Dinghy Power

[wincrasher]

My new Ranger is coming with an 8' RIB. I was thinking about powering this with a trolling motor.
I've been reading on the forum about people using the Torqeedo electric outboard, but frankly I don't want to spent that much.

I have this Group 31 lithium deep cycle battery. I bought it for an RV, but now it's free for other uses. It only weighs 28 lbs, so not so tough to deal with. I also have a 110v 20 amp charger for it. The battery is 110 AH, so it should give alot of service for a trolling motor (you can draw them down 90% without hurting them).

In my research of these motors, they seem to have a limit of 65 lbs thrust for 12v. The ones above that are 24 volts.
Is 55-65 lbs thrust "good enough" to move a RIB? Is there any recommendations on which brand to get? Amazon reviews are all over the place.

 

[ozoner]

There would definitely be some compromises. As you pointed out, they are limited in thrust, and with 100-120b RIB, a 30 pound battery, and 2 people (450-500 pounds total weight), my rough guess is that you would never see over 4-4.5 knots, and that is at full power, and less against a good headwind or some waves.

The main appeal here is the lesser cost as compared to a Torquedo, as well as the fact that you have an existing (fairly expensive) Lithium battery. As long as that battery stays in decent condition, you would be fine. But as soon as it fails, you are faced with maybe $1000 to replace it (ballpark from a really quick google search for Group31 Lithium RV battery). If you throw in the cost of a good saltwater trolling motor (the bass-boat freshwater ones will fall apart in 3 months in salt water), along with a replacement battery in 1-3 years, I would guess you are at least getting into the range of a Torquedo - Defender usually has the 2 HP version for somewhere around the $1700 area, and the 4 HP model for around 2K.

If the expense is your controlling factor, I bet you could find a good newer used 2-4HP gas motor for less than half the cost of a quality saltwater electric trolling motor, and throw your Lithium Group31 into your house bank on the Ranger for extra capacity.

 

[stwendl]

I made some measurements a few years ago.

My 9.9hp yamahas had 289 lbs of thrust each and powered a 14000 lbs catamaran to up 7.5kn. My dingy motor, a 5hp 4 stroke had about 105 lbs of thrust. I was at the time experimenting with an electric motor attached to the lower unit of a Honda outboard and reached the same thrusts. With tha motor I was able to get a lightweight dinghy with one person on a plane. As soon as a second person got on board it was not as easy any more. I was also carrying 4 automotive 12v batteries to power that electric drive with 48v

I had a torquedo at one time too with similar thrust as the 5hp. Batteries weight and capacity will be your biggest issue. But it all depends what your want to use it for. If you just come from anchor spot to shore it might be just fine. However, anytime you face currents, winds or so you wish you had those extra pounds of thrust and less weight. A decent charger and power source will also be required to replenish. Until those batteries come down in price it may not be as suitable for marine use.

You might be better off with a small two stroke motor

 

[Crewdog]

This big storm cloud, with what ended up to be 40 mph gusts, blew up suddenly in Bahamas 2014.

Tommy and Ashley Gaston racing back in their gasoline powered dinghy.

I guess powering a dinghy is choosing the right tool for the right job; in this case the gas outboard was correct.

I suppose one would have to consider the prime usage of dinghy; protected waters, ocean fishing, capacity, speed, etc.

If we get a dinghy, right now i'd probably go with a small outboard, if only that you can always carry and easily purchase more gasoline a lot quicker than recharging batteries or buying spares.

 

[Scuffy]

We have a small (55 pound) Bombard Max 2 dinghy with an air floor. We use a 2.5 hp Yamaha. This seems fine for us. The Suzuki 2.5 is a little lighter and has received good reports. Both engines above are four strokes. If two stroke is what you wanted I would look for a used Mercury are one the lightest at 3.5 hp and I think 27 pounds. At four horse power I think you start to be able to get an engines that have forward and reverse gears instead of the 180 spin of the motor for reverse action which is common in the smaller motors. Most four horse power motors also will accept a separate fuel tank instead of just the built in tanks in the smaller motors. The real enemy is the weight for us. I always tie a line to the dinghy motor when I'm hanging off the back of the swim platform trying to hang the 37 pound motor on the transom of the dinghy. That's when I am reminded that smaller motor is just fine.

 

[Cruzerboy]

We've had our 29s and factory dinghy for only one season, but we love our Lehr propane outboard. Light, simple, and no smelly additional fuel source to mess with. Propane canisters are available everywhere.

 

[jcarrdive]

We have an 8' dinghy with a 2.5hp Suzuki.
With me alone, it will not plane. (190 pounds). With Ela and 2 large dogs we have learned to plan a lot of time to get places.
If I had it to do over, I would get the 4 HP, or maybe a 6 hp. Our little 2.5 is easy and light though.
jeff

 

[wincrasher]

Well, I went ahead and ordered a 55 lb thrust salt water rated troller for $199. We'll see how it goes. If it's not adequate, I can always use it...as a trolling motor.

I did find that the Torqueedo is only 67 lbs of thrust. For $2000.

If I find I must have a gas outboard, the Honda 2.3 looks pretty good for $900.

 

[TomFoolery]

The 2.5 Suzuki can be bought for $760 shipping included. Weighs 30 lbs.

 

[Namaste]

We run a Portland Pudgyhttp://www.portlandpudgy.com/ powered by a Lehrs 2.5 propane outboard and are very happy with this combo. The pudgy rides on the swim platform on Weaver davits. This is not a fast dinghy setup but it works for us. We did not want to store gasoline on the tug.

 

[Two killicks]

In january, the yamaha outboard will have a new feature, you will be able to lay the motor on any side that you want! I am going to get the 4stroke 2.5hp from Yamaha for my Zodiac (7.5 feet).