cruising with only one twin in the water

[pcg]

I’d like comments on the strategy of cruising with just one outboard, on a boat with twins. I hope to be a C-Dory owner at some point and I’m in the early stages of deciding what I want, economical cruising being at the top of the list.

All other issues (reliability, safety, maneuverability, weight aft, etc.) aside, I’m wondering how much more efficient (looking at mpg here) it is to cruise at displacement speeds with one twin in the water and the other out, as opposed to a larger single outboard in the water and a kicker out of the water. I’m also looking for comments on prop selection in this scenario, and how that will impact the ability to get up and plane. Thanks in advance for sharing your experience and knowledge.

 

[DavidM]

You will probably only see a small improvement, maybe 5-10% in fuel economy with one engine vs two. And you will be significantly loading the in the water engine, so don't even think about planing with one.

The same thing is true with prop selection. If you prop at the lower end of the engine manufacturer's recommendations, typically 5,000-6,000 rpms you will pick up 5-10% better fuel economy. That is true for slow speed cruising and shouldn't hurt the engines. But if you cruise at higher rpms it will be better for the engines if they are propped closer to 6,000.

The foregoing is taken from my experience and reading about cruising diesel boats. But I think it is generally true of outboards as well.


David

 

[pcg]

Thank you. To clarify, I would not attempt to plane with one motor. Just wondering how propping for low speeds will affect the ability to plane with both motors.

 

[ssobol]

Like most things engine powered, one factor is how much weight you are moving. If your key requirement is economical cruising a single lower HP motor is the best solution. For instance, the Nimble Nomad is designed to cruise at displacement speeds and usually has a single 40 or 50hp motor. You could do the same with a C-Dory.

The Nomad is comparable to a 25 in terms of size and weight.

 

[pcg]

If your key requirement is economical cruising a single lower HP motor is the best solution.

The reason I'm looking at the twin scenario is I still need the hp to get over bars safely. We plan to fish in the ocean with this boat as well. I would go with a single 90 and a kicker, unless I can get a substantial savings in cruising economy by having twins and cruising with just one.

 

[rogerbum]

If your key requirement is economical cruising a single lower HP motor is the best solution.

The reason I'm looking at the twin scenario is I still need the hp to get over bars safely. We plan to fish in the ocean with this boat as well. I would go with a single 90 and a kicker, unless I can get a substantial savings in cruising economy by having twins and cruising with just one.

If substantial savings is a concern, the 90 + kicker is the better overall plan. Total cost of ownership for twins is higher and if you spend a lot of time trolling (or cruising at displacement speeds), it's better to put those hours on the less expensive kicker.

 

[Hunkydory]

In 16 seasons we’ve cruised our CD22 with twin Honda 40’s over 20,000 miles with over 80% at near displacement speed or very near at under 7 smph. During this time, I’ve tried both motors down & running, both motors down with only one running & one out of the water with the other down & running. At under 8 mph away from the docks or needing both motors for increased maneuverability, the best combination was both down with one running. I have found only a one to two tenths of a mph loss in speed at a set rpm for displacement speed with this combination vs one motor down & the other lower unit out of the water. The gain is a much larger increase in steering control in the chop. If sea conditions get rough enough where more speed is needed for a better ride & boat control, I then run both motors up to a boat speed that gives the best ride for conditions with fuel savings a secondary consideration. Fuel savings isn’t primarily for money savings, but rather extension of cruising range between fueling in remote cruising areas.

Because we run either light at very high elevation, Yellowstone Lake or at sea level very heavy, I run a 10 pitch 12 inch prop. Running light at sea level the boat will plane on one motor within acceptable motor rpm. Running heavy while long distance cruising at sea level with one motor at 2900 rpm will = 6.6 mph. Both motors must run at about 2300 rpm to = the same speed. It would take me running more time then I cared to prove a point to see what the exact difference would be between running one or two motors, but it makes sense to me one motor at 2900 will burn less fuel then two at 2300, while doing equal work, but I’ll listen to others who have good substantiated data to say otherwise.

It’s hard to get a accurate fuel burn rate for mileage, when cruising with a mix of displacement speed & plane, along with current & wind changes. My best estimate using the see through fuel tanks for fuel used over distance is around 7 mpg on the one 40 hp motor at 6.6 mph, which is slightly over a gallon per hour. I’ve been told by other C-brats that they do much better on a single 90 hp Honda, so perhaps this isn’t where the twins shine.

During our short cruises on lake Powell, I run a 11 pitch prop & see a noticeable difference of lower rpm & quieter on plane & displacement, but I didn’t see a noticeable fuel gain other then what would be gained anyway by running a much lighter boat.

With the 10 pitch 12 inch props the boat cruises when heavy at 15 mph at 4500 rpm & if my memory is accurate tops out near 21 mph at 5500 rpm.

Jay

 

[Hunkydory]

If your key requirement is economical cruising a single lower HP motor is the best solution.

The reason I'm looking at the twin scenario is I still need the hp to get over bars safely. We plan to fish in the ocean with this boat as well. I would go with a single 90 and a kicker, unless I can get a substantial savings in cruising economy by having twins and cruising with just one.

If substantial savings is a concern, the 90 + kicker is the better overall plan. Total cost of ownership for twins is higher and if you spend a lot of time trolling (or cruising at displacement speeds), it's better to put those hours on the less expensive kicker.

On the other hand cruising with a kicker at displacement speed will give much less control of the boat compared to the much larger propped & displacement sized twin. I wouldn’t want to cruise the way I do with our CD22 on a kicker. Also with 2300 hours now on our twins, if those hours had all been on a single with 80% of the running at displacement, that single motor would now have 4140 hours, so unless one is primarily using the kicker for back up & fishing, the economics could come out closer to even. With out a very high percentage of displacement speed cruising, I agree with the overall cost being higher with twins.

Jay

 

[Pacificcoast101]

I've had to drive my Tomcat a couple of times on one motor. It barely moves unless I have the working motor wide open throttle, sucking down gas. Having the second motor out of the water makes steering difficult because you're trying to move the weight of the entire motor with the hydraulics rather than the weight in water.