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Author Topic: Three are better than one?  (Read 5703 times)

GG

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Three are better than one?
« on: March 10, 2011, 09:09:54 pm »

The latest model started as a suggestion that I ought to look at the US Navies new LCS vessels in particular the USS Independence.  I duly looked and, boy, is that one ugly vessel! Well, it might have some functional attraction and frighten America's enemies but would not look out of place on an industrial estate amongst the other warehouses.

But, the idea of a slim hull with two sponsons for stability intrigued me and out came the sketchpad.  My original idea was to use a small waterjet unit to propel the model.  No matter how I tried, no practical way could be seen to fit it into the hull.  The use of a "surface piercing" propeller was then considered, the spray they generate would have made the model look quite impressive.  However, with the experimental nature of the trimaran style hull, a surface piercing prop was going to be an extra complication I did not need.  So the model ended up with a conventional submerged propeller.

A lot of paper was consumed until a sensible layout of internal items was achieved with a simple but hopefully strong construction.  The centre hull is about 30 inches (75cm) long with a beam of 2 inches (5cm).  The deck to which the sponsons are attached is 8 inches (20cm) wide.  The model was designed with the idea that the hull would not attempt to plane at high speeds, hard to do with such a narrow beam anyway, and the sponsons should just "kiss" the surface and supply the necessary stability.
Built mainly out of balsa with a liteply deck, the completed model weighed in at around 3 pounds (1.4 kg).  A "545" motor with direct drive to a 30mm diameter (P30) propeller and a six cell battery provided the "umph". The final appearance was aimed at a fast coastal craft, the sort that smaller nations might employ to protect their coastlines.  I also enjoyed the excuse to build a plastic helicopter kit for the flight deck and try my hand at a camouflage scheme.

In terms of handling it was perfectly normal at low speeds and could even be steered when moving astern.  The first high speed runs showed a tendency for a sponson to drop with no warning causing the model to suddenly heel over at an alarming angle.  Luckily the instinctive responce of cutting the power always allowed the model to recover. Numerous experiments were carried out and it was found that by angling the sponsons outwards, the model was stable at high speeds.  Well, stable in a straightline or gentle turns but too tight a turn and the sponson would start to drop.  This effect occurs with turns in either direction and just in case the rudder was the cause, a smaller rudder was tried.  This did not cure the problem and actually lost the astern steering, so the big one when back on.  On reflection this type of handling is not unknown in planing hulls, especially when overpowered.  In fact it seems quite realistic to avoid tight turns when travelling at full speed anyway.

I remain impressed with the effortless way the slim hull slices through the water at speed producing no significant bow wave.  It does however generate a nice wake wake behind the three transoms.
Glynn Guest



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Grub

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Re: Three are better than one?
« Reply #1 on: March 11, 2011, 07:39:13 am »

Hey Glynn,
Do you remember your book'Boat modelling made easy'
Because me and my Dad have made the hauler boat. Yesterday me and my dad went back to howes model shop to get some balsa for the Sentinel ,here is a link :-)

http://www.youtube.com/watch?v=0QAng1UYJ-I

Jack, age 9

by the way i love the new boat!!!!!!!!! :D
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unbuiltnautilus

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Re: Three are better than one?
« Reply #2 on: March 11, 2011, 02:14:06 pm »

Very effective looking model. Have you considered som sort of large chines on the inner or outer faces of the sponsons? It may help to provide a bit of lift when the hulls dip into the water.
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GG

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Re: Three are better than one?
« Reply #3 on: March 11, 2011, 08:06:02 pm »

Unbuiltnautilus,
                     I toyed with quite a few ideas but as I said, this sponson dropping only becomes a problem if you try to turn too tightly at high speed.  This is much the same as most of the high speed scale types of models I have built previously. It has never been a problem provided you learn what the model can and cannot do.  I guess the only difficulty might occur if it was sailed by someone who has never developed any rapid "feedback" in the model-modeller loop.
So, I'm happy to leave it as it is, exciting to sail and safe, providing you keep your eyes open and brain engaged in the task.
Glynn Guest
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Jack.H

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Re: Three are better than one?
« Reply #4 on: March 26, 2011, 09:19:52 am »

Absolutely perfect looking boat, Have you had any problems with the design because the front does not look very stable!

Jack.H :} 
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tweety777

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Re: Three are better than one?
« Reply #5 on: March 27, 2011, 02:07:06 pm »

You could build some sort of tunnel around the prop with a water inlet to make it a waterjet.
You can still use the rudder that I asume will be mounted right now by having the tunnel stop either besides the rudder (tunnel stops at the place where the rudderstock is), or have the tunnel stop just in front of the rudder.

Greetings Josse
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GG

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Re: Three are better than one?
« Reply #6 on: March 28, 2011, 08:19:59 pm »

Jack.H,
It's stable which was the whole idea of the trimaran layout.  The only thing to watch is not to try and turn too tightly when running at speed when a sponson can start to drop.  Reducing the speed and/or rudder angle allows the model to recover, a habit soon learnt.  In fact this is not dissimilar to a few overdriven planing hull models I've built.

Josse,
I fear that any tunnel around the propeller will only reduce the efficiency of the propeller.  I'm saving the waterjet for a more suitable model.

Glynn Guest
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tweety777

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Re: Three are better than one?
« Reply #7 on: March 29, 2011, 08:03:41 am »

Kort nozzles also improve efficiency of the prop, so if you build a long kort nozzle then you will only win efficiency.

Greetings Josse
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andyn

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Re: Three are better than one?
« Reply #8 on: March 29, 2011, 09:23:34 am »

What are you on tweety??
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tweety777

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Re: Three are better than one?
« Reply #9 on: March 29, 2011, 10:45:08 am »

Building a waterjet by building a tight fitting tunnel around a prop.
That is basically what a real waterjet is as well.
In this case the waterinlets will need to be bigger and also need to be fitted to the sides in order to remain have the ability to sail backwards.

Greetings Josse
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andyn

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Re: Three are better than one?
« Reply #10 on: March 29, 2011, 11:44:13 pm »

A waterjet will not run in reverse in any case, they only suck in air and blow bubbles. I ask again, what are you on?
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pugwash

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Re: Three are better than one?
« Reply #11 on: March 29, 2011, 11:46:06 pm »

Andy its just that dutch Happy Baccy

Geoff
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andyn

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Re: Three are better than one?
« Reply #12 on: March 29, 2011, 11:47:50 pm »

Comment removed. Admin.
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boater12

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Re: Three are better than one?
« Reply #13 on: April 06, 2011, 10:48:02 am »

Been on that for the last 3 years by the look of that Well Enhancer build........

Another un-called for comment  <*<

Jim.
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DickyD

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Re: Three are better than one?
« Reply #14 on: April 06, 2011, 11:22:34 am »

Been on that for the last 3 years by the look of that Well Enhancer build........
Get off his back Andy, what's he done to upset you this time ? >:-o
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boater12

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Re: Three are better than one?
« Reply #15 on: April 06, 2011, 05:42:19 pm »

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GG

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Re: Three are better than one?
« Reply #16 on: April 07, 2011, 09:06:08 pm »

Managed to get a few sailing photographs of this model.  This was not the easiest of tasks as I used a local canal, the only way to go was to hold about half throttle with one hand on the transmitter, operate the camera with the other hand whilst praying it did not hit any thing!
Hopefully they show the level trim at speed, little bow wave and a nice "bubbly" wake.



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unbuiltnautilus

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Re: Three are better than one?
« Reply #17 on: April 08, 2011, 12:04:52 pm »

Looks great, I want one :-))
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Jack.H

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Re: Three are better than one?
« Reply #18 on: April 08, 2011, 05:22:12 pm »

Is this a scratch build or did you use plans?

Jack.H
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GG

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Re: Three are better than one?
« Reply #19 on: April 09, 2011, 05:58:24 pm »

Jack.H,
            It's scratch built, very scratch built.
Glynn Guest
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farrow

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Re: Three are better than one?
« Reply #20 on: April 13, 2011, 09:53:44 pm »

I thought water jets could go in reverse, seen some real life ones that do, the army had some and some ribs are fitted with them, had one on my old vessel as a MCA approved rescue boat.
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triumphjon

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Re: Three are better than one?
« Reply #21 on: April 13, 2011, 10:13:43 pm »

the motors & props are only designed to rotate in one direction , in order to steer or reverse you need a pair of directional " buckets "
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malcolmfrary

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Re: Three are better than one?
« Reply #22 on: April 13, 2011, 10:20:47 pm »

Tweety, a Kort only increases efficiency at low speeds, there comes a point fairly low in the speed map where the extra submerged drag beats any gain.  A water jet, on the other hand, sucks water in through a duct under the hull, accelerates it enormously, and shoots it out of the back.  A bare prop has a lot going for it in this case.  The top speed of the boat is limited by the speed of this water flow.  For a boat to move its own length, a long, narrow, fast boat needs a long, narrow fast moving stream of water (small prop going like the clappers), something like a tug needs to shift a wide stream, in both cases to ensure that the water moved in the time at least equals the submerged hull volume (large, slow prop turning more slowly).  Then you add the extra to counter inefficiencies.
Old Dodes, reversing water jets have a thrust reversing bucket that drop over the jet outlet, causing the outlet stream to be diverted forward.
GG, would adding angled foils to the sponsons to give lift when submerged improve stability in gentle turns at speed, or would the extra drag make matters worse?
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Welsh_Druid

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Re: Three are better than one?
« Reply #23 on: April 14, 2011, 10:01:13 am »

I only noticed this thread today and I was immediately reminded of the Nigel Irens 1988 design the Ilan Voyager      (Its Long And Narrow )[/color

This was followed by the Cable and Wireless Adventurer which set a world record in 1998.
 Departing from Gibraltar on 19th April, the vessel returned on the 3rd July having covered the 24,500 nautical miles in 74 days 20 hours, spending some 62 days at sea. The average speed achieved was 16.5 knots and the average fuel consumption about 1mpg (4.5 litres/nm).

It seems the concept of a long narrow hull with sponsons has something going for it ?

With regard to the comments that jet drives do not work in reverse - my Stena Sealink ferry model (seen  last year at Wicksteed) is driven by two Graupner waterjets fitted with reversing buckets. Using the buckets to reverse results in the model doing so slowly. If I run the motors in reverse the model goes backwards much more rapidly.   Whatever the theorists say, in practice  jet drives DO work in reverse (well on my model anyway  :-) )

Don B.




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Welsh_Druid

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Re: Three are better than one?
« Reply #24 on: April 14, 2011, 10:03:52 am »

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