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Author Topic: Boat sits too low in the water  (Read 606 times)

drawknife

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Boat sits too low in the water
« on: October 13, 2021, 12:08:45 pm »

Hi folks, my latest boat is sitting too low in the water but still needs more weight on a lead bulb attached to the fin under the hull to stop it tipping over. Ok, I know I have put a lot of weight inside the hull but this cannot be removed. This is the problem with scaling up drawings from the internet. What can I add to the outside of the hull below waterline to raise the hull in the water please? Any suggestions other than make the hull lighter would be appreciated. Thanks.
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Geoff

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Re: Boat sits too low in the water
« Reply #1 on: October 13, 2021, 03:05:49 pm »

Depending on what the model is you can add a 4mm to 5mm layer of obechi to the underside of the hull and then fair it in at the bilges. Once sealed and painted it will hardly show at all. This will provide additional buoyancy albeit low down but should then allow you to add more weight to the keel.


The only other way is to widen the hull in the same way by 4mm 5to 5mm each side but the fundamental issue is the superstructure is too heavy.


Hope this may assist





Geoff
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chas

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Re: Boat sits too low in the water
« Reply #2 on: October 13, 2021, 03:32:48 pm »

In  practical terms, you can't do much without re building or major modification of the hull. It might be better to post some pictures of the model, and ask how to make it float properly. How big is the model and how much weight have you put inside? How bulky and heavy is the superstructure? And why can't the internal weight be removed and re cast as an external keel?
Chas

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justboatonic

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Re: Boat sits too low in the water
« Reply #3 on: October 13, 2021, 03:53:22 pm »

Hi folks, my latest boat is sitting too low in the water but still needs more weight on a lead bulb attached to the fin under the hull to stop it tipping over. Ok, I know I have put a lot of weight inside the hull but this cannot be removed. This is the problem with scaling up drawings from the internet. What can I add to the outside of the hull below waterline to raise the hull in the water please? Any suggestions other than make the hull lighter would be appreciated. Thanks.
The only way to make the hull lighter is to remove weight. Yes, obviously but unfortunately in reality it would mean rebuilding decking, deck houses and detail etc.
If it is fibre glass and painted, your options are to sand off this weight and use lighter coats.

You say you've added a lot of weight inside the hull. If this is lead weight, why cannot you remove some of this and add it to the 'fin' you mention?

You mention a 'fin' under the hull, is this model a yacht? If so, apart from a rebuild, the only solution I can see is extending the depth of the keel (or fin) downwards. This will mean adding less weight to the bottom of the fin because the longer moment arm will counteract the tendency to heel over.
Hope this helps but unfortunately its a hard situation to correct.
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drawknife

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Re: Boat sits too low in the water
« Reply #4 on: October 13, 2021, 09:41:39 pm »

Hi again,
Just to clarify the build. The boat is a 15 century warship with 10 (hopefully) firing cannon. It is motor powered, not wind and all the internal weight is the pyro cases, batteries and there is no internal ballast. It does have a forecastle and 2 raised rear decks which make it  slightly (!!!) top heavy. I uploaded a plan from the internet and forgot to add more substance to the depth of the hull. It does have a 150mm skeg beneath the hull with lead at the bottom but more lead will make it sit too low. Just hoping someone may have a solution. Thanks.
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warspite

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Re: Boat sits too low in the water
« Reply #5 on: October 14, 2021, 12:47:17 am »

one option is to put the bulb lower in the water, in other words to turn it into more of a yacht, dropping the bulb and taking some weight off to reduce the pull down may only work if the fin with the smaller bulb equals out the listing, also making a fin with a bulb at the top that looks part of the hull and adds flotation might help, I am thinking of doing this for my HMS Sovereign
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malcolmfrary

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Re: Boat sits too low in the water
« Reply #6 on: October 14, 2021, 09:47:49 am »

Adding lightness below the waterline will only make it less stable.  The only answer is a long fin and the right weight concentrated at the very bottom of the fin - see what is done with racing yachts.  That, and losing whatever weight can be lost above the waterline.  Masts and rigging contribute a lot of weight and side-wind collecting ability even without sails added. 
The longer the fin, the less weight is needed for stability, again look at the difference between an R36R and an IOM.
If a fin is used, it will need a mount that goes as far as possible up the inside of the hull.  If it is just fastened to the hull bottom, the twisting forces that keep the boat upright will try to break it off.
It is a real life problem - look up "Wasa" and "Mary Rose".

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GG

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Re: Boat sits too low in the water
« Reply #7 on: October 14, 2021, 10:44:05 am »

Sadly, another example of how a few simple calculations could have shown that the proposed model would be overweight.  Maybe I'm unusual in that I try to make an estimate of a new models displacement and compare it with the items it is planned to carry, batteries, motors, radio gear, etc, plus an estimate of the models weight.  This is no more than the proverbial "Engineers back of the envelope" calculation.


If this gives me a sensible margin of safety, for scale type of models this is usually a realistic amount of ballast, I start building.  If not, it's back to the drawing board (more accurately another envelope) and try some changes rather than waste time and effort on something that is likely not to work.  After all, the Government (which means all you tax payers) isn't rewarding me whether I succeed or fail..!


Glynn Guest 
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Colin Bishop

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Re: Boat sits too low in the water
« Reply #8 on: October 14, 2021, 11:04:20 am »

If you are really desperate then detachable outriggers with floats below the waterline where they won't be noticed too much are a possible option.

As Malcom says,  inherent instability is bad enough but people often overlook the windage effect which is scaled up enormously in a model boat even in a light breeze. So a boat which seems to float OK in the bath can easily be blown flat when subjected to pond conditions.

Colin
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tonyH

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Re: Boat sits too low in the water
« Reply #9 on: October 14, 2021, 12:14:19 pm »

The stuff you've installed must weigh a fair amount, especially if you've over-engineered matters. You haven't mentioned what batteries you've got etc. and what can you actually lose by doubling up etc.Could you, for example, put the batteries in a pod under the hull, or even, as the weight at the end of the keel. Generally they're the heaviest item and the work involved would be, frankly, a doddle! :-))
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Nordlys

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Re: Boat sits too low in the water
« Reply #10 on: October 14, 2021, 12:59:07 pm »

Sadly, another example of how a few simple calculations could have shown that the proposed model would be overweight. 


What do these weight calculations tell you then? It's easy enough to weigh the boat


N
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Colin Bishop

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Re: Boat sits too low in the water
« Reply #11 on: October 14, 2021, 01:12:03 pm »

Quote
What do these weight calculations tell you then? It's easy enough to weigh the boat

I think the point Glynn is making is that you roughly calculate the displacement before you start building. This will give you a permissable all up weight for the boat itself and anything you put in it. You can then decide if it is a viable project.

Colin
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warspite

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Re: Boat sits too low in the water
« Reply #12 on: October 14, 2021, 03:18:27 pm »

In the Sovereign above, the original counter weight was twice as long and half the weight, but I managed to reduce it to about 268 g and as shown, the current incarnation is still not right, the threaded bolt used is 100 mm long and the weight facing forward, so not ideal as it would be a weed catcher, I was going to alter it during furlough but as previously mentioned elsewhere I had other priorities.
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Geoff

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Re: Boat sits too low in the water
« Reply #13 on: October 14, 2021, 04:18:40 pm »

As previously suggested, if you double the depth of the keel you can effectively halve the lead weight as it will still give the same righting moment which would make the model lighter without effecting stability. You could also add some buoyancy where the keel attaches to the model if needed but this in not ideal.


Cheers


Geoff
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Geoff

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Re: Boat sits too low in the water
« Reply #14 on: October 14, 2021, 04:28:27 pm »

I just had an off the wall thought - it would take some modification at the bow and the stern but could you turn it into a two-decker rather than a three decker and paint over the lower stripe. This would give you more hull depth to play with but may offer a novel solution!


Cheers


Geoff
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drawknife

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Re: Boat sits too low in the water
« Reply #15 on: October 14, 2021, 10:21:38 pm »

Hi everyone,
Thanks for the replies and yes I know I should have done my maths but I didn't so please do not berate me.
I have taken all your suggestions into account and I am going to extend the skeg to give it a deeper lead weight.
I may even try to add buoyancy at the keel/hull joint. I will post results. If it does not work, I will have a static model.
Thanks again.
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RST

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Re: Boat sits too low in the water
« Reply #16 on: October 14, 2021, 11:30:18 pm »

Hi, not to jump-in or belittle but from the following...
Quote
I may even try to add buoyancy at the keel/hull joint.

...you might be missing the basic physics of what's going on there and what's been pointed out on this thread so far.  If you want to try that I suggest not to do it permanently and maybe to check it in the bath first with only an inch or two of water under the keel.  It's almost the worst place to put it.

Many full size ships have been split in two longitudinally and widened, plenty of others have had sponsons fitted to the shell at either side.  The latter looks "uncomfortable", sponsons will look rubbish and if they're not high enough, submergence might cause more hassle than worth.

It's a bit more difficult when you start-off with a difficult protoype from the start plus models operate in scale gale force on an almost flat calm day to us.  I fear you have a Mary Rose on your hands.  I've been there before, sometimes it just doesn't work trying to correct if if your distribution of weight is fixed.


Rich

PS:  GG is absolutely right and the basic mathematics of working out displacement are actually quite simple one you know.  Very simple if just estimating.  I'm bad enough for ignoring them and occasionally finding sometimes it's just a nightmare to correct afterwards, or using materials too heavy for the project.  Something top heavy is quite difficult to correct afterwards.
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tigertiger

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Re: Boat sits too low in the water
« Reply #17 on: October 15, 2021, 01:18:10 am »

l can only see two options that require no modification to the hull.
one. if you are not using Li-po batteries switch them.If you are running more than one battery for more than one voltage do some circuit play with resistors.You might get away with a single higher voltage Li-po battery.
Two. build the battery in to the bottom of the drop keel..It won't be elegant or hydrodynamic but that won't be important.
The only other option would be to remove the working guns and save those for your Mk2 version.
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malcolmfrary

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Re: Boat sits too low in the water
« Reply #18 on: October 15, 2021, 08:13:33 am »

I think Playmobil did the battery below thing a few years ago, a submersible pod underneath their pirate galleon. It also contained the radio bits and the motors, with some arrangement for getting the aerial above water.  A bit like one of those sucker fish, only doing something useful.
Combining a longer fin with buoyancy aids at the top does the job of lowering the centre of gravity while adding buoyancy without having the underwater buoyancy tipping the boat over.  Or at least has a good chance.


The force of the wind on any model boat is such that you multiply what you are feeling by the square root of whatever scale your boat is.  With a large scale model, a gentle breeze becomes a reasonable blow.  As the scale gets smaller, it soon becomes a biblical event.


"Add more lightness" - Colin Chapman.  He was into making things go faster, but it applies equally to making things float upright.
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chas

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Re: Boat sits too low in the water
« Reply #19 on: October 15, 2021, 12:09:40 pm »

I may even try to add buoyancy at the keel/hull joint.
] Imagine the model in the water, barely staying upright. You put your hand in the water and push upwards under the keel. The boat tips over doesn't it. This is why you can't add buoyancy to the keel
Chas.

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warspite

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Re: Boat sits too low in the water
« Reply #20 on: October 16, 2021, 11:07:04 am »

I would suggest remove the battery and weigh it, then enclose the battery in a submersible streamlined container and suspend it below the hull, make the container and connection to the hull as light as possible, this wont then add more weight to the downward force on the hull, the deeper you suspend the weight, the better the righting moment will be, the issue is connecting the battery back into the boat and ensuring water tightness.


Drawknife - could you post a picture of the boat, the battery and any heavy item that could be transferred to a submersible container external to the boat and then connected by extended cables. Also how big a boat is it, as it has a lead bulb and fin, just moving the battery to under the vessel will significantly improve the situation for the same weight. So changing the fin to have a large bulb which contains the battery and the lead weight below that will improve the situation.


If you look at the picture above you will see the fin and bulb (which is removable for transporting), the original was virtually twice as deep and was a brass tube with a square plate flat soldered to the bottom, it had 3 x 53g square washers on top of that plate, you can just see it under the hull in this picture. The righting moment of this was really good.
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