18'x54" GDSD boat 35GDSD motor only getting 21mph empty and will hardly plain out w/ 3 people and a cooler, This boat is the same speed as my 18'x44" w/35LT with no load and slower with the same load. I changed the 3 blade out for the 2 blade it helped in the mud but not with the speed. This rig has sixty hours on it. I always hear about these fast SD's and the one I have is a turd, could this be a deffected motor, boat to heavy or do you have to mod. the SD's to get speed out of them ? I have talked to a couple of shops about the lack of performance, they all recommend I mod. the motor in one way or another. I just don't think this rig should be as slow as my LT if there is nothing wrong with the motor.

That boat should be running atleast 26 mph empty if not 28-29. What rpm's are u running in the water?

Ya, Thats what the dealer said. Turning 3900 in the water

whats the bottom of the boat look like? if it has some good hooks around the transom that will kill the performance of the rig

If it's turning 3900 and you've gotten another prop. I would guess that the boat is hooked also.

Hooked ? what am I looking for ?

is the bottom of the boat dented significantly? usually right in front of the transom but not always.

It has a slight wave to it nothing major, you really have to look to see it. I will slide it back on the trailer to get a better look.

just a little dent can cause quite a bit

Have a compression and leak down test done. Leaking valves don't affect your top RPM but rob you of power. 1hr or 60 hrs- stock valves suck. If its still under warranty (2yrs) Briggs should deal with it.

I got a 16x60 with a 35 gdsd. 3 weeks old with a blind bag tackle box and 2 poles, I'm only getting 28 mph on gps thats open water with the wind in my favor. I was really hopeing for more. maybe they just bite! How much would a carb, air filter and pipes add? Is it worth the money?

you were hoping for more, why cause a bunch of guys claim they run 30 with the same setup, but can't back it up with a GPS. I think that is a damun good speed. Dead on IMO

FYI - guys a new prop is slower than a modestly worn one ! ! !

That is why I polish my new props....

Also a new engine is tight and max performance will be reached in about
30 to 50 hours too.....

1660gdsd and 35gdsd I can't break 30 either ready for some methanol and a hot shot!

my 18x60 ran 28.5 stock when it was new.i now have 388 hours on it and have MarshMellow exhaust,BPS intake and Mikuni carb with a yost power tube and have ran 30.2 with it

Sum of yall really need to quit believin what people say they run. I could tell you I run 39 mph, but it don't mean it's true. I hear so many people talk bout runnin 30+ mph, but da reality of it is just cause they bumped dat speed one day while runnin down current wit a tail wind don't mean they run it steady. Hell I bumped 34 once but I run a steady 31.5 when empty. And truth of da mater is how often do you run your boat wit just you and a gas tank. Ussually I have at least one ice chest another person and bout 200 lbs of other shit in da boat. So for a stock 35 hp surfave drive avg runnin speeds are 24 - 28 mph dependin on the hull. Also keep in mind theese engines are cheaply built from da B&S factory and performance does vary from engine to engine.

exactly, on any actual mud hull with some weight to it, in the 18x48 range, I'd expect steady unloaded speed of the stock 35/36s to be 25-28 mph. If anyone is getting more than that, their full must either be a lot lighter or they got a little something done to their motor. Unless they bumped 30 for 6 seconds when they shot through a weir that was blowing them out at an extra 2 mph.

if i jerk my gps really fast i can bump 45

I get 28-29 all day with the GPS sitting at my feet. I have held 30-31 a few times with the right conditions (I have witnesses). Now that's with 6 gallons of fuel and the things I keep in my storage box. As far as the comment about the prop. I ran faster with less rpm's with a new prop. I started gaining rpm's and losing speed as my prop started to lose surface area. The guys at go-devil will tell you the same thing. I don't know about other brands, but the GDSD's are sensitive on prop wear. I do agree with the comment about the compression. I had another motor that would turn the rpm's but didn't have any power.