F-Ribs And Sibs - Inflatable Boat Community

is anybody running a mercury 420 on a 20hp motor , ive been told its not a big enough engine,well i have been out on such a set up and although i was only a passenger it seemed to nip along. so if anyone has this setup please give me your opinion ,actually lets have everyones thoughts. ive got to make up my mind in the next couple of weeks.

Are you going for an open boat and tiller steer, or fitting a console, wheel, and seats?

In my opinion, you're way underpowered with a 20hp even on tiller.
On a rib weighing in around 120kg, I'd be looking at 30-40HP.
I think perhaps you'll have a better idea when you get out with some of the guys and try launching the said boat too.

I reckon a 420 in open trim weighing in at 113kg will go ok lightly loaded one up with a tiller steer 20hp motor. It ain’t gonna set the world alight but should see 19/20 knots on a good day.

Add a console, seating, battery etc. then it will be a different matter. A 25hp min, 30hp ok and 40hp a perfect match.

viewtopic.php?f=59&t=3169&start=10&hilit=420

All you need to know.... perhaps ?

thanks all for your input ,more sleepless nights methinks,but at least tellum seems happy with his 420 with 20at the back, its now a question of how far down i can reach in my pocket

Martin wrote: ↑

07 Apr 2019, 10:10

In my opinion, you're way underpowered with a 20hp even on tiller.
On a rib weighing in around 120kg, I'd be looking at 30-40HP.
I think perhaps you'll have a better idea when you get out with some of the guys and try launching the said boat too.

Having 1st hand experience of a friends Honwave T40 Sib with a 20hp 4 stroke I can vouch that on a tiller steer, it flies along nicely.
A Mercury 420 O.R is similarly weighted, but has a way more efficient smooth / solid hull so it will easily match the T40s planing performance on tiller - in fact it will out perform it in all respects.
The Merc 420 will also plane on a console set up (with 20hp) so long as it's not laden with lead or fat barstewards.
Seems to be a propensity to power boats to the max or over power boats on the forum at present
I reality when you only manage to get to get down to the basics and putting the thing on the water several times a year (as most members only manage to do) the minimum HP on high - ish revs will do the job just fine.
Think of the work your poor car does Daily / annually!
If like me you like to mix displacement with planing speeds then a 20hp will do the job just adequately on a 420 O.R. so don't be put off
I'll swap you for a 380 Sib on 20hp if u want !

I think that a set up that does minimal speeds one up and on a decent day may well be a completely out of its depth animal if with a bit of load on board encounters a change in weather that maybe produces a couple of meters of head sea or worse not being able to keep ahead of a following. I personally would much rather have power in reserve that I might never use than struggle trying to keep up with following swell ( I have been there its no fun feeling like your trying to power through treacle)

well thanks to kamen and beerbelly there, two totally different takes on the same subject which seems to be par for the course .im trying to make my mind up here, cant you all just say the same thing just to make it easier for me, lol. , watch this space , i am about to decide and when i do i will tell you what ive plumped for and my reasoning behind it. thanks all for your input

beerbelly wrote: ↑

08 Apr 2019, 17:14

I think that a set up that does minimal speeds one up and on a decent day may well be a completely out of its depth animal if with a bit of load on board encounters a change in weather that maybe produces a couple of meters of head sea or worse not being able to keep ahead of a following. I personally would much rather have power in reserve that I might never use than struggle trying to keep up with following swell ( I have been there its no fun feeling like your trying to power through treacle)

Hmmmm?
Doubt you would be able to use 8 of the 20 hp in the conditions you describe in a 4.2m boat.
Displacement speed for me and a cautious run back to port if there's a couple of metre high head seas or following sea.
Not all boats are designed to be planing so they have to use their modest power in conjunction with the sea / conditions to make a safe passage.
Ploughing thru a following sea in a small rib is very risky in my opinion as you risk going side on in the breaking wave and barrel rolling the thing over
Each to their own tho.
A 20hp on an Ocean runner 420 is a very capable and safe sea going Rib in competent hands in my opinion.