grael

Mineralvan, this comment isn't about the V or the intake, but about the bow section.

You have, essentially, two almost straight sections with what I look at as being a bit of an abrupt "knee" joining them, the long bit underside (the keel), and the section at the bottom of the bow.

It's usually better to flow that knee over a longer distance, that way you lengthen your planing area, reduce the unsupported bow area (and weight), reduce the wind assisted flip tendency, and reduce the tendency of the boat to bog when it hits a wave. (because it has to dip quite a bit before getting lift)

However, it's also good to do things the other way sometimes, to learn

Graham.

sundogz

Yes, the Delta is the intake pad. Your last drawing is good. The v-hull (keel) continues towards the intake where it flattens and angles up to the pad. This arrangement discourages cavitation and gives good steering control. Here is a pic of the jet ski hull showing that angle and delta. On the Bat boat, the angle is elongated but is the same principle.

marshr32

Really, the delta is the triangle shaped (like the greek letter delta) bit that goes from the front of the intake pad to the keel. I wouldnt angle it at 30 degrees, instead make it as long as possible, to where the bow starts to curve up.

Personally Id move the windscreen forward to get more access to the hull and pull the chines up a bit higher at the front to stop it plowing in rough stuff.
Otherwise I think the lines of the hull are fine.

marshr32

Full size sprint boats are actually pretty similar to your design (see pic)
The chines join the bow about where the winch hook is.

sundogz

Thanks Marshr, I've been calling the intake pad the delta. Sorry for the confusion Mineralvann (my bad). I got that from the triangle you get when you taper a V-hull to the transom for an intake pad. The delta (now being the transition 'triangle' from the keel to the intake) with the 30 degree abrupt change is already used on the knockoff and the jet ski with success. My point being that the intake should be recessed from the level keel line and not be incorporated in a tapered delta. On the Batboat, the Powerjet's installation instructions suggested that the transition (delta) start much more forward on the keel (like Marshr's drawing) which may turn out to be the more desireable design. Both designs have merits.

sundogz

I've been calling the intake pad a 'delta' ever since ISIRC10 built his first boat with the large tapered triangle (delta) intake pad (which he later sent me). Sorry if I confused anyone - live and learn. Anyway, that design doesn't allow much of the keel to be in contact with the water which may cause drifting or slipping when turning - and may require a fix such as a chine down the center to the keel. That's why the recessed intake design works so well - the keel stays in contact with the water. Does any of this make sense?

sailindude

hello,
i have a pair of jet drives ( mini) and before i fit them id like to know how to work out the position of the servo and how to attach it rods ect and drilling holes... im stuck

pleese help
regards
matt...

TwinJetMax

Hey

I had a redesign of my hull after reading some of the delta/intake pad stuff.
I just want to know which design people see as the better out of these two.

Cheers
Max

sundogz

Twinjet, I'm not too crazy about either one of those, but the one with the flat keel has reportedly been done with good results. Check out the past few posts for some worthy ideas. Sdg.

grael

Personally I'd go for the one with the long flat strip. It's got less drag.

However, the other one will track better, except when it is spinning out, and it will spin out more than the one with the long flat strip.

That's because when it(with the triangle) is drifting, the intact keel section will give more drag, and the stern will drift faster. Because the triangle forms a different angle than the two main planing surfaces, that uses up slightly more energy running. (looking side on.)

marshr32

Yep, I'd go for the new one too, i like it.
As Grael said it will be faster in a straight line and will probably not want to do the hook and spin thing so much. Plus thats the way fullsize jetsprint boats are done and they must do it for a reason!
Downside is it wont be so good on rougher water, might catch a bit more air?

Sundogz, your pic is exactly what I meant with making it longer, means the water doesnt have to turn so sharply into the intake.

To be honest with the power to weight we have it probably doesnt make a huge difference if you have a long delta, or short delta or no delta; just cut the intake of the jet unit into a vee shape.
Think of sitting the hull level on a table, cutting a hole big enough for the jet to go right thru to sit flat on the table, then cutting off all the bits of jet that are sticking out under the hull. This would also give you a huge V shaped scoop at the back of the intake and mean that the jet is really low so it primes quicker.

sundogz

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By the way, if you ask them when placing an order, they'll send you free HobbyKing decals. Good policies, both.

headhunter23

Ummm.. just a tip on extending the delta. On a couple boats I had tried this, I never had luck with it. When I did so, 2 boats became skippers. And one of the same boats started to wag, which is more than likely due to the delta not being completely flat. It is very hard to get it perfect. It looked flat to me but obviously not.

My suggestion, don't do it. Stay with a short delta. It's very messy and can also mess up ur hull quite easily when you cut so much of the hull and weaken it before trying to glue/epoxy it back together correctly.

Ivan.

grael

cool add Saw the shirts before. 20 cents off also !

sundogz

Twinjet, here is a video of a jet boat with a hull like your #2 hull with the triangle intake pad. Not a good choice either. <object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/Gjls32JnX9k&color1=0xb1b1b1&color2=0xcfcfcf&feature=player_embedded&fs=1"></param><param name="allowFullScreen" value="true"></param><param name="allowScriptAccess" value="always"></param><embed src="http://www.youtube.com/v/Gjls32JnX9k&color1=0xb1b1b1&color2=0xcfcfcf&feature=player_embedded&fs=1" type="application/x-shockwave-flash" allowfullscreen="true" allowScriptAccess="always" width="425" height="344"></embed></object>

grael

That looks like a pretty frustrating run. Do you know if it's using an outrunner ?

This one of mine, if you watch very carefully, twitches clockwise,anticlockwise around the long axis of the boat each time it takes off and lands.

I've redone the bottom of this hull for more edge lift at the stern, and higher up centre lift from the stern to 3/5 towards the bow in a stretched withdrawn V section. The shape of the stern to 2/3 towards the bow seems to be critical, also moving the jet intake forwards does a lot to calm a poorly trimmed jet unit. Running the jet in a large shroud has a number of positive effects:
~Preventing forward spray into/over the boat
~Reducing rear lift for a softer ride.

I have a fairly powerful motor on delivery from HK by slowmail, which I'm going to pair with 3S 2200mAHr 35C batteries, boat weight is 149 grams without any gear,
I'm going for a build with another 400 grams of gear weight inside, 109gm motor, 199gm battery, 80gm besc, receiver, servo and jet unit. Not sure how it will run... this is my 350mm long skinny boat.

TwinJetMax

To be quite frank, the hull in that vid is a very dissimilar shape. Kind of comparing apples with oranges.

It doesn't matter though because I am going with the other shape. it has less freeboard, more lift and should be faster.
I have started my 4th attempt at a plug, and I am very happy so far. Have been planking balsa frames that were printed out and cut.
After it has all been finished, including the deck, I will cover it in epoxy resin with black pigment and polish it until is shines. then take my mold, in two pieces.
will get some pics up later tonight.

grael

Hope it goes well for you Max. I find the resin tends to round off corners I want sharp.
The other thing to remember, is that if you want planing strakes on your huls, then it's easier (on a good mould), to create them in the mould in the first place. Most of the model shops have small PVC extrusions, I use a small triangular cross section for mine.

However, you can get more strength, better mouldability, and less weight if you do a stepped hull instead for the strakes. The fibreglass doesn't have to deform as far, and when it's set, the different levels of the steps reinforce each other longtitudinally better than normal strakes do.

I'm a bit in limbo with my boats now, I picked up a 50amp 2s-4s brushless ESC, new RC set (6 ch, 2.3G) 2S 1AHr batteries and a small 3000kv motor, I'm waiting for my big motor and an even bigger ESC to arrive, but the supplier was out of connectors.
I've figured out that my newest small jet unit needs a redesigned outlet, because I was getting more power with the previous model on a 2S battery, and the motor's hardly loaded on this one.

I tried loading my small skinny boat with 400grams weight, and it's sitting pretty low[]

Looks like I'd better do some more work on my CNC/extruder machine !

sundogz

No, it was one of those inrunners with the integrated cooling jacket. Here's the link [link=http://www.rcgroups.com/forums/showthread.php?t=1126424&page=2]triangle intake pad hull[/link]
He finally resorted to a trim tab which tamed it down. Similar to my BBY OMC21 project. I've since stripped it for other projects (if anyone wants the hull, pm me).

I'm convinced that a hull with a recessed intake pad (with a delta of whatever length) won't have those problems - at least not to that extent. Examples are the NQD Tear Into (Graupner knockoff), the ABC Jet Ski and the Viper Bat boat hull. You can overpower these hulls to varying degrees of success without trim tabs (via weight distribution, nozzle alignment, battery voltage choices, etc).

I've had my share the porpoising blues to be sure. Ought to start a club. I know a few choice members [sm=wink_smile.gif] Ha!

grael

I think the raised intake height lets the intake suck the back of the boat down more firmly.

I had a play with one of my 350mm hulls yesterday, with a ridicously overlarge prop in surface drive mode, and it certainly was not being "stuck down". About 1/4 throttle, it would come unstuck, zip onto the plane, accelerate to about 4 x the speed, and go pretty wild. No chance of stability over 1/3 throttle. It's flat today, so I'll give it another try.

sundogz

Yes, and when it sucks the rear of the hull downwards, the keel is well into the water able to provide resistance to side slippage. All good. Sounds like you've got a steep power to weight ratio going on there!

grael

I've figured out that the little jet unit I made needs to be running at at least 30,000 RPM to give good performance, because it doesn't load my 3000kv motor ( http://www.hobbyking.com/hobbyking/s...idProduct=5453 ) sufficiently under 2S, but it's pretty wild at 3S. Unfortunately, that motor is only good for 2S, it gets hot too fast on 3 cells. I think the motor would be great for the next size up jet I made, but I have priming problems on that design,and I didn't do a very good end to the outlet. Raining this weekend though, so I'll be working again on my CNC machine software, and maybe do another hand reconstruction of the larger jet unit.

I can get all different sizes of metric drill bits here, but some of the shaft sizes of the inrunners are hard to find drillbits to bore holes into couplings for. However, the 1.5mm and 2mm motor shaft sizes are OK, so next time I get small inrunners, I'll be going for those. I have an order for some 2200mA/HR 35C 3S batteries , a 70amp ESC and a 60 amp inrunner I'm waiting for, and just put in another order for an RC set for my stepson. I also ordered an extra 6 channel reciever and a cheap 2S 5000mA/HR LiPo for the rescue boat I plan to build.

The 60 amp inrunner has more than two poles on the rotor, and the idle current is listed at 2.8 amps. It also has a cooling impeller built in. I've been trying to find a large inrunner that will run up to 6S, that runs cool and is efficient, but I'm not much impressed with any other than this one, which only goes to 3S. The alternative for a big boat is an outrunner, but with very low rotor mass. (don't want the boat corkscrewing in the air.) The small cheap outrunners with the short rotors don't seem to have that problem.

sailindude

hello...
i am fitting my jet drives in and ive got to the point were im attaching the impeller shaft to the moter with the graupner connector wich fith them moth together and i dont know what size allen key i need to screw the grub screws in

in need of help...
matt

damfurst

they are either 1.5mm or 2mm. I can't remember for sure but it's one or the other.

grael

Newest hull almost done in balsa. Mega steps ! [8D]
I thought it would look ugly, but it's actually looking like it means serious business. [X(] Space for two very small jets under the shroud, or one bigger one. even though it has the steps, I think I can fit a saddlepack of 2200mA/Hr 35C 3S to the sides of the motor. Need to find my camera again...

Lots of aeration designed in under the hull, so it's a great one to show once and for all if it's really a problem for jet intakes.