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

[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?

[ranger58sb]

Don't know speed, but we have a relatively lightweight-ish 10' RIB (135-lbs) and a 5-hp 2-stroke (46-lbs), and it won't really plane even with just me (190-lbs) onboard.

-Chris

[Sailmonkey]

Our 5hp 2 stroke will plane off with only myself in an 8' rib with a speed of 14 knots. Two people in the same boat and we're just shy of planing with a speed of 5 knots.
I have plopped the same outboard on a 12' rib and with four people aboard we were doing around 8 knots.
Fwiw

[Tellie]

I'd always go bigger if possible. For my dingy I have a 5hp, a 10hp and a 18hp. All around I like the 10hp for planning and speed and fighting currents. The 18hp is sweet on long distance trips and diving. Except for the weight, I don't know why I still have the 5hp.

[Sailmonkey]

Originally Posted by Tellie

Except for the weight, I don't know why I still have the 5hp.

I was just saying the same thing.....but in favor of our 2 hp outboard!!

[stevensuf]

Normally 4hp per person or thereabouts for planning on smallish 10ft ft or so dinghies. our old 9ft rib would plane with two on 6hp but one of the 2 had to be light, our new 10ft rib needs 8hp to plane with 2 onboard.

[cfarrar]

The 11 ft aluminum boat should actually plane a little better than a shorter RIB. I'd venture the 5 hp will plane one person, maybe 2 if you're light. Start by heading downwind, get on a plane, and then make a gradual turn back into the wind if you need to go that way. It works!

[noelex 77]

You are right on the power limits between planing and semi displacement.

My prediction would be that you will just plane with 1 adult, but not with 2.

Minor differences with prop design, actual weight and power output of the motor will make a dramatic difference in speed in this transition zone between displacement and planning.

If it is important to be plane a small increase in horsepower would make a big difference, but any increase in weight means its difficult to pull up on the beach, in those areas where this is necessary.

[troppo]

Thank you folks for your replies, I found them all very interesting. It looks like it will come down to what will be the more important benefit: extra power of the bigger motor seeing as the 5hp seems quite close to the limit of planing or increase of portability with the smaller motor such as lifting up and down from my boat and hauling up beaches.

Today our river has a tidal difference of 4.9 meters so the current can be quite strong and I saw a croc just last week sunning itself only a few kms from the city centre so I think at times a little extra power would prove useful. On the other hand, at low tide there is a lot of sandbank to pull the dinghy up to get to the high tide mark.

I appreciate your answers as it is helping me come to a decision.

[cat man do]

Originally Posted by troppo

. On the other hand, at low tide there is a lot of sandbank to pull the dinghy up to get to the high tide mark.

Thats what they invented wheels for
Somthing like these are simple to make

Bit of ally box and some ally channel
The set I made have 2 wheels on each leg, one on either side of the box section

[Cheechako]

for an extra 6kg, go with the 10hp.

[Bash]

Originally Posted by troppo

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.

Do you remember how much you weighed back then?

(Seriously, I had a 12' tinny with a 9.8, and it had no problem planing two adults and gear.)

[Boracay]

We have a 5hp 2 stroke on our 9' tinnie and get 7-8 knots GPS in smooth water flat out - enough to get us noticeably over the Qld. limit of 6 knots. 5 is about the maximum rated power for the boat.

It probably depends what you mean by plane. We were not bouncing along the top of the water, but were well over displacement speed.

As to how useful it would depend on what you're using the boat for. We found that for runs of up to 4nm the 5hp is fine, any further it's starting to be a long trip.

If I were buying again right now I'd go for one of the lightweight 2.2 two strokes, and take the reduced speed. I found the 5hp was too heavy to manhandle on and off the dinghy, and we missed out on some nice trips because it was just too much hassle to get our act together.

[ranger58sb]

Originally Posted by ranger42c

Don't know speed, but we have a relatively lightweight-ish 10' RIB (135-lbs) and a 5-hp 2-stroke (46-lbs), and it won't really plane even with just me (190-lbs) onboard.

-Chris

I owe a correction.


Turns out the boat MIGHT plane, but only under certain circumstances. Yesterday, the 90-lb First Mutt and I went for a quick trip, me aft, him standing at the bow. This was the first time I had a speedometer app with me, so I was checking performance, in generally calm (flat) waters. In addition to the boat, motor, me, and the dog, I had 1/2-gallon of gas in the internal tank, and 1 gallon of gas in the external tank (which was also aft).


Normal putter-along speed was about 4 mph, max non-planing speed was 7 mph... and that seemed to be pretty much the limit of what we could do.


But I perservered, and we got a little unanticipated help when overtaking the (very small) wake of a large boat that had just passed... and climbing over that little wake was apparently just enough to bump us up on plane. First time, ever.


Max speed on plane was 11 mph before I had to back off (traffic, ran out of room, etc.). I didn't have enough space in the area to experiment with potentially higher speeds (if any), or backing off to see how slow I could go while remaiing on plane.


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

[Sailmonkey]

Originally Posted by ranger42c

I owe a correction.


Turns out the boat MIGHT plane, but only under certain circumstances. Yesterday, the 90-lb First Mutt and I went for a quick trip, me aft, him standing at the bow. This was the first time I had a speedometer app with me, so I was checking performance, in generally calm (flat) waters. In addition to the boat, motor, me, and the dog, I had 1/2-gallon of gas in the internal tank, and 1 gallon of gas in the external tank (which was also aft).



-Chris

What I neglected to mention was that in our 8' rib, while planing at 14 knots with the 5 hp 2 stroke I sit well forward with a pvc pipe tiller extension. My chosen seat for almost top speed is the seat, roughly 1/4 of the way aft from the bow. If i'm in a real hurry, I'll sit on the bottom of the boat, just aft of the seat, but the ride is better with my weight further fwd, it presses the bow down and forces the hull to split the seas rather than slap on the flat after sections of the hull.

This is not my boat, but is identical.
2006 Avon 260 Lite RIB