How fast does a hard dinghy motor?

[Cap Bill Des]

We've cruised the Caribbean for 5 years with an inflatable dinghy, high pressure floor, and have become very happy with our 3.5hp 2-stroke because it weighs 25 lbs and my wife can start it herself.

But I miss high-quality rowing and having a small sailing dinghy, so we are considering changing to a rigid dinghy.

How fast does an 8-foot dinghy, like a Fatty Knees or Bauer, or others, motor with a 3.5hp motor? (I know that some spec 2 hp max, but I asked if my 3.5hp was OK since it is same motor as 2.5hp retuned and was told it's OK since their concern is weight)

2 knots, 3,4,5,6 knots? Does anyone have data? I'm assuming flat water, no wind conditions. I asked Fatty Knees and they said they'd never measured. What I'm really hoping is that someone had taken a GPS out in their dinghy. ..

Thanks for any data on this!

[Cheechako]

Not sure the answer. But I had an 8 ft Dyer Dhow on my first long cruise. Great little boat, but really, in a chop, downright dangerous. With 2 aboard you don't have much freeboard. a 1 ft chop can get pretty wet and pretty iffy. Shot trip to shore not bad. long trip... I felt uncomfortable...

[Kim Gregory]

Cap Bill Des,
My 10 foot Danny Green Chameleon nesting dinghy ( plywood chine ) does 5 knots with 4 aboard with my 3.5 hp motor. Does 7 - 8 knots with just me in it.

See Forum thread:
http://www.cruisersforum.com/forums/...hy-156370.html

- Kim

[Mike OReilly]

I use a 3.5 hp (4-stroke) with my portabote. It will easily plane with one person on board. From distant memory (so might be wrong) my handheld GPS measured me moving at about ~8-9 knots (~15 km/h).

[Stu Jackson]

Wouldn't the answer depend on what specific boat you have? Or will get. I'm sure you know all about planing vs. displacement hulls.

[Adelie]

Depends on that and how the prop is pitched and how much the operator weighs . . .

Figure 8ish knots 1 person in 8-10' rigid or folding dinghy.

[sailorcherry]

Not a 3.5 but I've cruised with a 10'3" rowing/sailing dink (f/g). With a 2 stroke 9.9 with one person it was really fast, I think 11kts? Fully loaded down 6/7. Electric trolling motor (40lb thrust) 5kts, 3.5kts loaded.

I find the rowing sailing dinghy leaves you wanting a true sailing dinghy (planing, hanging over the side decks, surfing downwind). Adding an electric trolling motor helped speed up poor upwind performance.

If you have a large flushdeck my experience of cruising with an inflatable w/15hp o/b and a sailing dinghy (laser, sunfish etc) is amazing - lots of fun at anchor or marina.

[oldsalt_1942]

Dumbest question of the day....Why not ask:
How long is a piece of rope?
How much does a car cost?

[John Holbrook]

The question is entirely sensible and why this forum exists. Yours is the rudest response of the week...

Originally Posted by oldsalt_1942

Dumbest question of the day....Why not ask:
How long is a piece of rope?
How much does a car cost?

[phydeaux]

Harbor Patrol clocked me at 7.1k in an eight foot Maus pushed by a 2 hp Honda. Will cop to a stiff following breeze.

[Seadoc777]

It's a dumb question but the best answer is to use your portable GPS and measure your speed.

[Adelie]

Originally Posted by oldsalt_1942

Dumbest question of the day....Why not ask:

How long is a piece of rope?

How much does a car cost?

You wasted your time responding to a thread you thought was dumb in the first place and you made yourself look rude to other forum members.

Tell me again about what's dumb.

[Mike OReilly]

Originally Posted by oldsalt_1942

Dumbest question of the day....Why not ask:
How long is a piece of rope?
How much does a car cost?

I guess you’re the kind of person who only reads the headline, and not the story. If you read the actual question it provides a clear context: "But I miss high-quality rowing and having a small sailing dinghy, so we are considering changing to a rigid dinghy”, and then poses a very reasonable question:

How fast does an 8-foot dinghy, like a Fatty Knees or Bauer, or others, motor with a 3.5hp motor? (I know that some spec 2 hp max, but I asked if my 3.5hp was OK since it is same motor as 2.5hp retuned and was told it's OK since their concern is weight)

They don’t have one, so can’t use their GPS to get an answer, hence the very reasonable question here. This is why I provided an answer that gives them some relevant experience.

BTW, I forgot to add that my portabote is the 10’ model.

[seasick]

I agree on the face that the question could be more aptly phrased. As asked, my first thought was probably not faster than the speed of sound as breaking that barrier would open up a giant can of worms. My wife wanted me to look up speed/displacement formulas but not many skiffs are limited thus. So when I read what Cap Bill Des was really looking for it seemed a more appropriate question.

In another thread I mentioned my brother and I both had Yamaha 2 HPs but they may have been 2.5 HPs, regardless, his was mounted on a Avon Red Crest and mine was mounted to a fg klinker built sailing skiff. No comparison. The inflatable bagel could barely motor out of its own way and forget about rowing it against any amount of head wind. The versatility of being able to row, sail, skull, a hard dinghy far outweighs the speed answer you're seeking. As well, all else being equal, the hard dink is far less likely to get stolen. Only drawback in my estimation is scrambling in and out of while snorkeling.

[KP44]

Originally Posted by Cap Bill Des

How fast does an 8-foot dinghy, like a Fatty Knees or Bauer, or others, motor with a 3.5hp motor? (I know that some spec 2 hp max, but I asked if my 3.5hp was OK since it is same motor as 2.5hp retuned and was told it's OK since their concern is weight)

2 knots, 3,4,5,6 knots? Does anyone have data? I'm assuming flat water, no wind conditions. I asked Fatty Knees and they said they'd never measured. What I'm really hoping is that someone had taken a GPS out in their dinghy. ..

Thanks for any data on this!

There is a big difference between a sailing dingy and a dingy.
This is what you want to understand -
.
Most sailing hulls, even on a dingy, are displacement hulls. No amount of power is going to push it past the hull speed. I tried an outboard on my Minto 9' hard dingy and on a Boston Whaler Orca 'sailing' dingy. Both just raise their bows and leave a very big wake but they do not plane.
My 10' RIB and my 12' Porte-a-Bote (folding) both plane easily with an 8hp and fly with anything larger.
.
If you buy a "rowing" or "sailing" dingy, it probably will not go past its hull speed, and I use an electric trolling motor which works well.
Only a planning hull will take advantage of the power from an outboard over about 2hp.

As a side note - Since a displacement hull will only move at "rowing" speed and will not plane, I use an electric motor for them. The electric is quiet and about the same power as a strong oarsman. Just right. And no gasoline and no bulky, heavy engine.