24 replies to this topic

[Redline]

Have a good light downwind raceboat, that isnt so hot upwind. IT has a 120kg clip on bulb, I have access to a mould for a much bigger torpedo, so was thinking of removing the old bulb, and using a newly poured torpedo on the existing fin. Im trying to work out what difference adding a 240-300 kg bulb will have on the righting moment on the boat. The draft is 1.92M. And IM wondering if this is say the equivalent of 1,2 or 70 KG crew on the rail. Anyone know what the maths is to work this out??

[SemiSalt]

I'm sorry to hear you missed physics in high school.

Here is the first approximation.

Righting moment from keel bulb at heel angle A

Keel moment = sin(A) * depth of bulb * weight of bulb.
For A=25, bulb = 250 lbs, depth = 6
Keel moment = 0.42 * 6 * 250 = 630 foot pounds


Righting moment from W pounds of crew on the rail

Crew Moment = cos(A) * W * 0.5 * beam.
For A=25, 1 crew at 175 lbs, and beam = 9
Crew Moment = 0.9 * 175 * 0.5 * 9 = 708 foot pounds.

So at 25 degrees heel, the bulb is worth a little less than one man.

[Redline]

Hey thanks thats exactly what I was looking for as can compare different scenarios:

1) Water ballast (100kg/side) again only worth about 1 crewman

2) Bigger 240kg Clip-on Bulb (worth about a single crewman) above the std 120kg one.

3) New deeper foil 2.4M with a 750kg Bulb worth about 7 crewman on the rail!!

So the most effective and (most expensive) would be a new foil with 2.4M draft as it would provide the largest righting moment when heeled.


I did do my 'o' level physics but that was 35 yrs ago!! Thks for ur help.

[SemiSalt]

Option 3: I'd worry if that's too heavy for the blade.

[One eye Jack]

1342477058[/url]' post='3788105']
Have a good light downwind raceboat, that isnt so hot upwind. IT has a 120kg clip on bulb, I have access to a mould for a much bigger torpedo, so was thinking of removing the old bulb, and using a newly poured torpedo on the existing fin. Im trying to work out what difference adding a 240-300 kg bulb will have on the righting moment on the boat. The draft is 1.92M. And IM wondering if this is say the equivalent of 1,2 or 70 KG crew on the rail. Anyone know what the maths is to work this out??

Why not deepen your draft..that way it will lower your center of gravity.. It's not so much as how much but where your weight is..remember that teter toter..

[MSA]

Increased draft with a newer foil profile will not only help RM but theoretically should help your speed VMG as well. Its amazing what a new foil can do!

[Heavy Metal]

Ditch the bulb completely and attach 5 trapeze wires.

You'll be faster everywhere

[Cheesy]

Have a good light downwind raceboat, that isnt so hot upwind. IT has a 120kg clip on bulb, I have access to a mould for a much bigger torpedo, so was thinking of removing the old bulb, and using a newly poured torpedo on the existing fin. Im trying to work out what difference adding a 240-300 kg bulb will have on the righting moment on the boat. The draft is 1.92M. And IM wondering if this is say the equivalent of 1,2 or 70 KG crew on the rail. Anyone know what the maths is to work this out??

If it is a sport boat which it kind of sounds like it may be, adding weight will slow it down and possibly load the rig more than it was designed for, think light boat accelerating quickly in a gust, the loads drop off quickly. Now imagine it trying to accelerate another 200kg.... On the other hand if it is something like a R930 with a bulb on the keel a bigger bulb may make sense

[Redline]

The boat is a Ross 930

1/ It only has a 120kg bulb, but has a big keel structure that contains lead nealy all the way to the top of the stub. This is the class keel.
2/ The existing structure is around 950kg.

3/A new Foil around 150kg and a 750 new bulb (total 900kg) at 2.4 M instead of the old draft of 1.92M will give significant improvements on RM? And will likely have less wetted surface on the new foil,

IT shouldnt increase the weight of the boat either, but will dramatically affect upwind performance (hopefully!!)



Thanks

[MSA]

If you can get more RM with less weight.. would you not go that route??

[Scarecrow]

1. Don't forget to allow 20kg for the extra structure you need to build in around the bottom of the boat to take the extra moment.
2. Then talk to your friendly neighborhood rigger and check your rig (and chain plates) are up to the extra righting moment.

If you're chasing upwind speed you might be quite disappointed by the results. Except in some very narrow weather conditions, unless your boat is a pig which the Ross 930 isn't, a bit more righting moment won't make a big difference to your upwind speed as this is more dependant on your waterline length and sails than it is on stability. Your time and money may be better spent on a new sail wardrobe.

[Ncik]

1. Don't forget to allow 20kg for the extra structure you need to build in around the bottom of the boat to take the extra moment.
2. Then talk to your friendly neighborhood rigger and check your rig (and chain plates) are up to the extra righting moment.

If you're chasing upwind speed you might be quite disappointed by the results. Except in some very narrow weather conditions, unless your boat is a pig which the Ross 930 isn't, a bit more righting moment won't make a big difference to your upwind speed as this is more dependant on your waterline length and sails than it is on stability. Your time and money may be better spent on a new sail wardrobe.

+1

[Ned]

In addition to your righting moment you will also greatly increase your pitching moment. I suggest packing the rail, and cooler.

[quasi-expert]

The boat is a Ross 930

1/ It only has a 120kg bulb, but has a big keel structure that contains lead nealy all the way to the top of the stub. This is the class keel.
2/ The existing structure is around 950kg.

3/A new Foil around 150kg and a 750 new bulb (total 900kg) at 2.4 M instead of the old draft of 1.92M will give significant improvements on RM? And will likely have less wetted surface on the new foil,

IT shouldnt increase the weight of the boat either, but will dramatically affect upwind performance (hopefully!!)



Thanks

The old keel produces a max bending moment at the keel root of approximately (950kg*1,5m/2+120kg*1,5m)*9,81m/s²=8750Nm (canoe body draft=0,42m)
The bulb Version with a draft of 2.4m would come in at (150kg*2m/2+750kg*2m)*9,81m/s²=16190Nm, so almost twice as much. The boat's structure won't be strong enough to take that load!
If you really want to build a new keel I would keep the righting moment (or only slightly increase) and make the boat lighter!
And don't forget that increasing draft is usually good for upwind performance (RM and induced resistance), but will slow you downwind where you don't need the RM but still have the additional keel area.

[Cheesy]

The boat is a Ross 930

1/ It only has a 120kg bulb, but has a big keel structure that contains lead nealy all the way to the top of the stub. This is the class keel.
2/ The existing structure is around 950kg.

3/A new Foil around 150kg and a 750 new bulb (total 900kg) at 2.4 M instead of the old draft of 1.92M will give significant improvements on RM? And will likely have less wetted surface on the new foil,

IT shouldnt increase the weight of the boat either, but will dramatically affect upwind performance (hopefully!!)



Thanks

Have you had a look on crew.org I think there is a bit of information about all the keel mods to the R930s on there

[SemiSalt]

Except in some very narrow weather conditions, unless your boat is a pig which the Ross 930 isn't, a bit more righting moment won't make a big difference to your upwind speed as this is more dependant on your waterline length and sails than it is on stability.

This is true assuming the size of the rig is fixed and the baseline stability is adequate, which is true in this case. It's not true in general. A dinghy with crew on trapezes will be faster than the same boat with the crew on the rail, assuming the rig is big enough.

[Steve Clark]

As you can see it gets complicated fast.
In general, the loads on a sailing boat are all driven by stability.
Increase stability and you increase the loads.
Depending on the factors of safety used to engineer the boat, you may be able to get away with adding ballast or draft to increase stability. But in so doing, you will be reducing some of those factors of safety, and this may come back to bite you when you least want it. A mast that could be laid over and caned in a broach before the modification may not endure. Flor timbers and laminate adequate for the shorter higher CG keel may not be up to the task and can start to fail progressively.
That kind of thing.
Tread with care.
SHC

[Redline]

Hi Thanks for all the feedback.

Was just considering options at this stage. but there other 930's that have this new deeper draft keel/bulb arrangement. But as has been pointed out this involves beefing up the keel floors, strengthening chain plates and their attachment points. I have sailed against modifed versions of these boats, and they are very much better upwind than with the std keel. in 15 kts true the modifed boats truck upwind nicely, the class boats stagger and need early reefing or headsail changes, wheras the modifed boats just keep trucking. So I know it does work. Just a q of is a small bulb & water ballast more cost effective, but the calcs show them not to be very effective righting moment wise. Having a lighter but much deeper draft structure seem to be the way to go, but you need to spend the money on making sure the structure can cope with the additional loads. Just need deep pockets to do the mods I guess!

[Redline]

Ok Ive bitten the bullet and am getting the plans to make a new foil section and a cnc'd bulb (to make a new bulb of around 720 kgs) to be poured. New draft overall will be 2.4M. Weight of new foil & Bulb=830kg.

Allegedly the old 1.9M Fin and small 120kg bulb were around 950 kgs. Existing fin is attached with 6 x 18mm studs. New foil attaches with 8 x 25 mm bolts.

The question now is how much more floor strengthening should I put in place. The existing keel floors look reasonably substantial, but was thinking of glassing in some 3/4" marine ply between the stringers floors, with tri-axial cloth, across the width of the boat. upto bunk bases on both side. Prob help if I posted some pictures. Dont know if this will be strong enough or not!

[Redline]

Attached is the existing keel floors. The horizontals are spaced approx every 300mm. The stiffeners are every 300mm, and spread the load around 400 mm each side so 800mm overall. They are approx 110 h x 60 w in section.


IM planing on stiffening the whole area by adding some extra webs shown in the photo in red (on both sides). And also I intend to sandwich the centre spine in each cell with a double sheet of 3/4 ply. (so there will we 38mm extra all the way down the spine.

I will glass the whole area with 2-3 layers of 600g Tri-Axial cloth with epoxy.

Any suggestions on a better alternative would be welcolme.

Attached Files

•  KeelFloors Proposedsmall.jpg   491.06K   80 downloads

[Ncik]

[sarcasm]
it'll be fine, what's the worst that could happen?
[/sarcasm]

ask the designer of the keel to check for you. worth every cent.

[quasi-expert]

IMO the extra webs and upsized plates you are planning will not do the job. They increase longitudinal stiffness next to the keel fin, but don't do anything for the transverse stiffness and strength plus they create notches at the ends through high stiffness gradients. It looks like the existing structure is limited in height by the the floor boards. What I would do is max out the height and add additional transverse stiffeners where possible (between the first to bolts in the pic for example). the additional webs you drafted should not only run along the length of the keel root, but along the whole keel structure. Right now they end where they are needed the most i.e. at leading and trailing edge of the keel. That is where the highest loadings occur when the boat pitches or you run aground.

[BalticBandit]

Ok Ive bitten the bullet and am getting the plans to make a new foil section and a cnc'd bulb (to make a new bulb of around 720 kgs) to be poured. New draft overall will be 2.4M. Weight of new foil & Bulb=830kg.

Allegedly the old 1.9M Fin and small 120kg bulb were around 950 kgs. Existing fin is attached with 6 x 18mm studs. New foil attaches with 8 x 25 mm bolts.

The question now is how much more floor strengthening should I put in place. The existing keel floors look reasonably substantial, but was thinking of glassing in some 3/4" marine ply between the stringers floors, with tri-axial cloth, across the width of the boat. upto bunk bases on both side. Prob help if I posted some pictures. Dont know if this will be strong enough or not!

Well remember doing this is going to ding your rating as well. So you may never get what you are looking for. The Ross 930s I've raced against have been plenty quick. it may be that you would be better off spending the $$ on a Pro Coach to come and sort out what your upwind style should REALLy be like.

I'm always amazed at how much money folks will spend on gear and not invest $1 in a serious coaching session.


OTOH, righting moment DOES have a lot to do with upwind speed regardless of what Scarecrow posted. I remember reading that IMS did a calculation about slack lifelines and figures that a 4" droop beyond IMS regs on a J35 was the equiv of 4 sec/mi of extra boatspeed on the upwind leg

[Redline]

Thks for the reply.

The boat is going pretty well, and in the light I dont have any complaints, Ive won spring sailing series and one or two others in the boat, which goes well in the light, and am currently leading our winter series. Fully crewed with 6 on the rail it can go ok upwind in 15+ knots. But short handed its not so good. And I'm wanting to do some of the long distance stuff either single or double handed, hence wanting the boat itself to have better stability. These semi offshore races are a lot different than harbour racing, hence the desire to go for a deeper foil/bulb. ITs cheaper than buying a new boat! And probably half the Ross 930's in NZ have new foils, so its not that un-common.


Cheers

[Loco.]

Attached is the existing keel floors. The horizontals are spaced approx every 300mm. The stiffeners are every 300mm, and spread the load around 400 mm each side so 800mm overall. They are approx 110 h x 60 w in section.


IM planing on stiffening the whole area by adding some extra webs shown in the photo in red (on both sides). And also I intend to sandwich the centre spine in each cell with a double sheet of 3/4 ply. (so there will we 38mm extra all the way down the spine.

I will glass the whole area with 2-3 layers of 600g Tri-Axial cloth with epoxy.

Any suggestions on a better alternative would be welcolme.

You really need to get the designer involved imho.

Who's going to sign off the structural integrity so you can race or get insurance?