what power outboard for rigid tender?

[Tranona]

That won't plane but if you are going to load it with 4 people and gear then a 6 would be best. Traditionally of course it would have had a Seagull Silver Century Plus which would have given you 5 knots and an accompanying smoke haze!

 

[Rappey]

Pretty unlikely it will plane

A surf board planes yet that has no chines?
Rigid dingys usually have fairly flat floors, that is what they plane on. Terrible ride though as it slams on every tiny ripple.
You have to shift your body weight to help it get on the plane.
Can't get a pram dingy to plane ,due to the curve from bow to stern .

 

[nestawayboats]

The main difference between a Torqeedo 1103 or Epropulsion Spirit - both rated as "3hp equivalent" - and an actual 3hp petrol motor, is that a 3hp petrol might get some small, lightly loaded (1 person) dinghies on the plane. I would guess not the OP's round bilge hard dinghy though.

The 1kW electric outboards won't get anything on the plane as their maximum shaft speed/propeller means that they'll never drive anything at more than about 5.5 knots.

BUT if it's a boat that won't plane with 3hp, or would plane with one person but you have two or more people in it - ie running at displacement speeds - the 1kW electric motor will most likely do a better job than a 3hp petrol. Both the Torqeedo and the Epropulsion turn a relatively large propeller, relatively slowly (compared with combustion motors), to take advantage of the high torque output from electric motors. And this is exactly what you want at displacement speeds. There are many variables but generally the electric motors will get a more heavily loaded dinghy up to (or nearer to) displacement hull speed than a 3hp petrol. A 6hp petrol would probably win on top speed, by sheer brute force, but you're still unlikely to get anywhere near planing with 4 people on board and only 6hp (not on any boat).

"1kW electric outboard is Equivalent to 3hp petrol" is different to saying "1kW electric outboard is Equal to 3hp petrol". They are roughly equivalent but with very different delivery.

Going much higher up the scale, the 1kW electric motors will drive a 4 tonne sailing yacht (Sadler 29) at 3.3 knots, we've tried it. There's a lot of "shove" there.

IMHO, trying to drive any small open boat with 4 people in it against a 5 knot adverse tide (and headwind) would be "pushing your luck", regardless of the motor. I'd wait for the tide to slow down a bit.

Ian, Nestaway Boats Ltd

 

[Chae_73]

Thanks Ian, very informative.

The 5 knot bit is a slight exaggeration. Yes, it can run up to 5 knot in the narrow section at the mouth but there is no particular reason to go there in a tender.

Launching off the Bawdsey Haven slip which is in a bit of an eddy, and then heading up river, my guess would be you'd likely see no more than 3 knots of tide.

It does seem that electric has a lot to recommend it. I'm assuming that I'll need two outboards so likely that one will be electric.

 

[penfold]

You can get nearly anything to plane if you throw enough power at it ,excluding keeled boats.. but it may become very unstable and scary.. body weight positioning is very important in a small boat which often means you can't reach the outboard.
Your Orkney is certainly not designed to plane, but put a 30 on it and I would think 30 knts or death.?.. an oversized engine may be too heavy for the boat and the transom may struggle with being overpowered.
The hull is a v shape on planing hulls.

Not really, put more cowbell on a rowing dinghy it's likely to submarine long before it planes; rowing dinghys have rocker(hull aft of midships curves up to transom) to give low drag at displacement speeds, this punt has rocker(quite exaggerated rocker but it illustrates the point). It's sometimes feasible get such a hull to plane if you sit right aft so the wetted area is mostly flat but this isn't terribly safe.


Planing hulls have little or no rocker, this allows low drag at planing speed by facilitating flow detachment. They do not need a vee form, just no rocker. Planing dinghies are a giant pain in the bum to row as a result.

 

[Arcady]

it looks like this

View attachment 113551

Will you be lifting the outboard off regularly? If so, I’d choose the smallest model. 2.5 hp will get that up to hull speed.

I previously used a Honda 2.3 on a 10’ clinker dinghy and it was fine - even with five people on board (no comments please) against a strong flood tide.

Bigger will just be heavier and more costly.

 

[CAPTAIN FANTASTIC]

Dont underestimate the capability of the small 4 stroke modern outboards. The Honda 2.3, Suzuki 2.5hp, Tohatsu 3.5h and the Honda 5hp are all good engineered engines and reliable. The Honda 5hp will certainly get you on the plane. If you want something more powerful but heavier then go for the Tohatsu 6hp. Reliability and easy to start are the basic requirements for me , I therefore choose four stroke outboards.

 

[Chae_73]

Out of what is currently available locally, it looks like we'll be going for a Suzuki 2.5 or Honda 2.3. There is also a Suzuki 6 but its nearly twice the weight and price so probably over the top for our needs.

 

[Stemar]

Can't see a Suzuki 3.5 hp 4 stroke. Mariner 3.5 is 17 kg vs 23 kg for the Suzuki 6. I think I can live with the extra 6 kg.

The extra weight is fine until you have to take it off the tender, and you will, if only for servicing. I find my Mariner 3.5 quite heavy enough to lug around. I tow my tender, usually with the OB on it but, when things get bumpy, I put the OB on a bracket on Jissel's transom. When things get to the point you need to do that, 25+Kg (I've added some oil and fuel) could be quite interesting.

Given the way my old 3.5HP pushed Jissel (24ft, 3 tons) along when the engine didn't want to play, I can't imagine that wouldn't be enough to push a 12 ft GRP dinghy at hull speed without raising a sweat. If you need another half a knot, you'd probably get it, but more would need a lot of extra power to plane it and, as Rappey said, that may be something you'd only do once

 

[nestawayboats]

If you're only trying to reach displacement hull speed in a small dinghy then a 1kW electric outboard would do the job well. As mentioned before, they are largely interchangeable, in terms of propulsive effort, with petrol motors up to 3hp or so.

Plus they are quiet, smooth, childsplay to start, and easier to move up and down to the big boat than any petrol outboard - because the battery can be handed up separately leaving the shaft at about 10kg. And far more able to cope with infrequent use (no carb to clog up as petrol evaporates), stored on their sides (even upside down if you like), and other "abuse" that's inflicted on yacht tender motors.

I'm talking about the Torqeedo or Epropulsion types which have an integral but easily removable lithium battery roughly where you'd expect the fuel tank to be. Not the trolling motor type, where you need a 25kg separate battery in the boat... that's not easier to hand up and down!

My comments on the small petrol motor market, personal experience /opinion, not "official" advice etc etc:
- The Honda 2.3 is air cooled, simple and less to go wrong yes but you can hear the abysmal noise for miles, and has a centrifugal clutch which takes some getting used to in slow speed docking etc... and apparently there is a man on ebay who does very well out of selling the kit of stainless bits to replace the bits that might go rusty. 13kg I think, but that's "dry" so with petrol and oil in it more like 15kg.
- The Suzuki 2.5 is water cooled and does have a clutch (F-N gearbox) but is a bit notorious for running so lean that sometimes it barely runs. There is a simple fix if you know how, but only by making it emissions non-compliant. It also seems to be quite prone to carb clogging, probably just because everything's so small. Similar weight to Honda 2.3.
- The Tohatsu 3.5 and its identical Mercury Mariner sisters I quite like, if I can't have an electric one! They are water cooled, have F-N gearboxes, and seem less prone to carb clogging. They are 18kg ish (20-ish with oil and fuel), roughly the same as an assembled Torqeedo 1103 or Epropulsion Spirit, but without roughly half of that 18kg (the battery) being removable as it is on the electrics.
- The Honda 5 I don't have experience of but lots of Suzuki and Tohatsu motors of that size over the years. They are all single cylinder carb motors with F-N-R gearboxes. Yes it should get one person maybe two on the plane in a planing hull form. However motors of this size (4-6hp range) weigh 24- 27kg, which is more than a lot of people are happy lifting from wobbly dinghy transom to yacht pushpit.

We do still sell petrol motors below 4hp in theory, but very rarely nowadays... a large percentage of electric outboard customers are purchasing to replace petrol ones. Often after a season or two of paying a mechanic to fix their petrol motor several times (quite likely often just a carb clean, but not everybody can DIY that). Above that size it's perfectly possible to get larger electric outboards but cost (mainly of batteries) becomes a very significant deterrent.

Ian, Nestaway Boats Ltd

 

[Arcady]

...The Honda 5hp will certainly get you on the plane.

I’m sure it would do on most inflatables or RIBS, but the OP’s photo shows a reasonably narrow, easily driven displacement hull form. That won’t plane - but it will cut along nicely with even the smallest 4 stroke.

 

[Rappey]

That won’t plane -

So are you suggesting that if a 30 hp was placed on the rear it would still only do a displacement speed of about 5 knts ? I don't think so.
It could easily do 15 plus knts.

 

[LONG_KEELER]

Noise can be a problem on 2 Strokes . My downloaded decibel meter measures 90 decibels
for a 5HP . Great for power to weight but very thirsty and just too loud for me and the neighbours. Most places have a speed limit anyway.

I have given up speed and my Honda 2.3hp seems to last forever on 1lt fuel . If struggling against wind and tide, you can still row with the engine going as a turbo boost.

 

[Chae_73]

Well we have gone for the Honda 2.3 on the basis of price, simplicity, weight and availability. Not necessarily in that order of priority.

Thank you everyone for your informative replies. I'll report back after the next round of sea (river) trials.

 

[geem]

We run a nesting 12’5” dinghy. We use either 9.8hp Tohatsu 2 stroke or a Yam Enduro 15hp 2 stroke. With the 15hp the boat will plane with four people. With two onboard it will do over 20kts.
Hull shape is important to stability. A good set up is a vee section bow running in to a flattish stern section with some width in the transom. The width in the transom resists role and provides buoyancy where you need it. Another good investment if you are carrying load frequently is a large planing plate. This will lift the stern and assist with planing and provide some stability.
The interesting thing is my Tohatsu 9.8hp weighs the same as a Tohatsu 6hp four stroke. The 9.8hp 2 stroke is such a nice engine compared to the four stroke. It starts super easily being a twin cylinder. The remote tank keeps fuel weight out of the engine.
For your boat that looks quite round in the transom I wouldnt expect too much. I really don't think you want to try planing with lots of weight in that hull. I think in any chop it could end in tears. I think you may end up in a death roll?.
A 5hp Yamaha two stroke or 8 hp (twin cylinder) would be my choice for lightness and power. Having tried the four stroke route, I wont be going there again any time soon?

 

[jwilson]

Dont underestimate the capability of the small 4 stroke modern outboards. The Honda 2.3, Suzuki 2.5hp, Tohatsu 3.5h and the Honda 5hp are all good engineered engines and reliable. The Honda 5hp will certainly get you on the plane. If you want something more powerful but heavier then go for the Tohatsu 6hp. Reliability and easy to start are the basic requirements for me , I therefore choose four stroke outboards.

The dinghy owned by the OP (see post 17) is NOT ever intended to to plane. The hull form is just plain wrong for planing, though probably quite good as a rowing boat. A decent two-stroke 3.3/3.5 will get the OPs' dinghy going at hull speed easily. A four-stroke 4 to 6 hp will weigh a fair bit more and maybe get another 0.5 to 1 knot out of it, pulling along a lot more wash with it. It may however be more reliable, though a well maintained two-stroke should also be reliable. Much above 6hp is for practical purposes pointless.

You COULD make it plane, but it might need 20 hp plus, and when stopped the transom might sink. Actually using it planing with that weight engine would be near-suicidal, as it would be horribly unstable.

 

[Rappey]

) is NOT ever intended to to plane

There is certainly no argument about nearly every rigid dingy not being designed as a planing vessel, but some do it rather well when carrying minimal weight.
I once had a catamaran dingy. 9ft long but proper stepped planing hulls.
Having a fore and aft tunnel to sit on and a flat foredeck made it virtually impossible to carry as there was nothing to grab hold of and it was very heavy.
Beautifully smooth ride and very fast. I was sad to sell it but dragging it up and down the slipway from the public rack was just going to ruin it. Trolley not an option as nowhere to keep it.
I have a dingy with a fully rounded bow and pretty flat bottom ,it's just on the verge of planing with a 3.5 hitting 7.5 knts, I just need a little more power ?

 

[LONG_KEELER]

There is certainly no argument about nearly every rigid dingy not being designed as a planing vessel, but some do it rather well when carrying minimal weight.
I once had a catamaran dingy. 9ft long but proper stepped planing hulls.
Having a fore and aft tunnel to sit on and a flat foredeck made it virtually impossible to carry as there was nothing to grab hold of and it was very heavy.
Beautifully smooth ride and very fast. I was sad to sell it but dragging it up and down the slipway from the public rack was just going to ruin it. Trolley not an option as nowhere to keep it.
I have a dingy with a fully rounded bow and pretty flat bottom ,it's just on the verge of planing with a 3.5 hitting 7.5 knts, I just need a little more power ?

Reminds me of a hard cathedral shaped hull dinghy that I once owned. It was extremely stable for getting on and off the mother ship in it's form stability. You don't seem to see them anymore but it planed ok in smooth water. Could possibly flip in the rough stuff though. Water ingress in the foam hulls became a slow cause of death.

 

[geem]

Here is our 12’5” nesting hard dinghy that planes superbly. We have a large planing plate fitted to our 15hp Yam Enduro. Going up wind in to a chop we trim the engine so the bow is down. This gives a comfortable and dry ride. She will do 10/15 kts in to quite big seas. Down wind in big waves we need to trim the nose up or she will bury the bow. With the bow trimmed up and good use of the throttle it can be pretty exciting surfing down waves at over 20kts.

In normal use we trim the bow up. Combined with the large planing plate, that lifts the stern, we have minimal wetted area. We ride on the plate such that you need a good secure engine mount on the transom or you risk losing the engine over the stern! With this set up she will run well with one to three people. Well over 20 kts with two people. She will plane with four but you have less manoeuvrability and you need more throttle to stay on the plane. Its a perfect hooligans boat! I love it?

 

[ashtead]

Well we have gone for the Honda 2.3 on the basis of price, simplicity, weight and availability. Not necessarily in that order of priority.

Thank you everyone for your informative replies. I'll report back after the next round of sea (river) trials.

As quoted above there are rust issues with these I have heard (bolt replacement in stainless ) as well as noise so maybe some early preventative measures might be in order. I believe a forumite sells Honda OB as Ian at Nestaway says so he might be along to update you soon.