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Old 03-03-2019, 08:45 AM   #1
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Dinghy power thoughts

Hey all, hoping to get your thoughts on what to power a dinghy with.

A little background - I'm "practicing" up north on the Mississippi and St. Croix rivers up in MN/WI. Currently have a 30 foot cruiser. We are generally weekend cruisers (2 small kids) with several long weekends and 1-2 week long trips per year.

I'd love to get a Highfield 310 Classic PVC dink. Tiller steer only. Part of the appeal is backwater exploring, fishing, just general messing around. 15 to 20 mile trips to neighboring towns for fun a possibility too. Not sure how I'd move it with the boat yet, towing is easy (slow), and haven't thought a lot about the davit thing yet. Anyway, really after opinions on powering a dinghy.

The Highfield 310 is rated for 20HP max. I've got an old mid 80's Mariner 8HP, and while I may start with that, not sure long term that will suffice.

Dinghy dealer (that's all he sells, 2-3 brands) says most guys put on 9.9 Merc's.

9.9 Merc - 84lbs.
15 Merc EFI - 99lbs.
20 Merc EFI - 99lbs.

The 15/20 Merc EFI's are brand new, and the same engine, same weight. Magical pixie dust makes the 20HP a 20 somehow...don't care really.

Is it nuts to get the 20HP? I'm pretending price isn't a concern. Will the dink be a death trap, and sketchy to run at WOT? Or will I be happy with the extra oomph (I'm not a small dude) and top end speed for those longer fun runs? Another concern is youth handling of it, right now my son is 6, I can see another 6 he'd want to take it out on his own. Should I be sticking to the lower HP to build in a safety factor?

Thanks for your awesome opinions. I'm a trawler wanna-be. Like I said, currently practicing via cruiser.


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Old 03-03-2019, 09:02 AM   #2
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Within the same outboard class, HP often increases when the manufacturer increases the upper limit or the RPM and more fuel. I would start with what you have and change as you see the need. In a perfect world, borrowing a 15 and 20 HP engines would be the best way to determine what you need.


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Old 03-03-2019, 09:31 AM   #3
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It has little to do with HP, but I would GREATLY favor an EFI engine over a carburetor. In my experience they are much, much less prone to fuel problems when left unused for prolonged time, like over the winter.

Now that said, I'd be a bit leery of any vendor's brand new model. I prefer things that have a few years of proof behind them.

As for HP, I'd probably go for the 15 or the 20. With any load, I expect it will be hard to get the 8/9 up on a plane. With one person the 15/20 will probably be grossly over powered, but with 2 people plus gear, it would still be nice to be able to get up on plane..

As for your kid, I wonder if there are devices to limit the throttle? That would guard against a sudden, unexpected goose of the throttle that could end in disaster, and give him time to get progressively more comfortable operating the boat. With just him in the boat, it would be easy to get in trouble with a 15/20hp. And be certain to always use the engine kill lanyard. If you have ever seen a run-away boat where the operator got tossed out, you will never run without a lanyard again.
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Old 03-03-2019, 09:52 AM   #4
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The 9.9 will seem a little weak when you have two or more people on the boat ,the 15 and the 20 weigh the same. If we're up to me in the money was not an issue I would go with the 20
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Old 03-03-2019, 10:40 AM   #5
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If the dinghy capacity rating plate says 20, 20 should be OK.

If you ever intend to lift the thing, solve that before you buy the dinghy and motor. Really important!

EFI good, carbs suck. (Or not, as the case may be.)

Weight is an issue if you intend to mount and dismount the outboard often... or depending on whether you decide to lift it... and in that case, HOW you will lift it (motor attached? or not?). Some models are slightly lighter in weight than others -- maybe the Suzuki DF20A, for example, which is also an EFI system; they were lighter at least when they were introduced. (Not a recommendation, just an observation... although we have a DF15A and it seems to work well enough.)

Or if you never intend to lift it, might put it on a trailer for winter, etc... then weight might not be such a big deal.

I dunno about Highfields, but I can tell you an internal "floor" is nice... I wouldn't go backwards on that feature again, FWIW...

As for safety with kids... a 15 and a 20 on that weight dinghy are equal... both can make that dink get squirelly in a heartbeat, so you'll have to solve that with training or other external solutions.

Were it me, I'd probably start with what you have while you think about it... and then if I were to go larger, I'd likely go with whatever the max on the capacity plate says.

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Old 03-03-2019, 10:52 AM   #6
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Originally Posted by mncruiser View Post
Is it nuts to get the 20HP?
No, not only is it not nuts and given your size and 15 to 20 mile possible trips, it's a good choice!

I weigh 240lbs. and have a 20hp Honda on my dink that goes faster than I want to.
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Old 03-03-2019, 11:34 AM   #7
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I like more horsepower and like EFI. However, there's another item you stated quickly and I want to comment on. Thinking to your your safety and to your son in the future, I would not go with tiller only. I want a steering console. As others stated, obviously a kill cord is in order. I don't want 20 hp on a tiller.
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Old 03-03-2019, 12:07 PM   #8
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Def go with the 15 or 20, and def EFI.

Does the Merc EFI require a battery? My Suzuki 15hp EFI is battery-free which is nice.
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Old 03-03-2019, 12:07 PM   #9
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Here's how I solved my similar conundrum on my 34. It's a 3.3m/10'11" PVC soft bottom dink that's rated for max 15HP. I run it with a 15HP 2-stroke Merc and it's very stable and can reach speeds up to 20 mph. 10-15 is a comfortable cruising speed for this setup. The Garhauer lifting davit works great for lifting the motor and dink.

Since then, I have added a dinghy float referenced and pictured here.

Trawler Forum - View Single Post - PVC vs Hypalon dinghy
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Old 03-03-2019, 12:42 PM   #10
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We have an 8hp on an AB Lammina 9.5. I think it is rated for a 15. The 8hp (Mercury) came with the mothership, otherwise I would have been eyeing a 15.

The 8 performs fine. It is very quiet and very fuel efficient. We can do 16 knots on plane. Would still go with a 15...
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Old 03-03-2019, 01:40 PM   #11
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I definitely would go with the largest HP that the manufacturer allows, but I also would go with a dinghy that has a console. I can't imagine handling a tiller for a 15-20 mile trip. I would think that if you had to hang onto a tiller for that long it would be a painful experience and you would stop taking longer trips because of the inconvenience and pain involved with the tiller.

The longest trip we have taken with our Whaler is 60 miles round trip. If I had to hang onto a tiller for that long I'd probably never had done it. Tillers are just not comfortable.

My suggestion is to buy the dinghy you REALLY want then buy a davit system that will handle it.
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Old 03-03-2019, 02:23 PM   #12
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How far and how fast do you want to go? I replaced a 2 cycle outboard with an electric and am very happy. It's quiet, no more carrying gas, and no mixing of the fuel.

I can go about 10 kts in a Livingston w/55lbs. of thrust. If I want to go far, I take a 2nd battery, but have never used it.
As others said, know what weight you're going to lift before installing davits.
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Old 03-03-2019, 03:40 PM   #13
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I'm going to second (or third, fourth...) what others have said here. The only reasons to not go with the 20 are cost and weight. As it weighs the same as the 15hp, to me if you can handle the weight of the bigger block go with the 20hp. That's exactly what I did with our current setup, a 20hp EFI suzuki that replaced a 15hp carb 2 stroke. It's on an 11' Caribe light.

It's honestly pretty easy to handle a 20 on a tiller, you can quickly and easily adjust the tightness of the steering so there is no feedback. I run our dinghy on long freedive trips all the time, generally 5 to 10 miles, but sometimes longer, it's not so bad. Twenty miles may be pushing it, that's pretty far in a small boat. The thing that gets me after a long trip is more my back from sitting down and banging in the seas, not the size weight of the motor.
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Old 03-03-2019, 04:13 PM   #14
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If you havenít taken these trips yet- stick with what you have and get a feel for what you really need. I had a blast with my 3.5 on our little river in WI- and never stayed out long to go very far- buying more power would have been overkill. Iím glad I didnít as I moved the main boat and didnít need the dinghy any more...
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Old 03-03-2019, 04:18 PM   #15
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Whether you go with a 20,15, or even a 10 hp outboard, buy it with a flatter pitched prop than standard and consider adding a Doel Fin.

Some years ago with a 9.9 hp Toahatsu 2 cycle on a 9.5' RIB I couldn't get up on plane with more than one person aboard. I changed the prop to a flatter pitched one and that helped. A Doel Fin helped even more, about equal to the flatter pitch. After both changes I could easily plane with 350 lbs of people, a 40 lb dog, 30 lbs of groceries and 50 lbs of gas.

With today's 4 cycle engines which inherently have less low speed torque, it is even more important to use a flatter pitch prop which lets the engine rev up into its higher torque zone to get up on plane easier.

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Old 03-03-2019, 06:24 PM   #16
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I would go with the higher hp. Keep your 8 hp for when you want your child to run the boat or pick up a trolling motor for the kids to learn with before you let them loose with the larger motor.
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Old 03-03-2019, 08:56 PM   #17
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I've been running a Highfield 360 Classic with a 20hp Suzuki tiller for the past 3 years. Great little boat. I run 18kts lightly loaded and will plane with 4 adults and 2 kids in salt water, but won't quite do it in freshwater with that much load.

PVC is fine for up north, but if you plan on taking it to warmer climates spring for the hypalon.

EFI is a game changer- almost all fuel problems go away, significantly better fuel economy at less than full throttle cruising. I'll never own another outboard without EFI.
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Old 03-04-2019, 11:31 AM   #18
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Something i've learned owning a dozen smaller fishing boats... Always install max hp.

The manufacture wouldn't rate it at 20hp if it was going to be dangerous.
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Old 03-04-2019, 02:27 PM   #19
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Here's an article that helped inform me, basically bigger isn't always better, there are other factors to consider as well:
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Old 03-04-2019, 03:26 PM   #20
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Since you have an 8hp motor already, try it with that and see how it does. Go from there.

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