How heavy can one go with an outboard?
I have a reasonably old tub (Bell Boy) that was renovated somewhat before I bought it to be a decked side console. It came with a 1991 oil injected 70hp yamaha 2 stroke that runs well etc etc, but in the near future the plan is to go 4 stroke or maybe 2 stroke DI.
Initially my thought was "thats easy, replace like with like". So I hunted around to see what my options where around this hp rating (70/75/80). On the average I was looking at around 160 - 180kg of engine weight, except the 75hp E-TEC which came in at 145kg (and I could even head up to the 90hp E-TEC as it weighs the same as the 75hp). I even have the option of going the optimax in this hp bracket.
So then I hunted around to find out what my current engine weighs and uh oh, it around 102kg. So thats got me thinking, is it feasible to hang an up to 80% heavier engine off the transom? Will it be too heavy (gut feeling is yes)? How do I find out what my transom could safely handle?
If I have to go with 100kg or thereabouts, it somewhat limits my options to be honest. Honda and Suzuki (my two initial choices) will need to drop back to 50hp engines to break weight (100/110kg), leaving me with 60hp Mariner/Mercury/Yamaha/E-TEC all hovering around that 110kg mark.
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Tim
Posts: 2497
Date Joined: 26/09/06
HP
Might be worth looking at where they rate the HP on the new and old engines.
I had thought the old ones were rated at the flywheel and the new ones at the prop. If so a 60hp ETEC for example may be putting out more HP than your current 75.
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jersey
Posts: 393
Date Joined: 12/06/08
On the money
Tim is totaly correct,the old,and I'm not sure how far back that is ,moters were rated at the fly wheel, so by the time the power got too the prop you lost a fair % the new ones are all at the prop,I think the importance of moter-boat ratio is more the transome strenght,and structor,If you over power a boat,the transome might,collaps ratings are usually posted by the maker of the boat so go by that,get there a bit slower but stay afloat
Ryan Thipthorp (not verified)
Posts: 16
Date Joined: 01/01/70
my take!
Huggy, knowing that boat & original HP i'd repower with a 4 stroke or e-tec of similar size, dont wanna push the transoms strength!
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HuggyB
Posts: 2515
Date Joined: 03/08/08
figured as much
looks like the yammie 60 just hit the lead....
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diji
Posts: 404
Date Joined: 31/03/07
Dont worry about engine HP
Its transom capabilities, dont exceed it without beefing it up with engineer certs.
Insurance would love a over powered boat.
Tim
Posts: 2497
Date Joined: 26/09/06
Correct
Diji is right - Insurance will love it if you go over HP or transom weight. There should be a plate on it which gives both so atleast youll know what the max weight you can go to is.
** Rockingham Offshore Fishing Club **
Commodore Marine
Posts: 96
Date Joined: 12/05/08
Torque
All you boys are correct, but another issue that effect transom capability is torque.
A boat sitting stationary and underway at a trolling speed with the weight it will not be an issue, assuming that the transom is healthy in its construction and no degeneration has occured.
Where it is put under strain, when you accelerate heavily, thus exerting enormous amounts of pressure on the transom, upper deck and sub frame. This also occurs when you come off a wave hard or become airborne, and also when you are towing the boat on a trailer, as the motor is not supported by anything other than the transom.
So the wash up is that weight will be a problem when towing the boat and the jarring that occurs, it is then a problem when you accelerate due to the stresses is exerts on the transom when underway.
We have seen the result of to much torque on other boats, even when they motor is within the hp recommendation, ie the 350 Yamaha V8 has bent and twisted transoms (not ours) due to so much torque even though they may be rated for up to 400hp.
So the point of hp rating at the prop runs true and based on the age of the old motor care should be taken
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Andrew Welch
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till
Posts: 9358
Date Joined: 21/02/08
How long ago did motors go
How long ago did motors go from being flywheel to prop rated for poke?
davebarry (not verified)
Posts: 16
Date Joined: 01/01/70
What condition is the old 70 in?
I want to replace my old Suzuki 75 - '85 model with something "newer" without having to sell my house..
Your old yammer still any good?
Subsidise your deposit on the new one......
HuggyB
Posts: 2515
Date Joined: 03/08/08
the yammie is fine
but my upgrade is a fair way down the track yet. Finances are MIA and a new trailer is top of the list before a re-power.
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Faulkner Family
Posts: 18029
Date Joined: 11/03/08
as commodore marine mentioned
watch the h p . seen a cracked transome let in a bucket load of water due to old transome with to much hp.i would get it checked out before you invest your $ on a new motor .transoms from my understanding can be reinforced if needed
RUSS & SANDY
"A family that fishes together stays together"
RUSS and SANDY. A family that fishes together stays together