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

[dancrane]

Thank you, that's very useful!

I had a feeling it wasn't purely a matter of weight and output per kilo.

I suppose in a light boat, if at top speed with one engine, you're going as fast as the propeller is pitched for, then another similar engine (or another three) turning the same props at the same speed, won't move the boat much faster. It might increase the barge-pull effect but not the speed.

How skinny is your planing friend? It'd encourage my weight loss if I knew.

 

[Rappey]

Hes a stick insect. I would be surprised if he is close to 60kg..
My inflatable used to plane with me. i posted a while back about this wondering what i can do to get just a little more power to get on the plane, suggesting my tohatsu has probably lost a little power over time. The reply was not to do anything to the outboard but to diet and get back to the weight i was when it planed Guess thats the harsh reality.

 

[billskip]

if at top speed with one engine, you're going as fast as the propeller is pitched for, then another similar engine (or another three) turning the same props at the same speed, won't move the boat much faster.

That's about the size of it.....but...it would possibly give you wriggle room to go over pitch....

 

[Hot Property]

I've had a selection of inflatables from a Redcrest to a Honwave.

Thee Honwave was 3.2 meters long with an inflatable V floor. Fitted with a 15 EFI Mariner it would top out at 19 knots on flat water. The floor would start to deform which limits the speed.

Same engine on an Avon 3.1 rib gives 22 knots.

My brother has a similar V hull so. and with my 3.5 Mercury 4 stroke can just get on the plane at about 11 knots.

A bracket to hold a 3.5 tohatsu 2 stroke on a paddle board would be fun to try out. I would ratchet strap a seat so I could sit for better stability.

In the video the guy was unstable and couldn't put the full 6.5 HP into effect....

No kill cord but it was in Australia

 

[Iliade]

I have a vague memory of a 1960s/70s BBC program about a UK version of these hydroplanes, but with less top hamper, even smaller motors, and the kids lying down?

We had a water-jet longboard back in the 1990s, but that took something like 15hp to get up on the plane.

 

[dancrane]

More than a month later, all I've done is think about it, but I wouldn't want to do anything daft and hasty.

I'm still tempted to construct a lightweight plywood slight-V hull that I can strap under the Redcrest, incorporating a transom that will eliminate the torque-driven hogging of the inflatable's hull.

I don't know why I didn't find the following video, much sooner. The creator seems unresponsive to enquiries as to how he did it - but (aside from the fact I could simply buy a new or secondhand transom inflatable and launch it next weekend) is there any reason or consideration I should bear in mind, before attempting to do what this fellow has achieved so neatly?

The significant part seems to be that it had been basically a floppy old round-tail Avon, not a transom style with inflatable keel; and that the outboard is no longer attached to the standard Avon bracket, but to a structural part of the rigid hull that is constructed beneath the Redcrest.

I had thought about using Polymarine hypalon-repair adhesive to attach plenty of loops to the Redcrest's side, enabling the hard hull to be roped onto the inflatable part. The boat in the video seems to have had the hull glued direct to the hypalon, hence it looks more like a factory product.

Given that the intention would be to limit the power to that provided by my lightweight Tohatsu 3.5hp, I hope I could get away with not gluing the hard hull to the hypalon boat. Keeping the whole thing deflatable would be preferable, even if the rigid part couldn't be made any more compact. But would green water go surging through the gap between the Redcrest's floor and the ply hull, if they aren't permanently bonded together?
EDITED: Polymarine, not Polygrip.

 

[Mistroma]

I'd agree that he must have mounted the engine on a bracket that's effectively part of the hull he's made. That transfers all the thrust to the hull without it pushing the inflated stern at all. It does get around the prop. simply trying to drill up through the floor when power is applied.

You could bond the hull to something you could pull over the top of the bow. A few darts would probably give it enough shape to follow the Avon's bow section. That should get around the problem of bonding to your Avon and still prevent water getting into the gap. It would also give you something rigid going back to the stern where you'd attach a bracket for the outboard. All very string and chewing gum.

I wonder if you could cut a few sections of ply to give a reasonable V shape (oblong sides and triangles at bows). Stitched and taped, perhaps wood along the keel to hold the angle. I wonder if you could get around the problem of sealing the void by encouraging air flow. Leave stern open and put a hose up either side of the bow between outer skin and the Avon. Water would then rush out as the dinghy got up speed. A bit like self draining sailing dinghies.

The dinghy in the video looks a lot slower than mine. However, my Lodestar is only 2.1m, rigid transom, no thwart with inflated keel and floor. I only weigh 60kg and need to sit right at the bows. No chance of sitting back where the guy in the video was sitting. No sharp turns either as it's very twitchy and borderline uncontrollable. Calm weather, straight runs and only gentle turns (mostly weigh shift with tiny tiller adjustment using a long extension).

 

[dancrane]

Thanks Mistroma, great reply. Lots there, to consider. I wish this wasn't a working week.

I need to get some ply. Is 5mm likely to be strong enough for the hull, with support behind? I have a 5mm sample and it feels terribly slender but I believe Mirror dinghies (and others) use it.