Ribcraft 5.85 max design speed.

olisquire · 05 September 2010, 18:10

Hi,
Just wondering what the max speed you could squeeze out of a 5.85?
I have one with a 150hp ficht and with 3 on board with around 40ltres of fuel fully trimmed it hits 43.5kts, only 1.5kt faster than what the same boat with a 115 4 stroke goes. Have I hit the max design speed or does the engine need some attention?

Poly · 05 September 2010, 18:13

Quote:

Originally Posted by olisquire

Hi,
Just wondering what the max speed you could squeeze out of a 5.85?
I have one with a 150hp ficht and with 3 on board with around 40ltres of fuel fully trimmed it hits 43.5kts, only 1.5kt faster than what the same boat with a 115 4 stroke goes. Have I hit the max design speed or does the engine need some attention?

I'd guess you might not be running the optimal prop set up. Either that or you've made the fatal mistake of believing someone else's data was accurate.

willk · 05 September 2010, 18:19

Quote:

Originally Posted by Polwart

I'd guess you might not be running the optimal prop set up. Either that or you've made the fatal mistake of believing someone else's data was accurate.

That doesn't sound too far off, three up. If your revs are high at WOT, you might look at the prop.

However, I agree with P. - FIBnet strikes again

martini · 05 September 2010, 18:21

Yes the Fibnet phenomenon is all around us, particularly where top speed is concerned

What is your rpm at 43.5 knots?

Downhilldai · 05 September 2010, 18:23

IBWET's got one with a DF150. He gets 45kts

DJL · 05 September 2010, 18:29

43.5kts sounds right too me. I'd be surprised if anyone can get 42kts with a 115!

On a good day with the wind behind me I can get 45kts with a Yamaha F150.

willk · 05 September 2010, 18:33

Top speed on a RIB seems to be a touchy subject. I've been told some whoppers by people that I didn't expect to hear them from. Lads on piers express disbelief when I tell them my real top speed - they expect it to be higher at 50kts or something 'cos the others are lying feckers

Mollers · 05 September 2010, 18:40

Quote:

Originally Posted by willk

Top speed on a RIB seems to be a touchy subject. I've been told some whoppers by people that I didn't expect to hear them from.

Mine did show 49.1 knots once, it's not gotton near it since. "What's it's top speed mate?" "49 knts".

A 50knt+ rib is a rare thing.

tinker · 05 September 2010, 18:42

Doesn't engine height (relative to transom) have alot to do with max speed, in a straight line, in a flat sea? (going down hill, with the wind behind you)

Chris · 05 September 2010, 18:56

Quote:

Originally Posted by tinker

Doesn't engine height (relative to transom) have alot to do with max speed, in a straight line, in a flat sea? (going down hill, with the wind behind you)

Yes, and this also is dependant on prop type,some props like to run higher up and then will give that little bit more speed however you are potentialy risking how well the prop will "grip" once the going gets rough.

Its all a bot of a trade of and the whol prop thing is never cur and dried.

43.5 knots for a 150 with 3 up on a solid boat like a Ribcraft sounds about right to me. I get similar speeds on my 6.5 with the same HP and just me on board (so three up basically

)

philb · 05 September 2010, 19:30

Just for further comparison, our 585 with a Suz 140, need to check prop size before you ask, hits 42ish, slack tide, mind you need some weight in her to stop in flying around as small console layout.....

martini · 05 September 2010, 19:39

Quote:

Originally Posted by willk

Top speed on a RIB seems to be a touchy subject. I've been told some whoppers by people that I didn't expect to hear them from. Lads on piers express disbelief when I tell them my real top speed - they expect it to be higher at 50kts or something 'cos the others are lying feckers

I get asked every 2 mins what my top speed is with twin 225's, the look of dissapointment when I tell them...

Chris · 05 September 2010, 19:44

Quote:

Originally Posted by martini

I get asked every 2 mins what my top speed is with twin 225's, the look of dissapointment when I tell them...

yep I think people generally assume that MPH on the road equals MPH on the Sea. Why dont you offer to demonstrate what 50 knots feels like in heavy weather

BogMonster · 05 September 2010, 19:48

Quote:

Originally Posted by Mollers

Mine did show 49.1 knots once, it's not gotton near it since. "What's it's top speed mate?" "49 knts".

A 50knt+ rib is a rare thing.

Likewise with the old Humber - I got over 40 knots about 3 or 4 times by chucking absolutely everything out, breathing in and jumping up and down to reduce weight, therefore it did 40 knots

though 36-37 was more like it when normally loaded.

Some people talk mph and some talk knots which confuses things more. Personally I think it's a boat, therefore it uses nautical miles. If I need statute miles, I've made a right royal cock up

martini · 05 September 2010, 20:01

Quote:

Originally Posted by Chris

Why dont you offer to demonstrate what 50 knots feels like in heavy weather

It'll only do that downhill, 40 knots in a force 6 usually has the desired effect though

max recorded on the gps is 46 kts, from back when I tried a pair of 3 blades with maybe a knot or 2 of tide. Rarely goes over 40 now

Pikey Dave · 05 September 2010, 20:27

My previous RIB was a Ribcraft 5.85 with a 175 ficht, on a good day I could get 47kn.
I currently have a 6.4m Ribcraft with twin 115 etecs, if I'm lucky I can get 38kn

DJL · 05 September 2010, 20:28

Quote:

Originally Posted by chris

why dont you offer to demonstrate what 50 knots feels like in heavy weather :d

jambo · 05 September 2010, 20:59

Well as a Ribcraft 585 owner I suppose I better enter the debate under TRUENET rules just for Willk's benefit. I asked the builders of the boat and they were of the opinion depending on all the variables position of engine , prop, wt on board & sea conditions that mid forties was no problem, and on occasion on the mill pond we sometimes get up here I have achieve just that. Anyway these days I am far to bother about the pound in my pocket , or lack of it to go WOT all the time I also like to see where I ve been now and again!!

J

Hightower · 05 September 2010, 23:15

I once got sucked into all this my RIB does this speed and mine does that. . . Who cares! It costs money and time to extract a tiny bit of top end performance from a boat and for what? Higher fuel bills and sacrifice to the handling.

Much better to concentrate on getting the RIB to WOT in the correct range and to cruise at a decent speed vers revs to decrease fuel consumption and to have correctly balanced handling in all conditions IMHO

gotchiguy · 05 September 2010, 23:46

Quote:

Originally Posted by Mollers

Mine did show 49.1 knots once, it's not gotton near it since. "What's it's top speed mate?" "49 knts".

A 50knt+ rib is a rare thing.

70 litres fuel, just little me and 73 on the trim gauge = two way average of 52knts dead. Best one way was 53.08knts, this was the first time I have been on my own and with a completely spotless bottom

(she's been in the water since the week before the ALderney trip).

She shoots up immidiatly straight to 48knts even before trimming to normal setting. Talking to people I have learnt that most leisure boats have the potential to go alot faster than they do, but that it's a very tricky balancing to get everything to an optimum, for most people top speed is not the most important consideration when doing this