| T O P I C R E V I E W |
| Mad Harold |
Posted - 10 Mar 2017 : 19:34:33
When I bought Hereward, [Norman 20],I had the yard brokering it fit an auxillary outboard mount,thinking that an engine failure on a tidal river could be serious.
I have now bought a new 6hp Yamaha and running it up while moored,I was quite alarmed to see the transom flexing when I engaged forward and gave it a burst of power.It was pushing on the bottom bolts of the mount and pulling on the top ones,the movement was at a guess,about an eighth of an inch.The mount is on the port side midway between the main outboard and the side.
Had a look inside,and found the mount has been secured with just bolts and not very big washers.
Is it usual to bolt a steel plate inside and out,larger than the footprint of the mount,or to fibreglass the port inside of the transom,or both? Any advice? Thanks in advance. |
| 7 L A T E S T R E P L I E S (Newest First) |
| philihun |
Posted - 19 Mar 2017 : 16:05:15
Not guilty for that one mate.Bit off topic but at our club i organize the Lift outs and back in for the boats who winter on the hard. I hire a 100t mobile and get it at a Good rate. One of the owners asked why didnt i save money and get a 25t crane as the heaviest lift was only 14 t at 20 metre radius.He's supposed to be a retired engineer who has worked all over the place.
pr hunt |
| cliveshep |
Posted - 19 Mar 2017 : 10:31:58
quote:
Originally posted by philihun
Just to ease the pain mate. About 30 yrs ago in Barrow in Furness. Vickers Shipbuilding fitted a 180 ton nosecone section on a Polaris submarine upside down and fully welded it. I think they were advised to mark them top and bottom in the subsequent inquiry. They don't do transoms,too complicated.lol
pr hunt
Own up - were you the Foreman at that time?
Finally living the dream!
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| philihun |
Posted - 16 Mar 2017 : 21:15:45
Just to ease the pain mate. About 30 yrs ago in Barrow in Furness. Vickers Shipbuilding fitted a 180 ton nosecone section on a Polaris submarine upside down and fully welded it. I think they were advised to mark them top and bottom in the subsequent inquiry. They don't do transoms,too complicated.lol
pr hunt |
| Mad Harold |
Posted - 16 Mar 2017 : 19:42:42
Thanks for the reply Cliveshep,I read your last post after I did the job.I used 11mm ply for the pattress as large a piece as would fit in the available space.[Don't know why I said 5mm ply originally] I still think in imperial and get confused.Because of the time it took doing a "dry"assembly,it was a dreadful fiddle,I decided not to use P38,thinking it would go off before the job was done.Instead,I used Stixall,[2+tubes]to bed the pattress and a 3/16 square of steel to bolt to.Outside,a piece of 3/16 steel 18 inches by 6 inches holding the outboard mount,again, bedded with Stixall.Reading your last post,I realise that this is a bit "belt and braces"but it's done now.All that now remains to be done there,is to slacken the nuts a couple of flats in a couple of days when the Stixall has gone off to allow a bit of "give"and to paint it.Then on with the dozens of other jobs that need doing.The ad for this boat read;"ready to go after a wash and brush up,and a little TLC". Talk about a euphemism,I'm glad it wasn't advertised as a project. |
| cliveshep |
Posted - 14 Mar 2017 : 02:20:14
If your pattress more or less fills the space above the transom stiffener as you describe and if you are using only 6hp on one of those trapezium type lowering auxiliary brackets there would be no need for an outside plate. The pattress if stiff enough or beefed up with a steel plate inside would take the shear and torsional loads and transfer them to the outboard well side and hull side as well as to the side deck above the bracket and the stiffener below.
You'll find in such circumstances the aluminium mounting angles of the bracket itself would be sufficient spread.
Finally living the dream!
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| Mad Harold |
Posted - 11 Mar 2017 : 15:26:18
Many thanks for the advice Cliveshep,went to the boat this morning to make a template for the pattress out of an old piece of ply.It was in and out a dozen times to cut bits off to make it fit.There is actually a bulge running across the transom,which I assume is the timber you referred to about level with the bottom of the outboard well,but my aux outboard mount is well above this.Its one of those swing out and up type,and when lowered sticks out 10 inches plus 27 inches of outboard shaft,the twisting force on a small area must be considerable.
My inside ply pattress will extend from the top down to the timber cross piece,and the full width of the port side transom,using 4 or 5mm ply.I have aquired a piece of 3/16" sheet steel to fit to the outside and will make a pattress that extends down past the cross timber.This,hopefully will contain the pull on the top of the outboard mount,and the push on the bottom. Once again,many thanks. |
| cliveshep |
Posted - 11 Mar 2017 : 08:18:34
Totally correct - you'd need a large pattress inside to spread the loads if the auxiliary bracket is on an otherwise unsupported panel. I'm surprised though that your transom stiffening is not side to side all the way across?
Folks always express surprise at the torsional loads imposed by an outboard on a transom but the engine really is a long lever with the moment some way away from the shear-line of the "beam" of the transom. In the case of a Norman with a long-shaft motor it is a lever some 27-odd inches long so what you have witnessed is a graphic depiction of torsional loads in practice. That is why outboard or outdrive boat builders make transoms so sturdy, and add what elements they can to resist these torsional eccentric loads, such as motor wells and or seating with abutting bulkheads or knees to reduce the rotation.
All the boats I have worked on in the range have had the transom board going right across and not just for the width of the outboard well moulding. Remember the original design specs called for up to a 100hp long-shaft engine driving the boats. The hull sides and the sides of the motor well in almost all outboard boats are intended to resist these torsion loads.
I advise bedding your plywood pattress on a big dollop of P38 polyester filler to ensure it mates evenly with the grp if you don't have a full width transom timber moulded in and then bolt your bracket through it with large "penny" washers or a couple of 38 x 6 purpose made plates cut and drilled to match the bracket. Don't forget to use stainless steel bolts marine grade and bed the bracket on mastic to seal the bolt holes.
Finally living the dream!
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