Engines for a 3.2m RIB?

[Obsidian]

What is your opinion on the ideal engine for a small RIB?

Currently got a venerable 15HP 2-S and it seems to be the wrong combo, no sure whether to go slightly smaller and lighter (9.9) or up the ante!

[Pikey Dave]

Originally Posted by Obsidian

What is your opinion on the ideal engine for a small RIB?

Currently got a venerable 15HP 2-S and it seems to be the wrong combo, no sure whether to go slightly smaller and lighter (9.9) or up the ante!

I’d have thought 15-20hp would be the sweet spot. A friend has a 3m AB ally rib with a Honda 15hp 4s & it’s a well balanced outfit.

[Fenlander]

In what way is it the wrong combo?

Re going smaller and lighter to a 9.9 there is only the Tohatsu 9.8 2-stroke that is lighter than a 15hp 2-stroke. The 9.9 2-strokes are usually the same OB as the 15 just with a smaller carb and the 9.9 4-strokes are heavier than a 15 2-stroke.

[Obsidian]

Thanks both, I think maybe the motor I have is tired and under performing for its spec

Its either all or nothing, tickover gives 3-4kn and then its hump all the way until full throttle and then max speed is 16kn

I have seen a 2HP 4-S that is a similar weight, which looks appealing. But also like the idea of the 9.8HP

[Fenlander]

What motor do you have? And do you mean it lingers for ages getting over the hump to plane then it's straight up to 16kts?

[Pikey Dave]

Originally Posted by Obsidian

Thanks both, I think maybe the motor I have is tired and under performing for its spec

Its either all or nothing, tickover gives 3-4kn and then its hump all the way until full throttle and then max speed is 16kn

I have seen a 2HP 4-S that is a similar weight, which looks appealing. But also like the idea of the 9.8HP

That just sounds like typical planing hull behaviour. Planing hulls are either pushing a lump of water in front of them at displacement speed & then bang! Once over the hump you should be able to throttle back until the hull is just planing.

[treerat]

My 3.83 had a 25 on the back - that was fun

[Steve509926]

Hi, before investing in a new outboard I think I'd be looking at your Sib. You say 3.2m, but a 3.2m what? Are the pressures absolutely bang on? I think that once over the hump your speed is 16knts would indicate that there not much wrong with the ob.

[Pikey Dave]

I think it’s a RIB

[Steve509926]

Just reread the op, I think you're right

[jeffstevens763@g]

Originally Posted by Obsidian

Thanks both, I think maybe the motor I have is tired and under performing for its spec

Its either all or nothing, tickover gives 3-4kn and then its hump all the way until full throttle and then max speed is 16kn

I have seen a 2HP 4-S that is a similar weight, which looks appealing. But also like the idea of the 9.8HP

You’re spot on in my experience once on the plane and throttle back 10/12 knots depending on conditions weight etc

[Obsidian]

Originally Posted by Pikey Dave

That just sounds like typical planing hull behaviour. Planing hulls are either pushing a lump of water in front of them at displacement speed & then bang! Once over the hump you should be able to throttle back until the hull is just planing.

It feels like it is struggling though, but I would have expected a 15 to have ample oomph

There is a badly fitted (and hence wrinkly) keel guard fitted, I have a feeling that is causing a lot of drag and will try and remove it

[Fenlander]

Genuinely we need a bit more info.

Is this a GRP hull RIB or soft bottom SIB and if SIB hard or air floor. What make/model is the outboard and is it on steering or tiller? How many folks are you carrying?

[Peter_C]

How does it plane if you move weight forward? Everything sounds kinda normal to me.

Although some folks have a disdain for wings on the outboard, I have found them to be very beneficial when carrying heavy loads, and for helping keep the bow down while getting up onto plane. I used to have a Zodiac and it planned at a very low speed with the wing. The downfall to the wing is often you loose a little top end speed, and they can cause the prop to blow out on aggressive turns.

The propeller sizing is important too.

[Obsidian]

Originally Posted by Peter_C

How does it plane if you move weight forward?

So far its only been me in the RIB, but bought a tiller extension today and have invited friends round to sit in the bow over the weekend

Updates and photos of the impending calamity to follow

[Peter_C]

Originally Posted by Obsidian

So far its only been me in the RIB, but bought a tiller extension today and have invited friends round to sit in the bow over the weekend

Updates and photos of the impending calamity to follow

Calamity

Where is the fuel tank located? Might try moving it to the bow, along with any other loads, like the anchor, etc.

[Obsidian]

Originally Posted by Peter_C

Calamity

Where is the fuel tank located? Might try moving it to the bow, along with any other loads, like the anchor, etc.

All of the ballast is forward, apart from my 14st

It feels like the 15hp does not have the grunt to get over the hump, but I would expect it to do so easily. Not had a RIB this small before, but have had SIBs this size (and bigger) and found that 10-15 is a well performing combo. Hence the question about sweet spots and Goldilocks zones

It’s a 2006 motor, so it may be that the HP is not there any more, other than removing the keel guard I will give the carb a good clean and fresh GB oil

And of course try out the tiller extension

[Fenlander]

I will say again... more info needed. What make/model is the RIB and how heavy is it? Similarly with the OB, make and 2 or 4-stroke. All these things could make a difference.

[Obsidian]

Mariner

Originally Posted by Obsidian

15HP 2-S (two stroke)

Its an Avon, not sure of the weight yet, still getting round it

[Fenlander]

Is it a silver cowl, black leg model? They are usually long lasting. I wonder what pitch prop it has? Usually a 9" pitch is standard on those which covers most uses.