Dinghy Engine - How Big?

Simple,
Depending on the capabilities of your boat....
Crane or davit.. as big or heavy with as much HP as the crane/davit will take..

No davit/crane?.. as big or heavy with as much HP as you will be able to lift..

There is no substitute for Hp or length to make a dinghy more useful and stable. We take two dinghy's along a lot of the time.. one for kids.. one for the skipper. The kids used to have a 3hp tohatsu, now replaced with a 15hp yamaha. The yamaha uses more gas but is way quieter. Ran the small motor for years so they could run it all day and not use much fuel, start it themselves, and couldn't go too far and get into too much trouble. The 15hp is now on a 11' high pressure floor Zodiac and its rated for 10hp.

On my Avon 3.45 sportboat I run either a 40hp Johnson or a 25hp Mariner.. the 25hp is ok and will plane 4 adults.. the 40hp is dangerous and makes the skipper smile!.. and uses a significantly larger amount of fuel.

Attached pics in different configurations.

HOLLYWOOD

This is good info!

It seems that if you don't have a way to mechanically hoist the motor / boat, you shouldn't plan on planeing.

You're not going to be happy with the minimum recommended HP. This often seems to be about 60% of max hp.

A lot of manufacturers don't set a minimum recommended hp.

You're not going to be unhappy with the maximum recommended hp.

I'm getting more comfortable telling people that if they expect to plane the boat, they should install an engine of at least 70% of the max rated hp. More is better.

Has anybody put a 5 hp. on a 12 or 14 foot aluminum boat? While a 5 won't plane a large inflatable (10 feet +) will it plane a hard boat?

"The 15hp is now on a 11' high pressure floor Zodiac and its rated for 10hp."
Hollywood, you're a naughty boy!

What is the Avon rated for?

We have a Zodiac RIB with center console 11 feet. The model is YL340

It has a 30 hp honda on it and gets moving pretty good even when loaded down.

We store the skiff on the boat deck, and use the Bayliner factory stock crane to deploy it. The total process takes less than 5 minutes for one person.

10.5 Avon air deck with 8hp 2 stroke, planes nice with two persons and the dog, but over that it struggles onto plane.

Having witnessed a number of setups, one thing I've noticed the longer the raft like in the 10' catagory the better at planing they are, the short rafts really need a lot of power to get on top, which I'm guess is the limited lift area.

75% of our time is displacement speed, but it sure opens up a lot of options if you can get on plane.

High Wire said:

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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I have the same size, different brand, RIB with the same engine. I am very disappointed in the 4s Tohatsu. It is even difficult to get on plane with one person. Only has about 5 hours on it but I am trying to sell it and will pick up a two stroke. I also have a davit system and stayed small to keep the weight down.

Minimum useful size

For years I got along with a 10 Zodiac rib on weaver snap davits. Powered it first with the 25hp Mercury which was fun but dangerously fast. That's when I sold that 25 and bought the 15hp mercury which was much lighter and planed the rib nicely to 28knts. I also used a 3.5 hp Mercury as a go to shore easy easy to take on and off. The rib was rated for 15hp and was very light. My kids could water ski and knee board behind the little rib. I had a engine mount in the cockpit for the 15 and with the stability of the snap davits it was possible to hang the 15 on the back of the rib off the swim platform. The dingy was usually carried on the snap davits on the platform even though I had a davit to put it on the salon roof. This little rib rowed very well and this is how it was often used. It took nano seconds to launch with the davits and was very easy to drag up above the tide line. this was an extremely useful boat. At the same time I had a 10', Avon sport boat with a 25hp Yamaha, center council with power trim, running lights, power antenna and aluminum fuel tank. This was a totally useless boat for me. It came with the boat and after a couple of years I traded it in for a open 12' Achilles rib with wide beam and large tanks. This boat is the boat with a 20 hp Yamaha which is the max rated hp. This is now the work horse as the 10' Zodiac was given away because the fabric was wearing out. This boat rows well but because of it's greater weight it is seldom rowed. It's launched with deck crane from the salon roof. I will mount snap davits to the swim platform as a way to stabilize the dingy for loading and unloading but not as a way to carry and launch.

caltexflanc said:

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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+1 to that.

You can always throttle back a higher hp motor and cruise slower. And with the higher hp motor you can throttle up and go faster or move a heavier load if need be.

Algae is 7' fiberglass dinghy circa 1972. She's got a 30 pound thrust trolling motor (fresh water variety) that has seen some tough times. But she still works. And when this motor goes belly up I'll drop another $100 bill in the Amazon coffers and buy another 30 pound Minn-Kota trolling motor for fresh water.

For slipping along little creeks and bayous, Algae is so quiet Skipper and I can glide by birds and get up close and personal with the wild life. I think we see more too at a slower pace.

The thing I like best about the trolling motor is I simply flick a switch and she goes. I do not have the upper body strength to start a regular outboard.



For the most part though, I'd rather go faster.

Go fast or go home.

Ours is a Caribe DL-12 Rated at 40HP with a 40HP E-Tec. I can get it to do 33-35kts trimmed right.

janice142 said:

The thing I like best about the trolling motor is I simply flick a switch and she goes.

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How do you keep it charged, Janice?

-Chris

LaBomba said:

I have the same size, different brand, RIB with the same engine. I am very disappointed in the 4s Tohatsu. It is even difficult to get on plane with one person. Only has about 5 hours on it but I am trying to sell it and will pick up a two stroke. I also have a davit system and stayed small to keep the weight down.

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I've only used the motor a month and I'm already looking for a replacement. 2nd least favorite OB motor owned. The buzzing tiller makes my arm numb.

ranger42c said:

How do you keep it charged, Janice?

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Hello Chris. Not so much charged, as equalized with Seaweed.

First of all I ran a pair of wires to the transom from my panel. Like everywhere on Seaweed, the neutral is on the left and Red is Right.



Just to the left was a hole (came with the boat -- tucked under the rub rail)
Eventually that will be a light for the swim platform for Skipper but until I find a fixture...



That's the wire from an old battery charger that bit the dust with a pair of terminal rings at the bitter ends. Because there was little to differentiate between positive and ground I painted one of the wires red. (That's acrylic paint from Walmart -- craft section, 50 cents for a little plastic bottle.)

Shove terminal ends through the hole and then attach to the posts with wing nuts. Then snap on to the battery. Done. By mid-day the boat is at absorption charge and I take off the battery cables to Algae.

Currently I was using a Group 24 (40 amp hours, so usable 20 amps) however it's not really enough for extended trips. A Group 27 or 29 would be better but that's the size I use for Seaweed and priorities being what they are...

Regarding power to run the Minn-Kota 30: (from the company)
"While we do not have exact amp draws at speeds 1-4, but I can give you some estimates. Speed 1 will draw about 6 amps, 2 will be about 10, 3 about 14 and 4 about 18. It takes a big jump from there; when it is on speed 5, the motor will draw up to 30 amps. In reverse, speed 3 is about equal to 5 in forward. I really do not have any information about speeds 1 and 2 in reverse, but you can make some estimates based on the forward speed."

Two articles on my website regard the trolling motors on Algae:
janice142 article Trolling Motor Woes
and
Janice142 article More Trolling Motors

So I don't really "charge" per se, I merely bring the battery in Algae up to Seaweed's level. For someone (me!) without the strength to pull a cord on an outboard, this is the answer that works. And it's not expensive.

janice142 said:

For someone (me!) without the strength to pull a cord on an outboard, this is the answer that works. And it's not expensive.

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Actually pulling a cord on an outboard or a lawn mower is one of the most dangerous things you can do. The nature of the motion across your heart. The exertion. Lots of bad has come when people have had trouble starting something and continued to pull for a while.

Life is terminal

Like I said before, it depends where you cruise as to what dinghy fits your needs.

Parks: Mine is a Caribe 9, glass bottom, but the lightweight model with no storage box and single floor. With a 9.9 two-stroke Merc she runs like a raped ape with one or two, and just get up on plane with three. My plan is to buy one of your two hp Lehrs when we are cruising the loop for most of our needs and keep the 9.9 aboard for a backup when we get to the islands and have those longer stretches from the anchorages to the beach.

Larry, Do you know what is the max hp. rating for the Caribe?
I'll give you the special Trawler Forum price on the 2.5 Lehr.

HopCar said:

Larry, Do you know what is the max hp. rating for the Caribe?
I'll give you the special Trawler Forum price on the 2.5 Lehr.

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It's 15 HP, but my neighbor has one with a 4 stroke 9.9 and you really gotta be careful putting on the brakes cuz she's squatted quite a bit. I think the 2 stroke 15 is nearly the same weight as the 9.9, but a 15 four-stroke....I dunno, not me. Boat weight is said to be 125 lbs., engine probably 60 or so. A four stroke would be more like 90 I think.

Speaking of tenders, I am headed to the PNW this weekend for some family business. I'll have my truck up there as well. I thought I might go dinghy shopping in Anacortes on Thursday. Any recommendations of where I might pick one up? I am looking for an inflatable center console under 500 lbs suitable to hoist on a davit system, 12 ft max. A deal on a used one would be even better.
Scott

Larry, 9.9 on a boat rated for a max of 15 is about 66% of max. That's good info.

The 2 cycle engines seem to use the same block for 9.9 and 15 hp. The 4 cycles use the same block for 15 and 20 hp. Your 9.9 2 cycle weighs about 30 pounds lighter than a 9.9 4 cycle. Probably wouldn't have much performance difference but the 4 cycle is more likely to hurt you when you lift it off!

bligh said:

Speaking of tenders, I am headed to the PNW this weekend for some family business. I'll have my truck up there as well. I thought I might go dinghy shopping in Anacortes on Thursday. Any recommendations of where I might pick one up? I am looking for an inflatable center console under 500 lbs suitable to hoist on a davit system, 12 ft max. A deal on a used one would be even better.
Scott

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...no sales tax in Oregon!

bligh said:

Speaking of tenders, I am headed to the PNW this weekend for some family business. I'll have my truck up there as well. I thought I might go dinghy shopping in Anacortes on Thursday. Any recommendations of where I might pick one up? I am looking for an inflatable center console under 500 lbs suitable to hoist on a davit system, 12 ft max. A deal on a used one would be even better.
Scott

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Look at seattle craigslist...always a few there. You cannot go wrong with a used avon.
Hollywood

HopCar said:

"The 15hp is now on a 11' high pressure floor Zodiac and its rated for 10hp."
Hollywood, you're a naughty boy!

What is the Avon rated for?

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The avon is rated 30hp..I grew up in drag and circle boat racing...500 lb hulls with 700hp+..a 200lb+ with 40hp boat ..no problem!
Hollywood

dink power

We have a 11.5 RIB with a 25hp Mercury. 18in tubes and no seats. Seawise davit system. With the tank or person in the bow it gets on plane quickly and is very seaworthy smooth water or not. I like to be able to leave the the boat at anchor and go to some other places with a picnic lunch and explore while having the power to get there and back with out a problem with current or wind. 30+mph on good water. I have a 18# mushroom anchor that iI drop just before beaching, then go on in and unload the lunch and mate, tie off the tow line and the boat drifts back out a little and stays off the gravel. Get the biggest boat and motor that you can carry and expand your experience.

janice142 said:

So I don't really "charge" per se, I merely bring the battery in Algae up to Seaweed's level. For someone (me!) without the strength to pull a cord on an outboard, this is the answer that works. And it's not expensive.

Click to expand...

Thanks, Janice. Interesting solution.

Apologies to OP for the short diversion.

-Chris

Slowboat, how do you like the swivel mount for the outboard, is that the one from Weaver?

Slowboat 37 said:

We have a 11.5 RIB with a 25hp Mercury. 18in tubes and no seats. Seawise davit system. With the tank or person in the bow it gets on plane quickly and is very seaworthy smooth water or not. I like to be able to leave the the boat at anchor and go to some other places with a picnic lunch and explore while having the power to get there and back with out a problem with current or wind. 30+mph on good water. I have a 18# mushroom anchor that iI drop just before beaching, then go on in and unload the lunch and mate, tie off the tow line and the boat drifts back out a little and stays off the gravel. Get the biggest boat and motor that you can carry and expand your experience.

Click to expand...

I've got a 9.5 ft Achilles LEX-96 dink with inflatable keel and wood floor. I run at 5 mph or less with a 2 hp Honda. It easy to move around at 27 lbs, is very efficient at about an hour on a 1 qt tank and starts easily. But it's sloooowwww and being air-cooled, it's rather noisy, especially at half-throttle and more.

Here's a short video I made when trying out an over-sized engine on my dink. The dink is rated for 8 hp and this is a 15 hp Merc 2-stroke. If 8hp is good, 15hp is better, right? Well, it's squirrelly and a handful at full throttle. At half throttle, it runs just great!

(Click the photo to play the video)


I'm meeting my friend today to pick up this same motor for some summer carousing. I'm looking for a better-suited dink to mount it on. A 3.1M RIB should be just about right, but without a davit, I'm concerned that it'll be too much to handle. In the meantime, I'll run it at half-throttle on my LEX-96.

Go ahead....Nomex suit is on...flame away!

"Apologies to OP for the short diversion. "

Chris, the OP enjoyed the short diversion as he uses a Minnkota 30 to push his canoe and a strange little flats boat.

Ok, we now have a new data point for my survey. We know it takes 133% of max rated hp. to make Hollywood happy. That's my kind of guy!

I understand the need for speed. Following is a photo of my dinghy circa 1971.

Fly Wright signs in with 187% of max rated hp.! I think I see a trend here.

HopCar said:

"Apologies to OP for the short diversion. "

Chris, the OP enjoyed the short diversion as he uses a Minnkota 30 to push his canoe and a strange little flats boat.

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Canoe was what I was thinking about. 19' square stern; moves out right smartly with the 5-hp 2-stroke. Quieter would be better. I kinda wonder if an electric trolling motor with some kind of solar charger would work over long-ish distances... but paddling, mostly, with the motor being more like an auxiliary... or an "I'm tired, but we gotta keep moving for a while."

-Chris