What speed for my dinghy if it had 5hp motor?

[troppo]

Thanks for the useful info Chris. I also admired the care with which you treated a very sensitive topic. I wish I was as tactful, still learning.

Originally Posted by ranger42c

Subsequent test with The Admiral aboard, her at the bow (undisclosed weight), First Mutt amidships, and me aft... max speed was 6 mph, no planing.


-Chris

[ranger58sb]

-Chris

[deckofficer]

If planing speed is important for you with minimum hp, then the only way to go is a minimum wetted surface, air entrapment tunnel hull type of dinghy. Most are for racing (I don't know about you, but I don't need 70 mph in the dinghy), but some are for us as yacht tenders, being able to plane with just 3 hp.
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[troppo]

Update: I just bought a 9.8 hp 2 stroke, weighs 26 kg.

With the floods we get in the river where my boat is moored, having an outboard with extra grunt (compared to 5 hp) means I can handle it in my dinghy rather than rely on others. That decided me over carrying the extra 6kg compared to a 5 hp.

The 9.8 hp takes the 11 foot aluminium dinghy (with a deep v bow not a punt) up on the plane at about half throttle (or a little over, then throttle back) at which it does about 20-24 km/hr or so (11 knots). Full throttle I got up to 34.5 km/hr (18.6 knots). That's in calm water, one person, still running the motor in. I don't know if those speeds are good or poor performance but I am happy with them.

[Suijin]

It's really a matter of load that you're carrying and what it takes to get it on plane and keep it there in the conditions you are preparing for. Once on plane, it takes less hp to keep the boat moving on plane, as you've noticed based on your experience. A 5 hp on an 11' aluminum dinghy is going to get you up on plane just fine...with one person or maybe two. Add more weight and the story changes. Could you have done OK with a 5 hp? Hard to say without knowing more about what you carry, and the conditions you carry it in.

[troppo]

Hi Suijin, it was mainly the flood this year that persuaded me to go bigger. This last summer we had two floods where everyone moved their boats down river to a bend where the water was not running so fast and the logs and weed tended to go past. Boats were lost, one boatie drowned, many boats drifted and had to be chased down the river and caught. I didn't have too many dramas but it was close. At one point I was standing on shore waiting for my friend to pick me up in his fast dinghy while watching the water rise and my boat on its mooring slowly getting pulled under. He didn't turn up. Luckily I hitched a lift with another bloke. Later I found my mate and son-in-law had been busy rescuing one of their boats that was in trouble. With the bigger motor I should not have any problems coming up against the flood with two people on board. It's just insurance against some of the problems here with the river.

If my boat was moored at the coast I would probably go for a 5 hp as most of the time only me and less tricky conditions.

[44'cruisingcat]

Originally Posted by troppo

I've just bought an 11 foot lightweight aluminium dinghy (1980's Savage Gull) as a tender for my boat. Rated for 10 hp max and thinking about what size outboard. 5 hp 2 stroke weighs about 20 kg and the next jump up in weight is to about 26 kg and 9.8 hp (IMO when the 6 hp weighs 26 kg, I may as well go for the 9.8 hp if I decided to go bigger than 5 hp).

As a kid we had a similar sized tinny with a 6 hp and I can remember it got up on the plane with one person but I can't recall much else about performance.

What speed do you think I would get out of a 5 hp 2 stroke with one person on board? Do you think it would plane with two people?

Years ago I actually had a Savage Gull, with a 7.5 hp motor. It would plane easily one up. (I'm pretty big, 2m tall and about 115kg. Was fitter then so was even heavier, maybe 120kg)

With me and another largish adult, it would still plane, but took quite a while to get there. Any additional gear and it wouldn't.

When we were up in Kakadu, the combination of big tides and crocs made me decide to trade the 7.5 in on a 15.

Huge difference!

[troppo]

Originally Posted by 44'cruisingcat

When we were up in Kakadu, the combination of big tides and crocs made me decide to trade the 7.5 in on a 15.

Huge difference!

Haha, big tides and crocs, nothing like a bit of extra hp to help handle them. We had a 5.5 metre tide over the weekend, I am glad I wasn't down the river trying to row against that! And a bloke I know was hauling a crab pot out when he saw a croc coming towards him from the front. He quickly put the motor into reverse and had the fun of a croc chasing the boat while he was racing in reverse.

[44'cruisingcat]

Yep. I'm assuming your 9.8 is a Tohatsu, if so, I really like those motors. Smooth, quiet and reliable. Good choice!