Which engine for my dinghy?

[sand6000]

2 strokes banned in Aust? I find that hard to believe/understand. Can you explain.

[fursoc]

Thanks everyone for your thoughts. First of all, 2 strokes are already banned to be imported into Australia. They will be banned from selling from next year, so buying one now is a bit of a risk.

The mothership has a capable davit system which does not require removal of the motor when underway.

The main reason for my question, which a64pilot hit on, is that for the same weight and almost cost I can get a 9.9hp, 15hp, or 20hp. Considering I’d like to be able to do a bit of water sports with the dinghy and that we will have large loads when provisioning, why would I get a 9.9 or 15hp when I can get a 20hp. The only reason I can think is that the dinghy designer said 15 is the max.

Dragging the dinghy up the beach will be no different if I get the 15hp as the weight is that he same. Same goes for lifting up on the davits.

Only difference is power.

[fursoc]

Oh and just because I would have a larger engine doesn’t mean I would be disrespectful to fellow boaters in a mooring field. Limiting engine size to force respectful driving is not a good idea in my opinion. Better to teach how to drive properly and considerably.

[a64pilot]

Our little Suzuki will idle for extended times, smoothly and almost silently. It never loads up and needs a run to clean the plugs out.
We often “dinghy cruise” where we will run for a significant distance at 20 kts or so, find a nice neighborhood and idle through it so she can look at houses and me drink beer .
The fuel burn at low speeds is incredible, it doesn’t seem to burn any at all, and it will putt putt all day and never stumble.

As much as I love 2 strokes, I wouldn’t have one now.
Weight? The venerable Yamaha Enduro 15 is 5.7 lbs per HP, the Suzuki 20 HP is 4.9 lbs per HP.
Yes it weighs a little more, but it’s also 5 more HP.

[model 10]

[QUOTE=a64pilot;
As much as I love 2 strokes, I wouldn’t have one now.
Weight? The venerable Yamaha Enduro 15 is 5.7 lbs per HP, the Suzuki 20 HP is 4.9 lbs per HP.
Yes it weighs a little more, but it’s also 5 more HP.[/QUOTE]

But what if it quits? Third world guy's use Enduros. Sure, you see some other brands but the Enduro is King.

[Moontide]

Bit of a different viewpoint, If living on a boat and exploring is part of your program then my 2 cents says make sure it will plane with the whole family (and other gear) on board. Your play world will be much smaller if you can't plane with a full load.


One thing I have heard about 4 strokes is that they are much harder to start than a 2 stroke for the same horsepower. If you want to spend a few more $, an electric start will be appreciated by the wife and kids (and yes, I believe young kids can run dinghies with higher horsepower safely - just have to teach them right).


Bill

[SteveSadler]

yes horsepower is great! 4 adults can go 20mph with my 12'rib and 20hp honda.
Put that on a cat tender with 33% of my weight capacity, and it would be scary to open the throttle.The stern will squat, it wont plane, and just throw a huge wake at 10 knots. Now if you put 2 foils between the hulls, you could literally fly at 30 kts. Another subject altogether. Google hysats. For example I had a 22' catamaran power boat with a 115 hp engine. It went 18knots flat out. with a 25 hp it went 12knots. Displacement hulls have speed limits. A cat tender will perform like any cat that has far too much weight than designed to carry.

[boom23]

Get the biggest engine you and the dinghy can handle. It will give you more options exploring.

I've had a 11ft aluminum inflatable with a 20hp fuel injected Suzuki and it was great. I plan to get the same setup for my next boat.

[Uncle Bob]

Originally Posted by fursoc

Thanks everyone for your thoughts. First of all, 2 strokes are already banned to be imported into Australia. They will be banned from selling from next year, so buying one now is a bit of a risk.

The mothership has a capable davit system which does not require removal of the motor when underway.

The main reason for my question, which a64pilot hit on, is that for the same weight and almost cost I can get a 9.9hp, 15hp, or 20hp. Considering I’d like to be able to do a bit of water sports with the dinghy and that we will have large loads when provisioning, why would I get a 9.9 or 15hp when I can get a 20hp. The only reason I can think is that the dinghy designer said 15 is the max.

Dragging the dinghy up the beach will be no different if I get the 15hp as the weight is that he same. Same goes for lifting up on the davits.

Only difference is power.

Re the highlighted, I believe that this statement is missing one word, carbie.
"The final date for carbie two-stroke imports is announced as June 30, 2018"

[fursoc]

Originally Posted by SteveSadler

Put that on a cat tender with 33% of my weight capacity, and it would be scary to open the throttle.The stern will squat, it wont plane, and just throw a huge wake at 10 knots.

Hi Steve, can you elaborate? I probably don't understand the dynamics of boat building, this is my first foray into that world. Why will this happen?

The boat I am building has a carrying capacity of 325kgs (700lbs). Are you saying that if I open the throttle up with a full load, the boat won't plane? Or are you saying that if the boat is empty and I open up the throttle it wont plane?

Thanks.

[vanderd]

One final thought for “dragging it up on the beach” my 20hp fuel injected Suzuki has power tilt so beaching it is a breeze.

[SteveSadler]

I would ask someone with a similar dingy for a test ride before committing.Then try a few other designs, rib, inflated floor, hard boat. See how it planes with a load. 2 narrow hulls have 1/2 the planing area of 1 wide hull.
Also the planing area needs to be mostly aft.
Best to test drive a major investment before buying.

[ranger58sb]

Originally Posted by fursoc

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.

Why wouldn't I get the 20HP engine?

Why limit myself to 15HP?

Questions for the designer.

Could be as simple as his design statement dates from when 9.9s and 15s were on the same platform (and weight), but 20s were a platform step up in in weight, not just power.

Here, there's a max passengers/max horsepower/max weight (something like all that) plate attached to the dinghy... and given I have no inside info about the design and naval architecture (such as it is, for a dinghy), it's easy enough to respect the data plate.

FWIW, we have a Suzuki 15, same weight as their 20. Our dinghy's data plate says max 15-hp, so that's what I got. Can't tell that an extra 5-hp would be useful for our dinghy.

(Surprising they made a 9.9 version of the same thing; that seems a little goofy to me, but it might have been the absolute smallest platform they could fit the fuel injection system to...)

We have litigation issues here, too. Presumably is something bad happens, having exceeded the horsepower posted on the data plate, it could end up being cause for (further?) liability at judgement time. You may not have those kinds of issues in Oz...

-Chris

[Sberg]

Hi Fursoc. I’ve built this exact tender for our cruising cat. I have a Honda 15 long leg on it. I got this motor as it was on special but like you say, it could have the 20hp as weight is nearly the same. The 15 is like a de rated 20 so I often run it flat out. There is no squatting when you throttle up, it planes quickly. It is a great family tender. Myself and wife and three kids (7,9,11) have used it for exploring a lot of Australian rivers and creeks and reefs away from the main vessel.
I like many things about this tender, one being it is the best ride into chop that I have ever had. It never slams,as it comes back down off the tops of waves the tunnel creates a cushion of air.
I made some modifications and would make some more if building again. PM me if you want any more info

[sand6000]

9.9 and 15 hp are the same engine but with different exhaust tubes. The 9.9 as I recall was to evade the tax issue which became applicable at 10hp.