Which engine for my dinghy?

[dougrobbie]

I used to have a 1939 1/2 hp Evinrude outboard. Fuel efficient, pint of gas every 18 miles. It pushed the dink as about as fast as you could row it. Then at night we would unmount it and use it for a Martini Mixer! Dual use, couldn’t be better.

[CaptFigmo]

8 hp Mercury best for you.

[Ike]

a couple of comments:
In the US a boat manufacturer has to determine safe HP based on a formula the US Coast Guard publishes. However, it doesn't apply to multihulls. But for liability reasons the builders put a capacity plate on them anyway. Some states have laws which say you cannot exceed the rating on the Capacity Label (in Australia the builders plate) Check to see if there is such a law there. In Australia I believe they use The ABYC and ISO standards as the HP standard. See part 9.2 of
http://www.anzsbeg.com.au/files/7214...8/ABP_Ed_4.pdf

In the US, exceeding the HP rating will definitely void your insurance and open you for liability should you be in an accident. In other words, no bucks from your insurance company. Check with your insurance company.

From a purely practical stand point, I have a 12 foot dinghy (a monohull) my 2.5 HP Suzuki can push it faster than is safe. I used to have a 2 HP Johnson (2 stroke) that would do the same. Unfortunately after 30 years it died. I also have a 4 HP Mercury (a Tohatsu in disguise) that I use on my 18 foot runabout for trolling. Won't plane it but pushes it up to hull speed. On the 12 foot it would be a definite hazard. Besides it weighs 58 lbs. The 2.5 weighs 28 lbs. Both engines sip gas. I can run either one for 2 hours on their built in tank which holds about 3/4 of a liter. I have a 3 gal portable tank for the 4 HP and I ran it all Summer and still have at least a gal in the tank.

Also I think what was said about towing someone is true. It will bog down dramatically even at 20 HP. That"s a lot of drag to overcome.

As for comments about difficulty starting these engines, the only time I have had a problem is when I didn't follow the instructions. They are right there on the engine. They both start on the second or third pull. When Hot, the first pull, unless you do something dumb like choking them. (like I did and flooded them) But after 15 minutes they started right up. Did this twice! (not too smart) However I would love to have electric start on the 4 HP. I'm not getting any younger.

[Bulawayo]

What are your longer term plans? If staying in home waters then I would ensure that what ever I bought I could lift on/off the mother vessel.
If you are going off-shore, globe trotting, then I really believe in two motors. A baby (2 or 3hp) chug-chug engine for ship to shore / harbour duty and the biggest engine your dinghy can handle for those long journeys that liveaboards experience more regularly then initially thought. I love my Yamaha Malta (3.5hp, forward and neutral gears only) and my Yamaha Enduro 25hp. Both are two stroke and the 25hp really hops. I also have a rigid RIB lookalike dinghy - built in Aussie; its an all aluminium Ocean Craft, 3.3m long (a shade under 11ft) and will take a 30hp motor. I am very wary of most rigid dinghies and what happens to them in rough water when they are swamped - it will happen if you live aboard. Personally (and you are not me) I would look for a decent second hand dinghy - preferably a RIB, that can handle off-shore work and take at least a 15hp motor. I am big enough to manage a 25hp 2-stroke but in rough weather our goal post was used to lower it onto the dinghy transom. The Malta is a doddle to carry and see's as much use as the 25hp. To go back to the start, if staying close to home, then I would still go with your 20hp idea.

Originally Posted by fursoc

Hi, I am building myself a catamaran tender for our liveaboard 12m catamaran that my family (2 adults, 2 kids) will be living on from early next year (Link to the plans. I'm building the 3.2m).

The designer of the tender says the max power is 15HP. Since 2 stroke engines are pretty much banned in Australia, I am looking at 4 strokes.

Every 4 stroke engine that I have researched (suzuki, yamaha, honda, tohatsu) in the 15HP range is basically a cut down version of the 20HP. This means that the 15HP is the same weight and practically the same price as it's 20HP big sister.

Why wouldn't I get the 20HP engine? Isn't more power always going to be better? Fully loaded with provisions and the whole family we could probably plane back to the mother ship. With 20HP I could drag the kids on a donut and have a passenger watching on.

Why limit myself to 15HP?

[Nickodemus]

I agree with what is said above - make sure your wife can start the dinghy. My husband bought a brand new 9.9 hp Yamaha and there is no way I can start it. Now we have to sell it and buy a battery start since it really is a safety hazard. Anyone looking for a 2016 9.9 hp Yamaha in great condition?

[sparrowhawk1]

Originally Posted by Nickodemus

I agree with what is said above - make sure your wife can start the dinghy. My husband bought a brand new 9.9 hp Yamaha and there is no way I can start it. Now we have to sell it and buy a battery start since it really is a safety hazard. Anyone looking for a 2016 9.9 hp Yamaha in great condition?

Are you sure a starter cannot be added to this engine?

[oldtom]

[QUOTE=fursoc;2727642]Hi, I am building myself a catamaran tender for our liveaboard 12m catamaran that my family (2 adults, 2 kids) will be living on from early next year (Link to the plans. I'm building the 3.2m).

The designer of the tender says the max power is 15HP. Since 2 stroke engines are pretty much banned in Australia, I am looking at 4 strokes.

Every 4 stroke engine that I have researched (suzuki, yamaha, honda, tohatsu) in the 15HP range is basically a cut down version of the 20HP. This means that the 15HP is the same weight and practically the same price as it's 20HP big sister.

Why wouldn't I get the 20HP engine? Isn't more power always going to be better? Fully loaded with provisions and the whole family we could probably plane back to the mother ship. With 20HP I could drag the kids on a donut and have a passenger watching on.

Why limit myself to 15HP?[/QUO well i would (not bay a Honda) till Honda,Aust start fixing there cam gear problem I no of two boaties, my self that come to grief when the plastic gear come loose from the cam with moter stopping at the inappropriate time.

[NoTies]

I've always found it a good idea to completely ignore designer and manufacturer recommendations and just do your own thing. After all, what do they know?

[SailFastTri]

Lots of good advice above, and as usual, many points of view. IMHO- You should consult with the designer about HP ratings. You might need to reinforce the transom if you go larger.


My 2 cents: I prefer a tender to be light and portable, and especially the tender engine. Launch and retrieve is done in every new destination. In a harbor, you must minimize wake, and in open water you usually need go slow in a tender-sized boat to stay dry in the chop. Going with a heavy, powerful rig will have disadvantages more frequently than advantages. Either way; there are compromises.

[a64pilot]

Originally Posted by NoTies

I've always found it a good idea to completely ignore designer and manufacturer recommendations and just do your own thing. After all, what do they know?

If your referring to the max recommended HP, it’s only very rarely either of those.
It’s an arbitrary USCG formula is all it is, it has absolutely zip to do with that particular boat, that Boat may not be safe at the recommended HP or it may have the stability to handle much more.
RIBs as a general rule are exceptionally stable, and are I’m afraid to say, slow. The drag from the tubes slow them down. Of course there isn’t a different formula for a “tinnie” and a RIB.
However a manufacturer if they chose to can challenge the formula and conduct testing and set a higher HP number and thereby I’m sure open themselves up to future lawsuits, so it’s rarely if ever done.
In the US where the formula originated, it’s almost universally ignored, go onto any lake in Tennessee, Alabama or Georgia and look at the max HP plate, then look at what’s on the transom.

However I guess a person who has no self control needs a speed Governor on their car and maybe they do need to not ever exceed the HP on the plate.
However don’t assume that the rest of us can’t control ourselves and not go at excessive speeds, we like the extra HP so we can get onto plane easily with a heavy load and not play games with tiller extensions and everybody lean forward until we can crawl onto plane and once on plane we back off and cruise.

[a64pilot]

Originally Posted by Nickodemus

I agree with what is said above - make sure your wife can start the dinghy. My husband bought a brand new 9.9 hp Yamaha and there is no way I can start it. Now we have to sell it and buy a battery start since it really is a safety hazard. Anyone looking for a 2016 9.9 hp Yamaha in great condition?

Maybe have someone look at it? Maybe the compression release isn’t working correctly? My Wife who even when she was younger could not start a Lawnmower, can start our 20 HP Suzuki. I would hope that an engine of half the HP would be even easier to start?
Although the Suzuki 9.9 and 20 HP are the same engine, maybe it’s the same with Yamaha?

[NoTies]

Originally Posted by a64pilot

If your referring to the max recommended HP, it’s only very rarely either of those.
It’s an arbitrary USCG formula is all it is, it has absolutely zip to do with that particular boat, that Boat may not be safe at the recommended HP or it may have the stability to handle much more.
RIBs as a general rule are exceptionally stable, and are I’m afraid to say, slow. The drag from the tubes slow them down. Of course there isn’t a different formula for a “tinnie” and a RIB.
However a manufacturer if they chose to can challenge the formula and conduct testing and set a higher HP number and thereby I’m sure open themselves up to future lawsuits, so it’s rarely if ever done.
In the US where the formula originated, it’s almost universally ignored, go onto any lake in Tennessee, Alabama or Georgia and look at the max HP plate, then look at what’s on the transom.

However I guess a person who has no self control needs a speed Governor on their car and maybe they do need to not ever exceed the HP on the plate.
However don’t assume that the rest of us can’t control ourselves and not go at excessive speeds, we like the extra HP so we can get onto plane easily with a heavy load and not play games with tiller extensions and everybody lean forward until we can crawl onto plane and once on plane we back off and cruise.

Funny, I had no idea the USCG had jurisdiction in Australian waters. That FTA they signed sure isn't working in Australia's advantage. God save us from the average man in the street who claims to have more knowledge than scientists, engineers and designers in their specialties.

[Russ333]

On our 44 cat we use a 11.5" aluminium RIB with 20HP Yamaha 4 stroke. Pull-cord starting in less ideal circumstances can be a challenge for the less experienced or physically strong. Living wild we often use the dinghy for more than short hops to shore, so find the high speed planing capably very useful. Of course when heavily loaded, planing takes longer and is slower, but nonetheless it's a very useful workhorse.
Obviously your tender will be much easier driven than this, so the extra HP will probably only be needed for carrying heavy loads or towing.
I don't see a safety issue for a competent operator from the extra HP. But I'd also ring the designer for an off the record chat.

[Bone Fish]

First, cool little boat, what are you building it from?
I sold catamaran dinghies for a while, Livingston from Washington State.
Weight does matter because there is less buoyancy in the stern than a mono hull.
I notice your tender has a round hull form in the stern, much like a Glacier Bay. I doubt the boat planes, there doesn't appear to be enough horizontal wetted area. This isn't a bad thing our GB is great.
back to tenders we rigged a 14' Livingston with a Yamaha 40 4 stroke, 230 pounds if I recall. Two people at the transom would almost swamp it.
I extended the hulls 16" with bolt on swim-platform-flotation-pods and it made a huge difference. Attached is a picture showing the pods before glassing.
If I could get my hands on a 2 stroke Yamaha Enduro I would snap it up. Great weight to HP! best of luck with the build, post pictures!

[newhaul]

Why not go diesel so you only have to carry one fuel
Diesel Outboard Motors 5 to 80hp 4stroke 2,3,4 Cylinders water cooled