What size outboard for a deckboat?

[Piaro]

Hi all,


About a year ago one of my kids decided he wants to go on a long trip down some rivers (Illinois, Ohio, Mississippi) so we agreed when he graduates from college in 2 years or so we'll build a river cruiser. I was planning on getting something like a pontoon boat and we'd build a cabin on it. I wasn't really looking at all but mentioned it to a friend and the backstabber went out and found me a project boat already. So now I get to look at it for the next couple of years while my son is still in school.



It's a 24' Rinker with a low time 5.7 ltr Mercruiser I/O. I'm planning on having a gross weight of 6500-7500 lbs when all finished.


I am wanting to put an outboard on it that will do the lions share of the cruising work which I'm hoping to be 8-10 kts at approx 80% throttle if that sounds reasonable. I'll always have the main engine available to deal with crappy winds or current but no sense in using it if I don't need it at that time. I have a couple spare 20 HP engines laying around but they're 2 stroke and I'm wanting a 4 stroke here so I can plumb it into the main tank and not have to worry about it. Anyway, for most purposes how would a 10 HP work in this situation? Since I'm not in any sort of hurry at this point I'm just going to be keeping an eye out for something and I figure worse case scenario put something on that I already have and from there guesstimate what I should look for as the permanent engine. I am planning on connecting it to the stern drive via a linkage for steering.



Anyway, interested in hearing what people here have experiences with and suggestions.



Thank you for reading this far! Hope everyone is having a great day!




Piaro

 

[psneeld]

I doubt a 10hp engine will move that boat weighted down much faster than 6 or so knots at 80% throttle.

That's about what 20+/_ center consoles (my old one and some others I know) make with a 10 hp kicker as a trolling/get home engine.

 

[Gdavid]

I would stick with running the 260 hp mercruiser that is in the boat, when cruising at the hull speed it will sip fuel, but be aware that hull speed is probably about 6 knots or less, depending on the actual water line.

We had a 25' wellcraft with that power plant for years, it is about the most common recreational powerplant afloat, parts are very easy to find and it is relatively easy to work on, maintaining outdrives that live in the water is a pain but it is less of a challenge in fresh water. As long as you are going to have to maintain the main engine anyway, you might as well use it. Running it at about 10 knots is close to the worst efficiency you will find with that boat, it will be pulling a massive wake and is not an efficient speed for a planning hull. If the boat has the appropriate prop on it and can run nice and level on plane, it won't kill you on fuel burn, however if bury the throttle and the secondary barrels on the carburetor open up, it can burn some fuel in a hurry.

A small outboard for a backup is a nice idea but having to rig up remote steering and engine controls is a hassle on a relatively small boat. Not the route I would go.

 

[Piaro]

There are a couple reasons I am wanting a second engine. The first one is for redundancy on a boat I'm wanting to take trips of a month or two where an engine issue isn't a trip killer, I'd like to be able to continue on with whatever it is I'm doing. That also includes going upstream on whatever river system I'm exploring if need be. Oh, I should also mention that I'm a certified master mechanic, formerly certified master engine machinist, and state certified welding instructor. I have 6 seasons experience working as a marine mechanic/electronics tech on Lake Michigan and the Mississippi & Illinois rivers and a couple decades now in the transportation industry.



Anyway, I want 2 engines and since my mercruiser Bravo requires 100 hour changes for both engine oil and gear oil if I'm on a long trip I'm going to have to do them mid trip somewhere. PITA and another reason for having a different motor do most of the boat pushing. So if I have another motor to do most of the cruising work, how big should I be looking at? As mentioned I want it to be a 4 stroke for a few reasons but being plumbed into the main tank is a big one.



For the rebuild here, everything including the existing deck is going to be removed. I am going to be moving the helm from midships to all the way to the front, so I am going to have to lengthen the control cables from 20 ft to 35 ft for starters. Not really planning on saving any of the existing engine instrumentation.



At any rate, it's a couple years from being in the water and I can toss a 2 stroke 20 hp on for a start to see how that works and go from there either up or down. The main thing I mentioned is it was one of my kids ideas, not mine, and he's going to be building just as much of it as I am. I think that with how today's generation seems to be wary of anything involving hard physical work he's going to be developing a skill set hard to match in his age group, not to mention that we're going to be doing some cool trips around the US with this.