Engine Performance

[gerrytok]

Hi ....I have recently changed from a Suzuki 25 hp 3 cylinder 2 stroke to a Mariner 40 hp efi 4 stroke
fitted to a Shetland Suntrip 535. Can anyone tell me how many knots I should be able to reach ?
On a slight swell in open water ( Lough Neagh) I managed 17 knots, is that typical or should there be
more speed capable ?
Gerard

[Coddy]

Hi Gerrard

What you need to look at is the revs from the engine at WOT (wide open throttle)

It may be that the prop pitch/diam is too big or small for your craft.

It should have a maxium rev range between 5500 & 6000rpm

A lot may also depend on how many people on the boat and the amount of weight in the boat when testing.

If the revs are below 5500rpm the pitch may need reducing or if above 6000 then the pitch is to small and needs to be increased.

If could can advise further and find out what prop size you have fitted at the moment I am sure we can give more help

Dave

[daveinglasgow]

the speed is out by a lot so do a quick compression test as its sounds like compression is low or you are not running on all cylinders . if its the latter its an easy fix , if its the former its v v bad news.

I would check the HD leads, spark plugs and check it is sparking first and if this is ok a compression test. Oh and remember to use insulated pliers when you do this if you have not done it before!


Dave
ps if it is the prop it is miles out so I dont it is this as the boat the engine was last pushing must have been mega heavy or v v v v light

[dellquay13]

Hi,

If you could tell us the max revs you are getting at WOT we could give advice more confidently. A good tacho is a so helpful on any planing boat and gives better info about engine performance than a speedo.

Chris

[gerrytok]

Thanks for all your advice, I hope to check it out at the weekend ,weather permitting
gerry