Adding bilge plates to the runners to dry out upright?

[Swg]

I have a cornish shrimper and have just been offered a drying, harbour mooring after a very long wait. Want to dry our upright (snug space) - the shrimper leans over a lot. Legs are not allowed so new boat or attach bilge keels to the bilge plates. Size? Materials? Method of attachment etc ?

 

[dylanwinter]

I have a cornish shrimper and have just been offered a drying, harbour mooring after a very long wait. Want to dry our upright (snug space) - the shrimper leans over a lot. Legs are not allowed so new boat or attach bilge keels to the bilge plates. Size? Materials? Method of attachment etc ?

fenders

D

 

[MoodySabre]

They will probably slow you down when sailing.

 

[Swg]

Not snug enough for fenders Dylan and never in the mood for going fast so don't mind dragging around...

 

[prv]

Some of the Cornish Yawls, including ours, had bilge fins which allowed them to sit upright. You might get some ideas from photos of them?

The fins were basically steel plate, 10 or 12mm at a guess, with the tips turned outwards into little horizontal "toes" about 2" long to stop them sinking in. At the top a strip of the same plate was welded on to form a T cross-section, also with a fore-and-aft curve to fit the hull. Four pairs of bolts went through these T-flanges and the hull, with individual backing plates (like big square washers) on the inside.

Pete

 

[Tiller Girl]

Wot, like this?



Effectively these are just a flat steel plate with flanges top and bottom; top flange drilled both sides for fixing bolts which go through planks and the longitudinal stringer - a runner internally which runs the whole length of a wooden craft. My experience is that the flexing of the plates is to be taken in mind so I have had to double the frames at the rear end of the bilge plates. they slow me significantly and when hard on the wind in a reasonable breeze, I have a second bowwave from the leading edge of the windward bilge keel. I am very close to removing them.

 

[Wooden Boat Fittings]

Alan Buchanan's YM 3-Tonner, a bilge-keeler with a full-length displacement hull, was designed like this. As Tiller Girl says, you really need a full-length stringer to fasten the bilge-keels to -- I think backing plates on their own would not sufficiently-well distribute the grounding loads to the hull. I never experienced the flexing problems Tiller Girl mentions., but even if I had I would have put up with them for the sake of the ease of drying out --

Note that the trailing edge is nearly vertical. I would make both leading and trailing edges quite angled, to avoid fouling mooring lines.

Mike

 

[prv]

I think backing plates on their own would not sufficiently-well distribute the grounding loads to the hull

I never had the slightest worry about this on Kindred Spirit. But then she was very heavily built of GRP (only a tonne less than our new boat that's ten feet longer!) - I assume the OP's boat by the same builder will be similar.

Agree you wouldn't want to fasten them just to the planks, on a wooden boat.

Pete

 

[Seajet]

Beware longitudinally short bilge plates only keep the boat upright on firm ground; a Hunter Europa at my club with racy looking plates about 1' long has them just dig in as if they weren't there on the soft mud of Chichester Harbour; I seem to remember the Crabber Yawl had short plates like this, it would pay to make them longer - longitudinally - with a significant foot, but if using a simple turned edge don't make it so big as to hold the boat down in mud !

Compromise as ever...

 

[prv]

Beware longitudinally short bilge plates only keep the boat upright on firm ground; a Hunter Europa at my club with racy looking plates about 1' long has them just dig in as if they weren't there on the soft mud of Chichester Harbour; I seem to remember the Crabber Yawl had short plates like this, it would pay to make them longer - longitudinally - with a significant foot, but if using a simple turned edge don't make it so big as to hold the boat down in mud !

I dried out in KS on various surfaces from soft ooze to hard sand and always ended up bolt upright (or parallel to the bottom, anyway). In soft bottoms, the feet would sink in but so would the central keel, so we stayed level. On firmer surfaces, it would all sit on top (or sink very slightly all round). I've just googled pics of a Hunter Europa and I can well imagine those slim foils piercing through the mud. Ours were (from memory) about 2'6" or 3' long and with the turned-up bottom.

Pete

 

[Lakesailor]

The beauty of grp is that you can try some and if it doesn't work you can remove them. Make good the holes and sell the boat to get a more suitable one.

I can't see the difference between a bottom lip and the plate edge would matter much. A steady pressure on the plate with a lip would still push it into the mud if it were not up to holding the straight edge plate.

 

[Seajet]

Unless using horrible draggy thick plates they will dig in, the Europa I mentioned has short thin plates, no doubt designed to keep wetted area drag down, but they knife into the mud like butter; I've seen this with more pedestrian boats using simple steel plates too.

Conversely, if the ground was firm enough for these plates to work I'd be worried about the hull unless it had very beefy internal reinforcement.

 

[prv]

I can't see the difference between a bottom lip and the plate edge would matter much. A steady pressure on the plate with a lip would still push it into the mud if it were not up to holding the straight edge plate.

Very simple physics - pressure is force divided by area. The force (weight of the boat over that fin) remains the same, so having eight times the area (2" toe vs 1/4" plate edge, length remains the same) means an eighth of the ground pressure.

Pete

 

[Tiller Girl]

Alan Buchanan's YM 3-Tonner, a bilge-keeler with a full-length displacement hull, was designed like this. As Tiller Girl says, you really need a full-length stringer to fasten the bilge-keels to -- I think backing plates on their own would not sufficiently-well distribute the grounding loads to the hull. I never experienced the flexing problems Tiller Girl mentions., but even if I had I would have put up with them for the sake of the ease of drying out --

Note that the trailing edge is nearly vertical. I would make both leading and trailing edges quite angled, to avoid fouling mooring lines.

Mike

I think I agree with all that. A word about the flexing. It's not excessive but sufficient if I sail her hard to 'crack' the putty to wood seal. Also if I settle in totally soft mud, there is considerable pressure outwards onb the two bilge plates as the mud is compressed by 7 tons of wood, screws, paint, beer etc. I think how much of a concern it is for a GRP construction depends on the depths of the plates and the possible leverage. TG's survived since 1964.

 

[Lakesailor]

Very simple physics - pressure is force divided by area. The force (weight of the boat over that fin) remains the same, so having eight times the area (2" toe vs 1/4" plate edge, length remains the same) means an eighth of the ground pressure.

Pete

By the same simple physics all the lip will do is slow down the rate that the mud escapes sideways. It there is no underlying shale or sound bottom it will still sink in. But more slowly.

 

[Tranona]

Looking at photos of Shrimpers, they seem to have bilge runners already so the angle of tilt when it dries out could be reduced by making them deeper rather than going to the trouble of fabricating sheet metal ones. If they were laminated hardwood they could be shaped and either bolted through so that they could be removed in the future, or glassed over to make a permanent structure.

 

[prv]

By the same simple physics all the lip will do is slow down the rate that the mud escapes sideways.

If the mud is firm enough, it won't try to escape. How firm it needs to be before that happens depends on how much pressure you're applying to it. A wider footprint means you apply less pressure, so the mud doesn't need to be as firm to support it.

I can't imagine how this is in any way controversial.

Pete

 

[Lakesailor]

I'm not thinking that it makes any practical difference, but the assumption that ground pressure is the same on solid ground as on a viscous medium cannot be counted on. Witness the thread about lifting Elizmor.
Just saying.