Bristol Channel Pilot Cutter

[Dave Cooper 6Participant @davecooper6]

Just pondering main mast positions as most line drawings don't show it. I want to get this right as I think the deck-supporting frames in this area will need to be quite close in to carry the longitudinal and lateral sail loads.

From various photos I've studied, a figure of 5/13ths of deck length back from the stem head seems about right ?

I think this will mean frames at around the 4/13th and 6/13th 'datums' thus enclosing a fairly stout keel box for the mast step. This may be over-kill as the standing rigging will probably take most of the loads in tension (my usual tendency to over-engineer here !). Anyway, I'll try it out on the mock-up and see how it looks.

Any thoughts welcome…

[Dave Cooper 6Participant @davecooper6]

Hi All

A brief update : Have decided on 40" Hull length, around 11" beam and 9-10" draught. There will be plenty of space for interior lead /steel /epoxy ballast and for the RC gear plus a small auxiliary motor.

To begin with all sails will operate from one servo. Later, the main(s) will be on one servo with the jibs on another.

Half-size mock-up to begin as soon as I can lay my hands on some good quality card for the frames. 10 stations, 1 frame per station (over-kill ?). Mast at frame 4. Stem post, keel and transom in balsa – these to be transformed to 3mm marine ply on the finished model.

The model will be 'semi-scale' as I won't be slavishly following any existing lines drawings. (Mainly for reasons of hull volume). It will also feature an 'overscale' rudder to assist with tacking.

Hope to lay out the 50% frame plan next week with some photos.

That's it for now

[Malcolm FraryParticipant @malcolmfrary95515]

Posted by Dave Cooper 6 on 04/03/2021 23:26:53:

Just pondering main mast positions as most line drawings don't show it. I want to get this right as I think the deck-supporting frames in this area will need to be quite close in to carry the longitudinal and lateral sail loads.

From various photos I've studied, a figure of 5/13ths of deck length back from the stem head seems about right ?

I think this will mean frames at around the 4/13th and 6/13th 'datums' thus enclosing a fairly stout keel box for the mast step. This may be over-kill as the standing rigging will probably take most of the loads in tension (my usual tendency to over-engineer here !). Anyway, I'll try it out on the mock-up and see how it looks.

Any thoughts welcome…

A very rough and ready way is to draw your sail plan on card, and see where it balances. Do the same for the side profile of the submerged part of the hull. The first indicates the probable centre of effort, the other the probable centre of lateral resistance. On a 40" boat, having the sail plan centre about 1" or so forward of the hull centre seems to work.

If going for internal ballast, lead. Anything else lets the centre of gravity rise too much, and in a scale hull, there just isn't the space available low down for anything much lighter than lead. Cast ingots if that can be managed, a wide variation of shot sizes if not.

[Dave Cooper 6Participant @davecooper6]

Thanks Malcolm

I'll certainly plot the C of E and the CLR as you suggest. On the pilot cutter, the bowsprit will be adjustable in length, so this should allow some movement of the C of E to, hopefully, achieve balance. Failing this, I think I can also build in some 'range' to the mast step position.

In the past, I have used lead shot from shotgun cartridges for ballast (model gliders mainly) encased in epoxy. I have also got some lead blocks left over from my son's karting days – so these could be pressed into service as well…

Dave

[Dave Cooper 6Participant @davecooper6]

Following some basic research started work today on a half-size hull mock-up. Frame positions and trial fitting of stem and transom post to get a feel for things :

[Dave Cooper 6Participant @davecooper6]

A bit more progress: Keel and frames 1, 5 and 9 in position. (The eagle-eyed will have spotted the stern post being reversed in the previous photo – oops !)

[Dave Cooper 6Participant @davecooper6]

Sighting shot along the frames. Next step will be to try a stringer or two to see how the lines flow. Slowly getting used to this upside-down way of working !

[Dave Cooper 6Participant @davecooper6]

Stringer lines photos to follow as soon as possible.

 

Edited By Dave Cooper 6 on 14/03/2021 13:44:28

Edited By Dave Cooper 6 on 14/03/2021 13:46:15

[Dave Cooper 6Participant @davecooper6]

Some extra frames and experiments with strength /stiffness and internal access. Rudder may need to be enlarged.

[Ray Wood 3Participant @raywood3]

Hi Dave,

Your bulkhead no.5 needs to be a lot deeper and fuller and no.6,or your boat will be all keel and no hull displacement bouyancy at the stern, IMHO

Regards Ray

[Dave Cooper 6Participant @davecooper6]

Hi Ray

Yes, quite agree. No. 5 is also quite flimsy at present and needs a better section definitely.

Not tackled no. 6 yet as the stringer lines aren't flowing as I would like…more work to do ! (This may resolve itself when F5 is corrected).

What about the rudder ? Many full-size lines drawings show it going to (near) full keel depth – I'm inclined to follow suit.

Thrust line for the motor shaft is also a first attempt. I guess this is not so critical as it will be just a small 6v, 'get-you-home' motor.

Cheers for the feedback,

Dave

[Dave Cooper 6Participant @davecooper6]

Ten frames in. Starboard has better profiles than Port at present. Black dots indicate estimated static waterline. Frames 9 &10 are above the w/line. Motor thrust line probably between T1 and T2. Next step a trial 100% frame.

[Dave Cooper 6Participant @davecooper6]

From observations of full-size craft, they tend to 'sit' bows up and stern down. There is a 15% scale model of the "Jolie Brise" on Youtube which seems to follow this overall trend.

Dave

[Dave Cooper 6Participant @davecooper6]

Hi All, a quick update on progress :

The first two 'full-size' frames are now in 3mm ply. Decided to stay with the 6mm pine keel for now.

Main waterline stringer line not quite flowing as well as I would like, so, I'll tweak the mid-station frames as I go along to correct this. (Will probably increase hull volume as a side benefit.).

Some good news is that the 42" hull length will fit my standard size building board (also used for model aircraft). Initial thoughts on hull planking are two layers of 1/32 resin-bonded ply strips ? Hoping someone like SLEC can supply these…not yet decided between carvel or double-diagonal planking – any thoughts welcome !

As I'm hand-fretting /shaping the frames, these will probably take about 2 weeks to complete all 10. Photos to follow.

Dave

[Ray Wood 3Participant @raywood3]

Hi Dave,

This is the mock up ?? the transom looks quite tiny compared to you drawing you posted and is shown coming down to the waterline and much wider, which would mean you don't need to add the extension to the rudder. The drawing also shows the frames being being convex to no.3 and slightly more concave working to the stern, I think you need a lot more buoyancy in the stern frame shapes to support a 7 – 8 lbs additional keel or ballast in the bilges, Eddie Lancaster has just finished and sailed one of these with a removable 10lbs keel, planking in ply is is not great idea, and SLEC don't sell it in strips.

Sorry to have all these comments but at 40" she will be big and heavy so you want it right

Regards Ray

Edited By Ray Wood 2 on 07/04/2021 08:46:54

[Dave Cooper 6Participant @davecooper6]

Thanks for the comments Ray – yep it's the mock-up. Currently having a re-think on a few areas now (frames 6 to 10 especially).

There will probably be some extra buoyancy in the keel 'dead-drop' together with some revised frame profiles and heights. The mock-up is loosely based on "Kindly Light", I think I'm now leaning towards "Jolie Brise" or, "Cours – Apres" ie with fuller hull shape and volume.

With regards planking – do you have any recommendations ? ie Non-ply strip wood, Glass etc. ?

Regards,

Dave

[Ray Wood 3Participant @raywood3]

Hi Dave,

I'm great fan of balsa as you can cut your own planks from a sheet, say 3/16" cut to 3/8" widths, or obeche which I use a lot for framing. If your going to glass cloth on the outside & inside with ply keel & frames dont worry about it not being strong enough, it will be my 39" Cullamix is balsa planked and has 25 lbs of cast iron for ballast.

Keep us posted, Regards Ray

[Dave Cooper 6Participant @davecooper6]

Nice model Ray.

I'll give balsa and obechi a test on the mock-up…..glass also sounds promising – if only for it's 'ding-proof' protection.

Dave

[Dave Cooper 6Participant @davecooper6]

Build progress :

Just finishing some amendments to the stern frames (7, 8, and 9). This has necessitated a revised transom /rudder on the 50% mock-up.

First five half-frames in progress at 100%. Mainly using the re-profiled 50% starboard side ones as templates.

Made contact with an on-line colleague in Oz who has nearly completed a 56" pilot cutter. He reported that he had far too much hull displacement and now wishes he'd gone for a lower displacement design…(he needs a trolley to move it around and to launch it !).

Hopefully, some progress photos tomorrow.

Regards to all,

Dave C

[Malcolm FraryParticipant @malcolmfrary95515]

Posted by Dave Cooper 6 on 01/05/2021 17:32:35:

Made contact with an on-line colleague in Oz who has nearly completed a 56" pilot cutter. He reported that he had far too much hull displacement and now wishes he'd gone for a lower displacement design…(he needs a trolley to move it around and to launch it !).

Archimedes knew what he was talking about.

A 50% increase in size of a boat makes it over 3 times as heavy, if it was a heavy model to begin with…………..

[Dave Cooper 6Participant @davecooper6]

Hi Malcolm

Yes, it's the old linear-to-area-to-volume game.

I did suggest a removable keel to him (as he can't lift the boat without a car jack and ramps), but, he thinks they look "ugly".

I think they can be made to look acceptable – if properly engineered. (I intend using one as opposed to 100% on-board ballast). Anyway, it'll be underwater so, unless you're a scuba diver….!

Regards,

Dave

[Dave Cooper 6Participant @davecooper6]

Nearly there with the 50% mock-up. Frames 6, 7, 8, 9 and 10 have been re-profiled and re-sized.

[Dave Cooper 6Participant @davecooper6]

Work has begun on the 100% frames, 1 thro' 5. As I'm 'hand-fretting', have decided to bolt the frame halves together to shape identical port and starboard profiles.

[Dave Cooper 6Participant @davecooper6]

At last, a full kit of parts for the hull. Now where did I put that building board ?

[Dave Cooper 6Participant @davecooper6]

Blast and Damnation ! : Wouldn't you know it – after setting up the keel, stempost and sternpost bang on datum, I installed the two Frame 5 halves to give the whole structure some stability…(largest frame mid-ships).

Despite 'pegging' the two halves whilst the glue went off, the frame halves slipped 3mm out of register. I think a fix will be a small amount of re-profiling to re-match the halves. Also, for the other frames, I think I'll join them first, prior to installation !

Oh well, lesson learned. Thinking back, as I was packing the rocker at Station 5, there might have been a small amount of 'out-of-flat' on one of the packers – I guess this could have set up some internal stress against the keel and then the slippage was the result.

Now I know why Ray does his frames as in the photo for the Cullamix

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