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

Pretty certain my 3.3hp 2.1m lodestar dinghy is quite a bit faster than the paddle board setup. It isn't a RIB, just inflatable keel and floor. RIBs performance should be more predictable. I think total weight is around 105 kg (me 58kg, dinghy, 33kg & outboard, 14kg). I suspect another 15kg weight would be about the limit to get it up on the plane easily with my 3.3hp.

dancrane said:

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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A friend made a small 'hydroplane'. The bottom was a flat sheet of ply, curved up at the bow. Either side was a 25mm batten, then flat sides, dory style. The battens trapped air under the hull, it went like the proverbial.

Think there was a worn-out Dart catamaran at my sailing club...

...if I could salvage one good hull...they're pretty light...I could make one or two amas, proa or trimaran-style, and a sliding seat allowing variable trim...

...reinforce the transom and build up an outboard bracket...and lose enough weight to fit in my wetsuit again. It'd surely be a wet ride.

But interesting to find how much more or less efficient (and whether faster or slower) a long narrow hull would be, than a small RIB.

World record for human powered craft is 18.5 knts. Would assume this design must be the best?
I'm towing this foil board at a whole 4 knts..

Saw a small home made cat with outboard on the river. Very narrow hulls, ten by five feet or so. I imagine it would be relatively quick until it started to tail drag. Easy to form from ply. Then, the hydroplanes I saw in the maritime museum yesterday are quite primitive and basic design.

Hot Property said:

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

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Paddleboards are wide and have a lot of surface area compared to even a low-end canoe/kayak.

Foiling electric boards are quite fast, but some of them are several kW apparently?

I reckon something like a double racing kayak or surf ski might be low enough drag that the OP's outboard would run out of RPM.

Boats like the 49er are not particularly light, but the power to drive them is not huge.

Might be fun to have a crack at this oneday:
Lerryn Seagull Race | Fowey Harbour

Would be interesting to build a mini sea sled….

https://www.gerrmarine.com/Articles/SeaSleds.pdf

Is that not we call a cathedral hull, or dory? Trouble with those is that they can porpoise at speed.

The Hickman Sea Sled was the forerunner of the Boston Whaler, with some modifications to avoid the issues with prop ventilation in rough water that plagued some of the Sea Sleds.

Classic Whaler: Boston Whaler: Reference: 13 Foot: Original Hull Design

In Venezuela many would use the striped clean hul only of old jetski

This is the hull form Phil Bolger developed for cheap-to-build powerboats that would run fast on low power. This particular example is 25' long and built with a gorgeous vintage finish so wouldn't suit the OP's needs but I do think that something along these lines, scaled down to fit, would be very suitable. The hull is a flat-bottomed plywood box, with just a little rocker forwards to bring the hull bottom clear of the water at the bow.

That is, indeed, a thing of remarkable beauty.

I wonder if a half-size version could be arranged, perhaps a tad beamier for stability, but keeping the slender form and the crew weight low and central, canoe-style.

Despite dozens of daydreams I've never even built a pram dinghy. Is it possible for a DIY shed-worker to make something so sleek and efficient-looking, just with sheets of ply?

dancrane said:

Is it possible for a DIY shed-worker to make something so sleek

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It's very possible, whether it will be waterproof is another matter...

Bolger's designs for simple home-built boats are remarkably simple and neat (he also designed amazing bigger boats for professionals to build). I built his June Bug design, a 14' plywood rowing/sailing boat, in 25 hours, not counting painting.
Here are a couple of pics from the plans for that boat - it's just a set of simple, rectangular frames with plywood wrapped around them. Your boat could be 6mm ply; the frames could be ply with thin softwood battens on the edges to get the fastenings into. I would build it with external chines, ie make the frames; fasten on the sides; glue and screw battens along the lower edge of the sides on the outside, then glue and screw the bottom onto the chines. I reckon two people could have something looking like a boat in an easy weekend. But I'd build it pretty quick and rough and see how it went under power before devoting any time to fine paintwork and finishing details...

A daredevil child required. Simplify and add lightness.

dancrane said:

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?

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Not necessarily. Since you plus motor are likely to be 80% of the total weight I cant see the hull weight making much difference. Inflatables dont need to be flexible - thats a function of air pressure. At full speed you are likely to be planing so the area needed to support you is likely to be the same in both cases. My guess is that the wood boat being narrow will have better directional stability which certainly was the issue when my 3.5bhp motor got the rubber duck onto the plane.

Someone I met had an old Laser dinghy hull on which he had mounted a 5hp outboard. That certainly shifted!

I very much doubt that an inflatable dinghy would provide the best platform for speed (though would be fun anyway). You may want a flat bottom to aid planing, but for speed you don't want either flexing nor that amount of width, and you certainly don't want that amount of air resistance above the waterline.

You have focused on top speed, but acceleration is good fun, too! Flat bottoms are poor for speed in until you plane. (I have a flat bottomed Canadian canoe and that is hard work to paddle compared to something with a more curved cross section.) Note also that a flat bottomed boat, like the Bolger one above, has high initial stability, but terrible secondary stability. Not good in any significant waves.

A half circle hull section presents the least friction when immersed (but zero lateral stability), and hence you see something closely approaching that in racing sea kayaks and, I assume, racing rowing boats. Long and thin is the way to go, I think. (My sea kayak - not a racer, but 16ft long and very narrow - would go like a rocket with just a 2hp outboard, I'm sure, but I'd have to saw the stern off to mount the outboard, or better still make a (faired) outboard well between my legs, - the weight of it mounted to one side of the hull would roll the kayak over.)

Thus I imagine that the deck of that Phil Bolger boat (minus the windscreen screen and pram hood), would make a good hull bottom for quickly reaching hull speed. Perhaps one could have the outboard bracket mounted on a horizontal shaft projecting from the transom, such that it can rotate through 360 degrees laterally, and when one reaches hull speed one hangs onto the outboard control arm, faces sideways, and takes a few steps, thus rolling the hull through 180 degrees onto its flat bottom. Or some inflatable downward projecting 'sponsons' that flatten the curved bottom out as you reach hull speed.

I am also imagining a two pairs of water-skis, one pair forward and one aft of a long narrow hull, which you can lower and raise on gull-wing shaped arms at the appropriate speeds (just until you get your foils sorted, you understand!).

My alternative design has a pair of Hobie Cat type hulls, mounted by hinges onto the upper cross beams, and with a sheet of flexible, say, ply between the bottoms of the hull. For launching etc. the ply bottom is deeply concave, giving you the stability of the two hulls. You can then have some mechanism (your feet?) which pushes the ply bottom flat (the bottom of the hull will be pushed outwards) or even convex for different speeds.

Whatever you do, do post some pictures of your endeavours!

I had always fancied a canoe, an outboard and foil from an international moth. The challenge of driving it will be part of the fun.

The problem with more speed from small outboards, is running out of revs.

Once on the plane they have topped out on revs so cannot go quicker. Unless there are high speed props available.

onesea said:

I had always fancied a canoe, an outboard and foil from an international moth. The challenge of driving it will be part of the fun.

The problem with more speed from small outboards, is running out of revs.

Once on the plane they have topped out on revs so cannot go quicker. Unless there are high speed props available.

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Why limit yourself to an outboard? Try a longtail outfit!

This guy starts off very fast, and then just gets faster and faster!

Stemar said:

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!

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Many years ago at Inveraray one lad had a dingy like above with a 4.5hp 2stroke outboard, his friend had a fiberglass dingy of similar size with a similar engine.
They could run rings round our 12ft clinker built mahogany over oak tender with a 1.5hp Stuart Turner inboard engine, which flat out would do about 3knts, they could plane without problem.
We met them at the pier in Inveraray and we were all heading back to the caravan site about 3 miles down the coast, the weather had turned bad, which we knew was going to happen but the lads didn't, they asked us to accompany and escort them back to the caravan site as the were not sure about the weather. The waves were about 1.5m in height and the two dinghies were on full chat bouncing off the tops of the waves and we were going as slow as we could I was on the tiller and dad was managing the engine. We had no problems with the conditions but the two dinghies really had their work cut out for them. Speaking with them after getting back to the caravan site they said they could hardly hold the tiller arms s they were getting so hot and they struggled to keep up with us, we were going far too slowly for the conditions and could not go any slower, our engine was on tickover all the way back, and dad was complainging we were going too slow, after thinking that we would not be able to keep up with those two "Hot Rods"
Different hull shape in different conditions and our full displacement boat weathered the sea in it's stride.