I’ve got a 25’ 2’ 1990 Larson Senza Spectre with the 454 mag, bravo 1 with a 23pitch prop (365hp and 350hp at crankshaft). With 3 people (about 550lbs) and 3/4 tank (80 gallon tank) I run at 53mph (on GPS) @5500RPM... the RPMs seem a little high and speed seems low (even for an older boat). The 300hp test boat ran at 58mph on radar and the 230hp test boat ran at 41mph. This step hull runs 10mph faster than conventional hulls. Haven’t had a ton of time to mess with the boat and really play around so this is pretty much all of the info I have. What can I do to regain some speed?

53 at 5500rpm sounds like a ton of slip if you have a 1.5 ratio drive.....something doesn’t add up.
I dont think the ECM will allow a 454mag to turn that fast unless it’s been reprogramed. Tach off?

Scratch that last statement, just realized you have a carbed engine.

Assuming your numbers are correct and it’s a 1.5 drive that’s a crazy high slip. I’d want to borrow a box stock prop and go from there.

I haven’t checked my gear ratio, I assume it’s 1.5 and it’s one step. The prop is the original 3-blade stainless steel quicksilver prop which seems to be flawless. I think everything is original on this boat, if it’s the 1.5 gear ratio, that’s means I’m getting like a 33% slip. I don’t know if the RPMs are right but that’s what I’m seeing. I can’t do anything right now as it’s winter here.

Get a good timing light with a tach and you can check the accuracy when you get better weather. I would still try a box stock prop. Easiest thing to do. To see if the numbers change. The prop may look fine to your eyes but people knock and ding them a lot and get them repaired. Very easy to ruin a prop. Next I would go to a waterlogged type situation.

I never noticed any vibration or anything like that from the prop. It’s not waterlogged or anything like that either. The hull does add 10mph over conventional, it’s fairly light for its size at 4,000lbs dry weight, while my dad’s 1994 225CCR weighs 3,900lbs (350 mag, alpha 1 and stainless steel prop) and tops out at 47mph. I’ll test the tach firstly in the summer since I live in Ny. I’ll look into different props as well. I found an old listing for my boat, the guy stated that it did 66mph top speed back in 2011 and I’m fairly certain that nothing was changed since then. I’ll have to check the gear ratio to make sure it’s 1.5, my guess is that’s what it is. This boat has all the extras besides captains choice exhaust, just has thru-hull. Just based on what other owners stated and the test boats, this should be running much faster. I also remembered that at slow no wake speeds, I get a lot of wandering, maybe a 4 blade would help this as well?

Sounds like a prop or a tach problem. 350 hp and a boat with fuel and passengers added to a dry weight of 4000 lbs would likely run in the mid 50’s. With a good bottom and lots of prop testing, you might break into the low 60’s. Good luck with your boat.

My drive was labeled 1.50 but when I pulled it off someone had changed the gears and scribed 1.65 on it. A lot of fountains came with 1.65 drives that may have found its way to your boat.

The boat doesn’t seem to have had that much use out of it, I would be very surprised to hear that it wasn’t all original. It’s a bit in accessible at the moment so checking the gear ratio will have to wait, checking the tachometer will have to wait till the summer and any other diagnostics as well. I’ve seen the same setup as I have in another spectre and it ran like 68 MPH on GPS. I’ll be back here when I figure out the gear ratio.

yeah, my speedometer actually matches up to my GPS speed but I’ll stick to GPS so I know it’s right. So I changed the spark plugs out which I don’t think has EVER been done before.... so bad, so old. Got some new ones in, changed out the impeller and the water pump housing ended up cracking on me. So, won’t be able to test it out till possibly this weekend when I get the new housing. With the way the sparks look, I should be considerably faster with better acceleration. I’ll probably will replace the wires and cap as well... wires look like it wouldn’t hurt to change out.

If the easy things such as plugs have been overlooked it would be a good guess the carbs have been untouched as well. A proper tune up oughta make that feel like an entirely new boat.

very good point, I’ll have to take a look at the carbs as well. The spark plug wires are Mercruiser suppression... can’t remember the part number.

Top end check in: ran the boat with the new spark plugs and saw a 7.4% increase in top speed! This boat just hasn’t been maintained the way it was suppose to be. The spark plug wires are original and made back in 1989! Gonna put new spark plug wires and a new distributor cap on. Top end speed aside, I’m sure this could use one for the sake of good maintenance. Ran 52.8mph last year and 56.7mph today and I might be able to even get a little more out of it since I don’t think the trim was fully down. I didn’t know at the time but I had bumped the power wire on the outdrive and it stopped working. I doubt I’ll be hitting 68mph but I might be able to hit low 60’s with the trim all the way down and new wires and a new cap.

Trim down to get on plane the. Start trimming up. At least neutral with the hull bottom. Then go to positive trim for speed increase. Now I’m really confused on how plugs made the boat faster. We’re the RPMs the same? Plugs/tuneup usually come into play when the boat isn’t running right. Speed is a simple product of RPM, drive ratio, pitch, and slip. So if you had the same rpm and all else being equal something seems very odd.

Step hulls don’t work like that with trimming. Plugs can get you better acceleration, fuel economy and speed. I had a similar problem with a jet ski I use to own and the spark plugs made a massive difference. Didn’t pay attention to RPM but I could also feel the difference in speed.

I have a stepped hull and they do work like that. Lol. But good luck.

Have yet to see a boat that goes faster the more the bow is buried. And I have a straight bottom and a step.

I really haven’t had much time to mess with the boat but hopefully tomorrow I’m gonna mark the prop and check to see if I get any spinning. Switching the battery to the starboard side as well for listing to port issue. I’ll mess with the trim more but it seems pointless like it doesn’t effect the boat at all.

Well I got about 59mph at 4500RPM today on slightly choppy waters, 1/2 a tank and a total of 4 people onboard. Gained about 5mph trimming up a little bit, it’s a bit weird trimming this boat because you can’t feel when you hit the sweet spot on the trim. Just watched the speed when I was trimming and it didn’t take much trimming.

what propeller you running now?