Planing Dinghy: Hp vs Dinghy type vs #Passengers

[wingssail]

Interesting observations Mc HughV

Originally Posted by MicHughV

...To get that boat up on plane quickly requires judicious use of the trim tabs out back, the engine trim and full on throttle...
It will also get on plane without all that "trim" effort, but struggles a bit...

I too have a backgound in powerboats. I found a lot of extra power, and the right props, and full trim, got us up on a plane quickly.

Once on plane, it has a "happy place"...which is around 3,500 engine rpm. It will be on plane at at around 3,000 rpm, but it tends to be squirrelly, it's on a plane, but just. Less than 3,000 rpm it will just wallow. I've experimented with 3,100 pm....3,200 rpm, etc...I actually get better fuel consumption at 3,500 rpm, than I do at 3,000 rpm.

Then we could trim out, raise the tabs, and let the power loaf a little

You can hear it in the engines. At 3,000 rpm, engines are not "happy"...they feel like they are lugging. At 3,500 and above, the engines just sound better. I can generally tell from the the location of the spray alongside the boat, when I've got everything dialed in correctly.

You'd have to be there.

I'm with you.

Sea state is another issue. To keep the boat on plane in rough seas, the bow needs to come down, so as to keep the hull as flat as possible, so the trim tabs must be down. How much down, varies a bit...

I really avoid putting the bow down, it screws up the steering and prevents the boat from running easy. Find the speed which keeps it on plane and reduces the pounding, then work the throttle and the steering. With the seas, try to go just a bit faster them they are..

Working a deep "V"powerboat in big waves is an art, (other than running full bore and just taking it, I like that too, but I've broken some boats that way)

Too bad we don't have these options on a dingy. I run my zodiac at full throat in a chop occasinally, not for long, but it wails.

[Adelie]

Latest update.

[MicHughV]

Wing....outboard engines for dinks can have some limited trim induced by varying the tilt arm, but most people don't do this, but it does make a difference.

I have some friends at the marina that run the big boy fishing boats. Impressive boats. Flat out running in just about any sea state, but insane fuel consumption.....in order to chase after the big fish requires a good 50 mile run offshore..no sweat for them.
I tend to pick my weather for fishing excursions and limit my offshore distance to around 30 miles or less, mostly less.

Adelie's spread sheet seems to bear out that 15 hp is the sweet spot for a dink motor. In my neck of the woods, it's almost 99.9% Yamaha.

[Ween]

It looks like I inherited a generic solution from the PO.

10' Aluminum RIB - 9.9HP four - planes with 2 people + stuff

[Adelie]

Originally Posted by MicHughV

...

Adelie's spread sheet seems to bear out that 15 hp is the sweet spot for a dink motor. ......

It depends on what you mean by sweet spot.

It looks like 10-11' RIB, 15hp is what most people ultimately get. Lets them carry partner and guests so 3+ or 4, and has a higher top end speed with 2+.

9.9hp will consistently get 2+ on a plane in a 9-10' RIB.
8hp may get 2 or 2+ on a plane, and probably will if you pitch the prop down sacrificing top speed.

Looks like planing with a 5 or 6hp is possible for 2 maybe 2+ in a Hard or folding dinghy if the boat and passengers are kept light and the hull is optimized for planing. I don't see good data for this either way.

[MicHughV]

My knowledge on small outboard engines is that a 9.9 and a 15 hp engine are usually the identical engine, but the 9.9 is tuned to run about 500 rpm less, hp being rpm derived, so much the same power, but lower top end speed.

The 9.9 came about to satisfy some interior lake requirements of 10 hp or less..that's why you get that oddball number of 9.9.

It's interesting to note that engine hp usually goes up in increments of 5 hp for the smaller engines, and 50 hp for the larger hp engines.

You never see an outboard with 12.4 hp.....

Your above statement about sums up what I've found.

Other than the hassle of getting a 15 hp on and off the dink, it's a sweet motor.

One other thing to consider is that most small engines are started with a pull cord. Larger engines, 25 hp and up are mostly, not all, electric start..more weight and complexity.

The 8 hp 2-stroke Yammie comes in at around 60 lbs...

I don't have any knowledge or experience with the modern 4 stroke 8 hp engines..

Manufacturers toy around with hp numbers to fit a market. My 150 Yammies are 2.7 litre 4 cylinder engines...the 200 hp 4 cylinder Yammie is a 2.8 litre engine....a 100 cc gets you 50 hp ?? hmmmmm. You can get a 200 hp engine in both 4 and 6 cylinder varieties, but obviously more torque with the 6. Off course the 100 cc difference does not expand to the price difference between the 150 and 200 hp engines, that little extra 25 cc per cylinder translates to $1,000's.

Mechanics tell me the 150 Yammie should really be rated at about 175 hp...but the 150 just has a better ring to it from a marketing point of view.

Manufacturers never discuss or market engine torque...it's always hp talk.

As you say, prop selection is a key component. Our area bass fisherman go for the instant pop up plane, which would not work so good in the ocean.

It''s an interesting thread. If these were car engines, you could put them on a dyno to sort it all out.

[Adelie]

Originally Posted by MicHughV

My knowledge on small outboard engines is that a 9.9 and a 15 hp engine are usually the identical engine, but the 9.9 is tuned to run about 500 rpm less, hp being rpm derived, so much the same power, but lower top end speed.
In the US the 8 & 9.9hp 4-strokes are the same engine. I suspect that you are correct of 9.9 & 15hp 2-Strokes in Australia

The 9.9 came about to satisfy some interior lake requirements of 10 hp or less..that's why you get that oddball number of 9.9.
Yep. In the US there are some inland waters limited to "Under 10hp". I don't know if it started here or elsewhere.

It's interesting to note that engine hp usually goes up in increments of 5 hp for the smaller engines, and 50 hp for the larger hp engines.
Medium sized engines go up in 5hp increments, the small engines the increments can be as low as 0.2hp. At the low end there is 1.4,2.0,2.3,2.5,3.5.4.0, (I have an old 4.5), 5,6,7.5 (Ok, it's been a while since Honda made this size),8,9.8/9.9.
.....

It''s an interesting thread. If these were car engines, you could put them on a dyno to sort it all out.

I'm researching budget cruising. I know that the most common dinghy setup among long term cruisers outside US & EU is 10-11'RIB with 15hp 2-stroke. That's not what folks should start out with, at least not those starting in the US or EU.

I figure most cruisers will need/want to plane at least occasionally. Most cruisers are a couple. That means they need/want to plane 2+ (2 adults plus some gear). For folks starting in the US there are several reasons not to start with that.
A. They're just starting and making the full investment in a semi-durable item when they may reconsider cruising itself within a year is not wise.
B. Can't get 4-strokes on the cruising trail.
C Can't even get good service for the 4-stroke.
B&C mean that they will be changing motors at least within several years, so minimizing up front cost seems like a good move.
There are several other reasons I've thought of but don't remember off the top of my head.

[chasfgr]

Women go faster than men - my daughter proved that to the disgust of her two (lighter) brothers, when they were all sub-teens.
She could plane with a 2hp Johnson on a soft-floor inflatable.

She's now a senior academic in a Russell Group University.

[Jimmyhenry]

Highfield 3.1m. RIB. 1981 15hp Yamaha 2stroke. Planes very easily with 2 adults plus all the fishing gear. 4 if they are not all bigger men. 2 couples normalise weight is no problem.
I think RIB and length is the key.
Pete

[sv Stella Maris]

I remember couch cruising…hehehe.

Buy a dinghy, buy a motor. Spend time with other boats. Figure it out.

We had WestMarine 3.1 meter dual floor. Stepped fiberglass bottom. 15hp Yamaha Enduro. Fair seas it planed with our young family of 8. About 700 pounds for people + fuel + clothing…fyi.

[David Mathis]

Our 10' hard bottom inflatable with a 9.9 hp will easily plane and pull the grandkids (ages 5, 7,&10) on a kneeboard or water skis with two adults aboard but will not plane when hauling five adults. An extra 5hp (the boat is rated for 15 hp) would probably plane five adults but the boat gets skittish at max speed with only one person aboard. Although some small boats are safe at high speeds (a relative term), I'm skeptical about the stability of many inflatables at the normal speeds attained by rigid dingies when pushed by engines at the max rated horsepower.

Inflatables are in style and a great asset if one has a need to fold the boat and suitcase it. However, after having one for six years and not deflating it for the past four, the next dingy will not be inflatable. A little poke on a rock or some other hidden sharp object won't cause a crisis and the concerns about stability will be reduced.

[Jon Hacking]

Originally Posted by Adelie

I'm compiling info about how much horse power it takes to get a dinghy on a plane...

Great survey Adelie! We have a cat, so we prefer lighter alloy RIBs.

We started this cruise in 2001 with our 2 adult-sized teens, so we got an:
AB alloy RIB, 3.4m (I think), 25hp Merc 2-stroke, 6 adults

This lovely beast had a deep V, so the dealer said it needed the 25, but it was only $200 more than the 20. It would plane 4 adults PLUS all of our scuba-dive gear (heavy) into a stiff ocean chop. A bit twitchy at full throttle with only 1 person, but I towed & rescued big boats with it. Thirsty, but fat tubes made it dry(-ish). A lovely beast but after 8 years the tubes started coming away from the floor. Had to stick them back on with 5200 & alloy strips pop-riveted to the floor, pinching the Hypalon in between.

After the kids left we downsized. We now have a:
Highfield alloy 2.9m RIB, 15hp Merc 2-stroke, 4 adults

When we first bought it, it would struggle to get up even just with 3, & with 4 we all had to crowd forward & it would take a long time (& they better be smaller adults). Then we added a pair of Duol-Fins to the motor. These are fabulous! It now jumps on the plane, & it stays flat while it does so. No more crowding forward unless we're 4-up. It's a nice runabout (but there are times when I miss my 25hp beast, like when I'm trying to rescue yachts)

[Adelie]

Latest up date including a bunch of folks with hard dinghies I PM'd..... Most of whom only sail or row.

[teddy326]

8ft RIB, 5HP Yamaha 2 Stroke, 1Adult + small amount of gear. 2 Adults no.

[Jammer]

Originally Posted by Adelie

I'm researching budget cruising. I know that the most common dinghy setup among long term cruisers outside US & EU is 10-11'RIB with 15hp 2-stroke. That's not what folks should start out with, at least not those starting in the US or EU.

I figure most cruisers will need/want to plane at least occasionally. Most cruisers are a couple. That means they need/want to plane 2+ (2 adults plus some gear). For folks starting in the US there are several reasons not to start with that.
A. They're just starting and making the full investment in a semi-durable item when they may reconsider cruising itself within a year is not wise.
B. Can't get 4-strokes on the cruising trail.
C Can't even get good service for the 4-stroke.
B&C mean that they will be changing motors at least within several years, so minimizing up front cost seems like a good move.
There are several other reasons I've thought of but don't remember off the top of my head.

The life-cycle cost of having a 3.1m RIB+15hp outboard is large. Theft is a huge problem. I didn't write down a source but in my notes is a comment that the average full-time cruiser has an outboard stolen once every two years; most of the time the RIB goes with it. RIBs themselves are expensive and short-lived: hypalon helps, chaps help, but build quality is variable and warranty service poor across all brands. You end up spending $6,000 or more, boat and motor. Theft, short life, and other costs and causes of losses make this a $2000 annual expense. You could pay for a day at the marina every two weeks for laundry and shopping and it would be cheaper.


Hard dinghies with displacement hulls designed mainly for rowing are more accepted in the northeastern USA, Canadian maritimes, Great Lakes, and PNW. A 3 HP or smaller motor can be added. This is what I have now and it's a path I'm planning on staying on for the foreseeable future. Consider the advantages:

1. Lower overall cost profile due to longer useful life and reduced likelihood of theft.
2. Can be rowed, sailed, or motored with a lightweight 2 to 3 hp motor being most suitable. Such motors are inexpensive and of no interest to thieves. Also, can be powered with electric outboards if desired.
3. Can be towed and left in the water overnight except in severe weather or on long passages. (Some of the extra travel time required by the lower operating speed is recovered because there is no need to hoist the outboard and dinghy every night and launch it every morning)
4. Not susceptible to puncture. Easy to repair with epoxy, fiberglass, and paint.
5. Significantly larger interior volume although capacity is limited by weight.
6. Nesting designs are available for easier foredeck stowage.

The speed on these is going to be 3-4 knots regardless of propulsion source which is very reasonable for 1/2 a mile or maybe one mile but most people are going to get frustrated with longer trips especially on a repeat basis. So it depends how far from shore you're going to anchor, and in the areas where rowing dinghies are popular it is usually possible to anchor within 1/2 a mile of wherever you're going.


There are also dinghies designed for planing that are lightweight and short. They are unusual and hard to find. They provide a hull that is feasible to row (because it's stiff enough), though more difficult to drive than a displacement dinghy, and offer the speed of a RIB and the long life and puncture resistance of a hard dinghy.