25hp merc prop ??????

[fluidjuice]

I have just bought a 2006 two stroke 25hp mercury with an aluminium 10" by 13pitch prop(factory standard), Its on a 14foot 4 inch tri hull fibreglass day boat.
At wot with just me on it i was getting around 4700-4800rpm ( manuel says 5000=6000) so far i have changed it to an 11 inch pitch and got it up to about 5350rpm's 18mph again with just me in,
Do you think this is the right prop for my applicaion as over here in the uk the next pitch size down is a 9.5inch pitch, giving me maybe another 300=400rpms taking it to 5650. or do you think the eleven is fine,as i dont want to be revving the balls off of it.or is 5300 not enough.Thanks

 

[steelespike]

Re: 25hp merc prop ??????

Do you generally run alone?
You could try raising the motor vertically if you can gain an inch or 2 it could add some rpm and a little speed.The faster you can go the less gas you will use.

 

[fluidjuice]

Re: 25hp merc prop ??????

I do go out with a friend but havent with this engine yet,Do you think this prop will be ok, i can plane at around the 4200-4500 mark but i dont want to strain the engine, do you think 9.5 pitch would be to much.

 

[steelespike]

Re: 25hp merc prop ??????

Actually with boats more rpm is better as long as you stay within the rpm range.It is best to prop toward the max rpm with your typical load.
In a way boat motors are allways going up hill if the prop has too much pitch
the motor will lug ,operate below its range. With the right pitch the motor revs freely and doesn't work so hard.I think I would go with the 9 1/2 or see what raising the motor does for you.The antivent (cavitation) plate just above the prop even with the bottom of the boat is considered a starting point.Most motors end up about an inch above the bottom.A 2 inch improvement can net close to 2 mph and some rpm.
Was your speed by gps?The 25 can do about 32 on a light reasonably fast boat.Of course your boat is pretty heavy.18 with you by yourself seems a little low.Maybe a 9 1/2 will get you closer to 20.
weight disribution/elimination and motor trim when dealing with a heavy boat.
are critical.

 

[fluidjuice]

Re: 25hp merc prop ??????

Yes it was by gps and apart from the norm there was was not to much weight on the boat.I have ordered the 9.5 so will see how it fares

 

[junior1113]

Re: 25hp merc prop ??????

where is cavitation plate in relation to the bottom of the boat. like steel said moving the motor up on the transom will give off more rpms and performance would like to see plate about 1-1 1/2" up from bottom of transom less drag the better. if this helps 11" pitch should be good. good luck

 

[matt88]

Re: 25hp merc prop ??????

Does your boat have foam in the bottom? Sounds like it could be waterlogged. 18mph for your setup seems pretty slow. Where is your cavitation plate in relation to the bottom of the transom?