what prop to buy?

[octane]

I have a merc 4 stroke 115 efi, wot rpm should be between 6000 and 6400rpm.

with the stock 17 pitch prop I do 41mph @ 5600rpm (gas full, only me on board)

I will mostly use the boat for skiing/wakeboarding and tubing so I would like a better hole shot.

I was thinking a 15 pitch in the same size would bring me to the 6000rpm range and still give me good top end when the boat is loaded with more weight. Does this make sense, or should I aim for the 6400rpm range with a 13 or 14 pitch?

thanks

 

[steelespike]

Re: what prop to buy?

What boat are we running this on?
Generally they like to prop a 4 stroke for the top of their rpm range.
So it would seem you need to drop 4 inches.Hard to believe a 4 stroke can hold together for much time at those revs.My subaru is redlined at 6,400 but has never been there it cruises at 2,900 at 74mph.
But before doing a prop check your setup you may need to raise the motor a couple of holes.If no ventilation problems you may have room to raise it.
Won't gain too much but will give a better indication of what prop you really need.

 

[octane]

Re: what prop to buy?

this is for my 16'10" fiberglass open bow runabout (weights about 1300lbs + 400 for the engine)

what would lifting the engine do exactly? since the engine weights a lot, it's not something that I would want to do/have done unless there were some decent gains.

thanks

 

[steelespike]

Re: what prop to buy?

this is for my 16'10" fiberglass open bow runabout (weights about 1300lbs + 400 for the engine)

what would lifting the engine do exactly? since the engine weights a lot, it's not something that I would want to do/have done unless there were some decent gains.

thanks

I've never done it but raising the motor is actually fairly easy.
As I understand it you loosen the hight adjustment carefull rest the skeg on a block of wood and raise the bow of the boat until the motor reaches the next highest hole.As I understand it this is done without comepletely releasing the motor.Depending on the present setup raising the motor could gain 2 to 3 mph with a possible slight improvement in hole shot and planing speed and mileage.
Generally if your boat performs all maneuvers without ventilation problem there could be room to raise it some.Some will tollerate slight venting problems for the improvement in performance.Generally a stainless prop will combat venting and allow higher motor adjustment.