Re-Powering old Boat

Current have an old 79 Guardian, licensed at 5.7 meters running a 1995 115HP Johnson 2 stroke - 136kgs.
Transom is still solid, no flex at all, ass end has some small gel coat cracking as expected for a boat that age. Pass Inspection late last year.

Ive Just bought...Pick up today a second hand 2005 135HP Merc Optimax - 205kgs so a jump of 70kgs. Found the below calculation on line

Australian Standards Calculation AS1799
Length of boat X waterline Beam width at transom x 16 less 67 then x 1.34 = HP rating.

so I'm getting 5.7 x 1.85 = 168.72 less 67 = 101 x 1.34 = 136HP so basically its on it limit but not massive over kill.

Anyone with shipwright / boat building experience as such on a change out like this on the older boats ?

Yes, inspection and insurance will need updating.

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Such is Life - Ben Cousin 2007


pelagicyachts's picture

Posts: 1113

Date Joined: 23/02/11

70kg increase for 20hp more

Wed, 2021-02-24 08:44

70kg increase for 20hp more doesnt sound right - ar eyou sure your 135 merc weight is correct - 205 sounds a lot?

pelagicyachts's picture

Posts: 1113

Date Joined: 23/02/11

70kg increase for 20hp more

Wed, 2021-02-24 08:47

double post - quick google shows weight of 195kg - just got you 10 back :-)

Brock O's picture

Posts: 2567

Date Joined: 11/01/08

ha ha Cheers Pelagic! 2005

Wed, 2021-02-24 08:56

ha ha Cheers Pelagic!

2005 135XL SWB tag has LB 452 KG 205....I cant make out serial number or find it on any of the part catalogue pages.

Pretty sure it reads 1BD55524 or it could be 1BDSSS24....5's or S's I'm not use.

I dont see the 60 - 70kg being an issue...but will be interesting to see how it sits in the water once fitted which ill be tackling myself over the next few weekends.

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Such is Life - Ben Cousin 2007

Posts: 583

Date Joined: 28/11/16

 Only 20hp more but I think

Wed, 2021-02-24 10:53

 Only 20hp more but I think the 115hp is a V4 the 135hp is V6. 

Brock O's picture

Posts: 2567

Date Joined: 11/01/08

Not sure of configuration on

Wed, 2021-02-24 10:57

Not sure of configuration on the Merc yet....I'm hoping a change from a 95 carby to 05 direct injection will see a difference in fuel usage.

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Such is Life - Ben Cousin 2007

Posts: 583

Date Joined: 28/11/16

 The merc will have

Wed, 2021-02-24 10:59

 The merc will have significantly more torque 

Brock O's picture

Posts: 2567

Date Joined: 11/01/08

Thats brings a smile to my

Wed, 2021-02-24 11:28

Thats brings a smile to my face, the old johnno is great in good conditions but does lag when loaded plus chews the juice.

Jumping from 85 kw to 99 kw.

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Such is Life - Ben Cousin 2007

Faulkner Family's picture

Posts: 16493

Date Joined: 11/03/08

 With the extra weight at the

Wed, 2021-02-24 11:20

 With the extra weight at the back you may need to throw a bit of weight up front. Couple of Jerry cans of fuel or even just water should counteract . Or just tried more chain on the anchor rope

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RUSS and SANDY. A family that fishes together stays together

Brock O's picture

Posts: 2567

Date Joined: 11/01/08

yeah exactly, plenty of cabin

Wed, 2021-02-24 11:29

yeah exactly, plenty of cabin up front for extras which is were I load the dive gear and eski etc.

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Such is Life - Ben Cousin 2007