Lund SSV/ 40 hp Merc.

[Lund SSV]

09' 14' Lund SSV with a 98' 40 hp Mercury. Currently getting 36 mph, with a 14 pitch prop, Dont know diameter I think 10 something. max rmp for engine is 5500.

Im looking for more speed. Could I get away with putting a 17 pitch/ 10" diameter prop on it, without losing to much rpm?

Boat Weighs 285lbs, motor 200 lbs, gas 40 lbs, 2 batteries 80 lbs, trolling motor 20 lbs, and me 150lbs. Total- just under 800 lbs.

yes, I know boat is rated for 35 hp, with a max weight on transom of 230lbs. I know its 5 hp over rated but dnr doesnt care. boat can easily handle the 40 hp.

 

[steelespike]

Re: Lund SSV/ 40 hp Merc.

Hard to see but it looks like you could raise the motor a little.
Usually end up with the antivent plate slightly above the bottom.
Could gain a little there. I assume your saying you make 5,500rpm.
If my figures are right your setup with 1.83 ratio should make 35. mph at 10% slip.
So your doing darn good.Do you have power trim?
A similar prop in a 16" may make 5100 rpm. if my figures are right at 10% slip you should net 38 mph.Of course this assumes the boat can continue to get up out of the water.
And the motor makes the 5100.
If you often run with a load you may put more stress on the motor, hole shot may lag and
speed drop more.

 

[Black River]

Re: Lund SSV/ 40 hp Merc.

Everything I am about to say will not hurt the life or dependability of the motor. If you want to go fast, raise the motor up until the cavitation plate is dead level with the bottom of the boat. Next get a powertech SCB3 prop in a 15p, this prop will help lift the stern and in addition to the trim which will give you bow lift will get the boat up and out of the water which is faster.

Now for the kicker that the by the book guys aren't going to like.

Remove the rev limiter! That motor will safely turn 7000 rpms all day long. Next, put fiber reeds on the motor. Boyesen's are a good choice and the motor will digest these without damage as opposed to the steel reeds which will cause damage if they break. While you have the reed plate off, get a set of reed retainers for a 60 hp mercury. This will let the reeds open up which allows more fuel and air in. The 40hp reed stops are very restrictive and this is the same 59 cubic inch block that the 40, 50, and 60 use. Finally, get a set of 60 hp carburetors.

You can thank me later.

 

[Silvertip]

Re: Lund SSV/ 40 hp Merc.

Without a tach any changes you make will result in not knowing what other aspects you are changing. Merc specifies a 5500 RPM wide open throttle rpm maximum for a reason. Why -- only they know that. But here's the rub. I ran your numbers through a prop calculator and with a 1.83 gear ratio, 14P prop, and 36 mph using a 12% slip factor, the engine is already running 5647 rpm. If those numbers were verified with a tach, then (and only then) adding a 17P prop would lower the engine rpm 600 (or just over 5000 rpm) so you would be borderline "lugging" which is not good at all for a two stroke (or any engine for that matter). Speed may increase only if (repeat only if) the engine has the power to spin the 17P beyond 5000. Just because it spins a 14P at 5600 does not mean it can spin a 17P at the same rpm. Your next investment before doing anything is to buy a TinyTach so you at least know exactly what the current setup is doing. RPM in all of this is the critical number. I might add also that neither the 40, 50, or 60 are reated to run at 7000 rpm. Do people do it? Apparently! But it depends on how long you want this very nice rig to last and how well you want it perform at slow speeds.