Nesting, planing, sailing, rowing dinghy

[BobbingAboutBob]

For the last five years I've been using a Spindrift 11N as my yacht tender. I love it. It rows like a dream, it out sails every other sailing tender I've encountered, it's a great load carrier. An all round great practical boat.

There is only one thing it can't do: it won't plane. I think the hull has too much rocker, and I'm worried about the structure of a nesting boat carrying a bigger, heavier engine and hitting waves at higher speeds.

What would I need to change about the design to allow it to plane under motor? Ideally I'd be looking at fitting a 9.8hp two stroke (26kg) and have it plane with two adults (all up weight around 230kg).

I presume that a wider, flatter stern shape would be required. And that this would create a compromise and make rowing less efficient.

 

[Skip Johnson]

Adding a "whale tail" fin to the motor might do the job.

 

[BobbingAboutBob]

Adding a "whale tail" fin to the motor might do the job.

That's an interesting idea, and would be very easy to try out.

 

[marlinspike]

It's a sailing dinghy optimized for efficiency at below hull speed by keeping the aft buoyancy low, it's never going to motor well, I wouldn't even bother with a motor on that boat, just get a different dinghy if you absolutely need to plane, but then don't expect it to row or sail well.

Regardless, just about anything will plane with enough power... how much weight and how big of a motor did you have where it wouldn't plane?

And how much did that motor weigh? It's true that you don't want a heavy motor on a boat this size, but older 2-strokes are about half the weight for the same power rating, you can likely find 10-15 horse motors that are only about 50lbs.

 

[BobbingAboutBob]

It's a sailing dinghy optimized for efficiency at below hull speed by keeping the aft buoyancy low, it's never going to motor well, I wouldn't even bother with a motor on that boat, just get a different dinghy if you absolutely need to plane, but then don't expect it to row or sail well.

Regardless, just about anything will plane with enough power... how much weight and how big of a motor did you have where it wouldn't plane?

And how much did that motor weigh? It's true that you don't want a heavy motor on a boat this size, but older 2-strokes are about half the weight for the same power rating, you can likely find 10-15 horse motors that are only about 50lbs.

The biggest I've used it with is a 3.5hp two stroke, weighing about 13kg. It would get up to 6.5kt with just me aboard in good conditions.
I also own a 9.8hp two stroke and that's what I want to use on the new, modified design.

 

[marlinspike]

The biggest I've used it with is a 3.5hp two stroke, weighing about 13kg. It would get up to 6.5kt with just me aboard in good conditions.
I also own a 9.8hp two stroke and that's what I want to use on the new, modified design.

Nothing is really going to plane well with a motor that small, especially with two adults. I have a dinghy designed for planing and with just me aboard (a pretty big guy) it still won't plane with a light 3hp 2-stroke motor.

Consider a planing inflatable like the Sea Eagle Paddleski which is only 57lbs and will plane with 2 adults using a 5 horse motor, and rolls up small- you could probably still also bring the current dinghy for rowing and sailing.

 

[BobbingAboutBob]

Nothing is really going to plane well with a motor that small, especially with two adults. I have a dinghy designed for planing and with just me aboard (a pretty big guy) it still won't plane with a light 3hp 2-stroke motor.

Consider a planing inflatable like the Sea Eagle Paddleski which is only 57lbs and will plane with 2 adults using a 5 horse motor, and rolls up small- you could probably still also bring the current dinghy for rowing and sailing.

I don't expect anything to plane with the 3.5hp.

The new dinghy would be designed for the 9.8hp. I believe with the right design that this should be possible, for two adults weighing a combined 140kg.

In reality we motor more than we sail or row, so I'd be looking to bias the design in that direction.

 

[BobbingAboutBob]

One thing I'd be very open to is to add extra buoyancy or surface area to the stern of the boat, to convert it in to a more motor-friendly hull.

 

[skaraborgcraft]

Little Harrier from here: Motor Boats up to 16' https://www.selway-fisher.com/Mcup16.htm#RIER

20 knots with 10hp......allegedly. There was a pretty much identical boat in a 1950s book, with the same performance.

 

[marlinspike]

I don't expect anything to plane with the 3.5hp.

The new dinghy would be designed for the 9.8hp. I believe with the right design that this should be possible, for two adults weighing a combined 140kg.

In reality we motor more than we sail or row, so I'd be looking to bias the design in that direction.

If you don't sail much, is it critical that it still sail? I frequently see small open motorboats in aluminum and fiberglass that would plane with a 10hp motor ready to go for under $100, and they all come with oarlocks also. It sounds like you're planning to cut up a nice little sailboat to radically reshape it's hull, which doesn't seem like a good option to me, when you can probably find a ready to go dinghy in the shape you're looking for, for less price than the paint you will use for that project, and you'll still have the sailboat intact to use or resale.

A Robb White style sportboat, basically a canoe with an outboard transom, will readily plane with two adults and a 3hp motor. Lightweight, but the length could be a problem for stowing as a dingy.

 

[BobbingAboutBob]

If you don't sail much, is it critical that it still sail? I frequently see small open motorboats in aluminum and fiberglass that would plane with a 10hp motor ready to go for under $100, and they all come with oarlocks also. It sounds like you're planning to cut up a nice little sailboat to radically reshape it's hull, which doesn't seem like a good option to me, when you can probably find a ready to go dinghy in the shape you're looking for, for less price than the paint you will use for that project, and you'll still have the sailboat intact to use or resale.

A Robb White style sportboat, basically a canoe with an outboard transom, will readily plane with two adults and a 3hp motor. Lightweight, but the length could be a problem for stowing as a dingy.

I'm not cutting up my existing dinghy. I'm talking about building another one that's similar but I want to incorporate some difference features. I don't really care how much it costs.

And yes sailing is critical, but so is planing.

I know it's possible to achieve this- the OC does it.

 

[marlinspike]

I'm not cutting up my existing dinghy. I'm talking about building another one that's similar but I want to incorporate some difference features. I don't really care how much it costs.

And yes sailing is critical, but so is planing.

I know it's possible to achieve this- the OC does it.

Got it, that sounds good, I misunderstood.

I agree a boat can plane and sail, after all, small racing dinghies in that size often plane quite well. The Club FJ used by college sailing teams is a hull shape that feels very stable and comfortable for two adults, and planes readily under sail, but I don't think the transom design on an FJ would accommodate the weight and stress of an outboard. Still, that basic underwater hull shape might work for what you're trying to do.

The OC is a nice design, I was ferried around in one by some cruisers I met down in Baja. Certainly motors well, but I didn't get a chance to sail it.

 

[Dolfiman]

In case this example can be helpful your new design : Velocette, a nesting planing (with one 86 kg sailor) sailing dinghy, though I don’t know how much hp in needed to also go planing with an outboard.

Thread 'Velocette : a nesting dinghy for fun sailing'

Jun 19, 2022

We open this thread together with @Veloceruss as we have collaborated on this project since Russ started his thoughts late 2019, shared with you through 2 former threads :
I would like suggestions for design books | Boat Design Net
Hot Rod Nesting dinghy | Boat Design Net

Russ wanted to build a dinghy for sailing around anchorages to add the pleasure of small boat sailing to his cruising programme, when at mooring in beautiful bays with his First 42s7. First specifications were :
** A nesting dinghy to use for fun sailing, not a yacht tender (i.e. not a rowing or...

• Dolfiman
• nesting dinghy planing strip planking
• Replies: 1
• Forum: Sailboats

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[BobbingAboutBob]

Got it, that sounds good, I misunderstood.

I agree a boat can plane and sail, after all, small racing dinghies in that size often plane quite well. The Club FJ used by college sailing teams is a hull shape that feels very stable and comfortable for two adults, and planes readily under sail, but I don't think the transom design on an FJ would accommodate the weight and stress of an outboard. Still, that basic underwater hull shape might work for what you're trying to do.

The OC is a nice design, I was ferried around in one by some cruisers I met down in Baja. Certainly motors well, but I didn't get a chance to sail it.

The OC is an interesting dinghy but I've never seen one under sail. An owner told me that taking the 15hp off the back is too much of a pain.

That's something I'll have to consider myself. Twin rudders with the outboard in the middle could work, but I may have just reinvented the MacGregor 26 in miniature

 

[montero]

I've been planing with two adults on a 4hp 2T OMC. I'd pay attention to the L/B ratio.

Going further, maybe a stabilized mono (tri) with a center planing hull would be quite fast. Also with sail.

The drawback is size.