Repowering with small mercury outboard

[logged in]

Hello All,

I am looking for advice on mounting a new 2015 mercury 40hp in place of a 1979 140hp inline six

First a little back ground, the boat is a 1979 Larson fiberglass open bow runabout, The boat is only used for trolling for trout in the eastern sierras.
The existing motor runs fine, but to get the boat to go slow enough to troll .5 to 1 mph target speed I have installed a 14x 9 inch pitch prop and a trolling plate.
Idle is about 550 rpm. The trolling plate works for slowing the boat down but the steering sucks when I have to deal with any type of head wind.

I am looking into repowering to a 40hp 4 stroke with a 12.5 x 8 prop. The new outboard will have a different mounting pattern than the old 140.

I would like to re-drill a manual jack plate to accommodate the old odd ball mounting pattern on the transom side then mount the new motor using the standard mounting pattern. The vertical adjustment on the jack plate should allow me to dial in the mounting height to maximize efficiency.

I would like to avoid drilling any addition holes in the transom if I can.

Let me know what you think?

 

[steelespike]

My very rough figuring 750 rpm 8" prop 54% slip = about 1.3 mph
600 rpm 64% slip= about .8.
Then you have to deal with an under powered boat as well.How big is the boat.

 

[Chris1956]

Check the width of the jack plate. It may be too narrow for your existing IL6 mounting holes. If it is a 2 piece Jackplate, you maybe can make it work.

Of course, you could install a prop with lots less pitch on the IL6, to slow the troll down w/o the trolling plate. Then when you want to ski, simply change props.

 

[ondarvr]

Even in the best situation I can see trolling with a 140 Merc at 1 MPH enjoyable.

Two options, buy the 40HP Merc, might still troll too fast though, or buy a kicker for far less money and have the best of both worlds, great top speed and very quiet fuel sipping trolling at the low speeds you desire.

 

[Teamster]

Look for a 4 stroke 9.9 kicker,.....

And check out trolling bags made by Amish Outfitters,......

Two tools any troller should have, IMO,......

 

[Silvertip]

You should probably look for a Big Foot motor in that HP range (40, 5, 60 HP). They run a deeper gear ratio than a standard engine so it can run a wider range of props including pontoon-specific props. The large diameter low pitch varieties on the Big Foot lower unit may work well for you yet still provide reasonable push for the larger boat.

 

[ondarvr]

About the other part, Drilling new holes is a none issue, it's easy and will cause no problems if you seal them, and the old ones.