Speed under power - Merc 50hp 4 stroke - what is your speed?

[MarkSouth]

Finally got my 26X all fixed enough to go out. The engine has just been gone over at the Mercury dealer as the carbs were all gummed up. It tests well on the dyno.

The person I bought the boat from said the previous owner had changed the props, but I didn't think much of it at the time. Now I know what he was talking about.

The WOT (Wide Open Throttle) will go to about 4500 RPM. I understand the engine should go to 5500. The prop currently is a 12.5 x 8. The old prop (in a box) is a 12.5 x 10 which I think is the original.

The boat does 9 knots according to the GPS. The Merc dealer says that the engine is ok like it is and I shouldn't be concerned. But one of the the reasons I got the 26X was the under power speed. I was expecting at least 12 knots or more.

Is this a concern? The ballast tanks were full at the time. After reading here, not sure if they are supposed to be full or not for under power? Does anyone know?

What speed do you power at?

[Russ]

Ballast full will sink the boat a few inches and drastically reduce your speed. Empty the ballast (by opening it at WOT) until you feel the boat lift up and speed up.

I can't speak about the RPMs. That might be related to the ballast also.

[Tomfoolery]

The WOT (Wide Open Throttle) will go to about 4500 RPM. I understand the engine should go to 5500. The prop currently is a 12.5 x 8.

I believe it's 5500-6000 for the EFI 4-stroke. The gear ratio is only 1.83:1, so it's got a relatively fast prop at 3280 rpm. I can see why you'd have an 8" pitch on a 12.5" prop.

Dump the ballast and see what it does. In case you didn't know, you can open the vent in the bow, and open the valve on the transom as you're motor bow-high (like 6 kts+), and it'll empty itself. Close the stern valve before you slow down, or it will refill.

Mine will do about 16 mph (~14.5 kts) WOT with my Honda BF50 4-stroke with 3-blade 11-3/4" x 10" pitch, for reference. That's with 3 men on board and the mast up (sails on but stowed), by the way, but ballast tank empty and really no other gear. I travel light with this boat. I just splashed it with a new 4-blade Solas 11.8" x 9" pitch, but I don't know what it will do at WOT yet. I have it mostly to pull my kids on a wakeboard. Prop speed is 2885 rpm at WOT.

[MarkSouth]

Is there a vent in the bow too? I know there are vents under the step. Mine is a 1997 26X, wonder if they moved the vents?

[Tomfoolery]

Actually, I think they did move the vent, but I don't know when. My removable step is just a place to hold the watertight box that holds the registration card.

[Russ]

I believe the X had the vent hole under the steps where you described. The M has the vent hole under the V-berth.

Yea, empty the ballast and you'll see a big speed improvement. Just don't have fins down or sails up.

[gabid]

Is there a vent in the bow too? I know there are vents under the step. Mine is a 1997 26X, wonder if they moved the vents?

Mine is a 1998 X and the vent is under vberth on the port side. There is nothing under the step.

[Tomfoolery]

Well, that answers my question as to what year the vent was moved. '97 - under the step. '98 - in the v-berth.

[Catigale]

ISO 911 allows Roger to make changes to the boat spanning over 3 model years in an effort to spur future forum traffic

4500 rpm WOT is over propped for that boat and motor Mark, and the other prop is even more pitch ( second number) which will be worse...

Good news is.. You don't have to fix this this season...just be aware you will be lugging the engine at WOT so I would limit use at full throttle.

[Tomfoolery]

ISO 911 allows Roger to make changes to the boat spanning over 3 model years in an effort to spur future forum traffic

Bada-bum - crash

[dive4it]

I have an earlier model with an Evinrude (made by Suzuki) 50 HP on mine. I was amazed when I put a 11 3/8" X 10 pitch 4 blade and only was able to develope 4600 RPM....the amazing part was that with mast up, 3 POB's, and empty ballast, in turned 20.8 mph (not knots). This was done on a relativly flat lake a couple of weeks ago. I know my motor should turn up to 6,000 rpm or a little more so I'm kicking around the idea of going with an 8 pitch, but I don't want to mess with a good thing since 20mph seems too good to be true.

JT

[Tomfoolery]

No bottom paint? Tach not reading correctly? Tailwind? What's your secret?

The Suzuki DF50 is rated at 5500-6500 rpm, if I'm not mistaken. Whatever it is you're doing right, let us know, as I want to do it, too.

I'm getting 16 mph with small waves, 3 POB, mast up, no ballast, and a Honda BF50 with 11-3/4" x 10" 3-blade prop. Maybe this weekend I'll be able to take my new Solas 11.8" x 9" pitch 4-blade prop for a spin at speed.

Oh, and I have old bottom paint, and a bit of slime, all the time.

[dive4it]

No bottom paint? Tach not reading correctly? Tailwind? What's your secret?

The Suzuki DF50 is rated at 5500-6500 rpm, if I'm not mistaken. Whatever it is you're doing right, let us know, as I want to do it, too.

I'm getting 16 mph with small waves, 3 POB, mast up, no ballast, and a Honda BF50 with 11-3/4" x 10" 3-blade prop. Maybe this weekend I'll be able to take my new Solas 11.8" x 9" pitch 4-blade prop for a spin at speed.

Oh, and I have old bottom paint, and a bit of slime, all the time.

I don't know what the heck was up either? I have old bottom paint on mine, I checked the speed going in both directions (oposite each other). There was no wind at the time, it was my first time out with the boat. I had no cargo to speak of, just 5 gallons of water, 10 gallons of fuel, 500 lbs. of people. My eyes about bugged out when I watched the speed on the gps. Anything after 3/4 or 2/3 throttle didn't increase the speed, so I'm sure I'm still overpropped. I also know that it will be compounded when I have a full boat of gear, 20 gallons water, 29 gallons fuel, dive gear, dinghy, etc. That's why I'm thinking the lighter pitch for cruising as I don't want to bogg the motor too much (not that I'll be wanting to go WOT all the time or ever for that matter).

JT

[dive4it]

Of course, it doesn't matter how fast our boats can go, what really matters is that it's just one mile an hour faster than our buddy's boat that we're alongside.

JT

[Tomfoolery]

Anything after 3/4 or 2/3 throttle didn't increase the speed, so I'm sure I'm still overpropped. I also know that it will be compounded when I have a full boat of gear, 20 gallons water, 29 gallons fuel, dive gear, dinghy, etc. That's why I'm thinking the lighter pitch for cruising as I don't want to bogg the motor too much (not that I'll be wanting to go WOT all the time or ever for that matter).

That's what I'd be worried about - not that you're going to run WOT all the time, but that at less throttle, it's still overpropped. Lose and inch or two on the pitch, and let the engine breathe. Unless it's already at full speed, and the tach is wrong, and the throttle control isn't adjusted correctly.

I'd check engine speed with an independent tach, even at the dock (in neutral), just to verify the boat's tach. If it were me. I had a Yanmar inboard that would give two different readings for the same engine speed, depending on what it felt like showing at the time. I sold the boat before I went looking for the problem, but from the sound, the engine speed bands that vibrated, and the boat speed, I knew what the engine was doing (verified via hand-held tach down below with someone else at the helm), and it would sometimes be off by something like 5-600 rpm. Electric tach, so probably poor connections somewhere, but I never got around to finding the problem. Point being, I don't trust tachometers, especially those I don't know very well.