False keel for a 1905 sailing trawler

[Phil LawsonParticipant @phillawson23218]

I have built warships and service launches but never a sailing boat before. Maybe the choice of a plank on rib, 60cm. Brixham mumblebee is over ambitious.

Racing yachts have keels that must hang vertically but should the keel on this type of model be slightly inclined? In order to achieve the correct waterline, the bow needs to be 5.5 cms higher than the stern and it seems that the keel should act as a lever to achieve this.i.e. be angled.

Any advice on this and how to reasonably find the centre of effort would be much appreciated.

[Phil LawsonParticipant @phillawson23218]

[Tim RoweParticipant @timrowe83142]

Hello Phil and welcome.

Forget the angle of the keel for the moment. The only variable that is going to determine the correct wateline level is the longitudinal position of the centre of gravity and this needs to be on the same vertical line as the centre of buoyancy when the boat is on its correct waterline. It is a quite difficult to calculate unless you have naval architecture experience. It is very easy to find though by practical methods once the hull is built. If you have a look at my Galileo thread you can follow how it can be done.

Are you building direct from a plan?

You can do an approximation of the centre of effort by finding the geometric centre of each of the sails and using moments using the area of each sail and the distance of the geometric centre from a chosen datum. The centre of lateral resistance needs to be behind the centre of effort an on your size of boat I would think somewhere around 40 mm.

Can you post a picture of the plan if you have one.

Tim R

[Phil LawsonParticipant @phillawson23218]

Tim, thank you for your help. I will access your Galileo thread.

Am building from plans of "Nisha" as drawn by Edgar March. They were then enlarged. There are lines at The National Maritime Museum but this was a cheaper option.

Will set up a gallery later with plan. Thanks once again.

Phil

[Tim RoweParticipant @timrowe83142]

Nice subject Phil

If you look up this thread on the Thames Barges you will find some useful stuff about keels and Ray Wood also used one very successfully on a boat called Wild Duck.

Ray is on holiday at the moment so I bet he will get into trouble if he starts posting but he has lots of experience and is very helpful.

Once you have some pictures in a gallery you can always post them in your thread.

Looking forward to seeing them.

Tim R

[Phil LawsonParticipant @phillawson23218]

Tim, thank you for the link to Galileo. Page 9 was particularly relevant but your account was so helpful: it answered some questions I hadn't yet formed.

She's pretty vessel and with the benefit of only one mast. Stubby fingers got the better of me hence duplicate picture.

Phil

[Tim RoweParticipant @timrowe83142]

Hello Phil

You are well advanced so you will need to think what sort of keel (fin type or long). Fin with a bulb weight will be more efficient but possibly harder to engineer into your existing structure. The stresses can be quite high.

A long detachable keel will probably be easier to fit but won't be anything like so efficient and may make turning a bit sluggish. You will need to increase the rudder area. You could do this be having a larger deeper rudder for sailing keeping the scale version for display, of have a detachable extension which some people do in clear Perspex so it is invisible in the water.

Nice job by the way and looking very sturdy.

Tim R

[Phil LawsonParticipant @phillawson23218]

Hello Tim, Thanks for your further post and generous encouragement. I value your advice and help.

Yes, it is the keel that's taxing me. The main keel is only 5mm ply and I take your point about stresses. Had thought of a bulb but would have to reinforce (bulk up) present keel and add GRP. Had even considered adding a brass keel band. Very much liked your Galileo set up. Long keel could be pipe with lead filling or,preferably, lengthy fin with thinnish bulb to spread weight but hadn't given that too much thought. Freeboard is fairly low so would not be sailing in gusty conditions thus would welcome ability to turn fairly readily If possible/ compromise can be found.

Anticipated rudder problem at least. It's so small on the plan and have "liberated" Perspex to use with high torque servo.

Cheers,

Phil

[Malcolm FraryParticipant @malcolmfrary95515]

60cm is on the small side for a radio control sailboat, so it will need all the help it can get.

The "racing fin and torpedo shape bulb" is probably simplest. It will need a threaded metal rod to supply the rigidity (and if it pokes out the bottom of the fin, is handy for mounting the weight), and will need to go up to deck level via a tube. Threaded so you can get a nut on the top. This arrangement spreads the load beteen keel and deck beams, more or less eliminating the twisting forces. About 6" -9" of fin with the bulb on the end should work, try for about 5-10% of the sail area so the fin can help with reducing leeway. Positioning the fin – it needs to be just astern of the line of the center of effort of the sailplan, as it determines the center of lateral resistance ad whether or not it will ever steer straight. The fore and aft position of the torpedo on the end of the fin determines the center of gravity, and thus the static trim in water.

A friend, many years ago, built a 3 foot version. Looked great, sailed very fast, was unable to noticeably steer unless he used his auxiliary power, so a rudder with plenty of authority is good. His was big enough to use internal ballast, as was another boat (Bristol Channel pilot cutter about 4 feet long), whose owner didn't reckon that it had been a good days sailing unless he managed to get water into the on deck lifeboat.

[Phil LawsonParticipant @phillawson23218]

Yes, 60cms is small and could still come back to bite me when installing a sail winch. However, thanks for writing and taking me one step further.

I'll have to add doublers along the keel and reinforce them.

Is anything to be gained by using 2 rods in the fin to spread the weight slightly? Possibly the benefits of 2 tubes may be outweighed by drilling the keel in more than one place.

I have some 6mm threaded rod and tube to suit which seems adequate. Would not want to go much larger than this if using a single rod.

Hope to have as much enjoyment with this boat as your friend did with his. Really like the lines of the Bristol Channel Cutter. Looking at a launch next summer as there are many challenges but I'm enjoying them.

[Malcolm FraryParticipant @malcolmfrary95515]

I would have thought M3 or M4 for the rod if single, M3 if 2 of them. It will need some form of location to stop it trying to twist. Most yachts have a socket for the top of the fin to sit in for this. The thinner the rod, the slimmer the fin, and the less drag generated. Also, the narrower the tube can be, so that much less doubling on the main backbone where it passes through. My preferred metod of mounting the mast is to sit it in another tube going from deck to keel. This arrangement transmits the load between sails and fin without stressing the hull between them. A deck stepped mast relies on the rigging to do this, and can be a bit critical on a gaff rig if it needs to be erected at the pool side if it can't be transported ready rigged.

The construction looks solid and roomy enough for a high toque servo with an arm for sail control – if space proves to be tight for 3" of arm to swing about in, one of the new breed of 2 turn winches (about 6" travel) should be very adequate for the amount of sail area it needs to handle. I've tried using a standard servo in the past when it was the only thing available at a sensible price – despite the kit manufacturers claims, it wouldn't hold if there was enough wind to make the boat go. Both high torque servos and small inches are wonderful.

[Tim RoweParticipant @timrowe83142]

Phil

Two studs is going to soak up weight that you could put in the bulb. As Malcolm says, at this size it is going to need all the help it can get. One stud then with perhaps a dowel or peg to make sure the keel registers correctly inline with the centerline.

The boat already has quite a lot of lateral area and therefore resistance to leeway so I would go with Malcolm's lower end of the scale again to get as much weight as you possibly can into the bulb. This should be a bit of a mission because:

a) you have quite a lot of top hamper
b) it doesn't look like there will be a huge amount of weigh needed to get it down to the waterline so use it to best advantage.

Tim R

[Phil LawsonParticipant @phillawson23218]

Thank you both for your continued, helpful advice. I'm much happier with smaller rods than a Titanic 6mm. The keel protrudes about 6mm so perhaps I could use that to align the fin by adding a small aero foil section at the fin's top for support and reducing drag. The tube/ tubes would then be "boxed" and fastened between hull and deck former. This would obviate the need for a fin box within the hull?

Will use a fabricated tube joining deck and keel, as you suggest, to hold the Mast. I have a lathe so am wondering whether to make the wooden mast circular along its length or to have a square section running down from deck level for positive and convenient registering. There would be a weak spring at the mast base for tensioning the rigging.

On the subject of rigging, I am considering the use of 50 lb. breaking strain black Dacron. The mast is a shade under 60cms. Do you think this is "man enough"/ desirable , please? That's a long way ahead. I need some lead and to do some calculations first.

Phil

[Malcolm FraryParticipant @malcolmfrary95515]

50 lb is probably more than enough without being excessive, main thing is that you dont want it stretching. The spring idea is good for a display model, but not for a working model where a) it will get wet b) the rigging is doing a job. Many like a fixed length of stay with a turnbuckle for final adjustment. I tend to go with a line and bowsie arrangement (in my case, for bowsie read " cut off bit of servo horn&quot

Underwater, everything needs to be a nice smooth shape for easy water flow. Building over the keel should be fine as a way of locating the fin and keeping it in place, as long as it doesnt present any square faces to the flow of water. It isn't a racer, but you still need it to perform as well as possible.

[Phil LawsonParticipant @phillawson23218]

Thanks for the valuable confirmation re breaking strains and Spring advice. Seems pre wetting and stretching rigging cord could be the way to go. The Bowie idea certainly appeals in keeping with things as simple as possible.

You're right it won't be a racer but I wonder whether I should be building an ark given the present weather. 😀

[Tim RoweParticipant @timrowe83142]

Phil

Dacron doesn't really respond to wetting and is quite resistant to stretching. That is why sails are made out of it rather than Nylon which is very stretchy. If it is a braid rather than a rope it will stretch even less.

As you have a lathe you could easily make some annular bowsies in a black material that would closely mimic the deadeyes used on the original.

Looking forward to seeing how you sort out the keel. Keep us posted.

Tim R

[Phil LawsonParticipant @phillawson23218]

Looks like I will use Dyneema for strength and lack of stretch. but will try to avoid cut fingers, Tim. Am looking forward to lathe work on mast (ash) and bowsies – the latter in oak pickled with iron filings for darkness.

Thoughts on Keel similar to present yacht plan in current Model Boats: 4mm rod, carbon fibre inner then shaped balsa covered in lightweight grp (Eze-Kote). Now sourcing rod (B & Q) and CF. Probably 18cms to bulb from keel depending on how strong rod seems once bought.

Phil

[Author]

Posts