How fast can you go with 3.5hp (on water)?

[dancrane]

A thought with nought behind it.

Just wondering whether, with a sheet of 6mm ply and a willingness to get first dusty, then sweatily frustrated, then covered with epoxy, then finally soaked when it sinks...

...I could make a mini speedboat - very mini. Mainly I'm thinking how much speed potential is lost by inflatables, in pursuit of versatility, stability and buoyancy. Instead of that, a very simple ply hull would be stiff, light and flat - or possibly very narrow...

...so, what's the best, fastest, most efficient shape and form to be driven by a very small outboard? Canoe maybe? Or a cat, or something like a skiff?
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[BobnLesley]

Both our 3.3hp Mercury and 3.5hp Nissan 2T engines were able to get a 2 4m rib up onto the plane; that said we rarely bothered as the noise levels when we did were horrendous.

 

[Martin_J]

Maybe a two with a zero behind it if you try. I know my standard inflatable dinghy reaches 12 knots on the plane with the 3.3 on the back.

When I think of you trying, all I can imagine is these guys in their longtail river boats..

And an outsider view of them here.. Is this you next year Dan?

 

[dancrane]

I was definitely thinking of a home-built hull to suit the 3.5 two-stroke I already have, rather than a wild, deranged, expensive, unlimited (and perilous) pursuit of speed!

Things like foiling Moths manage to be extremely fast through lightness - and a lot of underwater engineering cleverness too, I realise.

But a low-drag, light stiff hull must be a better route to getting the most out of a tiny outboard, than a broad, bulbous, flexible inflatable, no?

 

[wombat88]

Why not try and old Finn hull?

Finn was the first one to come to mind but almost any old dinghy hull could produce a fun machine and cheap to do but re-enforce the transom first!

 

[jwilson]

Not a Finn: too heavy and not flat enough for good o/b planing. Maybe an old Topper hull with CB slot blanked off and and outboard bracket fitted - and a light person on board. The limiting factor probably the gearing on the prop.

 

[AntarcticPilot]

Or maybe something long and thin, like an eight or a dragon boat? Eights do around 15 knots with about 4hp, assuming a rower can sustain about half a horsepower.

 

[dancrane]

Those are all very appealing ideas.
It had occurred to me to experiment with a ropey old Mirror. How much of a Mirror's boxy structure is only for stiffening the hull for sailing?

It's often struck me that the inside of a Mirror would be much more useful if it wasn't all boxed in.

I'll get a buoyancy aid for my engine.

 

[Rappey]

But a low-drag, light stiff hull must be a better route to getting the most out of a tiny outboard, than a broad, bulbous, flexible inflatable, no?

The hard part for low power is getting on the plane. A flat bottomed boat will need a lot less power than something with a v shape. Pull a flat bottomed inflatable along on water and its extremely light.
12 knts is the most I've achieved with a 3.5 tohatsu 2t on a very small inflatable. It also has an inflatable keel which i would assume causes drag.
We tried it on a 3.2m rib and it did 9 knots which surprised me as its a heavy dingy.
Ive found a wave piercing catamaran hull can be very fast compared to a mono hull of the same length for a 8ft and 14ft boat.
Zap cats are tubed, light and fast.
Certainly interesting as to what speed could be extracted from a 3.5
I used to have a 9ish foot speedboat with a 25hp. It needed power to get on the plane but was great fun.
A possible advantage of a rib is that the hull is already some way out of the water due to the tube buoyancy.

 

[Fimacca]

I have 2 dinghies - a flat bottom avon and a v shaped new honda airdeck. The 3.5 tohat I use, planes with one but not the other at full pelt with just myself aboard.
V is the way !

 

[Stemar]

Here you go - a one sheet dinghy

I think I'd want to reinforce the transom, and I'm definitely wearing my life jacket - and the kill cord - and tying a couple of fenders to the OB I'd also wait until the water's a bit warmer!

 

[Chiara’s slave]

Depending how much compromise on stability, getting the length to beam ratio over 8 is the way to go, with a planing section. Think of a very narrow Optimist. It’s why our sailing boat will top 20kn.

 

[AntarcticPilot]

The hard part for low power is getting on the plane. A flat bottomed boat will need a lot less power than something with a v shape. Pull a flat bottomed inflatable along on water and its extremely light.
12 knts is the most I've achieved with a 3.5 tohatsu 2t on a very small inflatable. It also has an inflatable keel which i would assume causes drag.
We tried it on a 3.2m rib and it did 9 knots which surprised me as its a heavy dingy.
Ive found a wave piercing catamaran hull can be very fast compared to a mono hull of the same length for a 8ft and 14ft boat.
Zap cats are tubed, light and fast.
Certainly interesting as to what speed could be extracted from a 3.5
I used to have a 9ish foot speedboat with a 25hp. It needed power to get on the plane but was great fun.
A possible advantage of a rib is that the hull is already some way out of the water due to the tube buoyancy.

Getting on the plane isn't the only way to go fast - the right form factor will do it. Long and thin is fast, too, as borne out by eights, catamarans and a lot of military vessels. It also avoids the need for excess power to get over the "hump" and on to the plane.

 

[MisterBaxter]

Phil Bolger designed a number of boats to run fast on low power. I'm looking at a design of his called Firebrand now, 26' long by 4'3" wide, plywood with a completely rectangular cross section. It has very little rocker apart from.a gentle rise to get the forefoot out of the water so that the bottom of the boat meets the water not the sides. The taper to the bow starts at around the mid point of the boat, and behind that the sides are parallel. Basically a long, slim plywood box, pointy at the bow and square at the stern. The design is intended to make 12 knots with a 6hp steam engine that the owner wanted to build a fast boat around.
I would have thought that something along those lines, perhaps 14' x 3' and as light as possible, might be a good bet.

 

[Hot Property]

I've often though what a paddleboard will do with a 3.5 Tohasu on the back.

I just need to design and make a suitable bracket.... I would think it would fly

 

[Mistroma]

Both our 3.3hp Mercury and 3.5hp Nissan 2T engines were able to get a 2 4m rib up onto the plane; that said we rarely bothered as the noise levels when we did were horrendous.

Same with our 3.3hp Mariner on 2.1m Lodestar. The dinghy has a decent V at bow and flat aft section with inflatable keel and floor. It won't plane with 2 people but I only weigh about 58kg and it goes like a rocket with just me. I use a tiller extension and need to sit right at the bows. It seems as if it's just the prop in the water in calm conditions and gets close to being uncontrollable. Very shallow turns only.

 

[dancrane]

I suppose I ought to find a very small, wrecked RIB, on the lightweight side. By which I mean an actual rigid hulled design, rather than the many inflatables which call themselves RIBs on the strength of their inflatable keels.

If the tubes are destroyed it wouldn't matter, I'd find a way to substitute. I can already see it's going to look terrific.

Perhaps I'm being unfair to inflatable-rigid-inflatable hulls...are they pumped to the high pressures that stand-up paddleboards use?

It has me thinking...Coast Guard look away...if only someone sold outboard brackets for stand-up paddleboards...I bet they'd sell almost as well as the boards themselves.

EDIT: Hot Property, you must have been posting that same thought while I was fooling about with YouTube links.

I'm now thinking where does one source a discarded stand-up paddleboard?

But it's interesting that in spite of the low drag and greater power used in the video below (I realise his set up probably isn't making best use of its output) I doubt the fellow is going significantly faster than some of you have said your RIBs go with 3.5hp.

Great noise though.
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[TSB240]

Or maybe something long and thin, like an eight or a dragon boat? Eights do around 15 knots with about 4hp, assuming a rower can sustain about half a horsepower.

As a junior international in a single scull I could average about 1.5 minutes over 500 metres during training sessions of 10 runs with a 2 minute recovery period between.

I reckon that's about 10knots or more. Not bad for single man power or about 0.5 hp!

No chance of extending range beyond 500m without either oxygen, performance enhancing drugs or risk of cardiac arrest.

I never trained on an Ergo!

A double scull over short bursts was were more than a match for 8 heavies in an eight!

Op forget wide flat or V bottomed boats too much drag. The International Moth evolution went from scow designs to narrow easily driven lightweight hulls before the foiling revolution.

 

[Chiara’s slave]

Getting on the plane isn't the only way to go fast - the right form factor will do it. Long and thin is fast, too, as borne out by eights, catamarans and a lot of military vessels. It also avoids the need for excess power to get over the "hump" and on to the plane.

The message is becoming clear the planing ‘hump’ does not exist if you have the right form factor. All modern performance multis that don’t have foils use this. Our speed is perfectly linear to windspeed as a result. The only way to tell if you’re planing in a Dragonfly is to look at the sternwave.

 

[dancrane]

This is all fascinating and funny, thanks for so many intelligent responses. I will go through it again and take note of what I've only breezed through at first reading.

It occurred to me (within the last ten minutes) that I could make a very light ply 'sled' with a robust transom, that would carry the engine well enough at speed, but which would simply sink, at idle...

...but the sled could be shaped to carry an old Redcrest or Redstart...mine, for instance...held securely with ratchet straps, to multiple new heavy-duty rubber loops glued to the Avon's hull with approved Polymarine adhesive...

...so the old dinghy's abominable inefficient waste of horsepower and severe flexing, wouldn't be an issue. And the sled wouldn't go to the bottom when the engine is stopped.

And think how slick it will look, setting it up at the beach! I'll be Sonny Crockett himself.
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