| T O P I C R E V I E W |
| Tony846 |
Posted - 15 Jan 2012 : 12:06:12
Hi to you, my son and I have just bought a Norman 20 with a 30hp 2 stroke Yamaha main engine.
We are using the boat for coastal sea fishing and general leisure trips, the engine is a very good starter and will push the boat along quite well but we have noticed that there is very little power in reserve if the conditions become rough, so we are looking at a slightly bigger engine a 40hp to 50hp max for a degree of safety.
My questions to you are, will the transom take the extra weight and power if needed, should we re-enforce the transom, if so do you know if anyone has re-enforced a transom on a Norman 20 before and could they offer some advice and method.
T cunningham |
| 8 L A T E S T R E P L I E S (Newest First) |
| cliveshep |
Posted - 19 Jan 2012 : 22:37:07
This is going to be a long one!
The only way to structurally reinforce a transom to take the loads would be by adding a metal exoskeleton to it that transfers the weight and rotational loads of a large engine at full power back into the hull and spreads it over the entire structure.
However, in practice that so adds to the girth of the transom that no motor would ever fit on it not to mention the aesthetics of lumps of metal bolted on everywhere. So in reality it is unworkable.
You have one basic force at work here - the motor is trying under load to tear the transom off outwards at the top whilst attempting to screw itself under the hull and the motor weight is contributing to the effect by being offset outside of the transom line.
In plain English the downward forces on the transom top are relatively small and a rotten transom cold still handle those with a margin to spare, but the rotational forces imparted by the lever effect of the better part of 30 inches of lever (outboard leg) pushing at the lower end by 50hp of thrust are considerable. In such a case the motor weight contributes but is not significant. What your engine is trying to do is rip the top of the transom off outwards using the lower part as a fulcrum. Given that the top is where the water is certain to have penetrated due to the connection detail simply butted at the angle and covered with an aluminium angle, it is there that the rot will be. You can see that the part of the transom originally designed to resist the imposed forces the most is now likely to be the weakest part.
What a transom is essentially is a beam designed to resist the deflection and twisting forces imposed onto it and transmit those forces into the entire hull by means of the connection to the hull abutting the transom and other associated structure. To achieve this builders make the transom the better part of 50 mm thick, extend that thickening laterally in both directions across the stern to the hull sides, downwards to the bottom of the hull (in most cases) and reduce the span of this beam by the motor well sides and bottom, and stiffen those by the side decks thus triangulating the entire structure. The effect of this is to spread the loads into the hull.
Ok - that's how it works and if you haven't fallen asleep at this point you will now appreciate why other members are suggesting that you drill down through the top of the transom grp and then probe with a screwdriver to see how far the inevitable rot has gone. Failing that find a marine surveyor with a moisture meter and get him to run it all over the outside of the whole transom - trust me - you'll get a shock!
Dealing with the problem effectively is a bit of an exercise. The transom is glassed in to the hull at sides and bottom, and to the motor well moulding which is - dependent on age and model of your 20, either integral to the entire topsides moulding on later boats or attached to the cockpit coaming undersides on earlier models. Ideally you would remove the motor well and stern decks and also the cockpit lockers which on many models also stiffen the transom and hack the whole lot off and re-glass with new timber bonding it all back into hulls sides and bottom, re-bonding the motor well and side decks and glassing the lockers back in.
Easier said than done and it is a major rebuild that most would not be able to undertake successfully.It doesn't help that on the Normans much of the effective strength of the transom beam comes from that part nicely obstructed by the cockpit locker berths. One member has advised that he managed to open up the top of the transom and dig out the rotten cores and fill the whole void back in but if that were possible I would advise introducing some steel into the void running at least beyond the motor well sides and encapsulating that with resin and CSM stuffed well in. (CSM - Chopped Strand Mat)The steel perhaps construction rebar - the knobbly high tensile stuff at least 20mm thick. That will at least take the forces back to where they are transferred. I would suggest at least 3 bars being introduced. That will at least deal with the majority of the structure at its weakest point.
The cost of boating is insignificant compared to costs of a young wife and two teenage kids! |
| adi-n-chez |
Posted - 18 Jan 2012 : 20:49:35
quote:
Originally posted by Boatmad
Thought the 20 was rated for 80hp outboard.
pagreenwell
Maybe when she was new.........
Dont fancy a 80HP never mind a 50HP on a 30yr old 20 without an exploratory
Adi
Sewer Tubes ? I'd rather have a D**ncr*ft (Only Kidding) |
| Boatmad |
Posted - 18 Jan 2012 : 19:53:31
Thought the 20 was rated for 80hp outboard.
pagreenwell |
| Tony846 |
Posted - 18 Jan 2012 : 14:14:41
Many thanks to you Knighty, very helpful.
T C
T cunningham |
| Knighty |
Posted - 18 Jan 2012 : 13:07:07
As Dave, DF said above, get a few small holes and have a prod around to see if the wood sandwiched between the grp has rotted. If not you are good to go as the transom will take a 40-50 hp outboard. If it's rotted it going to be a case of opening it up from the top removing the rot and re-inforcing it all with new wood and resin etc.
You would prob get away if there is a bit of rot in a small 10hp river boat but a 50hp would prob end up cracking the transom or worst case breaking away.
Have a search for rotten transoms, has been covered before.
http://www.normanboats.co.uk/topic.asp?TOPIC_ID=6285&SearchTerms=rotten |
| Tony846 |
Posted - 18 Jan 2012 : 10:52:28
Not getting many takers re, advice re-enforcing the transom on a Norman 20.
T cunningham |
| Tony846 |
Posted - 15 Jan 2012 : 13:06:01
quote:
Originally posted by df
If the filler between the grp is in good condition it should take 50hp easily, I would drill a few test holes from inside to see how wet the wood is(not all the way trough to outside), don't forget to fill the holes afterwards
Thanks for the reply, I will try the drill test but as a precaution and given the age of the boat I think we would like to re-enforce the transom anyway, any suggestions on method
NBAS--The communal colostomy bag of the boating community.
Visit leomagill.co.uk
T cunningham |
| df |
Posted - 15 Jan 2012 : 12:11:25
If the filler between the grp is in good condition it should take 50hp easily, I would drill a few test holes from inside to see how wet the wood is(not all the way trough to outside), don't forget to fill the holes afterwards
NBAS--The communal colostomy bag of the boating community.
Visit leomagill.co.uk |
