Full throttle?????

[Northsearibber]

Hi

I have a small 3.6 m Ali hull rib with centre console and a new Yamaha 20hp four stroke

Problem is that although boat and engine Set up is fairly light ( about 130 kgs) with me (13 stone) wife 10 ish! Stone and kids 11 and 7 it just doesn't seem to quite get on the plane . Even at full throttle the yam engine doesn't seem to be revving that much - could it have been set up wrong meaning that full throttle on the remote lever is not translating to full on the engine. How can I check this?? Is there a simple way ??

I would have thought 20 hp would've this set up along at a reasonable pace but I think I am only getting about 13 knots?

Thoughts????

[Festinghouse]

you should easily plane with that. remove airbox cover and check the butterflies (round plates) are opening fully when control is fully forward

[PeterM]

Thats a lot of weight for 20hp to get on the plane.....

seriously - with 40hp it may still take some time to get going ....

You need a very low pitch prop to spin the engine to full revs - at present its like trying to pull away in third gear in a car full of people......

[boristhebold]

Easy way of checking, leave the wife and kids on the pontoon and go out for a test run on your own. Full throttle and record what revs and speed if you have a guage.

[Northsearibber]

Cheers guys

The problem is i dont have a rev gauge so difficult to tell - i am just going by ear and pace!

The rib goes better ( obvious i suppose) with just me on it but i still thought it would have had more go loaded up - some guys in the same bay have a3.4 m with 20 hp tiller steer and it flies with 4 , 16 year old boys on it?!

If i need to reprop are there any recommendations on pitch or is it a bit trial and error?

Festinghouse-thanks i will certainly check the butterflies

[SteveHall]

and see how long you can leave them on the side for....

[chipko]

Hi,
Would be surprised if standard prop as supplied on a new 20hp engine with your set up is unsuitable, but something probably not quite right. Plenty of cheap induction tach's available on e-bay to check revs at wot.
Before considering changing prop I would check remote throttle adjustment, engine height / trim, and weight distribution (some up front to get bow down while getting onto plane).
Not wishing to rub salt in but our 3.5m sib with 20hp 4 stroke planes effortlessly with three adults aboard so can understand your disappointment. With two aboard we can pull a ringo and on a good day a knee board - just!
Hope you get it sorted.

[Northsearibber]

Thanks all for comments -some stuff for me to try out and see where i get to

Cheers

[Poly]

Originally Posted by PeterM

seriously - with 40hp it may still take some time to get going ....

No way. That package sounds very similar in weight and power to mine. 2 Adults, 2 kids probably similar total load to mine gets on the plane fine. 4 adults gets there without two much difficulty. All with 20HP. The only way a 40 could struggle is if you massively over-propped it in the hope of achieving ridiculous speeds on a flat calm day: speeds that you'd probably never attain because a wee boat at silly speed is terrifying.

Northsearibber - you need to give us more info -->
- engine make/model (and recommended rev range)
- current prop size
- speed (take a smartphone in a waterproof bag) with you and with you and the family
- picture of the engine height
- details of the engine trim
- an idea how much experience you have
- ideally a picture of it "not quite on the plane"

People will then be able to give you a better idea. In terms of prop selection without a tacho you'll be stabbing in the dark.

[Northsearibber]

Cheers for that

Engine is a Yamaha f20 2013. Prop size is 9.25 x12.

Speed - would have to let you know exactly but about 12/13 mph.

Engine height is fine I think- cav plate is level with hull bottom

Trim - is a difficult one - I have played about but one hole out seems to be best - I think according to the manual each hole is 4 degrees?

The pictures I will need to get next time - but not sure if the answers above spark anything obvious?

I have ordered a cheap induction tach but cant really believe that the standard prop is not within its range on this size boat/load, although at least now I can check

[PeterM]

I'm running a 25 hp 4 stroke on a 3.6m rib.

I just think your asking/expecting too much of 20hp.

[Locozodiac]

My 430 rib with just a 18 HP tuned to perfection with 3 adults planes easily on most sea conditions. Unless you have a built in tach inside yoiur ear get a physical one, are cheap, will make your boating day.

If you're lost not knowing where to start rigging a small sib/rib with a 20 HP check this : + add an induction tach

http://www.rib.net/forum/f50/abc-sib...nes-58373.html

Happy Boating

[Poly]

Originally Posted by Northsearibber

Engine is a Yamaha f20 2013. Prop size is 9.25 x12.Speed - would have to let you know exactly but about 12/13 mph.

OK - Yamaha says your engine max rpm is 5000-6000 rpm. With a 2.08 gear ratio. So at 5000 rpm your prop should be spinning 2400 times per minute. Assuming no slip that would mean your prop moves forward 12x2400" a minute = 23.7 knots. Bearing in mind that 10% slip is about the best you will get you should be getting 20ish knots (23 mph). On a small boat it feels like more. You think you are only getting 1/2 that - so I think there is a problem.

Is it sitting bow up?
Is it generating a huge wake behind it?

Its going to take actual tacho and speed data to work out more - and ideally some video footage so people can see how she is sitting.

Originally Posted by PeterM

I'm running a 25 hp 4 stroke on a 3.6m rib.

I just think your asking/expecting too much of 20hp.

I'm running a 20 HP 2-str on a 3.9m and think he's not!

[dvoyer]

I have a Brig 360 with the same engine (2013 Yamaha 20 HP 4 strokes). Two adults and three kids (total weight around 200 Kg) I get 25 mph with a 9 1/4 x 10 propeller. When I tow a large 3 persons towable tube with the three kids on it I get 20 mph with a 9 1/4 x 9 propeller. I never had problem to get on plane. I think your engine is appropriate for the size of your rib, you just have to find a good setup.

Dominic
Quebec, Canada

[Northsearibber]

Thanks guys - all those comments are really useful and have been a lot of help.

The general opinion seems to be that boat/engine combo should be capable of 20 mph with a couple of adults on plus 1 maybe two kids. That is what I am after really so it seems the issue is in the set up . I have ordered a induction tach and with advice given will do a few tests when I next get the rib out - hopefully next weekend. Hopefully I will be able to sort the issue now but I may need more views and advice! Either way I will repost with some results

Again - many thanks to all who have offered the benefit of their experience

Cheers for now!