Which engine for my dinghy?

[fursoc]

Hi, I am building myself a catamaran tender for our liveaboard 12m catamaran that my family (2 adults, 2 kids) will be living on from early next year (Link to the plans. I'm building the 3.2m).

The designer of the tender says the max power is 15HP. Since 2 stroke engines are pretty much banned in Australia, I am looking at 4 strokes.

Every 4 stroke engine that I have researched (suzuki, yamaha, honda, tohatsu) in the 15HP range is basically a cut down version of the 20HP. This means that the 15HP is the same weight and practically the same price as it's 20HP big sister.

Why wouldn't I get the 20HP engine? Isn't more power always going to be better? Fully loaded with provisions and the whole family we could probably plane back to the mother ship. With 20HP I could drag the kids on a donut and have a passenger watching on.

Why limit myself to 15HP?

[a64pilot]

I didn’t.
Your building one, so it’s up to you to determine what is safe and what isn’t.

[john61ct]

May want to be careful opening the throttle all the way, especially with kids aboard.

[admiralslater]

The advantage of a hard bottom dingy is that you dont need a ot of horsepower for a good speed . I would go with somthing smaller for that boat maybe 6-8 . Also that size of motor can be man handled

[travellerw]

Personally I would see if you could borrow a couple of motors before making your purchase. I think you might be shocked at how easily the hard dink is pushed.

If I was a betting man, I would guess you will find a 9.8Hp more than adequate.

[admiralslater]

Originally Posted by travellerw

Personally I would see if you could borrow a couple of motors before making your purchase. I think you might be shocked at how easily the hard dink is pushed.

If I was a betting man, I would guess you will find a 9.8Hp more than adequate.

I agree

[John_Trusty]

I've seen issues with dingy engines that only one crew member can operate. If only "dad" can mount it on the dingy (if taken off for passage), or start the pull-start, then dad has to do this task. In an emergency, or just convenience for who does things when arriving, sometimes dad is not available. Think about future use.

[vanderd]

I had the same dilemma for my 11ft dingy. We had a 15 hp Yamaha. The 20hp Suzuki is fuel injected. In fact it’s the only remote steering 20 hp w/o a carburetor. The motor is great very fuel efficient and even has pull start should the battery fail. The only down side it’s only available with a long leg so I needed to add a riser block. Couldn’t be happier

[LeeV]

Torqeedo? If you’re building in foam and don’t need the “zoom,” it may suffice, as well as being green (no fuel, battery...).

[jkindredpdx]

I use an electric with a couple lawnmower batteries... It's nice because it is light and easy to handle. It always starts as long as the battery is charged, that's why I have two batteries. I have a small solar panel to keep the batteries topped off. I don't have any gasoline onboard... just diesel.

[S/V Alchemy]

Originally Posted by admiralslater

I agree

There is also the element of a short leg reducing weight and the beauty of a properly pitched prop. If said dink is always going to be cargo and passengers, you don't want speed. You want thrust. You figure out what HP from there. We have a 50 pound Portabote that goes 4 knots with a Honda 2.3 HP air cooled engine that my compact wife can one-arm onto the transom. That suits us as we are never in that much of a hurry with jerry cans, groceries and other shoreside stuff. Your mileage, pun aside, may vary, but 15 HP is a lot of power for a hard dink.

[DrDelmas]

I had a 10 foot hard dingy and a 2 cycle 15 hp. I pulled the kids many hours behind the boat until they got into there early teens.

[SteveSadler]

Thats a small dingy for 4 people. A rib with large tubes will have 3x the capacity per ft and not take waves over the bow. I did see an inflatable cat tender 12', 18" tubes, trampoline. and sailing rig. Looked good to me.
A daily problem is trying to open a spot at the dingy dock with 2 hulls. Stepping onboard by the bow with so little buoyancy.
I used to row my 12' hobie cat to shore daily, thinking it was faster than a mono hard dingy but couldn't keep up with the girl on the mooring next to me in her 8' ply tender.
More relevant to your question, an electric trolling motor will push you along nicely. As you won't plane, small and light is best.

[a64pilot]

His point is I believe that using the Suzuki as an example, the 9.9, 15 and 20 HP are all the same motor and all weigh 99 lbs.
So yes you may can get by with a 9.9, but why not have twice the HP for exactly the same weight?
I don’t understand why people keep saying, oh no, it might be unsafe at full throttle.
I can absolutely tell you that if I were to run my Wife’s car at full throttle it would be unsafe as it would be approaching 200 MPH.
However we don’t, we enjoy the acceleration at times, but have enough sense to not run wide open.
Why can’t people accept the same for a dinghy?

[finefurn]

For a Dinghy I 'd put more focus on weight than horsepower. Are you you going to tow the dinghy when underway with the mother ship, and mount the motor on the rail of your mother ship. In a mooring field you don't want to be throwing up a wake so speed is not necessarily a desirable thing. We have a 10 ft hard bottom inflatable with the Honda 2.3 weights 29 lbs. I had a Mercury 5 hp that I gave to my son that weighed easily twice as much. Much happier with the 2.3 hp