[IANK]

I have a Mariner 5hp 4 stroke as an auxillary for my Raider 18. I tried it the other day and it gave me an average speed (with and against tide) of 5mph at about 75% throttle (full throttle did not achieve any greater speed).
It is currently fitted with a size 8 prop (stamped on it).

Are there any experts on props out there who can advise as to whether I am likely to achieve an increased speed with a different sized prop. And if so which size?

Any advise would be appreciated

 

[richstrat]

Hulls operating in displacement mode (i.e. not planing) have a 'top speed'. This is known as the hull speed and is calculated by the formula:

speed (knots) = 1.34 x square root of the waterline length (ft).

The hull speed of an 18ft planing hull is likely to be 5 - 5.5 knots (depending on the exact waterline length). The only way to beat this is put in massive ammounts of power to get up on the plane, which a 5hp will not achieve - no matter what prop...

If you're hitting 5 mph, you're only a fraction away from the hull speed, so I would doubt it's worth spending any extra on a prop. Actually, if increasing throttle does not increase speed, that would indicate you are at hull speed...

The only real way to tell if your prop is right is to use a rev counter to see if the engine is operating in it's optimum range - your manual would tell what this range is - revs too high, increase the prop pitch, revs too low, decrease the prop pitch...

HTH

 

[IANK]

Thanks for the info.
I had assumed that as the engine revs could be increased from 75% to 100% without increase in speed that a bigger prop would give an increased speed. I had not considered that the boat could be at its maximum displacement speed.
It would also suggest that there would be no benefit in increasing the size of the engine to say 10hp as this would not achieve planing speed.
Thanks again