetted area of the hull; they deflect the water outwards reducing friction on the hull.

The downside is that more or larger spray rails reduce the 'vee' of the boat, giving it a harsher ride.

The different styles of rail/bow design are different speed/handling compromises.

Probably.
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Old 02-08-2006, 01:18 PM   #9
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Pretty much. A correctly designed spray rail adds lift and stability in all 3 axis'.

A hull with none will plane OK, but would likely be more difficult to get on the plane, and be less stable the faster it goes. If you look at quick, light boats, (Chris Dodges P28 - see the poker run photos for example) pretty much runs on the rails.

What you need depends largely on what you've got - ie deadrise, warp, pad, keel, length, beam, weight & speed all have an impact.
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Old 02-08-2006, 09:00 PM   #10
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so whats the difference between ones that run under the hull and ones that run along with the hull?? like an ocke v a sorcerer or whats the point of ones that sweep up in the bow or ones that stay levelish??
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Old 02-08-2006, 09:35 PM   #11
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You choose to have ones that stay level, if you like the exitememnt of a violent 'hook' every once in a while.

Ring style



Some Cougars/Phantom style



Some Cougars/Shead/Ocke style

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Old 02-08-2006, 10:19 PM   #12
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Does anyone know of a good book that covers this stuff? (Design of small, high speed motorboats)

I'm guessing the answer will be that there isn't one (too small a potential market)
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Old 02-08-2006, 10:19 PM   #13
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ive only got 4
but more like the second pic..... i think
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Old 03-08-2006, 06:18 PM   #14
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yeah but whats the difference??
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Old 03-08-2006, 06:50 PM   #15
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Are there any design books on the subject?
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Old 03-08-2006, 07:38 PM   #16
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Quote:
Originally posted by glen76
yeah but whats the difference??
Designers preference.

I personally don't like type 1 (horizontal), I think it makes for a boat with 'hooking' tendencies.

You get a situation where one side of the bows, or the other takes a major deflection, and the horizontally mounted, and thus curved round towards the centreline type rail, 'grabs' and tries to steer you into a roll & broach.

The mid style is pretty neutral in it's behavour, and the vertical style, in theory, should try to recover, rather than exagerate the roll.

'mid' & 'vertical' do it for me.
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Old 03-08-2006, 07:43 PM   #17
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There are lots of general boat design books, but very few specifically covering hi performance planing hulls. Dhows and Deltas would be a good one (rare as rocking horse shit), other than that I don't know of any I'm afraid. I'm sure there are some out there.
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Old 03-08-2006, 08:56 PM   #18
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Quote:
Originally posted by Matt
Dhows and Deltas would be a good one (rare as rocking horse shit),
Found a copy here .

Only USD 325 (172)
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Old 04-08-2006, 01:15 PM   #19
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by roll do you mean side stabilty or bow steer?? Jon can i nick ur pics in a quest for more info cause i have a bit of an obsession with not understanding spray rails!
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Old 04-08-2006, 02:32 PM   #20
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I believe, (based on real events & theory) that with the horizontal style rails, especially if they're particularly pronounced toward the stem, that 'bow-steer' & 'roll' are directly linked.

Picture the style, with the rails staying horizontal, meaning, that to join at the stem as they do, they have to curve around the forefoot.
Now imagine that in a seaway, at speed, your nose goes in hard-ish, but as there's a wave in front of you, one side (we'll say the port side for now) gets the lions share of the wave.
Now, those heavy rails, curved from out near the chine, right round and into the stem, dig in and grip the water and want to follow their lead as best they can, trying to take the bow with them.
As they're low down on the boat, with plenty of 'mass' up above them, the boat will tend to roll 'out' in the motion, or high-side' for you bikers, as it does this, those offending rails are presented deeper, and firmer into the wave, a vicious circle!
Kind'a like catching the outside edge on a ski, before you know it, the ski's, and your legs, have gone from under you, and the high up mass (your body) want's to carry on the way it was going, so you effectively 'roll out', just as I said above.

If this motion is violent enough, and the back end loses grip, the heavy end will eventually wanna overtake the light end, which now has it's well pronounced rails dug firmly in the wave like brakes.

Of course, all this happens in a split second. Ask Steve Salmon


In 1997 on Windermere, I had Tony Donald in the boat with me (donziman on here) for a record run.
I was attempting to show him how the hull of the 26 (with it's 18 degree transom, and megga in-trim) could be quite alarming if the power were applied, whilst fully trimmed in. (see 'glove' thread)

Once we'd finished the first run, and were making our way to the far end ready for the run-up & return run, I demonstrated this by trimming fully in, and opening the throttle fully.
The torque reaction of the prop, lays the boat over on it's port side, digs the rails in (aided by the nose weight of being trimmed in), and the 'process cycle' as described above happens so quickly, that on a flat lake, in a straight line, I get the rubrail wet, and end up facing about 45 degrees to starboard, all in a split second.
I knew it was going to react, that's why I did it, but never for one second though such a violent reaction was possible in flat water.

It genuinly felt like had I continued with the power on, it would have gone over.

It's noteworthy, that the transom angle on that particular model is well beyond that reccomended by Mercruiser, and that we did have 460hp on tap, but the theory is still the same.
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