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Old 06-23-2014, 10:56 PM   #1
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Dinghy Engine - How Big?

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?
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Old 06-23-2014, 11:08 PM   #2
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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.
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Old 06-23-2014, 11:26 PM   #3
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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.
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Old 06-23-2014, 11:29 PM   #4
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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.

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Old 06-23-2014, 11:34 PM   #5
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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.....
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Old 06-23-2014, 11:39 PM   #6
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10 1/2' RIB with big tubes. 15hp 2 stroke. I just found out it goes like the Clappers (whatever that is).
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Old 06-23-2014, 11:42 PM   #7
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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.
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Old 06-23-2014, 11:46 PM   #8
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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.
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Old 06-23-2014, 11:53 PM   #9
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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.
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Old 06-23-2014, 11:56 PM   #10
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"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.
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Old 06-24-2014, 12:06 AM   #11
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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.
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Old 06-24-2014, 12:12 AM   #12
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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.
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Old 06-24-2014, 12:21 AM   #13
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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.
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Old 06-24-2014, 01:20 AM   #14
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Quote:
Originally Posted by Moonstruck View Post
... 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".
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Old 06-24-2014, 01:29 AM   #15
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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?
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Old 06-24-2014, 02:07 AM   #16
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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.
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Old 06-24-2014, 02:13 AM   #17
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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.
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Old 06-24-2014, 02:22 AM   #18
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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.
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Old 06-24-2014, 06:30 AM   #19
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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.
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Old 06-24-2014, 06:30 AM   #20
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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.
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