Dinghy Engine - How Big?

[HopCar]

I'm often asked how big an engine do I need for my dinghy. I'm trying to come up with a rule of thumb answer. If you are happy traveling at displacement speeds, it's surprising how small an engine you can get away with.

The problem comes if you want to get up on plane. How small an engine can you get away with and still be happy with the performance?

It seems to me that most inflatable dinghies need at least 70% of their max rated power before people are happy with the performance on plane.

Tell me about your dinghy / engine combo.

Do you feel the need for speed or are you happy at displacement speeds?

What kind of dinghy do you have, Hard, Hard Bottom Inflatable, Fabric Bottom Inflatable?

How big is your engine?

What is the max HP rating for your dinghy?

Are you happy with the power you have?

 

[BruceK]

2.7 meter inflatable, big tubes, inflatable floor, very stable," rubber" bottom, using 5hp Tohatsu, (dinghy rated to 8hp). Good to use, very economical at low speeds, easily planes with 2 well positioned persons, or one using a tiller extension. I`d be ok with less hp and less speed/no planing. Motor weighs 18kg, gets taken on and off the dinghy,a less awkward to lift weight of say a 3.5hp, would be better.

 

[HopCar]

Thanks Bruce. You're just under my 70% of max and you're happy. Good info.

"Motor weighs 18kg, gets taken on and off the dinghy,"
That's the other thing that comes into the equation. Do you install and remove the engine each time? Most people I talk to don't like to lift a motor of more than about 50 pounds. That limits you to about a 5 hp. 4 cycle engine. A little more if you can find a 2 cycle.

 

[Tidahapah]

I have a 3.7 mtr (12 ft) tinny with Aero Floats a powered by a 15 hp Yamaha 2 stroke.
1 up goes like the clappers, but carries a load well when we have a few people or a load of groceries.
This also serves as my fishing boat so has a sounder and GPS plotter.
Bare boat weight is about 75 lb but comes in about 100 lbs with extras like full floor , anchor, chain and rope etc.

Cheers
Benn

 

[BruceK]

Could be at cross purposes, but 2 strokes are/were lighter than 4 strokes.
Yes, the engine comes on and off, a combination of weight and awkward positions when lifting it(and deep water but I often use a securing line while doing it) pushes me towards less motor/less weight.
Good luck with the survey, look forward to the results, and the drifts.....

 

[Moonstruck]

10 1/2' RIB with big tubes. 15hp 2 stroke. I just found out it goes like the Clappers (whatever that is).

 

[caltexflanc]

Get the highest HP it is rated for if you want to make some time. Anything else and you are not getting the most from the craft.

 

[Ski in NC]

I have a 8' inflatable. I've used three motors: 4hp two stroke two cyl, it would plane off with one POB. Two POB, it plowed. 6hp, it would plane with two, but engine was heavy and made the dink tow badly (bounced). The 4hp died (stripped driveshaft splines), then got a 1976 2hp two stroke. It will NOT plane. But since I almost never planed anyway, it does the job well. Advantage is being able to carry easily one hand and stash in lazarette. Simple simple. Craigslist find from a freshwater county. Happy with it.

 

[Rusty]

Our boat came with a 12' Flexboat fiberglass hull RIB and an 8hp Mercury 2 stroke motor. With one person, and a fair bit of effort, it would barely plane. We added a St. Croix seat and steering wheel along with a 20hp Tohatsu with electric tilt and start. We can coax 24 knots out of the boat now with two people in it. We are very happy with the dinghy now.

The Flexboat is rated for a 15 to 25 hp motor.

 

[HopCar]

"Get the highest HP it is rated for if you want to make some time. Anything else and you are not getting the most from the craft."

That's kind of how I feel but a lot of people worry about weight and money so are looking for a minimum acceptable engine.

Don, don't worry, they can cure clappers now.

I personally have an 8 ft. Avon soft bottom. I don't want to lift an engine big enough to plane it so I settled for displacement speeds and 2.5 hp.

 

[HopCar]

Rusty, I make that to be 80% of max hp. and you are happy with the performance. If they recommend a minimum of 15 hp. for your Flex Boat. that would be about 60% of max rated hp.

 

[MYTraveler]

14' Caribe RIB. Rated for 60 hp I believe. Equipped with 60 hp Yamaha 4stroke. Use all the power frequently but could live with 40 hp.

 

[HopCar]

Traveler, If you think you could be happy with 40 hp., that's about 66% of max hp.

I have a larger boat rated for 115 hp. I only have a 70 on it. I'm not happy. I can get it to plane when it's light but it's marginal. 70 hp. is about 60% of max hp.

 

[BruceK]

... I just found out it goes like the Clappers (whatever that is).

In Aussie terms: "faster than a fast thing", "fast as" or, just plain "fast".

 

[eyschulman]

Hopcar from looking at all posts it seems it depends on how fast you want to go and if you need to lift motor by simple manpower. I also find it interesting that some dyed in the wool displacement speed trawler owners insist on high speed dinghies is it a blow of the frustration of going slow thing?

 

[Scary]

I have three dingys with two engines

I have a Achilles 12. rib power by a 20 hp Yamaha 4 cycle. 24knts with 2 people and two large dogs. I have 12' Zodiac Rib that is currently powered by a 15hp Mercury 2cycle. Both boats are rated for 20hp. The Zodiac and Achillies perform about equal for speed and load. The weight savings of the Mercury over the Yamaha easily makes up for the 5 hp difference. The Mercury is my favorite, easier to start better torque and acceleration. The Yamaha is more fuel efficient and has a smoother idle, but that's it. The Yamaha's weight makes it not a portable motor. The Mercury weighs around 78lb right on the edge of portability in my mind. The third boat is a soft bottom inflatable keel Achilles 14' rated for 25hp. The 15 hp Mercury planes it fine with two people. I had a 25hp Mercury on this boat for years, my kids learned to ski behind it. That 25 Mercury weighed less than the Yamaha. I wish I had it back. I vote for the max rated power.

 

[BandB]

I'm only going to talk about 11' and up ribs and a Whaler as I have no experience with smaller. Also, I'm talking about davit systems or cranes to handle them.

We have a bit of a love of dinghies, maybe a dinghy fetish. We have Williams Jets and have a 14' with a 120 hp Weber (Gas), a 11' with a 100 hp Weber (Gas) and have on order a 17' with a 100 hp Yanmar (Diesel). It's a disease, can't help it.

We did look at outboard dinghies and in all cases would have chosen max hp. It's not top speed that we'd most do it for, it's just planing with a full load of people or gear. If anything we found the max under powering some of the smaller ones we looked at. Some people love Whalers and the 11' Sport model use to be one of the most popular tenders. But it's only rated for 25 hp. The boat only weighs 460 pounds so with one person maybe ok. But it's rated for four people and you put four in it and it just plows through the water. You've added more weight than the boat itself. An 11' Novurania weighs 420 pounds and is rated for 30 hp which means 25 hp with most brands. It's rated for 5 people. You can imagine how it will sit with 5 so any less motor would be a greater problem.

Now it does become a matter of use. If you anchor and then just use the dinghy to make a short trip to shore, then small engine and 5 knots is fine. And if you only have two people ever using it then the power for most is fine. We use ours to explore. We will take it from a marina and explore all nearby canals and coves and beaches. We've been known to run one a total over over 100 nm in a day. In the Bahamas we anchor off an island and then with the shallow draft of a dinghy just explore the entire island, circling it and enjoying.

It's just like any boat, tailored to your use. But underpowering will disappoint you when carrying more and also in rougher conditions if it's plowing on you. The other thing is that in most choices the weight between the motors you're considering is very minimal difference. Now if it means a significant step up in weight, maybe it's a concern. A Yamaha 9.9 weighs 93 pounds. A 15 around 120 pounds, a 20 weighs the same as it's the same block so I see nothing to justify the 15 ever. Now the 25 is a different engine and weighs around 180 pounds, so I can see an argument for the 20. The 30 is another step up to around 220 pounds but then the 40 is the same block and weight as it. So, if the boat is approved for a 40, really can't see why one would choose a 30.

I know we're talking sizes above those many here use and others will have to address the choices below 11'. Also, the engine weights I used are Yamaha and it would be different with other makes.

 

[Insequent]

I have a 14' aluminium RIB with a 40HP 4 stroke, max power allowed. It runs to about 28kn, but with a load of people on board (carries up to 8) I think a shorter pitched prop would be better. Or a bit more power.

Like Benn I have it setup for fishing - Simrad chartplotter with GPS and sounder. Having a bit of speed available is good for supply runs, pickups or drop offs.

My old 14' Novurania FRP RIB had a 50HP 2 stroke. Interestingly the Honda 4 stroke is lighter than the old Yamaha 50 2 stroke. Of course the aly RIB is quite a lot lighter boat.

 

[Vashon_Trawler]

Achilles LT2 with a ~2HP British Seagull Model 45 Featherweight. Perfect for getting to and from shore. The dinghy and outboard store nicely in my engine compartment.

 

[High Wire]

I have an 8'6" AB Inflatables RIB and a 6 hp Tohatsu 4S. It will not plane with 2 adults at WOT. A 3.5 hp 2S would do the same job with a lot less weight. Fortunately I have a davit system so I do not have to mount and dismount the engine each use.

 

[Bay Pelican]

Bay Pelican has a 3.2 meter AB RIB with a 9.9 hp two stroke. The 9.9 hp two stroke is a compromise. We can plane with two people on board (the usual) but not with three. The two stroke uses more gasoline (4 US gallons per week usually) but requires less pull to start so that my wife can start it. It also weights significantly less than a four stroke so lifting the dinghy and motor to the upper deck is reasonable.

If I change the carburetor it would be a 15 hp. Even though we frequently travel a couple of miles to a dinghy dock I have not felt the need for additional speed (except when it is raining of course).

 

[timjet]

I've powered my 10ft Avon RIB with a Merc 3.5 2 cycle and a Johnson 8 hp 2 cycle. With the Merc it will not plane. IMHO the small 2 cycle 3.5 and under are not reliable enough to carry on a cruising boat. My 8 hp will plane with me and the dog and is at the max weight for me to carry. I've installed a Weiver leaver system so I don't have to carry it often.
Ideally I would like a 15 hp Yamaha 2 cycle, but it is at the max weight limit for the Weaver leaver.

 

[Northern Spy]

Roll up 10' WM inflatable with a 2hp Honda suits our needs fine.

Maybe because rotomolded kayaks or Minto dinghy with oars is first choice for shore excursions.

 

[jleonard]

We have a 10-4 Achilles RIB powered by a 15 HP 2 cycle Yamaha. It's quiet at idle speeds, and will easily get the boat on plane with 4 adults. At top speed its fast (26 mph) and very stable.
What I can't believe is how little fuel it uses esp when cruising at low speed. And how easy it starts.
By far the best dinghy we have had.
I have a 2.2 Merc 2 cycle as a backup engine.
I also have a Humminbird portable fish finder for exploring, works great.

 

[Andy G]

OK I'll play.

10' RIB(hyperlon) with 15 hp Yamaha/ 2 stroke, great combination lives on the davit off the back of the boat.It planes with 4 up, can tow the kids behind at a fair clip, and goes likes the proverbial clappers with one or two up.

The two stroke Yamaha's seem indestructible.

 

[ranger58sb]

HopCar, I think it's pretty easy to just start with the dinghy manufacturer's min and max recommendations, and then work from there. Discussion follows.

We started with a West Marine RU-285 inflatable; roll-up "teak and holly" floor with an air keel underneath. Minimum recommended HP was 5, max was 8. We used a 5-hp Johnson 2-stroke (built by Suzuki) which weighs 46-lbs... partly because I could also use the same motor on our square-stern canoe (max: 5-hp), and partly because we carried the dinghy deflated, installed the motor on the spot, brought it all home is separate pieces for winter storage, etc. Maybe the 5-hp would plane that boat, with only me on board, if I sat in the right place, held my tongue just right, and flogged the snot out of the motor. Otherwise, it made a very reliable putt-putt to take the various First Mutts to shore and back.

Added a davit mount to the big boat's swim platform (not pictured in the avatar), leaving dinghy inflated with motor mounted for the season. Still needed to mount/remove the motor in spring and fall, but otherwise, much easier to handle.

Changed to a Walker Bay Genesis 310 RIB, using the same motor. Min recommended is 10-hp, max is 15-hp. It planed once -- and only once -- in the 5-hp configuration. And that was a bit of a fluke. I was flogging the outboard, 90-lb First Mutt was in hood-ornament position up forward, and an overtaking boat's wake lifted us up on plane. What a surprise! But mostly, it made a good putt-putt for taking... etc.

In the meantime, we acquired a trailer; no further need to lift the outboard as a separate unit. Yay!

Have since changed to a Suzuki 15-hp fuel injected motor. (Still in the break-in period, actually.) Big difference. Planes in a heartbeat, runs like a scalded cat. Weighs 106-lbs (electric start version) but I don't really care, since that's well within the limits of our davit system, and we still have that nifty trailer.

It happens Suzuki's 9.9-hp EFI version is built on the same platform as the 15s and 20s... sort of a departure from previous outboard practices. In that case, I found no benefit to using Walker Bay's minimum HP recommendation. Other 9.9s are lighter, although the gap is closing; still other brands may offer a weight advantage to some.

All this has led me to believe the dinghy manufacturer's probably have a bit of a clue, and their min and max recommendations for a given boat are useful as a starting point. If motor weight is an issue, go lower (and maybe go 2-stroke), and expect to go slower with the reduced HP. If weight is only partially an issue, the min recommended HP will likely plane the boat under some circumstances, but not necessarily all the time. If weight is not much of an issue, the max recommended HP is likely to work 80-90% of the time... and when it doesn't, other contributing factors (overloading, unfortunate load distribution, etc.) might be the cause.

But I think you knew all that

-Chris

 

[cafesport]

We carry 2. An 11.2 ab roll up with a 2 cycle Yamaha 15. Planes with two and four aluminum 80's. Rated for 20hp. The other is a nautica 12.5 wide body with a 4 stroke Yamaha 40, which is rated up to a 60hp. Fast enough and better range than running a 60.


Via iPhone.

 

[BandB]

As much as we use dinghies, we do understand boats limited to smaller sizes and less motor, but the concept of having to pull it up and remove the motor is one that would greatly lessen our use. Most boats will accommodate a davit of some sort and we'd certainly go that route with capacity for boat and motor. I say this only because that plays a role in motor selection and removing the requirement to be able to life the motor off and on is a big step.

 

[Capthead]

First, I say the main factor is where you use your boat and dink. If you only use it in a protected bay you have many options and can go as big or small as you wish.

Here in the Pacific if you go cruising you will encounter anchorages where going ashore means crossing the surf line and pulling your dinghy up on the beach away from the rising tide. Also towing isn't a guaranteed option when crossing the channel to our islands you can encounter high winds and seas.

This meant for me, an 11' Avon RIB light with a 2 stroke 8 HP Johnson. I can take that engine on and off and pull the dink up on the swim step. I don't want davits because I have a fishing cockpit and love to kill bait.

I don't know what the rated HP of the RIB is but my guess would be 15. We can plane it with the both of us but it isn't easy to do.

 

[jwnall]

Just a little 2.5 Tohatsu on a small inflatable. Gets me around as fast as I want to go, sips gas as daintily as a hummingbird, and is light enough to handle easily.