speed??????

[chuddster]

hi there

i have a 4.3m SIB with a 40hp mariner 2 stroke on the back my prop is 15 degree pitch and i'm only getting 20-21 knots at WOT can anyone suggest what could be wrong as i'm led to believe i should be nearer 28-30 knots with this engine?

any help appreciated

chuddster

[Fenlander]

I'm not being flippant but unless you know the revs at WOT you will be stabbing in the dark. It could for example be over propped, the engine could be worn, perhaps the throttle linkage is slack so WOT tiller isn't WOT at the engine and so many more things.

So if you don't have one buy a cheap Tinytach type thing.

[chuddster]

Thanks for your info, yes i do need to know rpm so will try invest in something. But i do know im getting full throttle on the carbs, compression is 125psi in both cylinders and it revs right up when not in gear.

I guess im after info if anybody has a similar set up and what speed are they getting.

[jeffstevens763@g]

We assume short shaft engine 15 inch transom? As fenlander says tachometer needed.

[Fenlander]

Chuddster I'm guessing your boat is an alloy floor Quicksilver??

There's a member on here, The Gurnard, who uses one with a 25hp 2-stroke. I think he's adventuring at the moment but I'll PM him a link to this so he can report on what speed he gets with his outfit.

[The Gurnard]

Hi Chuddster .. yup.. at first thought ...there appears there be something not quite correct with your setup .. I get approx the same speed with my Quicksilver powered by a 25HP outboard.


However that is with me only in it. I recall a guy recently saying his planes with seven folks in it.. but mine would be stuck at fast displacement speeds ( around 10 mph) with that amount of folks. Im mentioning this simply because you have not given any info other than top speed and 40HP...therefore we cant offer you suggestions to try.

That is why Fenlander or anyone else requires more info from you. Your engine may be fine ..but if over propped.. or if the boat is heavy loaded .. it may only be putting out around 3K revs at full throttle ..instead of 5K or so revs.. (if that is what the 40HP should put out) Remember..boat outboards don’t have gears like a car.. so if load or prop is too much..it cant reach max revs. That is why you need the rev counter to help identify the issue.

An example is the FRIB with a 6HP goes at 16mph with one person on it. With two folks..it cant get on the plane..it only gets 2.5K revs instead of 5.5K as recommended by manufacturer. However ..with a smaller prop..it gets 5.5K revs again but around 13mph with two on board. Cant keep the smaller prop on when only one on board though..as it will over rev the OB at WOT.

Good luck with getting the boat trimmed for your requirements.

[Markcamill]

Hi Gurnard, my engine is an evinrude etec 75. It does around 4000 revs with just me on it. How can i know how much Revs should be ?

[69cmw]

Originally Posted by Markcamill

Hi Gurnard, my engine is an evinrude etec 75. It does around 4000 revs with just me on it. How can i know how much Revs should be ?

Check the manual .....

[The Gurnard]

Yup..the owners manual should say in the Specifications page

Example my Yam F6CMH says
Performance:
Full Throttle operating range 4500-5500 revs per min

Therefore when it was only doing 2500 revs per min at full throttle ...because of the heavy weight .. a smaller sized prop was like dropping a gear in a car.. it then did 5000 revs per min.. but obviously as it was a smaller prop .. it could not go as fast as the bigger prop ..if all that makes sense

Trimming a boat for peek performance can be a bit of hit and miss and can be because of many things ..but the rev counter helps identify issues

[chuddster]

Thanks for your info, my friend also has a 3.8m quicksilver with 25hp and he gets 22knots with two people and a bit of fishing gear. Spoke with a few marine shop engineers and they all say i should be around 30knots with my set up. I,m taking it in to let someone have a look at it.