Hey all! I've been staring at propeller numbers for a while now and thought I'd see if I can get some input from some more experienced minds here. I've recently bought my first boat, a 1993 Baja Islander 240 with a stock Merc 7.4L (non Mag, about 650 hours) and Bravo 1 Outdrive. I've only had one successful trip thus far where I got to really open it up, so I don't have a ton of data, but this is what I'm looking at. Last trip it was 2 people on board and a full tank (60 gallons) of fuel. Holeshot is pretty comfortable, I had GREAT success popping out of the water super easily with a little steering to rock the hull. Even without that, holeshot was pretty decent. For speed, farthest I pushed it was about 5200rpm, which got me right about 50 mph. IF I'd stayed on it, I think it had a bit more to go, too. Now, a few problems I have with this:

Pretty dang uncomfortable with the motor turning over 5k. General consensus I've found is that the 4200-4600 WOT Mercruiser specifies may be a tad conservative, but pushing 5k is really rolling the dice on stock internals. I'd probably be happy if I was at 4700-4800 WOT with a light load in the boat, as it'll usually have a few more people on board.

My numbers put into a prop slip calculator show slip at over 27%. This seems like its pretty darn wrong. This is with GPS speedo, and the tach gauge on the dash, NOT a timing light.

This is all on a 3 blade SS Stiletto Advantage II 14.25x21P prop. Thinking of looking for a 4 blade 23P, hoping that it will keep a decent holeshot thanks to the extra blade, and steeper pitch maintaining top end speed at lower RPM? And I'd suspect the extra blade would help with prop slip? I'm not trying to race anybody, more comfortable cruising around 40 than going balls out all day long. Just looking to make sure the motor lasts, only wanting to keep this boat 2-3 years. I'm happy with the current performance of the boat, but want to make sure it doesn't over-rev without losing the holeshot too terribly bad.

Any thoughts from those wiser than me would be greatly appreciated! Thanks!

Welcome to the forum.

Your 240 Islander has a 20 degree deadrise at the transom hull, so it is going to be fairly fast for the power you have. I had a 208 Islander with a 454 (in the very unique Yamaha Hydra-Drive configuration) running a labbed Mirage 21" that ran high 60s (on GPS) at 5200 - 5400 rpm. That boat had a 17 degree deadrise and was about 500 pounds lighter (dry) than yours, so was probably a bit faster. Regardless, your prop slip numbers are atrocious...something isn't right. Either your Faria tachometer is way off (very common, I had to 'thump' mine to get it to settle down) or...something else major is going on. I would expect no more than 10% - 12% maximum with that hull.
Have you verified your speed by the way, using a hand-held GPS or equivalent?

I have no experience with that prop. The only other prop I tried on my 208 was a 23" pitch Laser II, and it sucked out of the hole, would only pull about 4700 rpm and was about 5 -6 mph slower than the Mirage.

My 252 Islander, 454MPI is running a 21" Revolution 4 and I love that prop. Runs about 54 mph @ 4500 rpm is all, but hell, most of my cruising is done around 36 - 40 mph these days. 36 mph is about 3500 rpm so...perfect for me. The hole shot is impressive for that boat/power...even with 90 gallons of fuel and a few passengers, I mean it just gets right with the program! If you can get your hands on one of those, I think you'd like it. It sounds like your engine is getting a bit tired and down some on power, I don't think you'd be able to twist a 23" four blade very successfully.

Super great info, thanks man! This gives me a lot to think on.

Yeah, I definitely think something is up, somewhere. Whether it's in reality or bad calculations. You bring up a good point that I haven't verified speed with a second GPS, nor am I 100% on the tach reading. Good to know the Faria tach inaccuracy is common, that's something to look into for sure. It is worth noting though, that it's not the original gauge. It is Faria, but the whole dash was refreshed a few years back with all new Faria gauages.

So, the speeds you mention are good to know. I'm not expecting any more than 60, but mid 50's would be awesome. As you say, I'm much more comfortable cruising at 35-40 than wide open all day long. I'm out here for fun with family and friends, not beating the crap out of my passengers through chop because I wanna go fast, and I do want to baby this motor a bit. I just want to be able to do that cruising speed at decent rpm and have a bit of headroom to go when I want it. It sounds like your 252 Islander setup is pretty spot on for what I'm looking for, so I'll definitely check out that prop. I do have my concerns about the motor being tired, but for now I'm happy with how it runs, as long is it keeps running! It's got about 650 hours on it, I *think*. Original hour meter is unfortunately wired to key-on, not ignition-on, so I'm not sure how far off that could be (I realized this after I left the key on overnight and woke up to 10 more engine hours, without the motor running; who knows how many other times that might've happened over the last 30 years). Motor does have some good blow by, and not as much oil pressure as I'd really like... But, for now, she's running like a champ.

I'm unsure of brand, but I've got a friend with a spare 4 Blade 21P prop that I'm going to try first, hopefully that'll be great. Or at least give me more info to go from!

it will be great to see your numbers/feedback if you get to try that 4 blade prop. I put on a Mirage Plus three blade, 21" pitch a few months ago to do a comparison test against the Rev 4 and it failed miserably...and the Mirage Plus is a fantastic all-around prop in most applications.
650 hours is probably more like 550 - 600 given the 'key on-meter on' condition, which isn't a lot for a 330 hp 454. A lot of people say to start thinkiing rebuild at the 750 hour range, but I've known owners with these engines that got far more hours than that on them. Do a compression test someday if it interests you, you may find it is in very good shape indeed.
My 252 Islander is 4400 pounds dry and it has a butt-ton of 'stuff' in it, at least another 400 pounds I'd guess between all my tools and skis and bumpers and towels and stereo stuff, etc. I know it and the trailer weigh in at 6,893 pounds on the local scales. So I would say your goal of 60 is certainly achievable, My gut tells me that prop you have on now just isn't a good fit for the boat. Or maybe the hub is spun. Your comment about using a little steering to rock the boat to help it pop on plane is perhaps noteworthy.

It would also be good to get some pics or ideally video of your boat trimmed out perfectly at top speed to see how much of your hull is in the water. My 252 has maybe 4' - 5' in ideal conditions; my 208 was damned near running on just the outdrive, lol! Maybe a foot or two in the water. As they say, a picture says a thousand words.

Yeah, fingers crossed it's decent. I'm planning on keeping this boat maybe 3 years or so, so we'll see how far I want to dig into it. As long as she keeps running, I'm happy!

Yeah, that's a bit heavier. They dry number I've been able to find on the 240 is around 3950. I don't have too much junk in it, yet, maybe 100 pounds of stuff, and it's only a 60 gallon tank.

I like this idea too. I'm guessing it *should* be about that 4' like your 252? Definitely curious to see this on mine.

I can send you a scan of the 1994 Sales Brochure if you'd like.

Since your running an off brand prop it's really hard to make comparisons. All props are different in how they perform. So a three blade this won't be anything like a three blade that. Most with Bajas run Merc props of the Mirage or Bravo designations. So comparing your prop to one of them will be a crap shoot. Find someone that'll let you borrow a 23 pitch Mirage for an hour. Then you'll have a solid base line for us to go on. It might even fit your desires. Once you've ran that Mirage, or even a Bravo, it's easy to say what will put you where you want to be from experience of known brand props.