Repower time: Replacing Twin 70HP Carb Two Strokes

I'm replacing twin 70HP two strokes with a single four stroke. How much HP do I need?

I have a 6m aluminium planing trailer boat. It has always had twin 70hp carburetted Johnson two stroke outboards (pod mounted). Runs well with this setup but squats a little in the stern. The boat is about 1,000 kg loaded (1,666 kg on trailer). It currently planes at 15 knots and tops out at about 34 knots with new props.

One of the engines has just died. I've scrapped it and sold the other engine.

I want to repower with four stroke. Due to weight and servicing cost, I can't go with twins. So I'm left with a buying single. The question is how much HP? I'm, thinking I'll need a little more HP in a four stroke, so I was thinking 175HP?

 

You are sure to get many opinions and arguments here.
If it were me, I'd look for a 150 but you could certainly go higher.
Would you consider a 150 2-stroke?

 

Does the boat have a capacity label.? You should go with whatever HP is on the label. If it had two 75's then 150 is probably the best choice.

 

That's a tough one. Did a few searches online and let's ballpark the 70s around 230 lbds each (fair average of most the 70s I could find data on).

Using the suzuki outboards you would be lighter in the back end with a 140, but giving up prop surface area it might underwhelm. The 175 is an extra 50 pounds over the current setup, but a liter bigger and with the torque and power swings a full 16 inch prop. That extra inch and almost full liter of engine displacement might be worth the 50 pound penalty.

Friend had one on a spare work skiff and we beat the fire out of it, was a solid little outboard.

 

Not really. If I'm going to spend all that money then I want 4S for the economy, reduced and absence of blue smoke. Also higher WAF (Wife Approval Factor), which is important for a purchase of this size.

Edit: If I was to go with 2s again I'd go with twin Yamaha 90HP 2s. Those are basically bullet proof and have some rigging advantages over my current Johnsons. Also a bit more power wouldn't hurt.

 

No capacity plate. It currently has 2 x 70HP

An identical version was built and powered with 150HP 2S. So I know at least 150HP from single engine is an option.

 

The existing 70HP 2s engines are 113kg each (250 lbs). Currently swinging 14" props.

I've looked around and 70 - 90HP 4S options are around the 160kg mark. So adding about 100kg (220 lbs). As the boat already stern squats (even with the outboards trimmed in), I'm reluctant to add stern weight. Repowering with a 150HP - 175HP is the same weight as my current setup +/- 10kg.

The 175 Zuk is one of the outboards I'm considering. It comes in at 235kg/240kg so 9kg/14kg more. But I'd be shedding some other weight (rigging and some spares). I think it would be roughly even.

If I go single, I'll have room for trim tabs, or a perma-trim to lift her bum. I'd struggle to accommodate either with my current twin setup.

I will miss twins. The current set up runs shallow, tracks beautifully and is very maneuverable.

 

Embrace it and you'll get used to a single quickly.
I much prefer it (And I've driven a bunch of twins).
Use it's disadvantages to your advantage (get to know your boat).

The 175 sounds better with more info.
You are the best to know however.

Are you buying new? After sale service is a huge factor.
What's available?

EDIT: I drove a Zodiac Mark 5 with twin Yamaha 90's 2s but they weren't counter rotating. Very fun boat.

 

I've had single engine boats before too. Both wheel and tiller steer. I also drive shaft driven and jet boats at work - single and twin.

I can drive either. However, I like twins - being able to turn the boat in its own length and better for beach launching and bar crossings. But you're right I'll get used to a single soon enough.

 

These factors may influence your purchase choice.
We have a shortage of o/b's available here these days.

EDIT: New and used.

 

New or near new. There are pros and cons to both. In NZ with something bought new, there are strong consumer law protections and suing over something<$30,000 is very cheap.

 

If it matters to you, Mercury's 3.4L V6 platform is amazingly quiet. Almost too quiet for my liking, lol.

 

That could work. 3.4L is a beast of an engine.