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May 04, 2013, 11:35 PM
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Discussion

Keel weight?.


I'm new to sailing and have wanted one for a long time. but didn't know if I would like it...
But I hade a chance to get a H/K phantom 1.89m at a steal.. so I pulled the trigger.

my question is how much weight should I put in the keel. my friend says the more the better for a noob.. is that the case?.. or should I put in the 1.3 kg that it calls for.. any advice would help thx for your time
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May 05, 2013, 03:05 AM
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KoneWone's Avatar
General rule of thumb is that the bulb should be some where between 60-70% of the total weight of the boat for good righting moment, the lower the bulb weight ratio the more boat will be prone to heel too far.

Having said that, most commercially made RC yachts are not designed to carry high bulb weight ratios, so you may then wish to keep adding weight until you get the hull sailing on its lines correctly, too much weight will sink the hull in the water and increased wetted surface area and slow the boat down (noticeably in light wind)

To compound the problem further many commercially made boats have too much sail area & under weight bulbs (double whammy) as result are very unstable in decent wind, only answer is to trim down the sail area and or increase bulb weight.

If you can use lead it has higher density per unit volume compared to most other metals, meaning more weight for less wetted surface area = less drag.

Example two bulbs same shape & size 1) Steel weight = 1.5 kgs and 2) Lead weight = 2.8 kgs.

Hope it helps

Cheers Alan
May 05, 2013, 05:59 AM
Registered User
The other advantage of using a denser material for a submerged ballast bulb is that it will displace less water for a given weight. Not a heck of a lot, but every little helps.
Although the box shifters selling the boat call it a "1.89m" because thats how tall it is, it can be regarded pretty much as a 1 metre boat, and you can use a lot of the information from that class. The information recommends using lead shot in the moulding - its unlikely you will get much more that the 1.3Kg in there, almost certainly not enough to have any ill effects. I would guess that the 2200g maximum for fin and bulb mentioned in the class rules will not be exceeded, whatever you do filling the bulb, so sinking the boat is not too likely, and any performance loss due to increased weight/wetted area will probably be more than made up for with improved handling in a wider range of wind conditions.
May 05, 2013, 09:42 AM
I DS slower than I build!
Cory's Avatar
I don't have an operating sailboat yet so I have no practical experience in the matter, but my understanding of physics is that a longer keel with the same weight will give the same righting effect as a heavier weight on a shorter keel because of the longer lever arm. I realize that hydrodynamic forces will be altered with the resulting increase in keel area, so that may affect things that haven't had experience with. I would think that increased keel area may not be a bad thing other than the increase in drag. A compromise could be a longer keel with a more narrow chord keeping area the same as the original design and resulting an more efficient, increased aspect ratio.
May 06, 2013, 05:06 AM
Registered User
Your physics is correct. There are engineering limits, though. It is widely accepted that fin area should be at least 5% of sail area, this to allow it to work to cut leeway. A longer arm will have greater forces acting on it, and will need greater strength to maintain rigidity, since a bendy fin is not a lot of use since it loses the righting moment. This extra strength usually means either more bulk, or more exotic build material. Too long, and you might not have water deep enough to sail in. It's all a question of getting the right balance, and manufacturers usually come pretty close. Apart from when they miss the point altogether.
A longer fin can have another drawback - the drag from the bulb is on a longer arm, and with a following wind its drag and inertia can form a fulcrum for the hull to pivot on, helping the boat to try to nose dive.
May 06, 2013, 08:53 AM
Still showing up for breakfast
lilleyen's Avatar
Quote:
Originally Posted by mfr02
Your physics is correct. There are engineering limits, though. It is widely accepted that fin area should be at least 5% of sail area, this to allow it to work to cut leeway. A longer arm will have greater forces acting on it, and will need greater strength to maintain rigidity, since a bendy fin is not a lot of use since it loses the righting moment. This extra strength usually means either more bulk, or more exotic build material. Too long, and you might not have water deep enough to sail in. It's all a question of getting the right balance, and manufacturers usually come pretty close. Apart from when they miss the point altogether.
A longer fin can have another drawback - the drag from the bulb is on a longer arm, and with a following wind its drag and inertia can form a fulcrum for the hull to pivot on, helping the boat to try to nose dive.
Excellent post, answered several of my questions at once.
May 16, 2013, 04:00 PM
Registered User
thx everyone for your help..
Mar 13, 2018, 06:21 AM
Registered User

keel weight


Hello Guys, I am trying to built a rc sail boat (ketch) 1.5 meters long with 3 diferent sails .
I need a matterial for the keel to be heavy and not to take lots of space..... because of the displacement
lead is not an option for my construction since we need something heavier....any sugestions??
Last edited by platon terzakis; Mar 13, 2018 at 06:26 AM. Reason: keel weight
Mar 13, 2018, 07:25 AM
Suspended Account
There are materials heavier than lead. But the problem with most is cost and danger. Think Gold, Uranium, Plutonium. etc.

Tungsten is heavier than lead and supposedly is readily available. But I don't know what something like that would cost.

Then with some of these thing you run into various rules regarding pollution and such. I doubt the local park authority would see the humor in your placing a pound or so of uranium into the local pond. Even if it was painted over (with lead paint).

My suggestion is to stick with lead. Make an external keel weight which you clamp to the boats keel for sailing purposes. Then remove it for display. The up side of this is if done right, you can adjust the weight fore and aft on the keel for optimum sailing performance. Once the right spot is found, make the attachment more secure. Nothing like having an external keel weight drop off during a gale.
Mar 15, 2018, 03:58 PM
Registered User
Quote:
Originally Posted by Cory
I don't have an operating sailboat yet so I have no practical experience in the matter, but my understanding of physics is that a longer keel with the same weight will give the same righting effect as a heavier weight on a shorter keel because of the longer lever arm. I realize that hydrodynamic forces will be altered with the resulting increase in keel area, so that may affect things that haven't had experience with. I would think that increased keel area may not be a bad thing other than the increase in drag. A compromise could be a longer keel with a more narrow chord keeping area the same as the original design and resulting an more efficient, increased aspect ratio.


Well, longer keel means you need deeper water, so there's that...
Mar 15, 2018, 08:24 PM
The wind is free, go sailing!
Scratchy101's Avatar
Spent uranium, if you don't mind a little radiation.
Latest blog entry: My DC6 Sailing and Mods
Mar 16, 2018, 04:48 AM
Registered User
Its been a few years, but what I said in post #5 is still true.
For density, for all practical purposes, lead is the limit. Any other metal that is readily obtainable will need more volume to give the same weight, but with good design it should not give any real problems. There is little to choose in performance between my two Victorias, one is an early one with (probably) a lead weight, the other (something else, maybe zinc) is a bit more bulky, but the same weight.
Last edited by mfr02; Mar 16, 2018 at 05:04 AM.
Mar 16, 2018, 01:22 PM
Thomas Armstrong
Quote:
Originally Posted by Scratchy101
Spent uranium, if you don't mind a little radiation.
Not an expert here, but I think depleted uranium may cause some government issues...
Mar 17, 2018, 04:30 AM
Registered User
Quote:
Originally Posted by tarmstro
Not an expert here, but I think depleted uranium may cause some government issues...
............might warm the pool water up a bit, though.
Mar 17, 2018, 09:51 AM
Registered User
Tungsten is another 50% denser than lead, but it's expensive. Tungsten shot is about $50/lb vs lead which is $2/lb.


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