Slo mo Shun

[Dave MilbournParticipant @davemilbourn48782]

Ref the props, Dave Jones’ Perkasa uses a 55mm 2-blade prop, so the ones Bob has can’t be too far off the mark. Not wanting to make “I told you so” noises but I wonder how much power those aluminium paddles are taking away from the
drive train?

DM

[Bob Abell 2Participant @bobabell2]

Morning Dave and Phil

 

It`s early days yet!

 

Spoke to the prop man yesterday and he agreed, the props are the correct size.

He also said, the motors were underpowered, which they are if you want to tear around the lake at break neck speed………………But, to my mind, they are ok for my slowish application

 

The hull is of large area, but the nose sinks to a lowish depth to reach the floatation point and to get on the plane, the heavy hull has to rise out of it, and at the mo, it`s a bit too deep.

 

So now I`m filling in the cavity area to reduce the depth of emersion……..and it should be able to slide out………up a gradual incline

 

Dave, what is the best dip switch setting for this application, please.

And I still like my turbo fans!

 

It keeps me off the streets!…………………………..Bob

[Dave MilbournParticipant @davemilbourn48782]

 

Dave, what is the best dip switch setting for this application, please.

And I still like my turbo fans!

Whatever works best for you, Bob – like it says in the manual!

Those paddles must be sapping a significant amount of the available power, but it’s your model.

DM

 

Edited By Dave Milbourn on 18/03/2010 07:08:32

[Bob Abell 2Participant @bobabell2]

This is the void that needs filling and should allow the hull to rise up this shallow incline and hopefully……………Get on the plane!

 

Will it work. chaps?

 

Bob says it will!

[Phil Winks 1Participant @philwinks1]

Of course it will help Bob wether it will totally cure it is another question. what cubic volume is it Remember 1ltr of water excluded equates to a Kilo of weight loss in flotation terms. On Daves favourite subject of the fans something you should consider is that an efficient aero prop of  4″ dia will require approx 80w to turn it at the speeds your motors are doing so roughly at 24v you are expending about 3amps per motor to cool everything !!

 

Phil

[Phil Winks 1Participant @philwinks1]

Thinking a little further Bob the hulldesign is based on a very small footprint when on the plane the spread will surely help with stability so when filling the void perhaps a shallow inverted v shape at the rear may help allowing the foot print to slowly decrease as it gets on the plane thus as speed increases so drag is reduced!!

 

 

Just a thought

Phil

[Bob Abell 2Participant @bobabell2]

Thank you, Einstein!

 

80 watts hey?…………….Well, when I`ve got a few things sorted out……………..I`ll remove the fans and see what an extra 80 watts per motor does to the performance on a short run without any cooling……………..And see what happens?…….eh?

 

The volume of water, being excluded is 357 cu ins

 

Bob

[Phil Winks 1Participant @philwinks1]

WOW that equates, according to my “High school” maths, to 12.9lbs of excluded water what would you have to do to lighten the boat by that much (Take the bty’s out maybe) 

 

 

Phil

[Phil Winks 1Participant @philwinks1]

On the fans issue try reducing them to about 1/8th over the motor dia that will reduce consumtion significantly and I think you’ll besuprised just how much air a diminuative little fan will shift

 

Phil

[ashley needhamParticipant @ashleyneedham69188]

Bob, do not faff around, take the fans off then run it.

 

The side sponsons are of course what the boat sits on when it is at speed. I have a feeling that filling in the void will just create significant surface contact area and increase drag. You will then be left with a thumping big step also creating turbulence and more significantly a trough in the water that the props will be running in, and cavitate (even more according to the spectators??)

 

I think that you should try the boat fanless, after all, we appear to be talking of a fair amount of extra power, and it may just be that the extra oomph gets the boat a bit higher and this will have a knock on effect…the higher it lifts the less drag, it gets a bit higher still…etc etc

 

it sounds good to me anyway.

 

Ash. expert….on anything..and never having made something as ,er,complicated as this.

[ashley needhamParticipant @ashleyneedham69188]

Actually having just looked at your hull again, a better,possibly, hydrodynamic fix would be to get rid of the small step at the front, rather than add more weight and surface area underneath

 

This would achieve, a) a flat bottom, enabling the water to flow under the hull, and also be less turbulent. b) lose weight at the front end.

 

I realise this may be a bit tricky to do, but……

 

Looks very very nice on the water…pleased? or success overshadowed by perceived lack of oomph?

 

Ash

[Phil Winks 1Participant @philwinks1]

Hi Ashley can we just look at that again ?

 

By filling that void the adjusted bouancy will have the same effect pretty much of lightening the boat by nearly 13lbs less the weight of a sheet of lite ply.

Bit of a no brainer I’d say!

 

However I do agree with you on the surface area issue, and so suggested the shallow inverted V shape between the rear of the sponsons, providing a smooth transition from the bow to the rear of the sponsons which should solve the potential turbulance issue 

 

About the cavitation Issue have a look at last augusts issue of MB the S.S. Noggsund build article where the issue of defining the difference between cavitation and aeration is disscussed. Given the power Bob has available I guess cavitation is near impossible, and what is actually happening is more likely to be the props aerating (sucking air from the surface) in the turbulent water caused by that step and cavity acting together.

 

Phil (Prob teaching Ashley to suck eggs)

Edited By Phil Winks 1 on 18/03/2010 22:07:19

[ashley needhamParticipant @ashleyneedham69188]

Phil. Not at all…me, no expert. Only of the armchair variety ! just putting in an idea, and will readily bow to superior knowledge.  mutter  mutter mumble mumble

 

The hull is designed for very high speed, and on the step. As we deviate from the original design, so anything can happen. It may be that just a bit more power is required and it will happily skim across the surface, problem solved.

 

I can see where you are coming from (to use management speak) with filling in the void, but I just dont like the idea of that huge wedge at the front of the boat.

 

Cavitation or aeration…  the props may be near enough the surface to be sucking in air, but the boat is not going as fast as the props would like to run (my guess) and thus could be busy spinning around cavitating. OR are the props NOT reaching sufficient speed to cavitate with the motor /battery combination?

 

Ashley

 

[Bob Abell 2Participant @bobabell2]

Thank you , chaps for getting so involved with the nitty gritty of the problem

 

I don`t like the new big step either, but it`s worth a try, having gone this far

 

My very knowldgeABELL brother ( Ami Mec E )…..was watching the first boat trial and straight away said that a sort of “lid” was required above the props to stop said props from sucking air!………………He could be right there?

 

Ashley………………The fans are staying for the moment…………….Definitely don`t want any more junke inside the hull…………………They are easily removed

 

All good fun, chaps………………Bob

[Phil Winks 1Participant @philwinks1]

The big step developed at the back of the sponsons by filling the void could well aggravate the prop problem Bob which is why I’m suggesting flaring it out with an inverted V shape to create the start of a tunnel to A; remove sudden changes of water pressure, and B; funnel a cleanish stream of water towards the props. Also as the boat gets on the plane the top of the inverted V will possibly leave the water and suddenly the boat is on 3 points as designed, hopefully making it stable “on the plane” and reducing wetted area/drag to a min.

 

The Issue with whether the props are cavitating/aerating Ash is a huge discussion point and as both will cause similar problems, although with different solutions required, perhaps we’d best not clog Bob’s thread with that one just yet. I’m pretty convinced that the suggestion I’ve made will go some 99% of the way to solving the planing issue.

 

Just out of interest Bob what is the all up sailing weight of her (Bty’s included) ?

[Bob Abell 2Participant @bobabell2]

Hello Phil

 

To be honest……..Don`t understand your inverted vee idea………Need a little sketch please

 

The hull carcass weighs……………….19 lb

Four batteries weigh……………………..   7 lb

 

Total weight……………………………………26 lb

 

Due to this excessive weight…imho…..the nose section is deep in the water

 

The cavity fill mod will spread the load and raise the nose……..As the photo shows, the back end is too deep in the water…………..and the motors are flogging their guts out!……Driving against the water load at the nose

 

I`m hopeful, the fill, Phil, will improve the situation

 

Bob

 

 

 

 

 

[Paul TParticipant @pault84577]

Hello Bob

 

The model is not going fast enough to get up on to the plane and from looking at the photos the props are thrashing around and just mixing the water rather than producing forward thrust.

In effect the boat is ploughing the water and pulling itself under rather than getting above it, try moving more weight toward the rear and using graduated increasing power into the wind to get the model to lift up.(sometimes less is more)

 

The theory about suction being caused by the hollow hull is correct as in this type of boat the bow has to lift so that the void can fill with air, which is then compressed by the forward movement of the hull creating an area of increased pressure that the boat then rides upon.

 

You could always let the model reach its true potential by ditching the puny electric motors and fit a decent 2 stroke.

 

All the best

 

Paul 

[Phil Winks 1Participant @philwinks1]

Paul has a very valid point on  the “less is more” idea as the immediate cure for aeration is less throttle so allowing the props to bite properly and then  gradually accelerate this  would prob involve a long run up to get her on the plane!

 

 

further on my idea’s on filling the void Bob does this help

Front sect viewed inverted rear part of hull left out for clarity

 

The curved out sect at the back is to smooth the flow of water over this area and as the hull rises the V sect will hopefully fill with air more easily than is at present happening.

 

Rough maths suggests that this shape will give you about 4 to 5lbs of extra floatation right where you need it, under the nose.

 

Phil.

[Bob Abell 2Participant @bobabell2]

Thank you, Paul and welcome back to the thread.

 

Thank you, Phil…………….I see what you mean now, thanks for the sketch…………….But “too late” he cried!………..It`s all glued up now!

 

We`ll soon see next Tuesday…………The fruits of my labour……….at the next planned sail test

 

Paul. we tried going into wind and against the wavelets, but the hull was glued to the water and stayed put!

 

Bob

[Phil Winks 1Participant @philwinks1]

My only worry is that you have simply moved the step aft. However there is also no fully enclosed void and the trim has been moved significantly aft, both good things, so as they say, the proof of the pudding will be in the eating, or in this case the sailing, of  best off luck mate

 

 

Ps Not at all suggesting that this boat is Pudding

 

 

Phil (Finally of to carve some lifeboats for the SS Tonendale)

[Bob Abell 2Participant @bobabell2]

What does the team think about this idea?

 

We`ve now got a big floating blob at the front and a long heavy tail hanging down at the back…………..How about a sponson at the rear to balance out the floatation points?

 

I can make it a screw-on affair……………So, I`m on the job already!

 

 

Don`t know how I do it for the money!…………………….Bob

[Phil Winks 1Participant @philwinks1]

Hmm I guess it can’t hurt to have a contingency should the aft end require lightening however would it not alter the appearance significantly?

[Bob Abell 2Participant @bobabell2]

I`ve been talked out of it already!…………………But will bear it in mind for future!

Here`s the latest mod………..Feel sure a box at the rear would be helpful

 

Bob

[Phil Winks 1Participant @philwinks1]

That should help her up a bit Bob From the pictures it doesn’t look like its to far down at the back but if it is there’s some room now to move bty’s fwd a tad

 

Phil

[Phil Winks 1Participant @philwinks1]

Well if you’ve been able to stick to your plans Bob, then you’re prob putting the final coat of paint on the modification in preperation for tomorrow. Hope you get good weather and more satisfying results.

 

We had a cracking day out at the club lake yesterday, the sun shone and attendance was good (for us) and only one real disaster. A plastic magic battle ship went down in fine style. Unfortunately I missed that on the video. However the good newsis one very determined member, using my 6m roach pole managed to find it 12′ out in 4′ of murky water with very little apparent damage. So all’s well that ends well.

 

Phil

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