Dingy Horsepower, need minimum weight!

[rslifkin]

The fast/slow thing to me is one of the up-sides of modern 4 strokes. A bigger outboard will be a little less efficient when putting along slowly, but not dramatically worse within the realm of outboards reasonable to put on a dinghy. But 4 strokes are typically pretty smooth and quiet at low RPM, plus they don't foul plugs when running slowly for a while. So you can have a fast dinghy, but still putt along slowly much of the time and only use the speed when you have a larger distance to cover and nothing close by to be bothered by the extra noise and wake.

[Hudson Force]

Sometimes, on a lower hp outboard, the "Delta wing" accessory attached to the foot will allow for the dinghy to plane and increase performance. We also used our mizzen halyard to lift our 5hp outboard.

[GordMay]

Originally Posted by Hudson Force

Sometimes, on a lower hp outboard, the "Delta wing" accessory attached to the foot will allow for the dinghy to plane and increase performance. We also used our mizzen halyard to lift our 5hp outboard.

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[Evergreen]

Keep it simple --- Stay with what you have. Patience will get you there just fine.

[Ike]

Originally Posted by Evergreen

Keep it simple --- Stay with what you have. Patience will get you there just fine.

Good advice. The KISS principle. I had a 2 HP 2 cycle Johnson that lasted almost almost 30 years (I bought it in 1980). It would push my wooden rowing dinghy at about 7 mph max. Always easy to start. The tank held about 1 quart and would run for 2 hours. When it died, I went for a 4 hp. Merc (tohatsu) It was hard to start, unreliable and weighed too much for my wife to lift. I traded it in on a Suzuki 2.5 which was reliable, weighed about 25 lb, and started on the first or second pull. Most dinks can't go much faster than 5 or 6 mph anyway unless you have a RHIB, or a planing hull, so a bigger engine is just a waste of gas.

[Talbot]

I had a 2hp yam as my first dinghy engine. Lovely motor, and sufficient to move my 30ft catamaran when secured alongside. Was a bit warm after a couple of miles!
That engine finally needed to be replaced as I could no longer repair it, and I got a 3.5 Tohatsu. great engine and sold as a 3.3 by a lot of other "makers",

That engine went when I sold my cat. My next boat had a honda 6 hp 4-stroke on a 3.1m rubber with an aluminium floor. They were so heavy that I could not lift the dinghy even without the engine onto the pontoon on my own. The 6hp was an appalling engine. very unreliable and very heavy and short on power. I got rid of both and purchased a 2.7m aluminium RIB and a 9.8 2stroke Tohatsu. Both a reasonable weight and yet able to plane.

[donradcliffe]

I went around the world wit a 3.5 hp 2 stroke plus an 18 hp 2 stroke, both
Tohatsu. I used the 28 pound 3.5 hp 98% of the time, and would buy another if I could.

[daydreamer]

It is not usually a matter of need. But a fast dink opens up a lot of possibilities. Calmer or quieter or safer anchorage and still able to access shore. Snorkeling and hiking opportunities without a nearby anchorage. Saving a day not having to move the big boat to provision food or find boat parts. Anchor in daylight and push the limits of darkness in the dink - for happy hour, dinner, shopping, guests, etc. Not having to give up the good spot just to move the big boat closer for any of the above.

But if you're the friendly sort, just anchor next to someone with a fast dink!

[SOLAR SUPPORT]

Electric outboard motors are quite light. Those with a power of 500-600Watt produce power slightly below 2Hp internal combustion engine. Internal combustion engines are much worse in terms of efficiency compared to electrically powered engines. It should also be added that the electric engine does not have extra problems such as fuel, maintenance or malfunction. More powerful electric motors may be preferred. For example, those with 1000 Watt power are more powerful than a 4-5 HP internal combustion engine. The ones with an integrated battery attached are generally expensive because they use a very light Lithium-Ion type battery. When you connect the 60Ah Lifepo4 battery weighing 10 lbs to the 400 Watt electric troling motor weighing ~16 lbs with a plug, you divide the load in half. Their total cost is aprox. 400$. I can say that I have no trouble putting these on the dinghy and putting them back on the boat. As for performance, I can sail for an hour with my 2.00 m boat at full power at a speed of 3-4 knots.

[Mickeyrouse]

I’ve been using a 5 hp 2 stroke for 30+ years. We have a 9’6” Achilles sport boat. It planes easily with my wife and I plus groceries- my wife usually tells me to slow down. Any more would a weighty overkill.

[Jim Cate]

Those with a power of 500-600Watt produce power slightly below 2Hp internal combustion engine. Internal combustion engines are much worse in terms of efficiency compared to electrically powered engines. It should also be added that the electric engine does not have extra problems such as fuel, maintenance or malfunction. More powerful electric motors may be preferred. For example, those with 1000 Watt power are more powerful than a 4-5 HP internal combustion engine.

Ahh, once again the magic of electric power somehow being better that nasty ol' IC power rears it's ugly head.

This subject has been beaten to death here on CF, but no matter how you manipulate the numbers a horsepower is still just over 745 watts. The "efficiency" you mention is hard to understand... do you mean that it takes more chemical energy to generate one HP in an ICE than in an electrical generator/battery charger/battery/electric motor? Or what?

But it does not matter! 1000 watts in an electric motor can't "be more powerful than a 4-5 hp ICE". More convenient, sure. Quieter? Yep! Less polluting (at least at the actual engine)? Most likely! Simpler mechanically? You bet! But more powerful... can't happen.

Propeller mismatching can bog either type of engine and cripple it's performance, put the power head under water and both suffer, and both rely upon a sort of limited "fuel", either petrol or amp hours, that can run out and leave you stranded, so neither is perfect. But the relative power is constrained by the well known relationship: 1 hp=745.7 watts and no advertising can alter that.

Jim

[barcoMeCasa]

Originally Posted by Jim Cate

Do be aware that one can use a halyard to hoist o/b motors and other things without t he need for a dedicated hoist. We've been using our main halyard to deal with a 15 hp motor for many years. At 86 and 84 years of age, Ann and I are hardly athletes,but still manage this with little issue.

Jim

^^^^
Hmmm... great minds and all that!

Feed the halyard through a block which can be pulled to the outside of the pole it's even easier.

[jjernigan1972]

I used a 7'7" soft bottom inflatable with 2.5hp 4 stroke OB. This worked for me for 4 years of sailing around the world. I love that I could set it on the inside of the dinghy with the dinghy along side and just reach over one handed and pick it up and place it on the stern rail. If you are avid spear fishing or want to run a long ways for snorkeling then I could see the bigger engine being useful. If it's mainly transport on and off the boat and you have patience the small motor is fine.

[BenBowSirocco]

I agree with DayDreamer!



We cruised 6+ years with hard dinghies and small outboards and it worked just fine.. But switching to a fast planing dinghy REALLY opened up our world!



We could be miles from town anchored in the peace and quite, but still zip over in less than 5 minutes.

We could run 5-8nm to a good dive or snorkel spot without moving the mothership.

We felt more comfortable in tight passes (read: high current) and when in busier areas because we felt we could get out of the way should be need to.

We could bring friends along anywhere and at the same speed we preferred to travel.
We dont often concern ourselves with what we kept or throw in the dink- as it rarely makes much difference to our performance.



Overall a larger outboard and big planing dinghy greatly increased what we refer to as our "radius of exploration" and I know for a fact, that we saw more and had more adventures for it: so personally I will never go back.



We use a 15hp outboard on a 10.5 foot boat. We handle the ourboard with a removable "crane" (aluminum pipe) and the main halyard handles the boat.

[Dockhead]

Originally Posted by Jammer

Anyway, back to your question.


Assuming that your dinghy is not a RIB and does not have a high-pressure air floor, you are unlikely to go much faster than 4 knots and will only realize marginal benefits from a move from 5 hp to 8 hp. The 5hp may offer benefits over your existing 2 HP on a windy day.

This.


In my experience, there's not much to gain over 3 or 4hp until you get to whatever level of power is needed to get the dink on a plane with a reasonable load. If your dink is not a RIB then you won't be able to plane it period, so you are not likely to have any use at all for any more power than 3 or 4hp.


And as you've seen, a bit more power may be a bunch more weight which can greatly complicated handling the motor.


So your 2hp might be optimal already. Possibly 3.5 or 4 hp could be a useful increase depending on the load. I have two outboards -- a 3.5hp Mercury and an 8hp Selva (neither of which I recommend to anyone). The Selva will plane my 3.1m light RIB if it's lightly loaded, just. But has to be handled with a tackle off my boom. The 3.5 can be manhandled. Guess which one gets used almost all the time?


Wind can be a factor in the power you need, but note that you don't "overcome" current. Current just is, and you can either motor faster than that or you can't, to make any actual speed over ground. This is going to be determined very much by hull speed of your dink. I would not go out in a dink like yours in a 4 knot current regardless of the motor. If you want to go faster, you will need not only a bigger motor, but you will need some kind of RIB.


That's a big tradeoff in terms of weight and handling. First 10 years or so I had my boat, I had a 3.4 heavy RIB with a 25hp two stroke motor. That was a proper little motorboat which I could use for all kinds of things -- including running across the Solent for parts, running out to sea (in good weather) to take a guest to the next port. It would plane with 4 people and gear on board, and make miles fast. But it could only be carried in davits, which was a big PITA, electric davits which were constant trouble, and which actually cracked once on a long passage in rough water.


I finally ditched this for a much lighter 3.1 folding RIB and much smaller motor. This I could fold up and carry on the foredeck in its bag, or carry on much simpler and more reliable manual davits. Motor stored on the pushpit. It's far less capable but worth it 1000x over for my use case.


Dinghy capability vs. handleability and storeability is one of the those trade offs which all cruisers face. There is no good answer to it; you just have to pick whichever compromise is the least painful for you.