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Author Topic: Marx Inboard/outboard (stern drive)  (Read 5544 times)

redpmg

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Marx Inboard/outboard (stern drive)
« on: August 11, 2012, 05:34:31 PM »

I have an old Marx model Z stern drive model 45 with monoperm super motor and wanted advice on how high to fit it on the transom (of an equally old 19 inch sports/fisher based on a Les Rowell design, kitted in the then Rhodesia now Zimbabwe). Should the prop be below the bottom of the transom ? or just little bit below the waterline? The model sits about 5/8 inch deep in the water. My full scale seagull runs with the prop only about 2 inches below the surface which only confuses me more. Can anyone help please
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dodgy geezer

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Re: Marx Inboard/outboard (stern drive)
« Reply #1 on: August 14, 2012, 10:52:54 AM »

Hi,

I see the issue..  Normally, you want to get the prop as low in the water as possible - the deeper it is, the better it works. As it gets closer to the surface it is working under less water pressure, and it can waste more energy in pushing the water upwards. So I would normally think of mounting the unit at the bottom of the transom, and letting the vertical part of the unit project as low as possible.

However, I do not know whether these drives have a positive water seal around the shaft. If not, I guess that water may leak into the boat if it's set too low? In this case it may be better to raise it a bit. In all cases, the top of the propeller must be below the base of the transom, of course.

There's my tuppence worth - anyone else got any ideas? Or even an old instruction leaflet providing advice?
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Circlip

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Re: Marx Inboard/outboard (stern drive)
« Reply #2 on: August 14, 2012, 12:25:23 PM »

Before fitting, you may care to read this :- http://www.rcgroups.com/forums/showthread.php?t=1702455

   Regards   Ian.
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redpmg

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Re: Marx Inboard/outboard (stern drive)
« Reply #3 on: August 14, 2012, 08:01:06 PM »










 :-)) Thanks for the advice - uploaded some pics now - not the same as the other unit which is a proper Z drive/ floor mounted .

The boat kit I think was based on a Sea Nymph which in turn was based on a Fairey Huntress and not a Broads Cruiser as some would have us believe !
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Circlip

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Re: Marx Inboard/outboard (stern drive)
« Reply #4 on: August 14, 2012, 10:37:57 PM »

Remember the bayonet fitting but couldn't find an advert for it. This unit replaced the "Bongo" but it's worth checking the gears for signs of the Bongo weakness problem. The Graupner boats of the time that this unit fitted were the first of the Vac formed hull types so weren't very heavy so put less strain on the gears than a wooden hull would. For installation heights look at the "Mercruiser" full size drives. Even the plate above the prop would be submerged on full size.

  Regards  Ian.
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redpmg

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Re: Marx Inboard/outboard (stern drive)
« Reply #5 on: August 15, 2012, 01:14:11 PM »

 Hi,

thanks Ian for the advice  :-)) , the boat is actually very light being made out of ' Mugongo' used as a balsa substitute during the time of sanctions. Its a very light wood, bit more brittle than balsa but drier and does not fur or string. Great to work with being quite soft, sands & carves easily ,and if you use brown boot polish comes up like beautifully polished Teak. So the Zimbabweans now use it to make masks and other curios !

If you use a very thinned coat of Polyester resin it soaks in and plasticises the wood , giving a very tough skin , without much of a weight gain. Wish I could lay my hands on more as its really great stuff.( Using Polyester as its far more easily obtainable here than Epoxy)

Was thinking of using 6v , but thanks to the good advice on the gears will stick to 4.5 volt and avoid full throttle if possible.

Now my next problem is how to work the steering as the leg tilts as well ......... {:-{

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phillnjack2

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Re: Marx Inboard/outboard (stern drive)
« Reply #6 on: August 30, 2012, 01:17:54 AM »

I dont know where dodgy geezer got his advice on deeper the prop the better.

you realy want the plate above the prop to be about 1/4 inch above the bottom of the transom for best performance.

When running full throttle, the plate that so many people call anti- cavitation plate (correct name is anti-ventilation)
should run on top of the water not below or level.

If the drive is too low you will get lots of water comming up the leg and into the back of the boat.
The drag of a outdrive leg or outboard leg is very high when too low and its gonna kill performance.


phill...........
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dodgy geezer

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Re: Marx Inboard/outboard (stern drive)
« Reply #7 on: August 30, 2012, 12:08:07 PM »


I dont know where dodgy geezer got his advice on deeper the prop the better.


Within reason, of course, it's a fairly standard position for displacement hulls. The deeper the water the more the thrust goes backwards rather than up. I don't think that the unit we're talking about is a modern high-powered one, so I wasn't thinking that it would plane unless the boat was very light...
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phillnjack2

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Re: Marx Inboard/outboard (stern drive)
« Reply #8 on: August 31, 2012, 02:13:42 AM »

even if it dont plane the lower it goes the more thrust is lost and more drag from the leg..
if you go too low with a outboard or outdrive the extra length of casing makes more dirty water,i.e more turbulant water.

the more disturbed the water the less efficiency the drive gives.

with a small motor and by the looks of it small scale type prop, this needs as much benefit as possible to get the boat going.




phill
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dodgy geezer

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Re: Marx Inboard/outboard (stern drive)
« Reply #9 on: August 31, 2012, 09:48:22 AM »

There is obviously a balance to be achieved in any particular situation, and that will be different for each boat. Nevertheless, I have never heard anyone claim that "the lower it goes the more thrust is lost".  I can find references which suggest the opposite - for instance the Wiki:

"A similar, but quite separate issue, is ventilation, which occurs when a propeller operating near the surface draws air into the blades, causing a similar loss of power and shaft vibration, but without the related potential blade surface damage caused by cavitation. Both effects can be mitigated by increasing the submerged depth of the propeller: cavitation is reduced because the hydrostatic pressure increases the margin to the vapor pressure, and ventilation because it is further from surface waves and other air pockets that might be drawn into the slipstream...."

or this item:  http://www.modelboatmayhem.co.uk/forum/index.php?topic=35264.0  (which was what I was thinking about when I made the first statement)
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Circlip

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Re: Marx Inboard/outboard (stern drive)
« Reply #10 on: August 31, 2012, 11:01:00 AM »

Quote
the boat is actually very light being made out of ' Mugongo' used as a balsa substitute during the time of sanctions. Its a very light wood, bit more brittle than balsa but drier and does not fur or string.

  Don't look like weight is  going to be a problem, in fact the opposite could be.

  Regards  Ian.
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phillnjack2

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Re: Marx Inboard/outboard (stern drive)
« Reply #11 on: September 01, 2012, 12:11:53 PM »

http://www.iboats.com/Transom-Elevators/dm/keywords.elevator--session_id.457648839--view_id.40157

This type of thing is sold to those poor folk who have such old engones that cannot get the engine up due to not having
any adjustment holes in the engine bracket, and for those who want to use a outboard that has no holes, I.E smaller outboards of say 4/5/6 hp etc.

notice the amount of lift it puts the engine bracket up by is 1 3/4 inch and also a 2 inch model. !!!!!!!!!!

now these are not made to destroy your engine thrust and performance, they are designed for gains.
this is on normal transom heights.

the transom heights on outboard boats is normaly 15 inch for short shaft and 20 inch for longshaft.
during the later part of the 19080's extra long shafts became available for bigger boats with 25inch transoms.
and now we see xxl shaft boats with 30inch transom.
outboard engines from around 90hp onwards now come in 3 sizes, longshaft 20inch,xlongshaft = 25 inch, and xxlongshaft= 30inch.
The shortshaft engines have mainly died out apart from the under 25hp range you can still get them.

Now to explain the 20 inch transom outboard ..
the height of a engine transom length is measured from the bracket of the outboard engine to the anti-ventilation plate....not the prop.

the 20 inch (longshaft) is realy for boats that have a 20 inch transom.
BUT
the pic below shows a longshaft engine that is now used on a short shaft transom.
The boat is 15 inch but the engine is a 20 inch.
YOU CANNOT JUST BOLT ON A LONGER H#SHAFT ENGINE AND EXPECT IT TO GO WELL.
the bracket now fitted is a raised setback.
the raise is 5 inch at the boat plus 2 inch at engine = 7 inch.
this is = to fitting 20 inch engine to 22inch trasom thus gaining a 2 inch lift. :-))
THE ANTI-CAVITATION PLATE IS NOW 2 INCHES HIGHER THAN THE BOTTOM OF THE BOAT.
Speed gain ove 6mph over having it deeper in the water plus steering far more precise, fuel consumption less and no silly drag causing water to come back into the boat even at cruise speed.
Accelleration better as well so an all round improvement.
the bracket is not an home made bodge up, its a professional bracket costing around 200


also notice the steering tube is now 2 inches higher than original if fitted to longshaft transom

see pic below
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phillnjack2

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Re: Marx Inboard/outboard (stern drive)
« Reply #12 on: September 01, 2012, 12:36:15 PM »

i could post thousands of pics of this type of thing.
But here is what is called a setback bracket that realy is in common use to give more cockpit room without an outboard splashwell.

Now notice that the engine anti-ventilation plate is much higher than the keel of hull bottom of the boat.
If the plate was the same height as the boat it would never get on the plane, power would be dramatically
reduced, just a big wave made behind and fuel consumption would be maximum with terrible mpg.

with this the engine plate is raised by 5 inches, this still only gives the normal type fitting.

on top of the setback will be a manual or hydraulic power lift to raise the engine up even further.

see pics below
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phillnjack2

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Re: Marx Inboard/outboard (stern drive)
« Reply #13 on: September 01, 2012, 01:18:00 PM »

Now dont take my word for what benefiits lifting outboard makes
(and remember the outdrive leg does exactly the same job as the outboard engine leg, and should be fitted likewise !!!

take a look at these boats
http://images.search.yahoo.com/search/images?_adv_prop=image&fr=altavista&va=outboard+engine+jack+plates

now also look at the video below.
at no time is there a mention of the engine anti-ventilation plate to be put lower !!!!!this is a definite no no.
but watch what happens when it gets raised and the plate is on top of the water.

http://www.youtube.com/watch?v=15QnJHiwkBw&feature=related

sometimes it needs little vids to show benefits of what can be done.


Now its ok to play around with outboards, but when fitting an outdrive that will be fixed in place we need to make sure its done once and only once as this will be a no go back type of thing.

do it right and put the plate slightly above the bottom of boat of youl get lots of drag and bad performance, even on a slow river river youl be wasting energy to push the boat.

the same goes for model boats as well, the more wasted energy = less time on water.
the little boat at the top of this topic is liable to turn over if the prop is mounted too low.

oh and heres one i made a long time ago for non believers




phill.. :-))
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Circlip

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Re: Marx Inboard/outboard (stern drive)
« Reply #14 on: September 01, 2012, 01:22:22 PM »

Couple of problems, Yer can't scale nature and we ain't got the ability to shuffle the inboard/outboard up and down the transom which is why I suggested looking at the positioning of a Mercruiser.

    Regards  Ian.
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phillnjack2

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Re: Marx Inboard/outboard (stern drive)
« Reply #15 on: September 01, 2012, 05:22:38 PM »


yeah i know what you mean CIRCLIP.

but most mercruisers were factory fitted at one level, and that is the anti-ventilation plate level with bottom of hull.
Idealy an inch or two up would be perfect, but they fit out so many different engine installations the level set up is sort of
run of the mill.
level with the bottom is just run of the mill, and what is known to be normal in the trade.

But even mercruiser NEVER EVER set up and outdrive with the plate lower than the hull bottom, it would cause just too much drag and reduce propulsion and use too much energy.

There are a few facts on the web about deep props doinga good job etc, but these are so wrong and out dated its unreal.

with todays know how many boats are able to go a lot longer on the gallon of fuel than before even using the same engine and outdrives.




phill...
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Circlip

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Re: Marx Inboard/outboard (stern drive)
« Reply #16 on: September 01, 2012, 05:39:41 PM »

Quote
There are a few facts on the web about deep props doinga good job etc, but these are so wrong and out dated its unreal.

   So is the Marx drive unit, we're talking 4.5volts and early 70's teknowlogy.

  Regards  Ian.
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dodgy geezer

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Re: Marx Inboard/outboard (stern drive)
« Reply #17 on: September 01, 2012, 07:01:08 PM »

More importantly, we're talking about small-scale model boats rather than full-size outboards, as Circlip said earlier. If the plate were put 1/4" above the base of the transom I would guess the prop might be less than 1/2" from the surface, which seems rather close to me. But we do have the boat owner, so perhaps we can actually see what happens in practice. It may be the case that a small amount of movement can be built into the hull, and some experiments carried out.

People mentioned the 'Bongo', which was the earlier version of this. That expected to be fully immersed - I have found a small plan for 'Moonsong', a little Vic Smeed model using the Bongo. Here are some pictures: 










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phillnjack2

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Re: Marx Inboard/outboard (stern drive)
« Reply #18 on: September 01, 2012, 10:52:24 PM »

glad you bought up the read up on the bongo, as something in that also relates to todays outdrives and stern drives.
the term kort nozzle !!!!!

today in full size powerboats ski boats runabouts and river cruisers, we have whats called the prop guard !!!!
these are a safety device but are based on the kort nozzle principle and realy do work very well.

funny thing is ive got one right infront of me that i have to repair tonight  ha ha
these work very well in model size too , as they help with slow speed and high speed manouvres and do help stop
unwanted debri hitting the prop.
they are sold as i said as a safety device to help stop swimmers and large mammals getting hurt by the prop.
But due to many of the prop guard slowing a boat down, they went to kort nozzle route and these should be made law everywhere.
A model with one will definitely improve things and will stop those nosey little fingers of the children getting hurt as well.

here is a pic of it and ill also post a pic of it on my evinrude 60hp engine.
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redpmg

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Re: Marx Inboard/outboard (stern drive)
« Reply #19 on: September 07, 2012, 07:52:08 PM »

Decided to try and have a small amount of adjustment on the height of the outboard leg - now made the mounting hole oval allowing the leg to be moved 5mm from its lowest setting -(plate level with the keel/bottom planks) and obviously 5mm up from that. That is the maximum that can be obtained with hopefully the mounting staying watertight, without repeated major surgery. {:-{

Only a small portion of the prop is shrouded by the bottom/keel at max height as there is a small gap between cavitation plate and prop , will be interesting to see what difference there is on performance (if any) there is between the two positions .  O0


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redpmg

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Re: Marx Inboard/outboard (stern drive)
« Reply #20 on: September 07, 2012, 08:08:40 PM »

HELP NEEDED :embarrassed: - forgot to ask for help re steering . All there is is a 2mm hole in the cavitation plate for something to be mounted . Obviously this will be underwater and initially thought of a bolt/rod mounted upright with a half moon steering arm on top way above waterline . Not sure whether to use pushrods or cables to an equal length arm on a servo. Can anybody make a better suggestion ?.Bearing in mind I need to make it adjustable in order to move the leg up by 5mm later, (had thought of simply changing the length of the upright rod/bolt.)
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dodgy geezer

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Re: Marx Inboard/outboard (stern drive)
« Reply #21 on: September 07, 2012, 08:17:58 PM »

I would have thought that a cable with a boot would be able to take the slight variation in height, if it were mounted half-way in between the two heights...
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